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We present a study of the upper critical field of the newly discovered heavy fermion superconductor UTe 2 by magnetoresistivity measurements in pulsed magnetic fields up to 60 T and static magnetic fields up to 35 T. We show that superconductivity survives up to the metamagnetic transition at H m ≈ 35 T at low temperature. Above H m superconductivity is suppressed. At higher temperature superconductivity is enhanced under magnetic field leading to reentrance of superconductivity or an almost temperature independent increase of H c2 . By studying the angular dependence of the upper critical field close to the b axis (hard magnetization axis) we show that the maximum of the reentrant superconductivity temperature is depinned from the metamagnetic field. A key ingredient for the field-reinforcement of superconductivity on approaching H m appears to be an immediate interplay with magnetic fluctuations and a possible Fermi-surface reconstruction. 1 arXiv:1905.05181v1 [cond-mat.str-el]

A fundamental issue concerning iron-based superconductivity is the roles of electronic nematicity and magnetism in realising high transition temperature (T c). To address this issue, FeSe is a key material, as it exhibits a unique pressure phase diagram involving non-magnetic nematic and pressure-induced antiferromagnetic ordered phases. However, as these two phases in FeSe have considerable overlap, how each order affects superconductivity remains perplexing. Here we construct the three-dimensional electronic phase diagram, temperature (T) against pressure (P) and isovalent S-substitution (x), for FeSe1−xSx. By simultaneously tuning chemical and physical pressures, against which the chalcogen height shows a contrasting variation, we achieve a complete separation of nematic and antiferromagnetic phases. In between, an extended non-magnetic tetragonal phase emerges, where T c shows a striking enhancement. The completed phase diagram uncovers that high-T c superconductivity lies near both ends of the dome-shaped antiferromagnetic phase, whereas T c remains low near the nematic critical point.

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In conventional metals, modification of electron trajectories under magnetic field gives rise to a magnetoresistance that varies quadratically at low field, followed by a saturation at high field for closed orbits on the Fermi surface. Deviations from the conventional behaviour, for example, the observation of a linear magnetoresistance, or a non-saturating magnetoresistance, have been attributed to exotic electron scattering mechanisms. Recently, linear magnetoresistance has been observed in many Dirac materials, in which the electron–electron correlation is relatively weak. The strongly correlated helimagnet CrAs undergoes a quantum phase transition to a nonmagnetic superconductor under pressure. Here we observe, near the magnetic instability, a large and non-saturating quasilinear magnetoresistance from the upper critical field to 14 T at low temperatures. We show that the quasilinear magnetoresistance may arise from an intricate interplay between a nontrivial band crossing protected by nonsymmorphic crystal symmetry and strong magnetic fluctuations.

Thermoelectric power (S) and Hall effect (R H) measurements on the paramagnetic superconductor UTe 2 with the magnetic field applied along the hard magnetization b axis are reported. The first-order nature of the metamagnetic transition at H m = H b c2 = 35 T leads to drastic consequences on S and R H. In contrast to the field dependence of the specific heat in the normal state through H m , S(H) is not symmetric with respect to H m. This implies a strong interplay between ferromagnetic fluctuations and a Fermi-surface reconstruction at H m. R H is very well described by incoherent skew scattering above the coherence temperature T m corresponding roughly to the temperature of the maximum in the susceptibility T χmax and coherent skew scattering at lower temperatures. The discontinuous field dependence of both S(H) and the ordinary Hall coefficient R 0 , at H m and at low temperature, provides evidence of a change in the band structure at the Fermi level.

The superconducting transition of FeSe1−xSx with three distinct sulphur concentrations x was studied under hydrostatic pressure up to ∼70 kbar via bulk AC susceptibility. The pressure dependence of the superconducting transition temperature (Tc) features a small dome-shaped variation at low pressures for x = 0.04 and x = 0.12, followed by a more substantial Tc enhancement to a value of around 30 K at moderate pressures. In x = 0.21, a similar overall pressure dependence of Tc is observed, except that the small dome at low pressures is flattened. For all three concentrations, a significant weakening of the diamagnetic shielding is observed beyond the pressure around which the maximum Tc of 30 K is reached near the verge of pressure-induced magnetic phase. This observation points to a strong competition between the magnetic and high-Tc superconducting states at high pressure in this system.

Ambient-pressure-grown LaO0.5F0.5BiS2 with a superconducting transition temperature Tc ∼ 3 K possesses a highly anisotropic normal state. By a series of electrical resistivity measurements with a magnetic field direction varying between the crystalline c-axis and the ab-plane, we present the first datasets displaying the temperature dependence of the out-of-plane upper critical field H ⊥ c2 (T ), the in-plane upper critical field H / / c2 (T ), as well as the angular dependence of Hc2 at fixed temperatures for ambient-pressure-grown LaO0.5F0.5BiS2 single crystals. The anisotropy of the superconductivity, H / / c2 /H ⊥ c2 , reaches ∼16 on approaching 0 K, but it decreases significantly near Tc. A pronounced upward curvature of H / / c2 (T ) is observed near Tc, which we analyze using a two-gap model. Moreover, H / / c2 (0) is found to exceed the Pauli paramagnetic limit, which can be understood by considering the strong spin-orbit coupling associated with Bi as well as the breaking of the local inversion symmetry at the electronically active BiS2 bilayers. Hence, LaO0.5F0.5BiS2 with a centrosymmetric lattice structure is a unique platform to explore the physics associated with local parity violation in the bulk crystal.