We theoretically investigate the effects of the lattice geometry on the nonequilibrium dynamics of photo-excited carriers in a half-filled two-dimensional Hubbard model. Using a nonequilibrium generalization of the dynamical cluster approximation, we compare the relaxation dynamics in lattices which interpolate between the triangular lattice and square lattice configuration and thus reveal the role of the geometric frustration in these strongly correlated nonequilibrium systems. In particular, we show that the cooling effect resulting from the disordering of the spin background is less effective in the triangular case because of the frustration. This manifests itself in a longer relaxation time of the photo-doped population, as measured by the time-resolved photo-emission signal, and a higher effective temperature of the photo-doped carriers in the non-thermal steady state after the intra-Hubbard-band thermalization.
Lattice effects on photo-excited states in interacting charge frustrated system are examined. Real time dynamics in the interacting spinless fermion model on a triangular lattice coupled to lattice vibration are analyzed by applying the exact diagonalization method combined with the classical equation of motion. A photo-induced phase transition from the horizontal stripe-type charge order (CO) to the 3-fold CO occurs through a characteristic intermediate time domain. By analyzing the time evolution in detail, we find that this characteristic dynamics are seen when the electron and lattice sect…
Geometrical frustration and spin-orbit coupling effect together play vital role to influence properties in pyrochlore based iridium oxides. Here we have investigated detailed structural, magnetic, thermodynamic and transport properties of pyrochlore iridate Y$_2$Ir$_2$O$_7$. Magnetization data show onset of magnetic irreversibility around temperature $T_{irr}$ $\sim$ 160 K, however, no sign of long-range type ferromagnetic ordering is observed below $T_{irr}$. Specific heat data show no visible anomaly across $T_{irr}$, and the analysis of data indicate sizable density of states across Fermi l…
We study the nonequilibrium dynamics of photoinduced phase transitions in charge ordered (CO) systems with a strong electron-lattice interaction and analyze the interplay between electrons, periodic lattice distortions, and a phonon thermal reservoir. Simulations based on a tight-binding Hamiltonian and Boltzmann equations reveal partially decoupled oscillations of the electronic order parameter and the periodic lattice distortion during CO melting, which becomes more energy efficient with lower photon energy. The cooling rate of the electron system correlates with the CO gap dynamics, respons…
Relaxation of electrons in a Hubbard model coupled to a dissipative bosonic bath is studied to simulate the pump-probe photoemission measurement. From this insight, we propose an experimental method of eliciting unoccupied part of the single-particle spectra at the equilibrium of doped-Mott insulators. We reveal first that effective temperatures of distribution functions and electronic spectra are different during the relaxation, which makes the frequently employed thermalization picture inappropriate. Contrary to the conventional analysis, we show that the unoccupied spectra at equilibrium ca…
Resonant inelastic X-ray scattering (RIXS) detects various types of high- and low-energy elementary excitations in correlated solids, and this tool will play an increasingly important role in investigations of time-dependent phenomena in photo-excited systems. While theoretical frameworks for the computation of equilibrium RIXS spectra are well established, the development of appropriate methods for nonequilibrium simulations are an active research field. Here, we apply a recently developed nonequilibrium dynamical mean field theory (DMFT) based approach to compute the RIXS response of photo-e…
We examine the effect of the magnetic frustration J' on the single-hole spectral function in the t-t'-t''-J model. At zero temperature, the exact diagonalization (ED) and the self-consistent Born approximation (SCBA) methods are used. We find that the frustration suppresses the quasiparticle (QP) weight at small momentum k, whereas the QP peak at k=(pi/2,pi/2) remains sharp. We also show the temperature dependence of the single-hole spectral function by using the ED method. It is found that the lineshapes at (pi/2,0) and (pi/2,pi/2) show different temperature dependence. These findings are con…
We present a formalism based on nonequilibrium dynamical mean field theory (DMFT) which allows to compute the time-resolved X-ray absorption spectrum (XAS) of photo-excited solids. By applying this formalism to the photo-doped half-filled and quarter-filled two-orbital Hubbard models in the Mott insulating regime we clarify how the time-resolved XAS signal reflects the nonequilibrium population of different local states. Apart from the missing broadening associated with continuum excitations, the atomic XAS spectrum computed with the nonthermal state populations provides a good approximation t…
Photo-excited charge dynamics of interacting charge-frustrated systems are studied using a spinless fermion model on an anisotropic triangular lattice. Real-time evolution of the system after irradiating a pump-photon pulse is analyzed by the exact diagonalization method. We focus on photo-excited states in the two canonical charge-ordered (CO) ground states, i.e., horizontal stripe-type and vertical stripe-type COs, which compete with each other owing to the charge frustration. We find that the photo-induced excited states from the two types of COs are distinct. From the horizontal stripe-typ…
We extend the nonequilibrium dynamical mean field (DMFT) formalism to inhomogeneous systems by adapting the "real-space" DMFT method to Keldysh Green's functions. Solving the coupled impurity problems using strong-coupling perturbation theory, we apply the formalism to homogeneous and inhomogeneous layered systems with strong local interactions and up to 39 layers. We study the diffusion of doublons and holes created by photo-excitation in a Mott insulating system, the time-dependent build-up of the polarization and the current induced by a linear voltage bias across a multi-layer structure, a…
We propose a multielectron approach to calculate superexchange interaction in magnetic Mott- Hubbard insulator La2CuO4(further La214) that allows to obtain the effect of optical pumping on the superexchange interaction. We use the cell perturbation theory with exact diagonalization of the multiband pd Hamiltonian inside each CuO6 unit cell and treating the intercell hopping as perturbation. To incorporate effect of optical pumping we include in this work the excited singlehole local states as well as all two-hole singlets and triplets. By projecting out the interband intercell electron hopping…
We use time-evolution techniques for (infinite) matrix-product-states to calculate, directly in the thermodynamic limit, the time-dependent photoemission spectra and dynamic structure factors of the half-filled Hubbard chain after pulse irradiation. These quantities exhibit clear signatures of the photoinduced phase transition from insulator to metal that occurs because of the formation of so-called $η$ pairs. In addition, the spin dynamic structure factor loses spectral weight in the whole momentum space, reflecting the suppression of antiferromagnetic correlations due to the buildup of $η$-p…
Photo-excited strongly correlated systems can exhibit intriguing non-thermal phases, but the theoretical investigation of them poses significant challenges. In this work, we introduce a generalized Gibbs ensemble type description for long-lived photo-doped states in Mott insulators. This framework enables systematic studies of photo-induced phases based on equilibrium methods, as demonstrated here for the one-dimensional extended Hubbard model. We determine the nonequilibrium phase diagram, which features $η$-pairing and charge density wave phases in a wide doping range, and reveal physical pr…
Competition between ordered phases, and their associated phase transitions, are significant in the study of strongly correlated systems. Here we examine one aspect, the nonequilibrium dynamics of a photoexcited Mott-Peierls system, using an effective Peierls-Hubbard model and exact diagonalization. Near a transition where spin and charge become strongly intertwined, we observe anti-phase dynamics and a coupling-strength-dependent suppression or enhancement in the static structure factors. The renormalized bosonic excitations coupled to a particular photoexcited electron can be extracted, which…
Photo-induced metallic states in a Mott insulator are studied for the half-filled, one-dimensional Hubbard model with the time-dependent density matrix renormalization group. An irradiation of strong AC field is found to create a linear dispersion in the optical spectrum (current-current correlation) in the nonequilibrium steady state reminiscent of the Tomonaga-Luttinger liquid for the doped Mott insulator in equilibrium. The spin spectrum in nonequilibrium retains the des Cloizeaux-Pearson mode with the spin velocity differing from the charge velocity. The mechanism of the photocarrier-dopin…
We show that the recently proposed cooling-by-doping mechanism allows to efficiently prepare interesting nonequilibrium states of the Hubbard model. Using nonequilibrium dynamical mean field theory and a particle-hole symmetric setup with dipolar excitations to full and empty bands we produce cold photo-doped Mott insulating states with a sharp Drude peak in the optical conductivity, a superconducting state in the repulsive Hubbard model with an inverted population, and $η$-paired states in systems with a large density of doublons and holons. The reshuffling of entropy into full and empty band…
We study the dynamics of screening in photo-doped Mott insulators with long-ranged interactions using a nonequilibrium implementation of the $GW$ plus extended dynamical mean field theory ($GW$+EDMFT) formalism. Our study demonstrates that the complex interplay of the injected carriers with bosonic degrees of freedom (charge fluctuations) can result in long-lived transient states with properties that are distinctly different from those of thermal equilibrium states. Systems with strong nonlocal interactions are found to exhibit a self-sustained population inversion of the doublons and holes. T…
We study the nonequilibrium dynamics of photoexcited electrons in the narrow-gap Mott insulator VO$_2$. The initial stages of relaxation are treated using a quantum Boltzmann equation methodology, which reveals a rapid ($\sim$ femtosecond time scale) relaxation to a pseudothermal state characterized by a few parameters that vary slowly in time. The long-time limit is then studied by a Hartree-Fock methodology, which reveals the possibility of nonequilibrium excitation to a new metastable $M_1$ metal phase that is qualitatively consistent with a recent experiment. The general physical picture o…
Intense laser pulses have recently emerged as a tool to tune between different orders in complex quantum materials. Among different light-induced phenomena, transient superconductivity far above the equilibrium transition temperature in cuprates is particularly attractive. Key to those experiments was the resonant pumping of specific phonon modes, which was believed to induce superconducting phase coherence by suppressing the competing orders or modifying the structure slightly. Here, we present a comprehensive study of photo-induced nonequilibrium response in underdoped YBa$_2$Cu$_3$O$_{6+x}$…
We study the dynamics of charge-transfer insulators after photo-excitation using the three-band Emery model and a nonequilibrium extension of Hartree-Fock+EDMFT and GW+EDMFT. While the equilibrium properties are accurately reproduced by the Hartree-Fock treatment of the full $p$ bands, dynamical correlations are essential for a proper description of the photo-doped state. The insertion of doublons and holons leads to a renormalization of the charge transfer gap %and $p$ bands and to a substantial broadening of the bands. We calculate the time-resolved photoemission spectrum and optical conduct…
We present time-resolved magneto-optical spectroscopy on the magnetic Mott-Hubbard-insulating Kitaev spin liquid candidate $α$-RuCl$_3$ to investigate the nonequilibrium dynamics of its antiferromagnetically ordered zigzag groundstate after photoexcitation. A systematic study of the transient magnetic linear dichroism under different experimental conditions (temperature, external magnetic field, photoexcitation density) gives direct access to the dynamical interplay of charge excitations with the zigzag ordered state on ultrashort time scales. We observe a rather slow initial demagnetization (…
We investigate theoretically nonequilibrium effects on photoluminescence and gain/absorption spectra of a driven-dissipative exciton-polariton condensate, by employing the combined Hartree-Fock-Bogoliubov theory with the generalized random phase approximation extended to the Keldysh formalism. Our calculated photoluminescence spectra is in semiquantitative agreement with experiments, where features such as a blue shift of the emission from the condensate, the appearance of the dispersionless feature of a diffusive Goldstone mode, and the suppression of the dispersive profile of the mode are ob…
We use density-matrix theory to calculate the ultrafast dynamics of unconventional superconductors from a microscopic viewpoint. We calculate the time evolution of the optical conductivity as well as pump-probe spectra for a d-wave order parameter. Three regimes can be distinguished in the spectra. The Drude response at low photon energies is the only one of those which has been measured experimentally so far. At higher energies, we predict two more regimes: the pair-breaking peak, which is reduced as Cooper-pairs are broken up by the exciting pulse; and a suppression above the pair-breaking p…
We study the role of excited phonon populations in the relaxation rates of nonequilibrium electrons using a nonequilibrium Green's function formalism. The transient modifications in the phononic properties are accounted for by self-consistently solving the Dyson equation for the electron and phonon Green's functions. The pump induced changes manifest in both the electronic and phononic spectral functions. We find that the excited phonon populations suppress the decay rates of nonequilibrium electrons due to enhanced phonon absorption. The increased phonon occupation also sets the nonequilibriu…
