ENERGY AND OPTICAL ABSORPTION SPECTRUM OF AN INCOMMENSURATE CHARGE DENSITY WAVE (POTASSIUM, FAR-INFRARED, PROFILES)

FRANCISCO EUGENIO FRAGACHAN GAUDENS, Purdue University

Abstract

The conduction-electron spectrum is calculated for Na and K taking into account the periodic potential 2(alpha)cos(Q('(.))r)(' )of the charge-density wave. Accordingly, the Schrodinger equation includes this term in addition to a pseudopotential 2(beta)cos(G('(.))r)(' )caused by the ionic lattice. Q(' )and G(' )are nearly parallel, and differ in magnitude by only a few percent. As a consequence three families of higher-order gaps in E(k)(' )arise: "minigaps", characterized by wave vectors (n+1)Q-nG;(' )"heterodyne gaps", with periodicities n(G-Q);(' )"second-zone minigaps", with periodicities (n+1)G-nQ.(' )The energy gap surfaces of the first two families truncate the Fermi surface and cause the open orbits observed by Coulter and Datars in high-field magnetoresistance. They also lead to new optical absorption edges in the far infrared. The absorption peaks associated with the first three minigaps are calculated for Na and K, and found to be an order of magnitude larger than both the interband absorption and the main charge density wave peak. The absorption peaks associated with the heterodyne gaps are found to be about two orders of magnitude larger than the normal interband. However, both the minigap and heterodyne absorption peaks, are much smaller than the room-temperature Drude absorption. Consequently a search for such edges must be carried out at low temperatures where the intra- band transitions are drastically reduced, and in samples for which the orientation of Q(' )allows the observation of the Mayer-El Naby anomaly. Presumably, the heterodyne absorption peaks should be observable in evaporated films, unlike the absorption from the minigaps and the main CDW gap (Mayer-El Naby). The optical absorption of the minigaps and the main CDW gap are uniaxial. It is shown that the shape and magnitude of the Mayer-El Naby anomaly suggests a gap, 2(alpha) (TURNEQ) 0.6 ev. Otherwise, the magnitude of the anomaly is drastically reduced.

Degree

Ph.D.

Subject Area

Condensation

COinS