Effect of Gaussian acoustic nanocavities in a narrow constriction on ballistic phonon transmission

We investigate the effect of Gaussian acoustic nanocavities in a narrow constriction on ballistic phonon transport through a semiconductor nanowire at low temperatures. When the transverse width of acoustic nanocavities takes a Gaussian function, it is found that the resonant peaks and band gaps in transmission spectra are obvious, indicating that the system has selective transmission and filters actions for ballistic phonons. The number and length of nanocavities have significant effects on the phonon transmission and thermal conductance. The results are compared with those in uniform width acoustic nanocavities. The Gaussian acoustic nanocavities are therefore a promising phononic device to manipulate ballistic phonons in nanophononics.     

Ballistic phonon transport through a Fibonacci array of acoustic nanocavities in a narrow constriction

We investigate the ballistic phonon transport through a Fibonacci array of acoustic nanocavities in a narrow constriction of a semiconductor nanowire at low temperatures. It is found that the transmission spectrum of such a system consists of quasiband gaps and narrow resonances caused by the coupling of phonon waves. Both phonon transmission and thermal conductance exhibit the similarity due to the Fibonacci sequence structure. The similarity is sensitive to the number n and parameters of nanocavities. The results are compared with those in a periodic acoustic nanocavities.     

Ballistic phonon thermal transport across topologically structured nanojunctions on Gold wires

We investigate the coherent phonon thermal transport at low temperatures in Gold nanowires, in order to study the effects of scattering on the lattice thermal conductivity. Three types of shaped joint nanostructures are employed in our calculation. We present a detailed study of the thermal conductance as a function of the temperature for different shaped joint. This is done by solving the phonon Boltzmann transport equation in the ballistic regime and calculating the transmission rates of the vibration modes through the consideration of the phonon group velocity modification in the system. The transmission properties are calculated by use of the matching method in the harmonic approximation with nearest and next nearest neighbor force constants. The results show that the transmission probabilities depend on the type of joint nanostructure. The pronounced fluctuations of the transmission spectra as a function of the frequency can be understood as Fano resonances. It is also found that the behavior of the thermal conductance versus temperature is qualitatively different for different nanostructures and depends sensitively on the width of the shaped joint.     

含双T形量子结构的量子波导中声学声子输运和热导

采用散射矩阵方法, 研究了含双T形量子结构的量子波导中声学声子输运和热导性质. 结果表明: 在极低温度, 双T形量子结构能增强低温热导; 相反地, 在相对较高的温度范围, 双T形量子结构能降低低温热导. 而在整个低温范围内, 增加散射区域最窄处的宽度能增强低温热导. 计算结果表明可以通过调节含双T形量子结构的量子波导结构来调控声子的输运概率和热导. 关键词： 声学声子输运 热导 量子结构     

固-固接触热传导的声子传递系数

声子传递系数是影响低温接触界面热传导的重要因素,文中对中间低温(20K~200K)接触传导模型的声子传递系数进行了讨论,分析了接触界面温差、弹性镜面传递和散射传递下的声子传递系数;还讨论了热流方向对声子传递系数的影响;指出了声失配理论预测值与实验值间存在差别的可能原因。该讨论对分析接触热传导有一定意义。     

异侧非重叠三封闭端量子波导中的声子输运与热导

研究了异侧非重叠三封闭端量子波导中的声学声子传输和热导率性质。结果表明：由于激发模的产生,总传输系数在整数约化频率的时发生跳跃;各个激发模所产生的温度条件不一样,温度越高,被激发的模越多,并且高阶模对热导的影响较小;声子传输和热导性质与不连续结构的形状和位置有直接的关系,声子传输和热导性质对量子线的温度环境相当敏感。     

Thermal conductance of thin silicon nanowires

The thermal conductance of individual single crystalline silicon nanowires with diameters less than 30 nm has been measured from 20 to 100 K. The observed thermal conductance shows unusual linear temperature dependence at low temperatures, as opposed to the T3 dependence predicted by the conventional phonon transport model. In contrast to previous models, the present study suggests that phonon-boundary scattering is highly frequency dependent, and ranges from nearly ballistic to completely diffusive, which can explain the unexpected linear temperature dependence.     

Thermal conductance associated with six types of vibration modes in quantum wire modulated with quantum dot

We study the ballistic phonon transport and thermal conductance of six low-lying vibration modes in quantum wire modulated with quantum dot at low temperatures. A comparative analysis is made among the six vibrational modes. The results show that the transmission rates of the six vibrational modes relative to reduced frequency display periodic or quasi-periodic oscillatory behavior. Among the four acoustic modes, the thermal conductance contributed by the torsional mode is the smallest, and the thermal conductances of other acoustic modes have adjacent values. It is also found that the thermal conductance of the optical mode increases from zero monotonously. Moreover, the total thermal conductance in concavity-shaped quantum structure is lower than that in convexity-shaped quantum structure. These thermal conductance values can be adjusted by changing the structural parameters of the quantum dot.     

Reduction of phonon mean free path: From low-temperature physics to room temperature applications in thermoelectricity

It has been proposed for a long time now that the reduction of the thermal conductivity by reducing the phonon mean free path is one of the best way to improve the current performance of thermoelectrics. By measuring the thermal conductance and thermal conductivity of nanowires and thin films, we show different ways of increasing the phonon scattering from low-temperature up to room-temperature experiments. It is shown that playing with the geometry (constriction, periodic structures, nano-inclusions), from the ballistic to the diffusive limit, the phonon thermal transport can be severely altered in single crystalline semiconducting structures; the phonon mean free path is in consequence reduced. The diverse implications on thermoelectric properties will be eventually discussed.     

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     

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.     

Single Metal Ohmic and Rectifying Contacts to ZnO Nanowires: A Defect Based Approach

Ohmic and rectifying metal contacts to semiconductor nanowires are integral to electronic device structures and typically require different metals and different process techniques to form. Here we show how a noble metal ion beam of Pt commonly used to pattern conducting contacts in electron microscopes can form both ohmic and Schottky/blocking contacts on ZnO nanowires by controlling native point defects at the intimate metal‐semiconductor interface. Spatially‐resolved cathodoluminescence spectroscopy on a nanoscale both laterally and in depth gauges the nature, density, and spatial distribution of specific native point defects inside the nanowires and at their metal interfaces. Combinations of electron and ion beam deposition, annealing, and sculpting of the same nanowire provide either low contact resistivity ohmic contacts or a high Schottky/blocking barrier with a single metal source. These results highlight the importance of native point defects distributed inside nanowires and their variation near interfaces with sculpting and annealing.     

Phonon Transport and Thermal Conductivity in a Four-Terminal Structure

With the help of scattering-matrix method, the acoustic phonon ballistic transmission and the thermal conductivity are studied detailedly in a four-terminal structure. We find that the transmission coefficients and the reduced thermal conductance for each region sensitively depend on geometric parameters, and are of quantum character, but the reduced total thermal conductance for all regions seems independent of structure parameters when the temperature is not very low. Our results show that one can control the thermal conductivity for each region to match practical requirements in devices by adjusting the geometric parameters.     

Phonon Transport and Thermal Conductivity in a Four-Terminal Structure

With the help of scattering-matrix method, the acoustic phonon ballistic transmission and the thermal conductivity are studied detailedly in a four-terminal structure. We find that the transmission coefficients and the reduced thermal conductance for each region sensitively depend on geometric parameters, and are of quantum character, but the reduced total thermal conductance for all regions seems independent of structure parameters when the temperature is not very low. Our results show that one can control the thermal conductivity for each region to match practical requirements in devices by adjusting the geometric parameters.     

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.     

Is the ‘Finite Bias Anomaly’ in planar GaAs-superconductor junctions caused by point-contact-like structures?

We correlate transmission electron microscope (TEM) pictures of superconducting In contacts to an AlGaAs/GaAs heterojunction with differential conductance spectroscopy performed on the same heterojunction. Metals deposited onto a (100) AlGaAs/GaAs heterostructure do not form planar contacts, but, during thermal annealing, grow down into the heterostructure along crystallographic planes in pyramid-like ‘point contacts’. Random surface nucleation and growth gives rise to a different interface transmission for each superconducting point contact. Samples annealed for different times, and therefore having different contact geometry, show variations in dI / dV characteristics of ballistic transport of Cooper pairs, wave interference between different point emitters, and different types of weak localization corrections to Giaever tunneling. We give a possible mechanism whereby the ‘finite bias anomaly’ of Poirier et al. [Phys. Rev. Lett. 79, 2105 (1997)], also observed in these samples, can arise by adding the conductance of independent superconducting point emitters in parallel.     

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.     

A study of the interface roughness effect in Si nanowires using a full 3D NEGF approach

The effect of interface roughness in ballistic Si nanowires is investigated using a full 3D non-equilibrium Green's Functions formalism. The current density, the electron density and the transmission function are calculated for nanowires with different interface roughness configurations. Interface roughness is randomly generated using an exponential autocorrelation function. The interface roughness profile in nanowires with 2 nm diameter and 6 nm length is reflected in the current density landscape showing macroscopic 3D patterns. These macroscopic patterns affect the transmission probability causing resonances coming from the constrictions in the channel. The shape of the electron density in cross-sections along the wire follows the distortion of electron transversal wave function.     

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.