Superfluidity and spin superfluidity in spinor Bose gases

\u3cp\u3eWe show that spinor Bose gases subject to a quadratic Zeeman effect exhibit coexisting superfluidity and spin superfluidity, and study the interplay between these two distinct types of superfluidity. To illustrate that the basic principles governing these two types of superfluidity are the same, we describe the magnetization and particle-density dynamics in a single hydrodynamic framework. In this description spin and mass supercurrents are driven by their respective chemical potential gradients. As an application, we propose an experimentally accessible stationary state, where the two types of supercurrents counterflow and cancel each other, thus resulting in no mass transport. Furthermore, we propose a straightforward setup to probe spin superfluidity by measuring the in-plane magnetization angle of the whole cloud of atoms. We verify the robustness of these findings by evaluating the four-magnon collision time, and find that the time scale for coherent (superfluid) dynamics is separated from that of the slower incoherent dynamics by one order of magnitude. Comparing the atom and magnon kinetics reveals that while the former can be hydrodynamic, the latter is typically collisionless under most experimental conditions. This implies that, while our zero-temperature hydrodynamic equations are a valid description of spin transport in Bose gases, a hydrodynamic description that treats both mass and spin transport at finite temperatures may not be readily feasible.\u3c/p\u3

Magnon spin Hall magnetoresistance of a gapped quantum paramagnet

\u3cp\u3eMotivated by recent experimental work, we consider spin transport between a normal metal and a gapped quantum paramagnet. We model the latter as the magnonic Mott-insulating phase of an easy-plane ferromagnetic insulator. We evaluate the spin current mediated by the interface exchange coupling between the ferromagnet and the adjacent normal metal. For the strongly interacting magnons that we consider, this spin current gives rise to a spin Hall magnetoresistance that strongly depends on the magnitude of the magnetic field, rather than its direction. This Letter may motivate electrical detection of the phases of quantum magnets and the incorporation of such materials into spintronic devices.\u3c/p\u3

Length scale for magnon-polaron formation from nonlocal spin transport

\u3cp\u3eWe develop a theory for nonlocal spin transport through magnetic insulators that treats the coherent magnetoelastic interaction on equal footing with incoherent relaxation processes. In particular, our theory is able to describe the formation of magnon polarons, hybridized spin and elastic waves, near an interface where spin is injected into the magnetic insulator. Our theory is based on the stochastic Landau-Lifshitz-Gilbert equation coupled to stochastic equations of motion for the lattice displacement. By solving these equations, we obtain the charge voltage generated in a detector on one side of the magnetic insulator in response to spin biasing with an injector on the other side. We find that though magnon-polaron formation causes anomalous features in the spin transport, a length scale exists, however, below which magnetoelastic coupling does not affect the nonlocal spin current. This finding may motivate experiments to explore this aspect of magnon-phonon coupling in magnetic materials.\u3c/p\u3

Landau-liftshitz theory of the magnon-drag thermopower

Metallic ferromagnets subjected to a temperature gradient exhibit a magnonic drag of the electric current. We address this problem by solving a stochastic Landau-Lifshitz equation to calculate the magnon-drag thermopower. The long-wavelength magnetic dynamics result in two contributions to the electromotive force acting on electrons: 1) An adiabatic Berry-phase force related to the solid angle subtended by the magnetic precession and 2) a dissipative correction thereof, which is rooted microscopically in the spin-dephasing scattering. The first contribution results in a net force pushing the electrons towards the hot side, while the second contribution drags electrons towards the cold side, i.e., in the direction of the magnonic drift. The ratio between the two forces is proportional to the ratio between the Gilbert damping coefficient α and the coefficient β parametrizing the dissipative contribution to the electromotive force

Microscopic theory of magnon-drag electron flow in ferromagnetic metals

\u3cp\u3eA temperature gradient applied to a ferromagnetic metal induces not only independent flows of electrons and magnons but also drag currents because of their mutual interaction. In this paper, we present a microscopic study of the electron flow induced by the drag due to magnons. The analysis is based on the s-d model, which describes conduction electrons and magnons coupled via the s-d exchange interaction. Magnetic impurities are introduced in the electron subsystem as a source of spin relaxation. The obtained magnon-drag electron current is proportional to the entropy of magnons and to α-β (more precisely, to 1-β/α), where α is the Gilbert damping constant and β is the dissipative spin-transfer torque parameter. This result almost coincides with the previous phenomenological result based on the magnonic spin-motive forces, and consists of spin-transfer and momentum-transfer contributions, but with a slight disagreement in the former. The result is interpreted in terms of the nonequilibrium spin chemical potential generated by nonequilibrium magnons.\u3c/p\u3

Magnon contribution to unidirectional spin Hall magnetoresistance in ferromagnetic-insulator/heavy-metal bilayers

\u3cp\u3eWe develop a model for the magnonic contribution to the unidirectional spin Hall magnetoresistance (USMR) of heavy metal/ferromagnetic insulator bilayer films. We show that diffusive transport of Holstein-Primakoff magnons leads to an accumulation of spin near the bilayer interface, giving rise to a magnoresistance which is not invariant under inversion of the current direction. Unlike the electronic contribution described by Zhang and Vignale [Phys. Rev. B 94, 140411 (2016)2469-995010.1103/PhysRevB.94.140411], which requires an electrically conductive ferromagnet, the magnonic contribution can occur in ferromagnetic insulators such as yttrium iron garnet. We show that the magnonic USMR is, to leading order, cubic in the spin Hall angle of the heavy metal, as opposed to the linear relation found for the electronic contribution. We estimate that the maximal magnonic USMR in Pt|YIG bilayers is on the order of 10-8 but may reach values of up to 10-5 if the magnon gap is suppressed and can thus become comparable to the electronic contribution in, e.g., Pt|Co. We show that the magnonic USMR at a finite magnon gap may be enhanced by an order of magnitude if the magnon diffusion length is decreased to a specific optimal value that depends on various system parameters.\u3c/p\u3

Many-body theory of spin-current driven instabilities in magnetic insulators

\u3cp\u3eWe consider a magnetic insulator in contact with a normal metal. We derive a self-consistent Keldysh effective action for the magnon gas that contains the effects of magnon-magnon interactions and contact with the metal to lowest order. Self-consistent expressions for the dispersion relation, temperature, and chemical potential for magnons are derived. Based on this effective action, we study instabilities of the magnon gas that arise due to spin current flowing across the interface between the normal metal and the magnetic insulator. We find that the stability phase diagram is modified by an interference between magnon-magnon interactions and interfacial magnon-electron coupling. These effects persist at low temperatures and for thin magnetic insulators.\u3c/p\u3

Magnonic black holes

\u3cp\u3eWe show that the interaction between the spin-polarized current and the magnetization dynamics can be used to implement black-hole and white-hole horizons for magnons - the quanta of oscillations in the magnetization direction in magnets. We consider three different systems: easy-plane ferromagnetic metals, isotropic antiferromagnetic metals, and easy-plane magnetic insulators. Based on available experimental data, we estimate that the Hawking temperature can be as large as 1 K. We comment on the implications of magnonic horizons for spin-wave scattering and transport experiments, and for magnon entanglement.\u3c/p\u3

Local thermomagnonic torques in two-fluid spin dynamics

\u3cp\u3eWe develop a general phenomenology describing the interplay between coherent and incoherent dynamics in ferromagnetic insulators. Using the Onsager reciprocity and Neumann's principle, we derive expressions for the local thermomagnonic torques exerted by thermal magnons on the order-parameter dynamics and the reciprocal pumping processes, which are in close analogy to the spin-transfer torque and the spin pumping at metallic interfaces. Our formalism is applicable to general long-wavelength dynamics and, although here we explicitly focus on ferromagnetic insulators possessing U(1) symmetry, our approach can be easily extended to other classes of magnetic materials. As an illustrative example, we apply our theory to investigate a domain wall floating over a spin superfluid, whose dynamics are triggered thermally at the system's edge. Our results demonstrate that the local pumping of coherent spin dynamics by a thermal magnon gas offers an alternative route - with no need for conducting components and thus devoid of ohmic losses - for the control and manipulation of topological solitons.\u3c/p\u3

Magnons versus electrons in thermal spin transport through metallic interfaces

\u3cp\u3eWe develop a theory for spin transport in magnetic metals that treats the contribution of magnons and electrons on equal footing. As an application, we consider thermally-driven spin injection across an interface between a magnetic metal and a normal metal, i.e. the spin-dependent Seebeck effect. We show that the ratio between magnonic and electronic contribution scales as √ T/T/\u3csub\u3ec\u3c/sub\u3e \u3csub\u3eF\u3c/sub\u3e/T\u3csub\u3ec\u3c/sub\u3e, with the Fermi temperature T\u3csub\u3eF\u3c/sub\u3e and the Curie temperature T\u3csub\u3eC\u3c/sub\u3e. Since, typically, T\u3csub\u3eC\u3c/sub\u3e≪ T\u3csub\u3eF\u3c/sub\u3e, the magnonic contribution may dominate the thermal spin injection, even though the interface is more transparent for electronic spin current.\u3c/p\u3