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Physical Review B

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

November 2009

Volume 80, Number 19 , partial issue

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BRIEF REPORTS

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

Wave propagation retrieval method for metamaterials: Unambiguous restoration of effective parameters
Andrei Andryieuski, Radu Malureanu, and Andrei V. Lavrinenko
Published 3 November 2009 (4 pages)
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In this Brief report we propose a direct method of effective-parameters restoration that is based on the wave propagation phenomenon. It is easy in implementation, has no unambiguity in retrieving effective properties and is applicable to thick metamaterial (MTM) slabs. The method is validated on the case studies of fishnet, split cube in carcass, and Jerusalem cross MTMs. The constraints of the method are designated.

Magnetization waves in split-ring-resonator arrays: Evidence for retardation effects
Manuel Decker, Sven Burger, Stefan Linden, and Martin Wegener
Published 5 November 2009 (4 pages)
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We excite low-symmetry planar arrays of nanoscale magnetic split-ring resonators oscillating at around 200 THz frequency under oblique incidence of light. Due to the in-plane coupling of split-ring resonators, classical magnetic-dipole waves result in the plane. We measure the dispersion relation of “antiferromagnetic” and “ferromagnetic” modes, revealing backward waves and a wave-vector-dependent damping. The latter provides evidence for retardation effects, which play no role in the quantum-mechanical counterpart of classical magnetization waves, i.e., magnons. Our experiments are in good agreement with both simple heuristic modeling and microscopic theory.

Anisotropic giant magnetoresistance near the Mott transition in pressurized Ca2RuO4
Fumihiko Nakamura, Ryuji Nakai, Tetsuo Takemoto, Mariko Sakaki, Takashi Suzuki, Patricia Lebre Alireza, Satoru Nakatsuji, and Yoshiteru Maeno
Published 11 November 2009 (4 pages)
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We have observed an anisotropic and giant magnetoresistance (MR) in the 4d-electron Mott transition system of Ca2RuO4. On the border of the Mott transition (~2  GPa), the MR effect at ~10  T reaches ~−55% at TC for longitudinal and ~+120% at low temperatures for the transverse effects. The negative MR is most likely interpreted as a reduction in a ferromagnetic (FM) fluctuation at TC. In contrast, the large positive effect is actually rare and is characteristic of the mixed state where the FM metallic islands are flecked with the insulating phases. We discuss the reason of the peculiar MR from the viewpoint of the “anisotropic magnetism,” the “tunnel MR,” and the “orbital physics.”

Momentum distribution of the hard-core extended boson Hubbard model in one dimension
Min-Chul Cha, Jong-Geun Shin, and Ji-Woo Lee
Published 17 November 2009 (4 pages)
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We investigate the momentum distribution associated with quantum phase transitions between a superfluid and a charge-density-wave state in the one-dimensional hard-core extended boson Hubbard model at half filling by using the Lanczos exact diagonalization method. The momentum distribution shows distinct features in different regions. At the Heisenberg point, it shows a universal behavior. In the superfluid phase, the Luttinger-liquid parameters are easily obtained from the finite-size scaling behaviors of the zero-momentum occupancy. Also the signature of a charge-density wave can be identified in the insulating phase.

Simultaneously optimizing the interdependent thermoelectric parameters in Ce(Ni1−xCux)2Al3
Peijie Sun, Tsuyoshi Ikeno, Toshio Mizushima, and Yosikazu Isikawa
Published 17 November 2009 (4 pages)
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Substitution of Cu for Ni in the Kondo lattice system CeNi2Al3 results in a simultaneous optimization of the three interdependent thermoelectric parameters: thermoelectric power, electrical, and thermal conductivities, where the electronic change in conduction band induced by the extra electron of Cu is shown to be crucial. The obtained thermoelectric figure of merit zT amounts to 0.125 at around 100 K, comparable to the best values known for Kondo compounds. The realization of ideal thermoelectric optimization in Ce(Ni1−xCux)2Al3 indicates that proper electronic tuning of Kondo compounds is a promising approach to efficient thermoelectric materials for cryogenic application.

Semiconductors I: bulk

Ferromagnetic properties of selectively Mn-doped (Ga,Mn)As quantum wells
Hye Jung Kim and Kyung Soo Yi
Published 2 November 2009 (4 pages)
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We investigate temperature dependence of spin-resolved subband structure and spontaneous magnetization for different doping profiles of magnetic impurities in diluted (Ga,Mn)As quantum well structures. The self-consistent hole subband energies and wave functions are determined by solving combined Schrödinger and Poisson equations numerically. We included the coupling effects among heavy-hole, light-hole, and split-off bands (six-band model) in our calculation and compared the result with the case of two-band model. We show that selective doping of magnetic ions in GaAs quantum well enhances the Curie temperature Tc significantly, but holes occupying light-hole subbands result to reduce spontaneous magnetization and hence the Curie temperature of the system. This reduction effect reveals more clearly near the T~Tc hindering an increase in the Curie temperature in a selectively Mn-doped system.

Hydrogen doping in indium oxide: An ab initio study
Sukit Limpijumnong, Pakpoom Reunchan, Anderson Janotti, and Chris G. Van de Walle
Published 9 November 2009 (4 pages)
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First-principles density-functional theory is employed to investigate the role of hydrogen impurities in In2O3. We find that both interstitial hydrogen (Hi) and substitutional hydrogen (HO) act as shallow donors. Our results support recent experiments by Koida et al. [Jpn. J. Appl. Phys. 46, L685 (2007)], which found hydrogen-doped In2O3 to be a good candidate for transparent conducting films.

Recombination lifetimes in InN films studied by time-resolved excitation-correlation spectroscopy
Horng-Chang Liu, Chia-He Hsu, Wu-Ching Chou, Wei-Kuo Chen, and Wen-Hao Chang
Published 10 November 2009 (4 pages)
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Recombination dynamics in degenerate InN were investigated by means of time-resolved excitation-correlation spectroscopy. The photoluminescence decay times are determined beyond the spectral response and temporal resolution limits of conventional photon-counting detectors. Spectral and temperature dependence of decay times reveal the effects of hole localizations on the recombination mechanisms. At low temperatures, the radiative lifetime taur is insensitive to temperature and significantly longer than that predicted for the radiative band-to-band recombination, indicative of a transition dominated by the free-to-bound recombination without k conservation. Above a certain temperature determined by the electron concentration, we find taur~T3/2, as expected for the band-to-band transition when the k-selection rule holds. We determine a lower limit for the bimolecular recombination coefficient B in InN at 300 K as 5.6×10−11  cm3/s.

Nonadiabatic spin-transfer torque in (Ga,Mn)As with perpendicular anisotropy
J.-P. Adam, N. Vernier, J. Ferré, A. Thiaville, V. Jeudy, A. Lemaître, L. Thevenard, and G. Faini
Published 17 November 2009 (4 pages)
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Current-induced magnetic domain wall motion has been investigated in microtracks made from a ferromagnetic semiconductor (Ga,Mn)As thin film with perpendicular anisotropy. In order to reveal the nature of this motion, small fields were additionally applied. The results demonstrate that, when driven by a low current density, the domain walls move under weak fields in a steady-state regime, ruling out models based on spin precession of the domain wall magnetization. The interpretation of these results requires a nonadiabatic contribution in the spin transfer, whose value is estimated and compared to recent theoretical calculations. This highlights the role of spin-orbit interaction in the carrier band on spin-transfer torque in continuous magnetic structures.

Electron spin decoherence in diluted magnetic quantum wells
P. M. Shmakov, A. P. Dmitriev, and V. Yu. Kachorovskii
Published 19 November 2009 (4 pages)
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We study electron-spin dynamics in diluted magnetic quantum wells. The electrons are coupled by exchange interaction with randomly distributed magnetic ions polarized by magnetic field B. This coupling leads to both spin relaxation and spin decoherence. We demonstrate that even very small spatial fluctuations of quantum well width dramatically increase rate of decoherence. Depending on the strength of exchange interaction and the amplitude of the fluctuations the decoherence can be homogeneous or inhomogeneous. In the homogeneous regime, the transverse (with respect to B) component of electron spin decays on the short time scale exponentially while the long-time spin dynamics obeys a power law. In the inhomogeneous case, the transverse spin component decays exponentially with the exponent quadratic in time.

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

Spin dynamics of electrons in the first excited subband of a high-mobility low-density two-dimensional electron system
Haihui Luo, Xuan Qian, Xuezhong Ruan, Yang Ji, and Vladimir Umansky
Published 3 November 2009 (4 pages)
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We report on time-resolved Kerr rotation measurements of spin coherence of electrons in the first excited subband of a high-mobility low-density two-dimensional electron system in a GaAs/Al0.35Ga0.65As heterostructure. While the transverse spin lifetime (T2<sup>*</sup>) of electrons decreases monotonically with increasing magnetic field, it has a nonmonotonic dependence on the temperature and reaches a peak value of 596 ps at 36 K, indicating the effect of intersubband electron-electron scattering on the electron-spin relaxation.

Interplay of dc current and microwaves in the magnetotransport of two-dimensional electron systems
Jesús Iñarrea
Published 3 November 2009 (4 pages)
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We theoretically study magnetoresistance oscillations in two-dimensional electron systems in the presence of microwaves and a dc electric field. We obtain that the microwave-induced resistance oscillations and zero resistance states are dramatically affected by a dc electric field of increasing intensity. The interplay of both fields produces a plasma wave which oscillates at the frequency of microwaves but with a phase difference of pi radians. This plasma wave interferes with the microwave-induced electronic motion changing gradually the resistance oscillations profile: maxima evolve to minima and vice versa. We introduced in our model anharmonicity corrections to magnetoresistance oscillations in order to explain the peculiar biased profile that experimental results present. The theoretical outcomes are in agreement with experimental evidence.

Electric-field induced modulation of the magneto-optical Kerr effect in a (Zn,Be,Mn)Se/GaAs spintronic device
J. H. Versluis, A. V. Kimel, A. Kirilyuk, P. Grabs, F. Lehmann, G. Schmidt, L. W. Molenkamp, and Th. Rasing
Published 4 November 2009 (4 pages)
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It is shown that when spin-polarized electrons are injected from a (Zn,Be,Mn)Se spin-aligner into GaAs, the magneto-optical Kerr effect from (Zn,Be,Mn)Se/GaAs spintronic device is strongly affected by a large electric-field at the (Zn,Be,Mn)Se/GaAs interface. This field causes the magneto-optical signal to be extremely sensitive to the sample temperature and the wavelength of the probing light. The observed temperature dependencies are explained in terms of the Franz-Keldysh effect at (Zn,Be,Mn)Se/GaAs interface. The findings demonstrate that the results of magneto-optical detection of electrically injected spins in spintronic devices can be easily misinterpreted if electric-field induced effects are not taken into account.

Quantum phase transition in Hall conductivity on an anisotropic kagome lattice
Shun-Li Yu, Jian-Xin Li, and Li Sheng
Published 5 November 2009 (4 pages)
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We study theoretically the quantum Hall effect (QHE) on the kagome lattice with anisotropy in one of the hopping integrals. We find an interesting quantum phase, in which the QHE exhibits the energy spectrum given by E(n)=±vFsqrt((n + 1/2)[h-bar]Be) (n is an integer) being different from the known types, though its quantization rule for Hall conductivity sigmaxy=2ne2/h is conventional. This phase evolves from the QHE phase with sigmaxy=4(n+1/2)e2/h and E(n)=±vFsqrt(2n[h-bar]Be) in the isotropic case, which is realized in a system with massless Dirac fermions (such as in graphene). The phase transition does not occur simultaneously in all Hall plateaus but occurs in a sequence from low to high energies, with the increase in hopping anisotropy.

Effect of electron-electron scattering on magnetointersubband resistance oscillations of two-dimensional electrons in GaAs quantum wells
A. V. Goran, A. A. Bykov, A. I. Toropov, and S. A. Vitkalov
Published 12 November 2009 (4 pages)
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The low-temperature (4.2<T<12.5  K) magnetotransport (B<2  T) of two-dimensional electrons occupying two subbands (with energy E1 and E2) is investigated in GaAs single quantum well with AlAs/GaAs superlattice barriers. Two series of Shubnikov-de Haas oscillations are found to be accompanied by magnetointersubband (MIS) oscillations, periodic in the inverse magnetic field. The period of the MIS oscillations obeys condition Delta12=(E2−E1)=k·[h-bar]omegac, where Delta12 is the subband energy separation, omegac is the cyclotron frequency, and k is the positive integer. At T=4.2  K the oscillations manifest themselves up to k=100. Strong temperature suppression of the magnetointersubband oscillations is observed. We show that the suppression is a result of electron-electron scattering. Our results are in good agreement with recent experiments, indicating that the sensitivity to electron-electron interaction is the fundamental property of magnetoresistance oscillations, originating from the second-order Dingle factor.

Statistics of quantum transfer of noninteracting fermions in multiterminal junctions
Dania Kambly and D. A. Ivanov
Published 19 November 2009 (3 pages)
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Similarly to the recently obtained result for two-terminal systems, we show that there are constraints on the full counting statistics for noninteracting fermions in multiterminal contacts. In contrast to the two-terminal result, however, there is no factorization property in the multiterminal case.

Surface physics, nanoscale physics, low-dimensional systems

Coupling of graphene and surface plasmons
Norman J. Morgenstern Horing
Published 2 November 2009 (4 pages)
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We analyze the plasmon spectrum of a graphene sheet in the vicinity of a thick plasmalike substrate, finding linear dispersion in some parameter ranges.

Dithering-amplitude dependence of STM-regulated dynamic lateral force microscopy maps on Si(111)7×7
Naruo Sasaki, Shigeki Kawai, and Hideki Kawakatsu
Published 3 November 2009 (4 pages)
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Model simulation of dynamic lateral force microscopy (DLFM) regulated by scanning tunneling microscopy (STM) has been performed. The simulated STM/DLFM maps on Si(111)7×7 exhibit marked transitions depending on the lateral dithering amplitude, and they can successfully reproduce the experimentally acquired maps for a wide range of operating conditions. This work describes the direct calibration of dithering-amplitude-induced artifacts of STM/DLFM maps on Si(111)7×7 for a small time-averaged tunneling current corresponding to a tip-sample distance larger than 5  Å, where the atomic relaxation of the tip and the sample is sufficiently small.

Electronic structure of H/Ge(111)1×1 studied by angle-resolved photoelectron spectroscopy
I. Razado-Colambo, H. M. Zhang, and R. I. G. Uhrberg
Published 6 November 2009 (4 pages)
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The electronic structure of H/Ge(111)1×1 was investigated using angle-resolved photoelectron spectroscopy. Spectra were measured along the high-symmetry lines of the 1×1 surface Brillouin zone. In the [overline Gamma ]−[overline K]−[overline M] direction, two surface states, labeled a and a[prime], were found in the lower and upper band-gap pockets. The a and a[prime] surface states are associated with the Ge-H bonds and the Ge-Ge backbonds, respectively. In the [overline Gamma ]−[overline M] direction, only the Ge-H surface state, a, can be identified. It is found in the band-gap pocket around the [overline M] point. The two hydrogen-induced surface states on H/Ge(111)1×1 show strong similarities with the corresponding surface states on H/Si(111)1×1. Results from H/Ge(111)1×1 and H/Si(111)1×1 are compared in this Brief Report.

Spin-valve effect in zigzag graphene nanoribbons by defect engineering
Sankaran Lakshmi, Stephan Roche, and Gianaurelio Cuniberti
Published 6 November 2009 (4 pages)
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We report on the possibility for a spin-valve effect driven by edge defect engineering of zigzag graphene nanoribbons. Based on a mean-field spin-unrestricted Hubbard model, electronic band structures and conductance profiles are derived, using a self-consistent scheme to include gate-induced charge density. The use of an external gate is found to trigger a semiconductor-metal transition in clean zigzag graphene nanoribbons, whereas it yields a closure of the spin-split band gap in the presence of Klein edge defects. These features could be exploited to make charge- and spin-based switches and field-effect devices.

Trajectory fluctuations accompanying the manipulation of spherical nanoparticles
Akshata Rao, Marie-Luise Wille, Enrico Gnecco, Karine Mougin, and Ernst Meyer
Published 10 November 2009 (4 pages)
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We relate the frictional forces acting on spherical nanoparticles pushed by a scanning probe tip on a flat surface to the trajectories of the particles. Based on a simple collisional model, we predict that the average direction of motion of the nanoparticles is almost independent of the friction force, whereas the fluctuations of the particle directions are inversely proportional to friction. The model is applied to interpret the trajectory fluctuations and the apparent discontinuities observed when spherical gold particles are manipulated on a silicon-oxide surface by atomic force microscopy.

Adhesion and peeling forces of carbon nanotubes on a substrate
Makoto Ishikawa, Ryuichi Harada, Naruo Sasaki, and Kouji Miura
Published 12 November 2009 (4 pages)
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The adhesion and peeling of a multiwalled carbon nanotube (MWCNT) on a substrate have been studied. Nanoscale and mesoscale intermittent adhesion and peeling, and a conformational transition of an MWCNT appear in the vertical force-distance curve, which depends strongly on the length of the MWCNT, substrate, and velocities of adhesion and peeling. The elastic bending feature of the MWCNT as a nanospring appears during the adhesion and peeling.

Light-induced valley currents and magnetization in graphene rings
A. S. Moskalenko and J. Berakdar
Published 12 November 2009 (4 pages)
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We study the nonequilibrium dynamics in a mesoscopic graphene ring excited by picoseconds shaped electromagnetic pulses. We predict an ultrafast buildup of charge polarization, currents, and orbital magnetization. Applying the light pulses identified here, nonequilibrium valley currents are generated in a graphene ring threaded by a stationary magnetic flux. We predict a finite graphene ring magnetization even for a vanishing charge current; the magnetization emerges due to the light-induced difference of the valley populations.

Band-gap tunability of a (6,0) BN nanotube bundle under pressure: Ab initio calculations
S. S. Coutinho, V. Lemos, and S. Guerini
Published 19 November 2009 (4 pages)
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Ab initio calculations were performed to study the structural transformation on a boron nitride nanotubes bundle under pressure. Several bundles of boron nitride nanotubes, disposed into a hexagonal arrangement, were studied between which the (6,0) zigzag configuration was chosen to be detailed here. Upon compression the hexagonal arrangement as well as the circular cross section of the tubes were preserved up to a critical pressure value. At this pressure value the tubes deform to an oval cross section and the bundle shape diverge from the original hexagonal symmetry. The percent difference volume suffers a discontinuity with pressure demonstrating the discontinuous nature of the structural transition. The energy gap undergoes a continuous decrease up to the pressure of collapse of the tubes. The tunability of the gap is a fundamental requirement for engineering electronic nanodevices widening the perspectives for boron nitride nanotube applications.

Low-temperature observation of the softened C-H stretching vibrations of cyclohexane on Rh(111)
Takanori Koitaya, Atsushi Beniya, Kozo Mukai, Shinya Yoshimoto, and Jun Yoshinobu
Published 19 November 2009 (4 pages)
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The C-H stretching vibrations of cyclohexane on Rh(111) were investigated using infrared reflection absorption spectroscopy between 20 and 89 K. At 20 K, the softened C-H stretching band consists of several sharp peaks, ranging from 2500 to 2700  cm−1. The wide-range distribution of the softened C-H stretching peaks results from inhomogeneity of adsorption environments. With increasing the substrate temperature, each softened C-H stretching peak becomes significantly broadened, but the normal C-H stretching peaks are little changed. These results indicate that the local interaction between the softened C-H species and the Rh(111) surface is sensitive to a thermally excited low-energy mode. The temperature-dependent broadening of a soft mode at low temperature is analyzed using a vibrational dephasing model, where the softened C-H stretching mode is anharmonically coupled with a thermally excited frustrated translation mode.

Quantum confinement and thermal effects on the Raman spectra of Si nanocrystals
Giuseppe Faraci, Santo Gibilisco, and Agata R. Pennisi
Published 19 November 2009 (4 pages)
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In nanocrystals, first-order Raman spectra can exhibit large shift and width due to two overlapping effects: quantum confinement and thermal heating. In order to distinguish each contribution we studied silicon nanocrystals by Raman spectroscopy. From the measurements we extracted the dependence of the Raman shift and width as a function of the temperature. A model size and temperature dependence was developed for the interpretation of these data, demonstrating the negligible contribution of the quantum confinement for sizes higher then 6 nm. Excellent agreement between theory and experiment was obtained both for the energy shift and for the width broadening.

ARTICLES

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

Optical spectroscopy of PrFe3(BO3)4: Crystal-field and anisotropic Pr-Fe exchange interactions
M. N. Popova, T. N. Stanislavchuk, B. Z. Malkin, and L. N. Bezmaternykh
Published 2 November 2009 (13 pages)
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High-resolution polarized optical absorption spectra of PrFe3(BO3)4 in the paramagnetic and antiferromagnetic phases are reported. The measured energies of the crystal-field (CF) levels within the 4f2 configuration of Pr3+ in the paramagnetic PrFe3(BO3)4 are described by the CF model that involves the 4f2/4f5d and 4f2/4f6p configuration interactions. Ordering of Fe spins along the crystalline c axis below TN=32  K is confirmed by the analysis of the spectra of Er3+ introduced as a probe into PrFe3(BO3)4. To account for the observed changes in the optical spectra of Pr3+ at temperatures below TN, in particular, for the shift of the CF levels, splitting of the CF doublets, and the appearance of forbidden lines, the Pr-Fe exchange Hamiltonian defined by seven parameters is considered. The theoretical approach has been tested by calculating the temperature dependence of the magnetic susceptibility. A good agreement between theory and optical and magnetic experimental data is found demonstrating the validity of the model used. The obtained results confirm that the model of the iron dimers inside the spiral chains of Fe3+(O2−)6 octahedrons introduced by us earlier for NdFe3(BO3)4 and modified in the present work may serve as a basis for analyzing the low-temperature properties of other rare-earth iron borates.

Transport in a Luttinger liquid with dissipation: Two impurities
Leiming Chen, Zoran Ristivojevic, and Thomas Nattermann
Published 2 November 2009 (11 pages)
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We consider theoretically the transport in a one-channel spinless Luttinger liquid with two strong impurities in the presence of dissipation. As a difference with respect to the dissipation free case, where the two impurities fully transmit electrons at resonance points, the dissipation prevents complete transmission in the present situation. A rich crossover diagram for the conductance as a function of applied voltage, temperature, dissipation strength, Luttinger liquid parameter K, and the deviation from the resonance condition is obtained. For weak dissipation and 1/2<K<1, the conduction shows a nonmonotonic increase as a function of temperature or voltage. For strong dissipation the conduction increases monotonically but is exponentially small.

Electromechanical instability in vibrating quantum dots with effectively negative charging energy
Teemu Ojanen, Friedrich C. Gethmann, and Felix von Oppen
Published 4 November 2009 (8 pages)
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In quantum dots or molecules with vibrational degrees of freedom the electron-vibron coupling renormalizes the electronic charging energy. For sufficiently strong coupling, the renormalized charging energy can become negative. Here, we discuss an instability toward adding or removing an arbitrary number of electrons when the magnitude of the renormalized charging energy exceeds the single-particle level spacing. We show that the instability is regularized by the anharmonic contribution to the vibron energy. The resulting effective charging energy as a function of the electron number has a double-well structure causing a variety of novel features in the Coulomb-blockade properties.

Optical conductivity of a doped Mott insulator: The interplay between correlation and electron-phonon interaction
G. De Filippis, V. Cataudella, A. S. Mishchenko, C. A. Perroni, and N. Nagaosa
Published 4 November 2009 (6 pages)
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The optical conductivity (OC) of cuprates is studied theoretically in the low-density limit of the t-t[prime]-J model taking into account the hole-lattice coupling. By developing a limited phonon basis exact diagonalization method capable of treating the lattice of largest size ever considered (4×4), we are able to discern the fine features of the mid-infrared (MIR) part of the OC revealing a three-peak structure. The two lowest peaks are observed in experiments and the highest one is tacitly resolved in moderately doped cuprates. Comparison of OC with the results of semianalytic approaches and detailed analysis of the calculated isotope effect indicate that the middle-energy MIR peak is of mostly magnetic origin while the lowest MIR band originates from the scattering of holes by phonons.

Imaging properties of lossless double-negative metamaterial wedges: Analytical and numerical investigation
Dimitrios L. Sounas, Nikolaos V. Kantartzis, and Theodoros D. Tsiboukis
Published 5 November 2009 (14 pages)
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The superlensing characteristics of a lossless double-negative metamaterial wedge are examined in this paper via an analytical technique. The algorithm incorporates the Kontorovich-Lebedev (KL) transform in a mathematically consistent manner whereas the role of the radiation condition on its correct application is quantitatively elucidated. Implementing a ray-approximation algorithm, the proper radiation conditions are resolved and the Helmholtz equation as well as the boundary conditions are accordingly transformed. To this end, the prior formulation introduces a linear operator which, given the KL transform of a specific kind, constructs the corresponding one of a different kind. Thus, the field can be described analytically in the entire domain, showing the ability of the wedge to, perfectly, focus a line source in two points, inside and outside the metamaterial, just like the planar double-negative slab. To validate the proposed analysis, the analytical results are compared to those acquired by means of the finite-difference time-domain method for various geometrical parameters and wedge configurations.
Ultrahigh Purcell factors and Lamb shifts in slow-light metamaterial waveguides
Peijun Yao, C. Van Vlack, A. Reza, M. Patterson, M. M. Dignam, and S. Hughes
Published 6 November 2009 (11 pages)
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We introduce the complex band structure and a medium-dependent (Green's function) quantum-optics formalism to study the enhanced spontaneous emission factors and Lamb shifts from a quantum dot or atom near the surface of a slow-light metamaterial waveguide. Using a realistic loss factor of gamma/2pi=2  THz, Purcell factors of approximately 250 and 100 are found at optical frequencies for p-polarized and s-polarized dipoles, respectively, placed 28 nm (0.02lambda0) above the slab surface. For smaller loss values, we demonstrate that the slow-light regime of odd metamaterial waveguide propagation modes can be observed and related to distinct resonances in the Purcell factors. Correspondingly, we predict unusually large and rich Lamb shifts of approximately −1 to −6  GHz for a dipole moment of 50 Debye. We also make a direct calculation of the far-field-emission spectra which contains direct measurable access to these enhanced Purcell factors and Lamb shifts.

Optical conductivity and x-ray absorption spectra of the Mott-Hubbard compounds RVO3 (R=Sr, Ca, La, and Y)
R. J. O. Mossanek, M. Abbate, P. T. Fonseca, A. Fujimori, H. Eisaki, S. Uchida, and Y. Tokura
Published 6 November 2009 (6 pages)
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Core-level and valence-band optical spectra provide important information on highly correlated systems. The former corresponds to transitions from the core level to the conduction band, and it is usually related to the unoccupied electronic structure. The later corresponds to transitions from the valence band to the conduction band, and it is sometimes described in terms of the joint density of states. These spectra are usually treated separately due to the differences in the experimental and theoretical methods. We present here a combined description of the core-level and valence-level optical spectra of Mott-Hubbard compounds. In particular, we studied the O 1s x-ray absorption and the optical-conductivity spectra of SrVO3-CaVO3-LaVO3-YVO3. The experimental data were analyzed using an extended p-d cluster model solved by an exact diagonalization method. The results show that correlation effects alone cannot account for the experimental structures, and that crystal-field effects and exchange interactions are necessary to explain the spectra. We also show that there is a correspondence between the features in the charge-transfer region of both spectra.

Cancellation of spin and orbital moments in URhGe under pressure: A density-functional prediction
W. Miiller, V. H. Tran, and M. Richter
Published 10 November 2009 (6 pages)
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The local-spin-density approximation (LSDA) is used to perform density-functional total-energy calculations for the ferromagnetic superconductor URhGe in the magnetic and in the nonmagnetic states under high pressure. Our calculations show that the ferromagnetic ground state is energetically preferred in a wide pressure range, even though the nearest U-U interatomic distance is much below Hill's limit. An intriguing behavior of the total magnetic moment upon application of pressure is observed. Due to a compensation of spin and orbital contributions, the total moment vanishes at a pressure of about 12 GPa. If the pressure is further enhanced, the total moment reappears, before a nonmagnetic ground state is obtained at about 50 GPa.

Insight into the performance of GGA functionals for solid-state calculations
Philipp Haas, Fabien Tran, Peter Blaha, Karlheinz Schwarz, and Robert Laskowski
Published 12 November 2009 (13 pages)
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Many exchange-correlation functionals of the generalized gradient approximation (GGA) are available in the literature. More particularly, during the last few years several research groups have proposed GGA functionals for solids, which very often perform better (especially for the lattice constant) than the standard GGA functional of Perdew, Burke, and Ernzerhof (PBE) [J. P. Perdew, K. Burke, and M. Ernzerhof, Phys. Rev. Lett. 77, 3865 (1996)]. The improvement over PBE is not systematic, but trends among the different classes of solids can be observed [P. Haas, F. Tran, and P. Blaha, Phys. Rev. B 79, 085104 (2009)]. A better understanding of the trends obtained with the existing functionals can obviously be very helpful for the construction of more accurate functionals, and in the aim of this we studied the distribution of the Wigner-Seitz radius rs (related to the electron density rho) and the reduced density gradient s in a few selected solids and identified the relevant ranges of rs (rs<4) and s (s<2) in solids. We focus on the variation of the exchange-correlation energy with respect to the unit-cell volume (dExc/dOmega), which determines the equilibrium lattice constant and identify the “important regions” in the unit cell, where the differences of dExc/dOmega between two functionals are most pronounced. In metallic systems, these important regions coincide with the spatial separation of semicore and valence electrons, while for semiconductors (open structures) and insulators (inhomogeneous systems) the tails of the valence electrons become equally important or even dominate.

Temperature dependence of the electronic structure of the Jeff=(1/2) Mott insulator Sr2IrO4 studied by optical spectroscopy
S. J. Moon, Hosub Jin, W. S. Choi, J. S. Lee, S. S. A. Seo, J. Yu, G. Cao, T. W. Noh, and Y. S. Lee
Published 17 November 2009 (5 pages)
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We investigated the temperature-dependent evolution of the electronic structure of the Jeff=(1/2) Mott insulator Sr2IrO4 using optical spectroscopy. The optical conductivity spectra sigma(omega) of this compound has recently been found to exhibit two d-d transitions associated with the transition between the Jeff=(1/2) and Jeff=(3/2) bands due to the cooperation of the electron correlation and spin-orbit coupling. As the temperature increases, the two peaks show significant changes resulting in a decrease in the Mott gap. The experimental observations are compared with the results of first-principles calculation in consideration of increasing bandwidth. We discuss the effect of the temperature change in the electronic structure of Sr2IrO4 in terms of local lattice distortion, excitonic effect, electron-phonon coupling, and magnetic ordering.
Fast update algorithm for the quantum Monte Carlo simulation of the Hubbard model
Phani K. V. V. Nukala, Thomas A. Maier, Michael S. Summers, Gonzalo Alvarez, and Thomas C. Schulthess
Published 17 November 2009 (8 pages)
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This paper presents an efficient algorithm for computing the transition probability in auxiliary field quantum Monte Carlo simulations of strongly correlated electron systems using a Hubbard model. This algorithm is based on a low rank updating of the underlying linear algebra problem, and results in significant computational savings. The computational complexity of computing the transition probability and Green's function update reduces to [script O](k2) during the kth step, where k is the number of accepted spin flips, and results in an algorithm that is faster than the competing delayed update algorithm. Moreover, this algorithm is orders of magnitude faster than traditional algorithms that use naive updating of the Green's function matrix.

Localized atomic basis set in the projector augmented wave method
A. H. Larsen, M. Vanin, J. J. Mortensen, K. S. Thygesen, and K. W. Jacobsen
Published 18 November 2009 (10 pages)
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We present an implementation of localized atomic-orbital basis sets in the projector augmented wave (PAW) formalism within the density-functional theory. The implementation in the real-space GPAW code provides a complementary basis set to the accurate but computationally more demanding grid representation. The possibility to switch seamlessly between the two representations implies that simulations employing the local basis can be fine tuned at the end of the calculation by switching to the grid, thereby combining the strength of the two representations for optimal performance. The implementation is tested by calculating atomization energies and equilibrium bulk properties of a variety of molecules and solids, comparing to the grid results. Finally, it is demonstrated how a grid-quality structure optimization can be performed with significantly reduced computational effort by switching between the grid and basis representations.

Spin-charge decoupling and the photoemission line-shape in one-dimensional insulators
Valeria Lante and Alberto Parola
Published 19 November 2009 (8 pages)
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The recent advances in angle-resolved photoemission techniques allowed the unambiguous experimental confirmation of spin-charge decoupling in quasi-one-dimensional (1D) Mott insulators. This opportunity stimulates a quantitative analysis of the spectral function A(k,omega) of prototypical one-dimensional correlated models. Here we combine Bethe Ansatz results, Lanczos diagonalizations, and field theoretical approaches to obtain A(k,omega) for the 1D Hubbard model as a function of the interaction strength. By introducing a single spinon approximation, an analytic expression is obtained, which shows the location of the singularities and allows, when supplemented by numerical calculations, to obtain an accurate estimate of the spectral weight distribution in the (k,omega) plane. Several experimental puzzles on the observed intensities and line-shapes in quasi-1D compounds such as SrCuO2, find a natural explanation in this theoretical framework.

Semiconductors I: bulk

Photogeneration of coherent shear phonons in orientated wurtzite semiconductors by piezoelectric coupling
Yu-Chieh Wen, Tsung-Shine Ko, Tien-Chang Lu, Hao-Chung Kuo, Jen-Inn Chyi, and Chi-Kuang Sun
Published 6 November 2009 (6 pages)
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Using piezoelectric wurtzite semiconductor with accurate control of the crystal cut, we investigate the photogeneration of coherent shear acoustic phonons through anisotropic piezoelectric coupling. Theoretical study suggests the dominant contribution of the piezoelectric effect to the shear phonon generation in a-plane wurtzite GaN, which is also confirmed by the picosecond ultrasonic experiments with different dopant and excitation conditions. Piezoelectric transduction thus explains the observed remarkable coherent shear phonon signal. The influence of the ultrafast screening of surface electric field by photocarriers on the intensity and waveform of the generated shear acoustic pulse are discussed.

Spatially separated intrinsic emission components in InxGa1−xN ternary alloys
Yoichi Yamada, Takuya Saito, Nobuo Kato, Eiji Kobayashi, Tsunemasa Taguchi, Hiromitsu Kudo, and Hiroaki Okagawa
Published 10 November 2009 (8 pages)
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The characteristics of band-edge photoluminescence (PL) from Ga-rich InxGa1−xN ternary-alloy epitaxial layers with indium compositions of x=0.02, 0.03, 0.05, 0.06, and 0.09 have been comprehensively studied by means of scanning near-field optical microscopy (SNOM) in addition to conventional macroscopic PL spectroscopy. The band-edge PL from the ternary alloys consisted of two intrinsic emission components, a strong higher-energy component and a weak lower-energy component. The radiative recombination channels of the two components were not independent of each other and thermal population of carriers took place between the components. A spatial separation between the two components was clearly indicated by SNOM-PL measurements. The higher-energy component corresponded to islandlike regions with diameters of several hundred nanometers whereas the lower-energy component corresponded to dotlike regions with diameters of 50–80 nm. In addition, the regions corresponding to the lower-energy component were surrounded by a local potential maximum, which acted as a potential barrier for carriers between the higher- and the lower-energy components.

Single-frequency laser spectroscopy of the boron bound exciton in 28Si
A. Yang, M. Steger, T. Sekiguchi, D. Karaiskaj, M. L. W. Thewalt, M. Cardona, K. M. Itoh, H. Riemann, N. V. Abrosimov, M. F. Churbanov, A. V. Gusev, A. D. Bulanov, I. D. Kovalev, A. K. Kaliteevskii, O. N. Godisov, P. Becker, H.-J. Pohl, J. W. Ager, III, and E. E. Haller
Published 11 November 2009 (6 pages)
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While the first comparison of shallow bound exciton photoluminescence between natural Si and highly enriched 28Si dramatically demonstrated the importance of inhomogeneous isotope broadening, the transitions in 28Si were in fact too narrow to be resolved with the then available instrumental resolution of 0.014  cm−1. We report results for the boron bound exciton transition in highly enriched 28Si using a novel apparatus for photoluminescence excitation spectroscopy based on a tuneable single-frequency laser source with sub-MHz resolution. Twenty well-resolved doublets, exhibiting a 10B–11B isotope splitting, are observed in the new spectra for 28Si with isotopic enrichment >99.99%. Linewidths as narrow as 0.0012  cm−1 (150 neV) full width at half maximum are observed for the most highly enriched sample.

Electronic, vibrational, and thermodynamic properties of metacinnabar beta-HgS, HgSe, and HgTe
M. Cardona, R. K. Kremer, R. Lauck, G. Siegle, A. Muñoz, and A. H. Romero
Published 11 November 2009 (14 pages)
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We report ab initio calculations of the electronic band structure and the phonon dispersion relations of the zincblende-type mercury chalcogenides (beta-HgS, HgSe, and HgTe). The latter have been used to evaluate the temperature dependence of the specific heat, which has been compared with experimental data. The electronic band structure of these materials has been confirmed to have an inverted direct gap of the alpha-tin type, which makes HgSe and HgTe semimetallic. For beta-HgS, however, our calculations predict a negative spin-orbit splitting that restores semiconducting properties to the material in spite of the inverted gap. We have calculated the spin-orbit induced linear terms in k, which appear at the Gamma8 valence bands. We have also investigated the pressure dependence of the crystal structure and the phonons.

A hybrid density functional study of lithium in ZnO: Stability, ionization levels, and diffusion
A. Carvalho, A. Alkauskas, Alfredo Pasquarello, A. K. Tagantsev, and N. Setter
Published 12 November 2009 (12 pages)
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The properties of interstitial and substitutional Li in wurtzite ZnO are modeled using hybrid density functional calculations. We investigate the impact of the band-gap error on the formation energies of the two defects and their dependence on the Fermi level. It is found that within a local-density approximation, the acceptor level of LiZn is very close to the valence-band top but as the band gap is opened, the acceptor state becomes more localized and the respective level is shifted upward. Taking polaronic effects into account, we place the ionization level of LiZn between Ev+0.60  eV and Ev+1.1  eV. This deeper level explains the difficulty in realizing p-type ZnO using Li as monodopant. Further, the mobility of the defects was investigated. While interstitial Li is mobile at low temperatures, independent of the stoichiometry, the diffusion of LiZn depends on the concentrations of intrinsic defects. Our calculations show that in O-rich material, where the defect is more stable, the dominant diffusion process corresponds to a dissociative mechanism requiring a substantial activation energy.

Hydrogen diffusion in GaAs1−xNx
R. Trotta, D. Giubertoni, A. Polimeni, M. Bersani, M. Capizzi, F. Martelli, S. Rubini, G. Bisognin, and M. Berti
Published 16 November 2009 (10 pages)
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Hydrogen (or deuterium) incorporation in dilute nitride semiconductors modifies dramatically the electronic and structural properties of the crystal through the creation of nitrogen-hydrogen complexes. In this work, we investigate how the formation and dissociation of such complexes rule the diffusion of deuterium in GaAs1−xNx. The concentration depth profile of deuterium is determined by secondary ion mass spectrometry under a wide range of experimental conditions that comprise different N concentrations (x=0.09%, 0.40%, 0.70%, and 1.5%) and D irradiation temperatures (TD=200, 250, 300 and 350 °C). The experimental data are successfully reproduced by a diffusion model in the presence of strong D trapping. In particular, the deuterium diffusion and capture rate coefficients are determined, and a minimum decay length of the deuterium forefront is found at low TD (<250 °C) and high x (>0.7%). These parameters set the experimental conditions within which a nanostructuring of the physical properties of GaAs1−xNx is attainable.

Atomic nature of the Si-B4 paramagnetic center assessed by multifrequency electron spin resonance
K. Keunen and A. Stesmans
Published 17 November 2009 (13 pages)
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An electron spin resonance (ESR) study has been carried out in the range 4.2–120 K on the trigonal Si-B4 center in annealed (200–350 °C) neutron-irradiated p-type Si. Detailed observations reveal a rich 29Si hyperfine (hf) structure, indicating interaction of the unpaired electron with up to five shells of equivalent Si sites. Optimized consistent computer-assisted fitting of spectra observed at three ESR frequencies point toward a set of 1-2-2-3-3 equivalent Si sites for the five observed shells in decreasing hf splitting, respectively, with the 1-3-3-3-3 set, conforming with the trigonal symmetry of the defect, giving satisfactory results as well. The strongest interaction of the unpaired electron is with one Si atom. Full angular mapping of all five hf doublets enabled inference of the hybrid coefficients, i.e., localization of the unpaired electron and s-p ratio over the different Si sites. Stepping from previous theoretical work, two tri-interstitial models are discussed from where, although lack of theoretical results on the molecular wave-function coefficients prevents definite assignment, the tri-interstitial I3-I model is advanced as a plausible candidate. An uncommon drastic temperature dependence is observed in which all resolved 29Si hf doublet splittings collectively and evenly narrow with increasing temperature toward 1/4 of the low-temperature values along a bounded exponential (Boltzmann factor) decay, with activation energy Ea[approximate]0.0041  eV. There is an attendant upward shift in g||. An effect due to thermal dilatation as possible origin is excluded. Instead, it is ascribed to even redistribution (delocalization) of the low-temperature unpaired hybrid by 75% over next-neighboring sites, possibly within a four-defect cluster arrangement.

Variable range hopping transport in ferromagnetic GaGdN epitaxial layers
A. Bedoya-Pinto, J. Malindretos, M. Roever, D. D. Mai, and A. Rizzi
Published 17 November 2009 (5 pages)
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Electrical-transport properties of ferromagnetic GaGdN layers grown by molecular-beam epitaxy on highly resistive 6H-SiC(0001) substrates have been investigated. It is found that doping with low concentrations of Gd increases the resistivity by several orders of magnitude as compared to unintentionally doped GaN. In the measurable temperature range between 5 and 120 K two different temperature dependences of the resistivity (T−1/2 and T−1/4) are observed, both of which are characteristic of variable-range-hopping conductivity in an impurity band of localized states. The experimental evidence, that room-temperature ferromagnetism and impurity-band hopping transport are coexistent, is discussed in connection with recent models proposing defect-induced magnetism in wide-band-gap III-Nitrides.

Exciton diffusion in energetically disordered organic materials
Stavros Athanasopoulos, Evguenia V. Emelianova, Alison B. Walker, and David Beljonne
Published 19 November 2009 (7 pages)
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We implement a simple, continuous, analytical model for exciton hopping in an energetically disordered molecular landscape. The model is parameterized against atomistic and lattice Monte Carlo simulations based on quantum-chemical calculations. It captures the essential physics of exciton diffusion in disordered media at different temperatures and yields a universal scaling law of the diffusion length with the dimensionless disorder parameter given by the ratio of the energetic disorder width to the thermal energy.

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

Disorder effect of resonant spin Hall effect in a tilted magnetic field
Zhan-Feng Jiang, Shun-Qing Shen, and Fu-Chun Zhang
Published 3 November 2009 (6 pages)
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We study the disorder effect of resonant spin Hall effect in a two-dimensional electron system with Rashba coupling in the presence of a tilted magnetic field. The competition between the Rashba coupling and the Zeeman coupling leads to the energy crossing of the Landau levels, which gives rise to the resonant spin Hall effect. Utilizing the Streda's formula within the self-consistent Born approximation, we find that the impurity scattering broadens the energy levels and the resonant spin Hall conductance exhibits a double peak around the resonant point, which is recovered in an applied tilted magnetic field.

First-principles study of the rectifying properties of Pt/TiO2 interface
Tomoyuki Tamura, Shoji Ishibashi, Kiyoyuki Terakura, and Hongming Weng
Published 3 November 2009 (6 pages)
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First-principles calculations have been performed to study the interface electronic structure of Pt/TiO2 and to analyze the rectifying property of the Pt/TiO2/Pt structure. For the stoichiometric interface, the metal-induced gap states (MIGS) have amplitude appreciably only at the interface TiO2. We will show that the presence of MIGS makes oxygen-vacancy formation energy small at the interface. It is therefore expected that the interfacial TiO2 layer can be easily reduced. We will then demonstrate that the Schottky barrier height is strongly affected by oxygen deficiency. According to the present calculation, the interface is of Schottky-contact type for the fully oxidized interfacial TiO2 while it becomes almost ohmic for strongly reduced one.

Polaritonics in nanowires made from dispersive materials
Mahi R. Singh
Published 3 November 2009 (8 pages)
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We have studied the optoelectronics of a polaritonic nanowire which is fabricated by embedding a polaritonic crystal into another polaritonic crystal. It is considered that the band gap of the embedded crystal lies within the band gap of the host crystal. This band-gap engineering is satisfied by GaP and MgO crystals where MgO is the host crystal. Polaritons in the nanowire are confined within the embedded crystal. Bound states of the confined polaritons are calculated using the transfer-matrix method. The bound polariton energies are evaluated for a GaP-MgO nanowire. It is found that the number of bound states in the wire depends on its size, well depth, and the barrier height. The absorption coefficient of the system has also been calculated by using the time-dependent Schrödinger equation method. Numerical simulations for the GaP-MgO nanowire show that when the resonant energy of a quantum dot lies near the bound states the spectrum has several transparent states. The nanowire can be switched among the transparent or absorbing states by tuning the resonant state of the quantum dot. The present findings can be used to make types of polaritonic devices such as polaritonic switches and transistors.

Microscope of glassy relaxation in femtogram samples: Charge offset drift in the single electron transistor
Neil M. Zimmerman and William H. Huber
Published 3 November 2009 (4 pages)
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By measuring the long-term charge offset drift in single electron tunneling transistors, we have observed a transient relaxation after fabrication which is correlated with the presence of amorphous insulator. The temperature and time dependence of the transient relaxation are both in agreement with an extension of the standard model for two-level systems in glasses. This technique, which is sensitive to atomic scale motion in femtogram-sized samples, offers the possibility of a technique for investigation of glassy relaxation.

Disorder-induced incoherent scattering losses in photonic crystal waveguides: Bloch mode reshaping, multiple scattering, and breakdown of the Beer-Lambert law
M. Patterson, S. Hughes, S. Schulz, D. M. Beggs, T. P. White, L. O'Faolain, and T. F. Krauss
Published 5 November 2009 (6 pages)
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Through a combined theoretical and experimental study of disorder-induced incoherent scattering losses in slow-light photonic crystal slab waveguides, we show the importance of Bloch mode reshaping and multiple scattering. We describe a convenient and fully three-dimensional theoretical treatment of disorder-induced extrinsic scattering, including the calculation of backscatter and out-of-plane losses per unit cell, and the extrapolation of the unit-cell loss to the loss for an entire disordered waveguide. The theoretical predictions, which are also compared with recent measurements on dispersion engineered silicon waveguides, demonstrate the failure of the Beer-Lambert law due to multiple scattering. We also explain why the previously assumed group velocity scalings of disorder-induced loss break down in general.

Time dependence of the ground-state population statistics of condensed microcavity polaritons
Nguyen Duy Vy, H. Thien Cao, D. B. Tran Thoai, and H. Haug
Published 5 November 2009 (4 pages)
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The semiclassical approach consisting of Boltzmann equations for the excited states and the ground state supplemented by a Master equation for the probability distribution of the condensate population is solved for picosecond pulsed microcavity polaritons. With the simple birth- and death-type Master equation for the condensate population that disregards the condensate phase, one gets results, which, compared to quasistationary solutions, are in better agreement with the experiments. First, the time-dependent solutions show the influence of the change to the coherent state already at pump powers close above the condensate threshold. Second, and even more important for the interpretation of corresponding pulsed experiments, the time-dependent solution give results for the second-order correlation function with seemingly larger correlations above threshold, although no polariton-polariton interaction in the ground state has been included.

Theory of charge transport in ferromagnetic semiconductor/s-wave superconductor junction
Yoshihiro Mizuno, Takehito Yokoyama, and Yukio Tanaka
Published 10 November 2009 (7 pages)
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We study tunneling conductance in ferromagnetic semiconductor/insulator/s-wave superconductor junction where Rashba spin-orbit interaction (RSOI) and exchange field are taken into account in the ferromagnetic semiconductor. We show that normalized conductance at zero voltage has a maximum as a function of RSOI for high-transparent interface and finite exchange field. This is because Andreev reflection probability shows a nonmonotonic dependence on RSOI in the presence of the exchange field. On the other hand, for intermediate transparent interface, normalized conductance at zero voltage has a reentrant shape at zero or small exchange field with increasing RSOI but is monotonically increasing by RSOI at large exchange field.

Photoconductivity in aligned CdSe nanorod arrays
D. Steiner, D. Azulay, A. Aharoni, A. Salant, U. Banin, and O. Millo
Published 11 November 2009 (7 pages)
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The voltage dependence of the phototransport in colloidal CdSe nanorods (NRs) arrays was studied for different capping molecules and degree of NR alignment. The photocurrent was found to considerably enhance by exchanging the trioctylphosphine capping ligands by diamine molecules or upon annealing. The corresponding current-voltage characteristics were highly nonlinear, showing, in the aligned NR arrays, a notable decrease in the differential conductivity at a certain capping-molecule-dependent applied electrical field. This transition smears, however, when the degree of NR alignment is reduced. These findings are well described by an exciton field-ionization model, which also accounts for the correlation we observe between the photocurrent I(V) curves and the voltage dependence of the fluorescence quenching in seeded-grown CdSe/CdS core/shell NR films.

Stochastic polarization formation in exciton-polariton Bose-Einstein condensates
D. Read, T. C. H. Liew, Yuri G. Rubo, and A. V. Kavokin
Published 11 November 2009 (10 pages)
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We demonstrate theoretically the spontaneous formation of a stochastic polarization in exciton-polariton Bose-Einstein condensates in planar microcavities under pulsed excitation. Below the threshold pumping intensity (dependent on the polariton lifetime), the average polarization degree is close to zero, while above threshold the condensate acquires a polarization described by a (pseudospin) vector with random orientation, in general. It is shown that the polariton-polariton interaction leads to suppression of the linear polarization degree of the condensate due to the self-induced Larmor precession of the pseudospin. We establish the link between the second-order coherence of the polariton condensate and the distribution function of its polarization. We examine also the mechanisms of polarization dephasing and relaxation.

Möbius graphene strip as a topological insulator
Z. L. Guo, Z. R. Gong, H. Dong, and C. P. Sun
Published 13 November 2009 (8 pages)
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We study the electronic properties of the Möbius graphene strip with a zigzag edge. We show that such a graphene strip behaves as a topological insulator with a gapped bulk and a robust metallic surface, which enjoys some features due to its nontrivial topology of the spatial configuration such as the existence of edge states and the non-Abelian induced gauge field. We predict that the topological properties of the Möbius graphene strip can be experimentally displayed by the destructive interference in the transmission spectrum, and the robustness of edge states under certain perturbations.

Electronic reconstruction at a buried ionic-covalent interface driven by surface reactions
C.-T. Lou (羅中廷), H.-D. Li (李宏道), J.-Y. Chung (鐘仁陽), D.-S. Lin (林登松), and T.-C. Chiang (江台章)
Published 13 November 2009 (5 pages)
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Lattice-matched ionic NaCl films were grown layer by layer on covalent Ge(100) using cycles of two half reactions (HRs) that involved the alternative adsorption of Cl and Na. The Ge 3d photoemission spectra obtained after full cycles of growth resembled that of clean Ge(100), but came to resemble that of the polar Cl-terminated surface after the subsequent half reaction of Cl adsorption. Concurrently, the Na and Cl core levels of the nanofilms shifted by ~1.7  eV between these two interface configurations. Our results demonstrate that reactions on the NaCl surface drive periodic electronic reconstructions at the NaCl-Ge interface.

Radiative lifetimes in undoped and p-doped InAs/GaAs quantum dots
Edmund Harbord, Peter Spencer, Edmund Clarke, and Ray Murray
Published 13 November 2009 (6 pages)
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We investigate the effect of p doping on the luminescence properties of InAs/GaAs self-assembled quantum dots (QDs). Continuous-wave and time-resolved photoluminescence measurements are obtained as a function of temperature and used to extract the radiative lifetime of the QD ground state. We find that the low-temperature luminescence lifetime decreases from ~1200 to ~700  ps for QDs doped with 0 and 10 holes/dot, respectively. The radiative lifetime of the undoped QDs increases monotonically with temperature and is consistent with Boltzmann spreading over dark states. The luminescence intensity from the heavily doped QDs changes much less with temperature compared with the undoped QDs and we attribute this to the presence of holes in ground states at higher temperatures.

Exciton correlations in coupled quantum wells and their luminescence blue shift
B. Laikhtman and R. Rapaport
Published 13 November 2009 (12 pages)
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In this paper we present a study of an exciton system where electrons and holes are confined in double quantum well structures. The dominating interaction between excitons in such systems is a dipole-dipole repulsion. We show that the tail of this interaction leads to a strong correlation between excitons and substantially affects the behavior of the system. Making use of qualitative arguments and estimates we develop a picture of the exciton-exciton correlations in the whole region of temperature and concentration where excitons exist. It appears that at low concentration degeneracy of the excitons is accompanied with strong multiparticle correlation so that the system cannot be considered as a gas. At high concentration the dipole-dipole repulsion suppresses the quantum degeneracy. As a result there exists a temperature region where the system behaves a classical liquid; such a region does not exist in case of contact interaction. We calculate the blue shift of the exciton luminescence line which is a sensitive tool to observe the exciton-exciton correlations.

Defect-induced optical absorption in the visible range in ZnO nanowires
R. Michael Sheetz, Inna Ponomareva, Ernst Richter, Antonis N. Andriotis, and Madhu Menon
Published 17 November 2009 (4 pages)
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The optical properties of ZnO nanowires containing defects are investigated using first-principles density-functional theory incorporating the LDA+U formalism. Calculations include defects in the form of substitutional N, Zn, and O vacancies as well as +1 charged O vacancy. Our calculations reveal that the presence of vacancies contribute strongly to optical absorption in the visible. Furthermore, the presence of +1 charged O vacancy is found to result in a blueshift of the absorption peaks, reducing the number of wavelengths that can be absorbed in the visible. These findings can be a useful tool for the design of new generation of materials with improved solar radiation absorption.

Phonon-induced dephasing of singlet-triplet superpositions in double quantum dots without spin-orbit coupling
K. Roszak and P. Machnikowski
Published 17 November 2009 (5 pages)
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We show that singlet-triplet superpositions of two-electron spin states in a double quantum dot undergo a phonon-induced pure dephasing which relies only on the tunnel coupling between the dots and on the Pauli-exclusion principle. As such, this dephasing process is independent of spin-orbit coupling or hyperfine interactions. The physical mechanism behind the dephasing is elastic phonon scattering, which persists to much lower temperatures than real phonon-induced transitions. Quantitative calculations performed for a lateral GaAs/AlGaAs gate-defined double quantum dot yield microsecond dephasing times at sub-Kelvin temperatures, which is consistent with experimental observations.

Effects of repulsive and attractive ionized impurities on the resistivity of semiconductor heterostructures in the quantum Hall regime
A. Raymond, I. Bisotto, Y. M. Meziani, S. Bonifacie, C. Chaubet, A. Cavanna, and J. C. Harmand
Published 18 November 2009 (7 pages)
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We have investigated experimentally and theoretically the effect of repulsive and attractive ionized impurities on the resistivity components (rhoxx and rhoxy) in the quantum Hall effect regime. GaAs/GaAlAs asymmetric modulation-doped quantum wells with additional delta doping (by Si donor atoms or Be acceptor atoms) in the GaAs channel or at the AlGaAs/GaAs interface has been grown using molecular beam epitaxy technique. Magnetotransport experiments, performed on samples doped with Si-attractive atoms, showed a plateau width increasing toward lower magnetic field at even filling factor. However, when samples were delta doped with Be repulsive atoms, the increase was observed in the opposite side. Part of the results was explained using a model based on the fifth Klauder's approximations where we demonstrate that the asymmetrical increase of the Hall plateaus with even filling factor (Landau gaps) is related to the asymmetry induced in the density of states by the additional impurities: the resulting disorder short range potential broadens and shifts the Landau levels but also creates impurity bands on the lower energy side of the Landau levels in the case of donors and on the upper energy side of the Landau levels in the case of acceptors. We notice that this asymmetrical behavior was not experimentally observed for odd filling factor plateaus (exchange gaps). We have also experimentally underscored the screening effect by free two-dimensional electrons of this disorder short range potential. Moreover, for delta-doped Be samples, the whole nu=1 Hall plateau was shifted toward higher magnetic field with respect to the classical Hall effect. This shift, observed for all samples, cannot be explained by the asymmetry of the density of states but rather by a magnetic delocalization of electrons from the upper energy impurity band associated with the last Landau level (n=0) into the free n=0 Landau states when this impurity band overtakes the Fermi level at the end of the nu=2 plateau. This magnetic delocalization effect is the opposite effect of the magnetic freeze out.

Coherent transport in semiconductor heterostructures: A phenomenological approach
Ariel Gordon and Daniel Majer
Published 18 November 2009 (6 pages)
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We propose a theoretical method for describing coherent quantum transport in semiconductor heterostructures and particularly in quantum cascade lasers (QCLs). The method is an extension of standard rate-equation models to include coherence. Instead of building the model from microscopic considerations, we construct it from the following requirements: invariance under basis change, non-negativity of the density matrix, and compatibility with existing rate-equation models. The computational effort associated with the proposed method is very modest. It is shown that the role of coherence in QCLs is crucial and omitting it from the calculation leads to unphysical results. The discussion in the paper is focused on QCLs but the approach applies in general to semiconductor heterostructures.

GW investigation of interface-induced correlation effects on transport properties in realistic nanoscale structures
Changsheng Li, M. Bescond, and M. Lannoo
Published 19 November 2009 (6 pages)
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We report a theoretical investigation of long-range correlation effects induced by the presence of interfaces in realistic semiconductor based nanoscale structures. This is performed within the so-called GW approximation of Hedin and Lundqvist in which we isolate the contribution of the interfaces with dielectrics or metallic electrodes to the exchange-correlation self-energy. We incorporate these correlation effects self-consistently into the solution of the Schrödinger equation and calculate its influence on transport properties in realistic nanoscale transistors. Numerical results show that the self-energy correction due to dielectric mismatch can be comparable to the direct quantum confinement effect. With the decrease in size, this correlation effect has a significant impact on the current-voltage characteristics and contributes to the increase in variability in ultimate nanoscale transistors.

Surface physics, nanoscale physics, low-dimensional systems

Electric-field control of the band gap and Fermi energy in graphene multilayers by top and back gates
A. A. Avetisyan, B. Partoens, and F. M. Peeters
Published 2 November 2009 (11 pages)
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It is known that a perpendicular electric field applied to multilayers of graphene modifies the electronic structure near the K point and may induce an energy gap in the electronic spectrum which is tunable by the gate voltage. Here we consider a system of graphene multilayers in the presence of a positively charged top and a negatively charged back gate to control independently the density of electrons on the graphene layers and the Fermi energy of the system. The band structure of three- and four-layer graphene systems in the presence of the top and back gates is obtained using a tight-binding approach. A self-consistent Hartree approximation is used to calculate the induced charges on the different graphene layers. We predict that for opposite and equal charges on the top and bottom layers an energy gap is opened at the Fermi level. For an even number of layers this gap is larger than in the case of an odd number of graphene layers. We find that the circular asymmetry of the spectrum, which is a consequence of the trigonal warping, changes the size of the induced electronic gap, even when the total density of the induced electrons on the graphene layers is low.

Electric carrier concentration in graphite: Dependence of electrical resistivity and magnetoresistance on defect concentration
A. Arndt, D. Spoddig, P. Esquinazi, J. Barzola-Quiquia, S. Dusari, and T. Butz
Published 2 November 2009 (5 pages)
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We investigate the dependence of the electrical resistivity and magnetoresistance of single crystalline micrometer-sized graphite samples of a few tens of nanometers thick on the defect concentration produced by irradiation at low fluences. We show that the carrier density of graphite n is extremely sensitive to the induced defects for concentrations as low as ~0.1  ppm and follows n~1/RV<sup>2</sup> with RV the distance between defects in the graphene plane. These and Shubnikov-de Haas oscillations results indicate that at least a relevant part of the carrier densities measured in graphite is not intrinsic.

Comparative study of the surface layer density of liquid surfaces
E. Chacón, E. M. Fernández, D. Duque, R. Delgado-Buscalioni, and P. Tarazona
Published 3 November 2009 (11 pages)
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Capillary wave fluctuations blur the inherent structure of liquid surfaces in computer simulations. The intrinsic sampling method subtracts capillary wave fluctuations and yields the intrinsic surface structure, leading to a generic picture of the liquid surface. The most relevant magnitude of the method is the surface layer density ns that may be consistently determined from different properties: the layering structure of the intrinsic density profiles, the turnover rate for surface layer particles, and the hydrodynamic damping rate of capillary waves. The good agreement among these procedures provides evidence for the physical consistency of the surface layering hypothesis, as an inherent physical property of the liquid surfaces. The dependence of the surface compactness, roughness, and exchange rate with temperature is analyzed for several molecular interaction models.

Modeling of the deposition of Na Clusters on MgO(001)
M. Bär, P. M. Dinh, L. V. Moskaleva, P.-G. Reinhard, N. Rösch, and E. Suraud
Published 4 November 2009 (12 pages)
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We investigate the dynamics of deposition of small Na clusters on MgO(001) surface. A hierarchical modeling is used combining quantum mechanical with molecular mechanical description. Full time-dependent density-functional theory is used for the cluster electrons while the substrate atoms are treated at a classical level. We consider Na6 and Na8 at various impact energies. We analyze the dependence on cluster geometry, trends with impact energy, and energy balance. We compare the results with deposit on the much softer Ar(001) surface.

Dynamic polarization of graphene by moving external charges: Random phase approximation
K. F. Allison, D. Borka, I. Radović, Lj. Hadžievski, and Z. L. Mišković
Published 4 November 2009 (13 pages)
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We evaluate the stopping and image forces on a charged particle moving parallel to a doped sheet of graphene by using the dielectric-response formalism for graphene's pi-electron bands in the random phase approximation (RPA). The forces are presented as functions of the particle speed and the particle distance for a broad range of charge-carrier densities in graphene. A detailed comparison with the results from a kinetic equation model reveal the importance of interband single-particle excitations in the RPA model for high particle speeds. We also consider the effects of a finite gap between graphene and a supporting substrate, as well as the effects of a finite damping rate that is included through the use of Mermin's procedure. The damping rate is estimated from a tentative comparison of the Mermin loss function with a high-resolution reflection electron energy loss spectroscopy experiment. In the limit of low particle speeds, several analytical results are obtained for the friction coefficient that show an intricate relationship between the charge-carrier density, the damping rate, and the particle distance, which may be relevant to surface processes and electrochemistry involving graphene.
Cooling dynamics and thermal interface resistance of glass-embedded metal nanoparticles
Vincent Juvé, Mattia Scardamaglia, Paolo Maioli, Aurélien Crut, Samy Merabia, Laurent Joly, Natalia Del Fatti, and Fabrice Vallée
Published 5 November 2009 (6 pages)
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The cooling dynamics of glass-embedded noble metal nanoparticles with diameters ranging from 4 to 26 nm were studied using ultrafast pump-probe spectroscopy. Measurements were performed probing away from the surface plasmon resonance of the nanoparticles to avoid spurious effects due to glass heating around the particle. In these conditions, the time-domain data reflect the cooling kinetics of the nanoparticle. Cooling dynamics are shown to be controlled by both thermal resistance at the nanoparticle-glass interface and heat diffusion in the glass matrix. Moreover, the interface conductances are deduced from the experiments and found to be correlated with the acoustic impedance mismatch at the metal/glass interface.

Surface-plasmon polariton resonances in triangular-groove metal gratings
T. Søndergaard and Sergey I. Bozhevolnyi
Published 5 November 2009 (9 pages)
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Electromagnetic resonances of triangular-groove gold gratings illuminated with monochromatic light are studied theoretically. The calculations performed are based on the Green's-function surface-integral equation method with the periodic Green's function. Local-field-enhancement spectra and near-field calculations reveal three types of resonances, namely, geometric resonances determined by the shape of individual grooves, standing-wave surface-plasmon polariton (SPP) resonances due to SPPs reflected by the neighbor grooves, and very sharp resonances (Rayleigh anomalies) at wavelengths near the cutoff wavelength of higher grating-reflection orders, which can be tuned simply by changing the angle of incident light. These resonances are also found to be observable in the reflection spectra, whose minima correspond to peaks in the enhancement spectra. Typical enhancements of the electric field magnitude inside the grooves are larger than 20, reaching in some cases the level of ~35. In the case of Rayleigh anomalies, the total reflection can be almost completely suppressed. The resonances can be realized in the wavelength range from visible to infrared by varying the groove height, angle, and periodicity, a feature that makes this configuration promising for a wide range of practical applications, for example, within surface-enhanced spectroscopies.

Wave-scattering formalism for thermal conductance in thin wires with surface disorder
Gursoy B. Akguc and Jiangbin Gong
Published 6 November 2009 (7 pages)
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Elastic wave characteristics of the heat conduction in low-temperature thin wires can be studied via a wave scattering formalism. A reaction matrix formulation of heat conductance modeled by elastic wave scattering is advocated. This formulation allows us to treat thin wires with arbitrary surface disorder. It is found that the correlation in the surface disorder may significantly affect the temperature dependence of the heat conductance.

Thermoelectric effects in transport through quantum dots attached to ferromagnetic leads with noncollinear magnetic moments
R. Świrkowicz, M. Wierzbicki, and J. Barnaś
Published 6 November 2009 (10 pages)
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Charge transport accompanied by heat transfer through a single-level quantum dot coupled to ferromagnetic leads with noncollinear magnetic moments is studied theoretically in the linear and nonlinear transport regimes. Calculations performed in the framework of nonequilibrium Green's function formalism and the equation of motion method reveal a significant influence of Coulomb blockade on thermal transport processes. The thermopower S and thermal efficiency described by the figure of merit ZT depend on magnetic configuration of the system. Two physically different situations are considered; one appears when spin accumulation is excluded and the second one when spin accumulation is relevant. In the latter case we also calculate the corresponding spin thermopower. Apart from this, magnetothermopower is introduced and discussed.