Excitonic instability at the spin-state transition in the two-band Hubbard model

Abstract

Using linear response theory with the dynamical mean-field approximation we investigate the particle-hole instabilities of the two-band Hubbard model in the vicinity of the spin-state transition. Besides the previously reported high-spin-low-spin order we find an instability towards triplet excitonic condensate. We discuss the strong and weak coupling limits of the model, in particular, a connection to the spinful hard-core bosons with a nearest-neighbor interaction. Possible realization in LaCoO3 at intermediate temperatures is briefly discussed.

Competition between excitonic charge and spin density waves: Influence of electron-phonon and Hund's rule couplings

“…By contrast, in the iron-pnictide superconductors 26-28 and Co oxides [18][19][20] , the valence and conduction bands are formed by the d orbitals on the (same) transitionmetal ions, so that the Hund's rule coupling is expected to be strong. Hence, in these materials, the SDW phase, if really excitonic in origin, is rather triggered by the Hund's rule coupling than by electron-phonon coupling.…”

“…In Ca 1−x La x B 6 , the weak ferromagnetism was interpreted in terms of doped spin-triplet excitons [15][16][17] . The condensation of spin-triplet excitons was also predicted to occur in the proximity of the spin-state transition, 18 of which Pr 0.5 Ca 0.5 CoO 3 is an example. 19,20 Likewise, the structural phase transition of the layered chalcogenide Ta 2 NiSe 5 has been attributed to a spin-singlet EI [21][22][23][24] .…”

“…Excitonic phases in strongly correlated spinful systems can be discussed in the framework of two-band Hubbard-type models. Including thereby the Hund's rule coupling is known to stabilize the spin-triplet excitonic phase in the otherwise degenerate spin-singlet and spintriplet excitonic phases [18][19][20]28,42,43 . On the other hand, we have shown in our previous work 43 that taking into account electronic interactions only, a spin-singlet excitonic phase cannot be stabilized, which may however be realized in 1T -TiSe 2 and Ta 2 NiSe 5 , where the importance of electron-phonon coupling was recently pointed out [10][11][12][13][14]23 .…”

Spontaneous Spin Textures in Multiorbital Mott Systems

“…The numerical simulations using continuous-time quantum Monte-Carlo impurity solver 30,31 were performed with the density-density approximation for the interaction (γ = 0), which effectively introduces a magnetic easy axis in the present model. Analytic mean-field calculations as well as preliminary DMFT computations performed with SU (2) Studies 15,19,[24][25][26][27] performed without cross-hopping V 1,2 = 0 revealed formation of the exciton condensate below a critical temperature, which decreases with doping away from integer filling.In the strong-coupling limit the ground state wave function of a uniform condensate can be approximated by a product of local functions Π i |C i with each, describing a local hybrid between LS and HS states with amplitudes s, ξ 1 , ξ 0 , and ξ −1 , which provides a useful analytic reference for interpretation of the numerical results. In the DMFT calculations we characterise the thermodynamic phases by the order parameter φ (i) = αβ σ αβ a † iα b iβ , with Pauli matrices σ.…”

“…We point out that without doping the s-and d-wave systems are identical, in the strong-coupling limit, since the cross-hopping enters as a product V 1 V 2 on each bond. 15 The SCDW' phase is characterized by purely imaginary φ which gives rise to k-odd exchange field in (3). The odd cross-hopping pattern can be thought of as having p x + p y symmetry, which is imprinted in the spin texture, shown in Fig.…”