Symmetry-breaking perturbations in the Jahn-Teller-Hubbard model

TL;DR

This paper investigates how symmetry-breaking perturbations affect the Jahn-Teller-Hubbard model, revealing significant differences from static models and highlighting the role of odd-frequency pairs in magnetic susceptibilities.

Contribution

It compares static and dynamical Jahn-Teller interactions using Eliashberg theory, uncovering qualitative differences and the impact of retardation effects on susceptibilities and superconductivity.

Findings

Magnetic orbital susceptibility differs between static and dynamical models.

Magnetic field induces nonzero orbital susceptibility via retardation effects.

Odd-frequency pairs are crucial for understanding magnetic responses.

Abstract

We study the effect of symmetry-breaking perturbations in the multiorbital Hubbard model coupled to anisotropic Jahn-Teller phonons, which is relevant for the description of fulleride superconductors. This system is often approximated by a model with static antiferromagnetic (AFM) Hund's coupling, in which the coupling to the Jahn-Teller phonon is effectively described, but the retardation effect associated with phonon propagation is neglected. We compare the properties of the models with static AFM Hund's coupling and dynamical Jahn-Teller electron-phonon interaction by means of the Eliashberg theory. Considering the susceptibilities for the spin, magnetic orbital, electric orbital, and superconductivity, we reveal a qualitatively different behavior between the two models in the case of the magnetic orbital susceptibility. We further study the effect of a magnetic field on the $s$-wave spin-singlet superconducting state. In the presence of the field, the magnetic orbital susceptibility becomes nonzero due to a combination of multiorbital and retardation effects, while the spin susceptibility remains zero at low temperatures. By analyzing this phenomenon both numerically and analytically, we clarify that odd-frequency pairs induced by the magnetic field play a crucial role in the spin and orbital magnetic susceptibilities. Thus, the magnetic degrees of freedom produce interesting behaviors in the presence of retardation effects associated with electron-phonon coupling.