Visualizing the Odd-parity Superconducting Order Parameter and its Quasiparticle Surface Band in UTe2

TL;DR

This study uses scanning tunneling microscopy and quasiparticle interference imaging to identify the topological superconducting order parameter in UTe2, revealing non-chiral, odd-parity, spin-triplet pairing with nodal structure.

Contribution

It provides direct experimental evidence of the quasiparticle surface band and the nature of the superconducting order parameter in UTe2, distinguishing between chiral and non-chiral states.

Findings

Observation of zero-energy Andreev conductance peak indicating non-chiral superconductivity.

Visualization of quasiparticle interference patterns consistent with a nodal, odd-parity order parameter.

Detection of a characteristic sextet of interference wavevectors confirming bulk gap structure.

Abstract

A distinctive identifier of nodal intrinsic topological superconductivity (ITS) would the appearance of an Andreev bound state on crystal surfaces parallel to the nodal axis, in the form of a topological quasiparticle surface band (QSB) appearing only for $T < T_C$. Moreover, theory shows that specific QSB characteristics observable in tunneling to an s-wave superconductor can distinguish between chiral and non-chiral ITS order parameter $\Delta_k$. To search for such phenomena in $\text{UTe}_2$, s-wave superconductive scan-tip scanning tunneling microscopy (STM) imaging was employed. It reveals an intense zero-energy Andreev conductance maximum at the $\text{UTe}_2$ (0-11) crystal termination. Development of the zero-energy Andreev conductance peak into two finite-energy particle-hole symmetric conductance maxima as the tunnel barrier is reduced, then signifies that $\text{UTe}_2$ superconductivity is non-chiral. Quasiparticle interference imaging (QPI) for an ITS material should be dominated by the QSB for energies within the superconductive energy gap $|E| \le \Delta$, so that bulk $\Delta(k)$ characteristics of the ITS can only be detected excursively. Again using a superconducting scan-tip, the in-gap quasiparticle interference patterns of the QSB of $\text{UTe}_2$ were visualized. Specifically, a band of Bogoliubov quasiparticles appears as a characteristic sextet $q_i$ :$i = 1-6$ of interference wavevectors showing that QSB dispersions $k(E)$ occur only for energies $|E| \le \Delta_{max}$ and only within the range of Fermi momenta projected onto the (0-11) crystal surface. In combination, these phenomena are consistent with a bulk $\Delta(k)$ exhibiting spin triplet, time-reversal conserving, odd-parity, a-axis nodal, $B_{3u}$ symmetry in $\text{UTe}_2$.