Lattice-shifted nematic quantum critical point in FeSe1−xSx

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.

Evolution of lattice, spin, and charge properties across the phase diagram of FeSe$_{1-x}$S$_x$

“…In FeSe it becomes clearly visible below 200 K and fills the spectral gap below the magnon at 500 cm −1 . Below approximately 100 K an isolated peak may be observed for x = 0 which continuously softens above the structural transformation at T s = 90 K, fades away below T s , and almost vanishes at 21 K [22,31]. The line shape and the temperature dependence above T s can be described quantitatively in terms of critical fluctuations in a similar fashion as in Ba(Fe 1−x Co x ) 2 As 2 [22,30,32].…”

“…The most likely explanation of this coincidence is that the two phenomena have the same origin and result from spin excitations. However, there is no consensus on that view in the literature, and Zhang et al [33] and Chibani et al [34] interpret the same experimental observation in terms of quadrupolar charge fluctuations. Yet, one certainly has to answer the question as to why the fluctuations are not found in the simulations [23].…”

Nematic fluctuations mediated superconductivity revealed by anisotropic strain in Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$

“…Enhancement of T c close to a nematic QCP is not universally observed in Fe SC. A particular vexing case is FeSe 1−x S x where T c is even suppressed close to the nematic QCP [43,44]. This was attributed to the coupling to the lattice which cuts off nematic fluctuations, and can significant quench the expected T c enhancement found in electron-only models [18,44,45].…”

Defect-induced electronic smectic state at the surface of nematic materials

“…While a purely electronic mechanism was previously invoked to explain surface nematicity [36], in this Letter we focus on the role of the elastic degrees of freedom. The nemato-elastic coupling, hereafter denoted g, is known to significantly impact the nematic state, particularly in FeSC [15,[37][38][39][40][41][42]. For instance, coupling to elastic fluctuations, manifested as acoustic phonons, generates long-range nematic interactions that render the nematic transition mean-field like [41,[43][44][45][46].…”