NMR evidence for a Peierls transition in the layered square-net compound LaAgSb$_2$

TL;DR

This study uses NMR spectroscopy to reveal that the charge-density-wave transition in LaAgSb$_2$ is driven by a Peierls instability, evidenced by spectral line splitting and temperature-dependent behavior consistent with a BCS-like order parameter.

Contribution

It provides direct NMR evidence that the CDW transition in LaAgSb$_2$ is a Peierls transition caused by electronic instability near the Fermi level.

Findings

NMR line splitting at $T_{CDW1}$ indicates lattice distortion.

Knight shift shows pseudogap and Fermi surface gapping.

CDW transition follows BCS-like temperature dependence.

Abstract

We measured the central ($1/2\leftrightarrow -1/2$) and first satellite ($\pm3/2\leftrightarrow \pm1/2$) lines of the \la\ NMR spectra as a function of temperature in LaAgSb$_2$, in order to elucidate the origin and nature of the charge-density-wave (CDW) transitions at $T_\text{CDW1}=207$ K and $T_\text{CDW2}=186$ K. In the normal state, the Knight shift K reveals a fairly linear relationship with decreasing temperature, which is ascribed to a pseudogap in the spin excitation spectrum, pointing towards the material being an unconventional metal. Upon further cooling, K decreases more steeply below $T_\text{CDW1}$, indicative of the partial Fermi surface gap opening on top of the pseudogap. The most remarkable finding in our study is a clear splitting of the satellite lines at $T_\text{CDW1}$ observed for $H\parallel c$, whose temperature dependence behaves as the BCS order parameter in the weak-coupling limit, evidencing that the CDW transition induces the periodic lattice distortion. Our NMR findings therefore demonstrate that the CDW transition in LaAgSb$_2$ is of Peierls type, being driven by the electronic instability in the vicinity of the Fermi level.