Improved Domain Wall Dynamics and Magnonic Torques using Topological Insulators

We investigate the magnetization dynamics that arise when a thin-film ferromagnet is deposited on a topological insulator (TI), focusing in particular on domain-wall motion via current and the possibility of a spin-wave torque acting on the magnetization. We show analytically that the coupling between the magnetic domain wall and the TI removes the degeneracy of the wall profile with respect to its chirality and topological charge. Moreover, we find that the threshold for Walker breakdown of domain wall motion is substantially increased and determined by the interaction with the TI, allowing for higher attainable wall velocities than in the conventional case where the hard axis anisotropy determines the Walker threshold. Finally, we show that the allowed modes of spin-wave excitations and the ensuing magnetization dynamics in the presence of a TI coupling enable a magnonic torque acting even on homogeneous magnetization textures. Our results indicate that the TI-ferromagnet interaction has a similar effect on the magnetization dynamics as an intrinsic Dzyaloshinskii-Moriya interaction in ferromagnets.Comment: 4 pages, 1 figure. Accepted for publication in PRB Rapid Communications. Chosen as Editors' Suggestio

Chirality Sensitive Domain Wall Motion in Spin-Orbit Coupled Ferromagnets

Using the Lagrangian formalism, we solve analytically the equations of motion for current-induced domain-wall dynamics in a ferromagnet with Rashba spin-orbit coupling. An exact solution for the domain wall velocity is provided, including the effect of non-equilibrium conduction electron spin-density, Gilbert damping, and the Rashba interaction parameter. We demonstrate explicitly that the influence of spin-orbit interaction can be qualitatively different from the role of non-adiabatic spin-torque in the sense that the former is sensitive to the chirality of the domain wall whereas the latter is not: the domain wall velocity shows a reentrant behavior upon changing the chirality of the domain wall. This could be used to experimentally distinguish between the spin-orbit and non-adiabatic contribution to the wall speed. A quantitative estimate for the attainable domain wall velocity is given, based on an experimentally relevant set of parameters for the system.Comment: 7 pages, 2 figures. Accepted for publication in Phys. Rev.

Superconducting Proximity Effect in Silicene: Spin-Valley Polarized Andreev Reflection, Non-Local Transport, and Supercurrent

We theoretically study the superconducting proximity effect in silicene, which features massive Dirac fermions with a tunable mass (band gap), and compute the conductance across a normal/superconductor (N/S) silicene junction, the non-local conductance of an N/S/N junction, and the supercurrent flowing in an S/N/S junction. It is demonstrated that the transport processes consisting of local and non-local Andreev reflection may be efficiently controlled via an external electric field owing to the buckled structure of silicene. In particular, we demonstrate that it is possible to obtain a fully spin-valley polarized crossed Andreev reflection process without any contamination of elastic cotunneling or local Andreev reflection, in stark contrast to ordinary metals. It is also shown that the supercurrent flowing in the S/N/S junction can be fully spin-valley polarized and that it is controllable by an external electric field.Comment: 5 pages, 4 figures + supplementary informatio

Asymmetric Ferromagnetic Resonance, Universal Walker Breakdown, and Counterflow Domain Wall Motion in the Presence of Multiple Spin-Orbit Torques

We study the motion of several types of domain wall profiles in spin-orbit coupled magnetic nanowires and also the influence of spin-orbit interaction on the ferromagnetic resonance of uniform magnetic films. We extend previous studies by fully considering not only the field-like contribution from the spin-orbit torque, but also the recently derived Slonczewski-like spin-orbit torque. We show that the latter interaction affects both the domain wall velocity and the Walker breakdown threshold non-trivially, which suggests that it should be accounted in experimental data analysis. We find that the presence of multiple spin-orbit torques may render the Walker breakdown to be universal in the sense that the threshold is completely independent on the material-dependent Gilbert damping, non-adiabaticity, and the chirality of the domain wall. We also find that domain wall motion against the current injection is sustained in the presence of multiple spin-orbit torques and that the wall profile will determine the qualitative influence of these different types of torques (e.g. field-like and Slonczewski-like). In addition, we consider a uniform ferromagnetic layer under a current bias, and find that the resonance frequency becomes asymmetric against the current direction in the presence of Slonczewski-like spin-orbit coupling. This is in contrast with those cases where such an interaction is absent, where the frequency is found to be symmetric with respect to the current direction. This finding shows that spin-orbit interactions may offer additional control over pumped and absorbed energy in a ferromagnetic resonance setup by manipulating the injected current direction.Comment: 12 pages including 7 figure

Superconducting Spintronics with Magnetic Domain Walls

The recent experimental demonstration of spin-polarized supercurrents offer a venue for establishment of a superconducting analogue to conventional spintronics. Whereas domain wall motion in purely magnetic structures is a well-studied topic, it is not clear how domain wall dynamics may influence superconductivity and if some functional property can be harnessed from such a scenario. Here, we demonstrate that domain wall motion in superconducting systems offers a unique way of controlling the quantum state of the superconductor. Considering both the diffusive and ballistic limits, we show that moving the domain wall to different locations in a Josephson junction will change the quantum ground state from being in a 0 state to a $\pi$ state. Remarkably, we also show that domain wall motion can be used to turn on and off superconductivity: the position of the domain wall determines the critical temperature $T_c$ and thus if the system is in a resistive state or not, causing even a quantum phase transition between the dissipationless and normal state at $T=0$. In this way, one achieves dynamical control over the superconducting state within a single sample by utilizing magnetic domain wall motion

General solution of 2D and 3D superconducting quasiclassical systems: coalescing vortices and nanoisland geometries

An extension of quasiclassical Keldysh-Usadel theory to higher spatial dimensions than one is crucial in order to describe physical phenomena like charge/spin Hall effects and topological excitations like vortices and skyrmions, none of which are captured in one-dimensional models. We here present a numerical finite element method which solves the non-linearized 2D and 3D quasiclassical Usadel equation relevant for the diffusive regime. We show the application of this on three model systems with non-trivial geometries: (i) a bottlenecked Josephson junction with external flux, (ii) a nanodisk ferromagnet deposited on top of a superconductor and (iii) superconducting islands in contact with a ferromagnet. In case (i), we demonstrate that one may control externally not only the geometrical array in which superconducting vortices arrange themselves, but also to cause coalescence and tune the number of vortices. In case (iii), we show that the supercurrent path can be tailored by incorporating magnetic elements in planar Josephson junctions which also lead to a strong modulation of the density of states. The finite element method presented herein paves the way for gaining insight in physical phenomena which have remained largely unexplored due to the complexity of solving the full quasiclassical equations in higher dimensions.Comment: 16 pages, 8 figures. Added several new result

Giant Triplet Proximity Effect in π\pi-biased Josephson Junctions with Spin-Orbit Coupling

In diffusive Josephson junctions the phase-difference $\phi$ between the superconductors strongly influences the spectroscopic features of the layer separating them. The observation of a uniform minigap and its phase modulation were only recently experimentally reported, demonstrating agreement with theoretical predictions up to now - a vanishing minigap at $\phi=\pi$. Remarkably, we find that in the presence of intrinsic spin-orbit coupling a giant proximity effect due to spin-triplet Cooper pairs can develop at $\phi=\pi$, in complete contrast to the suppressed proximity effect without spin-orbit coupling. We here report a combined numerical and analytical study of this effect, proving its presence solely based on symmetry arguments, which makes it independent of the specific parameters used in experiments. We show that the spectroscopic signature of the triplets is present throughout the entire ferromagnetic layer. Our finding offers a new way to artificially create, control and isolate spin-triplet superconductivity.Comment: 6 pages, 6 figures. Accepted for publication in Phys. Rev.

Tunable Supercurrent at the Charge Neutrality Point via Strained Graphene Junctions

We theoretically calculate the charge-supercurrent through a ballistic graphene junction where superconductivity is induced via the proximity-effect. Both monolayer and bilayer graphene are considered, including the possibility of strain in the systems. We demonstrate that the supercurrent at the charge neutrality point can be tuned efficiently by means of mechanical strain. Remarkably, the supercurrent is enhanced or suppressed relative to the non-strained case depending on the direction of this strain. We also calculate the Fano factor in the normal-state of the system and show how its behavior varies depending on the direction of strain.Comment: 7 Pages and 4 Figures. To Appear in Physical Review

Graphene-based extremely wide-angle tunable metamaterial absorber

We investigate the absorption properties of graphene-based anisotropic metamaterial structures where the metamaterial layer possesses an electromagnetic response corresponding to a near-zero permittivity. We find that through analytical and numerical studies, near perfect absorption arises over an unusually broad range of beam incidence angles. Due to the presence of graphene, the absorption is tunable via a gate voltage, providing dynamic control of the energy transmission. We show that this strongly enhanced absorption arises due to a coupling between light and a fast wave-mode propagating along the graphene/metamaterial hybrid.Comment: 9 pages, 6 figure