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Physical Review B

(Condensed Matter and Materials Physics - 15 (II))

January 2006

Volume 73, Number 4 , Articles (04xxxx)

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RAPID COMMUNICATIONS

Electronic structure: wide-band, narrow-band, and strongly correlated systems
Rapid

Negative index materials using simple short wire pairs
Jiangfeng Zhou, Lei Zhang, Gary Tuttle, Thomas Koschny, and Costas M. Soukoulis
Published 4 January 2006 (4 pages)
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Negative refraction is currently achieved by a combination of artificial "electric atoms" (metallic wires with negative electrical permittivity epsilon) and artificial "magnetic atoms" (split-ring resonators with negative magnetic permeability µ). Both epsilon and µ must be negative at the same frequency, which is not easy to achieve at higher than THz frequencies. We introduce improved and simplified structures made of periodic arrays of pairs of short metal wires and continuous wires that offer a potentially simpler approach to building negative index materials. Using simulations and microwave experiments, we have investigated the negative index n properties of short wire-pair structures. We have measured experimentally both the transmittance and the reflectance properties and found unambiguously that n<0. The same is true for epsilon and µ. Our results show that short wire-pair arrays can be used very effectively in producing materials with negative refractive indices.
Rapid

Possible Anderson transition below two dimensions in disordered systems of noninteracting electrons
Yoichi Asada, Keith Slevin, and Tomi Ohtsuki
Published 18 January 2006 (4 pages)
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We investigate the possibility of an Anderson transition below two dimensions in disordered systems of noninteracting electrons with symplectic symmetry. Numerical analysis of energy level statistics and conductance statistics on Sierpinski carpets with spin-orbit coupling indicates the occurrence of an Anderson transition below two dimensions.
Rapid

Electronic state of a doped Mott-Hubbard insulator at finite temperatures studied using the dynamical mean-field theory
A. Camjayi, R. Chitra, and M. J. Rozenberg
Published 19 January 2006 (4 pages)
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We study the electronic state of the doped Mott-Hubbard insulator within the dynamical mean field theory. The evolution of the finite temperature spectral functions as a function of doping show large redistributions of spectral weight in both antiferromagnetic and paramagnetic phases. In particular, a metallic antiferromagnetic state is obtained with a low frequency Slater-splitted quasiparticle peak coexisting with Hubbard bands. In the high temperature paramagnetic metallic phase, upon reducing doping, the system has a crossover through a "bad metal" state characterized by an anomalous shift of the quasiparticle peak away from the Fermi energy. We find that the charge compressibility of the antiferromagnetic metal is dramatically enhanced upon approaching the second order Néel line.
Rapid

Dynamical mean-field study of model double-exchange superlattices
Chungwei Lin, Satoshi Okamoto, and Andrew J. Millis
Published 19 January 2006 (4 pages)
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A theoretical study of [001] "double-exchange" superlattices is presented. The superlattice is defined in terms of an ABO3 perovskite crystal. Itinerant electrons hop among the B sites according to a nearest-neighbor tight binding model and are coupled to classical "core spins." The A sites contain ionic charges arranged to form a [001] superlattice that forces a spatial variation of the mobile electron charge on the B sites. The double-exchange interaction is treated by the dynamical mean-field approximation, while the long-range Coulomb interaction is taken into account by the Hartree approximation. We find the crucial parameter is the Coulomb screening length. Different types of phases are distinguished and the interfaces between them classified.
Rapid

Linear scaling electronic structure calculations and accurate statistical mechanics sampling with noisy forces
Florian R. Krajewski and Michele Parrinello
Published 25 January 2006 (4 pages)
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Numerical simulations based on electronic structure calculations are finding ever growing applications in many areas of physics. A major limiting factor, however, is the cubic scaling of the algorithms used. Building on previous work [Phys. Rev. B 71, 233105 (2005)] we introduce a statistical method for evaluating interatomic forces, which scales linearly with system size and is applicable also to metals. The method is based on exact decomposition of the fermionic determinant and on a mapping onto a field theoretical expression. We solve the problem of an accurate sampling of the Boltzmann distribution with noisy forces. This novel approach can be used in such diverse fields as quantum chromodynamics, quantum Monte Carlo, or colloidal physics.

Semiconductors I: bulk
Rapid

Alignment of isovalent impurity levels: Oxygen impurity in II-VI semiconductors
Jingbo Li and Su-Huai Wei
Published 24 January 2006 (4 pages)
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The isovalent oxygen impurity levels in the II-VI semiconductors ZnSe:O, ZnTe:O, and CdTe:O are studied using a method based on first-principles total energy and large scale charge patching band structure calculations. We find that, unlike the general expectation that these levels line up in an absolute energy scale, the positions of the isovalent a1(O) level depend sensitively on the local environment around the impurity, thus, the a1(O) levels align approximately only in the common-cation systems, whereas in the common-anion systems, the levels do not align. These general chemical trends also apply to other isovalent impurity systems.

Semiconductors II: surfaces, interfaces, microstructures, and related topics
Rapid

Classical memory effects on spin dynamics in two-dimensional systems
I. S. Lyubinskiy and V. Yu. Kachorovskii
Published 3 January 2006 (4 pages)
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We discuss classical dynamics of electron spin in two-dimensional semiconductors with a spin-split spectrum. We focus on a special case when a spin-orbit-induced random magnetic field is directed along a fixed axis. This case is realized in III-V-based quantum wells grown in the [110] direction and also in [100]-grown quantum wells with equal strength of Dresselhaus and Bychko–Rashba spin-orbit couplings. We show that in such wells the long-time spin dynamics is determined by non-Markovian memory effects. Due to these effects a nonexponential tail 1/t2 appears in the spin polarization.
Rapid

Experimental estimation of the electric dipole moment and polarizability of titanyl phthalocyanine using ultraviolet photoelectron spectroscopy
H. Fukagawa, H. Yamane, S. Kera, K. K. Okudaira, and N. Ueno
Published 4 January 2006 (4 pages)
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Changes in the work function (WF) and binding energy of the highest occupied molecular orbital (HOMO) level in a titanyl phthalocyanine-(OTiPc) graphite system were studied as a function of OTiPc coverage by ultraviolet photoelectron spectroscopy. We observed that the WF increases with the coverage of the oriented OTiPc until the formation of the monolayer, while the energy position of the HOMO level stays almost unchanged with respect to the Fermi level. By analyzing the observed coverage dependence of the WF using the Topping model, we estimated the electric dipole moment and polarizability of OTiPc in a monolayer.
Rapid

Multiphoton processes in microwave photoresistance of two-dimensional electron systems
M. A. Zudov, R. R. Du, L. N. Pfeiffer, and K. W. West
Published 9 January 2006 (4 pages)
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We extend our studies of microwave photoresistance of ultrahigh mobility two-dimensional electron systems (2DES) into the high-intensity, nonlinear regime employing both monochromatic and bichromatic radiation. Under high-intensity monochromatic radiation omega we observe zero-resistance states (ZRSs) that correspond to the rational values of epsilon=omega/omegaC (omegaC is the cyclotron frequency) and can be associated with multiphoton processes. Under bichromatic radiation omega1,omega2 we discover a resistance minimum, possibly a precursor of bichromatic ZRSs, which seems to originate from a frequency mixing process, omega1+omega2. These findings indicate that multiphoton processes play important roles in the physics of nonequilibrium transport of microwave-driven 2DES, and suggest new directions for theoretical and experimental studies.
Rapid

Evidence for multiple impurity bands in sodium-doped silicon MOSFETs
T. Ferrus, R. George, C. H. W. Barnes, N. Lumpkin, D. J. Paul, and M. Pepper
Published 13 January 2006 (4 pages)
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We report measurements of the temperature-dependent conductivity in a silicon metal-oxide-semiconductor field-effect transistor that contains sodium impurities in the oxide layer. We explain the variation of conductivity in terms of Coulomb interactions that are partially screened by the proximity of the metal gate. The study of the conductivity exponential prefactor and the localization length as a function of gate voltage have allowed us to determine the electronic density of states and has provided arguments for the presence of two distinct bands and a soft gap at low temperature.
Rapid

Quantum state tomography with quantum shot noise
P. Samuelsson and M. Büttiker
Published 19 January 2006 (4 pages)
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We propose a scheme for a complete reconstruction of one- and two-particle orbital quantum states in mesoscopic conductors. The conductor in the transport state continuously emits orbital quantum states. The orbital states are manipulated by electronic beam splitters and detected by measurements of average currents and zero frequency current shot-noise correlators. We show how, by a suitable complete set of measurements, the elements of the density matrices of the one- and two-particle states can be directly expressed in terms of the currents and current correlators.
Rapid

Lateral diffusive spin transport in layered structures
H. Dery, L. Cywinski, and L. J. Sham
Published 20 January 2006 (4 pages)
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A one-dimensional (1D) theory of lateral spin-polarized transport is derived from the two-dimensional flow in the vertical cross section of a stack of ferromagnetic and paramagnetic layers. This takes into account the influence of the lead on the lateral current underneath, in contrast to the conventional 1D modeling by the collinear configuration of lead/channel/lead. Our theory is convenient and appropriate for the current in-plane configuration of an all-metallic spintronics structure as well as for the planar structure of a semiconductor with ferromagnetic contacts. For both systems we predict the optimal contact width for maximal magnetoresistance and propose an electrical measurement of the spin-diffusion length for a wide range of materials.

Surface physics, nanoscale physics, low-dimensional systems
Rapid

Second-harmonic generation from arrays of symmetric gold nanoparticles
M. D. McMahon, R. Lopez, R. F. Haglund, Jr., E. A. Ray, and P. H. Bunton
Published 11 January 2006 (4 pages)
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We show that second-harmonic light can be generated from a diffraction grating of gold nanoparticles with planar inversion symmetry. By measuring the angular distribution of second-harmonic light, we observe an effect in which the diffraction pattern of the grating is superimposed on the intrinsic second-harmonic radiation pattern of the nanoparticles. This result suggests that the second-harmonic generation may be used to study coherent nonlinear optical effects in symmetric as well as asymmetric metal nanoparticles.
Rapid

Elasticity of hexagonal boron nitride: Inelastic x-ray scattering measurements
Alexey Bosak, Jorge Serrano, Michael Krisch, Kenji Watanabe, Takashi Taniguchi, and Hisao Kanda
Published 19 January 2006 (4 pages)
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The five independent elastic moduli of single-crystalline hexagonal boron nitride (h-BN) are determined using inelastic x-ray scattering. At room temperature the elastic moduli are in units of GPa C11=811, C12=169, C13=0, C33=27.0, and C44=7.7. Our experimental results are compared with predictions of ab initio calculations and previously reported incomplete datasets. These results provide solid background for further theoretical advances and quantitative input to model elasticity in boron nitride (BN) nanotubes.
Rapid

Preferential growth of Pt on rutile TiO2
Hakim Iddir, Vladimir Skavysh, Serdar Ögüt, Nigel D. Browning, and Mark M. Disko
Published 25 January 2006 (4 pages)
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The characterization of a Pt/TiO2 (Degussa P25) catalyst system using atomic resolution Z-contrast images and electron energy loss spectroscopy has recently revealed that Pt particles have a strong tendency to nucleate on the rutile phase of TiO2 rather than on anatase. Comparative ab initio pseudopotential calculations for Pt and Pt2 on the stoichiometric and reduced TiO2 surfaces and for oxygen vacancy (VO) formation energies are performed to address the microscopic origin of this finding. The results, which show that Pt binds more strongly to anatase surfaces, indicate that the selective growth of Pt on rutile must be controlled by the lower formation energy of VO on rutile and possibly by the stronger tendency of larger Pt particles to nucleate at the VO sites of rutile compared to anatase.
Rapid

Mapping the electron correlation in two-electron photoemission
F. O. Schumann, C. Winkler, G. Kerherve, and J. Kirschner
Published 26 January 2006 (4 pages)
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Electronic correlations are manifested in many-body effects like superconductivity and magnetism. Established theoretical concepts show that the Coulomb and exchange interaction result in a tendency of two electrons to avoid each other, leading to an exchange-correlation (xc) hole. We will report on double photoemission (DPE) experiments using a time-of-flight setup consisting of a small central collector surrounded by a resistive anode. The first allows detection only within a narrow solid angle, therefore fixing the momentum. The resistive anode covers a solid angle of ~1  sr, the determination of the impact position results in momentum resolution. As a pulsed light source we used synchrotron radiation and we studied a NaCl(100) surface upon excitation with 34 eV photons. The very existence of coincidences is already a manifestation of the correlation. The onset of pair emission occurs when energy conservation allows the ejection of two electrons from the highest occupied level. We have made two key observations. If E1 and E2 are fixed such that a pair emission from the top of the valence band is possible, a zone of reduced intensity with a diameter of ~1.1  Å–1 is visible. Recent calculations on DPE from a Cu(100) surface display exactly such a feature due to the xc hole. Hence we prove experimentally the very existence of the xc hole in double photoemission. The zone of reduced intensity disappears whenever emission below the top of the valence band becomes possible, indicating the sensitivity of the xc hole to inelastic scattering.
Rapid

Self-organization and geometry control of Pb islands grown on anisotropic substrates
M. Hupalo and M. C. Tringides
Published 27 January 2006 (4 pages)
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The growth of uniform nanostructures requires discovering robust and reproducible ways to control the grown size and geometry. It is found that the growth of Pb on the anisotropic substrate Si(111)-In(4×1) leads to the control of both the island height and island width by exploiting the combined effects of two different stabilization mechanisms. Quantum size effects control the height and result in uniform four-layer islands while the strain anisotropy, due to the underlying reconstruction, controls the width selection.

ARTICLES

Electronic structure: wide-band, narrow-band, and strongly correlated systems

Spin and charge excitations in the anisotropic Hubbard ladder at quarter filling with charge-ordering instability
Y. Ohta, T. Nakaegawa, and S. Ejima
Published 3 January 2006 (8 pages)
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Quantum Monte Carlo and density-matrix renormalization group methods are used to study the coupled spin-pseudospin Hamiltonian in one-dimension (1D) that models the charge-ordering instability of the anisotropic Hubbard ladder at quarter filling. We calculate the temperature dependence of the uniform spin susceptibility and specific heat as well as the spin and charge excitation spectra of the system. We thereby show that there is a parameter and temperature region where the spin degrees of freedom are separated from the charge degrees of freedom and behave like a 1D antiferromagnetic Heisenberg model, and that, outside this parameter region and above a crossover temperature, the spin excitations are largely affected by the charge fluctuations. We argue that observed anomalous spin dynamics in the disorder phase of a typical charge-ordered material alpha[prime]-NaV2O5 may possibly be a consequence of this type of spin-charge coupling.

Boundary conditions for interfaces of electromagnetic crystals and the generalized Ewald-Oseen extinction principle
Pavel A. Belov and Constantin R. Simovski
Published 4 January 2006 (14 pages)
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The problem of plane-wave diffraction on semi-infinite orthorhombic electromagnetic (photonic) crystals of a general kind is considered. Boundary conditions are obtained in the form of infinite system of equations relating amplitudes of incident wave, eigenmodes excited in the crystal, and scattered spatial harmonics. The generalized Ewald-Oseen extinction principle is formulated on the base of deduced boundary conditions. The knowledge of properties of infinite crystal's eigenmodes provides an option to solve the diffraction problem for the corresponding semi-infinite crystal numerically. In the case when the crystal is formed by small inclusions which can be treated as point dipolar scatterers with fixed direction the problem admits complete rigorous analytical solution. The amplitudes of excited modes and scattered spatial harmonics are expressed in terms of the wave vectors of the infinite crystal by closed-form analytical formulas. The result is applied for the study of reflection properties of metamaterial formed by cubic lattice of split-ring resonators.

Resonance width distribution for high-dimensional random media
Matthias Weiss, J. A. Méndez-Bermúdez, and Tsampikos Kottos
Published 4 January 2006 (7 pages)
045103  Full Text: PDF (148 kB)  | Buy Article
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We study the distribution of resonance widths [script P](Gamma) for three-dimensional (3D) random scattering media and analyze how it changes as a function of the randomness strength. We are able to identify in [script P](Gamma) the system-inherent fingerprints of the metallic, localized, and critical regimes. Based on the properties of resonance widths, we also suggest a criterion for determining and analyzing the metal-insulator transition. Our theoretical predictions are verified numerically for the prototypical 3D tight-binding Anderson model.

Assessment of metageneralized gradient approximation and screened Coulomb hybrid density functionals on bulk actinide oxides
Ionut D. Prodan, Gustavo E. Scuseria, and Richard L. Martin
Published 6 January 2006 (10 pages)
045104  Full Text: PDF (1171 kB)  | Buy Article
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Recent advances in the field of density functional theory afford increasingly accurate and efficient studies on a wide range of materials, but validation of any new computational method requires comprehensive benchmarking of its performance on various classes of systems. In the present work, we assess two newly developed density functionals on bulk uranium and plutonium oxides, for which structural, magnetic, and one-electron properties are calculated. The new functionals are the metageneralized gradient approximation (meta-GGA) of Tao, Perdew, Staroverov, and Scuseria (TPSS) and the screened Coulomb hybrid functional of Heyd, Scuseria, and Ernzerhof (HSE). Their predictions are compared to those of the local spin density approximation (LSDA), the Perdew-Burke-Ernzerhof (PBE) realization of the GGA, and a hybrid implementation of the latter (PBE0). The nonhybrid density functionals LSDA, PBE, and TPSS generally fail to provide a satisfactory qualitative description of the electronic and magnetic structure of actinide oxides. TPSS improves upon the LSDA in the prediction of bulk parameters, but only to a level of accuracy comparable to that of PBE. It however outperforms both of them for one-electron properties, as it predicts a nonzero band gap for antiferromagnetic plutonium oxides. HSE is computationally more efficient than its parent functional PBE0, while being at least as accurate for structural, one-electron and magnetic properties. The predictions of HSE agree well with experiment where known, making it suitable for calculations on transitional and f-element compounds.

Coated nonmagnetic spheres with a negative index of refraction at infrared frequencies
Mark S. Wheeler, J. Stewart Aitchison, and Mohammad Mojahedi
Published 6 January 2006 (7 pages)
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A three-dimensional lattice of micron-scale coated spheres is shown to have an isotropic negative index of refraction at infrared frequencies. The materials used are entirely non-magnetic. The Mie scattering theory of the constituent spheres is used in the effective medium theory. The physical mechanisms and procedures are presented in the design of a negative effective permeability with solid polaritonic spheres, as well as a negative effective permittivity with solid Drude spheres. It is then shown that a collection of polaritonic spheres coated with a thin layer of Drude material can exhibit a negative index of refraction at infrared frequencies. Comparison with numerical photonic band structure calculations verifies the theory.

Effect of quantum lattice fluctuations on the Peierls broken-symmetry ground state
William Barford and Robert J. Bursill
Published 9 January 2006 (6 pages)
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We use the density matrix renormalization group method to investigate the role of gapless, dispersive quantized phonons on the Peierls transition in the extended Hubbard-Peierls model. We calculate the staggered phonon order parameter as a function of the Coulomb interaction and the phonon frequency. For all parameter ranges we show that the staggered phonon order parameter scales as sqrt( lambda ) (and the dimerized bond order scales as lambda) as lambda-->0 (where lambda is the electron-phonon interaction). We also show that the quantum phonon predictions for the bond order follow the classical (adiabatic) predictions as a function of inverse chain size for small lambda. We thus conclude that quantum fluctuations in the nuclear degrees of freedom do not destroy the Peierls state for this model.

Electronic structure of transition-metal impurities in p-type ZnO
L. Petit, T. C. Schulthess, A. Svane, Z. Szotek, W. M. Temmerman, and A. Janotti
Published 9 January 2006 (5 pages)
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The self-interaction-corrected local spin-density approximation is used to investigate the ground-state valency configuration of transition metal (TM=Mn,Co) impurities in p-type ZnO. Based on total energy considerations, we find a stable localized TM2+ configuration for a TM impurity in ZnO if no additional hole donors are present. Our calculations indicate that the (+/0) donor level is situated in the band gap, as a consequence of which the TM3+ becomes more favorable in p-type ZnO, where the Fermi level is positioned near the top of the valence band. When codoping with N, it emerges that the conditions for the applicability of the Zener model as proposed by Dietl et al. [Science 287, 1019 (2000)] are fulfilled only in the scenario where the N concentration exceeds the TM impurity concentration.

First-principles study of electronic structure and optical properties of heterodiamond BC2N
Jian Sun, Xiang-Feng Zhou, Ya-Xian Fan, Jing Chen, Hui-Tian Wang, Xiaoju Guo, Julong He, and Yongjun Tian
Published 10 January 2006 (10 pages)
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Heterodiamond BC2N, as a kind of superhard material expectable, is studied using the ab initio pseudopotential density functional method. All the calculations are performed after geometric optimization starting from an eight-atom zinc-blende structure unit cell. For all the structures possible, we calculate in detail the structural parameters, charge transfers, bond populations, band structures, density of states, and optical properties (dielectric function, refractive index, absorption coefficient, reflectivity, electron energy loss spectrum, and photoconductivity). In addition, the optical anisotropy of some structures is also discussed. Our calculated results show that all the structures are metastable and some of them tend to form graphitelike structures and exhibit semimetallic behavior leading to interesting optical properties.

Orbital-selective insulator-metal transition in V2O3 under external pressure
M. S. Laad, L. Craco, and E. Müller-Hartmann
Published 12 January 2006 (15 pages)
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We present a detailed account of the physics of vanadium sesquioxide (V2O3), a benchmark system for studying correlation-induced metal-insulator transition(s). Based on a detailed perusal of a wide range of experimental data, we stress the importance of multiorbital Coulomb interactions in concert with first-principles local-density approximation (LDA) band structure for a consistent understanding of the insulator-metal (IM) transition under pressure. Using LDA+DMFT (dynamical mean-field theory), we show how the IM transition is of the orbital selective type, driven by large changes in dynamical spectral weight in response to small changes in trigonal field splitting under pressure. Very good quantitative agreement with (i) the switch of orbital occupation and (ii) S=1 at each V3+ site across the IM transition, and (iii) carrier effective mass in the paramagnetic phase, is obtained. Finally, using the LDA+DMFT solution, we have estimated screening-induced renormalization of the local, multiorbital Coulomb interactions. Computation of the one-particle spectral function using these screened values is shown to be in excellent quantitative agreement with very recent experimental (photoemission and x-ray-absorption spectroscopy) results. These findings provide strong support for an orbital-selective Mott transition in paramagnetic V2O3.

O(N) LDA+U electronic structure calculation method based on the nonorthogonal pseudoatomic orbital basis
Myung Joon Han, Taisuke Ozaki, and Jaejun Yu
Published 13 January 2006 (11 pages)
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We report an implementation of the LDA+U method based on the state-of-the-art linear combination of pseudo-atomic orbital (LCPAO) method, which is suitable for large-scale O(N) electronic structure calculations based on the density functional theory. By introducing a dual representation of the occupation number matrix instead of the on-site or full representations, the LDA+U formalism is refined to be consistent with a nonorthogonal LCPAO basis in regard to the sum rule of the total number of electrons. For typical transition metal oxide bulk systems, the band gap, magnetic moment, and detailed electronic structures are investigated with the different choices of basis orbitals and effective U values as well as the definition of the occupation number matrix. The results are in good agreement with previous theoretical and experimental studies, indicating that the proposed LDA+U scheme combined with the O(N) method is a quite promising approach for the study of large-scale correlated material systems consisting of localized electrons. We discuss the electronic structure and magnetic properties of (NiO)m/(CoO)n superlattices as an application of our method.

First-principles calculations of hyperfine parameters with the all-electron mixed-basis method
M. S. Bahramy, M. H. F. Sluiter, and Y. Kawazoe
Published 13 January 2006 (21 pages)
045111  Full Text: PDF (467 kB)  | Buy Article
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Within density functional theory, an efficient and accurate method for calculating and analyzing hyperfine parameters has been developed. The so-called mixed-basis method expands the one-electron wave functions in terms of both localized nucleus-centered functions and plane waves and thereby affords an accurate representation for the spin density both in the immediate vicinity of the nucleus and in the bonding regions. The current method is compared with experiment and the best computational methods reported in the literature. The mixed-basis approach is shown to yield highly accurate isotropic and anisotropic hyperfine parameters with modest computational effort. The atom-centered radial representation of the potentials and spin densities allows us to analyze, within the context of density functional theory, the effect of the exchange interaction on the individual core levels in a physically transparent way.

Linear optical properties in the projector-augmented wave methodology
M. Gajdos, K. Hummer, G. Kresse, J. Furthmüller, and F. Bechstedt
Published 17 January 2006 (9 pages)
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In this work we derive closed expressions for the head of the frequency-dependent microscopic polarizability matrix in the projector-augmented wave (PAW) methodology. Contrary to previous applications, the longitudinal expression is utilized, resulting in dielectric properties that are largely independent of the applied potentials. The improved accuracy of the present approach is demonstrated by comparing the longitudinal and transversal expressions of the polarizability matrix for a number of cubic semiconductors and one insulator, i.e., Si, SiC, AlP, GaAs, and diamond (C), respectively. The methodology is readily extendable to more complicated nonlocal Hamiltonians or to the calculation of the macroscopic dielectric matrix including local field effects in the random phase or density functional approximation, which is demonstrated for the previously mentioned model systems. Furthermore, density functional perturbation theory is extended to the PAW method, and the respective results are compared to those obtained by summation over the conduction band states.

Negative refractive index in gyrotropically magnetoelectric media
Jian Qi Shen
Published 18 January 2006 (5 pages)
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The possibility to realize a negative refractive index in gyrotropically magnetoelectric materials is studied. Since the Tellegen (nonreciprocity) parameter may probably dramatically reduce the refractive indices, the negative refractive indices of a gyrotropic chiral material with positive permittivity and permeability can be achieved. By using the concept of equivalent isotropic medium, the respective negative equivalent permittivity and permeability corresponding to the eigenmodes inside the anisotropic material can be derived. This is a scheme to realize the negative refractive index (and hence the backward wave propagation) in artificial composite materials. Besides this scheme, we consider an alternative way to realize negative refraction by means of a magnetoelectrically anisotropic material.

Negative refraction in gyrotropic media
V. M. Agranovich, Yu. N. Gartstein, and A. A. Zakhidov
Published 18 January 2006 (12 pages)
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The gyrotropy (chirality) as one of the manifestations of spatial dispersion responsible, e.g., for the rotation of the plane of polarization of light can also lead to negative refraction (NR) in certain frequency ranges. We discuss this effect for homogeneous isotropic chiral media in a nonresonant frequency domain close to frequency omegaL of longitudinal excitations, where dielectric permittivity vanishes: epsilon(omegaL)=0. Basic phenomena of reflection and refraction at the boundary of a chiral medium are considered and NR is demonstrated for frequencies below omegaL as following from the negative group velocity of circularly polarized electromagnetic waves of one handedness. We show that specular reflection of the polarized waves should provide a means of direct experimental detection of this frequency domain. We also discuss dissipation-related limitations for realization of NR.

Fermi surface and interlayer transport in high-stage MoCl5 graphite intercalation compounds
Kengo Enomoto, Shinya Uji, Takahide Yamaguchi, Taichi Terashima, Takako Konoike, Mitsuka Nishimura, Toshiaki Enoki, Masatsugu Suzuki, and Itsuko S. Suzuki
Published 18 January 2006 (7 pages)
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Shubnikov–de Haas (SdH) and angular-dependent magnetoresistance oscillations (AMRO's) in high-stage MoCl5 graphite intercalation compounds are reported. Several SdH oscillations and AMRO's are observed; however, the background of the angular-dependent magnetoresistance cannot be interpreted by the semiclassical model based on the Boltzmann coherent transport theory. A resistance peak in magnetic fields parallel to the conducting layers is clearly observed, demonstrating coherent transport. The results are discussed in terms of an incoherent motion crossing the magnetic intercalate layers and a coherent motion only in the graphite layers.

Importance of many-body effects to the spectral function of 1T-TiTe2
G. Nicolay, B. Eltner, S. Hüfner, F. Reinert, U. Probst, and E. Bucher
Published 18 January 2006 (5 pages)
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We present a line-shape analysis of high-resolution angle-resolved photoemission spectra of the quasi-two-dimensional system 1T-TiTe2, measured with an energy resolution of 5 meV and an angular resolution of 0.3°. The analysis is based on the spectral function, containing contributions from electron-phonon and electron-electron scattering and a remaining contribution from electron-impurity interaction. Our results show that the low-temperature spectra near kF can only be adequately described by taking all three contributions into account. In particular, the electron-phonon interaction has a distinct effect on the shape of the spectral function close to the Fermi level, making the influence of the electron-electron interaction hardly visible for this material. Furthermore, we discuss the importance of the sample quality and the influence of impurities on the photoemission spectra.

Field dependent quasiparticles in a strongly correlated local system
A. C. Hewson, J. Bauer, and W. Koller
Published 19 January 2006 (15 pages)
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We show how quasiparticles in a magnetic field of arbitrary strength H can be described by field dependent parameters. We illustrate this approach in the case of an Anderson impurity model and use the numerical renormalization group (NRG) to calculate the renormalized parameters for the levels with spin sigma, epsilon-tilded,sigma(H), resonance width Delta-tildesigma(H), and the effective local quasiparticle interaction U(H). In the Kondo or strong correlation limit of the model the progressive de-renormalization of the quasiparticles can be followed as the magnetic field is increased. The low temperature behavior, including the conductivity, in arbitrary magnetic field can be calculated in terms of the field dependent parameters using the renormalized perturbation expansion. Using the NRG the field dependence of the spectral density on higher scales is also calculated.

Phase diagram for Anderson disorder: Beyond single-parameter scaling
Nigel Goldenfeld and Roger Haydock
Published 20 January 2006 (10 pages)
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The Anderson model for independent electrons in a disordered potential is transformed analytically and exactly to a basis of random extended states leading to a variant of augmented space. In addition to the widely accepted phase diagrams in all physical dimensions, a plethora of additional, weaker Anderson transitions are found, characterized by the long-distance behavior of states. Critical disorders are found for Anderson transitions at which the asymptotically dominant sector of augmented space changes for all states at the same disorder. At fixed disorder, critical energies are also found at which the localization properties of states are singular. Under the approximation of single-parameter scaling, this phase diagram reduces to the widely accepted one in one, two, and three dimensions. In two dimensions, in addition to the Anderson transition at infinitesimal disorder, there is a transition between two localized states, characterized by a change in the nature of wave function decay.

Ab initio study of the electronic properties and Fermi surface of the uranium dipnictides
S. Lebègue, P. M. Oppeneer, and O. Eriksson
Published 24 January 2006 (8 pages)
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The electronic structure of the uranium dipnictides UX2 (X=As, Sb, and Bi) is investigated by means of ab initio calculations based on density functional theory. The calculated Fermi surfaces are presented and compared to available experimental models obtained from de Haas–van Alphen experiments. In agreement with experiments they are found to have a significant two-dimensional character. Also, the change of the electronic properties through the series is discussed.

Role of hydrogen atoms in the photoinduced formation of stable electron centers in H-doped 12CaO·7Al2O3
Peter V. Sushko, Alexander L. Shluger, Katsuro Hayashi, Masahiro Hirano, and Hideo Hosono
Published 24 January 2006 (13 pages)
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In this work we investigate a variety of chemical and photoinduced processes in which different hydrogenous species including H2 molecules, H– ions, and H0 atoms interact with the bulk of a complex nanoporous oxide 12CaO·7Al2O3. Our results provide a detailed and consistent explanation of the recently observed phenomenon of photoinduced conversion of the insulating H-doped 12CaO·7Al2O3 to a conductor [K. Hayashi et al., Nature (London) 419, 462 (2002)]. The formation of a large and thermally stable concentration of electron centers in this process is facilitated by a large concentration (up to 1020  cm–3) of extraframework O2– naturally present in this material and homogeneously distributed in its bulk. We show that these species are able to split H2 molecules into pairs of H+ and H– ions and convert H0 atoms into H+ and e– promoting the photoinduced conversion process. The similarity of the mechanisms described in this work to those known for low-coordinated sites at MgO surfaces indicates that the formation of electronic centers in oxides interacting with hydrogenous species could be a generic feature.

Electronic structure and thermopower of Ni(Ti0.5Hf0.5)Sn and related half-Heusler phases
L. Chaput, J. Tobola, P. Pécheur, and H. Scherrer
Published 25 January 2006 (7 pages)
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In this paper we investigate the electronic structure and the thermopower for Ni(Ti0.5Hf0.5)Sn and related half-Heusler compounds. Two different methods have been used to calculate the electronic structure, i.e., full potential linearized augmented plane wave method for ordered compounds and the Korringa-Kohn-Rostoker method within the coherent potential approximation for disordered alloys. We show that these methods give very close results if comparing the density of states obtained in both cases. Moreover, no peculiarities in the band structure have been revealed upon alloying the parent compounds and therefore the large value of the thermopower reported experimentally for Ni(Ti0.5Hf0.5)Sn with respect to NiTiSn or NiHfSn, does not have an origin in the electronic structure behavior. The thermopower calculations performed for different half-Heusler compounds rather suggest that the carrier concentration itself could be predominantly responsible for the large thermopower in Ni(Ti0.5Hf0.5)Sn as well as in other half-Heusler phases. Therefore, the large negative values of the Seebeck coefficient are not limited to some specified half-Heusler semiconductors but seem to be the rule for the n-type samples with moderately low carrier concentrations.

Nonlinear lattice relaxation of photoexcited diplatinum-halide chain compounds
Jun Ohara and Shoji Yamamoto
Published 25 January 2006 (7 pages)
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In order to reveal the relaxation mechanism of photogenerated charge-transfer excitations in quasi-one-dimensional halogen-bridged diplatinum complexes, we calculate the low-lying adiabatic potential energy surfaces of a one-dimensional extended Peierls-Hubbard model. High-energy excitations above the electron-hole continuum may relax into polarons, while excitons pumped within the optical gap are self-localized and then either decay by luminescence or divide into solitons. Neutral solitons, charged solitons, and polarons may be simultaneously photogenerated in a diplatinum-halide chain, which has never been observed in any conventional platinum-halide chain. Optical conductivity is also simulated along the decay paths for experimental verification.

Collective variable-range hopping in the Coulomb gap: Computer simulations
A. M. Somoza, M. Ortuño, and M. Pollak
Published 26 January 2006 (6 pages)
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We studied by computer simulation the variable-range hopping regime in the presence of Coulomb interactions in the strongly localized limit. We took full account of many-body effects by considering the many-electron configurations of the system rather than single-particle states. We studied methodically systems of different sizes. The systematic size dependence allows us to extrapolate our results and to draw conclusions about the importance of many-body effects in macroscopic systems. Many-electron transitions were seen to be important at very low temperatures.

Fermi surface and edge-localized states in graphite studied by high-resolution angle-resolved photoemission spectroscopy
K. Sugawara, T. Sato, S. Souma, T. Takahashi, and H. Suematsu
Published 26 January 2006 (4 pages)
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We have performed ultrahigh-resolution angle-resolved photoemission spectroscopy on graphite single crystal (kish graphite). We have successfully determined the electronic band structure in the close vicinity of the Fermi level and found an extremely small hole-like Fermi surface centered at the K(H) point. We also found a weakly dispersive band near the Fermi level around the K(H) point, which is not predicted by the bulk band calculation. The origin of this anomalous feature is discussed in relation to the electronic states characteristic of the stepped surfaces.

Renormalization-group analysis of the one-dimensional extended Hubbard model with a single impurity
S. Andergassen, T. Enss, V. Meden, W. Metzner, U. Schollwöck, and K. Schönhammer
Published 26 January 2006 (13 pages)
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We analyze the one-dimensional extended Hubbard model with a single static impurity by using a computational technique based on the functional renormalization group. This extends previous work for spinless fermions to spin-1/2 fermions. The underlying approximations are devised for weak interactions and arbitrary impurity strengths, and have been checked by comparing with density-matrix renormalization-group data. We present results for the density of states, the density profile, and the linear conductance. Two-particle backscattering leads to striking effects, which are not captured if the bulk system is approximated by its low-energy fixed point, the Luttinger model. In particular, the expected decrease of spectral weight near the impurity and of the conductance at low energy scales is often preceded by a pronounced increase, and the asymptotic power laws are modified by logarithmic corrections.

Disordered electronic systems. III. Thermoelectric power in alloys with phase separation
Joachim Sonntag
Published 26 January 2006 (11 pages)
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Applying effective medium theory and the Boltzmann transport equation, a formula is derived for calculation of the Seebeck coefficient alpha (thermoelectric power) of alloys with phase separation (composite) under especial consideration of the electrochemical potential µ and its change with temperature T,  dµ/dT. dµ/dT is essentially determined by the electronic structure, carrier densities, and electron-electron-interaction in the phases. For metal-metal composites and metal-insulator composites, alpha can be calculated approximately by [summation]iupsiloni(sigmai/alphai–sigma/alpha)/(sigmai/alphai+2sigma/alpha)~=0 and alpha~=alphaA, respectively, where sigma is the specific electrical conductivity of the composite. sigmai, alphai, and upsiloni are the specific electrical conductivity, the Seebeck coefficient, and the volume fraction, respectively, of the phase i  (i=A,B). Both sigmai and alphai depend on concentration caused by the condition of a common electrochemical potential in the composite. The alpha formulas derived can also be applied to composites with strong scattering. alpha vs x is calculated for a-Cr1–xSix alloys and compared with experimental data.

Fermi surface fluctuations and single electron excitations near Pomeranchuk instability in two dimensions
Luca Dell'Anna and Walter Metzner
Published 27 January 2006 (18 pages)
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A metallic electron system near an orientational symmetry breaking Pomeranchuk instability is characterized by a "soft" Fermi surface with enhanced collective fluctuations. We analyze fluctuation effects in a two-dimensional electron system on a square lattice in the vicinity of a Pomeranchuk instability with d-wave symmetry, using a phenomenological model which includes interactions with a small momentum transfer only. We compute the dynamical density correlations with a d-wave form factor for small momenta and frequencies, the dynamical effective interaction due to a fluctuation exchange, and the electron self-energy. At the quantum critical point the density correlations and the dynamical forward scattering interaction diverge with a dynamical exponent z=3. The singular forward scattering leads to large self-energy corrections, which destroy Fermi liquid behavior over the whole Fermi surface except near the Brillouin zone diagonal. The decay rate of single-particle excitations, which is related to the width of the peaks in the spectral function, exceeds the excitation energy in the low-energy limit. The dispersion of maxima in the spectra flattens strongly near those portions of the Fermi surface which are remote from the zone diagonal. The contribution from classical fluctuations to the self-energy spoils (omega/T) scaling in the quantum critical regime.

Nonanalytic corrections to the specific heat of a three-dimensional Fermi liquid
Andrey V. Chubukov, Dmitrii L. Maslov, and Andrew J. Millis
Published 27 January 2006 (22 pages)
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We revisit the issue of the leading nonanalytic corrections to the temperature dependence of the specific heat coefficient, gamma(T)=C(T)/T, for a system of interacting fermions in three dimensions. We show that the leading temperature dependence of the specific heat coefficient gamma(T)–gamma(0)[proportional]T3  ln  T comes from two physically distinct processes. The first process involves a thermal excitation of a single particle-hole pair, whose components interact via a nonanalytic dynamic vertex. The second process involves an excitation of three particle-hole pairs which interact via the analytic static fixed-point vertex. We show that the single-pair contribution is expressed via the backscattering amplitude of quasiparticles at the Fermi surface. The three-pair contribution does not have a simple expression in terms of scattering in particular directions. We clarify the relation between these results and previous literature on both 3D and 2D systems, and discuss the relation between the nonanalyticities in gamma and those in spin susceptibilities.

Orientation-dependent electron-energy-loss spectroscopy of TiO2: A comparison of theory and experiment
Christian Heiliger, Frank Heyroth, Frank Syrowatka, Hartmut S. Leipner, Igor Maznichenko, Kalevi Kokko, Wolfram Hergert, and Ingrid Mertig
Published 27 January 2006 (8 pages)
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We present a comparison of calculated and measured orientation-dependent electron-energy-loss spectroscopy (EELS) spectra of TiO2. The measurements were carried out with a scanning transmission microscope equipped with a parallel EELS detector. The lateral resolution is about 1  nm and the energy resolution is about 0.3–0.4  eV. The calculations are based on density functional theory. Spin-orbit coupling is included in the theoretical description of the core levels. Many-body corrections are included by a Z+1 approximation. The influence of experimental parameters like the collector and divergence semiangles are discussed in detail. Very good agreement between theoretical and experimental results concerning the peak positions of the spectra is reached by means of the Z+1 approximation. The investigated rutile, anatase, and brookite modifications exhibit pronounced differences in the L3 edge of titanium. The orientation dependence is clearly visible in the oxygen K edge.

Omnidirectional two-dimensional photonic crystal band gap structures
Thomas Haas, Annett Hesse, and Theodor Doll
Published 31 January 2006 (7 pages)
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Omnidirectional (3D) band gaps can evolve in 2(1/2) honeycomb and squared photonic crystal lattices and also in hexagonal crystals [Li and Xia, Phys. Rev. B 64, 153 (2001)]. This study presents a study of the according full parameter space. The calculation of omnidirectional photonic bands is based on the plane wave method for a two-dimensional photonic crystal with an off-plane wave vector kz=k0  sin(theta). The results are fully respecting the parameters index of refraction, the angle of incidence, as well as the structure radii. Omnidirectional band gaps are found for radii between R=0.34 and R=0.5, starting with index values from 3.0 and show maximum relative band gap sizes ranging from 2.7% up to 10%.

Microscopic model for photoinduced magnetism in the molecular complex [Mo(IV)(CN)2(CN-CuL)6]8+ perchlorate
Rajamani Raghunathan, S. Ramasesha, Corine Mathonière, and Valérie Marvaud
Published 31 January 2006 (7 pages)
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A theoretical model for understanding photomagnetism in the heptanuclear complex [Mo(IV)(CN)2(CN-CuL)6]8+ perchlorate is developed. It is a many-body model involving the active orbitals on the transition metal ions. The model is exactly solved using a valence bond approach. The ground state solution of the model is highly degenerate and is spanned by five S=0 states, nine S=1 states, five S=2 states, and one S=3 state. The orbital occupancies in all these states correspond to six Cu(II) ions and one diamagnetic Mo(IV) ion. The optically excited charge-transfer (CT) state in each spin sector occurs at nearly the same excitation energy of 2.993 eV for the physically reasonable parameter values. The degeneracy of the CT states is largest in the S=3 sector and so is the transition dipole moment from the ground state to these excited states. Thus laser irradiation with light of this energy results in most intense absorption in the S=3 sector. The lifetime of the S=3 excited states is also expected to be the largest as the number of states below that energy is very sparse in this spin sector when compared to other spin sectors. These twin features of our model explain the observed photomagnetism in the [Mo(IV)(CN)2(CN-CuL)6]8+ complex.

Magnetic susceptibility anisotropies in a two-dimensional quantum Heisenberg antiferromagnet with Dzyaloshinskii-Moriya interactions
M. B. Silva Neto, L. Benfatto, V. Juricic, and C. Morais Smith
Published 31 January 2006 (6 pages)
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The magnetic and thermodynamic properties of the two-dimensional quantum Heisenberg antiferromagnet (QHAF) that incorporates both a Dzyaloshinskii-Moriya and pseudodipolar interactions are studied within the framework of a generalized nonlinear sigma model. We calculate the static uniform susceptibility and sublattice magnetization as a function of temperature and we show that (i) the magnetic response is anisotropic and differs qualitatively from the expected behavior of a conventional easy-axis QHAF; (ii) the Néel second-order phase transition becomes a crossover, for a magnetic field B[perpendicular]CuO2 layers. We provide a simple and clear explanation for all the recently reported unusual magnetic anisotropies in the low-field susceptibility of La2CuO4 [L. N. Lavrov et al., Phys. Rev. Lett. 87, 017007 (2001)], and we demonstrate explicitly why La2CuO4 cannot be classified as an ordinary easy-axis antiferromagnet.

Construction of model dielectric functions for two- and three-dimensional electron liquids from density functionals
A. P. Fávaro, K. Capelle, and João Vítor Batista Ferreira
Published 31 January 2006 (7 pages)
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The Thomas-Fermi (TF) approximation for the static dielectric function of a three-dimensional (3D) electron liquid can be derived by minimizing the TF local-density approximation for the kinetic-energy functional. Here we show that this connection between energy functionals and model dielectric functions is not an artifact, but a general paradigm. Four examples are worked out in detail, by calculating the model dielectric functions that follow, respectively, from (i) exchange corrections to TF theory in 3D, i.e., TF-Dirac theory, (ii) further correlation corrections to TF-Dirac theory in 3D, (iii) TF theory in two dimensions (2D), and (iv) exchange corrections to TF theory in 2D. Each of these cases has certain interesting features, revealing connections between independent many-body methods, that are discussed in some detail. As a by-product of these investigations we also find that a common textbook statement about the long-wavelength (k-->0) limit of the random-phase approximation is not fully correct.

Semiconductors I: bulk

Ab initio calculation of excitons in ZnO
Robert Laskowski and Niels Egede Christensen
Published 5 January 2006 (7 pages)
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The optical absorption and excitonic properties of wurtzite ZnO are investigated by means of an ab initio approach taking into account electron-hole correlations. This is done by solving the Bethe-Salpeter equation, using the results of density functional theory calculations as a starting point. Our main focus is the calculation of the band edge optical spectra. We have identified ground states for three excitons (A, B, and C), with binding energies around 68  meV. Excitons A and B are excited mainly by light polarized perpendicular to the c crystallographic axis. The C exciton absorbs mainly light polarized parallel to the c axis. Due to spin-orbit interactions, excitons A and C show a tiny absorption for the parallel polarization and perpendicular polarizations, respectively.

Ab initio calculation of the electronic structure and spectroscopic properties of spinel gamma-Sn3N4
W. Y. Ching and Paul Rulis
Published 5 January 2006 (9 pages)
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The electronic structure and physical properties of gamma-Sn3N4 in the spinel structure are investigated by first-principles calculations. The calculated band structure, electronic bonding, and optical properties are compared with two well-studied spinel nitrides gamma-Si3N4 and gamma-Ge3N4. gamma-Sn3N4 is a semiconductor with a direct band gap of 1.40 eV and an attractive small electron effective mass of 0.17. Its optical properties are different from that of gamma-Si3N4 and gamma-Ge3N4 because of the difference in the conduction band minimum. The Sn K, Sn L3, Sn M5, and N K edges of the x-ray-absorption near-edge structure spectra in gamma-Sn3N4 are calculated using a supercell approach and are found to be rich in structures. These spectra are discussed in the context of the electronic structure of the unoccupied conduction band in the presence of the electron core-hole interaction. These calculated spectra can be used for the characterization of this novel compound.

Exchange interactions in ZnMeO (Me=Mn,Fe,Co,Ni): Calculations using the frozen-magnon technique
L. M. Sandratskii and P. Bruno
Published 5 January 2006 (6 pages)
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We report the density-functional-theory study of the electronic structure and exchange interactions in four diluted magnetic semiconductors (ZnMe)O, Me=Mn,Fe,Co,Ni. The calculations are performed for two impurity concentrations of 25% and 6.25%. For all systems the interatomic exchange interactions are short range with only the interaction between the nearest 3d impurities being sizable. For (ZnMn)O, the leading exchange interaction is antiferromagnetic for both impurity concentrations. In (ZnCo)O, the leading exchange interaction is ferromagnetic for high concentration and antiferromagnetic for low concentration. (ZnFe)O and (ZnNi)O are ferromagnetic for both concentrations. In all cases of the antiferromagnetic exchange interaction, the energy bands are either completely filled or empty. This reveals the connection between the presence of the charge carriers and ferromagnetism.

Effect of intermolecular disorder on the intrachain charge transport in ladder-type poly(p-phenylenes)
P. Prins, F. C. Grozema, J. M. Schins, T. J. Savenije, S. Patil, U. Scherf, and L. D. A. Siebbeles
Published 11 January 2006 (10 pages)
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We have studied the real and imaginary parts of the complex intrachain mobility of charge carriers on solid samples of ladder-type polymers using time-resolved microwave conductivity measurements. Experiments on samples with a different average polymer chain length show that the motion of charge carriers is limited by the chain ends. The experimental results can be described by one-dimensional diffusive motion along the polymer backbone. The intrachain mobility deduced for an infinitely long ladder-type polymer chain (with no barriers to charge transport such as defects or conjugation breaks) subject to interchain interactions is 30  cm2/V  s. This value is 20 times lower than the intrachain mobility found for charges along infinitely long isolated ladder-type polymer chains in dilute solution. Thus we find that interchain interactions in solid samples severely decrease the intrachain charge carrier mobility. However, the intrachain mobility of 30  cm2/V  s is more than four orders of magnitude higher than mobility values obtained from time of flight measurements reported in the literature. Hence, the performance of ladder-type polymers in optoelectronic devices can be significantly improved.

Effective-mass approximation for shallow donors in uniaxial indirect band-gap crystals and application to 4H-SiC
I. G. Ivanov, A. Stelmach, M. Kleverman, and E. Janzén
Published 18 January 2006 (12 pages)
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The effective-mass theory is applied for description of the electronic states of shallow donors in indirect band-gap uniaxial crystals, which have three different components of the electron effective-mass tensor, and two different components of the tensor of the dielectric constant. The Hamiltonian in the resulting Schrödinger equation for the envelope function has D2h symmetry and, after proper parametrization, a nonvariational numerical method is used for its solution. Two particular cases of D[infinity]h symmetry are identified and discussed separately. The comparison between theory and experiment for the 4H polytype of silicon carbide is revised using the least-squares method to determine the binding energies of the ground state of the most shallow nitrogen donor in this material, its valley-orbit split-off counterpart, and the mean value of the dielectric constant, and completed with calculation of the theoretical transition probabilities. In addition, the lowest-lying binding energies of the states, between which optical transitions are allowed, are calculated on a grid of values of the two parameters describing the anisotropy and the tabulated values can be used for interpolation to describe other materials.

Analytic bond-order potential for the gallium arsenide system
D. A. Murdick, X. W. Zhou, H. N. G. Wadley, D. Nguyen-Manh, R. Drautz, and D. G. Pettifor
Published 20 January 2006 (20 pages)
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An analytic, bond-order potential (BOP) is proposed and parametrized for the gallium arsenide system. The potential addresses primary (sigma) and secondary (pi) bonding and the valence-dependent character of heteroatomic bonding, and it can be combined with an electron counting potential to address the distribution of electrons on the GaAs surface. The potential was derived from a tight-binding description of covalent bonding by retaining the first two levels of an expanded Green's function for the sigma and pi bond-order terms. Predictions using the potential were compared with independent estimates for the structures and binding energy of small clusters (dimers, trimers, and tetramers) and for various bulk lattices with coordinations varying from 4 to 12. The structure and energies of simple point defects and melting transitions were also investigated. The relative stabilities of the (001) surface reconstructions of GaAs were well predicted, especially under high-arsenic-overpressure conditions. The structural and binding energy trends of this GaAs BOP generally match experimental observations and ab initio calculations.

Dynamics of the coherent ground state in intermediate-valent YbB12
B. Gorshunov, P. Haas, O. Ushakov, M. Dressel, and F. Iga
Published 23 January 2006 (5 pages)
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The optical properties of the intermediate valence semiconductor YbB12 are studied at frequencies 8  cm–1–10  000  cm–1 (quantum energies 1 meV to 1.2 eV) and temperatures 5 K–300 K. Microscopic parameters of charge carriers are determined: effective mass, mobility, relaxation rate, relaxation time, plasma frequency. A strong decrease of the relaxation rate is observed when cooled down to 70 K, indicating that scattering is mediated mainly by phonons and not by the Kondo effect. Below the coherence temperature of T*=70  K a Drude-like response of renormalized quasiparticles is observed with a scattering rate gamma following the Fermi liquid behavior gamma[proportional]T2, an effective mass m*[approximate]34m0, and an enhanced relaxation time tau=4×10–13  s, implying that coherent effects play a crucial role in formation of the ground state in YbB12. At T=5  K an absorption peak is found at 2.7 meV whose origin can be ascribed to an exciton-polaronic bound state.

Characterization of E[prime]delta and triplet point defects in oxygen-deficient amorphous silicon dioxide
G. Buscarino, S. Agnello, and F. M. Gelardi
Published 27 January 2006 (8 pages)
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We report an experimental study by electron paramagnetic resonance (EPR) of gamma-ray irradiation induced point defects in oxygen deficient amorphous SiO2 materials. We have found that three intrinsic (E[prime]gamma, E[prime]delta, and triplet) and one extrinsic ([AlO4]0) paramagnetic centers are induced. All the paramagnetic defects but E[prime]gamma center are found to reach a concentration limit value for doses above 103  kGy, suggesting a generation process from precursors. Isochronal thermal treatments of a sample irradiated at 103  kGy have shown that for T>=500  K the concentrations of E[prime]gamma and E[prime]delta centers increase concomitantly to the decrease of [AlO4]0. This occurrence speaks for a hole transfer process from [AlO4]0 centers to diamagnetic precursors of E[prime] centers proving the positive charge state of the thermally induced E[prime]gamma and E[prime]delta centers and giving insight on the origin of E[prime]gamma from an oxygen vacancy. A comparative study of the E[prime]delta center and of the 10  mT doublet EPR signals on three distinct materials subjected to isochronal and isothermal treatments has shown a quite general linear correlation between these two EPR signals. This result confirms the attribution of the 10  mT doublet to the hyperfine structure of the E[prime]delta center, originating from the interaction of the unpaired electron with a nucleus of 29Si (I=1/2). Analogies between the microwave saturation properties of E[prime]gamma and E[prime]delta centers and between those of their hyperfine structures are found and suggest that the unpaired electron wave function involves similar Si sp3 hybrid orbitals; specifically, for the E[prime]delta the unpaired electron is supposed to be delocalized over four such orbitals of four equivalent Si atoms. Information on the structural model of the triplet center are also obtained indicating that it could consist of the same microscopic structure as the E[prime]delta but for a doubly ionized state.

Tight-binding study of the magnetic properties of Mn-doped Ge
Guoqiang Liu and Bang-Gui Liu
Published 30 January 2006 (5 pages)
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A generalized tight-binding bond Stoner model satisfying charge self-consistence is used to study Mn-doped Ge. We fit the tight-binding parameters to first-principles nonmagnetic band structures of zinc blende MnGe, and then calculate the ferromagnetic and antiferromagnetic band structures with the local stoner model. The predicted band structures match the first-principles ferromagnetic and antiferromagnetic calculations of the MnGe well and reveal that the MnGe is a half-metallic ferromagnet with integer magnetic moment in Bohr magneton. Applied to low concentrations of Mn doping, this model produces good magnetic moments, spin-dependent density of states, and energetics for Mn doping concentrations down to 0.03125. These results are in agreement with corresponding first-principles calculations. This method can be applied to other transition-metal doped semiconductors.

Electronic structure of intrinsic defects in crystalline germanium telluride
Arthur H. Edwards, Andrew C. Pineda, Peter A. Schultz, Marcus G. Martin, Aidan P. Thompson, Harold P. Hjalmarson, and Cyrus J. Umrigar
Published 31 January 2006 (13 pages)
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Germanium telluride undergoes rapid transition between polycrystalline and amorphous states under either optical or electrical excitation. While the crystalline phases are predicted to be semiconductors, polycrystalline germanium telluride always exhibits p-type metallic conductivity. We present a study of the electronic structure and formation energies of the vacancy and antisite defects in both known crystalline phases. We show that these intrinsic defects determine the nature of free-carrier transport in crystalline germanium telluride. Germanium vacancies require roughly one-third the energy of the other three defects to form, making this by far the most favorable intrinsic defect. While the tellurium antisite and vacancy induce gap states, the germanium counterparts do not. A simple counting argument, reinforced by integration over the density of states, predicts that the germanium vacancy leads to empty states at the top of the valence band, thus giving a complete explanation of the observed p-type metallic conduction.

Phonon and free-charge-carrier properties of Zn1–xMnxSe epilayers grown by molecular-beam epitaxy
K. C. Agarwal, B. Daniel, C. Klingshirn, and M. Hetterich
Published 31 January 2006 (9 pages)
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Far infrared and mid infrared (FIR-MIR) reflectivity measurements have been carried out on two different series of undoped and chlorine-doped Zn1–xMnxSe epilayers grown by molecular-beam epitaxy. The FIR reflection studies performed upon the series of undoped Zn1–xMnxSe samples suggest an intermediate-mode behavior for the optical phonon modes in the composition range (0<=x<=0.43). In addition to the known ZnSe-like and MnSe-like phonon resonances a "weak-mode" feature is found below the MnSe-like phonon band. The frequency of this feature shows a temperature and Mn dependent redshift. We suggest that this feature originates from the alloy disorder present in the samples. The shape of the reststrahlen band is found to change significantly with layer thickness. This is discussed in some detail for pure ZnSe epilayers. To extract some information about the electron effective mass, we performed room-temperature plasma edge measurements on chlorine-doped n-type Zn1–xMnxSe epilayers (0<=x<=0.13). Via Drude-Lorentz-type multioscillator fits to our data, we extracted the optical electron effective mass (mop<sup>*</sup>) in a series of doped Zn(Mn)Se:Cl samples with different Mn contents and free-electron concentrations. Our results indicate that mop<sup>*</sup> in Zn1–xMnxSe is lower than that for ZnSe. In n-type chlorine-doped ZnSe samples with different free-electron concentrations, mop<sup>*</sup> varied from 0.133  m0 to 0.152  m0, while in Zn0.87Mn0.13Se:Cl samples, we found a variation from 0.095  m0 to 0.115  m0 within about ±9% experimental accuracy. From theoretical calculations, we determined the extrapolated room-temperature band-edge electron mass in ZnSe and Zn0.87Mn0.13Se to be about 0.132  m0 and 0.093  m0, respectively. Additionally, from electrical Hall effect measurements on Zn1–xMnxSe:Cl epilayers, we found a drastic reduction in the free-electron concentration with increasing Mn content. The incorporation of Mn increases the resistivity and decreases the mobility of the free charge carriers in the samples.

Semiconductors II: surfaces, interfaces, microstructures, and related topics

Single-exciton spectroscopy of semimagnetic quantum dots
J. Fernández-Rossier
Published 3 January 2006 (7 pages)
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A photoexcited II-VI semiconductor quantum dots doped with a few Mn spins is considered. The effects of spin-exciton interactions and the resulting multispin correlations on the photoluminescence are calculated by numerical diagonalization of the Hamiltonian, including exchange interaction between electrons, holes, and Mn spins, as well as spin-orbit interaction. The results provide a unified description of recent experiments on the photoluminesnce of dots with one and many Mn atoms as well as optically induced ferromagnetism in semimagnetic quantum dots.

Chemical tuning of band alignments for metal gate/high-kappa oxide interfaces
Y. F. Dong, S. J. Wang, Y. P. Feng, and A. C. H. Huan
Published 3 January 2006 (5 pages)
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We report a method for chemical tuning of band alignments for metal gate/high-kappa oxide interfaces. A heterovalent metal interlayer was included between the metal gate electrode and high-kappa oxide. Based on our first-principles calculations for Ni/ZrO2(001) interfaces, a tunability as wide as 2.8  eV can be achieved for the effective work function of metal gate on high-kappa oxide which far exceeds the required tuning range. In addition, we found a simple linear relationship between the effective metal work function and the electronegativity of interlayer metal atom for most of the transition metals considered. The localized interfacial dipole was found to dominate the contribution to the formation of Schottky barrier heights at metal/dielectric oxide interfaces.

Magnetotransport in two-dimensional electron gas: The interplay between spin-orbit interaction and Zeeman splitting
W. Yang and Kai Chang
Published 3 January 2006 (11 pages)
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We investigate theoretically the interplay between Zeeman splitting, Rashba spin-orbit interaction (RSOI), and Dresselhaus spin-orbit interaction (DSOI) and its influence on the magnetotransport property of two-dimensional electron gas (2DEG) at low temperature. Our theoretical results show that the nodes of the beating patterns of the magnetoresistivity rhoxx for 2DEG with RSOI or DSOI alone depend sensitively on the total spin splitting induced by these three spin splitting mechanisms. It is interesting to find that the eigenstates in the presence of RSOI alone are connected with those in the presence of DSOI alone but with opposite Zeeman splitting by a time-reversal transformation. Consequently, the magnetoresistivities exhibit exactly the same oscillation patterns for these two cases. For strong RSOI or DSOI alone, the magneto-oscillation of rhoxx shows two distinct periods. For 2DEG with both RSOI and DSOI, the beating patterns vanish for equal RSOI and DSOI strengths and vanishing Zeeman splitting. They will appear again, however, when Zeeman splitting or the difference between RSOI and DSOI strengths increases.

Spin relaxation in lateral quantum dots: Effects of spin-orbit interaction
Marian Florescu and Pawel Hawrylak
Published 4 January 2006 (17 pages)
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We report results of calculations of the effect of spin-orbit interaction on electron spin relaxation in a lateral quantum dot. Our study is motivated by puzzling results of high source-drain transport measurements of singlet-triplet transitions of two electrons in lateral and vertical devices that show a strong asymmetry as a function of the applied magnetic field. Using exact diagonalization techniques, we investigate the influence of the spin-orbit interaction on the energy levels of a two-electron droplet and we show that the spin-orbit interaction strongly affects the expectation values of the total and z-projection spins of the two-electron system. We then evaluate the energy relaxation rates for the two-electron droplet through the emission of longitudinal acoustic phonons and show that they are strongly dependent on the spin energy levels involved in the process. Our study shows that the spin-orbit interaction provides an effective coupling between the spin-polarized triplet states and the singlet state. However, the transition involving the spin singlet and the unpolarized triplet component is very weak even in the presence of spin-orbit interaction. The calculated scattering rates from the excited states to the ground state of the two-electron system clearly confirm this picture and reveal a microsecond time scale for the single-triplet relaxation through spin-orbit-mediated acoustic phonon emission and the relaxation mechanism presents a built-in magnetic field asymmetry, in qualitative agreement with experimental findings.

Spin-lattice relaxation of Mn ions in ZnMnSe/ZnBeSe quantum wells measured under pulsed photoexcitation
M. K. Kneip, D. R. Yakovlev, M. Bayer, A. A. Maksimov, I. I. Tartakovskii, D. Keller, W. Ossau, L. W. Molenkamp, and A. Waag
Published 9 January 2006 (8 pages)
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The dynamics of spin-lattice relaxation of the Mn ions in (Zn,Mn)Se-based diluted-magnetic-semiconductor quantum wells is studied by time-resolved photoluminescence. The spin-lattice relaxation time varies by five orders of magnitude from 10–3 down to 10–8  s, when the Mn content increases from 0.4 up to 11%. Free carriers play an important role in this dynamics. Hot carriers with excess kinetic energy contribute to heating of the Mn system, while cooling of the Mn system occurs in the presence of cold background carriers provided by modulation doping. In a Zn0.89Mn0.11Se quantum well structure, where the spin-lattice relaxation process is considerably shorter than the characteristic lifetime of nonequilibrium phonons, also the phonon dynamics and its contribution to heating of the Mn system are investigated.

Electronic transport through a quantum-dot ring
Kuo Bao and Yisong Zheng
Published 10 January 2006 (11 pages)
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Electronic transport through a mesoscopic ring consisting of coupled quantum dots is theoretically studied. First, an analytical expression about the linear conductance is obtained as a function of the quantum-dot level, which can be adjusted by a gate voltage. Then the linear conductance spectrum (the linear conductance versus the quantum-dot level) is calculated numerically. The peaks and zero points in this spectrum, due to resonance and antiresonance, respectively, are discussed in detail. The following interesting results are found for even-numbered quantum-dot rings in some specific coupling configurations with two electron reservoirs. When all the quantum-dot levels are aligned with the Fermi level, a resonant peak occurs in the spectrum if the number of quantum dots in the ring is a multiple of four. But such a peak is so fragile that a very small magnetic field perpendicular to the ring plane can change it into an antiresonant zero point. Contrarily, for any other even-numbered quantum-dot rings, such a sudden resonance-antiresonance transition does not occur. Instead, an antiresonant zero point persists in the spectrum, irrespective of whether a magnetic field is present or not. In addition, when an even-numbered quantum-dot ring couples to two electron reservoirs symmetrically, an appropriate magnetic field can completely suppress the overall linear conductance spectrum. All these results can be well explained by the decoupling effect of some eigenstates of the ring from the leads. Finally, the persistent current in a quantum-dot ring is calculated. It is found that the extrema and the zero points of the persistent current correspond to the valleys and peaks of the linear conductance, respectively.

Optical initialization and dynamics of spin in a remotely doped quantum well
T. A. Kennedy, A. Shabaev, M. Scheibner, Al. L. Efros, A. S. Bracker, and D. Gammon
Published 10 January 2006 (6 pages)
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The excitation of electron spin polarization and coherence by picosecond light pulses and their dynamics in a wide remotely doped quantum well are studied theoretically and experimentally. Assuming that all electrons in the quantum well are localized, the theory considers the resonant interaction of light pulses with the four-level system formed by the electron spins of the ground state and the hole spins of the trion excited state. The theory describes the effects of spontaneous emission, a transverse magnetic field and hole spin relaxation on the dynamics detected by the Kerr rotation of a probe pulse. Time resolved Kerr rotation experiments were carried out on a remotely doped 14  nm GaAs quantum well in the frequency range of optical transitions to the heavy hole (HH) trion and to the light-hole (LH) trion degenerate with the HH exciton. The experiments on the resonant excitation of the HH trion show a very slow heavy hole spin relaxation and, consequently, a weak electron spin polarization after the trion relaxation. In contrast, the resonant excitation of the LH trion/HH exciton results in a fast hole spin relaxation that increases electron spin polarization.

Indium incorporation in In-rich InxGa1–xAs/GaAs layers grown by low-pressure metalorganic vapor-phase epitaxy and its influence on the growth of self-assembled quantum dots
Guillaume Saint-Girons, Isabelle Sagnes, and Gilles Patriarche
Published 11 January 2006 (8 pages)
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We report a study of the interaction between In-alkyls and Ga-alkyls during the low-pressure metalorganic vapor-phase epitaxy (LP-MOVPE) of In-rich InxGa1–xAs layers around 500  °C. The structural properties of the layers are determined by analyzing their transmission electron microscopy contrast. The evolution of growth rate and composition of the InxGa1–xAs layers grown under various Ga precursor flow rates is interpreted by modeling the growth kinetics. This approach allows us to identify the main alkyl species present at the growth interface. In particular, it is shown that the presence of monomethylgallium and dimethylgallium on the growth surface enhances the incorporation of In in the layers, promoting the removal of the methyl radical from the monomethylindium molecules by methyl-exchange reactions. This mechanism plays a dominant role in the incorporation of In in our growth conditions: more than 70% of the In atoms incorporated in the layers undergo a methyl-exchange reaction on the growth surface. The structural properties of quantum dots (QDs) grown by depositing InxGa1–xAs layers with various In compositions are compared. An increase of the gallium precursor flow rate during the growth of the QDs is shown to increase their density (between a few 109 and a few 1010  cm–2) and to modify strongly their structural properties.

Scanning tunneling spectroscopy and topography of Si(111)-c2×8 and coexisting 7×7 and 2×1 reconstructions: Surface electronic band structure
Franck Rose, Shigeki Kawai, Takanori Ishii, and Hideki Kawakatsu
Published 11 January 2006 (7 pages)
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The electronic band structure of the Si(111)-c2×8 surface is reported by means of scanning tunneling spectroscopy. The surface is found to be semiconducting with adatom and rest atom bands lying inside the 0.8  eV energy band gap. The local density of states as well as the surface reactivity toward residual hydrogen atoms adsorption are compared for the c2×8, 7×7, and 2×1 reconstructions coexisting on the same (111) quenched surface. We show that the rest atoms of the Si(111)-c2×8 surface are the sites for individual hydrogen adsorption, inducing a local reverse charge transfer from the reconstruction.

Single-photon-induced random telegraph signal in a two-dimensional multiple-tunnel-junction array
Ratno Nuryadi, Yasuhiko Ishikawa, and Michiharu Tabe
Published 11 January 2006 (7 pages)
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A single-photon detection using a Si-based two-dimensional (2D) multiple-tunnel-junction field-effect transistor (FET) is reported. The single photon is detected as a random telegraph signal (RTS) in the single-hole-tunneling current regime. The frequency of RTS events depends on the light wavelength and intensity, indicating that the RTS occurs due to the single-photon absorption in the Si dots forming the 2D multijunctions. Based on a Monte Carlo simulation using an equivalent circuit representing the 2D multijunction FET, the "on" state of RTS appears when the photogenerated charge in the dot sensitively shifts the current level, while the on state returns to the "off" state when the charged dot is neutralized. The simulation result also shows that the RTS is triggered not only by the charging of a dot near the current percolation path, but also by the charging of a dot distances away from the path due to the nature of the multijunction system, i.e., the long screening length of the charge-induced potential. These results open up the development of single-photon devices with 2D multijunction systems.

Thermal stability, atomic vibrational dynamics, and superheating of confined interfacial Sn layers in Sn/Si multilayers
B. Roldan Cuenya, W. Keune, W. A. Adeagbo, and P. Entel
Published 12 January 2006 (11 pages)
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Multilayers composed of materials with low (Sn) and high (Si) bulk melting points were grown at room temperature by ultrahigh vacuum deposition. 119Sn Mössbauer spectroscopy has been used to investigate the temperature dependence of the Debye-Waller factor f, the mean-square displacement, and the mean-square velocity of 119Sn nuclei in ultrathin (10  Å thick) alpha-like Sn layers embedded between 50  Å thick Si layers. The f factor was found to be nonzero with a value of 0.036±0.009 even at 450  °C. This provides unequivocal proof of the solid state of the confined alpha-like Sn layers at least up to 450  °C. Melting can only be achieved by superheating to T>450  °C. This temperature is significantly higher than the melting temperature of bulk beta-Sn (231.9  °C) and of a nonconfined epitaxial alpha-Sn single layer grown on InSb(111) (170  °C) previously reported in the literature [T. Osaka et al., Phys. Rev. B 50, 7567 (1994)]. Our molecular dynamics calculations show that melting of bulk-like alpha-Sn starts at ~380  °C and is complete at ~530  °C according to the Lindemann criterion. Since we still observe the solid state at 450  °C for the confined alpha-like Sn films, considerable superheating is observed for this system. The stability of the ultrathin confined alpha-like Sn layers arises from electronic interactions with the surrounding Si layers, as evidenced by the Mössbauer chemical shift.

Strain determination in MBE-grown InAs quantum wires on InP
A. Mazuelas, L. González, J. M. García, Y. González, T. Schuelli, C. Priester, and H. T. Metzger
Published 12 January 2006 (6 pages)
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We have determined the strain in the three crystallographic directions in InAs quantum wires (QWr) grown by molecular beam epitaxy on (001)InP substrate. We used triple crystal x-ray diffraction to make scans along and perpendicular to the QWr direction in reciprocal space, around InP(220) reflections. We use the shape and strain sensitivity of the different scans to deconvolute both contributions. We used the alphaf scan analysis in grazing incidence diffraction to measure the strain relaxation perpendicular to the QWr as a function of height in the wire. We finally compare these results with finite elements calculations of the strain tensor in InAs QWr on InP.

Spin filtering through magnetic-field-modulated double quantum dot structures
P. Brusheim and H. Q. Xu
Published 12 January 2006 (5 pages)
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