Magnetization Relaxation and Geometric Forces in a Bose Ferromagnet

Abstract

We construct the hydrodynamic theory for spin-1/2 Bose gases at arbitrary temperatures. This theory describes the coupling between the magnetization, and the normal and superfluid components of the gas. In particular, our theory contains the geometric forces on the particles that arise from their spin's adiabatic following of the magnetization texture. The phenomenological parameters of the hydrodynamic theory are calculated in the Bogoliubov approximation and using the Boltzmann equation in the relaxation-time approximation. We consider the topological Hall effect due to the presence of a skyrmion, and show that this effect manifests itself in the collective modes of the system. The dissipative coupling between the magnetization and the normal component is shown to give rise to magnetization relaxation that is fourth order in spatial gradients of the magnetization direction. In metallic ferromagnets, magnetization dynamics leads to forces on quasiparticles of geometric origin called spin motive forces, that have gained considerable attention recently [8][9][10][11][12]. Furthermore, spin textures with nonzero chirality, such as the skyrmion lattice observed recently [13,14] induce the so-called topological Hall effect [5,6]. In addition, the coupling between magnetization and quasiparticles has also been shown to give rise to novel forms of magnetization relaxation in this case. A prominent example is inhomogeneous Gilbert damping [15][16][17]. This effect is important in clean solid-state systems. We therefore expect these effects to be particularly important for gases of ultracold atoms, that, in contrast to conventional condensed-matter systems, are free of impurities.The field of ultracold atoms is characterised by exquisite experimental control [18][19][20]. Relevant for our focus is the great amount of recent activity on spinor Bose gases. Firstly, it has been discovered that these gases can be either ferromagnetic or antiferromagnetic depending on the details of the scattering lengths [21,22]. Furthermore, numerous studies at zero temperature, based on the Gross-Pitaevskii equation, have elucidated the longwavelength properties of spinor gases [23,24]. Other areas of current interest in the field include topological excitations, magnetic dipole-dipole interactions and nonequilibrium quantum dynamics [25]. The recent progress in the understanding of ferromagnetic spinor gases and their manipulation by light has also enabled detailed studies of magnetization dynamics [26,27].

Superfluidity and spin superfluidity in spinor Bose gases

“…Both of these situations can be achieved in a system of ultracold atoms [33]. In addition to the terms already present in the LL equation, various spin-relaxation terms may be added, such as the transverse spin diffusion [43] and Gilbert damping [44], as well as terms due to magnetic field inhomogeneities [45] and magnetic dipole-dipole interactions [46]. However, all these terms can be made small in an ultracold-atom system, hence, we do not consider them here.…”

Brownian motion of massive skyrmions in magnetic thin films

“…Skyrmions were originally conceived in nuclear physics to describe interacting pions [1], however have found versatile application in a variety of different areas in physics [3][4][5]. A series of works reports their recent observation in chiral magnets [6][7][8][9][10][11].…”

Spin Waves and Dielectric Softening of Polar Molecule Condensates

“…In this Letter, we demonstrate that such systems can also exhibit remarkable dielectric properties. In particular, we consider a BEC of heteronuclear diatomic molecules subject to a weak applied electric field [15,16], which couples low-lying states of opposite parity. For a large class of molecules, these states are rotational in nature and are separated by an energy 2B R , where B R is the molecular rotational constant.…”