Doping charge carriers into Mott insulators provides a pathway to produce intriguing emergent phenomena. In equilibrium systems, doping can be chemically controlled. On the other hand, photo-doping, where particles are excited across the Mott gap, provides an alternative way. Compared to chemical-doping, photo-doping creates a wider variety of charge carriers, which may lead to the emergence of fascinating nonequilibrium states. In particular, when the gap is large, the lifetime of photo-carriers is exponentially enhanced, leading to quasi-steady states after intraband cooling of photo-carriers.In this talk, we explore possible hidden phases that arise as quasi-steady states of photodoped Mott insulators using the quasi-equilibrium approach. Within this approach, we treat the photo-doped state as an equilibrium state of an effective model for a given photo-doping level. We apply the idea to the 1D extended Hubbard model. In the first part, we present our numerical results obtained with the infinite time-evolving block decimation. We show the emergence of the so-called η-pairing phase and the string charge-density-wave phase, and discuss their physical properties. In the second part, we reveal the analytical aspects of these photo-doped states.
The Hilbert space fragmentation, for which the Hamiltonian shatters into exponentially many blocks in the site occupation basis, can result in the breakdown of thermalization. In this presentation, we focus on the pair-hopping (PH) model, a paradigmatic model of the Hilbert-space fragmentation. Furthermore, it can be derived as an effective model of the Stark chain, imposing strict conservation of the dipole moment. Notably, the non-vanishing autocorrelation functions in the PH model, as reported in, suggests the existence of local or quasilocal integrals of motion (IOMs). Hence, we propose a numerical algorithm that establishes all IOMs linear in a given set of operators. We employ it to demonstrate that the density modes in the PH model are frozen and become strict IOMs in the thermodynanic limit. Nevertheless, these modes become subdiffusive after incorporating higher-order corrections the PH model. Finally, we make a connection with the Stark model. We demonstrate that although both energy and dipole moment are conserved in the thermodynamic limit, the Stark chain supports only one IOM.
Brilliant, ultrashort, and coherent X-ray free-electron laser (FEL) pulses allow for investigation of dynamics at the inherent time and length scale of atoms. I will illustrate this capability at the example of recent time-resolved X-ray diffraction data taken in the “hidden” phase of the Van der Waals material 1T-TaS2, hinting that out-of-plane restacking suppresses the optically-induced hidden state. Furthermore, I will also present preliminary static micro-beam X-ray diffraction data of electrically switched 1T-TaS2 cryomemory cells which indicate that also from a structural point of view the photo- and electrically-induced “hidden” states are closely related.
Richardson model, first introduced in nuclear physics as a simplified model of nucleon pairing, is also an appropriate description of a small superconducting island with fixed charge. Complex systems composed of interconnected superconducting islands and interacting quantum dots can be modelled using Hamiltonians that can be transformed into the matrix-product-operator form with small matrices that can be efficiently solved using the density matrix renormalization group. This approach allows to include without any approximations the effects of both the exchange interaction (Kondo screening and Yu-Shina-Rusinov subgap states) and the charge repulsion (Coulomb blockade, capacitive coupling) and thereby provide reference results for this family of Hamiltonians that are more general than regular quantum impurity problems. The theory results match well the experimental measurements on hybrid semi-super devices.I will describe how this approach can be extended to incorporate two further phenomena, the spin-orbit coupling and the proximity effect leading to level-dependent pairing strength. The combination of the two leads to a degeneracy of even and odd-parity ground states in the regime where the external magnetic field becomes strong enough to generate an increasing number of quasiparticles in the superconducting levels with the weakest pairing strength. This manifests as equal spacing of even and odd states in the charge stability diagrams.
Understanding the properties of real materials requires the incorporation of multiple degrees of freedom into the theoretical modeling. In this research, we focus on the coupling of electrons to phonons. We developed a comprehensive matrix-product-states based schemes that allows to compute spectral functions, optical conductivity and thermal conductivity of one-dimensional Holstein chains, both for the polaron case and half filling, and at finite temperatures. These techniques work well in the small-polaron regime and in intermediate regimes where phonon frequency, electron-phonon coupling and elecronic hopping matrix elements are of the same scale.
State-of-the-art approaches to extract transport coefficients of many-body quantum systems broadly fall into two categories: (i) they target the linear-response regime in terms of equilibrium correlation functions of the closed system; or (ii) they consider an open-system situation typically modeled by a Lindblad equation, where a nonequilibrium steady state emerges from driving the system at its boundaries. While quantitative agreement between (i) and (ii) has been found for selected model and parameter choices, also disagreement has been pointed out in the literature. Studying magnetization transport in the spin-1/2 XXZ chain, we here demonstrate that at weak driving the nonequilibrium steady state in an open system, including its buildup in time, can remarkably be constructed just on the basis of correlation functions in the closed system. We numerically illustrate this direct correspondence of closed-system and open-system dynamics, and show that it allows the treatment of comparatively large open systems, usually only accessible to matrix product state simulations. We also point out potential pitfalls when extracting transport coefficients from nonequilibrium steady states in finite systems.
