240 research outputs found
Magnon contribution to unidirectional spin Hall magnetoresistance
We 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)], 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 , but may
reach values of up to 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.Comment: 9 pages, 7 figure
Creep of current-driven domain-wall lines: intrinsic versus extrinsic pinning
We present a model for current-driven motion of a magnetic domain-wall line,
in which the dynamics of the domain wall is equivalent to that of an overdamped
vortex line in an anisotropic pinning potential. This potential has both
extrinsic contributions due to, e.g., sample inhomogeneities, and an intrinsic
contribution due to magnetic anisotropy. We obtain results for the domain-wall
velocity as a function of current for various regimes of pinning. In
particular, we find that the exponent characterizing the creep regime depends
strongly on the presence of a dissipative spin transfer torque. We discuss our
results in the light of recent experiments on current-driven domain-wall creep
in ferromagnetic semiconductors, and suggest further experiments to corroborate
our model.Comment: For figure in GIF format, see
http://www.phys.uu.nl/~duine/mapping.gif v2: (hopefully) visible EPS figure
added. v2: expanded new versio
Magnons versus electrons in thermal spin transport through metallic interfaces
We 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
, with the Fermi temperature and the Curie
temperature . Since, typically, , the magnonic contribution
may dominate the thermal spin injection, even though the interface is more
transparent for electronic spin current.Comment: Contribution to the Special issue on Spincaloritronics in Journal of
Physics D: Applied Physic
Interaction effects on dynamic correlations in non-condensed Bose gases
We consider dynamic, i.e., frequency-dependent, correlations in non-condensed
ultracold atomic Bose gases. In particular, we consider the single-particle
correlation function and its power spectrum. We compute this power spectrum for
a one-component Bose gas, and show how it depends on the interatomic
interactions that lead to a finite single-particle relaxation time. As another
example, we consider the power spectrum of spin-current fluctuations for a
two-component Bose gas and show how it is determined by the spin-transport
relaxation time.Comment: 9 pages, 3 figure
Current Induced Order Parameter Dynamics: Microscopic Theory Applied to Co/Cu/Co spin valves
Transport currents can alter alter order parameter dynamics and change steady
states in superconductors, in ferromagnets, and in hybrid systems. In this
article we present a scheme for fully microscopic evaluation of order parameter
dynamics that is intended for application to nanoscale systems. The approach
relies on time-dependent mean-field-theory, on an adiabatic approximation, and
on the use of non-equilibrium Greens function (NEGF) theory to calculate the
influence of a bias voltage across a system on its steady-state density matrix.
We apply this scheme to examine the spin-transfer torques which drive
magnetization dynamics in Co/Cu/Co spin-valve structures. Our microscopic
torques are peaked near Co/Cu interfaces, in agreement with most previous
pictures, but suprisingly act mainly on Co transition metal -orbitals rather
than on -orbitals as generally supposed.Comment: 9 pages, 5 figure
Probing the topological exciton condensate via Coulomb drag
The onset of exciton condensation in a topological insulator thin film was
recently predicted. We calculate the critical temperature for this transition,
taking into account screening effects. Furthermore, we show that the proximity
to this transition can be probed by measuring the Coulomb drag resistivity
between the surfaces of the thin film as a function of temperature. This
resistivity shows an upturn upon approaching the exciton-condensed state.Comment: 4 pages, 3 figure
Spin transport in a unitary Fermi gas close to the BCS transition
We consider spin transport in a two-component ultracold Fermi gas with
attractive interspecies interactions close to the BCS pairing transition. In
particular, we consider the spin-transport relaxation rate and the
spin-diffusion constant. Upon approaching the transition, the scattering
amplitude is enhanced by pairing fluctuations. However, as the system
approaches the transition, the spectral weight for excitations close to the
Fermi level is decreased by the formation of a pseudogap. To study the
consequence of these two competing effects, we determine the spin-transport
relaxation rate and the spin-diffusion constant using both a Boltzmann approach
and a diagrammatic approach. The former ignores pseudogap physics and finite
lifetime effects. In the latter, we incorporate the full pseudogap physics and
lifetime effects, but we ignore vertex corrections, so that we effectively
calculate single-particle relaxation rates instead of transport relaxation
rates. We find that there is qualitative agreement between these two approaches
although the results for the transport coefficients differ quantitatively.Comment: 9 pages, 10 figure
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