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We have studied the magnetization of the recently discovered heavy fermion superconductor UTe 2 up to 56 T in pulsed-magnetic fields. A first-order metamagnetic transition has been clearly observed at H m =34.9 T when the magnetic field H is applied along the orthorhombic hard-magnetization b-axis. The transition has a critical end point at ∼11 K and 34.8 T, where the first order transition terminates and changes into a crossover regime. Using the thermodynamic Maxwell relation, we have evaluated the field dependence of the Sommerfeld coefficient of the specific heat directly related to the superconducting pairing. From the analysis, we found a significant enhancement of the effective mass centered at H m , which is reminiscent of the field-reentrant superconductivity of the ferromagnet URhGe in transverse fields. We discuss the origin of their field-robust superconductivity.

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We have performed the 125 Te-nuclear magnetic resonance (NMR) measurement in the field along the b axis on the newly discovered superconductor UTe 2 , which is a candidate of a spin-triplet superconductor. The nuclear spin-lattice relaxation rate divided by temperature 1/T 1 T abruptly decreases below a superconducting (SC) transition temperature T c without showing a coherence peak, indicative of UTe 2 being an unconventional superconductor. It was found that the temperature dependence of 1/T 1 T in the SC state cannot be understood by a single SC gap behavior but can be explained by a two SC gap model. The Knight shift, proportional to the spin susceptibility, decreases below T c , but the magnitude of the decrease is much smaller than the decrease expected in the spin-singlet pairing. Rather, the small Knight-shift decrease as well as the absence of the Pauli-depairing effect can be interpreted by the spin triplet scenario.

We performed AC calorimetry and magnetoresistance measurements under pressure for H a-axis (easymagnetization axis) in the novel heavy-fermion superconductor UTe 2 . Thanks to the thermodynamic information, multiple superconducting phases have been revealed under pressure and magnetic field. The (H, T ) phase diagram of superconductivity under pressure displays an abrupt increase of the upper critical field (H c2 ) at low temperature and in the high field region, and a strong convex curvature of H c2 at high temperature. This behavior of H c2 and the multiple superconducting phases require a state for the spin-triplet superconducting order parameter more complex than an equal spin pairing. Above the superconducting critical pressure, P c , we find strong indications that the possible magnetic order is closer to antiferromagnetism than to ferromagnetism.The recently discovered heavy-fermion superconductivity (SC) in UTe 2 attracts much attention, 1, 2) because spin-triplet SC is most likely realized in this system, lying at the proximity of ferromagnetic (FM) order. SC coexisting microscopically with long-range FM order is already well studied 3, 4) in three uranium compounds, UGe 2 , 5) URhGe 6) and UCoGe. 7) Because of the strong internal field due to the FM moment, spin-triplet state with equal-spin pairing (ESP) is favoured. In this case, SC can survive even under strong internal exchange field. Furthermore SC can be even reinforced at high magnetic field (H), pressure (P) and uniaxial stress by tuning the FM fluctuations. When the field is applied along the intermediate hard-magnetization axis (b-axis) in URhGe and UCoGe, the FM Curie temperature is suppressed, and the FM fluctuations are remarkably enhanced. Then field-reentrant (-reinforced) SC is observed at high fields, which highly exceeds the socalled Pauli limit. 8,9) A quite similar situation might be also realized in UTe 2 . UTe 2 is a paramagnet with a body-centered orthorhombic crystal structure (space group: Immm, #71, D 25 2h ). The large Sommerfeld coefficient, γ ∼ 120 mJ K −2 mol −1 , indicates strong correlations in the electronic states. 10) The magnetization curves show a relatively large anisotropy between the easy-magnetization axis (a-axis) and the hard-magnetization axes (b and c-axes). The b-axis is the hardest magnetization axis at low temperatures; the magnetization curve shows sharp 1st order metamagnetic transition at H m = 35 T, where the effective mass is strongly enhanced. [11][12][13] The value of H m is well scaled by T χ max (∼ 35 K), at which a broad maximum of susceptibility χ is observed at low field.The SC properties of UTe 2 are spectacular. The SC transition occurs at T sc = 1.6 K with the large specific heat jump. The residual γ-value amounts to ∼ 40 % against the normal state γ-value for the best quality sample, 14) and the entropy balance is not satisfied, assuming a constant γ-value extrapolated from the normal state above T sc . Strong electronic correlations are dominant in this system, which is also indirectly con...

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