Abstract
We report the evolution of nematic fluctuations in FeSe1−xSx single crystals as a function of Sulfur content x across the nematic quantum critical point (QCP) xc ~ 0.17 via Raman scattering. The Raman spectra in the B1g nematic channel consist of two components, but only the low energy one displays clear fingerprints of critical behavior and is attributed to itinerant carriers. Curie–Weiss analysis of the associated nematic susceptibility indicates a substantial effect of nemato-elastic coupling, which shifts the location of the nematic QCP. We argue that this lattice-induced shift likely explains the absence of any enhancement of the superconducting transition temperature at the QCP. The presence of two components in the nematic fluctuations spectrum is attributed to the dual aspect of electronic degrees of freedom in Hund’s metals, with both itinerant carriers and local moments contributing to the nematic susceptibility.
Highlights
The link between quantum criticality and the emergence of unconventional superconducting (SC) states is ubiquitous among several families of materials including heavy fermion, cuprates, and iron-based (Fe SC) superconductors[1,2,3]
The initial focus was on the proximity of the AF phase which is expected to lead a sign changing s± SC pairing state[10,11], but recently the role of nematic fluctuations and criticality on the SC state has come under scrutiny[12,13,14,15,16,17,18,19,20]
Several theoretical works have argued that the SC pairing is generically enhanced near a nematic quantum critical point (QCP)[5,21,22,23,24], even if it is not the leading pairing glue
Summary
The link between quantum criticality and the emergence of unconventional superconducting (SC) states is ubiquitous among several families of materials including heavy fermion, cuprates, and iron-based (Fe SC) superconductors[1,2,3]. Coming from the tetragonal, non-nematic side of the FeSe1−xSx phase diagram, Tc is essentially flat upon approaching the nematic QCP at xc ~ 0.17 showing only a mild maximum well-inside the nematic ordered phase[42,43] (Fig. 1a) This apparent contradiction with theoretical expectations was recently argued to arise from the coupling between electronic nematic degrees of freedom and the lattice, which cuts off nematic quantum criticality at low temperature. This effect may suppress the expected enhancement of Tc found in electronic-only models[24]. Our study highlights the important role of electron–lattice coupling effects in both SC and normal state properties of FeSe, and more generally of Fe SC
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