极化子效应对量子盘中线性和非线性光吸收系数的影响

利用密度矩阵的方法,得出了考虑极化子效应的量子盘的线性和非线性光吸收系数的解析表达式,并以GaAs为例讨论了光吸收系数与不同的入射光子能量和量子盘的厚度之间的关系.结果表明,极化子效应对吸收系数有相当的影响 关键词： 量子盘 光学吸收系数 极化子效应     

Variational approaches to quantum impurities: from the Fröhlich polaron to the angulon

ABSTRACT

Problems involving quantum impurities, in which one or a few particles are interacting with a macroscopic environment, represent a pervasive paradigm, spanning across atomic, molecular, and condensed-matter physics. In this paper we introduce new variational approaches to quantum impurities and apply them to the Fröhlich polaron – a quasiparticle formed out of an electron (or other point-like impurity) in a polar medium, and to the angulon – a quasiparticle formed out of a rotating molecule in a bosonic bath. We benchmark these approaches against established theories, evaluating their accuracy as a function of the impurity-bath coupling.     

Niobium Doping in BiVO4: Interplay Between Effective Mass,Stability, and Pressure

We have applied density functional theory to study the electronic structure changes caused by Nb incorporation in BiVO₄ and the application of external pressure. The overall solubility of Nb in BiVO₄ is usually high, and the presence of oxygen vacancies affect the dopability of Nb in BiVO₄. Through the analyses of the chemical-potential landscape, we have determined the single-phase stability zone of BiVO₄ with the Nb doping. The most favorable Nb doping is simultaneous substitutions at both V- and Bi-sites. Even though Nb substitution at only V-site is next favorable, the band gap change is not very significant which agrees with an earlier experiment. However, it does change the electron effective mass by 20 % owing to the presence of Nb 4d bands in the conduction bands, which explains better catalytic activity by Nb-doped BiVO₄. In addition, application of external pressure the single-phase stability zone in the chemical-potential landscape. We have also focused on the local structural distortions near the Nb doping site, especially on the BiO₈ octahedra. We have shown here that pressure-induced symmetrization of BiO₈ dodecahedron lowers the electron's effective mass further and therefore can help to improve the photoconduction property of BiVO₄.     

量子棒中弱耦合杂质束缚极化子的性质

给出了具有椭球边界量子棒经过坐标变换成球形边界的哈密顿量。采用线性组合算符和幺正变换的方法,研究了在非均匀抛物限制势下量子棒中弱耦合杂质束缚极化子的振动频率、基态能量和基态结合能随库仑束缚势、电子-声子耦合强度和椭球的纵横比的变化关系。数值计算结果表明:振动频率、基态能量和基态结合能随库仑束缚势的增加而增大,基态能量和基态结合能随电子-声子耦合强度的增加而增加。当e′>1时,振动频率、基态能量和基态结合能随椭球的纵横比的增加而增加;e′<1时,随纵横比的减少,振动频率、基态能量和基态结合能都增大;当e′=1时,振动频率取极小值,基态能量和基态结合能也取较小的稳定值。     

电场下应变纤锌矿GaN/AlxGa1-xN异质结中束缚极化子的压力效应

对应变纤锌矿GaN/AlxGa1-xN异质结系统,引入简化相干势近似,利用改进的LLP变换和变分法计算了流体静压力和外场作用下束缚极化子的结合能。考虑由于晶格失配所致的单、双轴应变以及界面光学声子模和半空间光学声子模与电子和杂质之间的相互作用,讨论了结合能随压力、杂质位置和流体静压力的变化关系以及声子对于斯塔克能量移动的影响。数值计算结果表明,高频支界面声子模和类LO半空间声子模对于结合能和斯塔克能移的影响是主要的,且随压力的增加而显著增大,而低频支界面声子模和类TO声子模的作用则很小,且对于杂质位置和流体静压力的变化不敏感。计算还表明,导带的弯曲也不容忽视。     

量子阱中极化子的自能与电磁场和温度的关系

采用Ｌａｒｓｅｎ谐振子算符代数运算与变分微扰相结合的方法，研究处于电磁场中量子阱内电子一体纵光学声子耦合诉性质的曙依赖性，得到了有限温度下系统的自能。     

电场中束缚极化子

采用线性组合算符及幺正变换方法研究了电场对量子阱弱耦合束缚极化子的性质的影响.推导出量子阱中束缚极化子的基态能量和库仑束缚势、电场和阱宽的变化关系.数值计算结果表明,基态能量因电场和库仑束缚势的不同而不同,随电场和库仑束缚势的增大而增大,随阱宽的增大而迅速减小.     

Sub-Band Filling and Hole Transport in Polythiophene-Based Electrolyte-Gated Transistors: Effect of Side-Chain Length and Density

The relationship between hole density and conductivity in electrochemically gated polythiophene films is examined. The films are  integrated into electrolyte-gated transistors (EGTs), so that hole accumulations can be electrochemically modulated up to ≈0.4 holes per thiophene ring (hpr). Polythiophenes include poly(3-alkylthiophenes) (P3ATs) with four different side chain lengths – butyl (P3BT), hexyl (P3HT), octyl (P3OT), or decyl (P3DT) – and poly[2,5-bis(3-dodecylthiophen-2-yl)thieno[3,2-b]thiophene] (PBTTT) and poly(3,3′′′-didodecyl[2,2′:5′,2′′:5′′,2′′′-quaterthiophene]-5,5′′′-diyl) (PQT). Analysis of the drain current – gate voltage (I_D–V_G) and gate current – gate voltage (I_G–V_G) characteristics of the EGTs reveals that all six polythiophene semiconductors exhibited reversible conductivity peaks at 0.12 – 0.15 hpr. Conductivity is suppressed beyond ≈0.4 hpr.The maximum carrier mobilities of the P3AT semiconductors increase, and hysteresis of the conductivity peaks decreases, with increasing alkyl side-chain length. PBTTT and PQT with reduced side chain densities exhibit the largest hysteresis but have higher hole mobilities. The results suggest that at ≈0.4 hpr, a polaronic sub-band is filled in all cases. Filling of the sub-band correlates with a collapse in the hole mobility. The side-chain dependence of the peak conductivity and hysteresis further suggests that Coulombic ion-carrier interactions are important in these systems. Tailoring ion-carrier correlations is likely important for further improvements in transport properties of electrochemically doped polythiophenes.     

多原子半无限晶体中极化子的激发能量

研究多原子半无限晶体中电子与表面光学SO声子和体纵光学LO声子强耦合的极化子的激发态的性质。采用线性组合算符和幺正变换方法导出强耦合情形下极化子的基态能量、第一内部激发态能量和激发能量。结果表明,多原子半无限晶体中强耦合极化子的基态能量、第一内部激发态能量和激发能量不仅包含不同支LO声子和不同支SO声子与电子耦合的能量,而且还包含不同支LO声子之间和不同支SO声子之间相互作用贡献的附加能量。     

Correlation between large polarons in molecular chains

We studied few extra electrons in a molecular chain with respect of electron-phonon coupling in the adiabatic approximation. It is shown that the lowest state of two extra electrons in a chain corresponds to the singlet bisoliton state with one deformational potential well. Two electrons with parallel spins form a localised triplet state, which corresponds to the two-hump charge distribution function. Three extra electrons form an almost independent nonlinear superposition of a soliton and bisoliton states. In the case of four electrons, the two almost independent bisolitons are formed. These two states tend to separate in the chain at the maximal distance due to the Fermi repulsion, accounted for in the zero-order adiabatic approximation. This repulsion is partly compensated by the attraction between the solitons due to their exchange with virtual phonons, described by the non-adiabatic part of the Hamiltonian. The formation of solitons is characterised by the appearance of the bound soliton and bisoliton levels in the forbidden energy band. This constitutes the qualitative difference of the large polaron (soliton) states from the almost free electron states and small polaron states.