Quantum spin dynamics

Abstract:

The Stoner-Wohlfarth model of a single-domain grain is applied to a complex situation: magnetization of a solid multigrain particle embedded in an elastic medium. In this situation, application of a magnetic field establishes a specific magnetomechanical process: polarization and switching of individual grains change the net energy of the particle and, as a result, make it rotate as a whole relative to the matrix. Because of that coupling, the magnetic hysteresis loop of a particle composed of highly coercive grains progressively shrinks with the increase of the matrix compliance. The effect is studied theoretically by numerical simulations on a particle comprising several hundred magnetically uniaxial grains with randomly oriented easy axes. The results of the model are discussed with regard to magnetic measurements performed on dispersions of spherical NdFeB microparticles in PDMS matrices of varied stiffness.

Abstract:

We show that the macroscopic magnetic and electronic properties of strongly correlated electron systems can be manipulated by coupling them to a cavity mode. As a paradigmatic example we consider the Fermi-Hubbard model and find that the electron-cavity coupling enhances the magnetic interaction between the electron spins in the ground-state manifold. At half filling this effect can be observed by a change in the magnetic susceptibility. At less than half filling, the cavity introduces a next-nearest-neighbor hopping and mediates a long-range electron-electron interaction between distant sites. We study the ground-state properties with tensor network methods and find that the cavity coupling can induce a phase characterized by a momentum-space pairing effect for electrons.