Probing p-wave superconductivity in UTe2 via point-contact junctions

Abstract

Uranium ditelluride (UTe₂) is the strongest contender to date for a p-wave superconductor in bulk form. Here we perform a spectroscopic study of the ambient pressure superconducting phase of UTe₂, measuring conductance through point-contact junctions formed by metallic contacts on different crystalline facets down to 250 mK and up to 18 T. Fitting a range of qualitatively varying spectra with a Blonder-Tinkham-Klapwijk (BTK) model for p-wave pairing, we can extract gap amplitude and interface barrier strength for each junction. We find good agreement with the data for a dominant p _y -wave gap function with amplitude 0.26 ± 0.06 meV. Our work provides spectroscopic evidence for a gap structure consistent with the proposed spin-triplet pairing in the superconducting state of UTe₂.

Keywords: Superconducting properties and materials; Topological matter.

Fig. 1. Differential conductance spectra of UTe₂ junctions at 250−350 mK.

a Schematic of point-contact devices fabricated using single-crystal samples of UTe₂. b Graphical representation of facet directions of samples S1 and S2. c−f Normalized differential conductance (blue) and fits (magenta) using a p-wave BTK model as described in the main text. Obtained fit parameter values for gap energy Δ, scattering rate Γ, impedance Z, and temperature T are noted. Here, T is fixed to the experimentally measured temperature, and Z is a fitting parameter except for S2-A. Z is set to 30 for S2-A for convenience, since there is very little variation in the spectra with increasing Z upon entering the tunneling regime. Each junction resistance is shown in the figure.

Fig. 2. Temperature and magnetic field dependence of the differential conductance of UTe₂ junctions.

a−d Zero-field differential conductance di/dv measured at fixed temperatures between 0.3 K and 2 K. For each panel, the T = 1.5 K or 1.6 K data are labeled to denote the resistive transition temperature. e−h Magnetic field dependence of di/dv measured at 350 mK for S1 and 250 mK for S2. For S1-A and S1-B, the magnetic field is applied in-plane, and the upper critical field is obtained from the independently measured resistive transition. For S2-A and S2-B, the magnetic field is applied out-of-plane, and the upper critical field is estimated using the orientation of facets and the known angle dependence (see SM, section III).

Fig. 3. BTK simulation of normalized differential conductance using p-wave model.

Simulated conductance spectra are presented for junctions oriented along different facets of the orthorhombic crystal structure, plotted as a function of interface barrier strength Z (color scale) ranging between 0 (blue) and 5 (red). a−i show the BTK simulation with p_x, p_y, p_z, respectively. a, d, g are BTK simulation at the facet $\widehat{n}=\left[0,0,1\right]$. b, e, h are BTK simulation at the facet $\widehat{n}=\left[0.4,0.6,0.7\right]$. c, f, i are BTK simulations at the facet $\widehat{n}=\left[0.5,0.4,0.8\right]$. All spectra are plotted with fixed parameters k_BT = 0.1Δ and Γ = 0.4Δ.

Fig. 4. Extracted Superconducting energy gap magnitudes.

Circles represent the gap sizes extracted from the p-wave BTK fits. The uncertainty is defined by the 95% confidence interval for the fit. The dashed and dotted curves are the simulated temperature dependence of Δ for s-wave and p-wave models, respectively, with T_c fixed at 1.6 K. The red and orange curves, measured in S1 and S2, respectively, demonstrate the superconducting transition measured by four-probe resistance using ohmic contacts as current leads and point-contacts for voltage leads.