Pseudospin-Electron Model at Weak Coupling

Abstract

The static and dynamic dielectric susceptibility of the pseudospin-electron model is investigated in the random phase approximation. The possibility of the phase transition to the homogeneous phase, to the phase with doubly modulated lattice period and to the incommensurate phase is revealed in the regime µ = const depending on the value of the chemical potential. The phase separation on the homogeneous and phase with doubly modulated lattice period is established and the phase diagrams (n, h) are built. The influence of the tunnelling-like splitting on the phase transition picture is studied. The presence of pseudospin-wave excitations and the smooth excitation spectrum and the possibility of their superimposing are revealed.

Raman Light Scattering in Pseudospin-Electron Model at Strong Pseudospin-Electron Interaction

“…In this work we calculate Raman spectra for the system described by the pseudospin-electron model (PEM) using the method which is based on the construction of a polarizability operatorP in the framework of a microscopic approach; the method was developed in [6][7][8][9] and is an alternative to standard ones [2][3][4][5]. PEM describes the systems with locally anharmonic elements (anharmonic vibrations of apical oxygen O4 in HTSC of YBaCuO-type, the systems with hydrogen bonds, etc.).…”

“…PEM describes the systems with locally anharmonic elements (anharmonic vibrations of apical oxygen O4 in HTSC of YBaCuO-type, the systems with hydrogen bonds, etc.). In [9] we considered contributions to Raman spectra for the zz polarization case (z is a direction of local vibrations and it is perpendicular to the layer in the case of layered crystals of YBaCuO-type). Here we investigate the case of xx + yy polarization and compare these two cases.…”

“…Here we investigate the case of xx + yy polarization and compare these two cases. Like in [9], we consider the strong coupling case, when the interaction constants in the PEM are large in comparison with the width of electron band.…”

“…We use the expression for cross-section of Raman light scattering written in the following form ( [6][7][8][9][10]):…”

“…HereM α ( k) is a dipole moment of a crystal unit cell in the k-representation and the symbol {{M β ( k , t)|M α ( k, s)}} stands for "unaveraged" Green function defined in the following way ( [6,9]):…”