Long-range Electron-phonon Interaction Research Articles

Polaron features for long-range electron–phonon interaction

The polaron features for long-range electron–phonon interaction are investigated byextending a variational approach previously proposed for the study of systems with localcoupling. The ground state spectral weight, the average kinetic energy, the mean number ofphonons, and the electron–lattice correlation function are discussed for a wide range ofmodel parameters focusing on the adiabatic regime and comparing the results with theshort-range case (Holstein model). A strong mixing of electronic and phononic degrees offreedom for small values of the electron–phonon coupling constant is found in the adiabaticcase due to the long-range interaction. Finally a polaron ‘phase diagram’ is proposed.

Small adiabatic polaron with a long-range electron-phonon interaction

Two-site single electron system interacting with many vibrating ions of a lattice via a long-range (Fr\"{o}hlich) electron-phonon interaction is studied in the adiabatic regime. The renormalised hopping integral of small adiabatic Fr\"{o}hlich polarons is calculated and compared with the hopping integral of small adiabatic Holstein polarons.

Superlight High Tc Bipolarons in Cuprates

Most of the proposed models of high-temperature superconductivity (HTSC) are based on the short-range electron-electron correlations or/and on a short-range electron-phonon interaction. However, in the cuprates the screening is poor due to a low carrier density, layered crystal structure, and high ionicity of the lattice. We develop further the bipolaron model of HTSC, which explicitly takes into account the long-range origin of all interactions. The long-range electron-phonon (Fröhlich) interaction binds carriers into real space pairs (small bipolarons) with surprisingly low mass but sufficient binding energy, while the long-range Coulomb repulsion keeps them from forming larger clusters. The model has explained many key features of cuprates. It is shown here that real-space pairing takes place in many cuprates at variance with some (incorrect) criteria of the BCS-BEC crossover.

Isotope Effect on the Ferroelectric Phase Transitions in BaTiO 3, SrTiO 3 and PbTiO 3

We discuss theoretically the intriguing isotope effect (opposite trends for oxygen and for cations such as Ti or Ba) on the ferroelectric phase-transition temperatures in BaTiO 3, SrTiO 3 and PbTiO 3 . The transition temperature is defined as the balancing temperature of the two forces; the force reinforcing the ferroelectric (low-temperature) structure (the long-range electron-phonon interaction force), and the thermal fluctuations which will stabilize the paraelectric (higher temperature) structure. Oxygen (anion) isotope exchange induces mainly change in the thermal fluctuation; resulting in that with light isotope introduction the phase transition temperature lowers. The cation isotope exchange induces change in the long-range force; resulting in that with light isotope introduction the phase transition temperature rises.

Superlight Bipolarons Explain the High Superconducting Temperature

Over the last decade, several competing models of high-temperature superconductivity (HTSC) were proposed, none of which have succeeded to explain high values of the superconducting critical temperature T c without adjustable parameters. Most of the proposed models are based on the short-range electron-electron interaction or/and on a short-range electron-phonon interaction. Here we propose a model of HTSC which explicitly takes into account the long-range origin of both types of interaction. Remarkably, the long-range electron-phonon interaction binds carriers into superlight real-space pairs-bipolarons-with a relatively low mass but sufficient binding energy. The model quantitatively explains high T c values without any fitting parameters and other key features of the cuprates.

STATICS AND DYNAMICS OF COUPLED ELECTRON–PHONON SYSTEMS

We review recent variational methods that provide extremely accurate solutions for the problem of one and two electrons coupled to dynamical, quantum phonons, including retardation effects. The dynamics of these systems far from equilibrium is explored, including the dynamics of polaron formation after electron injection by an STM. An examination of three-point functions suggests a qualitative modification of the traditional picture of a polaron. Long-range electron–phonon interactions are studied in some generality, including vector and frustrated Fröhlich models, and bounds are obtained on the possible mass enhancement.

ELECTRON STRINGS IN OXIDES

We found that insulating and conducting electron strings may arise in oxide materials with not very wide bands, like HTSC, due to electron-phonon and electron-electron interactions including antiferromagnetic correlations. We estimate the string length and the number of particles self-trapped into a single string taking into account the typical parameters of cuprates, like La2CuO4. The conducting strings typically may have a small doping (n<0.3) and are created mostly due to the long-range electron-phonon interaction associated with ionic bonding of Oxides.

Angle-resolved photoemission in YBa 2Cu 3O 7 and YBa 2Cu 4O 8

We derive the coherent part of the ARPES spectra with the exact spectral function of a one-dimensional hole in a random potential, taking into account bipolaron formation due to the strong long-range electron-phonon interaction. Some unusual features of ARPES including the polarization dependence and spectral shape in YBa 2Cu 3O 7 and YBa 2Cu 4O 8 are described without any Fermi-surface, large or small. The theory is compatible with the doping dependence of kinetic and thermodynamic properties of cuprates as well as with the d-wave symmetry of the superconducting order parameter.

Polaron dynamics and bipolaron condensation in cuprates

Based on the exact cluster diagonalization and recent Quantum Monte Carlo simulations we analyze dynamic properties of small polarons and bipolarons formed by short-range (Holstein) and long-range (Fr\ohlich) electron-phonon interactions. We show that the exact results agree well with canonical Holstein theory for a cluster and with Lang-Firsov theory for a lattice. Lang-Firsov theory of a single polaron and our $1/\ensuremath{\lambda}$ perturbation expansion for a multipolaron system are practically exact in a wide range of the adiabatic parameter $\ensuremath{\omega}/t$ and the electron-phonon coupling \ensuremath{\lambda} for a long-range interaction. (Bi)polarons exist in the itinerant Bloch states at temperatures below the characteristic phonon frequency no matter which values the parameters of the system take. We show that recent claims by several authors with regards to a breakdown of Holstein-Lang-Firsov theory of a small polaron and the ``impossibility'' of bipolaronic superconductivity are the result of an erroneous interpretation of the electronic energy levels of the two-site Holstein model and a misunderstanding of the electron-phonon interaction in ionic solids with polaronic carriers. A ``phase'' diagram in $t/\ensuremath{\omega}\ensuremath{-}\ensuremath{\lambda}$ space is proposed to elucidate the BCS and (bi)polaronic domains. Bipolaron theory provides a parameter-free expression for the superconducting critical temperature of layered cuprates. Crystallization of the (bi)polaronic liquid is shown to be impossible in the range of the parameters typical for cuprates. The small Fr\ohlich polaron has spectral features compatible with single-particle tunneling and photoemission in cuprates.

Structural Phase Transformation and Phonon Softening in Iron-Based Alloys

The interrelation of Invar and Martensite in iron-based alloys like Fe 3 Ni is discussed on the basis of ab initio calculations for different crystal structures. Inelastic neutron scattering and ultrasonic experiments have shown that a soft transverse phonon mode in [110] direction acts as a precursor in the close packed fcc phase of these systems. Unfortunately there are no experimental results for phonon modes in the transformed bct phase. We have examined in detail the changes in the phonon dispersions of these systems by varying the ratio of the lattice constants along the Bain path. In recent calculations we have used the method of Varma and Weber in combination with the full potential LAPW (FLAPW) method to calculate frozen phonons for the tetragonal phase. Our new numerical results reproduce the experimental findings for the fcc phase and give an overview of the changes of the phonon energies when varying the c/a ratio towards the bct structure. In particular we observe that phonon softening is caused by the long-range electron-phonon interaction in systems with magnetovolume instabilities, and vanishes after the martensitic transformation.

Intervalley Gamma -X deformation potentials for top valence bands in III-V zincblende semiconductors by ab initio pseudopotential calculations

The authors have calculated Gamma -X hole-phonon scattering matrix elements for the top valence bands in nine III-V zincblende semiconductors AlP, AlAs, AlSb, GaP, GaAs, GaSb, InP, InAs and InSb, by ab initio pseudopotential calculations. At the same time, the authors have calculated the eigenfrequencies and eigenvectors of transverse acoustic (TA) and optic (TO) phonons at X in these materials, which agree very well with the available experimental results. They have evaluated the Gamma -X intervalley deformation potentials (IVDPs) for the longitudinal phonons, and transverse phonons, respectively. The IVDPs for the TAX phonon are very small, as expected from symmetry. The IVDPs for the TOX phonon do not vary appreciably from one semiconductor to another, averaging about 5 eV AA-1. IVDPs for the longitudinal phonon with X1 symmetry are generally larger than those of X3 symmetry. This is due to more screening to the long-range electron-phonon interaction for the X3 phonon by electron redistribution, resulting in smaller IVDPs.