Thermodynamic signatures of short-range magnetic correlations in ${\mathrm{UTe}}_2$

The normal state out of which unconventional superconductivity in ${\mathrm{UTe}}_2$ emerges is studied in detail using a variety of thermodynamic and transport probes. Clear evidence for a broad Schottky-like anomaly with roughly $R\text{ln}2$ entropy around $T^{*}\approx 12$ K is observed in all measured quantities. Comparison with high magnetic field transport data allows the construction of an $H\text{−}T$ phase diagram resembling that of the ferromagnetic superconductor URhGe. The low-field electronic Grüneisen parameter of $T^{*}$ and that of the metamagnetic transition at $H_m\approx 35$ T are comparable pointing to a common origin of both phenomena. Enhanced Wilson and Korringa ratios suggest that the existence of short-range ferromagnetic fluctuations cannot be ruled out.

Figure 1

(a) $\alpha /T$ for all three crystallgraphic directions together with the calculated volume expansion. The solid black line is a Debye fit to the phonon background with ${\mathrm{\Theta }}_D=260$ K. (b) Electronic contribution to $c$ axis thermal expansion of ${\mathrm{UTe}}_2$. The inset shows the electronic Grüneisen parameter. (c) Electronic contribution to the specific heat with $\gamma$ and $T_c$ indicated with the corresponding entropy shown in the inset. Panel (d) shows the temperature derivative of the $a$-axis resistivity. The arrow and the red dashed line indicate the position of the crossover temperature $T^{*}$.

Figure 2

(a)–(c) Temperature dependence of ${\alpha }_c$ in 0 and 10 T applied magnetic fields. (d)–(f) Derivative of the fit to the high-field resistivity data from [5] (see Supplemental Material [23]). (g)–(i) Electronic specific heat in 0 and 10 T. (j) shows the evolution of $T^{*}(H$) (circles) for the three field directions. $T^{*}(H$) is either determined by the position of the maximum in $d{\rho }_a/dT$, the maximum of $C_e$, or the minimum in ${\alpha }_c$ shown in the Supplemental Material [23]. The square symbols represent the first-order metamagnetic transition from Refs. [4, 5]. Lines correspond to the normalized field dependence of $1/\sqrt{A}$ and $1/\gamma$, taken from Refs. [5] and [34], respectively. Dashed lines serve as a guide to the eye.

Figure 3

(a) Magnetic susceptibility measured for various field amplitudes and directions. (b) Curie-Weiss fit for all three crystallographic directions with negative Curie-Weiss temperatures. (c) Susceptibility with subtracted Curie-Weiss fit indicating that deviations from the Curie-Weiss behavior develop together with the anisotropy between the $a$ and $b$ axis at low temperature.