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.     

含半圆弧形腔的量子波导中声学声子输运和热导特性

采用散射矩阵方法,研究了在应力自由和硬壁两种典型的边界条件下含半圆弧形腔的量子波导中声学声子输运和热导性质.结果表明在两种边界条件下声子透射谱和热导有着不同的特征.在应力自由边界条件下,能观察到普适的量子化热导现象,当结构为一理想的量子线时,在低温区域有一个量子化平台出现,而当半圆弧形结构存在时,非均匀横向宽度引发的弹性散射使得量子化平台被破坏;在硬壁边界条件下,不可能观察到量子化热导现象,热导随温度的增加单调上升;计算结果表明还可以通过调节半圆弧形结构的半径来调控声子的输运概率和热导. 关键词： 声学声子输运 热导 量子体系     

Universal quantized spin-Hall conductance fluctuation in graphene

We report theoretical investigations of the quantized spin-Hall conductance fluctuation of graphene in the presence of disorder. Two graphene models that exhibit the quantized spin-Hall effect (QSHE) are analyzed. Model I is with unitary symmetry under an external magnetic field B not = 0 but with a zero spin-orbit interaction, t(SO)=0. Model II is with symplectic symmetry where B=0 but t(SO) not = 0. The two models give exactly the same universal QSHE conductance fluctuation value 0.285+/-0.005e/4pi regardless of symmetry. We also examined a third model that exhibits QSHE but with quadratic dispersion and obtained the same results. Finally, all three models of QSHE have a one-sided log-normal distribution for spin-Hall conductance. Our results strongly suggest that the quantized spin-Hall conductance fluctuation belongs to a new universality class.     

Thermal conductance in a quantum waveguide modulated with quantum dots

We investigate the thermal conductance in a quantum waveguide modulated with quantum dots at low temperatures. It is found that the thermal conductance sensitively depends on the geometrical parameters of the structure and boundary conditions. When the stress-free boundary conditions are applied in the structure, the universal quantum of thermal conductance can be found regardless of the geometry details in the limit T→0. For an uniform quantum waveguide, a thermal conductance plateau can be observed at very low temperatures; while for the quantum waveguide modulated with quantum dots, the plateau disappears, instead a decrease of the thermal conductance can be observed as the temperature goes up in the low temperature region, and its magnitude can be adjusted by the radius of the quantum dot. Moreover, it is found that the quantum waveguide with two coupling quantum dots exhibits oscillatory decaying thermal conductance behavior with the distance between two quantum dots. However, when the hard-wall boundary conditions are applied, the thermal conductance displays different behaviors.     

The influence of boundary conditions on thermal conductance in semiconductor quantum wire with structural defect

Using the scattering-matrix method, we investigate the influences of boundary conditions on thermal conductance in quantum wire with structural defect. A comparison between the thermal conductances is made when stress-free, hard-wall, and mixed boundary conditions are applied for acoustic transport leads. The results show that the quantized thermal conductance plateau at very low temperature can be observed only in transport lead with stress-free boundary condition. For hard-wall or mixed boundary conditions, qualitatively different thermal conductance characteristics are found. Moreover, we find that the behavior of the thermal conductance sensitively depend on the geometric parameters and the position of the defect in quantum wire.     

Quantum transport in carbon nanoscrolls

The transport properties of carbon nanoscrolls in a uniform electric field are investigated by using the Landauer-Buttiker formula. The energy dispersions depend sensitively on the geometry and the field strength. The bandgaps may become zero and exhibit the semiconductor-metal transitions. Variations in the electronic structures with the geometry or the field strength are reflected in the conductance. The conductance exhibits a stepwise behavior, caused by the discrete number of quantized channels contributing to transport. The shoulders of the conductance are stretched out as temperature rises.     

Modulated thermal transport for flexural and in-plane phonons in double-stub graphene nanoribbons

Thermal transport properties are investigated for out-of-plane phonon modes(FPMs) and in-plane phonon modes(IPMs) in double-stub graphene nanoribbons(GNRs). The results show that the quantized thermal conductance plateau of FPMs is narrower and more easily broken by the double-stub structure. In the straight GNRs, the thermal conductance of FPMs is higher in the low temperature region due to there being less cut-off frequency and more low-frequency excited modes. In contrast, the thermal conductance of IPMs is higher in the high temperature region because of the wider phonon energy spectrum. Furthermore, the thermal transport of two types of phonon modes can be modulated by the double-stub GNRs, the thermal conductance of FPMs is less than that of IPMs in the low temperatures, but it dominates the contribution to the total thermal conductance in the high temperatures. The modulated thermal conductance can provide a guideline for designing high-performance thermal or thermoelectric nanodevices based on graphene.     

Phonon Transport and Thermal Conductivity in an Acoustic Filter

We investigate the phonon ballistic transmission and the thermal conductivity in a dielectric quantum structure. It is found that these observable quantities sensitively depend on geometric parameters, and are of quantum character. The total transmission coetfficient as a function of the reduced waveguide-length exhibits periodical behaviour and the reduced thermal conductance decreases below the ideal universal value for the low temperature. Our results show that one can control the thermal conductivity of the structure and make all kinds of acoustic filters to match practical requirements in devices by adjusting the geometric parameters.     

Electron transport in double-walled carbon nanotubes

The transport properties of finite length double-walled carbon nanotubes subject to the influences of a transverse electric field and a magnetic field with varying polar angles are investigated theoretically. The electrical conductance, thermal conductance and Peltier coefficient dependences on the external fields and symmetric configuration are studied in linear response regime. Prominent peak structures of the electrical conductance are predicted when varying the electric field strength. The features of the conductance peaks are found to be strongly dependent on the external fields and the intertube interactions. The heights of the electrical and thermal conductance peaks display the quantized behavior, while those of the Peltier coefficient do not. The conductance peaks are found to be broadened by the finite temperature.     

Phonon thermal conductance of disordered graphene strips with armchair edges

Based on the model of lattice dynamics together with the transfer matrix technique, we investigate the thermal conductances of phonons in quasi-one-dimensional disordered graphene strips with armchair edges using Landauer formalism for thermal transport. It is found that the contributions to thermal conductance from the phonon transport near von Hove singularities is significantly suppressed by the presence of disorder, on the contrary to the effect of disorder on phonon modes in other frequency regions. Besides the magnitude, for different widths of the strips, the thermal conductance also shows different temperature dependence. At low temperatures, the thermal conductance displays quantized features of both pure and disordered graphene strips implying that the transmission of phonon modes at low frequencies are almost unaffected by the disorder.     

Universal conductance of nanowires near the superconductor-metal quantum transition

We consider wires near a zero temperature transition between superconducting and metallic states. The critical theory obeys hyperscaling, which leads to a universal frequency, temperature, and length dependence of the conductance; quantum and thermal phase slips are contained within this critical theory. Normal, superconducting, and mixed (SN) leads on the wire determine distinct universality classes. For the SN case, wires near the critical point have a universal dc conductance which is independent of the length of the wire at low temperatures.     

Two-terminal conductance of a fractional quantum Hall edge

We have found a solution to a model of tunneling between a multichannel Fermi liquid reservoir and an edge of the principal fractional quantum Hall liquid (FQHL) in the strong-coupling limit. The solution explains how the chiral edge propagation makes the universal two-terminal conductance of the FQHL fractionally quantized and different from that of a 1D Tomonaga-Luttinger liquid wire, where a similar model, but preserving the time reversal symmetry, predicts unsuppressed free-electron conductance.     

Thermal enhancement of interference effects in quantum point contacts

We study an electron interferometer formed with a quantum point contact and a scanning probe tip in a two-dimensional electron gas. The images giving the conductance as a function of the tip position exhibit fringes spaced by half the Fermi wavelength. For a contact opened at the edges of a quantized conductance plateau, the fringes are enhanced as the temperature T increases and can persist beyond the thermal length l(T). This unusual effect is explained by assuming a simplified model: The fringes are mainly given by a contribution which vanishes when T→0 and has a decay characterized by a T-independent scale.     

Mesoscopic description of radiative heat transfer at the nanoscale

We present a formulation of the nanoscale radiative heat transfer using concepts of mesoscopic physics. We introduce the analog of the Sharvin conductance using the quantum of thermal conductance. The formalism provides a convenient framework to analyze the physics of radiative heat transfer at the nanoscale. Finally, we propose a radiative heat transfer experiment in the regime of quantized conductance.     

Fractional quantum conductance in gold nanowires

The conductance of a nanoscopic wire decreases in steps when it is stretched until it breaks. This is due to narrowing of the wire whereby quantized conductance channels sequentially close. However the conductance plateaus seldom occur at integral multiples of the quantum of conductance G₀ and most steps are much smaller than G₀. High speed precision measurements of the conductance steps reveal that the nanowires are composed of not one but several quantized conductors in series and that a step is caused by a quantized conductance change in one of the elements in the sequence. Resolved series conductance quanta up to 20 G₀ have been observed at room temperature. The effect is explained in terms of elastic electronic scattering and has important consequences for nano-electronic circuitry.     

Nonequilibrium Green's function approach to phonon transport in defective carbon nanotubes

We have developed a new theoretical formalism for phonon transport in nanostructures using the nonequilibrium phonon Green's function technique and have applied it to thermal conduction in defective carbon nanotubes. The universal quantization of low-temperature thermal conductance in carbon nanotubes can be observed even in the presence of local structural defects such as vacancies and Stone-Wales defects, since the long wavelength acoustic phonons are not scattered by local defects. At room temperature, however, thermal conductance is critically affected by defect scattering since incident phonons are scattered by localized phonons around the defects. We find a remarkable change from quantum to classical features for the thermal transport through defective carbon nanotubes with increasing temperature.     

螺旋纳米带中的声子输运

运用微分几何方法及形式散射理论,研究螺旋纳米带中的标量声子输运问题,计算了声子透射系数及热导率.数值结果表明,弯曲导致了声子模式之间的量子干涉,使总透射系数随能量变化的量子化台阶呈现振荡行为,有效地抑制了热导率. 关键词： 螺旋纳米带 声子输运 形式散射理论 微分几何方法     

Low-Temperature Thermal Conductance in Superlattice Nanowire with Structural Defect

Using the scattering-matrix cascading method, we investigate the effect of structural defect on the acoustic phonon transmission and thermal conductance in the superlattice nanowire at low temperatures. In the present system, the phonon transmissions exhibit quite complex oscillatory behaviour. It is found that a lateral defect in an otherwise periodic structure significantly decrease the thermal conductance and completely washes away the transmission quantization. However, the appreciable transmission quantization survives in the presence of a longitudinal defect whereas a good quantization plateau of thermal conductance emerges below the universal level in a wide temperature range with the lateral defect.