Microscopic theory of the nematic phase in Sr$_3$Ru$_2$O$_7$

TL;DR

This paper develops a microscopic mean-field theory explaining the nematic phase and metamagnetic transitions in Sr$_3$Ru$_2$O$_7$, accounting for experimental observations through band structure, Coulomb interactions, and spin-orbit effects.

Contribution

It introduces a realistic bilayer model with Coulomb interactions, van Hove singularities, and spin-orbit coupling to explain the nematic phase in Sr$_3$Ru$_2$O$_7$.

Findings

Explains the nematic phase with resistive anisotropy.

Accounts for the suppression of spin-density-wave order.

Describes the phase diagram near the metamagnetic quantum critical point.

Abstract

In an externally applied magnetic field, ultra-pure crystals of the bilayer compound Sr$_3$Ru$_2$O$_7$ undergo a metamagnetic transition below a critical temperature, $T^*$, which varies as a function of the angle between the magnetic field $H$ and the Ru-O planes. Moreover, $T^*$ approaches zero when $H$ is perpendicular to the planes. This putative "metamagnetic quantum critical point", however, is preempted by a nematic fluid phase with order one resistive anisotropy in the {\it ab} plane. In a "realistic" bilayer model with moderate strength local Coulomb interactions, the existence of a sharp divergence of the electronic density of states near a van Hove singularity of the quasi-one-dimensional bands, and the spin-orbit couplings permitted by the presence of multiple orbitals result in a mean-field phase diagram which accounts for many of these experimentally observed phenomena. Although the spin-orbit coupling is not overly strong, it destroys the otherwise near perfect Fermi surface nesting and hence suppresses spin-density-wave (SDW) ordering.