Theory of Excitonic States in CaB6

TL;DR

This paper investigates excitonic states in CaB6 using Ginzburg-Landau theory, predicting magnetic and optical properties that could confirm exciton condensation and identify the ground state.

Contribution

It introduces a theoretical framework for exciton condensation in CaB6 and predicts observable magnetic and optical phenomena to verify the theory.

Findings

Two possible excitonic ground states identified

Predicted antiferromagnetic spin structure and magnetoelectric effect

Optical phenomena like nonreciprocal effects and second harmonic generation predicted

Abstract

We study the excitonic states in CaB6 in terms of the Ginzburg-Landau theory. By minimizing the free energy and by comparing with experimental results, we identify two possible ground states with exciton condensation. They both break time-reversal and inversion symmetries. This leads to various magnetic and optical properties. As for magnetic properties, it is expected to be an antiferromagnet, and its spin structure is predicted. It will exhibit the magnetoelectric effect, and observed novel ferromagnetism in doped samples and in thin-film and powder samples can arise from this effect. Interesting optical phenomena such as the nonreciprocal optical effect and the second harmonic generation are predicted. Their measurement for CaB6 will clarify whether exciton condensation occurs or not and which of the two states is realized.