Field-induced tuning of the pairing state in a superconductor

Abstract

The recently discovered superconductor UTe2 with a Tc between 1.5 K and 2 K, has attracted much attention due to indications of spin-triplet and topological superconductivity. Its properties under magnetic field are also remarkable, with field-reinforced and field-induced superconducting phases. Here, we report the first complete thermodynamic determination of the phase diagram for fields applied along the three crystallographic directions. Measurements were performed up to 36 T along the hard b axis in order to follow the superconducting transition up to the metamagnetic transition at Hm = 34.75 T. They demonstrate the existence of a phase transition line within the superconducting phase, and drastic differences occurring between these two phases. Detailed analysis supports a different spin state between the two phases, possibly a spin-triplet to spin-singlet transition.

Magnetic field-induced transition with spin rotation in the superconducting phase of UTe2

“…The H kink anomaly, like the overall phase diagram, is consistent with the phase transitions determined by the recent specific-heat measurements [21]. Because the responses to H and θ are different between the LHSC and HHSC states, it is reasonable to consider that the kink at H kink marks a phase transition between the two SC states.…”

“…Because the responses to H and θ are different between the LHSC and HHSC states, it is reasonable to consider that the kink at H kink marks a phase transition between the two SC states. Such a transition between two bulk SC states was also confirmed in the work [21] by linear magnetostriction and thermal dilatation, evidencing anomalies due to vortex pinning in both phases. The results of the H-sweep measurement at 0.6 K revealed clear hysteresis behavior at µ 0 H * ∼ 4 T; it was found to be related to an anomaly of the vortex state, because the anomaly was not previously observed in the H dependence results for the electronic term in specific-heat measurements [22].…”

First Observation of the de Haas–van Alphen Effect and Fermi Surfaces in the Unconventional Superconductor UTe2

“…Superconductivity survives up to the first-order metamagnetic transition at H m = 35 T, 5) as bulk properties. 6) The values of H c2 highly exceed the Pauli limit, (∼ 3 T), for all field directions, suggesting a spin-triplet state. A microscopic evidence for a spin-triplet state is obtained from NMR experiments, in which the Knight shift is unchanged or decreases very slightly below T c for H a, b and c-axis.…”

Quantum-geometry-induced anapole superconductivity

“…While we focused on the superconducting state at the zero magnetic field in this paper, an anapole superconducting state with finite-q pairing may also appear in finite magnetic fields. The observation of the double superconducting transitions in UTe 2 under a magnetic field implies the superconducting phases with distinct symmetry 77 . If the superconducting state around H b 15T is the s + ip-wave pairing state as discussed 77 , finite-q anapole superconductivity or monopole superconductivity is expected.…”

“…The observation of the double superconducting transitions in UTe 2 under a magnetic field implies the superconducting phases with distinct symmetry 77 . If the superconducting state around H b 15T is the s + ip-wave pairing state as discussed 77 , finite-q anapole superconductivity or monopole superconductivity is expected.…”