Raman scattering in K2Pt(CN)4Br0.3 · 3H2O

Raman scattering experiments on K₂Pt(CN)₄Br_0.3 · 3H₂O are reported between 5 and 300 K as a function of temperature. A line of A₁ symmetry detected at 44 cm^?1 shows interesting temperature dependent properties. It is concluded from a comparison of the frequency, symmetry, and scattering intensity of this line with theoretical predictions that the excitation concerned represents the amplitude mode of the charge density wave (the line observed in infrared absorption being the phase mode). No Peierls transition is observed, but the results are consistent with a Peierls distortion present at all temperatures. The findings are correlated with inelastic neutron scattering and infrared studies. Finally, the CN stretching modes at 2189 and 2173 cm^?1 and the water mode at 3490 cm^?1 are studied as a function of temperature.     

Coulomb effects on the Peierls transition

The influence of the intrachain Coulomb interactions on the Peierls transition is examined. The change of the form of the electronic polarizability at twice the Fermi wavevector, to a power law divergence in the presence of Coulomb interactions, is found to enhance the mean field transition temperature for the Peierls transition.     

Intramolecular phase phonon absorptions as probes of the donor and acceptor chain contributions to the phase transitions of TTF-TCNQ

Below the metal-insulator transition the infrared absorption spectra (4000-200 cm^-1) of TTF-TCNQ powders display antiresonance dips above the electronic gap and two phase phonon absorptions at 317 and 253 cm^-1 attributable to the interaction of electrons with intramolecular modes of TTF and TCNQ. The temperature dependence of the two absorptions shows that the 54 K metal-insulator transition is driven by the Peierls distortion on the TCNQ sublattice whereas the distortion on the TTF chains increases markedly around the 49 K phase transition.     

Peierls mechanism of the metal-insulator transition in ferromagnetic hollandite K2Cr8O16

Synchrotron x-ray diffraction experiment shows that the metal-insulator transition occurring in a ferromagnetic state of a hollandite K(2)Cr(8)O(16) is accompanied by a structural distortion from the tetragonal I4/m to monoclinic P112(1)/a phase with a √2×√2×1 supercell. Detailed electronic structure calculations demonstrate that the metal-insulator transition is caused by a Peierls instability in the quasi-one-dimensional column structure made of four coupled Cr-O chains running in the c direction, leading to the formation of tetramers of Cr ions below the transition temperature. This provides a rare example of the Peierls transition of fully spin-polarized electron systems.     

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.     

Mechanism of conduction in TTF and TMTSF organic conductors from a study of their vibrational spectra

The charge transfer complexes of organic donors TTF and TMTSF have been prepared and studied with infrared spectroscopy. The nature of transition has been studied by analyzing features of absorption. TTF-TCNQ was found to be a Peierls semiconductor and not metallic. This shows that the mean field transition temperature is operative in TTF-TCNQ. TMTSF-TCNQ and TMTSF-DDQ showed lesser band gap than that of TTF-TCNQ. TTF-DDQ and TTF-I₂ also showed very small band gap and were more conducting than TTF-TCNQ. The band gaps could be assigned to either the Peierls gap or the pinning gap of charge density waves.     

Theory of the soft phonon mode and dielectric constant below the Peierls transition temperature

We discuss, within mean field theory, the behavior of the soft phonon mode and static dielectric constant below the Peierls transition temperature in a one-dimensional band conductor.     

Analytical approach to the quantum-phase transition in the one-dimensional spinless Holstein model

We study the one-dimensional Holstein model of spinless fermions interacting with dispersion-less phonons by using a recently developed projector-based renormalization method (PRM). At half-filling the system shows a metal-insulator transition to a Peierls distorted state at a critical electron-phonon coupling where both phases are described within the same theoretical framework. The transition is accompanied by a phonon softening at the Brillouin zone boundary and a gap in the electronic spectrum. For different filling, the phonon softening appears away from the Brillouin zone boundary and thus reflects a different type of broken symmetry state.     

Mass waves and the metal-semiconductor transition in one-dimensional systems of the type K2Pt(CN)4Br0.3 2.3H2O

A coupling mechanism of the chain electrons in partially oxydized Tetracyanoplatinates (TCNP) induced by the tunneling of ions in the intermediate medium can explain the existence of a Peierls instability in these compounds. Excitations of mass waves play the role of soft phonons inducing the electronic transition.     

Nonsphericity of atomic wave functions in the theory of Peierls-type structural phase transition

A theory of Peierls-type metal-semiconductor phase transition has been developed. The proposed model accounts for the cubic anharmonicity of interatomic interaction and the nonsphericity of atomic wave functions of electronic quantum states. The wave function overlapping results in the formation of a quasi-one-dimensional Peierls band in the electronic spectrum of the system. It is shown that structural transformations of the crystal lattice upon the phase transition represent a pairwise approach of atoms along the crystal axis C and a uniform strain of atomic chains taking on a zigzag shape. The theoretical results obtained are applied to the interpretation of experimental data on the metal-semiconductor phase transition in vanadium dioxide.     

Solving Energy Levels for SSH Hamiltonian Describing Peierls Phase Transition by Virtue of Invariant Eigen-operator Method

We show that the recently proposed invariant eigen-operator （IEO） method can be successfully applied to solving energy levels for SSH Hamiltonian describing Peierls phase transition. The electronic energy band of compound lattice is also studied by IEO method.     

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     

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.     

Peierls and spin-Peierls phase transitions in structurally unstable quasi-one-dimensional solids

The Peierls and spin-Peierls phase transitions are studied in solids in which a structural instability is already present. It is found that the presence of this intrinsic mode can increase considerably the critical temperature. For small values of the critical temperature, the transition is of the BCS-type, like the Peierls (or spin-Peierls) phase transition, but with an effective electron (or spin)-phonon coupling constant renormalized by the anharmonicity and by the instability of the phonon. Numerical results are also presented for larger critical temperatures. Then the BCS behaviour is no longer observed.     

The local field effect and metal-insulator transitions

Possible local field effects which accompany the metal-insulator transition have been investigated. It is shown that the intensity of light absorption by an impurity molecule may increase several times in dielectric phase due to local field corrections. A simple model of Peierls transition is explored.     

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.     

Orientation dependence of screw dislocation motion in b.c.c. transition metals

A tight-binding type electronic theory is used to calculate the Peierls stress of the screw dislocation in b.c.c. transition metals. The repulsive core-core interaction energies are simulated by Born-Mayer type potentials as well as the modified Born-Mayer potentials. It is shown that there are essential differences in the orientation dependence of the screw dislocation motion among the transition metals, in agreement with experiments.     

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。     

Electron correlation in C(4N+2) carbon rings: aromatic versus dimerized structures

The electronic structure of C(4N+2) carbon rings exhibits competing many-body effects of Huckel aromaticity, second-order Jahn-Teller (SOJT), and Peierls instability at large sizes. This leads to possible ground state structures with aromatic, bond angle, or bond length alternated geometry. Highly accurate quantum Monte Carlo results indicate the existence of a crossover between C10 and C14 from bond angle to bond length alternation. The aromatic isomer is always a transition state. The gap opening mechanism is the SOJT effect, which coalesces with the Peierls regime as N-->infinity.