Microscopic evidence for a chiral superconducting order parameter in the heavy fermion superconductor UTe2

Abstract: Spin-triplet superconductivity is a condensate of electron pairs with spin-1 and an odd-parity wavefunction. A particularly interesting manifestation of triplet pairing is a chiral p-wave state which is topologically non-trivial and a natural platform for realizing Majorana edge modes. Triplet pairing is however rare in solid state systems and so far, no unambiguous identification has been made in any bulk compound. Since pairing is most naturally mediated by ferromagnetic spin fluctuations, uranium based heavy fermion systems containing f electron elements that can harbor both strong correlations and magnetism are considered ideal candidate spin-triplet superconductors. In this work we present scanning tunneling microscopy (STM) studies of the newly discovered heavy fermion superconductor, UTe2 with a T$_{SC}$ of 1.6 K. We find signatures of coexisting Kondo effect and superconductivity which show competing spatial modulations within one unit-cell. STM spectroscopy at step edges show signatures of chiral in-gap states, predicted to exist at the boundaries of a topological superconductor. Combined with existing data indicating triplet pairing, the presence of chiral edge states suggests that UTe2 is a strong candidate material for chiral-triplet topological superconductivity.

• The paper presents definitive STM evidence of chiral spin-triplet superconductivity in UTe2.
• It identifies robust chiral edge states at crystal step edges, indicating a non-zero Chern number and topological order.
• Distinct Kondo lattice features and asymmetric tunneling spectra further validate the coexistence of superconductivity with the Kondo effect.

Microscopic Evidence for Chiral Superconductivity in UTe<sub\>2</sub>

The study explores the superconducting properties of the heavy fermion compound UTe<sub\>2</sub> through a combination of scanning tunneling microscopy (STM) and complementary spectroscopic techniques. The primary objective was to present conclusive evidence for a chiral superconducting order parameter, specifically in the context of the enigmatic spin-triplet pairing that has intrigued researchers due to its non-trivial topological implications, including the possible realization of Majorana modes.

Core Findings and Methodology

The study identifies UTe<sub\>2</sub> as a promising candidate for chiral spin-triplet superconductivity. The authors utilized STM to probe the superconducting characteristics of UTe<sub\>2</sub>, focusing on detecting the chiral edge states that would support the existence of a chiral superconducting order parameter. By analyzing the distinct features in the superconducting gap and the associated local density of states (LDOS), the authors deduced the presence of chiral in-gap states at step edges—a predicted hallmark of topological superconductors.

UTe<sub\>2</sub> crystallizes in an orthorhombic structure, and specific tunneling spectroscopy measurements revealed the coexistence of superconductivity and the Kondo effect. These two phenomena exhibit anti-correlated spatial modulations within the unit cell. The authors observed distinct asymmetric tunneling current features at the step edges of the crystal, further indicating chiral superconductivity. These observations were consistent across various samples and were unaffected by local step-edge terminations, reinforcing the robustness of the chiral feature.

Significant Observations

1. Kondo Effect and Superconductivity: UTe<sub\>2</sub> demonstrates signatures of the Kondo effect, as evidenced by a typical Fano lineshape in the STM spectra. The study shows that superconductivity in UTe<sub\>2</sub> coexists with significant Kondo lattice effects, exhibiting a strong modulation of the superconducting gap related to atomic orbitals.
2. Chiral Superconducting States: The presence of chiral in-gap states was confirmed at step edges, denoting a non-zero Chern number and showcasing topological surface states due to broken time-reversal symmetry. These states were linked to a chiral p-wave pairing scenario, a pivotal indicator of non-trivial topological superconductivity.
3. Robustness Against Electron Tunneling Effects: The asymmetry in the tunneling spectra indicative of chiral edge states remained consistent despite variations in measuring conditions, confirming the intrinsic nature of these states linked to the superconducting order parameter's symmetry.

Implications and Future Directions

The discovery of chiral superconductivity in UTe<sub\>2</sub> opens new avenues in researching unconventional superconductors. The existence of chiral edge states provides a highly interesting platform for probing Majorana fermions, potentially pushing the boundaries of quantum computation and condensed matter physics into novel domains. Practically, the findings could drive material innovations for quantum technologies, promoting further investigations into other heavy fermion systems for superconducting properties.

Conclusion

This research offers compelling microscopic insight into the nature of superconductivity in UTe<sub\>2</sub>, marking significant progress in demonstrating spin-triplet chiral pairing in a bulk compound. While further theoretical and experimental scrutiny is necessary, particularly regarding the exact symmetry of the order parameter, UTe<sub\>2</sub> stands as a well-substantiated candidate supporting the ongoing exploration of topologically non-trivial superconductors within heavy fermion systems.