Disordered magnets: ground state, excitations, and thermodynamics

Abstract

Disorder is ubiquitous in solids. In magnets with localized spin moments, typical examples of disorder are missing or extra spins, arising, e.g., from chemical substitution. In this project, we study two classes of disordered magnets: with and without long range magnetic order. For magnets which display a ground state with long range magnetic order, we consider the effects of impurities, generally vacancies, in spin-S frustrated antiferromagnets. We investigate the (singular) behavior of the impurity induced spin texture as a function of an external magnetic field as well as its signatures in several physical observables. For the class of magnets without long range magnetic order, we consider in particular quantum paramagnets with spontaneous or disorder induced dimerization. We propose here the development of a flexible method applicable both to finite disorder and to a temperature interval experimentally relevant. Specifically, our investigation is based on an extension of the bond operator formalism to inhomogeneous magnetic systems. We obtain not only thermodynamic quantities, but also the magnetic excitation spectra, as measured by inelastic neutron scattering, which have been increasingly experimentally accessed. (AU)