Abstract
Motivated by recent observation of magnetic field induced transition in LaCoO3 we study the effect of external field in systems close to instabilities towards spin-state ordering and exciton condensation. We show that, while in both cases the transition can be induced by an external field, temperature dependencies of the critical field have opposite slopes. Based on this result we argue that the experimental observations select the exciton condensation scenario. We show that such condensation is possible due to high mobility of the intermediate spin excitations. The estimated width of the corresponding dispersion is large enough to overrule the order of atomic multiplets and to make the intermediate spin excitation propagating with a specific wave vector the lowest excitation of the system.
Highlights
Motivated by recent observation of magnetic field induced transition in LaCoO3 we study the effect of external field in systems close to instabilities towards spin-state ordering and exciton condensation
We show that both spin-state order (SSO) and excitonic condensate (EC) can be induced by an external magnetic field and calculate the temperature dependencies of the critical field hc(T) together with other relevant physical observables close to the phase boundaries
We have used two-orbital Hubbard model to simulate the effect of an external magnetic field on the ordering transition in the vicinity of spin-state crossover
Summary
We have used two-orbital Hubbard model to simulate the effect of an external magnetic field on the ordering transition in the vicinity of spin-state crossover. We find that the dhc/dT > 0 slope observed in LaCoO3 is consistent with exciton condensation, but inconsistent with SSO. We show that the field-induced transition is of the insulator-to-insulator type. We have estimated dispersion of the IS and HS excitations of the LS ground state and found sizeable bandwidths for the IS excitations of the order of several 100 meV, while the bandwidth of the HS excitations is an order of magnitude smaller. We conclude that the field-induced transition is a Bose-Einstein condensation of the IS excitons. The mobility of IS excitations is a key property of the low-temperature regime that has to be taken into account in description of LaCoO3
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