Observation of Persistent Zero Modes and Superconducting Vortex Doublets in UTe${}_2$

Abstract

Superconducting vortices can reveal electron pairing details and nucleate topologically protected states. Yet, vortices of bulk spin-triplet superconductors have never been visualized. Recently, UTe${}_2$ has emerged as a nominative spin-triplet superconductor, but its superconducting order parameter is elusive, and whether time-reversal symmetry is broken remains unsettled. Here, we visualize superconducting vortices on the (011) surface of ultra-clean UTe${}_2$ single crystals ($T_c=2.1K$) using scanning tunneling microscopy (STM). We introduce $\frac{d^{2}I}{d{V}^2}$ imaging as an effective method for vortex visualization in superconductors with substantial residual zero-energy density of states (DOS), as in UTe${}_2$. Anisotropic single-flux-quantum vortices, with coherence lengths of $\sim$ 12 nm (4 nm) parallel (perpendicular) to the a-axis, form a triangular vortex lattice (VL) under small out-of-plane magnetic fields. The vortex size decreases at higher fields, suggesting multi-band superconductivity. The robustness of vortex structures and VL against magnetic field polarity and cooling history strongly supports time-reversal invariant superconductivity. Spatially non-splitting and spectroscopically narrow zero-bias peaks (ZBPs) of differential conductance at vortex cores (VCs) persist up $8T$, well beyond the Pauli limit ($3.9T$), consistent with Majorana zero modes (MZMs) in a topological vortex line protected by a combination of mirror, time-reversal, and particle-hole symmetries. Close examination of vortex structures reveals a mirror-asymmetric doublet $--$ one with a singular core and another with an enhanced energy gap. Such unconventional vortices in a non-chiral $p$-wave superconductor could originate from the induction of a subdominant order parameter close to the VCs.

11+17 pages, 4+14 figures. v2: Discussions added with minor changes in the acknowledgement

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Cited by in corpus (3)

• Two-dimensional flat band on the (011) surface of UTe${}_2$: Implication for STM measurements with a superconducting tip
• Nodal Superconductivity of UTe${}_2$ Probed by Field-Angle-Resolved Specific Heat on a Crystal with $T_{\mathrm{c}}=2.1$ K
• Quasiparticle Interference of Spin-Triplet Superconductors: Application to UTe${}_2$