Unconventional Superconductivity

The most sustained research effort in Prof. Liu’s lab has been the work on the symmetry properties of the superconducting order parameter (OP) in bulk single crystals of Sr2RuO4 and the eutectic phase of Sr2RuO4-Ru in collaborating with several other research groups. Prof. Liu’s group performed the first Josephson-effect-based phase-sensitive test on the symmetry properties of the orbital part of the superconducting OP in bulk Sr2RuO4 which yield evidence supporting the odd-parity, spin-triplet superconductivity in this material. Evidence for unconventional superconductivity was also found for the eutectic phase of Sr2RuO4-Ru. The experimental work on Sr2RuO4was extended to the detection of half-quantum fluxoid states and the chiral surface currents. Our current experiments bulk Sr2RuO4and eutectic Sr2RuO4-Ru are focused on determining their precise pairing symmetries and the exploration of the experimental consequences.  

The phase-sensitive test of the pairing symmetry was also performed on UTe2, a heavy Fermion superconductor discovered more recently. This material features modestly anisotropic electrical and magnetic properties in the normal state. However, strongly anisotropic superconductivity was found, including the grammatically large upper critical fields. Most surprisingly, as the magnetic field applied in certain direction increases, the superconductivity is suppressed first but returns as the field is increased further. These properties are difficult to understand without invoking spin-triplet pairing. Our group was the first to prepare planar Josephson junctions using UTe2 as an electrode and performed the phase-sensitive test of the pairing symmetry in it. Our data suggests that the superconducting OP is of the odd-parity and therefore must be spin-triplet. We are currently working to extend the tunneling measurements to higher magnetic fields.

Recent publications:

1. Anthony J. Leggett and Ying Liu, “Symmetry properties of superconducting order parameter in Sr2RuO4,” J. Supercond. Novel Mag. 34, 1647 (2021).

2. Xinxin Cai, Brian M. Zakrzewski, Yiqun A. Ying, Hae-Young Kee, Manfred Sigrist, J. Elliott Ortmann, Weifeng Sun, Zhiqiang Mao, and Ying Liu, “Magnetoresistance oscillation study of the spin counterflow half-quantum vortex in doubly connected mesoscopic superconducting cylinders of Sr2RuO4,” Phys. Rev. B 105, 224510 (2022).

3. Zixuan Li, Camilla M. Moir, Nathan J. McKee, Eric Lee-Wong, Ryan E. Baumbach, M. Brian Maple, Ying Liu, “Observation of odd-parity superconductivity in UTe2”, Proc. Nat. Acad. Sci. 122, e2419734122 (2025).