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arXiv:2412.19205 · Physical review. B./Physical review. B

Role of the phonon coupling in driving photoexcited Mott insulators towards a transient superconducting state

Sujay Ray, Martin Eckstein, Philipp Werner

Published May 27, 2025• 5 citations
arXivHTMLPDF

Abstract

Understanding light-induced hidden orders is relevant for nonequilibrium materials control and future ultrafast technologies. Hidden superconducting order, in particular, has been a focus of recent experimental and theoretical efforts. In this study, we investigate the stability of light-induced $\ensuremath{\eta}$ pairing. Using a memory truncated implementation of nonequilibrium dynamical mean field theory (DMFT) and entropy-cooling techniques, we study the long-time dynamics of the photoinduced superconducting state. In the presence of coupling to a cold phonon bath, the photodoped system reaches a quasi-steady state, which can be sustained over a long period of time in large-gap Mott insulators. We show that this long-lived prethermalized state is well described by the nonequilibrium steady-state implementation of DMFT.

DOI
10.1103/physrevb.111.174309

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•Sep 4, 2020•4 citations
Understanding light-induced hidden orders is relevant for nonequilibrium materials control and future ultrafast technologies. Hidden superconducting order, in particular, has been a focus of recent experimental and theoretical efforts. In this study, we investigate the stability of light-induced \ensuremathη\ensuremath{\eta}\ensuremathη pairing. Using a memory truncated implementation of nonequilibrium dynamical mean field theory (DMFT) and entropy-cooling techniques, we study the long-time dynamics of the photoinduced superconducting state. In the presence of coupling to a cold phonon bath, the photodoped system reaches a quasi-steady state, which can be sustained over a long period of time in large-gap Mott insulators. We show that this long-lived prethermalized state is well described by the nonequilibrium steady-state implementation of DMFT.
High-temperature ηηη-pairing superconductivity in the photodoped Hubbard model
We investigate superconductivity emerging in the photodoped Mott insulating Hubbard model using steady-state dynamical mean-field theory implemented on the real-frequency axis. By employing high-order strong-coupling impurity solvers, we obtain the nonequilibrium phase diagram for photoinduced ηηη-pairing superconductivity with a remarkably high effective critical temperature. We further identify a superconducting gap in the momentum-resolved spectral function and optical conductivity, providing spectroscopic signatures accessible to experiments. Our results highlight a route to a controllable form of high-temperature superconductivity in nonequilibrium strongly correlated systems, fundamentally distinct from the equilibrium sss-wave pairing state in the attractive Hubbard model or cuprate-like ddd-wave superconductors.
Photodoped states are widely observed in laser-excited Mott insulators, in which charge excitations are quickly created and can exist beyond the duration of the external driving. Despite the fruitful experimental explorations, theoretical studies on the microscopic models face the challenge to simultaneously deal with exponentially separated time scales, especially in multiband systems, where the longtime behaviors are often well beyond the reach of state-of-the-art numerical tools. Here, we address this difficulty by introducing a steady-state description of photodoped Mott insulators using an open-system setup, where the photodoped system is stabilized as a nonequilibrium steady state by a weak external driving. Taking advantage of the stationarity, we implement and discuss the details of an efficient numerical tool using the steady-state dynamical mean-field theory combined with the noncrossing approximation. We demonstrate that these stationary photodoped states exhibit the same properties of their transient counterparts, while being solvable with reasonable computational efforts. Furthermore, they can be parametrized by just a few physical quantities, including the effective temperature and the density of charge excitations, which confirms the universal nature of photodoped states indeed independent of the excitation protocols. As a first application, we consider the stationary photodoped states in a two-band Hubbard model with intertwined spin-and-orbital ordering and find a family of hidden phases unknown from the previous studies, implying an apparently unexplored time regime of the relaxation of the intertwined orders.
The study of nonequilibrium phenomena in interacting lattice systems can provide new perspectives on correlation effects, and information on metastable states of matter. Mott insulators are a promising class of systems for nonequilibrium studies, since they exhibit exotic phenomena and complex phase diagrams upon doping, and because a large Mott gap provides protection against fast thermalization and heating after photo-excitations. We can thus expect the emergence of interesting transient states and photo-induced phases in Mott systems. This review presents the current understanding of the mechanisms which control the time evolution of photo-doped charge carriers and the properties of photo-induced metastable states. We focus on recent theoretical progress, identify the relevant underlying concepts, and link them to experimental observations. The review starts with a general discussion of field-induced nonequilibrium setups and an overview of key experiments which revealed characteristic properties of photo-excited Mott states, proceeds with a compact overview of the theoretical tools which have been developed to investigate these strongly correlated nonequilibrium states, and then analyzes Mott insulators driven out of equilibrium by static electric fields, periodic fields, and short laser pulses. We also discuss the appearance of nonthermal electronic orders in photo-excited Mott systems, including nonthermal spin and orbital orders, ηηη pairing states, and novel types of excitonic orders.
The search for hidden orders in photoexcited lattice systems is an active research field driven by experimental reports of light-induced or light-stabilized phases. In this study, we investigate hidden electronic orders in strongly correlated two-orbital Hubbard models with orbital-dependent bandwidths. In equilibrium, the half-filled systems are antiferromagnetically ordered. Using nonequilibrium dynamical mean field theory we demonstrate the appearance of nonthermal ferromagnetic order in the photodoped state, if the two bandwidths are sufficiently different, and its coexistence with spin-singlet $\ensuremath{\eta}$ superconductivity in the high photodoping region. Spin-triplet $\ensuremath{\eta}$-superconducting order appears instead if the two bandwidths are comparable. The rich nonequilibrium phasediagram uncovered in this work shows that Mott insulating multiorbital systems provide an interesting platform for the realization of nonthermal electronic orders.
ηηη-pairing in metallic and particle-hole asymmetric systems
Light-induced superconducting-like states have been reported in several classes of correlated materials. From a theoretical point of view, the induction of ηηη-pairing is a promising route to nonthermal superconductivity. Numerical studies of photo-doped Mott systems revealed ηηη-pairing states with very high effective critical temperatures. These investigations were however restricted to particle-hole symmetric states in large-gap Mott insulators, while the experiments were performed on strongly correlated metallic systems. It is thus relevant to explore if ηηη-pairing also exists in non-particle-hole symmetric setups and in photo-excited metallic states. Here we use steady-state nonequilibrium dynamical mean field theory combined with a strong-coupling impurity solver up to third order to investigate this issue. We find that in the strongly correlated regime with large Mott gap, and for low effective doublon and holon temperatures, ηηη-pairing is robust against changes in the total filling and an imbalance in the doublon and holon density. An asymmetry in the effective doublon and holon temperatures can however strongly suppress the order parameter. In photo-doped metallic systems with a three-peak structure in the local density of states, ηηη-pairing can be realized in set-ups with positive doublon and holon temperatures and a population inversion in the low-energy quasi-particle band.
Ultrafast laser pulses can redistribute charges in Mott insulators on extremely short time scales, leading to the fast generation of photocarriers. It has recently been demonstrated that these photocarriers can form a novel η-paired condensate at low temperatures, featuring a staggered superconducting pairing field. In this conference paper, we discuss the origin of the η-paired hidden phase and its optical response, which may be detected in a pump-probe experiment. The hidden phase may be relevant for possible light-induced superconductivity in Mott insulators.