Characterization of two electronic subsystems in cuprates through optical conductivity

TL;DR

This study interprets optical conductivity in cuprates using a Fermi liquid framework, revealing a second gapped electronic subsystem that explains non-Fermi-liquid behavior and normal state properties across different doping levels.

Contribution

The paper introduces a method to separate Fermi-liquid and non-Fermi-liquid components in optical spectra of cuprates, providing a unified understanding of their normal state properties.

Findings

Identification of a gapped, non-FL spectral feature in cuprates.

Extension of FL/non-FL separation across the entire phase diagram.

Explanation of superfluid density attenuation in overdoped cuprates.

Abstract

Understanding the physical properties of unconventional superconductors as well as of other correlated materials presents a formidable challenge. Their unusual evolution with doping, frequency, and temperature has frequently led to non-Fermi-liquid (non-FL) interpretations. Optical conductivity is a major challenge in this context. Here, the optical spectra of two archetypal cuprates, underdoped HgBa$_2$CuO$_{4+\delta }$ and optimally-doped Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta }$, are interpreted based on the standard Fermi liquid (FL) paradigm. At both dopings, perfect frequency-temperature FL scaling is found to be modified by the presence of a second, gapped electronic subsystem. This non-FL component emerges as a well-defined mid-infrared spectral feature after the FL contribution -- determined independently by transport -- is subtracted. Temperature, frequency, and doping evolution of the MIR feature identify a gapped rather than dissipative response. In contrast, the dissipative response is found to be relevant for pnictides and ruthenates. Such an unbiased FL/non-FL separation is extended across the cuprate phase diagram, capturing all the key features of the normal state and providing a natural explanation why the superfluid density is attenuated on the overdoped side. Thus, we obtain a unified interpretation of optical responses and transport measurements in all analyzed physical regimes and all analyzed compounds.