Abstract
We report the magnetoresistance in the novel spin-triplet superconductor UTe2 under pressure close to the critical pressure Pc, where the superconducting phase terminates, for field along the three a, b and c-axes in the orthorhombic structure. The superconducting phase for H // a-axis just below Pc shows a field-reentrant behavior due to the competition with the emergence of magnetic order at low fields. The upper critical field Hc2 for H // c-axis shows a quasi-vertical increase in the H-T phase diagram just below Pc, indicating that superconductivity is reinforced by the strong fluctuations which persist even at high fields above 20T. Increasing pressure leads to the disappearance of superconductivity at zero field with the emergence of magnetic order. Surprisingly, field-induced superconductivity is observed at high fields, where a spin-polarized state is realized due to the suppression of the magnetic ordered phases; the spin-polarized state is favorable for superconductivity, whereas the magnetic ordered phase at low field seems to be unfavorable. The huge Hc2 in the spin-polarized state seems to imply a spin-triplet state. Contrary to the a- and c-axes, no field-reinforcement of superconductivity occurs for magnetic field along the b-axis. We compare the results with the field-reentrant superconductivity above the metamagnetic field, Hm for the field direction tilted by about 30 deg. from b to c-axis at ambient pressure as well as the field-reentrant (-reinforced) superconductivity in ferromagnetic superconductors, URhGe and UCoGe.
Highlights
The recent discovery of superconductivity in the heavy fermion paramagnet UTe2 has attracted a lot of attention1,2) UTe2 at ambient pressure does not show a longrange magnetic order down to low temperatures, many similarities to ferromagnetic superconductivity are pointed out
The upper critical field Hc2 for H c-axis shows a quasi-vertical increase in the H-T phase diagram just below Pc, indicating that superconductivity is reinforced by the strong fluctuations which persist even at high fields above 20 T
Field-induced superconductivity is observed at high fields, where a spin-polarized state is realized due to the suppression of the magnetic ordered phases; the spin-polarized state is favorable for superconductivity, whereas the magnetic ordered phase at low field seems to be unfavorable
Summary
The recent discovery of superconductivity in the heavy fermion paramagnet UTe2 has attracted a lot of attention1,2) UTe2 at ambient pressure does not show a longrange magnetic order down to low temperatures, many similarities to ferromagnetic superconductivity are pointed out. The upper critical field Hc2(0) for a and c-axes shows large values (7 and 11 T, respectively), exceeding the Pauli limit for all field directions, supporting a spin-triplet scenario, which is evidenced by the spin susceptibility in NMR experiments.22) Another important point in UTe2 is the emergence of multiple superconducting phases under pressure (P) clarified by AC calorimetry measurements.23) Tc initially decreases with pressure and splits into two transitions above 0.25 GPa. The lower Tc continuously decreases with further increasing pressure and extrapolates linearly to zero. AC calorimetry measurements under pressure with the magnetic field applied along the a-axis confirmed the occurrence of multiple superconducting phases in UTe2 and that the sudden increase of Hc2 at low temperatures is connected with the lower Tc.25) These experimentally observed superconducting phases imply different order parameters.12,27,28) This is consistent with a spin-triplet state, which could have multiple phases owing to the spin degree of freedom. The results indicate that superconductivity can survive in the spin-polarized state far above the critical field of the magnetic ordered state, while magnetic ordered state seems to suppress superconductivity
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