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Physical Review B

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

November 2009

Volume 80, Number 20 , partial issue

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RAPID COMMUNICATIONS

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

Electronic structure of FeCr2S4: Evidence of Coulomb enhanced spin-orbit splitting
Soumyajit Sarkar, Molly De Raychaudhury, I. Dasgupta, and T. Saha-Dasgupta
Published 9 November 2009 (4 pages)
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The electronic structure of the spinel compound, FeCr2S4, is studied using density-functional-theory-based calculations. Our calculations provide a microscopic understanding of the origin of the insulating behavior of this compound, which turn out to be driven by Coulomb enhanced spin-orbit coupling operative within the Fe-d manifold. We also investigate the possible role of the structural distortions and compare the calculated optical property data with that of the experimental one.
Rapid

Edge exponent in the dynamic spin structure factor of the Yang-Gaudin model
M. B. Zvonarev, V. V. Cheianov, and T. Giamarchi
Published 10 November 2009 (4 pages)
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The dynamic spin structure factor [script S](k,omega) of a system of spin-1/2 bosons is investigated at arbitrary strength of the interparticle repulsion. As a function of omega it is shown to exhibit a power-law singularity at the threshold frequency defined by the energy of a magnon at given k. The power-law exponent is found exactly using a combination of the Bethe ansatz solution and an effective-field theory approach.
Rapid

Resonant optical scattering in nanoparticle-doped polymer photonic crystals
J. J. Baumberg, O. L. Pursiainen, and P. Spahn
Published 10 November 2009 (4 pages)
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A broadband hyperspectral technique is used to measure the coherent optical backscatter across a wide spectral bandwidth, showing the resonant suppression of the photon transport mean free path around the photonic bandgap of a shear-assembled polymer photonic crystal. By doping with carbon nanoscale scatterers that reside at specific points within the photonic crystal lattice, the ratio between photon mean free path and optical penetration is tuned from 10 to 1, enhancing forward scatter at the expense of back-scatter. The back-scattering strength of different polarisations is not explained by any current theory.
Rapid

Experimental demonstration of a nonmagnetic metamaterial cloak at microwave frequencies
Boubacar Kanté, Dylan Germain, and André de Lustrac
Published 17 November 2009 (4 pages)
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Metamaterials have paved the way to unprecedented control of the electromagnetic field. The conjunction with space coordinate transformation has led to a “relativity inspired” approach for the control of light propagation. “Invisibility cloak” is the most fascinating proposed device. However, the realized structures up to now used a graded “metamagnetic” so as to achieve the cloaking function. Artificial magnetism is still very challenging to obtain in optics despite the currently promising building blocks, not suited for optical cloaking. We report here the experimental demonstration of a nonmagnetic cloak at microwave frequencies by direct mapping of the magnetic field together with the experimental characterization of a cloak in free space configuration. The diameter of the concealed region is as big as 4.4 in wavelength units, the biggest reported experimentally so far. The principle can be scaled down to optical domain while keeping the compatibility with current nanofabrication technologies.
Rapid

Switching the electric and magnetic responses in a metamaterial
Xiang Xiong, Wei-Hua Sun, Yong-Jun Bao, Ru-Wen Peng, Mu Wang, Cheng Sun, Xiang Lu, Jun Shao, Zhi-Feng Li, and Nai-Ben Ming
Published 20 November 2009 (4 pages)
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We demonstrate in this Rapid Communication that in an assembly of stacked metallic U-shaped resonators, pure magnetic and electric responses are, respectively, realized, and the magnetic and electric responses can be switched at the same frequency by changing the polarization of incident light for 90°. This unique feature originates from the topological symmetry of the structure. We suggest that this property opens a gateway to construct metamaterial with tunable permittivity and permeability.

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

Hyperfine mediated triplet-singlet transition probability in a double-quantum-dot system: Analogy with the double-slit experiment
Fernando Domínguez and Gloria Platero
Published 2 November 2009 (4 pages)
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We apply an elementary measurement scheme to calculate the electronic triplet-singlet transition mediated by hyperfine interaction in a double-quantum dot. We show how the local character of the hyperfine interaction and the nuclear back-action process (flip-flop) are crucial to cancel destructive interferences of the triplet-singlet transition probability. It is precisely this cancellation that differentiates the hyperfine interaction from an anisotropic magnetic field which mixes the triplet and the singlet eigenstates.
Rapid

Localization in an inhomogeneous quantum wire
A. D. Güçlü, C. J. Umrigar, Hong Jiang, and Harold U. Baranger
Published 2 November 2009 (4 pages)
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We study interaction-induced localization of electrons in an inhomogeneous quasi-one-dimensional system—a wire with two regions, one at low density and the other high. Quantum Monte Carlo techniques are used to treat the strong Coulomb interactions in the low-density region, where localization of electrons occurs. The nature of the transition from high to low density depends on the density gradient—if it is steep, a barrier develops between the two regions, causing Coulomb blockade effects. Ferromagnetic spin polarization does not appear for any parameters studied. The picture emerging is in good agreement with measurements of tunneling between wires.
Rapid

Interaction-tuned compressible-to-incompressible phase transitions in quantum Hall systems
Z. Papić, N. Regnault, and S. Das Sarma
Published 2 November 2009 (4 pages)
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We analyze transitions between quantum Hall ground states at prominent filling factors nu in the spherical geometry by tuning the width parameter of the Zhang-Das Sarma interaction potential. We find that incompressible ground states evolve adiabatically under this tuning, whereas the compressible ones are driven through a first-order phase transition. Overlap calculations show that the resulting phase is increasingly well described by appropriate analytic model wave functions (Laughlin, Moore-Read, Read-Rezayi). This scenario is shared by both odd (nu=1/3,1/5,3/5,7/3,11/5,13/5) and even denominator states (nu=1/2,1/4,5/2,9/4). In particular, the Fermi-liquid-like state at nu=1/2 gives way, at large enough value of the width parameter, to an incompressible state identified as the Moore-Read Pfaffian on the basis of its entanglement spectrum.
Rapid

Surface-induced polarity inversion in ZnO nanowires
Giancarlo Cicero, Andrea Ferretti, and Alessandra Catellani
Published 2 November 2009 (4 pages)
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In this work we use ab initio calculations to address the polar behavior of ZnO nanowires. Moving from a description based on Wannier functions, we employ a computational approach that allows one to express the polarization of a nanostructure in terms of local contributions. In particular, we discuss the changes in the nanostructure polarity in terms of two contributions, one related to changes in the equilibrium lattice parameters and the other related to surface effects. The former contribution is also interpreted on the basis of piezoelectric constants. Surprisingly, we find that for the smallest nanostructures, the average dipole is opposite to that of an infinite bulk structure.
Rapid

Dimensional crossover of the dephasing time in disordered mesoscopic rings
M. Treiber, O. M. Yevtushenko, F. Marquardt, J. von Delft, and I. V. Lerner
Published 10 November 2009 (4 pages)
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We study dephasing by electron interactions in a small disordered quasi-one-dimensional (1D) ring weakly coupled to leads. We use an influence functional for quantum Nyquist noise to describe the crossover for the dephasing time tau[cursive phi](T) from diffusive or ergodic 1D (tau[cursive phi]<sup>-1</sup>[proportional]T2/3,T1) to zero-dimensional (0D) behavior (tau[cursive phi]<sup>-1</sup>[proportional]T2) as T drops below the Thouless energy. The crossover to 0D, predicted earlier for two-dimensional and three-dimensional systems, has so far eluded experimental observation. The ring geometry holds promise of meeting this long-standing challenge, since the crossover manifests itself not only in the smooth part of the magnetoconductivity but also in the amplitude of Altshuler-Aronov-Spivak oscillations. This allows signatures of dephasing in the ring to be cleanly extracted by filtering out those of the leads.
Rapid

Interplay between superfluidity and magnetic self-trapping of exciton polaritons
I. A. Shelykh, T. C. H. Liew, and A. V. Kavokin
Published 10 November 2009 (4 pages)
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In dilute magnetic semiconductor microcavities, exciton polaritons self-localize in real space due to the magnetic polaron effect. The resulting circularly polarized classical condensates can be transformed into superfluids by increasing the temperature and applying an external magnetic field. The interplay between polariton-polariton repulsion and exchange coupling of polaritons with magnetic ions strongly affects the phase diagram for Bose-Einstein condensation of exciton polaritons.
Magnetoresistance in an asymmetric Ga1−xMnxAs resonant tunneling diode
Edward Likovich, Kasey Russell, Wei Yi, Venkatesh Narayanamurti, Keh-Chiang Ku, Meng Zhu, and Nitin Samarth
Published 13 November 2009 (4 pages)
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In a GaMnAs/AlGaAs resonant tunneling diode (RTD) structure, we observe that both the magnitude and polarity of magnetoresistance are bias dependent when tunneling from a three-dimensional GaMnAs layer through a two-dimensional GaMnAs quantum well. This magnetoresistance behavior results from a shift of negative differential resistance features to higher bias as the relative alignment of the GaMnAs layer magnetizations is changed from parallel to antiparallel. Our observations agree with recent predictions from a theoretical analysis of a similar n-type structure by Ertler and Fabian, and our results suggest that further investigation into ferromagnetic RTD structures may result in significantly enhanced magnetoresistance.
Rapid

Exciton polaritons in two-dimensional photonic crystals
D. Bajoni, D. Gerace, M. Galli, J. Bloch, R. Braive, I. Sagnes, A. Miard, A. Lemaître, M. Patrini, and L. C. Andreani
Published 16 November 2009 (4 pages)
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Experimental evidence of strong coupling between excitons confined in a quantum well and the photonic modes of a two-dimensional dielectric lattice is reported. Both resonant scattering and photoluminescence spectra at low temperature show the anticrossing of the polariton branches, fingerprint of strong coupling regime. The experiments show that the polariton dispersion can be tailored by properly varying the photonic crystal lattice parameter, which enlarges the possibility of engineering polariton dispersion.
Rapid

Spin injection across the Fe/GaAs interface: Role of interfacial ordering
B. D. Schultz, N. Marom, D. Naveh, X. Lou, C. Adelmann, J. Strand, P. A. Crowell, L. Kronik, and C. J. Palmstrøm
Published 16 November 2009 (4 pages)
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Spin injection efficiency is shown to strongly depend on the interfacial structure between Fe contacts and AlxGa1−xAs in spin-based light emitting diodes. Both the magnitude and sign of the injected carriers are dependent on the atomic structure of the contacts and can be controlled through changes in temperature both during and following growth. We propose that the observed dependence is due to phase formation resulting from Fe/GaAs interfacial reactions. This proposed mechanism is consistent with electronic structure calculations, which show that thin layers of DO3 Fe3Ga at the Fe/GaAs interface can produce the observed sign reversals in the spin polarization of injected carriers.
Rapid

Charge ratchet from spin flip: Space-time symmetry paradox
Sergey Smirnov, Dario Bercioux, Milena Grifoni, and Klaus Richter
Published 16 November 2009 (4 pages)
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Traditionally the charge ratchet effect is considered as a consequence of either the spatial symmetry breaking engineered by asymmetric periodic potentials, or time asymmetry of the driving fields. Here we demonstrate that electrically and magnetically driven quantum dissipative systems with spin-orbit interactions represent an exception from this standard idea. In contrast to the so far well established belief, a charge ratchet effect appears when both the periodic potential and driving are symmetric. We show that the source of this paradoxical charge ratchet mechanism is the coexistence of quantum dissipation with the spin-flip processes induced by spin-orbit interactions.
Rapid

Phonon-assisted transitions from quantum dot excitons to cavity photons
Ulrich Hohenester, Arne Laucht, Michael Kaniber, Norman Hauke, Andre Neumann, Abbas Mohtashami, Marek Seliger, Max Bichler, and Jonathan J. Finley
Published 18 November 2009 (4 pages)
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For a single semiconductor quantum dot embedded in a microcavity, we theoretically and experimentally investigate phonon-assisted transitions between excitons and the cavity mode. Within the framework of the independent boson model we find that such transitions can be very efficient, even for relatively large exciton-cavity detunings of several millielectron volts. Furthermore, we predict a strong detuning asymmetry for the exciton lifetime that vanishes for elevated lattice temperature. Our findings are corroborated by experiment, which turns out to be in good quantitative and qualitative agreement with theory.
Rapid

Electronic entanglement via quantum Hall interferometry in analogy to an optical method
Diego Frustaglia and Adán Cabello
Published 20 November 2009 (4 pages)
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We present an interferometric scheme producing orbital entanglement in a quantum Hall system upon electron-hole pair emission via tunneling. The proposed setup is an electronic version of the optical interferometer proposed by Cabello et al. [Phys. Rev. Lett. 102, 040401 (2009)] and is feasible with the present technology. It requires single-channel propagation and a single primary source. We discuss the creation of entanglement and its detection by the violation of a Bell inequality.
Rapid

Charge transfer statistics in symmetric fractional edge-state Mach-Zehnder interferometer
V. V. Ponomarenko and D. V. Averin
Published 20 November 2009 (4 pages)
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We have studied the zero-temperature statistics of charge transfer between the two edges of quantum Hall liquids with filling factors nu0,1=1/(2m0,1+1) forming Mach-Zehnder interferometer. The known Bethe ansatz solution for symmetric interferometer is used to obtain the cumulant-generating function of charge at constant voltage V between the edges. Its low-V behavior can be interpreted in terms of electron tunneling, while its large-V asymptotics reproduces the m-state dynamics (m[equivalent]1+m0+m1) of quasiparticles with fractional (for m>1) charge and statistics. We also analyze the transition region between electrons and quasiparticles.

Surface physics, nanoscale physics, low-dimensional systems
Rapid

Surface modes in plasmonic crystals induced by diffractive coupling of nanoantennas
G. Vecchi, V. Giannini, and J. Gómez Rivas
Published 4 November 2009 (4 pages)
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Lattice surface modes in two-dimensional plasmonic crystals of metallic nanoantennas are sustained by the diffractive coupling between localized plasmon resonances. We have investigated experimentally the dispersion of these modes by combining variable-angle transmittance and reflectance spectroscopy. Transmittance spectra through plasmonic crystals reveal quality factors for these modes exceeding 150 in the near infrared, 30 times higher than the quality factor associated to localized plasmon resonances. We have derived the characteristic lengths of the lattice surface modes, distinguishing a regime in which the group velocity is reduced while the mode intensity is strongly confined near the nanoantennas and a regime in which the surface mode propagates over several unit cells of the plasmonic crystal.
Rapid

Low-temperature specific heat of one-dimensional multicomponent systems at the commensurate-incommensurate phase transition point
T. Vekua
Published 6 November 2009 (4 pages)
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Low-temperature dependence of specific heat of one-dimensional multicomponent systems at the commensurate-incommensurate phase transition point is studied. It is found that for canonical systems, with a fixed total number of particles, low-temperature specific heat linearly depends on temperature with a diverging prefactor.
Gap opening in the zeroth Landau level of graphene
A. J. M. Giesbers, L. A. Ponomarenko, K. S. Novoselov, A. K. Geim, M. I. Katsnelson, J. C. Maan, and U. Zeitler
Published 11 November 2009 (4 pages)
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We have measured a strong increase of the low-temperature resistivity rhoxx and a zero-value plateau in the Hall conductivity sigmaxy at the charge neutrality point in graphene subjected to high magnetic fields up to 30 T. We explain our results by a simple model involving a field dependent splitting of the lowest Landau level of the order of a few Kelvin, as extracted from activated transport measurements. The model reproduces both the increase in rhoxx and the anomalous nu=0 plateau in sigmaxy in terms of coexisting electrons and holes in the same split zero-energy Landau level.
Rapid

Hyperfine-induced valley mixing and the spin-valley blockade in carbon-based quantum dots
András Pályi and Guido Burkard
Published 17 November 2009 (4 pages)
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Hyperfine interaction (HFI) in carbon nanotube and graphene quantum dots is due to the presence of 13C atoms. We theoretically show that in these structures the short-range nature of the HFI gives rise to a coupling between the valley degree of freedom of the electron and the nuclear spin, in addition to the usual electron spin-nuclear spin coupling. We predict that this property of the HFI affects the Pauli blockade transport in carbon-based double quantum dots. In particular, we show that transport is blocked only if both the spin and the valley degeneracies of the quantum dot levels are lifted, e.g., by an appropriately oriented magnetic field. The blockade is caused by four “supertriplet” states in the (1,1) charge configuration.
Rapid

Subpicosecond coherent nonlinear optical response of isolated single-walled carbon nanotubes
S. Tao, Y. Miyata, K. Yanagi, H. Kataura, and H. Okamoto
Published 17 November 2009 (4 pages)
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We report an excitation-energy dependence of ultrafast changes of exciton absorptions in isolated semiconducting single-walled carbon nanotubes. For a photoexcitation far below the exciton transition, a blueshift of exciton absorptions originating from the optical Stark effect due to exciton-photon coupling was observed. Under near-resonant excitation conditions, a broadening of exciton absorptions was discriminated from a blueshift signal in the time and frequency domains. The broadening can be attributed to virtual exciton-real exciton scattering, which is considered to be characteristic of strongly confined excitons in nanotubes.
Rapid

Stability and work function of TiCxN1−x alloy surfaces: Density functional theory calculations
H. Zhu, M. Aindow, and R. Ramprasad
Published 20 November 2009 (4 pages)
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Critical factors that control the vacuum work function of the TiCxN1−x ternary system surfaces were determined using detailed density functional theory calculations. Surface chemistry (i.e., orientation, stoichiometry, and defect density) was found to play the most important role in determining the work function value, far surpassing the impact of alloy composition (i.e., x value) on the work function. In general, Ti-deficient surfaces display larger work functions. Work function tuning may thus be effectively accomplished by controlling the surface chemistry rather than the composition.

ARTICLES

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

Quantum Hall effect in bilayer graphene: Disorder effect and quantum phase transition
R. Ma, L. Sheng, R. Shen, M. Liu, and D. N. Sheng
Published 2 November 2009 (5 pages)
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We numerically study the quantum Hall effect (QHE) in bilayer graphene based on tight-binding model in the presence of disorder. Two distinct QHE regimes are identified in the full energy band separated by a critical region with nonquantized Hall Effect. The Hall conductivity around the band center (Dirac point) shows an anomalous quantization proportional to the valley degeneracy, but the nu=0 plateau is markedly absent, which is in agreement with experimental observation. In the presence of disorder, the Hall plateaus can be destroyed through the float-up of extended levels toward the band center and higher plateaus disappear first. The central two plateaus around the band center are most robust against disorder scattering, which is separated by a small critical region in between near the Dirac point. The longitudinal conductance around the Dirac point is shown to be nearly a constant in a range of disorder strength, until the last two QHE plateaus completely collapse.

Generalized Slater-Jastrow trial function: Application to the electron gas at high density
James C. Porter
Published 3 November 2009 (14 pages)
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The function u(i,j)=u(|ri−rj|), in terms of which variational trial functions of the Slater-Jastrow type are expressed, is generalized to include dependence on the electron momenta as well as on the electron-pair separations. The resulting Euler equations are used to evaluate the ground-state coulomb correlation energy giving epsilonc=0.0622 log(rs)−0.1441  Ry, where rs is the ratio of the radius of a sphere, containing, on average one electron, to the Bohr radius. This correlation energy is 0.050 Ry lower than the corresponding result epsilonc=0.0622 log(rs)−0.094  Ry found by Gell-Mann and Brueckner. Also evaluated are the corresponding correlation corrections to the Hartree-Fock (HF) exchange potential energy and to the average exchange charge density as a function of distance. The correction to the HF exchange potential energy is shown to cancel the divergence of the HF exchange energy at the Fermi energy so that the electron effective mass remains finite.

Optimizing the thermoelectric efficiency of icosahedral quasicrystals and related complex alloys
Enrique Maciá
Published 3 November 2009 (7 pages)
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In this work we analyze the potential role of quasicrystals and related alloys in thermoelectric material research. Relatively large figure of merit values are expected for those samples exhibiting two properly located narrow features in the density of states close to the Fermi level. It is expected that optimized quasicrystals will perform better at relatively low temperatures, whereas the ZT curve of complex metallic alloys reaches its maximum at high temperatures. Among state-of-the-art quasicrystals most promising samples for thermoelectric applications are found in the AlPd(Mn,Re) system. Quasicrystalline and related approximants in the ScMgCuGa and CaAuIn systems, synthesized on the basis of pseudogap tuning concepts, appear as promising candidates as well.

Highly efficient method for Kohn-Sham density functional calculations of 500–10 000 atom systems
M. J. Rayson and P. R. Briddon
Published 4 November 2009 (11 pages)
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A method for the solution of the self-consistent Kohn-Sham equations using Gaussian-type orbitals is presented. Accurate relative energies and forces are demonstrated to be achievable at a fraction of the computational expense for large systems. With this approach calculations involving around 1000 atoms can easily be performed with a serial desktop computer and ~10 000 atom systems are within reach of relatively modest parallel computational resources. The method is applicable to arbitrary systems including metals. The approach generates a minimal basis on the fly while retaining the accuracy of the large underpinning basis set. Convergence of energies and forces are given for clusters as well as cubic cells of silicon and aluminum, for which the formation energies of defects are calculated in systems up to 8000 and 4000 atoms, respectively. For these systems the method exhibits linear scaling with the number of atoms in the presently important size range of ~500–3000 atoms.

Electronic stopping of low-energy H and He in Cu and Au investigated by time-of-flight low-energy ion scattering
S. N. Markin, D. Primetzhofer, M. Spitz, and P. Bauer
Published 4 November 2009 (4 pages)
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Electronic stopping has been investigated experimentally for slow H+ and He+ ions (v<0.6  a.u.) in polycrystalline Au and Cu, by means of time-of-flight low-energy ion scattering. Measurements were performed in backscattering geometry using polycrystalline films of several nanometers thickness; high-resolution Rutherford backscattering spectrometry was used for precise thickness calibration (<4%). Absolute stopping cross-section data for Cu and Au were obtained down to v~0.07  a.u. Both Au and Cu exhibit a rather sharp, distinctive threshold velocity of ~0.18  a.u. for excitation of d electrons by H+ and He+. Below this threshold, projectiles interact exclusively with s electrons.

Ligand isotope structure of the optical 7F0-->5D0 transition in EuCl3·6H2O
R. L. Ahlefeldt, A. Smith, and M. J. Sellars
Published 5 November 2009 (5 pages)
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The structure of the 7F0-->5D0 transition of Eu3+ in EuCl3·6H2O is investigated in crystals with three different isotopic compositions [natural abundance (0.2% 18O, 0.01%2H), 1.9% 18O, and 0.06% 2H]. Raman-heterodyne-detected NMR is used to identify shifts caused by isotopes of the ligands O and H in sites neighboring the Eu site in the lattice. The structure of the 7F0-->5D0 transition is explained as a combination of the hyperfine structure of 151Eu and 153Eu and isotope shifts caused by Cl, O, and H.

Effective Coulomb interactions within BEDT-TTF dimers
Edan Scriven and B. J. Powell
Published 6 November 2009 (9 pages)
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We calculate the parameters for Hubbard models of kappa-(BEDT-TTF)2X and beta-(BEDT-TTF)2X. We use density-functional theory (DFT) to calculate the interactions between holes in dimers of the organic molecule bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF) for 23 experimental geometries taken from a range of materials in both the beta and kappa polymorphs. We find that the effective Coulomb interactions are essentially the same for all of the compounds studied. We highlight the disagreement between our parametrization of the Hubbard model and previous results from both DFT and Hückel methods. We show that this is caused by the failure of an assumption made in previous calculations (which estimate the effective Coulomb interaction from the intradimer hopping integral). We discuss the implications of our calculations for theories of the BEDT-TTF salts based on the Hubbard model and use our calculated parameters to explain a number of phenomena caused by conformational disorder in these materials.

High-resolution hard x-ray photoemission investigation of La2−2xSr1+2xMn2O7 (0.30<=x<0.50): Microscopic phase separation and surface electronic structure of a bilayer colossal magnetoresistance manganite
S. de Jong, F. Massee, Y. Huang, M. Gorgoi, F. Schaefers, J. Fink, A. T. Boothroyd, D. Prabhakaran, J. B. Goedkoop, and M. S. Golden
Published 9 November 2009 (7 pages)
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Photoemission data taken with hard x-ray radiation on cleaved single crystals of the bilayered, colossal magnetoresistant manganite La2−2xSr1+2xMn2O7 (LSMO) with 0.30<=x<0.50 are presented. Making use of the increased bulk sensitivity upon hard x-ray excitation it is shown that the core-level footprint of the electronic structure of the LSMO cleavage surface is identical to that of the bulk. Furthermore, by comparing the core-level shift of the different elements as a function of doping level x, it is shown that microscopic phase separation is unlikely to occur for this particular manganite well above the Curie temperature.

Pairing dynamics in strongly correlated superconductivity
B. Kyung, D. Sénéchal, and A.-M. S. Tremblay
Published 9 November 2009 (8 pages)
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Confirmation of the phononic origin of Cooper pair formation in superconductors came with the demonstration that the interaction was retarded and that the corresponding energy scales were associated with phonons. Using cellular dynamical mean-field theory for the two-dimensional Hubbard model, we identify such retardation effects in d-wave pairing and associate the corresponding energy scales with short-range spin fluctuations. We find which frequencies are relevant for pairing as a function of interaction strength and doping and show that the disappearance of superconductivity on the overdoped side coincides with the disappearance of the low-energy feature in the antiferromagnetic fluctuations, as observed in neutron-scattering experiments.

Spin Coulomb drag in a spin-polarized Luttinger liquid
P. Schlottmann
Published 10 November 2009 (10 pages)
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The spin Coulomb drag is a distinctive feature of spin-polarized transport. The current of majority spins can induce a current of minority-spin carriers via the transconductivity. The friction is caused by the Coulomb interaction between up and down spin. This interaction reduces the current but does not change the spin polarization. We calculate the conductivities and the transconductivity for a spin-polarized interacting one-dimensional electron gas with nonmagnetic impurities using the Kubo formalism. Due to the Luttinger-liquid properties, the temperature dependence of the transport correlation functions follow power laws of T with nonuniversal exponents.

Electronic structure and magnetic properties of Li2ZrCuO4: A spin-(1/2) Heisenberg system close to a quantum critical point
M. Schmitt, J. Málek, S.-L. Drechsler, and H. Rosner
Published 11 November 2009 (9 pages)
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Based on density-functional calculations, we present a detailed theoretical study of the electronic structure and the magnetic properties of the quasi-one-dimensional chain cuprate Li2ZrCuO4 (Li2CuZrO4). For the relevant ratio of the next-nearest-neighbor exchange J2 to the nearest-neighbor exchange J1 we find alpha=−J2/J1=0.22±0.02 which is very close to the critical point at 1/4. Owing this vicinity to a ferromagnetic-helical critical point, we study in detail the influence of structural peculiarities such as the reported Li disorder and the nonplanar chain geometry on the magnetic interactions combining the results of local-density-approximation-based tight-binding models with LDA+U-derived exchange parameters. Our investigation is complemented by an exact diagonalization study of a multiband Hubbard model for finite clusters predicting a strong temperature dependence of the optical conductivity for Li2ZrCuO4.

Valence-state transition in SrMn1−xMoxO3 (0<=x<=0.5) investigated by soft x-ray absorption spectroscopy
Jieun Lee, Bongjae Kim, B. H. Kim, B. I. Min, S. Kolesnik, O. Chmaissem, J. Mais, B. Dabrowski, H. J. Shin, D. H. Kim, H. J. Lee, and J.-S. Kang
Published 13 November 2009 (5 pages)
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The electronic structures of perovskite SrMn1−xMoxO3 (0<=x<=0.5) have been investigated by employing soft x-ray absorption spectroscopy (XAS). Mn 2p XAS shows the systematic change in the valence states of Mn ions in SrMn1−xMoxO3 due to the substitution of hexavalent Mo6+(4d0) ions. With increasing x, the valence states and the spin configurations of Mn ions change from high-spin (HS) Mn4+ for x=0, to HS Mn3+(t2g<sup>3</sup>[up-arrow]eg<sup>1</sup>[up-arrow]) for x=0.3, and HS Mn2+(t2g<sup>3</sup>[up-arrow]eg<sup>2</sup>[up-arrow]) for x=0.5. The measured Mn 2p XAS spectra are described well with the configuration interaction (CI) cluster model, including the Jahn-Teller distortion effect for Mn3+ ions. The combination of the findings of Mn 2p XAS and the CI calculations provides the complete picture of the electronic structures of SrMn1−xMoxO3 for 0<=x<=0.5.

Role of Si and Ge as impurities in ZnO
J. L. Lyons, A. Janotti, and C. G. Van de Walle
Published 13 November 2009 (5 pages)
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The electronic and structural properties of Si in ZnO are studied using density functional calculations with both the generalized gradient approximation and a hybrid functional. Our calculations show substitutional Si on the Zn site (SiZn) to be significantly lower in energy than the Si interstitial (Sii) and Si on an O site (SiO). SiZn is predicted to be a shallow donor in ZnO with a formation energy low enough to lead to significant incorporation of Si in the ZnO crystal. Interestingly, we find that SiZn has a lower formation energy (and therefore higher solubility) under Zn-rich conditions than under O-rich conditions. We also study the properties of Ge in ZnO for comparison, finding behavior similar to the Si impurity, but with an even lower formation energy. Our results suggest Si and Ge as suitable n-type dopants in ZnO.

Kondo screening cloud in the single-impurity Anderson model: A density matrix renormalization group study
Andreas Holzner, Ian P. McCulloch, Ulrich Schollwöck, Jan von Delft, and Fabian Heidrich-Meisner
Published 13 November 2009 (8 pages)
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A magnetic moment in a metal or in a quantum dot is, at low temperatures, screened by the conduction electrons through the mechanism of the Kondo effect. This gives rise to spin-spin correlations between the magnetic moment and the conduction electrons, which can have a substantial spatial extension. We study this phenomenon, the so-called Kondo cloud, by means of the density matrix renormalization group method for the case of the single-impurity Anderson model. We focus on the question whether the Kondo screening length, typically assumed to be proportional to the inverse Kondo temperature, can be extracted from the spin-spin correlations. For several mechanisms—the gate potential and a magnetic field—which destroy the Kondo effect, we investigate the behavior of the screening cloud induced by these perturbations.

Resonant cavitylike modes in dielectric photonic crystals made of collections of subwavelength cylinders
Ruey-Lin Chern and Didier Felbacq
Published 16 November 2009 (6 pages)
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We investigate resonant cavitylike modes in dielectric photonic crystals made of collections of subwavelength cylinders with high permittivity. A large number of collective modes appear for TE polarization (where magnetic fields are oriented along the cylinder axis), which possess similar features of resonant cavity modes that occur in polaritonic structures. These modes are dispersionless in nature and intensively gathered around an asymptotic frequency that depends on the subwavelength cylinders. The typical resonant cavitylike modes are illustrated with the magnetic field distributions at the resonant frequencies. In particular, the field pattern in the building block shows a close resemblance to TEnm mode of an isolated waveguide. The respective cutoff frequency at large oscillation orders serves as the asymptotic frequency of resonant cavitylike modes.

Pressure-induced structural and electronic transitions in FeOOH from first principles
Katrin Otte, Rossitza Pentcheva, Wolfgang W. Schmahl, and James R. Rustad
Published 18 November 2009 (9 pages)
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Using density-functional theory, we investigate the stability, structural, magnetic, and electronic properties of the iron oxyhydroxide polymorphs [alpha-, beta-, gamma-, and hp(epsilon)-FeOOH] under hydrostatic pressure. At ambient conditions goethite (alpha) is the lowest energy phase, consistent with recent calorimetric measurements. Around 6–7 GPa we predict a transformation to the high-pressure hp(epsilon) phase. This structural transformation is followed by a high-spin to low-spin transition at 7.7 GPa, at much lower pressure than for other currently discussed iron-bearing minerals. While in the ground state the Fe3+ ions are coupled antiferromagnetically, at high pressures a strong competition to a ferromagnetic alignment is found in hp(epsilon)-FeOOH. Concerning the electronic properties, including an on-site Coulomb repulsion parameter U (LDA/GGA+U method) improves the size of the zero-pressure band gaps substantially but shifts the spin transition to higher pressure (56.5 GPa). The predicted spin crossover is associated with a blueshift of 0.4 eV.

Finite-temperature dynamics with the density-matrix renormalization group method
J. Kokalj and P. Prelovšek
Published 20 November 2009 (5 pages)
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We present a numerical method for the evaluation of dynamical response functions at finite temperatures in one-dimensional strongly correlated systems. The approach is based on the density-matrix renormalization group method, combined with the finite-temperature Lanczos diagonalization. The feasibility of the method is tested on the example of dynamical spin correlations in the anisotropic Heisenberg chain, in particular, it yields nontrivial results for the critical behavior in the isotropic case.

Semiconductors I: bulk

Ferroelectric nature and real-space observations of domain motions in the organic charge-transfer compound tetrathiafulvalene-p-chloranil
Hideo Kishida, Hisashi Takamatsu, Ken Fujinuma, and Hiroshi Okamoto
Published 6 November 2009 (7 pages)
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Ferroelectricity in an organic charge-transfer compound, tetrathiafulvalene-p-chloranil (TTF-CA), originating from the one-dimensional valence and lattice instabilities, has been investigated by an electroreflectance (ER) method. Microscopic ER spectroscopy in the visible region enables real-space observations of both ferroelectric domain structures with a few hundred micrometers in size and depinning of the domain walls under strong electric fields. In addition, from ER spectroscopy in the infrared molecular-vibration region, we demonstrate that field-induced changes in the dimeric molecular displacement as well as charge transfer between TTF and CA molecules play an important role in the large dielectric response in TTF-CA.

Temperature-dependent resistivity of ferromagnetic Ga1−xMnxAs: Interplay between impurity scattering and many-body effects
F. V. Kyrychenko and C. A. Ullrich
Published 6 November 2009 (6 pages)
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The static conductivity of the diluted magnetic semiconductor Ga1−xMnxAs is calculated using an equation of motion approach for the current response combined with time-dependent density-functional theory to account for Hartree and exchange interactions within the hole gas. We find that the Coulomb scattering off the charged impurities alone is not sufficient to explain the experimentally observed drop in resistivity below the ferromagnetic transition temperature: the often overlooked scattering off the fluctuations of localized spins is shown to play a significant role.
Gamma7 valence band symmetry related hole fine splitting of bound excitons in ZnO observed in magneto-optical studies
Markus R. Wagner, Jan-Hindrik Schulze, Ronny Kirste, Munise Cobet, Axel Hoffmann, Christian Rauch, Anna V. Rodina, Bruno K. Meyer, Uwe Röder, and Klaus Thonke
Published 18 November 2009 (6 pages)
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The symmetry ordering of the valence bands in ZnO is derived from high-resolution magneto-optical measurements of bound excitons. We report on the experimental observation of a hole state related fine splitting for bound excitons in the Voigt configuration. This splitting is related to a nonzero Landé g value g[perpendicular] for hole states from the A valence band. Based on theoretical considerations, the symmetry of the uppermost valence band is doubtlessly identified as Gamma7. This attribution is confirmed by polarization and angular resolved magnetophotoluminescence spectroscopy.

High-temperature ferromagnetism by means of oriented nanocolumns: Co clustering in (Zn,Co)O
N. Jedrecy, H. J. von Bardeleben, and D. Demaille
Published 19 November 2009 (6 pages)
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In order to provide insight into the magnetic properties of semiconductors with ferromagnetic inclusions, we present a thorough quantitative analysis of (Zn,Co)O films with high Co concentration, which give rise to high saturation magnetization values (~110  kA m−1) at room temperature. From the orientation-, temperature-, and field-dependent responses of the magnetization and of the Q-band ferromagnetic resonance modes, we give evidence of two types of nanosized ferromagnetic Co clusters and derive their respective contributions. The first type are spherical with diameters about 5 nm and blocking temperatures TB about 100 K. The second type are elongated along the surface normal and crystallographically oriented with respect to the ZnO host. Some are nanocolumns about 4 nm wide, whose height may reach up to 60 nm, leading to TB>=300  K. These results are confirmed by high-resolution transmission electron microscopy analysis. In addition to provide transition from ferromagnetism to superparamagnetism above 300 K, the nanocolumns lead to strong anisotropic magnetic properties. We believe that they could find applications in spintronic devices.

Manganese-hydrogen complexes in Ga1−xMnxN
C. Bihler, U. Gerstmann, M. Hoeb, T. Graf, M. Gjukic, W. G. Schmidt, M. Stutzmann, and M. S. Brandt
Published 20 November 2009 (10 pages)
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The effects of hydrogenation on Mn-doped GaN are studied with electron-paramagnetic resonance (EPR), local vibrational mode (LVM) spectroscopy, and density-functional theory (DFT) calculations. With EPR, we find two distinct Mn complexes which, in particular, differ in the size and orientation of the uniaxial crystal field and are attributed to Mn-H complexes oriented along the out-of-plane Ga-N bonds parallel to the c axis and to Mn-H complexes oriented along the “in-plane” Ga-N bond directions. DFT calculations in the local spin-density approximation taking into account the self-consistent correlation parameter U predict that the in-plane back-bonded configuration and the out-of-plane bond-center configuration of the Mn-H complexes have the lowest, nearly identical total energy, in good agreement with the EPR results. The hyperfine interactions with the Mn nucleus in the different complexes are fully reproduced by the theory. Using infrared-absorption and reflection measurements, we additionally observe the stretching mode of the in-plane configuration and report the effects of H/D substitution and measurement temperature on the LVM.

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

Change in the character of quasiparticles without gap collapse in a model of fractional quantum Hall effect
Csaba Tőke and Jainendra K. Jain
Published 4 November 2009 (11 pages)
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It is commonly assumed in the studies of the fractional quantum Hall effect that the physics of a fractional quantum Hall state, in particular the character of its excitations, is invariant under a continuous deformation of the Hamiltonian during which the gap does not close. We show in this article that, at least for finite systems, as the interaction is changed from a model three body interaction to Coulomb, the ground state at filling factor nu=2/5 evolves continuously from the so-called Gaffnian wave function to the composite fermion wave function, but the quasiholes alter their character in a nonperturbative manner. This is attributed to the fact that the Coulomb interaction opens a gap in the Gaffnian quasihole sector, pushing many of the states to very high energies. Interestingly, the states below the gap are found to have a one-to-one correspondence with the composite fermion theory, suggesting that the Gaffnian model contains composite fermions, and that the Gaffnian quasiholes are unstable to the formation of composite fermions when a two-body interaction term is switched on. General implications of this study are discussed.

Realizing singlet-triplet qubits in multivalley Si quantum dots
Dimitrie Culcer, Lukasz Cywiński, Qiuzi Li, Xuedong Hu, and S. Das Sarma
Published 5 November 2009 (6 pages)
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There has been significant progress in the implementation and manipulation of singlet-triplet qubits in GaAs quantum dots. Given the considerably longer spin coherence times measured in Si, considerable interest has been generated recently in Si quantum dots. The physics of these systems is considerably more complex than the physics of GaAs quantum dots owing to the presence of the valley degree of freedom, which constitutes the focus of this work. In this paper we investigate the physics of Si quantum dots and focus on the feasibility of quantum coherent singlet-triplet qubit experiments analogous to those performed in GaAs. This additional degree of freedom greatly increases the complexity of the ground state and gives rise to highly nontrivial and interesting physics in the processes of qubit initialization, coherent manipulation and readout. We discuss the operational definition of a qubit in Si-based quantum dots. We find that in the presence of valley degeneracy a singlet-triplet qubit cannot be constructed, whereas for large valley splitting (>>kBT) the experiment is similar to GaAs. We show that experiments on singlet-triplet qubits analogous to those in GaAs would provide a method for estimating the valley splitting in Si. A Zeeman field distinguishes between different initialized states for any valley splitting and provides a tool to determine the size of this splitting.

Measurement of the Knight field and local nuclear dipole-dipole field in an InGaAs/GaAs quantum dot ensemble
T. Auer, R. Oulton, A. Bauschulte, D. R. Yakovlev, M. Bayer, S. Yu. Verbin, R. V. Cherbunin, D. Reuter, and A. D. Wieck
Published 5 November 2009 (15 pages)
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We present a comprehensive investigation of the electron-nuclear system of negatively charged InGaAs/GaAs self-assembled quantum dots (QDs) under the influence of weak external magnetic fields (up to 3 mT). We demonstrate that, in contrast to conventional semiconductor systems, these small fields have a profound influence on the electron spin dynamics, via the hyperfine interaction. QDs, with their comparatively limited number of nuclei, present electron-nuclear behavior that is unique to low-dimensional systems. We show that the conventional Hanle effect used to measure electron-spin relaxation times, for example, cannot be used in these systems when the spin lifetimes are long. An individual nucleus in the QD is subject to milli-Tesla effective fields, arising from the interaction with its nearest neighbors and with the electronic Knight field. The alignment of each nucleus is influenced by application of external fields of the same magnitude. A polarized nuclear system, which may have an effective field strength of several Tesla, may easily be influenced by these milli-Tesla fields. This in turn has a dramatic effect on the electron-spin dynamics and we use this technique to gain a measure of both the dipole-dipole field and the maximum Knight field in our system thus allowing us to estimate the maximum Overhauser field that may be generated at zero external magnetic field. We also show that one may fine tune the angle which the Overhauser field makes with the optical axis.

Tunnel coupling in an ensemble of vertically aligned quantum dots at room temperature
V. V. Nikolaev, N. S. Averkiev, M. M. Sobolev, I. M. Gadzhiyev, I. O. Bakshaev, M. S. Buyalo, and E. L. Portnoi
Published 5 November 2009 (10 pages)
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We report unambiguous observation of the formation of mixed electronic states in an ensemble of self-assembled vertically aligned quantum dots at room temperature. Three closely spaced layers containing stacked In(Ga)As/GaAs quantum dots are placed in the active region of a two-section semiconductor device, and investigations of the quantum-dot optical properties at different applied electric fields are carried out by means of differential-absorption spectroscopy. A simple semianalytical model, which describes absorption of two layers of coupled quantum dots with an account of the size dispersion, is developed. A comparison between our experimental and theoretical results allows clear attribution of the observed low-photon-energy field-dependent spectral features to the four mixed optical transitions due to the two upper quantum-dot layers. Interpretation of the experimental results reveals an anticrossing of spatially direct and indirect transitions characterized by the energy splitting of approximately 30 meV.

Role of nonlinear effects in nanowire growth and crystal phase
V. G. Dubrovskii, N. V. Sibirev, G. E. Cirlin, A. D. Bouravleuv, Yu. B. Samsonenko, D. L. Dheeraj, H. L. Zhou, C. Sartel, J. C. Harmand, G. Patriarche, and F. Glas
Published 6 November 2009 (8 pages)
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We study theoretically and experimentally nonlinear effects during the “vapor-liquid-solid” growth of semiconductor nanowires. Nonlinear growth equation considered contains kinetic coefficients from the surface and sidewall diffusion which can be of either signs. We predict four possible growth scenarios: (I) infinite growth; (II) decomposition; (III) averaging growth to a finite length; and (IV) continuing growth such that nanowires of small initial length decay and longer nanowires grow infinitely. We present the experimental evidence of nontrivial scenarios (II) and (IV) during the Au-assisted molecular-beam epitaxy of GaAs nanowires. Scenario (II) corresponds to the evaporation of GaAs nanowires during the annealing. Scenario (IV) is observed during two-step growth procedure where a low-temperature growth is followed by deposition at 630 °C. We show that the growth via scenario (IV) enables to control the crystal phase and to obtain the stacking-fault-free sections of zinc-blende GaAs NWs having only 15–20 nm in radius.

De Haas-van Alphen effect and energy gaps of a correlated two-dimensional electron system in an AlAs two-valley pseudospin system
T. Windisch, X. Huang, S. Dasgupta, B. Rupprecht, Ch. Heyn, M. Bichler, A. Fontcuberta i Morral, M. Grayson, G. Abstreiter, M. A. Wilde, and D. Grundler
Published 9 November 2009 (6 pages)
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We report highly sensitive de Haas-van Alphen (dHvA) effect measurements on a high-mobility two-dimensional electron system in an AlAs quantum well. Here two valleys are occupied forming a pseudospin system. At 400 mK, the dHvA effect shows pronounced oscillations at filling factors nu=1 to four. In the quantum limit at nu=1 the data are consistent with an interaction-enhanced valley splitting, which exceeds the Zeeman spin splitting in a perpendicular field B. When tilting B the energy gap DeltaE at nu=1 shows first an unexpectedly strong angular dependence and then remains constant. This suggests a crossover in the energy gap, most likely from a spin to a pseudospin gap. We attribute the strong initial dependence of DeltaE on the tilt angle to skyrmion-type spin excitations. Surprisingly, the dHvA oscillation amplitudes do not display coincidence phenomena at higher filling factors. This is explained by the large valley splitting and avoided crossings of energy levels.

Delocalization-localization transition of plasmons in random (GaAs)m(Al0.3Ga0.7As)6 superlattices
Yu. A. Pusep, A. D. Rodrigues, and S. S. Sokolov
Published 9 November 2009 (5 pages)
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The transition of plasmons from propagating to localized state was studied in disordered systems formed in GaAs/AlGaAs superlattices by impurities and by artificial random potential. Both the localization length and the linewidth of plasmons were measured by Raman scattering. The vanishing dependence of the plasmon linewidth on the disorder strength was shown to be a manifestation of the strong plasmon localization. The theoretical approach based on representation of the plasmon wave function in a Gaussian form well accounted for by the obtained experimental data.

Magnetism in tunable quantum rings
G. Bårdsen, E. Tölö, and A. Harju
Published 10 November 2009 (5 pages)
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We have studied the spin structure of circular four-electron quantum rings using tunable confinement potentials. The calculations were done using the exact diagonalization method. Our results indicate that ringlike systems can have oscillatory flips between ferromagnetic and antiferromagnetic behavior as a function of the magnetic field. Furthermore, at constant external magnetic fields there were seen similar oscillatory changes between ferromagnetism and antiferromagnetism when the system parameters were changed. According to our results, the magnetism of quantum rings could be tuned by system parameters.

Optical spin orientation in strained Ge/SiGe quantum wells: A tight-binding approach
Michele Virgilio and Giuseppe Grosso
Published 11 November 2009 (8 pages)
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We present a theoretical investigation of electron-spin optical orientation in strained Ge/SiGe quantum wells. The atomistic sp3d5s* nearest-neighbor tight-binding model adopted, allows us to obtain the spin polarization of the excited electrons, as a function of frequency and direction of the incident radiation, in the presence of external fields, different strain conditions, and taking into account contributions arising from states out of the Brillouin zone center. Orbital projected densities of states further highlight spin mixing of the localized spin-orbitals composing the states involved in the optical transitions. Our results illustrate the potential of the adopted method as a theoretical tool to support experiments on optical manipulation of spins in group IV based heterostructures.

Quantum criticality near the Stoner transition in a system of two tunnel-coupled quantum dots with spin-orbit coupling
Oleksandr Zelyak and Ganpathy Murthy
Published 12 November 2009 (9 pages)
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We study a system of two tunnel-coupled quantum dots with the first dot containing interacting electrons (described by the universal Hamiltonian) not subject to spin-orbit coupling whereas the second contains noninteracting electrons subject to spin-orbit coupling. We focus on describing the behavior of the system near the Stoner transition. Close to the critical point quantum fluctuations become important and the system enters a quantum-critical regime. The large-N approximation allows us to calculate physical quantities reliably even in this strongly fluctuating regime. In particular, we find a scaling function to describe the crossover of the quasiparticle decay rate between the renormalized Fermi-liquid regime and the quantum-critical regime.

Internal electronic structure and fine structure of multiexcitons in semiconductor quantum dots
Vladan Mlinar and Alex Zunger
Published 12 November 2009 (15 pages)
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We perform a detailed theoretical study of the characteristic internal electronic structure of various multiexcitons (Nh,Ne), where Nh is number of holes and Ne is the number of electrons in the self-assembled semiconductor quantum dots (QDs). For each of the leading (Nh,Ne) excitonic complexes we start from the single-particle configuration (e.g., a specific occupation pattern of S and P electron and hole levels by a few carriers) and then show the many-particle multiplet levels for the initial state of emission (Nh,Ne) and the final state of emission (Nh−1,  Ne−1). We denote which states are dark and which are bright; the order and multiplicity, the leading single-particle character of each multiplet state, and the fine-structure splittings. These are of general utility. We also show explicit numerical values for distances between various transitions for four specific QDs. Here the presented information is important and potentially useful for a few reasons: (i) the information serves as a guide for spectroscopic interpretation; (ii) the information reveals non-Aufbau cases, where the dot does not have Aufbau occupation of carriers' levels; (iii) the information shows which transitions are sensitive to random-alloy fluctuations (if the dot is alloyed) and importance of this effect. We show that because of such alloy information, distances between peaks cannot be used to gauge structural information.

Temperature dependence of the spectral band shape of CdSe nanodots and nanorods
J. I. Climente, J. Planelles, and F. Rajadell
Published 12 November 2009 (5 pages)
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We study theoretically the effect of thermal population on the emission spectrum of single CdSe nanocrystals. Quantum confinement leads to nonsimple emission band shapes, which have different characteristics for excitons, biexcitons, positive, and negative trions. These effects are particularly pronounced in nanorods. The maximum of the emission band is not necessarily centered at the fundamental transition energy.

Biexciton oscillator strength
M. Combescot and O. Betbeder-Matibet
Published 13 November 2009 (12 pages)
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Our goal is to provide a physical understanding of the elementary coupling between photon and biexciton and to derive the physical characteristics of the biexciton oscillator strength, following the procedure we used for trion. Instead of the more standard two-photon absorption, this work concentrates on molecular biexciton created by photon absorption in an exciton gas. We first determine the appropriate set of coordinates in real and momentum spaces to describe one biexciton as two interacting excitons. We then turn to second quantization and introduce the “Fourier transform in the exciton sense” of the biexciton wave function, which is the relevant quantity for oscillator strength. We find that, like for trion, the oscillator strength for the formation of one biexciton out of one photon plus a single exciton is extremely small: it is one biexciton volume divided by one sample volume smaller than the exciton oscillator strength. However, due to their quantum nature, trion and biexciton have absorption lines, which behave quite differently. Electrons and trions are fermionic particles impossible to pile up all at the same energy. This would make the weak trion line spread with electron density, the peak structure only coming from singular many-body effects. By contrast, the bosonic nature of exciton and biexciton makes the biexciton peak mainly rise with exciton density, this rise being simply linear if we forget many-body effects between the photocreated exciton and the excitons present in the sample.

Nonlinear transport theory for negative-differential resistance states of two-dimensional electron systems in strong magnetic fields
A. Kunold and M. Torres
Published 13 November 2009 (7 pages)
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We present a model to describe the nonlinear response to a direct dc current applied to a two-dimensional electron system in a strong magnetic field. The model is based on the solution of the von Neumann equation incorporating the exact dynamics of two-dimensional damped electrons in the presence of arbitrarily strong magnetic and dc electric fields while the effects of randomly distributed impurities are perturbatively added. From the analysis of the differential resistivity and the longitudinal voltage, we observe the formation of negative-differential resistivity states that are the precursors of the zero-differential resistivity states. Both the effects of elastic impurity scattering as well as those related to inelastic processes play an important role. The theoretical predictions correctly reproduce the main experimental features provided that the inelastic-scattering rate obeys a T2 temperature dependence, consistent with electron-electron interaction effects.

Bilayer quantum Hall system at nut=1: Pseudospin models and in-plane magnetic field
O. Tieleman, A. Lazarides, D. Makogon, and C. Morais Smith
Published 17 November 2009 (8 pages)
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We investigate two theoretical pseudomagnon-based models for a bilayer quantum Hall system (BQHS) at total filling factor nut=1. We find a unifying framework which elucidates the different approximations that are made. We also consider the effect of an in-plane magnetic field in BQHSs at nut=1, by deriving an equation for the ground-state energy from the underlying microscopic physics. Although this equation is derived for small in-plane fields, its predictions agree with recent experimental findings at stronger in-plane fields, for low electron densities. We also take into account finite-temperature effects by means of a renormalization group analysis, and find that they are small at the temperatures that were investigated experimentally.

Role of electronic excitations in the energy loss of H<sub>2</sub><sup>+</sup> projectiles in high-kappa materials
S. M. Shubeita, R. C. Fadanelli, J. F. Dias, P. L. Grande, C. D. Denton, I. Abril, R. Garcia-Molina, and N. R. Arista
Published 18 November 2009 (6 pages)
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In the present work we report on the energy loss ratio Rn of fast H2<sup>+</sup> clusters in thin films (30–50  Å) of LaScO3 and HfO2. The medium energy ion scattering technique was employed covering a broad energy range (40–200 keV/amu). The energy loss ratio data showed no clear evidence of collective excitations in these materials. The experimental results were interpreted in terms of three different theoretical approaches: the dielectric formalism with the Brandt-Reinheimer theory for semiconductor materials; the detailed simulation of the molecular fragments dynamics through the target; and finally the unitary convolution approximation adapted for hydrogen molecules. Only the simulation agrees with the experimental results for both oxides. The unitary convolution approximation works quite well for HfO2 but overestimates slightly the LaScO3 data. The overall results indicate that the energy loss ratio depends critically on the description of the electronic properties of such oxides.

Extended excitons and compact heliumlike biexcitons in type-II quantum dots
Bhavtosh Bansal, S. Godefroo, M. Hayne, G. Medeiros-Ribeiro, and V. V. Moshchalkov
Published 18 November 2009 (5 pages)
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We have used magnetophotoluminescence measurements to establish that InP/GaAs quantum dots have a type-II (staggered) band alignment. The average excitonic Bohr radius and the binding energy are estimated to be 15 nm and 1.5 meV, respectively. When compared to bulk InP, the excitonic binding is weaker due to the repulsive (type-II) potential at the heterointerface. The measurements are extended to over almost 6 orders of magnitude of laser excitation powers and to magnetic fields of up to 50 T. It is shown that the excitation power can be used to tune the average hole occupancy of the quantum dots and hence the strength of the electron-hole binding. The diamagnetic shift coefficient is observed to drastically reduce as the quantum dot ensemble makes a gradual transition from a regime where the emission is from (hydrogenlike) two-particle excitonic states to a regime where emission from (heliumlike) four-particle biexcitonic states also becomes significant.

Controlling and enhancing terahertz collective electron dynamics in superlattices by chaos-assisted miniband transport
M. T. Greenaway, A. G. Balanov, E. Schöll, and T. M. Fromhold
Published 18 November 2009 (5 pages)
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We show that a tilted magnetic field transforms the structure and THz dynamics of charge domains in a biased semiconductor superlattice. At critical field values, strong coupling between the Bloch and cyclotron motion of a miniband electron triggers chaotic delocalization of the electron orbits, causing strong resonant enhancement of their drift velocity. This dramatically affects the collective electron behavior by inducing multiple propagating charge domains and GHz-THz current oscillations with frequencies ten times higher than with no tilted field.
Masses in graphenelike two-dimensional electronic systems: Topological defects in order parameters and their fractional exchange statistics
Shinsei Ryu, Christopher Mudry, Chang-Yu Hou, and Claudio Chamon
Published 18 November 2009 (32 pages)
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We classify all possible 36 gap-opening instabilities in graphenelike structures in two dimensions, i.e., masses of Dirac Hamiltonian when the spin, valley, and superconducting channels are included. These 36 order parameters break up into 56 possible quintuplets of masses that add in quadrature and hence do not compete and thus can coexist. There is additionally a sixth competing mass, the one added by Haldane to obtain the quantum Hall effect in graphene without magnetic fields, which breaks time-reversal symmetry and competes with all other masses in any of the quintuplets. Topological defects in these five-dimensional order parameters can generically bind excitations with fractionalized quantum numbers. The problem simplifies greatly if we consider spin-rotation invariant systems without superconductivity. In such simplified systems, the possible masses are only 4 and correspond to the Kekulé dimerization pattern, the staggered chemical potential, and the Haldane mass. Vortices in the Kekulé pattern are topological defects that have Abelian fractional statistics in the presence of the Haldane term. We calculate the statistical angle by integrating out the massive fermions and constructing the effective field theory for the system. Finally, we discuss how one can have generically non-Landau-Ginzburg-type transitions with direct transitions between phases characterized by distinct order parameters.

Terahertz photoresponse of a quantum Hall edge-channel diode
Christian Notthoff, Kevin Rachor, Detlef Heitmann, and Axel Lorke
Published 19 November 2009 (5 pages)
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The terahertz (THz) photoresponse of a two-dimensional electron gas in the quantum Hall regime is investigated. We use a sample structure which is topologically equivalent to a Corbino geometry combined with a cross-gate technique. This quasi-Corbino geometry allows us to directly investigate the THz-induced transport between adjacent edge states, thus avoiding bulk effects. We find a pronounced photovoltage at zero applied bias, which rapidly decreases when an external current bias is applied. The photovoltage and its dependence on the bias current can be described using the model of an illuminated photodiode, resulting from the reconstruction of the Landau bands at the sample edge. Using the sample as a detector in a Fourier-transform spectrometer setup, we find a resonant response from which we extract a reduced effective cyclotron mass. The findings support a nonbolometric mechanism of the induced photovoltage and the proposed edge-channel diode model.

Key role of the wetting layer in revealing the hidden path of Ge/Si(001) Stranski-Krastanow growth onset
Moritz Brehm, Francesco Montalenti, Martyna Grydlik, Guglielmo Vastola, Herbert Lichtenberger, Nina Hrauda, Matthew J. Beck, Thomas Fromherz, Friedrich Schäffler, Leo Miglio, and Günther Bauer
Published 19 November 2009 (9 pages)
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The commonly accepted Stranski-Krastanow model, according to which island formation occurs on top of a wetting layer (WL) of a certain thickness, predicts for the morphological evolution an increasing island aspect ratio with volume. We report on an apparent violation of this thermodynamic understanding of island growth with deposition. In order to investigate the actual onset of three-dimensional islanding and the critical WL thickness in the Ge/Si(001) system, a key issue is controlling the Ge deposition with extremely high resolution [0.025 monolayer (ML)]. Atomic force microscopy and photoluminescence measurements on samples covering the deposition range 1.75–6.1 ML, taken along a Ge deposition gradient on 4 in. Si substrates and at different growth temperatures (Tg), surprisingly reveal that for Tg>675 °C steeper multifaceted domes apparently nucleate prior to shallow {105}-faceted pyramids, in a narrow commonly overlooked deposition range. The puzzling experimental findings are explained by a quantitative modeling of the total energy with deposition. We accurately matched ab initio calculations of layer and surface energies to finite-element method simulations of the elastic energy in islands, in order to compare the thermodynamic stability of different island shapes with respect to an increasing WL thickness. Close agreement between modeling and experiments is found, pointing out that the sizeable progressive lowering of the surface energy in the first few MLs of the WL reverts the common understanding of the SK growth onset. Strong similarities between islanding in SiGe and III/V systems are highlighted.

First-principles study of back-contact effects on CdTe thin-film solar cells
Mao-Hua Du
Published 20 November 2009 (4 pages)
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Forming a chemically stable low-resistance back contact for CdTe thin-film solar cells is critically important to the cell performance. This paper reports theoretical study of the effects of the back-contact material, Sb2Te3, on the performance of the CdTe solar cells. First-principles calculations show that Sb impurities in p-type CdTe are donors and can diffuse with low diffusion barrier. There properties are clearly detrimental to the solar-cell performance. The Sb segregation into the grain boundaries may be required to explain the good efficiencies for the CdTe solar cells with Sb2Te3 back contacts.

Surface physics, nanoscale physics, low-dimensional systems

Anomalous finite size effects on surface states in the topological insulator Bi2Se3
Jacob Linder, Takehito Yokoyama, and Asle Sudbø
Published 2 November 2009 (5 pages)
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We study how the surface states in the strong topological insulator Bi2Se3 are influenced by finite size effects and compare our results with those recently obtained for two-dimensional topological insulator HgTe. We demonstrate two important distinctions: (i) contrary to HgTe, the surface states in Bi2Se3 display a remarkable robustness towards decreasing the width L down to a few nm thus ensuring that the topological surface states remain intact and (ii) the gapping due to the hybridization of the surface states features an oscillating exponential decay as a function of L in Bi2Se3 in sharp contrast to HgTe. Our findings suggest that Bi2Se3 is suitable for nanoscale applications in quantum computing or spintronics. Also, we propose a way to experimentally detect both of the predicted effects.

Capacitance of graphene nanoribbons
A. A. Shylau, J. W. Klos, and I. V. Zozoulenko
Published 4 November 2009 (9 pages)
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We present an analytical theory for the gate electrostatics and the classical and quantum capacitance of the graphene nanoribbons (GNRs) and compare it with the exact self-consistent numerical calculations based on the tight-binding p-orbital Hamiltonian within the Hartree approximation. We demonstrate that the analytical theory is in a good qualitative (and in some aspects quantitative) agreement with the exact calculations. There are however some important discrepancies. In order to understand the origin of these discrepancies we investigate the self-consistent electronic structure and charge density distribution in the nanoribbons and relate the above discrepancy to the inability of the simple electrostatic model to capture the classical gate electrostatics of the GNRs. In turn, the failure of the classical electrostatics is traced to the quantum mechanical effects leading to the significant modification of the self-consistent charge distribution in comparison to the noninteracting electron description. The role of electron-electron interaction in the electronic structure and the capacitance of the GNRs is discussed. Our exact numerical calculations show that the density distribution and the potential profile in the GNRs are qualitatively different from those in conventional split-gate quantum wires; at the same time, the electron distribution and the potential profile in the GNRs show qualitatively similar features to those in the cleaved-edge overgrown quantum wires. Finally, we discuss an experimental extraction of the quantum capacitance from experimental data.

Charge transfer satellite in Pr@C82 metallofullerene observed using resonant x-ray emission spectroscopy
H. Yamaoka, H. Sugiyama, Y. Kubozono, A. Kotani, R. Nouchi, A. M. Vlaicu, H. Oohashi, T. Tochio, Y. Ito, and H. Yoshikawa
Published 4 November 2009 (5 pages)
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Resonant x-ray emission spectroscopy (RXES) was performed on the metallofullerene Pr@C82 at the Pr L3 absorption edge. We verify not only nearly three-electron charge transfers from the metal to the cage but also back-electron transfer observed as a charge transfer satellite. The results are compared to theoretical calculations with a single-impurity Anderson model. Theory shows that the electronic structure of endohedral atom in the cage is atomiclike. The satellite structure originates from the charge transfer, i.e., dynamical screening effect, induced by the core-hole potential in the final state rather than from the valence fluctuation of the rare-earth metal in the ground state. We also performed the RXES measurement of Pr2O3 for comparison.

Infrared optical properties of chromium nanoscale films with a phase transition
Robert Lovrinčić and Annemarie Pucci
Published 5 November 2009 (6 pages)
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Thickness-dependent infrared spectra measured during growth of Cr on diamond C(100) under ultrahigh vacuum conditions reveal a structural phase transition from a discontinuous phase to the crystalline bulk one with Drude-type optical properties. The thickness of 2.5 nm of the observed phase transition well agrees with literature data on the theoretically predicted maximum size for stable fcc nanoclusters the formation of which should be supported on C(100) owing to a good lattice match. Below 2.5 nm, the spectral behavior surprisingly well corresponds to a Drude-Smith type dielectric function, which allows one to determine the conductivity for the discontinuous phase and to get quantitative information on the effect of coherent backscattering.

Plasmon modes of spatially separated double-layer graphene
E. H. Hwang and S. Das Sarma
Published 6 November 2009 (5 pages)
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We derive the plasmon dispersion in doped double-layer graphene (DLG), made of two parallel graphene monolayers with carrier densities n1 and n2, respectively, and an interlayer separation of d. The linear chiral gapless single-particle energy dispersion of graphene leads to DLG plasmon properties with several unexpected experimentally observable characteristic features such as a nontrivial influence of an undoped (n2=0) layer on the DLG plasmon dispersion and a strange influence of the second layer even in the weak-coupling d-->[infinity] limit. At long wavelengths (q-->0), the density dependence of the plasma frequencies is different from the usual two-dimensional (2D) electron system with quadratic energy dispersion. Our predicted DLG plasmon properties clearly distinguish graphene from the extensively studied usual parabolic 2D electron systems.

Atomic configuration, conductance, and tensile force of platinum wires of single-atom width
Tokushi Kizuka and Kosuke Monna
Published 6 November 2009 (9 pages)
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Platinum (Pt) wires of single-atom width were produced by the retraction of a Pt nanotip from contact with a Pt plate at room temperature inside a transmission electron microscope. The distance between the nanotip and the plate was controlled using a conductance feedback system, as a result of which wires showing certain conductance values were observed continuously by in situ lattice imaging. Simultaneously, the force acting on the wires was measured using a function of atomic force microscopy. The tip-plate distance was also increased with a constant speed, and the atomic configuration, force, and conductance were similarly investigated. The single-atom-width Pt wires were found to exhibit straight shapes with an interatomic distance of 0.28±0.03  nm. The wires were stable at a tensile force of approximately 1 nN; the observed interatomic distance resulted from elastic expansion. The present study demonstrated experimental evidence for the relationship between wire length and conductance; the wires extend from a three-atom length to a five-atom length as the selected feedback conductance decreases from 2.0 to 0.5G0 (where G0=2e2/h, e being the charge of an electron and h Planck's constant). Contacts exhibiting a conductance of 3.0G0 were two-atom-width contacts. In a conductance histogram constructed from the simple retraction, only one peak was observed at 1.3G0. Thus, it was found that the conductance of single-atom-width Pt wires is less than 3.0G0, with 1.3G0 being that of the most-stable state.