Charge Density Wave Instabilities And Incommensurate Structural Phase Transformations

Incommensurate structural phase transformations involve the appearance of modulated atomic displacements with spatial periodicity unrelated to the fundamental periodicity of the basic lattice. In the case of some quasi one- or two-dimensional metals such transformations are the result of Fermi-surface instabilities that also produce electronic charge density waves (CDW's) and soft phonon modes due to metallic electron screening singularities. Incommensurate soft mode instabilities have been found in insulators as well. Recent neutron scattering studies of both the statics and dynamics of incommensurate structural instabilities will be reviewed.

The latest addition to this series covers a field which is commonly referred to as charge density wave dynamics. The most thoroughly investigated materials are inorganic linear chain compounds with highly anisotropic electronic properties. The volume opens with an examination of their structural properties and the essential features which allow charge density waves to develop. The behaviour of the charge density waves, where interesting phenomena are observed, is treated both from a theoretical and an experimental standpoint. The role of impurities in statics and dynamics is considered and an examination of the possible role of solitons in incommensurate charge density wave systems is given. A number of ways to describe charge density waves theoretically, using computer simulations as well as microscopical models, are presented by a truely international board of authors.

Some of the most interesting structural phase transformations under current study involve modes of atomic displacement that are incommensurate with the underlying lattice. In some cases, these transformations are driven by Fermi surface instabilities that also produce electronic charge density waves and soft phonon modes due to dielectric screening singularities. The periodic potential of the underlying lattice causes complex non-sinusoidal distortions of the condensed displacement waves, and may lead to subsequent transformations which ''lock in'' the period of the displacements with that of the lattice. Recent neutron scattering studies bearing on both static and dynamic aspects of incommensurate structural instabilities will be reviewed. 42 refs.

This thesis presents analytical theoretical studies on the interplay between charge density waves (CDW) and superconductivity (SC) in the actively studied transition-metal dichalcogenide 1T-TiSe2. It begins by reapproaching a years-long debate over the nature of the phase transition to the commensurate CDW (CCDW) state and the role played by the intrinsic tendency towards excitonic condensation in this system. A Ginzburg-Landau phenomenological theory was subsequently developed to understand the experimentally observed transition from commensurate to incommensurate CDW (ICDW) order with doping or pressure, and the emergence of a superconducting dome that coexists with ICDW. Finally, to characterize microscopically the effects of the interplay between CDW and SC, the spectrum of CDW fluctuations beyond mean-field was studied in detail. In the aggregate, the work reported here provides an encompassing understanding of what are possibly key microscopic underpinnings of the CDW and SC physics in TiSe2.

Phase transitions in which crystalline solids undergo structural changes present an interesting problem in the interplay between the crystal structure and the ordering process. This text, intended for readers with some prior knowledge of condensed-matter physics, emphasizes the basic physics behind such spontaneous structural changes in crystals. Starting with the relevant thermodynamic principles, the book discusses the nature of order variables and their collective motion in a crystal lattice; in a structural phase transition a singularity in such a collective mode is responsible for the lattice instability, as revealed by soft phonons. This mechanism is analogous to the interplay of a charge-density wave and a periodically deformed lattice in low-dimensional conductors. The text also describes experimental methods for modulated crystal structures and gives examples of structural changes in representative systems. The book is divided into two parts. The first, theoretical, part includes such topics as: the Landau theory of phase transitions; statistics, correlations and the mean-field approximation; pseudospins and their collective modes; soft lattice modes and pseudospin condensates; lattice imperfections and their role in the phase transitions of real crystals. The second part discusses experimental studies of modulated crystals using x-ray diffraction, neutron inelastic scattering, light scattering, dielectric measurements, and magnetic resonance spectroscopy.

Neutron and x-ray scattering study on K0.3MoO3 and other quasi one dimensional conductors -- X-ray study of charge-density wave in K0.30Mo03 under electric fields -- Transmission electron microscopy for imaging and diffraction studies of low dimensional transition metal chalcogenides -- Aspects of charge-density waves in the TaTe4-NbTe4 structures and in 2H-TaSe2 -- Charge density waves, phasing, sliding and related phenomena in NbSe3 and other transition metal chalcogenides -- Structural and electrical properties interpretation through band structure calculations on the (MSe4)nI SERIES (M = Nb, Ta). -- Defects and charge density waves in irradiated layer and chain compounds -- Electron diffraction charge density wave studies in the chalcogenide compounds (MX4)nI -- Neutron studies of the blue bronzes K0.3MoO3 and Rb0.3MoO3 -- The effect of a magnetic field on the discotm1ensurate to commensurate transition in 2H TaSe2 -- High pressure investigation of the cdw phase diagram of 1T-TaS2 -- Landau theory of 2H-TaSe2 -- Multidomain structures of incommensurate phases in CDW states of 2H-TaSe2 -- Electron microscopy of charge density wave defects in 1T-TaS2 and 1T-TaSe2 -- Aspects of strong electron-phonon coupling related to the CDW transition at temperatures above it -- Elastic and other properties at the commensurate-incommensurate transition in 2H-TaSe2 -- CDW phase mode investigation in the FIR in K0.3MaO3 and band structure calculation -- 93Nb NMR study of CDW in (NbSe4)10/3I single crystal -- Electronic Properties and Fe57 Mössbauer measurements of T1+xNb3-xSe10 with T = Fe, Cr -- Transport and Mössbauer studies of the peierls transition in Fe-doped K0.30MoO3 -- Charge density wave instabilities in quasi two-dimensional oxides n-Mo4O11 and ?-Mo4O11 -- Thermal conductivity of layered dichalcogenides -- Tunneling study of commensurate charge density wave states in 1T-TaS2 -- Galvanomagnetic properties of the quasi-two dimensional purple bronze K0.9Mo6O17 -- Non-local elastic forces in charge-density wave systems -- Soliton model of charge-density-wave depinning -- Dynamics of incommensurate structures -- Some problems arising from electrostatic potential in CDW behavior -- The single domain model of charge-density wave transport -- On the microscopic theory of kinetic phenomena in peierls conductors -- Near commensurability effects on charge density wave dynamics -- Shift in the longitudinal sound velocity due to sliding charge density waves -- Microscopic local mechanisms for "Noises" generated by moving CDW -- Phase vortices and CDW conduction noise -- Damping of CDW-condensate motion by interaction with thermal phasons -- Microscopic theory of interaction of CDW with impurities -- Quantum effects in the Josephson approach to a CDW -- Fokker planck theory of the classical charge density wave model with current noise -- Travelling charge density waves : A mean field treatment -- Coherent and incoherent effects in charge density wave transport -- Threshold field, electrical conductivity and time-dependent voltage in transition metal tri- and tetrachalcogenides -- Solitons in TaS3 experiment -- Thermal gradient experiments on the charge-density-wave conduction noise spectrum -- Broadband noise in orthorhombic TaS3 -- High field I-V characteristics of orthorhombic TaS3 -- Inertial dynamics of CDW transport in NbSe3 -- Frequency dependent conductivity of CDW compounds -- AC conductivity of the blue bronze K0.3 MoO3 -- Subharmonic shapiro steps, devil's staircase, and synchronization in RF-driven CDW conductors -- Mode locking and chaos in sliding charge-density-wave systems -- Chaos in charge density wave systems -- Contribution of CDW motion to the hall effect and to the transverse conductivity in TaS3. experiment -- Contribution of CDW motion to the hall effect and to the transverse conductivity. Theory -- Dependence of the elastic modulus of TaS3 on the CDW current -- Low frequency elastic properties of materials containing a sliding CDW -- The conductivity of orthorhombic TaS3 under uniaxial strain -- Ohmic and nonlinear transport of (TaSe4)2I under pressure -- Pinning, metastability and sliding of charge-density-waves -- Distortion, metastability and breaking in charge-density wave transport: Recent experiments on niobium triselenide, suggesting a new mean-field approach -- Bistable configurations of the pinned charge density wave: Random-field-model dynamics observed in rearrangement prior to depinning -- Electric field induced relaxation of metastable states in TaS3 -- Remanent deformation of CDWs -- Relaxation of the deformed cdw state: electric and thermal hysteresis -- Thermal hysteresis in the thermopower of o-TaS3 -- Delayed switching between normal and CDW conducting states in o-TaS3 -- The effect of uniaxial strain on metastable states in TaS3 -- Influence of defects on the metastable states of o-TaS3 -- Charge density wave transport in the blue bronzes K0.30MoO3 and Rb0.30MoO3 : metastability, hysteresis and memory effects -- Effects of irradiation on the blue bronzes K0.30MoO3 and Rb0.30MoO3 -- Relaxation of metastable states in blue bronze K0.3MoO3 -- Incommensurate ferroelectrics -- Commensurate and incommensurate phases of a two-dimensional lattice of superconducting vortices -- (TMTSF)2X compounds: Superconductivity, spin-density waves and anion ordering -- Impurity pinning in quasi-1D superconductivity -- Numerical studies of the effect of a wall on SDW in a jellium -- Pinning of amplitude solitons in Peierls systems with impurities -- New resistive state in low dimensional superconductor TaSe3 -- Switching in cdw systems and in V02 -- A comparative study -- The effect of varying the bandfilling in a Peierls conductor -- Solitons and polarons in a spin density wave chain -- Charge density waves in superionic conductors -- Numerical study of impurity pinning in one-dimensional interacting electron systems -- Multivalued charge-density waves.

This NATO Advanced Study Institute was the fourth in a series devoted to the subject of phase transitions and instabilities with particular attention to structural phase transforma~ions. Beginning wi th the first Geilo institute in 19'(1 we have seen the emphasis evolve from the simple quasiharmonic soft mode description within the Landau theory, through the unexpected spectral structure re presented by the "central peak" (1973), to such subjects as melting, turbulence and hydrodynamic instabilities (1975). Sophisticated theoretical techniques such as scaling laws and renormalization group theory developed over the same period have brought to this wide range of subjects a pleasing unity. These institutes have been instrumental in placing structural transformations clearly in the mainstream of statistical physics and critical phenomena. The present Geilo institute retains some of the counter cul tural flavour of the first one by insisting whenever possible upon peeking under the skirts of even the most successful phenomenology to catch a glimpse of the underlying microscopic processes. Of course the soft mode remains a useful concept, but the major em phasis of this institute is the microscopic cause of the mode softening. The discussions given here illustrate that for certain important classes of solids the cause lies in the electron phonon interaction. Three major types of structural transitions are considered. In the case of metals and semimetals, the electron phonon interaction relie6 heavily on the topology of the Fermi surface.

The structural phase transition is one of the most fundamental problems in solid state physics. Layered transition-metal dichalcogenides provide us with a most exciting area for the study of structural phase transitions that are associated with the charge density wave (CDW). A large variety of structural phase transitions, such as commensurate and incommensurate transitions, and the physical proper ties related to the formation of a CDW, have been an object of intense study made for many years by methods employing modem microscopic techniques. Rather recently, efforts have been devoted to the theoretical understanding of these experimental results. Thus, McMillan, for example, has developed an elegant phenomenological theory on the basis of the Landau free energy expansion. An extension of McMillan's theory has provided a successful understanding of the successive phase transitions observed in the IT- and 2H-compounds. In addition, a microscopic theory of lattice instability, lattice dynamics, and lattice distortion in the CDW state of the transition-metal dichalcogenides has been developed based on their electronic structures. As a result, the driving force of the CDW formation in the IT- and 2H-compounds has become clear. Furthermore, the effect of lattice fluctuations on the CDW transition and on the anomalous behavior of various physical properties has been made clear microscopically.