Anomalous high-magnetic field electronic state of the nematic superconductors FeSe1−xSx

Abstract

Understanding superconductivity requires detailed knowledge of the normal electronic state from which it emerges. A nematic electronic state that breaks the rotational symmetry of the lattice can potentially promote unique scattering relevant for superconductivity. Here, we investigate the normal transport of superconducting FeSe1−xSx across a nematic phase transition using high magnetic fields up to 69 T to establish the temperature and field-dependencies. We find that the nematic state is an anomalous non-Fermi liquid, dominated by a linear resistivity at low temperatures that can transform into a Fermi liquid, depending on the composition x and the impurity level. Near the nematic end point, we find an extended temperature regime with ∼ T 1.5 resistivity. The transverse magnetoresistance inside the nematic phase has as a ∼ H 1.55 dependence over a large magnetic field range and it displays an unusual peak at low temperatures inside the nematic phase. Our study reveals anomalous transport inside the nematic phase, driven by the subtle interplay between the changes in the electronic structure of a multi-band system and the unusual scattering processes affected by large magnetic fields and disorder.Magnetic field is a unique tuning parameter that can suppress superconductivity to reveal the normal low-temperature electronic behavior of many unconventional superconductors [1,2]. High-magnetic fields can also induce new phases of matter, probe Fermi surfaces and determine the quasi-particle masses from quantum oscillations in the proximity of quantum critical points [1,3]. In unconventional superconductors, close to antiferromagnetic critical regions, an unusual scaling between a linear resistivity in temperature and magnetic fields was found [4,5]. Magnetic fields can also induce metal-toinsulator transitions, as in hole-doped cuprates, where superconductivity emerges from an exotic electronic ground state [2].FeSe is a unique bulk superconductor with T c ∼ 9 K which displays a variety of complex and competing electronic phases [6]. FeSe is a bad metal at room temperature and it enters a nematic electronic state below T s ∼ 87 K. This nematic phase is characterized by multi-band shifts driven by orbital ordering that lead to Fermi surface distortions [6,7]. Furthermore, the electronic ground state is that of a strongly correlated system and the quasiparticle masses display orbital-dependent enhancements [7,8]. FeSe shows no long-range magnetic order at ambient pressure, but complex magnetic fluctuations are present at high energies over a large temperature range [9]. Below T s , the spin-lattice relaxation rate from NMR experiments is enhanced as it captures the low-energy tail of the stripe spin-fluctuations [10,11]. Furthermore, recent µSR studies invoke the close proximity of FeSe to a magnetic quantum critical point as the muon relaxation rate shows unusual temperature dependence inside the nematic state [12].The changes in the electronic structure and magnetic fluctuations of FeSe can have profound implicatio...

Resistivity near a nematic quantum critical point: Impact of acoustic phonons

“…other hand, Ref. [56] reported a temperature dependence of α eff that is qualitatively consistent with our findings, increasing from close to 3/2 at higher temperatures to close to 2 at lower temperatures. The data points are also shown in Fig.…”

“…The bigger empty circles, indicated by κ exp latt , are the experimental data reported in Ref. [56] for the resistivity exponent α eff (T ) of "optimally" doped FeSe1−xSx with x = 0.18, which harbors a putative nematic QCP. Here, we used the value εF = 250 K for the Fermi energy of this material.…”

Non-Fermi liquid transport in the vicinity of the nematic quantum critical point of superconducting FeSe1−xSx

“…For x = 0.22, T -linear dependence of the resistivity changes to approximately T 2 dependence, as expected for a Fermi liquid at lower temperatures. Similar behavior has been reported now by several groups [10,32,33]. Figure 2 shows the color plot of the exponent α in the T dependence of the resistivity…”

“…Although the Fermi surface changes smoothly when crossing the nematic QCP in FeSe 1−x S x [29,30], the nematic QCP has a significant impact on both the superconducting [30,31] and normal state properties [10,[32][33][34]. The superconducting gap structure exhibits a dramatic change at the QCP.…”

“…The superconducting gap structure exhibits a dramatic change at the QCP. The in-plane resistivity exhibits a T -linear behavior, ρ xx ∝ T [10,32,33], at low temperature in the vicinity of the nematic QCP, suggesting that the critical fluctuations of nematic order can give rise to deviations from Fermi liquid behavior [23,35]. However, the T -linear ρ xx alone cannot be simply taken as conclusive evidence for the NFL behavior because it can appear as a result of multiband effects (see the Appendix).…”