Floquet DMFT Analysis of High Harmonic Generation in Mott Insulators

We study the high harmonic generation (HHG) in Mott insulators using Floquet dynamical mean-field theory (DMFT). We show that the main origin of the HHG in Mott insulators is the doublon-holon recombination, and that the character of the HHG spectrum differs depending on the field strength. In the weaker-field regime, the HHG spectrum shows a single plateau as in the HHG from gases, and its cut-off energy $\epsilon_{\rm cut}$ scales linearly with the field strength $E_0$ as $\epsilon_{\rm cut}=\Delta_{\rm gap} + \alpha E_0$, where $\Delta_{\rm gap}$ is the Mott gap. On the other hand, in the stronger-field regime, multiple plateaus emerge and the $m$-th cut-off scales as $\epsilon_{\rm cut,m}=U + m E_0$. We show that this difference originates from the different dynamics of the doublons and holons in the weak- and strong-field regimes. We also comment on the similarities and differences between HHG from Mott insulators and from semiconductors. This proceedings paper complements our recent work, Phys. Rev. Lett. 121, 057405 (2018), with additional results and analyses.Comment: 6 pages, proceedings of SCES2019(accepted

Nonthermal switching of charge order: Dynamical slowing down and optimal control

We investigate the laser-induced dynamics of electronically driven charge-density- wave (CDW) order. A comprehensive mean-field analysis of the attractive Hubbard model in the weak-coupling regime reveals ultrafast switching and ultrafast melting of the order via a nonthermal pathway. The resulting nonequilibrium phase diagram exhibits multiple distinct regimes of the order parameter dynamics upon increasing field strength, indicative of multiple dynamical phase transitions. Using an intuitive pseudospin picture, we show how the distinct dynamical regimes can be connected to the spin precession angle. We furthermore study the effects of electron-electron interactions beyond mean field to show that the main features of the phase diagram are robust against scattering or thermalization processes. For example, the nonthermal state with switched order is characterized by a particularly slow relaxation. The nonthermal phases can be stabilized by tailoring the pulse shape with optimal control theory. We also demonstrate how the dynamics allows to distinguish an electron-electron interaction driven CDW from an electron-phonon interaction driven CDW

Energy flow during relaxation in an electron-phonon system with multiple modes: A nonequilibrium Green's function study

We investigate an energy flow in an extended Holstein model describing electron systems coupled to hot-phonons and heat-bath phonons. To analyze the relaxation process after the photo-excitation of electrons, we employ the nonequilibrium dynamical mean-field theory (DMFT). We find the backward energy flow during the relaxation, where the direction of energy transfer between electrons and hot-phonons is reversed. To clarify the microscopic mechanism of the backward energy flow, we introduce the approximated energy flows, which are calculated with the gradient and quasiparticle approximations and are related to the nonequilibrium distribution functions. We compare these approximated energy flows with the full energy flows calculated from the nonequilibrium DMFT. We find that, in the weak electron-hot-phonon coupling regime, the full and approximated energy flows are almost the same, meaning that the relaxation dynamics can be correctly understood in terms of the nonequilibrium distribution functions. As the strength of the electron-hot-phonon coupling increases, the approximated energy flow fails to qualitatively reproduce the full energy flow. This indicates that the microscopic origin of the energy flow cannot be solely explained by the nonequilibrium distribution functions. By comparing the energy flows with different levels of approximation, we reveal the role of the gradient and quasiparticle approximations.Comment: 20 pages, 12 figure

High-harmonic generation in the Rice-Mele model: Role of intraband current originating from interband transition

We consider high-harmonic generation (HHG) in the Rice-Mele model to study the role of the intraband current originating from the change of the intraband dipole via interband transition. This contribution, which has been often neglected in previous works, is necessary for the consistent theoretical formulation of the light-matter coupling. We demonstrate that the contribution becomes crucial when the gap is smaller than or comparable to the excitation frequency and the system is close to the half filling.Comment: 6 pages, 2 figure

Multiple amplitude modes in strongly coupled phonon-mediated superconductors

We study collective amplitude modes of the superconducting order parameter in strongly coupled electron-phonon systems described by the Holstein model using the nonequilibrium dynamical mean-field theory with the self-consistent Migdal approximation as an impurity solver. The frequency of the Higgs amplitude mode is found to coincide with the superconducting gap even in the strongly coupled (beyond BCS) regime. Besides the Higgs mode, we find another collective mode involving the dynamics of both the phonons and the superconducting order parameter. The frequency of this mode, higher than twice the renormalized phonon frequency in the superconducting phase, is shown to reflect a strong electron-mediated phonon-phonon interaction. Both of collective modes are predicted to contribute to time-resolved photoemission spectra after a strong laser pump as vertex corrections to produce resonance peaks, which allows one to distinguish them from quasiparticle excitations