Signatures of nematic quantum critical fluctuations in the Raman spectra of lightly doped cuprates

TL;DR

This paper investigates how nematic quantum critical fluctuations influence Raman spectra in lightly doped cuprates, revealing material-specific roles of nematic and charge-ordering modes in spectral features.

Contribution

It introduces a phenomenological model linking nematic quantum critical fluctuations to Raman spectral anomalies in lightly doped cuprates, highlighting differences between materials.

Findings

Longitudinal nematic fluctuations explain Raman absorption in B2g channel.

Charge-ordering modes are needed for La2-xSrxCuO4 to match spectral shoulders.

Different modes' involvement suggests distinct doping evolutions of nematic states.

Abstract

We consider the lightly doped cuprates Y$_{0.97}$Ca$_{0.03}$BaCuO$_{6.05}$ and La$_{2-x}$Sr$_x$CuO$_4$ (with $x=0.02$,0.04), where the presence of a fluctuating nematic state has often been proposed as a precursor of the stripe (or, more generically, charge-density wave) phase, which sets in at higher doping. We phenomenologically assume a quantum critical character for the longitudinal and transverse nematic, and for the charge-ordering fluctuations, and investigate the effects of these fluctuations in Raman spectra. We find that the longitudinal nematic fluctuations peaked at zero transferred momentum account well for the anomalous Raman absorption observed in these systems in the $B_{2g}$ channel, while the absence of such effect in the $B_{1g}$ channel may be due to the overall suppression of Raman response at low frequencies, associated with the pseudogap. While in Y$_{0.97}$Ca$_{0.03}$BaCuO$_{6.05}$ the low-frequency lineshape is fully accounted by longitudinal nematic collective modes alone, in La$_{2-x}$Sr$_x$CuO$_4$ also charge-ordering modes with finite characteristic wavevector are needed to reproduce the shoulders observed in the Raman response. This different involvement of the nearly critical modes in the two materials suggests a different evolution of the nematic state at very low doping into the nearly charge-ordered state at higher doping.