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arXiv:2006.02096 · Physical review. B./Physical review. B

Nonthermal excitonic condensation near a spin-state transition

Philipp Werner, Yuta Murakami

Published Dec 4, 2020• 13 citations
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Abstract

Recent experiments demonstrate the induction of long-range order in correlated electron systems via external perturbations, which calls for a better understanding of nonthermal ordered states and nonequilibrium symmetry breaking. Here, we reveal a mechanism based on entropy cooling and entropy trapping in a strongly correlated multiorbital system. We consider a two-orbital Hubbard model with Hund coupling and crystal-field splitting and show that in the vicinity of a spin-state transition, crystal-field quenches can induce an excitonic condensation at initial temperatures above the highest ordering temperature in equilibrium. We furthermore identify a dynamical phase transition in the evolution of the order parameter and show that such quenches can result in long-lived nonthermal excitonic condensates which have no analog in the equilibrium phase diagram.

DOI
10.1103/physrevb.102.241103

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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.
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.
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.
Using the dynamical mean-field approximation we investigate the formation of excitonic condensate in the two-band Hubbard model in the vicinity of the spin-state transition. With temperature and band filling as the control parameters we realize all symmetry allowed spin-triplet excitonic phases, some exhibiting a ferromagnetic polarization. While the transitions are first order at low temperatures, at elevated temperatures continuous transitions are found that give rise to a multicritical point. Rapid but continuous transition between ferromagnetic and nonmagnetic excitonic phases allows switching of uniform magnetization by small changes of chemical potential.
Excitonic condensation and stripe formation are two distinct manifestations of electronic correlations. While excitonic order naturally arises in multi-orbital systems, stripe order is a prominent feature of doped correlated-electron models. Here, we investigate an excitonic analogue of stripe order in the two--orbital Hubbard-Kanamori model on the square lattice, characterized by a spatial modulation of inter-orbital particle-hole coherence and, in the orbital-parity-symmetric limit, spontaneous breaking of a relative orbital Z2\mathbb{Z}_2Z2​ symmetry. Using unrestricted real-space Hartree-Fock calculations complemented by random-phase-approximation instability analysis, we determine how the Kanamori interactions select different excitonic channels. The sign of Hund's exchange controls the spin character of the condensate: ferromagnetic Hund coupling favors triplet excitonic order, whereas antiferromagnetic Hund coupling stabilizes singlet excitonic order. Upon doping, commensurate excitonic density waves develop incommensurate textures, including excitonic stripes and, in the triplet sector, spiral excitonic density waves. We further show that crystal-field splitting strongly reorganizes the excitonic instability by modifying inter-orbital nesting and can stabilize triplet excitonic order even in the absence of pair hopping. Our results establish excitonic stripes as a distinct symmetry-broken state of multi-orbital correlated systems and identify microscopic routes for their stabilization.
We study dynamical phase transitions from antiferromagnetic to paramagnetic states driven by an interaction quench in the fermionic Hubbard model using the nonequilibrium dynamical mean-field theory. We identify two dynamical transition points where the relaxation behavior qualitatively changes: one corresponds to the thermal phase transition at which the order parameter decays critically slowly in a power law ∝t(-1/2), and the other is connected to the existence of nonthermal antiferromagnetic order in systems with effective temperature above the thermal critical temperature. The frequency of the amplitude mode extrapolates to zero as one approaches the nonthermal (quasi)critical point, and thermalization is significantly delayed by the trapping in the nonthermal state. A slow relaxation of the nonthermal order is followed by a faster thermalization process.