Phenomenology of a-axis and b-axis charge dynamics from microwave spectroscopy of highly ordered YBa2Cu3O6.50 and YBa2Cu3O6.993

TL;DR

This study uses microwave spectroscopy on high-quality YBa2Cu3O6.50 and YBa2Cu3O6.993 crystals to investigate intrinsic charge dynamics in a d-wave superconductor, revealing long-lived excitations and anisotropic conduction contributions.

Contribution

It introduces a phenomenological model capturing energy-dependent quasiparticle dynamics and distinguishes the roles of CuO chains and CuO2 planes in charge transport.

Findings

Detection of long-lived quasiparticle excitations in the superconducting state.

Validation of the model with the Ferrell-Glover-Tinkham sum rule.

Identification of anisotropic conduction contributions from chains and planes.

Abstract

Extensive measurements of the microwave conductivity of highly pure and oxygen-ordered \YBCO single crystals have been performed as a means of exploring the intrinsic charge dynamics of a d-wave superconductor. Broadband and fixed-frequency microwave apparatus together provide a very clear picture of the electrodynamics of the superconducting condensate and its thermally excited nodal quasiparticles. The measurements reveal the existence of very long-lived excitations deep in the superconducting state, as evidenced by sharp cusp-like conductivity spectra with widths that fall well within our experimental bandwidth. We present a phenomenological model of the microwave conductivity that captures the physics of energy-dependent quasiparticle dynamics in a d-wave superconductor which, in turn, allows us to examine the scattering rate and oscillator strength of the thermally excited quasiparticles as functions of temperature. Our results are in close agreement with the Ferrell-Glover-Tinkham sum rule, giving confidence in both our experiments and the phenomenological model. Separate experiments for currents along the $\hat a$ and $\hat b$ directions of detwinned crystals allow us to isolate the role of the CuO chain layers in \YBCO, and a model is presented that incorporates both one-dimensional conduction from the chain electrons and two-dimensional transport associated with the \cuplane plane layers.