声子之间相互作用和库仑势对量子点中束缚极化子性质的影响

研究了库仑场中抛物量子点中束缚极化子的性质，采用线性组合算符和微扰法,导出了量子点中束缚极化子的基态能量，在计及电子在反冲效应中发射和吸收不同波矢的声子之间的相互作用下,研究了其对量子点中束缚极化子的基态能量的影响。数值计算表明：当考虑声子之间的相互作用时,量子点中束缚极化子的基态能量随量子点的有效受限长度的减小而迅速增大，当l0〉1.0时,必须考虑声子之间的相互作用对基态能量的影响。     

磁场内双声子作用对量子点中极化子的影响

为突出双声子相互作用对磁场内量子点中极化子的影响,简化了理论模型,把库仑势对电子质量的影响用电子在能带中的质量来代替。使用线性组合算符和么正变换对系统进行理论计算,导出了半导体量子点中磁极化子的基态和各部分能量。当考虑电子在反冲效应中反射和吸收不同波矢的声子之间相互作用时,讨论了这种作用对半导体量子点中磁极化子基态能量的影响。通过数值计算表明,半导体量子点中磁极化子的基态能量随量子点的有效受限长度的减小而迅速增大,随磁场的增加而增加。当磁场较弱时,声子之间的相互作用对极化子性质的影响是不能忽略的。     

金属Ti热学性质第一原理研究

应用基于密度泛函与密度泛函微扰理论的平面波赝势方法计算一组不同晶格常数下六角密堆积（hcp）结构金属Ti的声子谱及相应的静态总能，由此得到不同晶格常数下的自由能，由准谐近似及自由能极小判据得到自由能与温度的关系，进而计算热膨胀系数、定容摩尔热容及定压摩尔热容与温度的关系，对热膨胀系数及定容摩尔热容的第一原理计算值与德拜理论计算值进行比较。结果表明：295K下声子谱理论值与实验值除在[001]方向上的光学纵模有少量偏差外，其余部分符合得很好；hcp结构金属Ti有一定程度的各向异性热膨胀，沿c轴方向与口轴方向的热膨胀系数比值为1．5左右；热膨胀系数、定容摩尔热容及定压摩尔热容第一原理计算值在较宽的温度范围内与已有的实验数据相符；热膨胀系数的德拜理论值仅在室温以下温区与实验结果相符；定容摩尔热容的第一原理计算值与德拜理论值在中温区有少量偏差，在低温及高温区非常接近。     

Observation of strain effect on the suspended graphene by polarized Raman spectroscopy

We report the strain effect of suspended graphene prepared by micromechanical method. Under a fixed measurement orientation of scattered light, the position of the 2D peaks changes with incident polarization directions. This phenomenon is explained by a proposed mode in which the peak is effectively contributed by an unstrained and two uniaxial-strained sub-areas. The two axes are tensile strain. Compared to the unstrained sub-mode frequency of 2,672 cm⁻¹, the tension causes a red shift. The 2D peak variation originates in that the three effective sub-modes correlate with the light polarization through different relations. We develop a method to quantitatively analyze the positions, intensities, and polarization dependences of the three sub-peaks. The analysis reflects the local strain, which changes with detected area of the graphene film. The measurement can be extended to detect the strain distribution of the film and, thus, is a promising technology on graphene characterization.     

Efficient Near Infrared to Visible and Near‐Infrared Upconversion Emissions in Transparent (Tm3+, Er3+)‐α‐Sialon Ceramics

Intense visible and near‐infrared frequency upconversion and a frequency downconversion photoluminescence have been reported for the first time in Tm³⁺ and Er³⁺ co‐doped transparent α‐Sialon ceramics under 980 nm excitation. The α‐Sialon ceramics were prepared by hot‐press sintering technique. Intense upconversion bands at 554, 678, 803 nm, and a downconversion band at 1530 nm were observed as a result of the efficient energy transfer between Er³⁺ and Tm³⁺ ions. The quadratic dependence of upconversion intensities on the excitation power indicates that the upconversion process is governed by two‐photon absorption process. The sintered samples of thickness 0.20 mm have transparency above 80% in the range of 2000 to 4200 nm and it reaches as high as 82% at 3350 nm. Moderately low phonon energy was found with the highest frequency band at 828 cm^?1. These novel properties manifest the potential applications of transparent α‐Sialon ceramics as a multifunctional material.     

Quantitative Identification of Phonon Modes Controlling the Multiphonon Relaxation in Heavy-Metal Oxide Glasses

The phonon mode(s) controlling the multiphonon relaxation (MPR) in PbO–Bi₂O₃–Ga₂O₃ glass was analyzed, and the effect of GeO₂ addition on the MPR process was investigated. MPR rates were obtained from the lifetimes of the Tm³⁺:³ H ₄ level in glasses over the temperature range 20–280 K. In PbO–Bi₂O₃–Ga₂O₃ glass, phonons from the bending vibration between GaO₄ tetrahedra (∼550 cm⁻¹) controlled the MPR process. On the addition of GeO₂, the phonon mode at ∼770 cm⁻¹ due to the stretching vibration of GeO₄ tetrahedra started to affect the MPR process. Phonon modes controlling the MPR process in PbO–Bi₂O₃–Ga₂O₃–GeO₂ glass were both 550 cm⁻¹ and 770 cm⁻¹.     

Anisotropic pairing caused by unscreened long-range interactions

Anisotropy of the order parameter in high-T _c superconductors is explained by unscreened interaction of charge carriers with long-wave optical phonons. Screening is absent due to the low frequency of long-wave plasmons in layered structures.The authors are grateful to G. Deutscher, A. Bill, and E. Joon for discussions. Thanks are also due to D. Nevedrov who did the numerical calculations. The research described in this publication was made possible in part by Grant No. LL4100 from the Joint Fund Program of Estonia. This work was also supported by Estonian SF Grant No. 369.     

The relationship between the elastic constants and the instantaneous normal modes of liquids

In the solid state the elastic properties of a substance are related quite simply and directly to its phonon spectrum. However, while it is well-known that liquids possess certain macroscopic elastic properties, their relationship with an analogous set of liquid phonons is far more tenuous. We show here that one candidate for what one might call the phonons of a liquid its instantaneous normal modes has a very precise connection with the liquid’s high-frequency macroscopic elastic moduli. This connection suggests, moreover, that there is at least some plane-wave-like character to the short-time, long-wavelength dynamics of liquids. Paper dedicated to Professor Edward A. Mason.     

Measurement of the Thermal Conductivity of Si and GaAs Wafers Using the Photothermal Displacement Technique

Thermal conductivity and thermal diffusivity of Si and GaAs wafers were measured using the photothermal displacement technique, and the temperature dependence of these two quantities was investigated. Thermal diffusivity was obtained from the phase difference between the heating source and the signal, and thermal conductivity was determined from the maximum value of the signal amplitude in the temperature range 80 to 300 K. It was verified that an increase in doping concentration gives rise to a decrease in thermal conductivity at low temperatures. The experimental results obtained on samples with different types and doping concentrations are consistent with those expected from theoretical considerations.