Son-Hsien Chen

Microwave-driven ferromagnet–topological-insulator heterostructures: The prospect for giant spin battery effect and quantized charge pump devices

We study heterostructures where a two-dimensional topological insulator (TI) is attached to two normal metal (NM) electrodes while an island of a ferromagnetic insulator (FI) with precessing magnetization covers a portion of its lateral edges to induce time-dependent exchange field underneath via the magnetic proximity effect. When the FI island covers both lateral edges, such device pumps pure spin current in the absence of any bias voltage, thereby acting as an efficient spin battery with giant output current even at very small microwave power input driving the precession. When only one lateral edge is covered by the FI island, both charge and spin current are pumped into the NM electrodes. We delineate conditions for the corresponding conductances (current-to-microwave-frequency ratio) to be quantized in a wide interval of precession cone angles, which is robust with respect to weak disorder and can be further extended by changes in device geometry.

Non-Abelian spin-orbit gauge: Persistent spin helix and quantum square ring

We reexpress the Rashba and Dresselhaus interactions as non-Abelian spin-orbit gauges and provide a perspective in understanding the persistent spin helix [B. A. Berneving et al., Phys. Rev. Lett. 97, 236601 (2006)]. A spin-orbit interacting system can be transformed into a free-electron gas in the equal-strength Rashba-Dresselhaus [001] linear model, the Dresselhaus [110] linear model, and a one-dimensional system. A general tight-binding Hamiltonian for nonuniform spin-orbit interactions and hoppings along arbitrary directions, within the framework of finite-difference method, is obtained. As an application based on this Hamiltonian, a quantum square ring in contact with two ideal leads is found to exhibit four states: insulating, spin-filtering, spin-flipping, and spin-keeping states.

Inverse quantum spin Hall effect generated by spin pumping from precessing magnetization into a graphene-based two-dimensional topological insulator

We propose a multiterminal nanostructure for electrical probing of the quantum spin Hall effect (QSHE) in two-dimensional (2D) topological insulators. The device consists of a ferromagnetic (FM) island with precessing magnetization that pumps (in the absence of any bias voltage) pure spin current symmetrically into the left and right adjacent 2D TIs modeled as graphene nanoribbons with the intrinsic spin-orbit (SO) coupling. The QSH regime of the six-terminal TI|FM|TI nanodevice, attached to two longitudinal and four transverse normal metal electrodes, is characterized by the SO-coupling-induced energy gap, chiral spin-filtered edge states within finite length TI regions, and quantized spin Hall conductance when longitudinal bias voltage is applied, despite the presence of the FM island. The same unbiased device, but with precessing magnetization of the central FM island, blocks completely pumping of total spin and charge currents into the longitudinal electrodes while generating DC transverse charge Hall currents. Although these transverse charge currents are not quantized, their induction together with zero longitudinal charge current is a unique electrical response of TIs to pumped pure spin current that cannot be mimicked by SO-coupled but topologically trivial systems. In the corresponding two-terminal inhomogeneous TI|FM|TI nanostructures, we image spatial profiles of local spin and charge currents within TIs which illustrate transport confined to chiral spin-filtered edges states while revealing concomitantly the existence of interfacial spin and charge currents flowing around TI|FM interfaces and penetrating into the bulk of TIs over some short distance.

Spin-charge conversion in a multiterminal Aharonov-Casher ring coupled to precessing ferromagnets: A charge-conserving Floquet nonequilibrium Green function approach

We derive a non-perturbative solution to the Floquet-nonequilibrium Green function (Floquet-NEGF) describing open quantum systems periodically driven by an external field of arbitrary strength of frequency. By adopting the reduced-zone scheme, we obtain expressions rendering conserved charge currents for any given maximum number of photons, distinguishable from other existed Floquet-NEGF-based expressions where, less feasible, infinite number of photons needed to be taken into account to ensure the conservation. To justify our derived formalism and to investigate spin-charge conversions by spin-orbit coupling (SOC), we consider the spin-driven setups as reciprocal to the electric-driven setups in S. Souma et. al., Phys. Rev. B 70, 195346 (2004) and Phys. Rev. Lett. 94, 106602 (2005). In our setups, pure spin currents are driven by the magnetization dynamics of a precessing ferromagnetic (FM) island and then are pumped into the adjacent two- or four-terminal mesoscopic Aharonov-Casher (AC) ring of Rashba SOC where spin-charge conversions take place. Our spin-driven results show reciprocal features that excellently agree with the findings in the electric-driven setups mentioned above. We propose two types of symmetry operations, under which the AC ring Hamiltonian is invariant, to argue the relations of the pumped/converted currents in the leads within the same or between different pumping configurations. The symmetry arguments are independent of the ring width and the number of open channels in the leads, terminals, and precessing FM islands, In particular, net pure in-plane spin currents and pure spin currents can be generated in the leads for certain setups of two terminals and two precessing FM islands with the current magnitude and polarization direction tunable by the pumping configuration, gate voltage covering the two-terminal AC ring in between the FM islands.

Spin and charge pumping in magnetic tunnel junctions with precessing magnetization: A nonequilibrium Green function approach

We study spin and charge currents pumped by precessing magnetization of a single ferromagnetic layer within FIN or FIF F-ferromagnet; I-insulator; N-normal metal multilayers of nanoscale thickness attached to two normal-metal electrodes with no applied bias voltage between them. Both simple one-dimensional model, consisting of a single precessing spin and a potential barrier as the “sample,” and realistic threedimensional devices are investigated. In the rotating reference frame, where the magnetization appears to be static, these junctions are mapped onto a four-terminal dc circuit whose effectively half-metallic ferromagnetic electrodes are biased by the frequency /e of microwave radiation driving magnetization precession at the ferromagnetic resonance FMR conditions. We show that pumped spin current in FIF junctions, diminished behind the tunnel barrier and increased in the opposite direction, is filtered into charge current by the second F layer to generate dc pumping voltage of the order of 1 V at FMR frequency 10 GHz in an open circuit. In FIN devices, several orders of magnitude smaller charge current and the corresponding dc voltage appear concomitantly with the pumped spin current due to barrier induced asymmetry in the transmission coefficients connecting the four electrodes in the rotating-frame picture of pumping.

Spin-Hall effect in a finite two-dimensional electron gas sample with a central defect

We study spin and charge accumulation in two-dimensional electron gas (2DEG) with spin-orbit Rashba interactions. The Landauer–Keldysh formalism is employed to numerically compute accumulations in a 2DEG finite square sample with and without an embedded spin-independent hard-wall impurity. Cases with two and four attached ideal leads are considered. We find that in the two directions perpendicular to the Rashba field the impurity induces, respectively, a dipole and a quadrupole. In the direction parallel to the Rashba field the spin accumulation is determined purely by the imposed boundary conditions.

Spin-Polarized Transport and Dynamics in Magnetic Tunneling Structures

In the first part of this paper, we report a systematic study on the structural evolution under rapid thermal annealing and the corresponding transport properties in magnetic tunnel junctions (MTJs) with a crystalline MgO barrier. The results clearly indicate that high tunneling magnetic resistance can be achieved by annealing MTJs at a very short time, and it is directly related to the formation of (001) crystalline structures. In the second part, we report the spin dynamics in tunneling structure through direct electrical detection. A surprisingly large voltage generation in F/I/N and F/I/F junctions was observed, which is contradictory to the prediction from the standard spin-pumping theory. We proposed a theoretical formalism to study spin-pumping effects in ferromagnetic multilayer structures. The formalism can yield a remarkably clean physical picture of the spin and charge pumping in tunneling structures. The calculated values are consistent with experimental results.

Competition between spin–orbit interaction and exchange coupling within a honeycomb lattice ribbon

Spin density patterns of a pinned magnetic impurity that is embedded in a honeycomb lattice with zigzag edges are investigated by employing a mean-field assisted Landauer–Keldysh formalism. Both the intrinsic spin–orbit coupling and the extrinsic localized magnetic moments are considered, and the effects of the pinning directions and the species of the sublattice on the electron spins are analyzed. A local time-reversal symmetry breaking cannot destroy the equilibrium edge-state spin accumulation that is induced by intrinsic spin–orbit coupling when the pinning field lies in the plane of the ribbon and the embedding position is the A-site at the edge. The induced local spin can be either parallel or antiparallel to the localized impurity moment, depending on the location of the pinned impurity, because itinerant electrons are found only at the B-site of the edge, but not at the A-site.

Spin flip of a single anisotropic magnetic impurity in four-terminal Landauer setup with Rashba spin-orbit coupling

The mean-field-assisted Landauer–Keldysh formalism is employed to study the flip of an impurity spin under a uniaxial anisotropic field in two-dimensional electron gas with Rashba spin-orbit coupling. The spin-flip process with uniaxial anisotropic axis set to three different directions is investigated in a four-terminal Landauer setup. We show that the spin flip follows a three-dimensional trajectory rather than in-plane motion. As bias voltage changes sign, the impurity spin will flip from one saturated state to another and move along a different trajectory, depending on the chosen initial saturated state.

Non-equilibrium study of spin wave interference in systems with both Rashba and Dresselhaus (001) spin-orbit coupling

We study the electron spin transport in two dimensional electron gas (2DEG) system with both Rashba and Dresselhaus (001) spin-orbital coupling (SOC). We assume spatial behavior of spin precession in the non-equilibrium transport regime, and study also quantum interference induced by non-Abelian spin-orbit gauge field. The method we adopt in this article is the non-equilibrium Greens function within a tight binding framework. We consider one ferromagnetic lead which injects spin polarized electron to a system with equal strength of Rashba and Dresselhaus (001) SOC, and we observe the persistent spin helix property. We also consider two ferromagnetic leads injecting spin polarized electrons into a pure Dresselhaus SOC system, and we observe the resultant spin wave interference pattern.