Excitonic transition in (Pr,Ca)CoO₃ family

The members of (Pr_1-yY_y)_1-xCa_xCoO₃ family exhibit a continuous phase transition accompanied by disappearance of the fluctuating moment of Co and increase of resistivity by several decades. Most intriguing feature of the low temperature phase is breaking of the time reversal symmetry without presence of ordered atomic moments. We will argue that the experimental observations are explained by condensation of atomic size excitons, which gives rise to ordered magnetic multipoles. We will present model calculations performed with dynamical-mean field theory, which demonstrate general features of the excitonic condensation. In addition, we will present results of material specific LDA+U calculations which uncover the excitonic order in Pr_0.5YCa_0.5CoO₃ and explain its low temperature behavior.