Optical Conductivity of Ortho-II YBa$_2$Cu$_3$O$_6.5$

TL;DR

This paper investigates the optical conductivity of Ortho-II YBa$_2$Cu$_3$O$_6.5$, revealing how band splitting and interband transitions influence anisotropic optical features and provide insights into quasiparticle behavior.

Contribution

It introduces a model linking interband transitions to optical anisotropy and quasiparticle spectra in Ortho-II YBa$_2$Cu$_3$O$_6.5$, incorporating band splitting effects from LDA calculations.

Findings

Interband transitions cause anisotropic features in optical conductivity.

Edge position of features relates to temperature-dependent quasiparticle spectrum.

Bilayer splitting could produce a double edge shape in optical spectra.

Abstract

The Ortho-II phase of YBa$_2$Cu$_3$O$_6.5$ is characterized by a periodic alternation of empty Cu and filled Cu-O b-axis doubling the unit cell in the a direction. The extra oxygen in the full chains gives rise to an attractive potential for the holes in the planes. The planar bands split in two with a gap opening at the new Brillouin zone boundary $k_x=\pm pi/2$, which we estimate from LDA calculations. Using a planar model which treats the d-wave superconductivity in a mean field approximation, we show that interband transitions produce a strongly anisotropic feature in the optical conductivity controlled by a region in k-space close to ($\pi/2,\pi/2$). The edge position of this feature gives information on the temperature dependence of quasiparticle spectrum in this region. Bilayer splitting would show up as a double edge shape.