Influence of charge density waves and spin density waves on optical absorption and lattice dynamics

Yashu Xuan, Purdue University

Abstract

An incommensurate charge density wave (CDW) arises from, and causes, an extra periodic potential acting on the conduction electrons. The presence of this potential in the one-electron Schrodinger equation results in energy gaps in the energy spectrum E$(\vec{\rm k})$ which, for jellium, would have been merely $\hbar\sp2$k$\sp2$/2m. New optical transitions from filled states (below the Fermi level) to empty states (above) lead to a uniaxial absorption (the CDW optical conductivity). This new optical absorption, which has explained the Mayer-El Naby anomaly in metallic potassium, can also explain the anomaly (similar to the Mayer-El Naby) found in bulk metallic sodium. A generalized treatment of the ellipsometric method which recognizes the anisotropy of the CDW absorption in evaporated films can explain the excess absorption just below the Wilson-Butcher threshold for Na films. Spin density waves (SDWs) have built-in charge modulations, equal in magnitude but opposite in sign, for the two spin states. A small shift in the relative phase of the spin-up and spin-down modulations creates an additional charge response, and causes a peak in the response function $Q(\vec{\rm q})$ for $\vec{\rm q}$ near $\pm\vec{\rm Q}$, where $\vec{\rm Q}$ is the SDW wave vector. When this spin-split-phase contribution to $Q(\vec{\rm q})$ is incorporated into the theory for phonon spectra of metals, the anomalous depression of the two lower modes near (1/3,1/3,0) for zinc and cadmium can be understood. The dynamic pseudopotential theory for phonons in metals involving the shell model, which divides each ion into an inner core and an outermost filled electron shell, is applied to calculate the phonon spectra of the divalent hexagonal-close-packed (hcp) metals Mg, Zn, and Cd. The outermost filled L shell of sodium, set into oscillation by the electric field of a photon, leads to an extra term in the interband matrix element. This term interferes with the $\vec{\rm A}\cdot\vec{\rm p}$ term in the Wilson-Butcher tail, and explains the extra optical absorption peak in the ultra-violet.

Degree

Ph.D.

Advisors

Overhauser, Purdue University.

Subject Area

Condensation

COinS