Universal features of Thermopower in High Tc systems and Quantum Criticality

TL;DR

This paper explores the universal relationship between thermopower and doping in high Tc superconductors, proposing a quantum critical point as the underlying cause, supported by theoretical calculations and the Kelvin formula.

Contribution

It interprets the OCTHH universality in high Tc systems through the lens of quantum criticality and provides a theoretical framework using exactly solved models and the Kelvin formula.

Findings

Thermopower vanishes at optimal doping in high Tc superconductors.

A possible quantum critical point underlies the observed universal correlation.

The Kelvin formula relates thermopower to entropy derivatives, supporting the interpretation.

Abstract

In high Tc superconductors a wide ranging connection between the doping dependence of the transition temperature Tc and the room temperature thermopower Q has been observed. A "universal correlation" between these two quantities exists with the thermopower vanishing at optimum doping as noted by OCTHH (Obertelli, Cooper, Tallon, Honma and Hor). In this work we provide an interpretation of this OCTHH universality in terms of a possible underlying quantum critical point (QCP) at Tc. Central to our viewpoint is the recently noted Kelvin formula relating the thermopower to the density derivative of the entropy. Perspective on this formula is gained through a model calculation of the various Kubo formulas in an exactly solved 1-dimensional model with various limiting procedures of wave vector and frequency.