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arXiv:2403.07737 · Physical review. B./Physical review. B

Photoinduced ferromagnetic and superconducting orders in multiorbital Hubbard models

Sujay Ray, Philipp Werner

Published Jul 8, 2024• 11 citations
arXivHTMLPDF

Abstract

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.

DOI
10.1103/physrevb.110.l041109

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Superconductivity in laser-excited correlated electron systems has attracted considerable interest due to reports of light-induced superconductinglike states. Here we explore the possibility of nonthermal superconducting order in strongly interacting multiorbital Hubbard systems, using nonequilibrium dynamical mean field theory. We find that a staggered $\ensuremath{\eta}$-type superconducting phase can be realized on a bipartite lattice in the high photodoping regime, if the effective temperature of the photocarriers is sufficiently low. The $\ensuremath{\eta}$ superconducting state is stabilized by Hund coupling---a positive Hund coupling favors orbital-singlet spin-triplet $\ensuremath{\eta}$ pairing, whereas a negative Hund coupling stabilizes spin-singlet orbital-triplet $\ensuremath{\eta}$ pairing.
Multiorbital Hubbard models with Hund coupling and crystal-field splitting exhibit an instability toward spin-triplet excitonic order in the parameter regime characterized by strong local spin fluctuations. Upon chemical doping, two distinct types of excitonic ferromagnetism have been reported. Using steady-state nonequilibrium dynamical mean-field theory, we demonstrate that photo-doped half-filled systems can host nonthermal counterparts of these excitonic phases and exhibit a rich phase diagram in the space of photo-doping and crystal field splitting. Photo-doping a spin-triplet excitonic insulator provides a route towards nonequilibrium control of magnetic order.
Nonequilibrium control of electronically ordered hidden phases may lead to the development of ultrafast switches and memory devices. In this study, we demonstrate tunable hidden orders in the half-filled photodoped two-orbital extended Hubbard model. Using steady-state nonequilibrium dynamical mean-field theory, we clarify the coexistence and interplay of nonthermal charge, spin, and orbital order. The hidden state at low effective temperature and sufficiently high photodoping is reminiscent of Kugel-Khomskii order in the two-orbital Hubbard model at 14\frac{1}{4}41​ and 34\frac{3}{4}43​ filling, but it emerges out of a nonequilibrium charge-ordered state and exhibits a different magnetic structure. A low-energy effective Hamiltonian is used to analyze the exchange processes that stabilize the nonthermal order.
An antiferromagnetic Hund coupling in multiorbital Hubbard systems induces orbital freezing and an associated superconducting instability, as well as unique composite orders in the case of an odd number of orbitals. While the rich phase diagram of the half-filled three-orbital model has recently been explored in detail, the properties of the doped system remain to be clarified. Here, we complement the previous studies by computing the entropy of the half-filled model, which exhibits an increase in the orbital-frozen region and a suppression in the composite ordered phase. The doping-dependent phase diagram shows that the composite ordered state can be stabilized in the doped Mott regime, if conventional electronic orders are suppressed by frustration. While antiferro-orbital order dominates the filling range $2\ensuremath{\lesssim}n\ensuremath{\le}3$ and ferro-orbital order dominates the strongly interacting region for $1\ensuremath{\lesssim}n<2$, we find superconductivity with a remarkably high Tc{T}_{c}Tc​ around n=1.5n=1.5n=1.5 (quarter filling). Also in the doped system, there is a close connection between the orbital-freezing crossover and superconductivity.
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.
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.