轨道诱导Peierls相变材料MgTi_2O_4的电子自旋共振研究

用变温电子自旋共振手段(Electron Spin Resonance,ESR),对轨道诱导Peierls相变MgTi_2O_4体系进行了研究.研究发现,轨道诱导Peierls相变所伴随的自旋二聚相变对ESR谱产生了影响.在相变温度以上,MgTi_2O_4的磁性为顺磁行为.而在相变温度以下,ESR谱显示MgTi_2O_4的磁性偏离了顺磁行为.对ESR谱线的参数拟合结果显示,MgTi_2O_4在发生轨道诱导Peierls相变时,自旋耦合作用逐渐增强.这说明:自旋耦合作用的增强很有可能是导致相变的一个重要的因素.     

轨道诱导Peierls相变材料MgTi2O4的电子自旋共振研究

用变温电子自旋共振手段,对轨道诱导Peierls相变MgTi2O4体系进行了研究。研究发现,轨道诱导Peierls相变所伴随的自旋二聚相变对ESR谱产生了影响。在相变温度以上,MgTi2O4的磁性为顺磁行为。而在相变温度以下,ESR谱显示MgTi2O4的磁性偏离了顺磁行为。对ESR谱线的参数拟合结果显示,MgTi2O4在发生轨道诱导Peierls相变时,自旋耦合作用逐渐增强。这说明:自旋耦合作用的增强很有可能是导致相变的一个重要的因素。     

氢链中的Peierls机理(Ⅱ) 电子关联对Peierls不稳定性的影响

本文用相关基函数理论研究电子关联对氢链中Peierls不稳定性的影响,结果发现,电子关联使得氢链中的二聚化程度从0.37au减小为0.32au,这说明电子关联只是减弱但不会完全破坏氢链中的Peierls不稳定性,本理论表明,在电子相互作用下链状体系中的Peierls机理依然成立。     

氢链中的Peierls机理(Ⅱ)——电子关联对Peierls不稳定性的影响

本文用相关基函数理论研究电子关联对氢链中Peierls不稳定性的影响,结果发现,电子关联使得氢链中的二聚化程度从0.37au减小为0.32au,这说明电子关联只是减弱但不会完全破坏氢链中的Peierls不稳定性,本理论表明,在电子相互作用下链状体系中的Peierls机理依然成立。 关键词：     

一维有机导体的Peierls相变研究

根据建立在电子-声子相互作用基础上的Peierls 相变理论，利用形变势模型和半经验晶体轨道方法计算的能带结构数据，对一维有机导体tetrathiafulvalene-tetracyanoquinodimethane的Peierls相变温度进行了计算.并对其金属-绝缘体相变机制进行了讨论. 关键词： 电子-声子相互作用 Peierls相变温度 一维有机导体     

扶手椅形碳纳米管的Peierls相变研究

根据描述电子-声子相互作用的Frhlich哈密顿量,推导出了扶手椅形碳纳米管(ABT(n))的Peierls相变温度的一般表达式,并利用该式计算了ABT(8)和ABT(10)的Peierls相变温度,结果表明ABT(n)的Peierls相变温度将随管径的增大而减小,并且ABT(n)在远离室温下都不会发生Peierls相变,仍然保持其金属性. 关键词：     

剪切应变下刃型位错的滑移机理的晶体相场模拟

针对刃型位错的滑移运动, 构建包含外力场与晶格原子密度耦合作用的体系自由能密度函数, 建立剪切应变作用体系的晶体相场模型. 模拟了双相双晶体系的位错攀移和滑移运动, 计算了位错滑移的Peierls势垒和滑移速度. 结果表明: 施加较大的剪切应变率作用, 体系能量变化为单调光滑曲线, 位错以恒定速度做连续运动, 具有刚性运动特征; 剪切应变率较小时, 体系能量变化出现周期波动特征, 位错运动是处于低速不连续运动状态, 运动出现周期“颠簸”式滑移运动, 具有黏滞运动特征; 位错启动运动, 存在临界的势垒. 位错启动攀移运动的Peierls势垒要比启动滑移Peierls势垒大几倍. 位错攀移和滑移运动特征与实验结果相符合.     

金属螺旋型碳纳米管的Peierls相变研究

根据描述电子声子相互作用的Fr?hlich哈密顿量,推导出了金属螺旋型碳纳米管(n₁,n₂)的Peierls相变温度的一般表达式,并利用该式计算了(6,3)的Peierls相变温度.结果表明金属螺旋型碳纳米管在远离室温下都不会发生Peierls相变,仍然保持其金属性 关键词： 螺旋型碳纳米管 电子声子相互作用 Peierls相变     

脉冲Doppler系统中混叠现象的克服和测速最大值的提高

脉冲Doppler系统混叠现象的存在使其最高速度的测量范围受到限制．本文先介绍一种克服混叠现象的方法：平均频率跟踪法，然后报导了我们研究的两种克服混叠现象的方法：最大频率跟踪法和混合跟踪法．通过计算机的模拟实验，指出：利用混合跟踪法不仅可较好地克服混叠现象，而且便于仪器实现．这对于脉冲Doppler仪器性能的提高，具有重要意义．     

TTF-TCNQ的Peierls相变研究

根据建立在电子-声子相互作用基础上的Peierls相变理论，利用形变势模型和半经验晶体轨道方法计算的能带结构数据，对一维有机导体TTF-TCNQ的Peierls相变温度进行了计算，结果表明,TTF链的相变温度低于TCNQ链的相变温度，从而说明前者的电子-声子耦合相互作用比后者要弱,TTF-TCNQ在54K的金属-绝缘体相变主要发生在TCNQ链上. 关键词： 一维有机导体 Peierls相变温度 电子-声子相互作用     

Mechanism of gap opening in a triple-band Peierls system: in atomic wires on Si

One dimensional (1D) metals are unstable at low temperature undergoing a metal-insulator transition coupled with a periodic lattice distortion, a Peierls transition. Angle-resolved photoemission study for the 1D metallic chains of In on Si(111), featuring a metal-insulator transition and triple metallic bands, clarifies in detail how the multiple band gaps are formed at low temperature. In addition to the gap opening for a half-filled ideal 1D band with a proper Fermi surface nesting, two other quasi-1D metallic bands are found to merge into a single band, opening a unique but k-dependent energy gap through an interband charge transfer. This result introduces a novel gap-opening mechanism for a multiband Peierls system where the interband interaction is important.     

Imperfect nesting and Peierls instability for a two-dimensional tight-binding model

Based on a half-filled two-dimensional tight-binding model with nearest-neighbour and next nearest-neighbour hopping the effect of imperfect Fermi surface nesting on the Peierls instability is studied at zero temperature. Two dimerization patterns corresponding to a phonon vector (π,π) are considered. It is found that the Peierls instability will be suppressed with an increase of next nearest-neighbour hopping which characterizes the nesting deviation. First and second order transitions to a homogeneous state are possible. The competition between the two dimerized states is discussed. Received 22 December 2000     

Lattice dynamics and electron-phonon interaction in (3,3) carbon nanotubes

We present a detailed study of the lattice dynamics and electron-phonon coupling for a (3,3) carbon nanotube which belongs to the class of small diameter based nanotubes which have recently been claimed to be superconducting. We treat the electronic and phononic degrees of freedom completely by modern ab initio methods without involving approximations beyond the local density approximation. Using density functional perturbation theory we find a mean-field Peierls transition temperature of approximately 240 K which is an order of magnitude larger than the calculated superconducting transition temperature. Thus in (3,3) tubes the Peierls transition might compete with superconductivity. The Peierls instability is related to the special 2k(F) nesting feature of the Fermi surface. Because of the special topology of the (n,n) tubes we also find a phonon softening at the Gamma point.     

Quasi-one Dimensional Topological Insulator: Möbius Molecular Devices in Peierls Transition

We show that, assisted by the Peierls transition of lattice, as a quasi-one dimensional (Q1D) tight binding system, a Möbius molecular device can behave as a simple topological insulator. With the Peierls phase transition to form a domain wall, the solitonary zero modes exist as the ground state of this electron-phonon hybrid system, which is protected by the Z₂ topology of the Möbius strip. The robustness of the ground state prevents these degenerate zero modes from their energy spectrum splitting caused by any perturbation.     

A study of Peierls instabilities for a two-dimensional t-t' model

In this paper we study Peierls instabilities for a half-filled two-dimensional tight-binding model with nearest-neighbour hopping t and next nearest-neighbour hopping t' at zero and finite temperatures. Two dimerization patterns corresponding to the same phonon vector (π,π) are considered to be realizations of Peierls states. The effect of imperfect nesting introduced by t' on the Peierls instability, the properties of the dimerized ground state, as well as the competition between two dimerized states for each t' and temperature T, are investigated. It is found: (i). The Peierls instability will be frustrated by t' for each of the dimerized states. The Peierls transition itself, as well as its suppression by t', may be of second- or first-order. (ii). When the two dimerized states are considered jointly, one of them will dominate the other depending on parameters t' and T. Two successive Peierls transitions, that is, the system passing from the uniform state to one dimerized state and then to the other may take place with decrease of temperature. Implications of our results to real materials are discussed. Received 31 July 2001     

Symmetry-breaking phase transition without a Peierls instability in conducting monoatomic chains

The one-dimensional (1D) model system Au/Ge(001), consisting of linear chains of single atoms on a surface, is scrutinized for lattice instabilities predicted in the Peierls paradigm. By scanning tunneling microscopy and electron diffraction we reveal a second-order phase transition at 585 K. It leads to charge ordering with transversal and vertical displacements and complex interchain correlations. However, the structural phase transition is not accompanied by the electronic signatures of a charge density wave, thus precluding a Peierls instability as origin. Instead, this symmetry-breaking transition exhibits three-dimensional critical behavior. This reflects a dichotomy between the decoupled 1D electron system and the structural elements that interact via the substrate. Such substrate-mediated coupling between the wires thus appears to have been underestimated also in related chain systems.     

Solitons and (spin-)Peierls transition in disordered quasi-one-dimensional systems

We study the (spin-)Peierls transition in quasione- dimensional disordered systems, treating the lattice classically. The role of kinks, induced thermally and by disorder, is emphasized. For weak interchain interaction the kinks destroy the coherence between different chains at a temperature significantly lower than the mean-field Peierls transition temperature. We formulate the effective Ising model, which describes such a transition, investigate the doping dependence of the (spin-)Peierls transition temperature and discuss several implications of the picture developed. The results are compared with the properties of the spin-Peierls system CuGeO₃.     

Mirror nesting and electron-hole asymmetry at repulsive superconducting pairing

The dependence of the superconducting order parameter Δ(k) on the momentum of the relative motion of a pair with a large total momentum K is numerically studied for the case of repulsive pairing with allowance for the kinematic and insulator constraints on the momentum transfer at scattering. The Fermi contour with nesting and mirror nesting, which is typical of cuprates and optimal for repulsion-induced superconductivity, lies in an extended vicinity of the saddle points of the dispersion law. A deviation from the mirror nesting cuts off the logarithmic singularity from below and bounds the pre-exponent in Δ(k). The effective coupling constant is determined by the degree of the electron-hole asymmetry. The suppression of the contribution of small momentum transfer processes by the impurity and electron-phonon scattering favors an increase in the order parameter amplitude. The nesting of the Fermi contour causes a Peierls singularity in the Coulomb interaction. The self-consistency equation allows the solutions that may be both antisymmetric and symmetric with respect to the momentum inversion. The maximum-amplitude antisymmetric solution in the case of a singlet pairing can be realized only for K ≠ 0.