Quantum Matter Seminars - Yakovenko

Loop currents and anomalous Hall effect due to time-reversal-breaking superconductivity

Sept. 9, 2020 (Wed.) at 3PM (ET)

Victor Yakovenko

Department of Physics, CMTC and JQI, University of Maryland, College Park

It was found experimentally that superconductivity spontaneously breaks time-reversal symmetry (TRS) in such materials as Sr2RuO4, UPt3, URu2Si2, and Bi/Ni bilayers. For the latter material, we argue that the superconducting order parameter has the winding number of +-2 around the Fermi surface, thus making Bi/Ni bilayers a rare example of intrinsic 2D topological superconductivity [1]. The experimental evidence for TRS breaking comes from the polar Kerr effect, which is rotation of polarization of normally incident light upon reflection from the sample. For a clean superconductor, theoretical studies indicate that this effect is possible only if electrons have more than one band. To clarify these conditions, we study a model of chiral TRS-breaking superconductivity on the honeycomb lattice with pairing between different sublattices [2]. We show that the experimental manifestations of TRS breaking can be characterized using the TRS-odd commutator of the superconducting pairing potential and its Hermitian conjugate. It generates persistent loop currents around each lattice site and opens a topological mass gap at the Dirac points with the corresponding chiral edge states, as in Haldane's model of the quantum anomalous Hall effect. It also generates the intrinsic ac Hall conductivity in the absence of an external magnetic field, which determines the polar Kerr effect. We also speculate on a possibility of breaking Z2 time-reversal and U(1) gauge symmetries in two separate phase transitions.

[1] X. Gong, M. Kargarian, A. Stern, D. Yue, H. Zhou, X. Jin, V. M. Galitski, V. M. Yakovenko, and J. Xia, "Time-reversal symmetry-breaking superconductivity in epitaxial bismuth/nickel bilayers", Science Advances 3, e1602579 (2017), arXiv:1609.08538

[2] P. M. R. Brydon, D. S. L. Abergel, D. F. Agterberg, and V. M. Yakovenko, "Loop currents and anomalous Hall effect from time-reversal symmetry-breaking superconductivity on the honeycomb lattice", Phys. Rev. X 9, 031025 (2019), arXiv:1802.02280

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