---
abstract: 'A tight binding model is introduced to describe the strong interaction limit of excitonic ordering. At stoichiometry, the model reduces in the strong coupling limit to a pseudo-spin model with approximate U(4) symmetry. Excitonic order appears in the pseudo-spin model as in-plane pseudo-magnetism. The U(4) symmetry unifies all possible singlet and triplet order parameters describing such states. Super-exchange, Hunds-rule coupling, and other perturbations act as anisotropies splitting the U(4) manifold, ultimately stabilizing a paramagnetic triplet state. The tendency to ferromagnetism with doping (observed experimentally in the hexaborides) is explained as a spin-flop transition to a different orientation of the U(4) order parameter. The physical mechanism favoring such a reorientation is the enhanced coherence (and hence lower kinetic energy) of the doped electrons in a ferromagnetic background relative to the paramagnet. A discussion of the physical meaning of various excitonic states and their experimental consequences is also provided.'
author:
- |
    Leon Balents\
    Physics Department, University of California, Santa Barbara, CA 93106
title: 'Excitonic order at strong-coupling: pseudo-spins, doping, and ferromagnetism'
---

Introduction {#sec:intro}
============

The unexpected discovery of high-$T_c$ itinerant ferromagnetism in doped hexaborides[@hexaborides] has re-ignited interest in the problem of excitonic ordering near the semiconductor–metal transition.[@Keldysh; @HalperinRice] 