CM Seminar: Janez Bonca – University of Ljubljana
Title: Equilibrium and far-from-equilibrium properties of bipolaron coupled to  dispersive phonons

Abstract: In the first part of my talk, I will discuss  a Holstein-like model with two electrons nonlinearly coupled to quantum phonons. Using an efficient method based on full quantum approach [1-4] we  simulate the dynamical response of a system subject to a short spatially uniform optical pulse that couples to dipole-active vibrational modes. Nonlinear electron-phonon coupling can either soften or strengthen the phonon frequency in the presence of electron density [5]. When two electrons are free to propagate on a lattice subject to non-linear coupling to phonons that soften phonon frequency, an external optical pulse with well tuned frequency can induce attraction between electrons. Electrons remain bound long after the optical pulse is switched off. Changing the frequency of the pulse the attractive electron–electron interaction can be switched to repulsive. Two sequential optical pulses with different frequencies can switch between attractive and repulsive interaction [6].

In the second part, I will discuss the phase diagram of the bipolaron in the Holstein – Hubbard model in the presence of dispersive phonons. We show that a finite dispersion can stabilize a bound bipolaron even at large Coulomb repulsion U [7]. The sign of the curvature of the optical phonon dispersion plays a decisive role on the bipolaron binding energy and the effective mass in the presence of  U. Finally, I will discuss the influence of U on the ARPES spectral function of the bipolaron.

Speaker Bio: Janez Bonca is a Professor and Dean of the Faculty of Mathematics and Physics at the University of Ljubljana, Slovenia.  His research interests include theory of incommensurate systems, theory of strongly correlated systems and high temperature superconductors, theory of heavy fermion systems, theory of mesoscopic systems and quantum dots, theory of frustrated spin systems, physics of electron – phonon interaction and theory of polarons and bipolarons, study of systems driven far from equilibrium, thermalization in many-body systems, theory of many-body localization. Prof. Bonca completed his PhD from the University of Ljubljana, and worked as a Post-doctoral Associate at  Los Alamos National Laboratory from 1992-1995.