Theory of Doped Excitonic Insulators

TL;DR

This paper develops a theoretical framework for doped excitonic insulators, explaining experimental observations in hexaborides, including ferromagnetism and complex phase diagrams with inhomogeneous states.

Contribution

It provides a new theoretical understanding of doped excitonic insulators, emphasizing intra-valley condensation and ferromagnetism in specific band structures.

Findings

Intra-valley condensation is energetically favored in certain band structures.

Doping induces ferromagnetism due to kinetic energy quenching.

The phase diagram features inhomogeneous density and magnetization states.

Abstract

The theory of doped excitonic insulators is reinvestigated in light of recent experiments on hexaborides. For the appropriate valley-degenerate X_3,X_3' band structure, ``intra-valley'' condensation is energetically favored. Ferromagnetism occurs upon doping due to the quenching of kinetic energy at the otherwise direct first-order excitonic insulator-metal transition. The phase diagram includes states of spatially inhomogeneous density and magnetization at low temperatures.