Effects of annealing on the electrical properties of HgTe crystals

The carrier mobility of HgTe crystals at 15°C increased to as much as $\text{33,000}\text{cm}{}^2\text{V sec}$, and the transverse magnetoresistance decreased, when the crystals were annealed long enough in mercury vapor. The results are compared with our calculations of the galvanomagnetic effects for mixed scattering by phonons and charged centers, which was made for a parabolic band. It is concluded that the density of charged centers decreases with annealing, and that acoustic phonons may be the dominant scattering sources near room temperature.     

Phonons in Ge/Si superlattices with Ge quantum dots

Ge/Si superlattices containing Ge quantum dots were prepared by molecular beam epitaxy and studied by resonant Raman scattering. It is shown that these structures possess vibrational properties of both two-and zero-dimensional objects. The folded acoustic phonons observed in the low-frequency region of the spectrum (up to 15th order) are typical for planar superlattices. The acoustic phonon lines overlap with a broad emission continuum that is due to the violation of the wave-vector conservation law by the quantum dots. An analysis of the Ge and Ge-Si optical phonons indicates that the Ge quantum dots are pseudoamorphous and that mixing of the Ge and Si atoms is insignificant. The longitudinal optical phonons undergo a low-frequency shift upon increasing laser excitation energy (2.54–2.71 eV) because of the confinement effect in small-sized quantum dots, which dominate resonant Raman scattering.     

多空穴错位分布对石墨纳米带中热输运的影响

利用非平衡格林函数方法研究了石墨纳米带中三空穴错位分布对热输运性质的影响.研究结果发现:三空穴竖直并排结构对低频声子的散射较小,导致低温区域三空穴竖直并排时热导最大,而在高频区域,三空穴竖直并排结构对高频声子的散射较大,导致较高温度区域三空穴竖直并排时热导最小;三空穴的相对错位分布仅能较大幅度地调节面内声学模高频声子的透射概率,而三空穴的相对错位分布能较大幅度地调节垂直振动膜高频声子和低频声子的透射概率,导致三空穴的相对错位分布不仅能大幅调节面内声学模和垂直振动模的高温热导,也能大幅调节垂直振动模的低温热导.研究结果阐明了空穴位置不同的石墨纳米带的热导特性,为设计基于石墨纳米带的热输运量子器件提供了有效的理论依据.     

Electron scattering and transport phenomena in small-gap zinc-blende semiconductors

We give expressions for electrical conductivity, thermoelectric power and thermal conductivity of conduction band electrons in small-gap zinc-blende semiconductors, obtained by solving the Boltzmann equation by a variational procedure. The term resulting from the phonon-drag is included in the Boltzmann equation. The following electron scattering mechanisms are investigated: inter and intraband scattering by optical phonons via polar and nonpolar interactions, scattering by charged centers (ionized defects and heavy holes) and by neutral centers, as well as scattering by acoustic phonons. Particular attention is paid to the screening of the electron-optical phonon polar interaction by free carriers, which is particularly important in the case of a linear energy band. The formula for the intraband RPA dielectric function for the case of the linear band is given.The general formulation of all the problems investigated permits direct application of the results given in this paper to both intrinsic or n-type HgTe-type and InSb-type semiconductors, including mixed crystals, e.g. Cd_xHg_1?xSe near the cross point.     

Electronic thermal conductivity in suspended and supported bilayer graphene

Electronic thermal conductivity κ_e is investigated, using Boltzmann transport equation approach, in a suspended and supported bilayer graphene (BLG) as a function of temperature and electron concentration. The electron scattering due to screened charged impurity, short-range disorder and acoustic phonon via deformation potential are considered for both suspended and supported BLG. Additionally, scattering due to surface polar phonons, is considered in supported BLG. In suspended BLG, calculated κ_e is compared with the experimental data leaving the phonon thermal conductivity. It is emphasized that κ_e is important in samples with very high electron concentration and reduced phonon thermal conductivity. κ_e is found to be about two times smaller in supported BLG compared to that in suspended BLG. With the reduced extrinsic disorders, in principle, the intrinsic scattering by acoustic phonons can set a fundamental limit on possible intrinsic κ_e.     

On the Theory of Brillouin Scattering in Piezoelectric Semiconductors with Acoustic Phonon-Conduction Electron Interaction

The theory of nonresonant Brillouin scattering in anisotropic piezoelectric semiconductors with deformation potential coupling and piezoelectric coupling between excited systems of acoustic phonons and conduction electrons is reviewed. The scattering efficiency is calculated using the appropriate dyadic electromagnetic GreeN′s function. The depletion of the scattered intensity arising from a non phase-matched scattering kinematics and from a spatial exponential decay of the sound amplitude is taken into account. The contributions to the Brillouin scattering from the free-carrier-screened indirect photoelastic effect and from the free-carrier density modulation are expressed in terms of the self-consistent electric field. This field is obtained from a Boltzmann-equation calculation of the effective ac conductivity tensor. The acoustic dispersion of the Brillouin-scattering efficiency is considered, and some possibilities of determining electronic transport properties by means of Brillouin scattering are outlined.     

Acoustic phonons transport in a quantum waveguide embedded double defects

By using scattering matrix method, we investigate the acoustic phonons transport in a quantum waveguide embedded double defects at low temperatures. When acoustic phonons propagate through the waveguide, the total transmission coefficient versus the reduced phonon frequency exhibits a series of resonant peaks and dips, and acoustic waves interfere with each other in the waveguide to form standing wave with particular wavelengths. In the waveguide with void defects, acoustic phonons whose frequencies approach zero can transport without scattering. The acoustic phonons propagating in the waveguide with clamped material defects, the phonons frequencies must be larger than a threshold frequency. It is also found that the thermal conductance versus temperature is qualitatively different for different types of defects. At low temperatures, when the double defects are void, the universal quantum thermal conductance and a thermal conductance plateau can be clearly observed. However, when the double defects consist of clamped material, the quantized thermal conductance disappears but a threshold temperature where mode 0 can be excited emerges. The results can provide some references in controlling thermal conductance artificially and the design of phonon devices.     

Acoustic phonons transport in a quantum waveguide embedded double defects

By using scattering matrix method, we investigate the acoustic phonons transport in a quantum waveguide embedded double defects at low temperatures. When acoustic phonons propagate through the waveguide, the total transmission coefficient versus the reduced phonon frequency exhibits a series of resonant peaks and dips, and acoustic waves interfere with each other in the waveguide to form standing wave with particular wavelengths. In the waveguide with void defects, acoustic phonons whose frequencies approach zero can transport without scattering. The acoustic phonons propagating in the waveguide with clamped material defects, the phonons frequencies must be larger than a threshold frequency. It is also found that the thermal conductance versus temperature is qualitatively different for different types of defects. At low temperatures, when the double defects are void, the universal quantum thermal conductance and a thermal conductance plateau can be clearly observed. However, when the double defects consist of clamped material, the quantized thermal conductance disappears but a threshold temperature where mode 0 can be excited emerges. The results can provide some references in controlling thermal conductance artificially and the design of phonon devices.     

Four-Wave Scattering of a Laser Beam by Induced Collective Oscillations of Solid Nanoparticles in Suspensions

Spectra of coherent scattering at four-wave mixing of two diode laser radiation in liquid suspensions of dielectric nanoparticles are obtained for the first time. Dependence of the scattering resonance width on the concentration of nanoparticles in the suspension is studied. It is shown that the experimentally observed dependences of the width of the scattered radiation spectrum on the concentration of nanoparticles in the suspension result from the collective excitation of acoustic phonons.     

A microscopic model of sequential resonant tunneling transport through weakly coupled superlattices

A microscopic model is developed for resonant tunneling transport in weakly coupled semiconductor superlattices in a constant external electric field. The model takes into account multiple subbands and electric-field dependence of scattering by acoustic and optical phonons, charged impurities, and interface roughness. The model is used as a basis for computing the resonant-tunneling profiles for structures with small size-quantization energies. The computed results are in good agreement with experiment. In structures of this type, an important role is played by electric-field dependence of scattering processes and the threshold behavior of elastic processes is strongly manifested. A substantial asymmetry is predicted not only for the first tunneling resonance, but also for higher order resonant tunneling processes.     

Electron mobility and electron scattering in CdxHg1−xSe mixed crystals

We report a detailed study, both experimental and theoretical, of electron mobility and Hall coefficient in small-gap Cd_xHg_1?xSe mixed crystals. The electron mobility is calculated by the variational technique. The results obtained with no adjustable parameter are within 15% of the experimental values in the range of temperature 4.2–300 K, electron concentrations 7 × 10¹⁶?5 × 10¹⁸cm^?3 and composition 0 < x ? 0.25.The scattering of electrons by charged centres, optical phonons (polar and nonpolar interaction), acoustic phonons as well as disorder (alloy) scattering is taken into account. It is shown that the composition-dependent dielectric function and the band edge symmetry play an important role in the explanation of the experimental results.     

Bond polarizability calculations of hyper-Raman scattering in sodium, potassium and rubidium halides

Bond polarizability calculations extended for hyper-Raman scattering by the optical LO and TO zone-centre phonons in sodium, potassium and rubidium halides are presented. The effect of the crystal ionicity on both of the independent hyper-Raman tensor components is taken into consideration. Contributions of rotation of crystals bonds and their stretching to hyper-Raman scattering are determined. For LO phonons the electrooptic part of the hyper-Raman tensor is found. The results obtained are compared, when possible, with experimental and theoretical data reported in the literature.     

Galvanomagnetic effects in n-type gallium antimonide

This paper reports the theoretical and experimental study performed on Hall mobility and free carrier Faraday rotation in a degenerate n-GaSb sample in the temperature range 77–300K. Following relaxation time approximation the mobility is estimated separately for the Γ- and L-valley taking into account the scattering of electrons due to ionised impurities, space charge, polar optical phonons, deformation potential, intervalley acoustic and optical phonons. The effective mobility is calculated considering a two valley model, degeneracy and non-parabolicity of the Γ-valley, and compared with these experimental results. Microwave Faraday rotation data at 77 and 300K is analysed generalising the d.c. magneto-conductivity tensor components as derived by Bordure and Savelli to the high frequency, and is used to confirm the scattering mechanisms and band parameters used in the analysis of d.c. galvanomagnetic results.     

Mechanism of strong resonant 1LO Raman scattering

The interaction of electrons with impurities and the quasi-elastic scattering of electrons by acoustic phonons highly enhances the efficiency of resonant 1 LO Raman scattering. As a result, for a wide range of parameter values the efficiency of resonant scattering becomes rather strong and does not depend on the strength of interaction of electrons with the impurities and acoustic phonons, and on the impurity concentration.     

Theory of Raman light scattering by nanocrystal acoustic vibrations

A theory of Raman scattering of light by acoustic phonons in spherical nanocrystals of zinc-blende and wurtzite semiconductors has been developed with the inclusion of the complex structure of the valence band. The deformation-potential approximation was used to describe the exciton-phonon interaction. It is shown that this approximation allows only Raman scattering processes involving spheroidal acoustic phonons with a total angular momentum F=0 or 2. The effect of phonon quantum confinement on linewidth in Raman scattering spectra and scattered polarization is analyzed. An expression for the shape of the spectral line corresponding to nonresonant scattering from F=0 phonons was obtained. Fiz. Tverd. Tela (St. Petersburg) 41, 1473–1483 (August 1999)     

低温下多通道量子结构中的弹性声子输运和热导

利用弹性近似模型和散射矩阵方法,研究了低温下多通道量子结构中的弹性声学声子输运的性质. 计算结果表明,对于低频声学声子,只要通道的横向宽度相同,各通道中最低阶模的透射概率几乎不受其他结构参数的影响,且其数值都接近于0.25;而高频声学声子在各通道中的透射概率与结构参数密切相关,不同通道中的透射概率不同;当温度非常低时,各通道的热导都接近于量子化热导π²k²_BT/(3h)的四分之一;随着温度的升高,各通道的热导增减 关键词： 声学声子输运 热导 量子结构     

Exciton-phonon interaction in mixed silver halides: Resonant Raman scattering vs photoluminescence

An investigation of resonant Raman scattering in mixed crystals of AgBr:Cl at 1.8 K shows that the zero-phonon and LO phonon-assisted exciton luminescence excited in the free indirect exciton absorption, exhibits an anomalous dependence on the exciton photon energy E_L. Close to the exciton gap, the bands show a Raman-like behaviour with their peaks at constant energetic distance from E_L. As E_L is tuned further into the absorption, the bands gradually develop into normal photoluminescence. The effect is explained by taking into account exciton relaxation via scattering by long-wavelength acoustic phonons, a process which is strongly energy dependent. In addition, resonant Raman scattering observed for excitation in the zero-phonon absorption suggests study for the first time of the mode behaviour of certain off-zone center phonons in this system.     

Ballistic magnon transport and phonon scattering in the antiferromagnet Nd2CuO4

The thermal conductivity of the antiferromagnet Nd2CuO4 was measured down to 50 mK. Using the spin-flop transition to switch on and off the acoustic Nd magnons, we can reliably separate the magnon and phonon contributions to heat transport. We find that magnons travel ballistically below 0.5 K, with a thermal conductivity growing as T3, from which we extract their velocity. We show that the rate of scattering of acoustic magnons by phonons grows as T3, and the scattering of phonons by magnons peaks at twice the average Nd magnon frequency.     

Electronic thermal conduction in suspended graphene

The electronic thermal conductivity (ETC), κ_e, of suspended graphene (SG) is studied for 15<T<400 K, following the Boltzmann transport formalism. The electrons are considered to be scattered from defects along with the intrinsic in-plane acoustic phonons, out-of-plane flexural phonons (FPs) and optical phonons. The ETC is evaluated by computing the first-order perturbation distribution function by directly solving the linearized Boltzmann equation by an iterative method. Numerical calculations of the temperature and concentration dependences of κ_e show the dominance of charged impurity scattering at lower temperatures (T<75 K) and of FPs at higher temperatures. The results are compared with the commonly used low-temperature and high-energy relaxation time approximations. Our calculations are in good agreement with recent κ_e data extracted for high-mobility SG samples. The validity of Wiedemann–Franz law is also discussed.     

Resonant raman scattering at the E1 energy gap of semiconductor crystals

We present a discussion of resonant Raman scattering by optical phonons at the E₁ energy gap of group IV and groups III–V compound semiconductor crystals (e.g., Ge and InSb). For allowed scattering by TO and LO phonons, the q-dependent “double resonant” two-band calculation of the Raman tensor may display destructive interference effects when the intermediate electron-hole pairs are uncorrelated. We also discuss the Franz-Keldysh mechanism of resonant electric field induced Raman scattering by LO phonons. The double resonance terms due to this mechanism will, for large electric fields, broaden and have its largest resonance enhancement at the energy gap.