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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.

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.

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.

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.

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.

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.

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.

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)
195408  Full Text: PDF (346 kB)  | Buy Article
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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)
195409  Full Text: PDF (638 kB)  | Buy Article
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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.