Excitonic condensation of strongly correlated electrons: the case of Pr$_{0.5}$Ca$_{0.5}$CoO$_3$

TL;DR

This paper investigates a low temperature phase transition in PCCO compounds, proposing that excitonic condensation of spin-triplet excitons explains observed phenomena like specific heat peaks, resistivity increase, and magnetic susceptibility drop.

Contribution

It demonstrates that excitonic condensation accounts for the phase transition and related properties in PCCO, linking it to spin-state transition physics.

Findings

Condensation of spin-triplet excitons explains the phase transition.

Explains exchange splitting and Pr valence transition.

Provides a unified theoretical framework for observed phenomena.

Abstract

We use a combination of dynamical mean-field model calculations and LDA+U material specific calculations to investigate the low temperature phase transition in the compounds from the (Pr$_{1-y}$R$_y$)$_x$Ca$_{1-x}$CoO$_3$ (R=Nd, Sm, Eu, Gd, Tb, Y) family (PCCO). The transition, marked by a sharp peak in the specific heat, leads to an exponential increase of dc resistivity and a drop of the magnetic susceptibility, but no order parameter has been identified yet. We show that condensation of spin-triplet, atomic-size excitons provides a consistent explanation of the observed physics. In particular, it explains the exchange splitting on the Pr sites and the simultaneous Pr valence transition. The excitonic condensation in PCCO is an example of a general behavior expected in certain systems in the proximity of a spin-state transition.