Excitonic condensation in systems of strongly correlated electrons

Abstract

The idea of exciton condensation in solids was introduced in the 1960s with the analogy of superconductivity in mind. While exciton supercurrents have been realised only in artificial quantum-well structures so far, the application of the concept of excitonic condensation to bulk solids leads to a rich spectrum of thermodynamic phases with diverse physical properties. In this review we discuss recent developments in the theory of exciton condensation in systems described by Hubbard-type models. In particular, we focus on the connections to their various strong-coupling limits that have been studied in other contexts, e.g. cold atoms physics. One of our goals is to provide a 'dictionary' that would allow the reader to efficiently combine results obtained in these different fields.

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) symmetric model 18 show only quantitative differences (e.g. reduction of the transition temperature).…”

“…14, 15 The basic ingredient is a crystal built of atoms with quasidegenerate singlet/triplet ground states. Under suitable conditions a spin-triplet exciton condensate 16,17 is formed, which may adopt a variety of thermodynamic phases with diverse properties 18 . Several experimental realizations of excitonic magnetism have already been discussed in the literature.…”

“…Note that on a bipartite lattice the t a t b > 0 case with a staggered φ-order can be mapped on the t a t b < 0 by the gauge transformation a i → (−1) i a i . 18 We consider two cross-hopping patterns at this point: V 1 = V 2 (even) and V 1 = −V 2 (odd). The two corresponding phase diagrams share the general features inherited from the 'parent' system with no cross-hopping studied in 24.…”

LDA+DMFT approach to ordering phenomena and the structural stability of correlated materials

“…An interesting feature of this transition, which can be well explained by the classical Blume-Emery-Griffiths model [109], is its reentrant character in parts of the phase diagram [110,111]. The above (classical) order is not the only instability of the model.…”

“…We choose the model parameters in the region dominated by the excitonic instability (arrow in Fig. 3), but unlike in Figure 3 we choose, for computational convenience, t a t b < 0 to get a uniform condensate (q = 0) [111]. The condensate is characterized by a finite expectation value…”