arXiv subjects

R. A. Duine

Publications and source records attributed to R. A. Duine.

At least 91 records · Page 5Linked to original sources

arXiv2010

Spin drag in an ultracold Fermi gas on the verge of a ferromagnetic instability

Recent experiments [Jo et al., Science 325, 1521 (2009)] have presented evidence of ferromagnetic correlations in a two-component ultracold Fermi gas with strong repulsive interactions. Motivated by these experiments we consider spin drag, i.e., frictional drag due to scattering of particles with opposite spin, in such systems. We show that when the ferromagnetic state is approached from the normal side, the spin drag relaxation rate is strongly enhanced near the critical point. We also determine the temperature dependence of the spin diffusion constant. In a trapped gas the spin drag relaxation rate determines the damping of the spin dipole mode, which therefore provides a precursor signal of the ferromagnetic phase transition that may be used to experimentally determine the proximity to the ferromagnetic phase.

cond-mat.quant-gas↗

arXiv2009

Current-induced torques in continuous antiferromagnetic textures

We study the influence of an electric current on a continuous non-collinear antiferromagnetic texture. Despite the lack of a net magnetic moment we find that the exchange interaction between conduction electrons and local magnetization generally results in current-induced torques that are similar in phenomenology to spin transfer torques in ferromagnets. We present the generalization of the non-linear sigma model equation of motion for the Néèl vector that includes these current-induced torques, and briefly discuss the resulting current-induced antiferromagnetic domain wall motion and spin-wave Doppler shift. We give an interpretation of our results using a unifying picture of current-induced torques in ferromagnets and antiferromagnets in which they are viewed as due to the current-induced spin polarization resulting from an effective spin-orbit coupling.

cond-mat.mes-hall↗

arXiv2009

Quantum vortex dynamics in two-dimensional neutral superfluids

We derive an effective action for the vortex position degree-of-freedom in a superfluid by integrating out condensate phase and density fluctuation environmental modes. When the quantum dynamics of environmental fluctuations is neglected, we confirm the occurrence of the vortex Magnus force and obtain an expression for the vortex mass. We find that this adiabatic approximation is valid only when the superfluid droplet radius $R$, or the typical distance between vortices, is very much larger than the coherence length $ξ$. We go beyond the adiabatic approximation numerically, accounting for the quantum dynamics of environmental modes and capturing their dissipative coupling to condensate dynamics. For the case of an optical-lattice superfluid we demonstrate that vortex motion damping can be adjusted by tuning the ratio between the tunneling energy $J$ and the on-site interaction energy $U$. We comment on the possibility of realizing vortex Landau level physics.

cond-mat.quant-gas↗

arXiv2009

Spin motive forces and current fluctuations due to Brownian motion of domain walls

We compute the power spectrum of the noise in the current due to spin motive forces by a fluctuating domain wall. We find that the power spectrum of the noise in the current is colored, and depends on the Gilbert damping, the spin transfer torque parameter $β$, and the domain-wall pinning potential and magnetic anisotropy. We also determine the average current induced by the thermally-assisted motion of a domain wall that is driven by an external magnetic field. Our results suggest that measuring the power spectrum of the noise in the current in the presence of a domain wall may provide a new method for characterizing the current-to-domain-wall coupling in the system.

cond-mat.mes-hall↗

arXiv2009

Spin drag in noncondensed Bose gases

We show how time-dependent magnetic fields lead to spin motive forces and spin drag in a spinor Bose gas. We propose to observe these effects in a toroidal trap and analyze this particular proposal in some detail. In the linear-response regime we define a transport coefficient that is analogous to the usual drag resistivity in electron bilayer systems. Due to Bose enhancement of atom-atom scattering, this coefficient strongly increases as temperature is lowered. We also investigate the effects of heating.

cond-mat.quant-gas↗

arXiv2009

Fluctuations of current-driven domain walls in the non-adiabatic regime

We outline a general framework to determine the effect of non-equilibrium fluctuations on driven collective coordinates, and apply it to a current-driven domain wall in a nanocontact. In this case the collective coordinates are the domain-wall position and its chirality, that give rise to momentum transfer and spin transfer, respectively. We determine the current-induced fluctuations corresponding to these processes and show that at small frequencies they can be incorporated by two separate effective temperatures. As an application, the average time to depin the domain wall is calculated and found to be lowered by current-induced fluctuations. It is shown that current-induced fluctuations play an important role for narrow domain walls, especially at low temperatures.

cond-mat.mes-hall↗

arXiv2009

A strongly interacting Bose gas: Nozières and Schmitt-Rink theory and beyond

We calculate the critical temperature for Bose-Einstein condensation in a gas of bosonic atoms across a Feshbach resonance, and show how medium effects at negative scattering lengths give rise to pairs reminiscent of the ones responsible for fermionic superfluidity. We find that the formation of pairs leads to a large suppression of the critical temperature. Within the formalism developed by Nozieres and Schmitt-Rink the gas appears mechanically stable throughout the entire crossover region, but when interactions between pairs are taken into account we show that the gas becomes unstable close to the critical temperature. We discuss prospects of observing these effects in a gas of ultracold Cs133 atoms where recent measurements indicate that the gas may be sufficiently long-lived to explore the many-body physics around a Feshbach resonance.

cond-mat.other↗

arXiv2009

Current-driven and field-driven domain walls at nonzero temperature

We present a model for the dynamics of current- and field-driven domain-wall lines at nonzero temperature. We compute thermally-averaged drift velocities from the Fokker-Planck equation that describes the nonzero-temperature dynamics of the domain wall. As special limits of this general description, we describe rigid domain walls as well as vortex domain walls. In these limits, we determine also depinning times of the domain wall from an extrinsic pinning potential. We compare our theory with previous theoretical and experimental work.

cond-mat.other↗

arXiv2009

Current-induced spin torques in III-V ferromagnetic semiconductors

We formulate a theory of current-induced spin torques in inhomogeneous III-V ferromagnetic semiconductors. The carrier spin-3/2 and large spin-orbit interaction, leading to spin non-conservation, introduce significant conceptual differences from spin torques in ferromagnetic metals. We determine the spin density in an electric field in the weak momentum scattering regime, demonstrating that the torque on the magnetization is intimately related to spin precession under the action of both the spin-orbit interaction and the exchange field characteristic of ferromagnetism. The spin polarization excited by the electric field is smaller than in ferromagnetic metals and, due to lack of angular momentum conservation, cannot be expressed in a simple closed vectorial form. Remarkably, scalar and spin-dependent scattering do not affect the result. We use our results to estimate the velocity of current-driven domain walls.

cond-mat.other↗

arXiv2008

Vortex-lattice pinning in two-component Bose-Einstein condensates

We investigate the vortex-lattice structure for single- and two-component Bose-Einstein condensates in the presence of an optical lattice, which acts as a pinning potential for the vortices. The problem is considered in the mean-field quantum-Hall regime, which is reached when the rotation frequency $Ω$ of the condensate in a radially symmetric trap approaches the (radial) trapping frequency $ω$ and the interactions between the atoms are weak. We determine the vortex-lattice phase diagram as a function of optical-lattice strength and geometry. In the limit of strong pinning the vortices are always pinned at the maxima of the optical-lattice potential, similar to the slow-rotation case. At intermediate pinning strength, however, due to the competition between interactions and pinning energy, a structure arises for the two-component case where the vortices are pinned on lines of minimal potential.

cond-mat.mes-hall↗

arXiv2008

Effects of non-adiabaticity on the voltage generated by a moving domain wall

We determine the voltage generated by a field-driven domain wall, taking into account non-adiabatic corrections to the motive force induced by the time-dependent spin Berry phase. Both the diffusive and ballistic transport regimes are considered. We find that that the non-adiabatic corrections, together with the contributions due to spin relaxation, determine the voltage for driving fields smaller than the Walker breakdown limit.

cond-mat.mes-hall↗

arXiv2008

Current-induced domain-wall motion in synthetic antiferromagnets

Domain-wall magnetoresistance and low-frequency noise have been studied in epitaxial antiferromagnetically-coupled [Fe/Cr(001)]_10 multilayers and ferromagnetic Co line structures as a function of DC current intensity. In [Fe/Cr(001)]_10 multilayers a transition from excess to suppressed domain-wall induced 1/f noise above current densities of j_c ~ 2*10^5 A/cm^2 has been observed. In ferromagnetic Co line structures the domain wall related noise remains qualitatively unchanged up to current densities exceeding 10^6A/cm^2. Theoretical estimates of the critical current density for a synthetic Fe/Cr antiferromagnet suggest that this effect may be attributed to current-induced domain-wall motion that occurs via spin transfer torques.

cond-mat.mtrl-sci↗

arXiv2008

Achieving the Neel state in an optical lattice

We theoretically study the possibility of reaching the antiferromagnetic phase of the Hubbard model by starting from a normal gas of trapped fermionic atoms and adiabatically ramping up an optical lattice. Requirements on the initial temperature and the number of atoms are determined for a three dimensional square lattice by evaluating the Neel state entropy, taking into account fluctuations around the mean-field solution. We find that these fluctuations place important limitations on adiabatically reaching the Neel state.

cond-mat.stat-mech↗

arXiv2008

Effective temperature and Gilbert damping of a current-driven localized spin

Starting from a model that consists of a semiclassical spin coupled to two leads we present a microscopic derivation of the Langevin equation for the direction of the spin. For slowly-changing direction it takes on the form of the stochastic Landau-Lifschitz-Gilbert equation. We give expressions for the Gilbert damping parameter and the strength of the fluctuations, including their bias-voltage dependence. At nonzero bias-voltage the fluctuations and damping are not related by the fluctuation-dissipation theorem. We find, however, that in the low-frequency limit it is possible to introduce a voltage-dependent effective temperature that characterizes the fluctuations in the direction of the spin, and its transport-steady-state probability distribution function.

cond-mat.mes-hall↗

arXiv2008

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.

cond-mat.mes-hall↗

arXiv2008

Spin pumping by a field-driven domain wall

We calculate the charge current in a metallic ferromagnet to first order in the time derivative of the magnetization direction. Irrespective of the microscopic details, the result can be expressed in terms of the conductivities of the majority and minority electrons and the non-adiabatic spin transfer torque parameter $β$. The general expression is evaluated for the specific case of a field-driven domain wall and for that case depends strongly on the ratio of $β$ and the Gilbert damping constant. These results may provide an experimental method to determine this ratio, which plays a crucial role for current-driven domain-wall motion.

cond-mat.mes-hall↗

arXiv2007

Current-Induced Torques in Magnetic Metals: Beyond Spin Transfer

Current-induced torques on ferromagnetic nanoparticles and on domain walls in ferromagnetic nanowires are normally understood in terms of transfer of conserved spin angular momentum between spin-polarized currents and the magnetic condensate. In a series of recent articles we have discussed a microscopic picture of current-induced torques in which they are viewed as following from exchange fields produced by the misaligned spins of current carrying quasiparticles. This picture has the advantage that it can be applied to systems in which spin is not approximately conserved. More importantly, this point of view makes it clear that current-induced torques can also act on the order parameter of an antiferromagnetic metal, even though this quantity is not related to total spin. In this informal and intentionally provocative review we explain this picture and discuss its application to antiferromagnets.

cond-mat.mes-hall↗

arXiv2007

Functional Keldysh Theory of Spin Torques

We present a microscopic treatment of current-induced torques and thermal fluctuations in itinerant ferromagnets based on a functional formulation of the Keldysh formalism. We find that the nonequilibrium magnetization dynamics is governed by a stochastic Landau-Lifschitz-Gilbert equation with spin transfer torques. We calculate the Gilbert damping parameter $α$ and the non-adiabatic spin transfer torque parameter $β$ for a model ferromagnet. We find that $β\neq α$, in agreement with the results obtained using imaginary-time methods of Kohno, Tatara and Shibata [J. Phys. Soc. Japan 75, 113706 (2006)]. We comment on the relationship between $s-d$ and isotropic-Stoner toy models of ferromagnetism and more realistic density-functional-theory models, and on the implications of these relationships for predictions of the $β/α$ ratio which plays a central role in domain wall motion. Only for a single-parabolic-band isotropic-Stoner model with an exchange splitting that is small compared to the Fermi energy does $β/α$ approach one. In addition, our microscopic formalism incorporates naturally the fluctuations needed in a nonzero-temperature description of the magnetization. We find that to first order in the applied electric field, the usual form of thermal fluctuations via a phenomenological stochastic magnetic field holds.

cond-mat.mes-hall↗