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

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 ${\text{UTe}}_2$.

27 pages, 6 figures

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