Electron interactions and charge ordering in La$_{2-x}$Sr$_x$CuO$_4$

TL;DR

This study uses inelastic light scattering to explore how electron interactions and charge order evolve across different doping levels in La$_{2-x}$Sr$_x$CuO$_4$, revealing charge-order fluctuations and their relation to superconductivity.

Contribution

It provides new insights into doping-dependent charge-order fluctuations and their symmetry properties in La$_{2-x}$Sr$_x$CuO$_4$, highlighting the proximity to static charge order.

Findings

Universal behavior in B2g spectra in superconducting phases.

Charge-order fluctuations are observed for 0.05 ≤ p < 0.16 in La$_{2-x}$Sr$_x$CuO$_4$.

Charge-order fluctuations disappear at B1g symmetry for p ≤ 0.05.

Abstract

We present results of inelastic light scattering experiments on single-crystalline La$_{2-x}$Sr$_{x}$CuO$_4$ in the doping range $0.00 \le x=p \le 0.30$ and Tl$_2$Ba$_2$CuO$_{6+\delta}$ at $p=0.20$ and $p=0.24$. The main emphasis is placed on the response of electronic excitations in the antiferromagnetic phase, in the pseudogap range, in the superconducting state, and in the essentially normal metallic state at $x \ge 0.26$, where no superconductivity could be observed. In most of the cases we compare B$_{1g}$ and B$_{2g}$ spectra which project out electronic properties close to $(\pi,0)$ and $(\pi/2, \pi/2)$, respectively. In the channel of electron-hole excitations we find universal behavior in B$_{2g}$ symmetry as long as the material exhibits superconductivity at low temperature. In contrast, there is a strong doping dependence in B$_{1g}$ symmetry: (i) In the doping range $0.20 \le p \le 0.25$ we observe rapid changes of shape and temperature dependence of the spectra. (ii) In La$_{2-x}$Sr$_{x}$CuO$_4$ new structures appear for $x < 0.13$ which are superposed on the electron-hole continuum. The temperature dependence as well as model calculations support an interpretation in terms of charge-ordering fluctuations. For $x \le 0.05$ the response from fluctuations disappears at B$_{1g}$ and appears at B$_{2g}$ symmetry in full agreement with the orientation change of stripes found by neutron scattering. While, with a grain of salt, the particle-hole continuum is universal for all cuprates the response from fluctuating charge order in the range $0.05 \le p < 0.16$ is so far found only in La$_{2-x}$Sr$_{x}$CuO$_4$. We conclude that La$_{2-x}$Sr$_{x}$CuO$_4$ is close to static charge order and, for this reason, may have a suppressed $T_c$.