Observation of out-of-phase bilayer plasmons in YBa_2Cu_3O_7-delta

TL;DR

This study investigates the temperature-dependent c-axis optical conductivity in YBa_2Cu_3O_7-delta, revealing a new out-of-phase bilayer plasmon mode that varies with doping and explains previously mysterious infrared features.

Contribution

It provides the first observation and explanation of out-of-phase bilayer plasmons in YBa_2Cu_3O_7-delta using a layered crystal structure model.

Findings

Identification of a new MIR peak as a transverse out-of-phase bilayer plasmon.

Plasmon frequency shifts to lower values with decreasing doping.

Correlation of the plasmon with previously unexplained FIR features in underdoped cuprates.

Abstract

The temperature dependence of the c-axis optical conductivity \sigma(\omega) of optimally and overdoped YBa_2Cu_3O_x (x=6.93 and 7) is reported in the far- (FIR) and mid-infrared (MIR) range. Below T_c we observe a transfer of spectral weight from the FIR not only to the condensate at \omega = 0, but also to a new peak in the MIR. This peak is naturally explained as a transverse out-of-phase bilayer plasmon by a model for \sigma(\omega) which takes the layered crystal structure into account. With decreasing doping the plasmon shifts to lower frequencies and can be identified with the surprising and so far not understood FIR feature reported in underdoped bilayer cuprates.