微尺度下壁面粗糙度对面向导热影响研究

随着空间尺度的缩小,界面对导热的影响就会越来越明显。本文采用非平衡分子动力学模拟方法模拟了壁面的粗糙度对面向导热的影响,并给出微尺度下的粗糙度定义。在模拟中构造了不同的壁面粗糙度,得到了面向热导率随着壁面粗糙度增加而减小的结果。分析认为壁面的镜面反射率随着粗糙度增加而减小,而镜面反射率减小将导致热导率下降．     

二维CdO的低晶格热导率研究

寻求具有较小晶格热导率k_lat的高热电性能的二维材料具有重要意义。基于从头计算和声子玻耳兹曼输运理论,该研究首先对二维CdO结构进行优化,并通过计算声子谱验证了单层CdO的动力学稳定性。在此基础上详细研究了单层CdO的声子输运性质。计算表明在室温下单层CdO的晶格热导率k_lat约为5.7 W/(m·K),低于单层石墨烯、磷烯、黑磷和MoS₂等二维材料的晶格热导率。其中,Z方向声学模式(Z-direction acoustic, ZA),横声学支(transverse acoustic,TA),纵声学支(longitudinal acoustic, LA),Z方向光学模式(Z-direction optical, ZO),横光学支(transverse optical, TO),纵光学支(longitudinal optical, LO)对k_lat的百分比贡献分别为73.7％、13.9％、3.7％、2.8％、4.7％和1.2％。研究发现,ZA、TA、LA声学支和光学支之间的强散射是导致单层CdO低热导率的原因。本文计算结果可用于指导基于CdO的低维热电器件的设计。     

Electrical and Thermal Transport Properties of n-type Bi6Cu2Se4O6 (2BiCuSeO + 2Bi2O2Se)

New thermoelectric materials, n-type Bi₆Cu₂Se₄O₆ oxyselenides, composed of well-known BiCuSeO and Bi₂O₂Se oxyselenides, are synthesized with a simple solid-state reaction. Electrical transport properties, microstructures, and elastic properties are investigated with an emphasis on thermal transport properties. Similar to Bi₂O₂Se, it is found that the halogen-doped Bi₆Cu₂Se₄O₆ possesses n-type conducting transports, which can be improved via Br/Cl doping. Compared with BiCuSeO and Bi₂O₂Se, an extremely low thermal conductivity can be observed in Bi₆Cu₂Se₄O₆. To reveal the origin of low thermal conductivity, elastic properties, sound velocity, Grüneisen parameter, and Debye temperature are evaluated. Importantly, the calculated phonon mean free path of Bi₆Cu₂Se₄O₆ is comparable to the interlayer distance for BiO─CuSe and BiO─Se layers, which is ascribed to the strong interlayer phonon scattering. Contributing from the outstanding low thermal conductivity and improved electrical transport properties, the maximum ZT ≈0.15 at 823 K and ≈0.11 at 873K are realized in n-type Bi₆Cu₂Se_3.2Br_0.8O₆ and Bi₆Cu₂Se_3.6Cl_0.4O₆, respectively, indicating the promising thermoelectric performance in n-type Bi₆Cu₂Se₄O₆ oxyselenides.     

Influence of Electron–Phonon Interaction on the Lattice Thermal Conductivity in Single-Crystal Si

Lattice thermal conductivity can be reduced by introducing point defect, grain boundary, and nanoscale precipitates to scatter phonons of different wave-lengths, etc. Recently, the effect of electron–phonon (EP) interaction on phonon transport has attracted more and more attention, especially in heavily doped semiconductors. Here the effect of EP interaction in n-type P-doped single-crystal Si has been investigated. The lattice thermal conductivity decreases dramatically with increasing P doping. This reduction on lattice thermal conductivity cannot be explained solely considering point defect scattering. Further, the lattice thermal conductivity can be fitted well by introducing EP interaction into the modified Debye–Callaway model, which demonstrates that the EP interaction can play an important role in reducing lattice thermal conductivity of n-type P-doped single-crystal Si.     

反铁磁链中Dzyaloshinskii–Moriya机制驱动矢量自旋手征束缚态的研究

Dzyaloshinskii-Moriya(DM)相互作用驱动的矢量自旋手征态,能和声子发生耦合,具备非常丰富的物理现象.本论文以一维反铁磁链中自旋手征-声子耦合模型为基础,研究不同声子环境下,耦合强度对自旋手征动力学演化过程的影响规律.结果表明,对自旋S=1/2的系统,在不同的声子浴(sub-Ohmic(0 1))中,均会在非相干到相干自旋涨落过程中产生无能隙一级相变,其来源是自旋手征束缚态的形成.相变发生的临界自旋-声子耦合强度正比于自旋涨落大小,反比于系统德拜频率.当自旋-声子耦合强度超过其临界值时,自旋手征束缚态的产生将极大地抑制非相干自旋涨落.     

Optical modes in nanoscale one-dimensional spin chains

The spin chain systems with one-dimensional magnetic ordering are promising candidates for quantum optical devices. This paper shows how the optical excitation can induce various phonon modes in an ideal Cu-O chain at various lengths. The calculation was carried out at different level theories including configuration interaction singles for excited states, density functional theory and second-order Möller-Plesset perturbation. In general, the number of modes increases with chain length due to growing asymmetry of atomic positions when chain exceeds 5 nm. There were, however, only two basic modes: one is associated with the symmetric oscillation of oxygen and another with the asymmetric motion of the same along the chain. At the length below 4.3 nm, the Raman activity of the symmetric mode (440 cm⁻¹) dominates. From analysis of density of states, this mode may be associated with the excitation across the lowest LUMO bands with changing in spin state.     

Temperature dependent phonon modes and ionicity of LiGaO2 single crystal

This paper reports that polarized far-infrared reflectivity measurements have been done on LiGaO2 single crystal along two crystalline taxes at different temperatures. The temperature dependent frequencies of the longitudinal and transverse optical phonon have been obtained from the real part of optical conductivity and the loss function respectively. A small Drude component is observed at frequency below 300 cm^-1 which could arise from Li ions or oxygen deficiencies. The ionicity of LiGaO2 has been studied from the analysis of the Born effective charge of different ions.     

Microscopic phonon theory of Si/Ge nanocrystals

Microscopic phonon theory of semiconductor nanocrystals (NCs) is reviewed in this paper. Phonon modes of Si and Ge NCs with various sizes of up to 7 nm are investigated by valence force field theory. Phonon modes in spherical SiGe alloy NCs approximately 3.6 nm (containing 1147 atoms) in size have been investigated as a function of the Si concentration. Phonon density-of-states, quantum confinement effects, as well as Raman intensities are discussed.      

Testing a New Class of Phonon Operators for RPA-Like Calculations Within an Exactly Solvable Model

Working in a boson formalism, we introduce a phonon operator different from that commonly used in RPA calculations. As a major feature, this new phonon operator has a vacuum carrying only a limited number of particle-hole excitations. Therefore, the form of this vacuum is quite different from the exponential one of the RPA. In addition, multiphonon excitations have a particle-hole structure never exceeding in complexity that of this vacuum, no matter how large is the number of phonons involved. We present the results of some preliminary tests performed within the exactly solvable two-level Lipkin model.     

Investigation of phonon anomalies in intermediate valence compounds SmS and Sm0·65Y0·25S

Phonon anomalies in two intermediate valence compounds (IVC), SmS and Sm_0·75Y_0·25S have been investigated using breathing shell model (BSM). The BSM includes breathing motion of electron shells of the rare earth atom due tof−d hybridization. The phonon dispersion curves of IVC, calculated from the present model, agree well with the measured data. One-phonon density of states calculated from the present model compares well with the Raman spectra.