ANISOTROPY OF CHARGE-CARRIERS AND DIELECTRIC FUNCTION OF YBA2CU4O8 (Y124)

Orientation dependent (momentum transfer resolved) electron energy loss spectrometry (EELS) has been utilized to probe the localization of free carrier plasmons in single crystals of YBa2Cu4O8 (Y124). Two low energy excitations are observed in EELS spectra, one at about 1.4 eV which is the free carrier plasmon,and another excitation at about 3.2 eV. The oscillator strength of the free carrier plasmon is observed to vary considerably with momentum transfer. Maximum oscillator strength of the free carrier plasmon is observed when the momentum transfer (q) is confined to the ab planes, while the minimum strength is observed for q parallel to the c-axis. Dielectric function calculated using the Kramers-Kronig analysis indicates that the major differences in the dielectric function of Y124 within the ab plane versus along the c-axis are confined only within the first few eVs. The O K core loss spectra exhibit subtle but important deviations from that of YBa2Cu3O7-delta (Y123). Possible EELS spectral signatures of the double chains in Y124 are discussed.