Unidirectional electron-phonon coupling as a "fingerprint'' of the nematic state in a kagome superconductor

TL;DR

This study visualizes how electron-phonon interactions in a kagome superconductor create a nematic state with directional electronic properties, revealing a competition between nematicity and superconductivity and expanding understanding of electron correlations.

Contribution

It uncovers unidirectional electron-phonon coupling as a fingerprint of nematicity in a kagome superconductor, highlighting its impact on electron dynamics and the interplay with superconductivity.

Findings

Electron-phonon coupling alters electron self-energy along one lattice direction.

Nematic transition affects quasiparticle scattering interference patterns.

Superconductivity is suppressed as nematic order develops.

Abstract

Electronic nematicity has been commonly observed in juxtaposition with unconventional superconductivity. Understanding the nature of the nematic state, as well as its consequence on the electronic band structure and superconductivity, has become a pivotal focus in condensed matter physics. Here we use spectroscopic imaging-scanning tunneling microscopy to visualize how the interacting quasiparticles organize themselves in the nematic state of kagome superconductor CsV$_{3-x}$Ti$_x$Sb$_5$, in which twofold symmetric (C$_2$) quasiparticle scattering interference of the vanadium kagome bands emerges below the bulk nematic transition temperature (T$_{nem}$). Surprisingly, we find that the coupling to collective modes, i.e., the phonon, dramatically alters the electrons self-energy and renormalizes the Fermi velocity of the in-plane vanadium d$_{xy/x^2-y^2}$ bands only along the C$_2$ direction, making the low-energy dispersion and electron dynamics highly nonequivalent along the three lattice directions. The anti-correlation between T$_{nem}$ and the superconducting transition temperature upon Ti substitution further suggests a possible competition between superconductivity and electron nematicity in this series, with a principal superconducting gap opening on the same V bands once the nematic state is totally suppressed. The organizing principle of these quasiparticles provides essential information for understanding the interplay between charge density wave and superconductivity in these kagome superconductors, and also reveals a previously unexplored form that expands the landscape for modelling electronic nematicity in systems where electron correlations and lattice degree of freedom act in concert.