Effects of Disorder on the Competition between Antiferromagnetism and Superconductivity

TL;DR

This paper investigates how disorder, like vacancies, influences the competition and coexistence of antiferromagnetism and superconductivity, revealing that impurities can induce magnetic order within the superconducting state.

Contribution

It presents a phenomenological Ginzburg-Landau model showing how impurities transform the AF-SC phase boundary and induce microscopic coexistence of AF and SC with distinct spin excitation spectra.

Findings

Single impurities induce local staggered magnetization in SC.

Finite impurity concentration leads to microscopic coexistence of AF and SC.

Spectral weight shifts from SC mode to AF mode in the coexistence state.

Abstract

Motivated by the observation of unusual magnetism in Ce_xCu_2Si_2 ($x\sim 1$), we study the effect of disorder, such as Ce vacancy, on the competition between superconductivity (SC) and antiferromagnetism (AF) on the basis of the phenomenological Ginzburg-Landau theory. Assuming that the AF-SC transition is of first order in clean system, we show that a single impurity in the SC state can induce staggered magnetization by suppressing the SC around it. For finite concentration of impurities, the first-order AF-SC boundary in the clean case is replaced by a finite region where the SC and the induced AF moments coexist microscopically with spatially varying order parameters. We argue that spin excitation spectrum in the coexistent state has a dual structure of SC gapped mode and the low-energy spin-wave mode. In accordance with the emergence of AF out of SC ground state, the spectral weight will be transferred from the former mode to the latter, keeping the structure of both modes basically unchanged.