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The hybridization between light and matter in the strong and weak coupling regime, which can be obtained by placing materials into optical cavities resonant to different material excitations, can determine the onset of macroscopic functionalities which are proper of the "light-matter" hybrids and are different from those of its constituents. We show here the experimental evidence of the reversible control of the metal-to-insulator phase transition in the correlated dichalcogenide charge density wave 1T-TaS$_{2}$ embedded in low energy Terahertz cavities. We reveal that, while long wavelength cavities (up to ~ 25 GHz) favour the metallic phase, the coupling with cavities at higher frequencies results in a stabilization of the insulating charge density wave phase. Overall, for the present setting the measured critical temperature associated to the metal-to-insulator transition in 1T-TaS$_{2}$ can be shifted by more than 70 K by tuning the cavity resonant frequency. Such a giant modification overcomes the intrinsic hysteresis in the material and enables a reversible touchless control of the metal-to-insulator phase transition. A switch between conductive and insulating behaviour is obtained by mechanically tuning the distance between the cavity mirrors and their alignment. Our finding and the indication that cavity modes can couple to charged domain wall fluctuations, and thereby selectively affect the properties of the conductive state, uncover a new path to control the macroscopic transport properties of quantum materials by changing their electromagnetic environment., Comment: 29 pages, 17 figures

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 to the full nonequilibrium DMFT result. This suggest a route to combine the accurate DMFT description of nonequilibrum states of solids with cluster calculations of the XAS signal.

We discuss a semiclassical approach to solve the quantum impurity model within non-equilibrium dynamical mean-field theory for electron-lattice models. The effect of electronic fluctuations on the phonon is kept beyond Ehrenfest dynamics, leading to a stochastic phonon evolution with damping and noise terms that are self-consistently determined by the electronic correlation functions in the fluctuating phonon field. Together with a solution of the electronic model based on a non-perturbative quantum Boltzmann equation, the approach can be used to address the coupled dynamics of the electrons and the lattice during photo-induced phase transitions. Results for the Anderson-Holstein model are benchmarked against numerically exact quantum Monte Carlo data. We find good agreement for the phonon distribution function at temperatures comparable to the charge ordering temperature. The general formulation can be extended to models with electron-electron interactions or multi-orbital systems., Comment: 17 pages, 4 figures

The optical conductivity contains information about energy absorption and the underlying physical processes. In finite-dimensional systems, vertex corrections to the bare bubble need to be considered, which is a computationally challenging task. Recent numerical studies showed that in the weak coupling limit, near an ordering instability with wave vector $��$, $��$-tons (or Maki-Thompson diagrams) yield the most relevant vertex corrections. This provides a route for including vertex corrections into, for example, dynamical mean field theory estimates of the optical conductivity. By implementing calculations on the Kadanoff-Baym contour, we reveal the characteristic spectral signatures of the $��$-tons and their evolution under non-equilibrium conditions. We consider interaction quenches of the weakly-correlated Hubbard model near the antiferromagnetic phase boundary, and analyze the evolution of the Drude and $��$-ton features. While the bubble contribution to the optical conductivity is found to thermalize rapidly, after some oscillations with frequencies related to the local spectral function, the $��$-ton contribution exhibits a slower evolution. We link this observation to the prethermalization phenomenon which has been previously studied in weakly interacting, quenched Hubbard models., 15 pages, 12 figures

Using ultrashort laser pulses, it has become possible to probe the dynamics of long-range order in solids on microscopic timescales. In the conventional description of symmetry-broken phases within time-dependent Ginzburg-Landau theory, the order parameter evolves coherently, with small fluctuations along an average trajectory. Recent experiments, however, indicate that some systems can support a different scenario, named ultrafast inhomogeneous disordering, where the average order parameter is no longer representative of the state on the atomic scale. Here we theoretically show that ultrafast disordering can occur in a minimal, yet paradigmatic, model for a Peierls instability if atomic scale inhomogeneities of both the electronic structure and the charge density wave order parameter are taken into account. The latter is achieved using a non-equilibrium generalization of statistical dynamical mean-field theory, coupled to stochastic differential equations for the order parameter., Comment: 13 pages, 7 figures

Quasi-one-dimensional systems exhibit many-body effects elusive in higher dimensions. A prime example is spin-orbital separation, which has been measured by resonant inelastic X-ray scattering (RIXS) in Sr$_2$CuO$_3$. Here, we theoretically analyze the time-resolved RIXS spectrum of Sr$_2$CuO$_3$ under the action of a time-dependent electric field. We show that the external field can reversibly modify the parameters in the effective $t-J$ model used to describe spinon and orbiton dynamics in the material. For strong driving amplitudes, we find that the spectrum changes qualitatively as a result of reversing the relative spinon to orbiton velocity. The analysis shows that in general, the spin-orbital dynamics in Mott insulators in combination with time-resolved RIXS should provide a suitable platform to explore the reversible control of many-body physics in the solid with strong laser fields., Comment: 13 pages, 4 figures

An elusive goal in the field of driven quantum matter is the induction of long-range order. Here, we demonstrate a mechanism based on light-induced evaporative cooling of holes in a correlated electron system. Since the entropy of a filled narrow band grows rapidly with hole doping, the isentropic transfer of holes from a doped Mott insulator to such a band results in a drop of temperature. Strongly correlated Fermi liquids and symmetry-broken states could thus be produced by dipolar excitations. Using nonequilibrium dynamical mean field theory, we show that suitably designed chirped pulses allow to realize this cooling effect. In particular, we demonstrate the emergence of antiferromagnetic order in a system which is initially in a weakly correlated state above the maximum N\'eel temperature. Our work suggests a general strategy for inducing strong correlation phenomena and electronic orders in light-driven materials or periodically modulated atomic gases in optical lattice potentials.

The nonequilibrium Green's functions (NEGF) approach is a versatile theoretical tool, which allows to describe the electronic structure, spectroscopy and dynamics of strongly correlated systems. The applicability of this method is, however, limited by its considerable computational cost. Due to the treatment of the full two-time dependence of the NEGF the underlying equations of motion involve a long-lasting non-Markovian memory kernel that results in at least a $N^3_t$ scaling in the number of time points $N_t$. The system's memory time is, however, reduced in the presence of a thermalizing bath. In particular, dynamical mean-field theory (DMFT) -- one of the most successful approaches to strongly correlated lattice systems - maps extended systems to an effective impurity coupled to a bath. In this work, we systematically investigate how the memory time can be truncated in nonequilibrium DMFT simulations of the Hubbard and Hubbard-Holstein model. We show that suitable truncation schemes, which substantially reduce the computational cost, result in excellent approximations to the full time evolution. This approach enables the propagation to longer times, making fundamental processes like prethermalization and the final stages of thermalization accessible to nonequilibrium DMFT., Comment: 14 pages, 16 figures

We investigate a quantum dot (Anderson impurity) coupled to metallic leads, with a time-periodic voltage bias across the device. Using a time-dependent Schrieffer-Wolff transformation, we show that the Floquet Hamiltonian of the model can be mapped onto a two-channel Kondo model, in which the impurity is screened by separate conduction bands corresponding to parity-even and odd superpositions of the metallic leads. By changing the frequency and amplitude of the perturbation, one can tune the system to a quantum critical point with symmetric coupling of the impurity to both channels. For the understanding of the driven state, energy absorption from the drive must be considered: Although the absorption at the impurity is balanced by the energy flow into the conduction band, locally it leads to non-thermal distribution functions which can have a detrimental effect on the Kondo physics. However, a numerical simulation demonstrates that the absorption can be systematically suppressed at a fixed value of the induced couplings by increasing the frequency, so that the time-averaged dynamics in the driven one-channel Anderson model can be used to study the critical behavior of the two-channel model., Comment: 11 pages, 6 figures

We demonstrate how electric fields with arbitrary time profile can be used to control the time-dependent parameters of spin and orbital exchange Hamiltonians. Analytic expressions for the exchange constants are derived from a time-dependent Schrieffer-Wolff transformation, and the validity of the resulting effective Hamiltonian is verified for the case of a quarter-filled two-orbital Hubbard model, by comparing to the results of a full nonequilibrium dynamical mean-field theory simulation. The ability to manipulate Hamiltonians with arbitrary time-dependent fields, beyond the paradigm of Floquet engineering, opens the possibility to control intertwined spin and orbital order using laser or THz pulses which are tailored to minimize electronic excitations., Comment: 7 pages, 4 figures

We use non-equilibrium dynamical mean-field theory to demonstrate the existence of a critical interaction in the real-time dynamics of the Hubbard model after an interaction quench. The critical point is characterized by fast thermalization and separates weak-coupling and strong-coupling regimes in which the relaxation is delayed due to prethermalization on intermediate timescales. This dynamical phase transition should be observable in experiments on trapped fermionic atoms., Comment: 4 pages, 3 figures