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

Area of Science:

• Condensed Matter Physics
• Superconductivity Research
• Materials Science

Background:

• Superconducting vortices offer insights into electron pairing and topological states.
• Visualizing vortices in bulk spin-triplet superconductors at the atomic scale remains challenging.
• UTe2 is a prime spin-triplet superconductor with an elusive order parameter and unsettled time-reversal symmetry (TRS) status.

Purpose of the Study:

• To visualize superconducting vortices in UTe2 at the atomic scale.
• To investigate the nature of superconductivity in UTe2, specifically regarding time-reversal symmetry.
• To explore the potential presence of topological states, such as Majorana zero modes, within the vortices.

Main Methods:

• Utilized scanning tunneling microscopy (STM) on ultraclean UTe2 single crystals (Tc = 2.1 K).
• Employed d2I/dV2 imaging, an effective technique for vortex visualization in superconductors with a significant zero-energy density of states (DOS).
• Applied small out-of-plane magnetic fields to induce vortex lattice formation.

Main Results:

• Successfully visualized anisotropic single-flux-quantum vortices with distinct coherence lengths along the a-axis (12 nm) and perpendicular to it (4 nm).
• Observed the formation of a triangular vortex lattice (VL), whose structure and lattice remained invariant under changes in magnetic field polarity and cooling history, supporting time-reversal invariant superconductivity.
• Detected persistent, nonsplit, spectrally sharp zero-bias conductance peaks (ZBPs) at vortex cores up to 8 T, consistent with symmetry-protected Majorana zero modes (MZMs).
• Identified a mirror-asymmetric doublet in vortex structures, with one component exhibiting ZBPs and the other an enhanced apparent gap, potentially indicating a field-induced multicomponent order parameter.

Conclusions:

• The study provides the first atomic-scale visualization of vortices in a bulk spin-triplet superconductor, UTe2.
• Experimental evidence strongly supports time-reversal invariant superconductivity in UTe2 under zero field.
• The observed zero-bias conductance peaks at vortex cores are consistent with the presence of Majorana zero modes, suggesting topological superconductivity in UTe2.