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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.

Multiple superconducting order parameters are extremely rare. Here we show that a very small pressure can induce this phenomenon in the recently discovered heavy fermion superconductor UTe2. This nearly ferromagnetic system shows several intriguing phenomena, including an extraordinary reinforcement of superconductivity in very strong magnetic fields. We find that pressure can tune the system to a more correlated state and probable magnetic order. The superconducting critical temperature is strongly enhanced, reaching almost 3K, a new record for Ce-and U-based heavy fermion superconductors. Most spectacularly under pressure we find a transition within the superconducting state, putting UTe2 among the very rare systems having multiple superconducting phases. UTe2 under pressure is a treasure trove of several of the most fascinating phenomena in unconventional superconductivity and may well be a keystone in their understanding.In most superconductors the superconducting order parameter is s-wave, meaning it has the same symmetry as the crystal lattice. However in the ever expanding family of unconventional superconductors, which includes such disparate members as high-Tc cuprates and pnictides, organic superconductors, and heavy fermions, the order parameter can assume a number of different symmetries, usually lower than the lattice symmetry. This opens the intriguing possibility that a given system could in principle exhibit different order parameters, each one being selected by changing an external variable like temperature or magnetic field. This scenario does in fact exist, but is extremely rare, having been really established only in superfluid 3 He[1] and in two superconductors: UPt3[2,3] and thorium-doped UBe13 [4]. The recently discovered superconductivity in the heavy fermion system UTe2 [5] shows several unusual properties, the most spectacular being re-entrant superconductivity when magnetic fields as high as 60 Tesla are applied in specific directions [6][7][8][9]. Another intriguing property is the temperature dependence of the specific heat. Indeed in all samples a large residual term, of about 50% of the normal state specific heat, seems to remain as the temperature approaches zero [5,6].In this report we show that the superconductivity is extremely sensitive to hydrostatic pressure as a tuning parameter and that UTe2 is probably another example of multiple superconducting phases. The superconducting state found at zero pressure is monotonously depressed with pressure but a second superconducting state is found to emerge as pressure is increased. Pressure increases the splitting between the 2 transitions and the high temperature superconducting transition reaches nearly 3K, a new record for a U-based heavy fermion superconductor. Pressure also drives the system towards a more correlated state, with evidence for a strong enhancement of the electronic effective mass. At a critical pressure of about 1.7 GPa both superconducting states are suppressed and a new order parameter, probably magnetic, is foun...

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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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