CMT Student Seminar: Zhi-Yuan Wei

** This is a new weekly seminar for local condensed matter theory students and postdocs to present their work to one another. Everyone is welcome! If you're interested in presenting at a future seminar, please send a message to This email address is being protected from spambots. You need JavaScript enabled to view it.. **
 
Time:  Thursday, March 6, 2025 - 1:00-2:00pm
Location:  ATL 3100A and Virtual Via Zoom: https://umd.zoom.us/j/94978519761

Speaker: Zhi-Yuan Wei, UMD
Title: Kondo Impurity in Attractive Hubbard Baths

Abstract: We investigate theoretically equilibrium and dynamical properties of a Kondo impurity coupled to either 1D or 2D superconductors, modeled by the attractive Fermi-Hubbard model. By employing a non-Gaussian variational approach, we go beyond the approximation of a constant superconducting (SC) gap. We show that the dynamical properties of the system can be modified qualitatively, when space and time dependent renormalization of the SC gap and electron-impurity hybridization are included. For the ground state, we find the singlet-doublet phase transition and π-phase shifts of the SC order parameter. For dynamics, first, we consider spin dynamics following an abrupt connection of the polarized impurity to the 2D bath. We find rapid relaxation of impurity polarization and directional emission of a magnetization pulse, which becomes damped as it propagates into the bulk. Then we analyze transport between two SC leads coupled through the impurity at finite bias voltage. Here we go beyond analysis of the steady state to investigate full-time dynamics following an abrupt application of the bias voltage. We uncover four distinct regimes in the transient dynamics and transport properties: (I) the AC Josephson effect regime; (II) dynamical competition between charge-density-wave (CDW) and SC orders with transient Kondo correlations; (III) the coexistence of AC and DC currents facilitated by partial Kondo screening and dynamical stabilization of the SC order; (IV) DC Kondo transport regime modified by the SC order. Regime II exhibits a dynamical transition from SC to CDW order that locally restores the U(1) symmetry. We argue that our findings for regime IV provide a theoretical explanation for the experimentally observed anomalous enhancement of DC conductance and suppression of the AC Josephson current. Finally, we discuss the potential experimental realization with ultracold atoms.
 
Journal Reference: https://arxiv.org/abs/2501.05562