Multiple nodal superconducting phases and order-parameter evolution in pressurized UTe$_2$

TL;DR

This study uses point-contact spectroscopy to reveal multiple superconducting phases in pressurized UTe$_2$, identifying the evolution of its order parameter and providing key insights into its unconventional spin-triplet pairing symmetry.

Contribution

It provides the first spectroscopic evidence of multiple SC phases and clarifies the order parameter symmetry in pressurized UTe$_2$ using a novel analysis approach.

Findings

Presence of a $p_z$ component in the SC order parameter.

Identification of $B_{2u}$ or $B_{3u}$ symmetry for the order parameter.

Distinction of multiple SC phases via the ratio of $p_z$ to $p_{x(y)}$ wave pairings.

Abstract

Spin-triplet superconductivity (SC) offers a unique avenue for realizing non-Abelian Majorana zero modes and thus the fault-tolerant topological quantum computation, and has attracted a broad audience for both fundamental research and potential applications. The recently discovered heavy-fermion spin-triplet superconductor candidate UTe$_2$ has sparked great interest for its ultrahigh upper critical field and reentrant SC phases in the proximity to a field-polarized magnetic state. Despite extensive studies on the phase diagrams and competing orders induced by pressure and magnetic field, limited has been known about its SC order parameters and their evolution with these control parameters, largely due to the lack of appropriate symmetry-sensitive detections. Here, we report comprehensive point-contact spectroscopy measurements of pressurized UTe$_2$ on the (0~0~1) surface. The observation of Andreev bound state strongly suggests the presence of a $p_z$ component in the SC order parameters. Quantitative analysis based on an extended Blonder-Tinkham-Klapwijk model unveils $B_{2u}$ or $B_{3u}$ as the most likely representation for both ambient and pressurized UTe$_2$, and remarkably, the multiple SC phases can be distinguished by a single parameter $\langle \Delta_{z}\rangle/\langle\Delta_{x(y)}\rangle$, the relative weight between the $p_z$-wave and $p_{x(y)}$-wave pairings. These findings not only impose stringent constraints on the superconducting order parameter in UTe$_2$, but also provide key spectroscopic evidence for the existence of multiple SC phases tuned through pressure.