# Unconventional Superconductivity in Heavy Fermion UTe2

## TL;DR

This study reports the growth and characterization of UTe2, revealing evidence of unconventional spin-triplet superconductivity with strong coupling and high upper critical fields, suggesting complex underlying electronic interactions.

## Contribution

First detailed experimental investigation of UTe2's superconducting properties, highlighting its unconventional nature and the influence of sample quality on critical fields.

## Key findings

- Superconductivity at Tsc=1.65K with a large specific heat jump.
- Unconventional superconductivity indicated by large Sommerfeld coefficient and entropy discrepancy.
- High upper critical fields along all axes, supporting spin-triplet pairing.

## Abstract

We grew single crystals of the recently discovered heavy fermion superconductor UTe2, and measured the resistivity, specific heat and magnetoresistance. Superconductivity (SC) was clearly detected at Tsc=1.65K as sharp drop of the resistivity in a high quality sample of RRR=35. The specific heat shows a large jump at Tsc indicating strong coupling. The large Sommerfeld coefficient, 117mJ K-2mol-1 extrapolated in the normal state and the temperature dependence of C/T below Tsc are the signature of unconventional SC. The discrepancy in the entropy balance at Tsc between SC and normal states points out that hidden features must occur. Surprisingly, a large residual value of the Sommerfeld coefficient seems quite robust (gamma_0/gamma ~ 0.5). The large upper critical field Hc2 along the three principal axes favors spin-triplet SC. For H // b-axis, our experiments do not reproduce the huge upturn of Hc2 reported previously. This discrepancy may reflect that Hc2 is very sensitive to the sample quality. A new perspective in UTe2 is the proximity of a Kondo semiconducting phase predicted by the LDA band structure calculations.