Observation of Persistent Zero Modes and Superconducting Vortex Doublets in UTe₂.

Abstract

Superconducting vortices can reveal electron pairing details and nucleate topologically protected states. Yet, vortices of bulk spin-triplet superconductors have never been visualized at the atomic scale. Recently, UTe₂ has emerged as a prime spin-triplet superconductor, but its superconducting order parameter is elusive, and whether time-reversal symmetry (TRS) is broken remains unsettled. Here, we visualize vortices on the (011) surface of ultraclean UTe₂ single crystals (<i>T</i>_c = 2.1 K) using scanning tunneling microscopy (STM). We introduce <math xmlns="http://www.w3.org/1998/Math/MathML"><mfrac><mrow><msup><mi>d</mi><mn>2</mn></msup><mi>I</mi></mrow><mrow><mi>d</mi><msup><mi>V</mi><mn>2</mn></msup></mrow></mfrac></math> imaging as an effective technique for vortex visualization in superconductors with substantial residual zero-energy density of states (DOS), as in UTe₂. Anisotropic single-flux-quantum vortices, with coherence lengths of ∼12 nm (4 nm) parallel (perpendicular) to the <b>a</b>-axis, form a triangular vortex lattice (VL) under a small out-of-plane magnetic field. The invariance of vortex structures and VL under changes of field polarity and cooling history strongly supports time-reversal invariant superconductivity under zero field. At vortex cores (VCs), nonsplit, spectrally sharp zero-bias conductance peaks (ZBPs) persist up 8 T that are consistent with symmetry-protected Majorana zero modes (MZMs) in a topological vortex line. Close examination of vortex structures reveals a mirror-asymmetric doublet─one with ZBPs and another with an enhanced apparent gap, possibly originating from a field-induced multicomponent order parameter.