Close proximity of FeSe to a magnetic quantum critical point as revealed by high-resolution $\hat{I}¼$SR measurements

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 $T_{\mathrm{s}}\approx$ 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 ($\hat{I}¼$SR) measurements. The $\hat{I}¼$SR Knight shift and the bulk susceptibility linearly scale at high temperatures but deviate from this behavior around $T^{\ast }\sim 10$ 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 $T^{\ast }$. In the temperature range $T_{\mathrm{s}}\gtrsim T\gtrsim T^{\ast }$ the muon spin depolarization rate follows a critical behavior $\hat{I}›\propto T^{-0.4}$. The observed non-Fermi liquid behavior with a cutoff at $T^{\ast }$ indicates that FeSe is in the vicinity to a antiferromagnetic quantum critical point. Our analysis is suggestive for $T^{\ast }$ triggered by the Lifshitz transition.

15 pages, 16 figures