Abstract
We report the magnetoresistance under pressure in the novel spin-triplet superconductor UTe2 close to the critical pressure P-c, where the superconducting phase terminates, for field along the three a, b, and c-axes in the orthorhombic structure. The superconducting phase for H parallel to a-axis just below P-c shows a field-reentrant behavior due to the competition with the emergence of magnetic order at low fields.
The upper critical field H-c2 for H parallel to c-axis shows a quasi-vertical increase in the H-T phase diagram just below P-c, indicating that superconductivity is reinforced by the strong fluctuations which persist even at high fields above 20 T. 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 H-c2 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, H-m for the field direction tilted by similar to 30 deg from b to c-axis at ambient pressure as well as the field-reentrant (-reinforced) superconductivity in ferromagnetic superconductors, URhGe and UCoGe.