Close proximity of FeSe to a magnetic quantum critical point as revealed by high-resolution $\mu$SR measurements

TL;DR

This study uses high-resolution muon spin rotation measurements to reveal that FeSe is close to a magnetic quantum critical point, showing non-Fermi liquid behavior and electronic changes near 10 K, despite lacking static magnetism.

Contribution

It provides evidence that FeSe is near a magnetic quantum critical point, linking its electronic properties to a Lifshitz transition and critical magnetic fluctuations.

Findings

Muon spin depolarization rate follows a critical power law $ ightarrow T^{-0.4}$.

Deviation of Knight shift and susceptibility around $T^{*} \\sim 10$ K.

FeSe exhibits non-Fermi liquid behavior near a quantum critical point.

Abstract

A nematic transition preceding a long-range spin density wave antiferromagnetic phase is a common feature of many Fe based superconductors. However, in the FeSe system with a nematic transition at $T_{\rm s} \approx$ 90 K no evidence for long-range static magnetism down to very low temperature was found. The lack of magnetism is a challenge for the theoretical description of FeSe. Here, we investigated high-quality single crystals of FeSe using high-field (up to 9.5 Tesla) muon spin rotation ($\mu$SR) measurements. The $\mu$SR Knight shift and the bulk susceptibility linearly scale at high temperatures but deviate from this behavior around $T^{*} \sim 10$ K, where the Knight shift exhibits a kink. This behavior hints to an essential change of the electronic and/or magnetic properties crossing the region near $T^{*}$. In the temperature range $T_{\rm s} \gtrsim T \gtrsim T^{*}$ the muon spin depolarization rate follows a critical behavior $\Lambda \propto T^{-0.4}$. The observed non-Fermi liquid behavior with a cutoff at $T^{*}$ indicates that FeSe is in the vicinity to a antiferromagnetic quantum critical point. Our analysis is suggestive for $T^{*}$ triggered by the Lifshitz transition.