Emerging new phases in correlated Mott insulator Ca2RuO4

TL;DR

This paper reviews the complex quantum phases of Ca2RuO4, highlighting recent theoretical insights, spin splitting phenomena, and potential routes to exotic states via doping and electric fields, with implications for advanced material applications.

Contribution

It introduces the concept of Ca2RuO4 as an altermagnetic candidate and explores how doping and electric fields can induce novel quantum states in this material.

Findings

Ca2RuO4 exhibits spin splitting with orbital selectivity.

Doping with 3d3 ions may induce negative thermal expansion.

Electric fields can modulate orbital fluctuations and magnetic order.

Abstract

The Mott insulator Ca2RuO4 is a paradigmatic example among transition metal oxides, where the interplay of charge, spin, orbital, and lattice degrees of freedom leads to competing quantum phases. In this paper, we focus on and review some key aspects, from the underlying physical framework and its basic properties, to recent theoretical efforts that aim to trigger unconventional quantum ground states, using several external parameters and stimuli. Using first-principle calculations, we demonstrate that Ca2RuO4 shows a spin splitting in the reciprocal space, and identify it as an altermagnetic candidate material. The non relativistic spin-splitting has an orbital selective nature, dictated by the local crystallographic symmetry. Next, we consider two routes that may trigger exotic quantum states. The first one corresponds to transition metal substitution of the 4d4 Ru with isovalent 3d3 ions. This substitutional doping may alter the spin-orbital correlations favoring the emergence of negative thermal expansion. The second route explores fledgling states arising in a nonequilibrium steady state under the influence of an applied electric field. We show that the electric field can directly affect the orbital density, eventually leading to strong orbital fluctuations and the suppression of orbital imbalance, which may, in turn, reduce antiferromagnetism. These aspects suggest possible practical applications, as its unique properties may open up possibilities for augmenting existing technologies, surpassing the limitations of conventional materials.