Spectroscopic evidence for a first-order transition to the orbital Fulde-Ferrell-Larkin-Ovchinnikov state

A Cooper pair in a superconductor may be driven into a distinct state with non-zero total momentum when the time-reversal symmetry is broken. Recently, a new type of finite momentum superconducting pairing, namely the orbital Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state, has been proposed in superconductors with strong spin-orbit coupling such as NbSe2. Experimental demonstration for this state is limited to resistance measurements, which is not enough to establish the full physical picture. In this talk, I will show the thermodynamic evidence for the orbital FFLO state. By carrying out tunneling spectroscopy measurements with atomically flat van der Waals tunneling junctions, a first-order phase transition from the Ising pairing to the orbital FFLO state in NbSe2 is directly observed. The phase transition manifests itself by two distinct features: (1) a discontinuity in the superconducting gap value as a function of the in-plane magnetic field. (2) a hysteresis behavior when sweeping the magnetic field back and forth. The observation matches our theoretic calculation by taking into account the melting of the Josephson vortex lattice.

Menghan Liao is a Collaborateur scientifique in the Department of Quantum Matter Physics at the University of Geneva. In 2020, he received a PhD degree from the Department of Physics at Tsinghua University, for his experimental work on 2D superconductors. From 2021 to 2023, he worked as a postdoc in Prof. Alberto Morpurgo Group at the University of Geneva and extended his research to van der Waals (vdW) 2D magnetic materials. In 2023, he received the Ambizione grant from the Swiss National Science Foundation and became an independent research scientist in the same department. His current research focuses on the electrical transport of vdW devices to investigate unconventional superconductivity and 2D magnetism. He has been cited more than 1800 times according to Google Scholar.

If you would like to meet with Menghan during his visit, please contact Susan LaMoreaux.