arXiv subjects

Arne Brataas

Publications and source records attributed to Arne Brataas.

At least 145 records · Page 8Linked to original sources

arXiv2012

Spin-Transfer and Exchange Torques in Ferromagnetic Superconductors

We consider how superconducting correlations influence spin-transfer torques in ferromagnetic superconductors. It is demonstrated that there is a novel torque arising from particle-hole interference that depends on the U(1) phase associated with the superconducting order parameter. We also show that there is an equilibrium exchange torque between two ferromagnetic superconductors in contact via a normal metal mediated by Andreev states. The latter equilibrium magnetic torque is also sensitive to spin-resolved phase differences in the superconducting order parameters as well as to an externally applied phase difference.

cond-mat.supr-con↗

arXiv2012

Triplet supercurrent in ferromagnetic Josephson junctions by spin injection

We show that injecting nonequilibrium spins into the superconducting leads strongly enhances the stationary Josephson current through a superconductor-ferromagnet-superconductor junction. The resulting long-range super-current through a ferromagnet is carried by triplet Cooper pairs that are formed in s-wave superconductors by the combined effects of spin injection and exchange interaction. We quantify the exchange interaction in terms of Landau Fermi-liquid factors. The magnitude and direction of the long-range Josephson current can be manipulated by varying the angles of the injected polarizations with respect to the magnetization in the ferromagnet.

cond-mat.supr-con↗

arXiv2012

Magneto-Josephson effects in junctions with Majorana bound states

We investigate 1D quantum systems that support Majorana bound states at interfaces between topologically distinct regions. In particular, we show that there exists a duality between particle-hole and spin degrees of freedom in certain spin-orbit-coupled 1D platforms such as topological insulator edges. This duality results in a spin analogue of previously explored `fractional Josephson effects'---that is, the spin current flowing across a magnetic junction exhibits 4π-periodicity in the relative magnetic field angle across the junction. Furthermore, the interplay between the particle-hole and spin degrees of freedom results in unconventional magneto-Josephson effects, such that the Josephson current is a function of the magnetic field orientation with periodicity 4π.

cond-mat.mes-hall↗

arXiv2012

Intrinsic Spin Swapping

Here, we study diffusive spin transport in two dimensions and demonstrate that an intrinsic analog to a previously predicted extrinsic spin swapping effect, where the spin polarization and the direction of flow are interchanged due to spin-orbit coupling at extrinsic impurities, can be induced by intrinsic (Rashba) spin-orbit coupling. The resulting accumulation of intrinsically spin-swapped polarizations is shown to be much larger than for the extrinsic effect. Intrinsic spin swapping is particularly strong when the system dimensions exceed the spin-orbit precession length and the generated transverse spin currents are of the order of the injected primary spin currents. In contrast, spin accumulations and spin currents caused by extrinsic spin swapping are proportional to the spin-orbit coupling. We present numerical and analytical results for the secondary spin currents and accumulations generated by intrinsic spin swapping, and we derive analytic expressions for the induced spin accumulation at the edges of a narrow strip, where a long-range propagation of spin polarizations takes place.

cond-mat.mes-hall↗

arXiv2012

Spin Pumping and Spin Transfer

Spin pumping is the emission of a spin current by a magnetization dynamics while spin transfer stands for the excitation of magnetization by spin currents. Using Onsager's reciprocity relations we prove that spin pumping and spin-transfer torques are two fundamentally equivalent dynamic processes in magnetic structures with itinerant electrons. We review the theory of the coupled motion of the magnetization order parameter and electron for textured bulk ferromagnets (e.g. containing domain walls) and heterostructures (such as spin valves). We present first-principles calculations for the material-dependent damping parameters of magnetic alloys. Theoretical and experimental results agree in general well.

cond-mat.mes-hall↗

arXiv2011

Magnetization Dissipation in the Ferromagnetic Semiconductor (Ga,Mn)As

We compute the Gilbert damping in (Ga,Mn)As based on the scattering theory of magnetization relaxation. The disorder scattering is included non-perturbatively. In the clean limit, the spin-pumping from the localized d-electrons to the itinerant holes dominates the relaxation processes. In the diffusive regime, the breathing Fermi-surface effect is balanced by the effects of interband scattering, which cause the Gilbert damping constant to saturate at around 0.005. In small samples, the system shape induces a large anisotropy in the Gilbert damping.

cond-mat.mtrl-sci↗

arXiv2011

Transport properties of non-equilibrium systems under the application of light: Photo-induced quantum Hall insulators without Landau levels

In this paper, we study transport properties of non-equilibrium systems under the application of light in many-terminal measurements, using the Floquet picture. We propose and demonstrate that the quantum transport properties can be controlled in materials such as graphene and topological insulators, via the application of light. Remarkably, under the application of off-resonant light, topological transport properties can be induced; these systems exhibits quantum Hall effects in the absence of a magnetic field with a near quantization of the Hall conductance, realizing so-called quantum Hall systems without Landau levels first proposed by Haldane.

cond-mat.mes-hall↗

arXiv2011

A unified first-principles study of Gilbert damping, spin-flip diffusion and resistivity in transition metal alloys

Using a formulation of first-principles scattering theory that includes disorder and spin-orbit coupling on an equal footing, we calculate the resistivity $ρ$, spin flip diffusion length $l_{sf}$ and the Gilbert damping parameter $α$ for Ni$_{1-x}$Fe$_x$ substitutional alloys as a function of $x$. For the technologically important Ni$_{80}$Fe$_{20}$ alloy, permalloy, we calculate values of $ρ= 3.5 \pm 0.15$ $μ$Ohm-cm, $l_{sf}=5.5 \pm 0.3$ nm, and $α= 0.0046 \pm 0.0001$ compared to experimental low-temperature values in the range $4.2-4.8$ $μ$Ohm-cm for $ρ$, $5.0-6.0$ nm for $l_{sf}$, and $0.004-0.013$ for $α$ indicating that the theoretical formalism captures the most important contributions to these parameters.

cond-mat.mtrl-sci↗

arXiv2011

Nuclear Dynamics During Landau-Zener Singlet-Triplet Transitions in Double Quantum Dots

We consider nuclear spin dynamics in a two-electron double dot system near the intersection of the electron spin singlet $S$ and the lower energy component $T_{+}$ of the spin triplet. The electron spin interacts with nuclear spins and is influenced by the spin-orbit coupling. Our approach is based on a quantum description of the electron spin in combination with the coherent semiclassical dynamics of nuclear spins. We consider single and double Landau-Zener passages across the $S$-$T_{+}$ anticrossings. For linear sweeps, the electron dynamics is expressed in terms of parabolic cylinder functions. The dynamical nuclear polarization is described by two complex conjugate functions $Λ^{\pm}$ related to the integrals of the products of the singlet and triplet amplitudes ${\tilde{c}}_{S}^{\ast}{\tilde{c}}_{T_{+}}$ along the sweep. The real part $P$ of $Λ^{\pm}$ is related to the $S$-$T_{+}$ spin-transition probability, accumulates in the vicinity of the anticrossing, and for long linear passages coincides with the Landau-Zener probability $P_{LZ}=1-e^{-2πγ}$, where $γ$ is the Landau-Zener parameter. The imaginary part $Q$ of $Λ^{+}$ is specific for the nuclear spin dynamics, accumulates during the whole sweep, and for $γ\gtrsim 1$ is typically an order of magnitude larger than $P$. $Q$ has a profound effect on the nuclear spin dynamics, by (i) causing intensive shake-up processes among the nuclear spins and (ii) producing a high nuclear spin generation rate when the hyperfine and spin-orbit interactions are comparable in magnitude. We find analytical expressions for the back-action of the nuclear reservoir represented via the change in the Overhauser fields the electron subsystem experiences.

cond-mat.mes-hall↗

arXiv2011

Magnetization Dissipation in Ferromagnets from Scattering Theory

The magnetization dynamics of ferromagnets are often formulated in terms of the Landau-Lifshitz-Gilbert (LLG) equation. The reactive part of this equation describes the response of the magnetization in terms of effective fields, whereas the dissipative part is parameterized by the Gilbert damping tensor. We formulate a scattering theory for the magnetization dynamics and map this description on the linearized LLG equation by attaching electric contacts to the ferromagnet. The reactive part can then be expressed in terms of the static scattering matrix. The dissipative contribution to the low-frequency magnetization dynamics can be described as an adiabatic energy pumping process to the electronic subsystem by the time-dependent magnetization. The Gilbert damping tensor depends on the time derivative of the scattering matrix as a function of the magnetization direction. By the fluctuation-dissipation theorem, the fluctuations of the effective fields can also be formulated in terms of the quasistatic scattering matrix. The theory is formulated for general magnetization textures and worked out for monodomain precessions and domain wall motions. We prove that the Gilbert damping from scattering theory is identical to the result obtained by the Kubo formalism.

cond-mat.mes-hall↗

arXiv2011

FMR and voltage induced transport in normal metal-ferromagnet-superconductor trilayers

We study the subgap spin and charge transport in normal metal-ferromagnet-superconductor trilayers induced by bias voltage and/or magnetization precession. Transport properties are discussed in terms of time-dependent scattering theory. We assume the superconducting gap is small on the energy scales set by the Fermi energy and the ferromagnetic exchange splitting, and compute the non-equilibrium charge and spin current response to first order in precession frequency, in the presence of a finite applied voltage. We find that the voltage-induced instantaneous charge current and longitudinal spin current are unaffected by the precessing magnetization, while the pumped transverse spin current is determined by spin-dependent conductances and details of the electron-hole scattering matrix. A simplified expression for the transverse spin current is derived for structures where the ferromagnet is longer than the transverse spin coherence length.

cond-mat.mes-hall↗

arXiv2011

Phenomenology of Current-Induced Dynamics in Antiferromagnets

We derive a phenomenological theory of current-induced staggered magnetization dynamics in antiferromagnets. The theory captures the reactive and dissipative current-induced torques and the conventional effects of magnetic fields and damping. A Walker ansatz describes the dc current-induced domain-wall motion when there is no dissipation. If magnetic damping and dissipative torques are included, the Walker ansatz remains robust when the domain-wall moves slowly. As in ferromagnets, the domain-wall velocity is proportional to the ratio between the dissipative-torque and the magnetization damping. In addition, a current-driven antiferromagnetic domain-wall acquires a net magnetic moment.

cond-mat.mes-hall↗

arXiv2010

Microwave response of a magnetic single-electron transistor

We consider a single-electron transistor in the form of a ferromagnetic dot in contact with normal-metal and pinned ferromagnetic leads. Microwave-driven precession by the dot induces a pumped electric current. In open circuits, this pumping produces a measurable reverse bias voltage, which can be enhanced and made highly nonlinear by Coulomb blockade in the dot. The dependence of this bias on the power and spectrum of microwave irradiation may be utilized to develop nanoscale microwave detectors analogous to single-electron transistor-based electrostatic sensors and nanoelectromechanical devices.

cond-mat.mes-hall↗

arXiv2010

Effective Magnetic Monopoles and Universal Conductance Fluctuations

The observation of isolated positive and negative charges, but not isolated magnetic north and south poles, is an old puzzle. Instead, evidence of effective magnetic monopoles has been found in the abstract momentum space. Apart from Hall-related effects, few observable consequences of these abstract monopoles are known. Here, we show that it is possible to manipulate the monopoles by external magnetic fields and probe them by universal conductance fluctuation (UCF) measurements in ferromagnets with strong spin-orbit coupling. The observed fluctuations are not noise, but reproducible quasiperiodic oscillations as a function of magnetisation direction, a novel Berry phase fingerprint of the magnetic monopoles.

cond-mat.mes-hall↗

arXiv2010

Feedback control of noise in spin valves by the spin-transfer torque

The miniaturisation of magnetic read heads and random access memory elements makes them vulnerable to thermal fluctuations. We demonstrate how current-induced spin-transfer torques can be used to suppress the effects of thermal fluctuations. This enhances the fidelity of perpendicular magnetic spin valves. The simplest realization is a dc current to stabilize the free magnetic layers. The power can be significantly reduced without losing fidelity by simple control schemes, in which the stabilizing current-induced spin-transfer torque is controlled by the instantaneous resistance.

cond-mat.mes-hall↗

arXiv2010

AC Josephson Effect Induced by Spin Injection

Pure spin currents can be injected and detected in conductors via ferromagnetic contacts. We consider the case when the conductors become superconducting. A DC pure spin current flowing in one superconducting wire towards another superconductor via a ferromagnet contact induces AC voltage oscillations caused by Josephson tunneling of condensate electrons. Quasiparticles simultaneously counterflow resulting in zero total electric current through the contact. The Josephson oscillations can be accompanied by Carlson-Goldman collective modes leading to a resonance in the voltage oscillation amplitude.

cond-mat.supr-con↗

arXiv2010

Focused Crossed Andreev Reflection

We consider non-local transport in a system with one superconducting and two normal metal terminals. Electron focusing by weak perpendicular magnetic fields is shown to tune the ratio between crossed Andreev reflection (CAR) and electron transfer (ET) in the non-local current response. Additionally, electron focusing facilitates non-local signals between normal metal contacts where the separation is as large as the mean free path rather than being limited by the coherence length of the superconductor. CAR and ET can be selectively enhanced by modulating the magnetic field.

cond-mat.mes-hall↗

arXiv2010

Scattering Theory of Charge-Current Induced Magnetization Dynamics

In ferromagnets, charge currents can excite magnons via the spin-orbit coupling. We develop a novel and general scattering theory of charge current induced macrospin magnetization torques in normal metal$|$ferromagnet$|$normal metal layers. We apply the formalism to a dirty GaAs$|$(Ga,Mn)As$|$GaAs system. By computing the charge current induced magnetization torques and solving the Landau-Lifshitz-Gilbert equation, we find magnetization switching for current densities as low as $ 5\times 10^{6}$~A/cm$^2$. Our results are in agreement with a recent experimental observation of charge-current induced magnetization switching in (Ga,Mn)As.

cond-mat.mes-hall↗