Electron effective mass and resonant polaron effect in CdTe/CdMgTe quantum wells

Cyclotron resonance in CdTe/CdMgTe quantum wells (QWs) was studied. Due to the polaron effect the zero-field effective mass is strongly influenced by the QW width. The experimental data have been described theoretically by taking into account electron-phonon coupling and the nonparabolicity of the conduction band. The subband structure was calculated self-consistently. The best fit was obtained for an electron-phonon coupling constant alpha = 0.3 and bare electron mass of m(b) = 0.092m(0).

High-field cyclotron resonance and electron-phonon interaction in modulation-doped multiple quantum well structures

A systematic study of electron cyclotron resonance (CR) in two sets of GaAs/Al0.3Ga0.7As modulation-doped quantum-well samples (well widths between 12 and 24 nm) has been carried out in magnetic fields up to 30 T. Polaron CR is the dominant transition in the region of GaAs optical phonons for the set of lightly doped samples, and the results are in good agreement with calculations that include the interaction with interface optical phonons. The results from the heavily doped set are markedly different. At low magnetic fields (below the GaAs reststrahlen region), all three samples exhibit almost identical CR which shows little effect of the polaron interaction due to screening and Pauli-principle effects. Above the GaAs LO-phonon region (B > similar to 23 T), the three samples behave very differently. For the most lightly doped sample (3 x 10(11) cm(-2)) only one transition minimum is observed, which can be explained as screened polaron CR. A sample of intermediate density (6 x 10(11) cm(-2)) shows two lines above 23 T; the higher frequency branch is indistinguishable from the positions of the single line of the low density sample. For the most heavily, doped sample (1.2 x 10(12) cm(-2)) there is no evidence of high frequency resonance, and the strong, single line observed is indistinguishable from the lower branch observed from sample with intermediate doping density. We suggest that the low frequency branch in our experiment is a magnetoplasmon resonance red-shifted by disorder, and the upper branch is single-particle-like screened polaron CR. (C) 1998 Elsevier Science B.V. All rights reserved.

Resonant magnetopolaron effects in GaAs/AlGaAs multiple quantum well structures

A detailed experimental study of electron cyclotron resonance (CR) has been carried out at 4.2 K in three modulation-doped GaAs/Al0.3Ga0.7As multiple quantum well samples in fields up to 30 T. A strong avoided-level-crossing splitting of the CR energies due to resonant magnetopolaron effects is observed for all samples near the GaAs reststrahlen region. Resonant splittings in the region of AlAs-like interface phonon modes of the barriers are observed in two samples with narrower well width and smaller doping concentration. The interaction between electrons and the AlAs interface optical phonon modes has been calculated for our specific sample structures in the framework of the memory-function formalism. The calculated results are in good agreement with the experimental results, which confirms our assignment of the observed splitting near the AlAs-like phonon region is due to the resonant magnetopolaron interaction of electrons in the wells with AlAs-like interface phonons. (C) 1998 Elsevier Science B.V. All rights reserved.

热带印度洋低频Rossby波及其对上层热结构的影响

Rossby波是地球物理流体动力学中非常重要的一种波动，海洋斜压长Rossby波在海洋动力过程中起着相当重要的作用。它维持并影响强西边界流，是海盆内能量传播的主要机制，它所携带的变异信号从大洋的东边界传播到内部，对海气耦合系统起到很重要的作用。热带印度洋是季风爆发的源地，对季风的年际变化具有重要影响。研究热带印度洋对理解季风变率和提高季风预测水平有重要的科学和应用价值。 本文利用TOPEX/Poseidon等高度计资料、美国国家海洋数据中心（NODC）的世界海洋图集（WOA05）长期气候态水文资料、美国Scripps海洋研究所的上层海温资料、中国Argo资料中心提供的Argo资料、美国国家环境预测中心（NCEP）的海表面温度、FSU（Florida State University）月平均风场和海气界面热通量等观测数据，全面分析了热带印度洋低频Rossby波的基本特征，并深入研究了低频Rossby波的生成机制及其对上层海洋热结构的影响。 采用相关分析等统计方法，结合1.5层约化重力模式，研究了热带南印度洋低频Rossby波的生成机制。结果表明： （1）热带南印度洋低频Rossby波分为东边界扰动产生的Rossby波和南印度洋中部风强迫Rossby波；东边界激发的为自由Rossby波，沿12°S波速大约13 cm/s，向西最远传播到80°E左右，之后被局地变量调整；强迫Rossby波在西传的过程中不断加强，波速较快，沿12°S能超过20 cm/s； （2）东边界扰动由印度尼西亚贯通流（ITF）导致的地转调整过程引起；内区风强迫Rossby波生成和加强的关键区为（70°E–95°E，15°S–5°S）；显著的西传Rossby波同太平洋上的厄尔尼诺/南方涛动（ENSO）事件紧密相连，ENSO通过大气的遥驱动机制激发热带南印度洋低频Rossby波； （3）作为东边界低频变量扰动的一个重要因子，ITF的变化与ENSO事件密切相关，总的来讲，El Niño年ITF偏弱，La Niña年ITF偏强，这与前人的研究结果一致；但它在ENSO的不同位相时期，存在一定差异，并具有夏季锁相特征：El Niño事件发生年的春季到秋季，ITF偏强，夏季最强；从El Niño盛期（冬季）到次年秋季，ITF持续偏弱，夏季最弱。上述夏季锁相特征与夏季风的强弱变化相对应。La Niña期间情况相反。 西南印度洋（SWIO）（50°E–75°E，15°S–5°S）以及苏门答腊-爪哇沿岸地区是海表面高度异常（SSHA）和海表面温度异常（SSTA）相关显著的区域，表明内部海洋动力过程在次表层和表层变量的相互关系中起重要作用。本文以2006–2008年期间三个连续的同El Niño或者La Niña同时发生的正印度洋偶极子（IOD）事件为背景，基于Argo观测资料研究了表层和次表层IOD的演变以及二者的区别和联系，并首次采用垂直模态分解方法探讨了Rossby波对上层海洋热力结构影响的动力学特征，得到如下主要结论： （1）在热带印度洋，海洋动力过程一般主要由第一和第二低阶垂直斜压模态控制，而第一斜压模态处于主导地位——在SWIO海区，第一斜压模态运动的方差解释率为第二模态的2–3倍，在赤道和东南印度洋也达到2倍左右；另外，赤道印度洋地区高阶斜压模态运动对该地区的海洋动力过程也具有一定的贡献； （2）低频斜压Rossby波能影响海洋的垂直层结，尤其是强暖Rossby波使同第二斜压模态运动紧密相连的海洋上层层结减弱，加强第二斜压模态的贡献量，导致上层各等压线向下垂直位移增大，最终通过垂直混合过程调整上层海洋的热力结构；而低频斜压冷Rossby波会加强上层垂直层结，抑制该层内变量变化，因此第二斜压模态的贡献依然很小； （3）表层IOD和次表层IOD分布形态不同：表层东部冷异常主要集中在东南印度洋Sumatra-Java沿岸，次表层冷异常基本关于赤道对称；表层西部暖异常基本关于赤道对称，而在次表层赤道以南海温扰动强度远远大于赤道以北； （4）正IOD事件中，东南印度洋冷SSTA首先出现于Java沿岸，沿岸东南风引起的潜热释放增加以及沿岸上升流是该初始冷异常建立的主要机制，与之相关的SSTA东西梯度加强大气环流变化，并进一步强迫随后的海洋运动；1–2个月后，SST冷异常中心北跳到Sumatra沿岸并向西扩展，同时不断增强，其中Sumatra沿岸上升流、来自赤道印度洋的冷Kelvin波及其反射的西传冷Rossby波是这一演变过程的动力机制，而沿岸上升流起决定作用。