2025/03/21 by Sharma, Nileema, Toole, Matthew, McKenzie, James +5 · 1 citation
#FOS: Physical sciences #Strongly Correlated Electrons (cond-mat.str-el) #Superconductivity (cond-mat.supr-con)
paper · doi:10.48550/arxiv.2503.17450
Superconducting vortices can reveal electron pairing details and nucleate topologically protected states. Yet, vortices of bulk spin-triplet superconductors have never been visualized. Recently, UTe2 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 UTe2 single crystals (Tc=2.1 K) using scanning tunneling microscopy (STM). We introduce (d2 I)/(dV2) imaging as an effective method 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 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.