Quasiparticle Interference and Strong Electron-Mode Coupling in the Quasi-One-Dimensional Bands of Sr$_2$RuO$_4$

TL;DR

This study uses advanced spectroscopic techniques to reveal strong electron interactions and mode couplings in the quasi-one-dimensional bands of Sr$_2$RuO$_4$, providing insights into its correlated metallic state and implications for its superconductivity.

Contribution

It demonstrates the presence of strong correlation effects and mode couplings in Sr$_2$RuO$_4$'s normal state using FT-STS and M-EELS, highlighting the quasi-1D band characteristics.

Findings

Strong renormalization of Fermi velocity indicating correlated metal behavior

Observation of kinks at ~10meV, 38meV, and 70meV energies

Coupling of higher energy features with collective modes

Abstract

The single-layered ruthenate Sr$_2$RuO$_4$ has attracted a great deal of interest as a spin-triplet superconductor with an order parameter that may potentially break time reversal invariance and host half-quantized vortices with Majorana zero modes. While the actual nature of the superconducting state is still a matter of controversy, it has long been believed that it condenses from a metallic state that is well described by a conventional Fermi liquid. In this work we use a combination of Fourier transform scanning tunneling spectroscopy (FT-STS) and momentum resolved electron energy loss spectroscopy (M-EELS) to probe interaction effects in the normal state of Sr$_2$RuO$_4$. Our high-resolution FT-STS data show signatures of the \beta-band with a distinctly quasi-one-dimensional (1D) character. The band dispersion reveals surprisingly strong interaction effects that dramatically renormalize the Fermi velocity, suggesting that the normal state of Sr$_2$RuO$_4$ is that of a 'correlated metal' where correlations are strengthened by the quasi 1D nature of the bands. In addition, kinks at energies of approximately 10meV, 38meV and 70meV are observed. By comparing STM and M-EELS data we show that the two higher energy features arise from coupling with collective modes. The strong correlation effects and the kinks in the quasi 1D bands may provide important information for understanding the superconducting state. This work opens up a unique approach to revealing the superconducting order parameter in this compound.