Spin-triplet superconductors potentially host topological excitations that are of interest for quantum information processing. We report the discovery of spin-triplet superconductivity in UTe2, featuring a transition temperature of 1.6 kelvin and a very large and anisotropic upper critical field exceeding 40 teslas. This superconducting phase stability suggests that UTe2 is related to ferromagnetic superconductors such as UGe2, URhGe, and UCoGe. However, the lack of magnetic order and the observation of quantum critical scaling place UTe2 at the paramagnetic end of this ferromagnetic superconductor series. A large intrinsic zero-temperature reservoir of ungapped fermions indicates a highly unconventional type of superconducting pairing.

Applied magnetic fields underlie exotic quantum states, such as the fractional quantum Hall effect 1 and Bose-Einstein condensation of spin excitations 2 . Superconductivity, on the other hand, is inherently antagonistic towards magnetic fields. Only in rare cases 3-5 can these effects be mitigated over limited fields, leading to reentrant superconductivity. Here, we report the unprecedented coexistence of multiple high-field reentrant superconducting phases in the spin-triplet superconductor UTe 2 6 . Strikingly, we observe superconductivity in the highest magnetic field range identified for any reentrant superconductor, beyond 65 T. These extreme properties reflect a new kind of exotic superconductivity rooted in magnetic fluctuations 7 and boosted by a quantum dimensional crossover 8 .

We report the novel preparation of single crystals of tetragonal iron sulfide, FeS, which exhibits a nearly ideal tetrahedral geometry with S-Fe-S bond angles of 110.2(2) • and 108.1(2) • . Grown via hydrothermal de-intercalation of KxFe2−yS2 crystals under basic and reducing conditions, the silver, plate-like crystals of FeS remain stable up to 200 • C under air and 250 • C under inert conditions, even though the mineral "mackinawite" (FeS) is known to be metastable. FeS single crystals exhibit a superconducting state below Tc = 4 K as determined by electrical resistivity, magnetic susceptibility, and heat capacity measurements, confirming the presence of a bulk superconducting state. Normal state measurements yield an electronic specific heat of 5 mJ/mol-K 2 , and paramagnetic, metallic behavior with a low residual resistivity of 250 µΩ·cm. Magnetoresistance measurements performed as a function of magnetic field angle tilted toward both transverse and longitudinal orientations with respect to the applied current reveal remarkable two-dimensional behavior. This is paralleled in the superconducting state, which exhibits the largest known upper critical field Hc2 anisotropy of all iron-based superconductors, with H ||ab c2 (0)/H ||c c2 (0) =(2.75 T)/(0.275 T)=10. Comparisons to theoretical models for 2D and anisotropic-3D superconductors, however, suggest that FeS is the latter case with a large effective mass anisotropy. We place FeS in context to other closely related iron-based superconductors and discuss the role of structural parameters such as anion height on superconductivity.

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We present details of materials synthesis, crystal structure, and anisotropic magnetic properties of single crystals of CeAlGe, a proposed type-II Weyl semimetal. Single-crystal x-ray diffraction confirms that CeAlGe forms in noncentrosymmetric I41md space group, in line with predictions of non-trivial topology. Magnetization, specific heat and electrical transport measurements were used to confirm antiferromagnetic order below 5 K, with an estimated magnon excitation gap of ∆ = 9.11 K from heat capacity and hole-like carrier density of 1.44 × 10 20 cm −3 from Hall effect measurements. The easy magnetic axis is along the [100] crystallographic direction, indicating that the moment lies in the tetragonal ab-plane below 7 K. A spin-flop transition to less than 1 µB/Ce is observed to occur below 30 kOe at 1.8 K in the M (H) (H a) data. Small magnetic fields of 3 kOe and 30 kOe are sufficient to suppress magnetic order when applied along the a-and c-axes, respectively, resulting in a complex T-H phase diagram for H a and a simpler one for H c.