Spin-lattice order in one-dimensional conductors: beyond the RKKY effect

TL;DR

This paper nonperturbatively studies magnetic order in 1D conductors, revealing the breakdown of the RKKY model and identifying conditions for spiral order and phase transitions.

Contribution

It provides the exact phase diagram for spin-lattice order in 1D conductors beyond the RKKY approximation, highlighting the role of lattice spacing and coupling strength.

Findings

Spiral order vanishes above a critical exchange coupling.

Critical coupling depends strongly on lattice spacing.

Breakdown of RKKY picture at commensurate lattice spacing.

Abstract

We investigate magnetic order in a lattice of classical spins coupled to an isotropic gas of one-dimensional (1d) conduction electrons via local exchange interactions. The frequently discussed Ruderman-Kittel-Kasuya-Yosida (RKKY) effective exchange model for this system predicts that spiral order is always preferred. Here we consider the problem nonperturbatively, and find that such order vanishes above a critical value of the exchange coupling that depends strongly on the lattice spacing. The critical coupling tends to zero as the lattice spacing becomes commensurate with the Fermi wave vector, signalling the breakdown of the perturbative RKKY picture, and spiral order, even at weak coupling. We provide the exact phase diagram for arbitrary exchange coupling and lattice spacing, and discuss its stability. Our results shed new light on the problem of utilizing a spiral spin-lattice state to drive a one-dimensional superconductor into a topological phase.