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     

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     

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.     

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     

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。     

电荷密度波材料K0.3MoO3及W掺杂样品的红外光学响应的研究

研究了准一维材料K03MoO3以及掺杂了3%和15%的W的样品在TCDW=180K以上和以下的温度时的光反射谱和电导率谱.在此基础上讨论了伴随着Peierls相变产生的相声子和单粒子能隙,以及W掺杂对CDW凝聚的影响 关键词： 电荷密度波 Peierls相变     

Peierls相变与磁场中碳纳米管的场发射

应用紧束缚模型和WKB方法研究碳纳米管的out-of-plane型Peierls相变,及其对碳纳米管的场发射的影响.结果发现Peierls相变会在室温出现,并使碳纳米管费米面附近出现能隙,导致碳纳米管发生金属—半导体转变,从而抑制碳纳米管的场发射.磁场也会抑制Peierls形变,Peierls相变和磁场相互竞争影响碳纳米管的能带结构,从而影响碳纳米管的场发射. 关键词： 场发射 碳纳米管 Peierls相变     

准二维电荷密度波导体钾紫青铜KxMo6O17单晶样品的制备

采用碳酸盐替代高钼酸盐电解还原的方法成功制备出了准二维电荷密度波导体钾紫青铜单晶.通过x射线衍射、透射电子显微术等方法对单晶进行结构分析表明：晶胞参数a=b=0.5540nm，c=1.3508nm，单晶为三角晶系，对称群为P3.电阻温度关系曲线的测量显示：钾紫青铜K_xMo₆O₁₇单晶在112K附近发生金属到金属的Peierls相变. 关键词： 钾紫青铜 电荷密度波 Peierls相变     

盐对DNA相变影响的非线性特性研究

在Prohofsky，Peyrard-Bishop等提出的描述DNA双螺旋分子结构模型以及实验测量的基础上 , 给出了与盐(指NaCl)浓度有关的哈密顿模型, 得到了非线性动力学方程及扭结孤波解.并 由此求出了DNA变性相变所需要的Peierls相变力. 进一步讨论了盐浓度对相界面和相变力的 影响, 得到的结果与实验测量一致. 关键词： DNA 盐浓度 相界面 相变力     

低维体系的相变和叠套

与一维体系不同，二维体系费密面的叠套是不完全的，此种叠套的偏离会显著地减弱体系的不稳定性．定量地研究了叠套偏离对Peierls相变的影响，并确定了叠套偏离的限度，超过此值，Peierls相变被抑制，这有利于进入超导态． 关键词：     

Superconductivity and Peierls instability in coupled linear chain systems

We study the superconducting transition temperature (T_c) and the Peierls instability temperature (T_p) using Eliashberg type equations for both T_c and T_p self consistently with finite interchain coupling. We show that T_c > T_p below a critical electron-phonon coupling constant which depends on the bare phonon frequency. This determines an upper bound on T_c so that for higher transition temperatures T_p > T_c and superconductivity is unlikely. Higher values of T_c are possible if the interchain coupling is increased above a critical value where the Peierls instability is suppressed.     

Correlation length of the Peierls distortion

A Peierls distortion incommensurate with the lattice constant is considered in the Fröhlich model of a linear conductor. A prediction for the correlation length of the distortion is obtained using a Ginzburg-Landau expansion of the free energy for temperatures well belowT _p , the mean field Peierls transition temperature. The correlation length is found to be strongly increasing at aboutT _p /4.     

Hysteresis in the electrical properties of orthorhombic tantalum trisulphide: Evidence for an incommensurate-commensurate charge-density wave transition?

A study of electrical conduction in orthorhombic TaS₃ has revealed the existence of thermal hysteresis throughout the temperature range 55 K < T < 205 K. This is attributed to variability in the wavevector q of the charge-density wave (CDW) which develops below T_p = 215 K, and confirms the recent finding, from electron diffraction, that at temperatures not too far below T_p the CDW is incommensurate with the underlying lattice. Evidence that q becomes commensurate, at least along the chain direction at 55 K is provided by the vanishing of hysteresis at that temperature, and also by a rise in the threshold field for continuous motion of the CDW.From its dependence on temperature it is concluded that between T_p and 55 K the conduction in the linear regime is better described as that of a Peierls semi-metal, rather than that of a Peierls intrinsic semiconductor. At most temperatures within that range electrical hysteresis also is observed, and a detailed study of this leads to the tentative conclusion that translation of the CDW conveys negative charge, carried presumably by negatively-charged discommensurations. The mechanisms of conduction below 55 K remain uncertain.     

The Peierls instability in one-dimensional conductors with arbitrary bandfilling

Mean field calculations of the Peierls instability have been carried out for arbitrary bandfilling both in the case of free electrons as well as for the tightbinding approximation. New expressions for Δ(0) and T_p are derived which change the experimentally derived λ to a more reasonable value. Δ(0)/k_BT_p may differ significantly from the BCS value 1.76 in the tightbinding approximation.     

Specific heat studies of the spin-peierls transition

An experimental study is presented of the specific heat of the spin—Peierls systems TTF-TBT(M), M = Cu, Au. The linear term, γT, expected for the uniform one-dimensional antiferromagnet at low temperatures is observed above the Peierls transition. Specific heat anomalies associated with the transition are found at T_p(Cu) = 12.4K and T_p(Au) = 2.06K. The transition for TTF-BDT(Cu) is mean-field like; whereas the transition for TTF-BDT(Au) is broadened by one-dimensional fluctuation effects.     

A New Type of Charge-Density-Wave Pinning in Orthorhombic TaS3 Crystals with Quenching Defects

Diminishing in the concentration of quenching defects during thermocycling of orthorhombic TaS₃ samples in the temperature range below the Peierls transition temperature T < T_P is observed. It makes it possible to study the character of pinning of the charge density wave by these defects. A number of fundamental differences from pinning by ordinary local pinning centers—impurities and point defects—have been found. We conclude that quenching defects are extended (non-local) objects (presumably, dislocations) that can diffuse from the crystal during low-temperature thermocycling due to their strong interaction with the charge density wave, which is intrinsic for Peierls conductors. The presence of these defects leads to a previously unknown non-local type of the charge-density-wave pinning that acts on T_P and the threshold field for the onset of charge-density-wave sliding, E_T, differently in comparison with the local pinning centers.

     

First-order calculation of the phonon propagator of a quasi-one-dimensional electron-phonon system

Using Green function technics the phonon propagator is calculated consistently in the first order with respect to both the square of the electron-phonon interaction matrix element and the matrix element of the Coulomb interaction of the electronic subsystem. It is shown that the so-called self-energy and exchange contributions lead to an increase of the Peierls temperatureT _p in comparison with the result of the usual random-phase approximation (RPA), whereas the Umklapp processes tend to lowerT _p .     

Energy gap of a charge density wave in NbSe3 induced by a high magnetic field above the Peierls transition temperature

The effect of a magnetic field on the energy gap of the charge density wave (CDW) in NbSe₃ near the temperature T _p2 of the lower Peierls transition has been investigated using interlayer tunneling spectroscopy. It has been shown that the magnetic field increases the energy gap and can even induce it at temperatures higher than T _p2 by 15–20 K. As the field strength increases, the peak amplitude of the gap singularity of the tunneling spectrum first increases, reaches its maximum at 20–30 T, and then decreases. The increase in the gap peak amplitude is attributed to the field-induced improvement of the condition of the CDW nesting, while the decrease in the amplitude in high fields, to the breakdown of the ground state caused by its Zeeman splitting.     

On the Peierls transition in a periodic potential

The properties of a one-dimensional conductor in the presence of an external periodic potential V_Q have been investigated. The low temperature gap is enhanced, the temperature dependence of the gap Δ(T) is smeared at T_PF, the Peierls transition temperature in the absence of V_Q, and hence there is no longer a real phase transition. We also relate the phase pinning frequency ω_ø to the size of V_Q.     

Fluctuation conductivity near a Peierls lattice distortion

There have been a number of recent theories for the fluctuation conductivity due to phonon softening near a one-dimensional Peierls lattice distortion. In these, the conductivity is evaluated for one or more Feynman response diagrams, with a resulting fluctuation conductivity which is positive, negative, or negligible. We calculate the conductivity using a conserving response theory which derives all of the response diagrams for any particular approximation to the electron propagator for the solid in thermodynamic equilibrium. Using this method in the mean-field theories to date, valid in principle for sufficiently high temperatures, the fluctuation conductivity due to phonon softening is positive and proportional to ($\text{T - T}{}_{\mathrm{p}}\text{)}{}^{\mathrm{<mtext>-</mtext><mtext>3</mtext><mtext>2</mtext>}}$, where T_p is the mean-field temperature scale in the one-dimensional Peierls system. This conductivity contribution is not due to the Fröhlich sliding mode.