Optical conductivity near finite-wavelength quantum criticality

TL;DR

This paper investigates how optical conductivity behaves near a finite-wavelength quantum critical point, revealing diverse responses depending on system parameters and challenging previous interpretations of experimental data.

Contribution

It demonstrates that coupling to collective modes introduces nonuniversal energy scales, leading to varied optical responses near criticality, which clarifies interpretations of cuprate experiments.

Findings

Optical conductivity vanishes in clean Galilean-invariant systems without coupling to collective modes.

Coupling to collective modes creates nonuniversal energy scales affecting critical behavior.

Diverse optical responses near criticality can include scaling peaks, fixed-frequency peaks, or no peaks.

Abstract

We study the optical conductivity sigma(Omega) of an electron system near a quantum-critical point with finite-wavelength ordering. sigma(Omega) vanishes in clean Galilean-invariant systems, unless electrons are coupled to dynamical collective modes, which dissipate the current. This coupling introduces a nonuniversal energy scale. Depending on the parameters of each specific system, a variety of responses arise near criticality: scaling peaks at a temperature- and doping-dependent frequency, peaks at a fixed frequency, or no peaks to be associated with criticality. Therefore the lack of scaling in the far-infrared conductivity in cuprates does not necessarily call for new concepts of quantum criticality.