Quantitative thermodynamic study of superconducting and normal states in UTe2 under pressure

Abstract

We report a quantitative calorimetric study of UTe2 under pressure with a direct measurement of the Sommerfeld gamma coefficient, showing a three-fold enhancement of electronic effective mass when approaching the critical pressure where superconductivity is suppressed and ordered states occur. We analyse the evolution of gamma with the amplitude of the jumps in the specific heat at the two superconducting transitions, and the superconducting critical temperature with pressure. This analysis would suggest that the high pressure superconducting phase nucleates only on a fraction of the Fermi surface. It also points to the possible major role of a quantum critical point of the unidentified phase that has been called weak magnetic order, rather than to the critical pressure of the antiferromagnetic phase. Just at the border of long-range antiferromagnetic order, where superconductivity emerges from the weak magnetic order phase, a significant increase in the specific heat jump for both superconducting transitions is found, accompanied by a noticeable change of their shapes.

Figures (3)

• Figure 1: Specific heat as a function of temperature at different pressures. Blue and orange triangles represent superconducting transitions (to resp. SC1 and SC2). Violet and grey triangles represent the magnetic phases (resp. AFM and WMO).
• Figure 2: Zero-field $p-T$ phase diagram of UTe$_2$ obtained from quantitative specific heat measurements in the piston cylinder cell (red squares), and ac calorimetry in the diamond anvil cell (green circles). The pressure is normalized to the critical pressure where the AFM order appears (1.45 GPa and 1.62 GPa for the piston-cylinder and DAC respectively. The WMO phase is stabilized in a significant pressure range before the critical pressure where the AFM state appears, with the maximum $T_\text{c}$ lying close to the extrapolated critical pressure $p_\text{WMO}$.
• Figure 3: (a) Relative evolution of gamma coefficient and entropy as a function of pressure. The red dotted line is a guide for the eye, the orange line a prediction from a strong coupling model with an increase of the fraction ($\alpha$ - blue dashed line on right axis) of the Fermi surface affected by the pairing mechanism of SC2 (see text) (b) Specific heat jumps for both superconducting transitions. The BCS jump expected from gamma is also shown in grey. The orange arrow shows the extrapolated crossing point of SC1 and SC2. (c) Zero-field phase diagram obtained from Fig. \ref{['fig:raw']}. Dashed lines: strong coupling constants (right axis) reproducing the pressure dependence of the critical temperatures of SC1 and SC2.