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arXiv:1410.5198 · Physical Review B

Phase diagram of exciton condensate in doped two-band Hubbard model

J. Kuneš

Published Dec 22, 2014• 36 citations
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Abstract

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.

DOI
10.1103/physrevb.90.235140

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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.
We present a dynamical mean-field study of two-particle dynamical response functions in a two-band Hubbard model across several phase transitions. We observe that the transition between the excitonic condensate and spin-state ordered state is continuous with a narrow strip of supersolid phase separating the two. Approaching transition from the excitonic condensate is announced by softening of the excitonic mode at the MMM point of the Brillouin zone. Inside the spin-state ordered phase there is a magnetically ordered state with $2\ifmmode\times\else\texttimes\fi{}2$ periodicity, which has no precursor in the normal phase.
The exciton Mott transition and electron‐hole (e‐h) pair condensation are studied using a two‐band Hubbard model with both repulsion and attraction. With the use of the dynamical mean‐field theory, we clarify the phase diagram for the phase transitions at zero and finite temperatures. Away from half‐filling we find an exciton insulating phase, which is followed by the formation of incoherent e‐h pairs, and the first‐order transition between a metallic phase and the exciton insulating phase. We also examine the transition temperature to an e‐h pair condensed phase where BEC‐BCS crossover takes place, and discuss characteristic features of the finite‐temperature phase diagram.
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.
We study a two-band Hubbard model using the dynamical mean-field theory combined with the exact diagonalization method. At the electron density n=2n=2n=2, a transition from a band-insulator to a correlated semimetal occurs when the on-site Coulomb interaction UUU is varied for a fixed value of the charge-transfer energy ΔΔΔ. At low temperature, the correlated semimetal shows ferromagnetism or superconductivity. With increasing doping ∣n−2∣|n-2|∣n−2∣, the ferromagnetic transition temperature rapidly decreases and finally becomes zero at a critical value of nnn. The second-order phase transition occurs at high temperature, while a phase separation of ferromagnetic and paramagnetic states takes place at low temperature. The superconducting transition temperature gradually decreases and finally becomes zero near n=1n=1n=1 (n=3n=3n=3) where the system is Mott insulator which shows antiferromagnetism at low temperature.
We generalize the mean-field theory for the spinless Bose-Hubbard model to account for the different types of superfluid phases that can arise in the spin-1 case. In particular, our mean-field theory can distinguish polar and ferromagnetic superfluids, Mott insulator, that arise at integer fillings at zero temperature, and normal Bose liquids into which the Mott insulators evolve at finite temperatures. We find, in contrast to the spinless case, that several of the superfluid-Mott insulator transitions are of first order at finite temperatures. Our systematic study yields rich phase diagrams that include first-order and second-order transitions and a variety of tricritical points. We discuss the possibility of realizing such phase diagrams in experimental systems.