Single thermodynamic transition at 2 K in superconducting UTe2 single crystals

Abstract

UTe2 is a newly-discovered unconventional superconductor wherein multicomponent topological superconductivity is anticipated based on the presence of two superconducting transitions and time-reversal symmetry breaking in the superconducting state. The observation of two superconducting transitions, however, remains controversial. Here we demonstrate that UTe2 single crystals displaying an optimal superconducting transition temperature at 2 K exhibit a single transition and remarkably high quality supported by their large residual resistance ratio and small residual heat capacity in the superconducting state. Our results shed light on the intrinsic superconducting properties of UTe2 and bring into question whether UTe2 is a multicomponent superconductor at ambient pressure.

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

“…The recent discovery of ferromagnetic (FM) superconductors [5][6][7] has made it possible to study the spin-triplet pairing state in the superconductors. Additionally, a spin-triplet SC candidate UTe 2 has been newly discovered [8]; the SC transition temperature T c is 1.6 ∼ 2.0 K [9,10]. Although UTe 2 undergoes no FM transition, it was considered to be an end member of FM superconductors owing to its physical similarity to FM superconductors [8,9].…”

“…Several possibilities were considered for the origin of the larger-K peak; they include the occurrence of a mosaic structure and/or minute Uatom deficiency in the single-crystal sample. In the former case, the misalignment of the mosaic was estimated to be 0.03 • (0.15 • ) on the a (c) axis; additionally, the 125 Te NMR measurement for the higher-T c single crystal is critical for the latter possibility because T c seems to be very sensitive to a U-atom deficiency [9,10,23]. Further experiments are required to clarify the origin of the larger K peak.…”

UTe2: A new Topological Superconductor?

“…Generically, Cooper pairs of different symmetry will condense at different temperatures, producing a two-stage jump in the specific heat capacity. While the specific heat does develop a double peak in a magnetic field, no such jump has been seen in zero field in samples grown by the prescription outlined in [14]. Moreover, the application of pressure [15] does not split the transition, ruling out the possibility of an accidental degeneracy.…”

Suppressed magnetic scattering sets conditions for the emergence of 40 T high-field superconductivity in UTe2

“…The nature of the superconducting ground state, the identification of the different field or pressure-induced superconducting phases, and their relation to topological superconductivity are still under debate, notably from a theoretical point of view [28][29][30]. Moreover, recent specific-heat measurements [31,32] contradict the proposal of multicomponent ambient SC [8,9]. Understanding the mechanisms responsible for the emergence of the hfSC and its relation to the low-field superconductivity (lfSC) is one of the key questions for current research in the field.…”