Easy-axis ferromagnetic chain on a metallic surface

TL;DR

This paper investigates the quantum phase transition and excitation spectrum of an easy-axis ferromagnetic S=1/2 chain on a metallic surface, revealing a transition from ferromagnetic order to a Kondo-screened paramagnetic phase.

Contribution

It provides a detailed analysis of the phase transition, excitation damping, and experimental implications for spin chains on metallic surfaces, including extensions to higher spins.

Findings

Quantum phase transition from ferromagnetic to paramagnetic phase.

Damped magnon excitations in the ferromagnetic phase.

Experimental predictions for SPEELS measurements.

Abstract

The phases and excitation spectrum of an easy-axis ferromagnetic chain of S=1/2 magnetic impurities built on the top of a clean metallic surface are studied. As a function of the (Kondo) coupling to the metallic surface and at low temperatures, the spin chain exhibits a quantum phase transition from an Ising ferromagnetic phase with long-range order to a paramagnetic phase where quantum fluctuations destroy the magnetic order. In the paramagnetic phase, the system consists of a chain of Kondo-singlets where the impurities are completely screened by the metallic host. In the ferromagnetic phase, the excitations above the Ising gap are damped magnons, with a finite lifetime arising due to the coupling to the substrate. We discuss the experimental consequences of our results to spin-polarized electron energy loss spectroscopy (SPEELS), and we finally analyze possible extensions to spin chains with S>1/2.