QUANTUM PERCOLATION AND BALLISTIC CONDUCTANCE IN A SYSTEM OF DOUBLE-COUPLED CHAINS

The quantum-mechanical calculation of electronic conductance in double-coupled chains as a function of the interchain bonding probability p is presented. The calculated results show that one still can see the basic plateaus in the ensemble-averaged conductance curves as a function of the Fermi energy for the weak disorder. In addition, dense irregularly oscillating structures are superimposed upon each plateau. The characteristics of the conductance are very sensitive to the presence of the interchain broken bonds. For the strong disorder (p≈0.5) the conductance quantization breaks down. The accuracy of the quantization conductance rapidly drops down as the value of p approaches 0.5. The ensemble-averaged value of the logarithmic conductance as a function of the sample length exhibits a linear variation, determining a localization length. Both the localization length and the root-mean- square (RMS) value of the conductance fluctuations depend on p and the Fermi energy of electrons. The variations of the localization length and RMS with p are both of an approximate parabolic function around p≈0.5. No percolation transition is found for this quasi-one-dimensional system, as expected.     

THE EFFECT OF DIRECT f-f HOPPING ON THE HYBRIDIZATION GAP INSULATING BEHAVIOR OF THE PERIODIC ANDERSON LATTICE

By using the well known slave-boson technique and within the framework of mean field approximation, we study the effect of the direct f-f hopping on the low-temperature hybridization gap insulating behavior of the periodic Anderson lattice. The results show that the direct f-f hopping will decrease the hybridization gap or lead it to vanish. In the latter case, the lattice will exhibit no hybridization gap insulating behavior even if the lower renormalized band is fully occupied.     

巡游电子反铁磁性的变分泛函积分方法

从双子晶格Hubbard模型出发,应用变分泛函积分方法,研究了巡游电子反铁磁性.该方法的关键在于:对Hubbard哈密顿量的相互作用部分作Stratonovich-Hubbard变换时,采用连续分解方案,导致自旋涨落与电荷涨落的分离,从而能更好地描述两类涨落所具有的相反倾向的性质.在连续分解中所引入的参数由变分原理确定,从而消除了泛函积分理论中的“不确定性”困难.本文还对半满带情形作了数值研究,计算了基态性质,奈耳温度,磁化强变以及顺磁磁化率等.与Hasegawa理论比较,在磁化率的温度依赖行为方面有一定 关键词：     

零温下掺杂浓度x=0.5时的钙钛矿型锰氧化物磁有序和电荷有序共存相的理论研究

为了解释实验上观测到的ｘ＝０．５掺杂时的钙钛矿型锰氧化物磁有序和电荷有序的共存，我们使用包含巡游电子近邻排斥作用的哈密顿量，运用ＨａｔｒｅｅＦｏｃｋ近似，引入两个序参量，得到了能带结构，并讨论了其填充情况；计算了几种不同位形下的能量，得到磁有序和电荷有序共存的必要条件；结合其他小组的工作，可以从理论上得出ＣＥ型反铁磁＋电荷有序在特定参数区将成为系统基态．     

关于单元系平衡的稳定条件的几点说明

关于热力学中单元。相系平衡的稳定性条件问题，我刊曾分别在１９８７年第８期和１９８８年第６期上发表过有关文章，后一篇是“来稿摘要”，对前一篇的观点提出了异议，但随后又收到讨论这一问题的来稿，提出了与前两者均为不同的看法，看来，对此问题还有进一步澄清之必要．为此，我们请林宗涵同志写了这篇短文，希望有助于这一问题的澄清．     

RESONANT TUNNELING THROUGH COUPLED DOUBLE-QUANTUM-WELL

A double-well resonant tunneling structure has been investigated carefully using the nonequilibrium Green's function method. We find that in the transmission probability two maxima appear even when the two levels have the same energy. This characteristic is at-tributed to the resonant tunneliug through mixed quasibound states. The tunneling current formula through this system under a dc voltage has been derived exactly. Three different cases are considered and several novel properties are found, which manifest coherent charac-teristics of the tunneling process.