Powerlaw optical conductivity with a constant phase angle in high Tc superconductors

TL;DR

This paper identifies a universal phase angle behavior in the optical conductivity of high Tc superconductors, suggesting the presence of an unconventional quantum phase transition near optimal doping.

Contribution

It reveals a universal phase angle pattern linked to quantum criticality in cuprates, providing new insights into their quantum phase transition nature.

Findings

Phase angle matches the optical conductivity exponent

Universal behavior observed in the quantum critical region

Supports the existence of an unconventional QPT in cuprates

Abstract

In certain materials with strong electron correlations a quantum phase transition (QPT) at zero temperature can occur, in the proximity of which a quantum critical state of matter has been anticipated. This possibility has recently attracted much attention because the response of such a state of matter is expected to follow universal patterns defined by the quantum mechanical nature of the fluctuations. Forementioned universality manifests itself through power-law behaviours of the response functions. Candidates are found both in heavy fermion systems and in the cuprate high Tc superconductors. Although there are indications for quantum criticality in the cuprate superconductors, the reality and the physical nature of such a QPT are still under debate. Here we identify a universal behaviour of the phase angle of the frequency dependent conductivity that is characteristic of the quantum critical region. We demonstrate that the experimentally measured phase angle agrees precisely with the exponent of the optical conductivity. This points towards a QPT in the cuprates close to optimal doping, although of an unconventional kind.