Persistent spin current in a mesoscopic hybrid ring with spin-orbit coupling

Universal Spin Superconducting Diode Effect from Spin-Orbit Coupling

We propose a universal spin superconducting diode effect (SDE) induced by spin-orbit coupling (SOC) in systems with spin-triplet correlations, where the critical spin supercurrents in opposite directions are unequal. By analysis from both the Ginzburg-Landau theory and energy band analysis, we show that the spin-↑↑ and spin-↓↓ Cooper pairs possess opposite phase gradients and opposite momenta from the SOC, which leads to the spin SDE. Two superconductors with SOC, a p-wave superconductor as a toy model and a practical superconducting nanowire, are numerically studied and they both exhibit spin SDE. In addition, our theory also provides a unified picture for both spin and charge SDEs.

Generation and manipulation of pure spin current in a conducting loop coupled to an Aharonov-Bohm ring

In this work we put forward a new prescription for the generation and manipulation of non-decaying pure spin current (SC) in a Rashba spin-orbit (SO) coupled conducting loop which is attached to an Aharonov-Bohm (AB) ring. In presence of a single link between the rings, a SC is established in the flux-free ring, without accompanying any charge current (CC). The magnitude and direction of this SC are controlled by means of the AB flux, without tuning the SO coupling, which is the central aspect of our study. Employing a tight-binding framework we describe the two-ring quantum system, where the effect of magnetic flux is incorporated through Peierls phase factor. The specific roles of AB flux, SO coupling and the connectivity among the rings are critically investigated which yield several non-trivial signatures in energy band spectrum and pure SC. Along with SC, the phenomenon of flux-driven CC is also discussed, and at the end, different other effects like electron filling, system size and disorder are analyzed to make the present communication a self contained one. Our detailed investigation may provide some key aspects of designing efficient spintronic devices where SC can be guided in an other way.

Generation of circular spin current in an AB magnetic ring with vanishing net magnetization: a new prescription

In this work we report for the first time the appearance of non-decaying circular spin current in a magnetic ring with vanishing net magnetization, even in absence of any spin chirality. Breaking the symmetry in hopping integrals we can misalign up and down spin electronic energy levels which yields a net spin current in the magnetic quantum ring, threaded by an Aharonov-Bohm flux. Along with spin current, a net charge current also appears, and we compute both these currents using the second quantized approach. A tight-binding framework is employed to describe the magnetic ring where each site of the ring contains a finite magnetic moment. Itinerant electrons get scattered from the localized magnetic moments at different lattice sites, and the moments are arranged in such a way that the net magnetization vanishes. The interplay between magnetic moments and asymmetric hopping integrals leads to several atypical features in energy spectra, especially the existence of vanishing current carrying energy eigenstates together with the current carrying ones. The formation of such states those do not contribute any current is the artifact of different kinds of on-site energies and/or hopping integrals in different segments of the magnetic ring. The atypical signatures of energy levels are directly reflected into the charge and spin currents, and here we critically investigate the behaviors of circular currents as functions of electron filling, hopping integrals, strength of spin-moment interaction and ring size. Finally, we discuss briefly the possible experimental realization to implement our proposed magnetic system. The present analysis may provide a new route of generating persistent spin current in magnetic quantum rings with vanishing net magnetization, circumventing the use of spin-orbit coupled systems.

New Pyridine-Bridged Ferrocene-Rhodamine Receptor for the Multifeature Detection of Hg2+ in Water and Living Cells

A challenge in the design of optical and redox-active receptors is how to combine a specific recognition center with an efficient responsive system to facilely achieve multifeature detection in biological and environmental analyses. Herein, a novel ferrocene-rhodamine receptor conjugated with a pyridine bridge was designed and synthesized. This receptor can sensitively sense Hg²⁺ in aqueous media via chromogenic, fluorogenic, and electrochemical multisignal outputs with a low detection limit and fast response time. Moreover, it can be qualified as a fluorescent probe for effectively monitoring Hg²⁺ in living cells. A plausible recognition mode was proposed and rationalized with theoretical calculations.

Anomalous spin Nernst effect in Weyl semimetals

The spin Nernst effect describes a transverse spin current induced by the longitudinal thermal gradient in a system with the spin-orbit coupling. Here we study the spin Nernst effect in a mesoscopic four-terminal cross-bar Weyl semimetal device under a perpendicular magnetic field. Because the spin current is a tensor, it has three elements with the spin direction pointing to the x, y  and z directions when the spin current flows along the transverse lead. By using the tight-binding Hamiltonian combined with the nonequilibrium Green's function method, the three elements of the spin current in the transverse leads and the spin Nernst coefficients are obtained. The results show that the spin Nernst effect in the Weyl semimetal has an essential difference to the traditional Nernst effect: we found that the z direction spin current is zero without the magnetic field while it appears under the magnetic field, and the x and y  direction spin currents in the two transverse leads flow out or in together, in contrast to the traditional spin Nernst effect, in which the spin current is induced by the spin-orbit coupling and flows out from one lead and flows in on the other. We call it the anomalous spin Nernst effect. In addition, we show that the Weyl semimetals have inversion-type symmetry, mirror-reversal-type symmetry and electron-hole-type symmetry, which lead to the spin Nernst coefficients being either odd or even functions of the Fermi energy, the magnetic field and the transverse terminals. Moreover, the spin Nernst effect in the Weyl semimetals are strongly anisotropic and its coefficients are strongly dependent on both the direction of thermal gradient and the direction of the transverse lead connection. Three non-equivalent connection modes (x-z, z-x and x-y  modes) are studied in detail, and the spin Nernst coefficients for three different modes exhibit very different behaviors. These strongly anisotropic behaviors of the spin Nernst effect can be used as the characterization of magnetic Weyl semimetals.

Equilibrium spin current in graphene with Rashba spin-orbit coupling

The existence of a background spin current under thermodynamic equilibrium is an interesting phenomenon in the two-dimensional electron gas with Rashba spin-orbit coupling (RSOC). Here we study the equilibrium spin current (ESC) in graphene with RSOC. For an infinite graphene with uniform RSOC, we found that the ESC is proportional to λ² with λ the Rashba strength and mainly comes from the energy window [−λ, λ] near Dirac points. In the regime of energy far away from Dirac points, the λ³ dependence as that in a normal two-dimensional electron gas is recovered. In a system with a normal graphene strip inserted between two Rashba graphene sheets, we found that the ESC can penetrate through the normal graphene layer (perpendicular to the interface). This unique effect can be understood by considering the spin-filtered scattering from the normal region to the RSOC region. The finding of the ESC through the normal region without RSOC advances the understanding of ESC and provides a new way to generate a pure spin current in graphene. For an experimentally accessible strength of Rashba spin-orbit coupling, the ESC remains over room temperature.

Maximum intrinsic spin-Hall conductivity in two-dimensional systems with k-linear spin-orbit interaction

We analytically calculate the intrinsic spin-Hall conductivities (ISHCs) (σ(z)(xy) and σ(z)(yx)) in a clean, two-dimensional system with generic k-linear spin-orbit interaction. The coefficients of the product of the momentum and spin components form a spin-orbit matrix β̃. We find that the determinant of the spin-orbit matrix detβ̃ describes the effective coupling of the spin sz and orbital motion Lz. The decoupling of spin and orbital motion results in a sign change of the ISHC and the band-overlapping phenomenon. Furthermore, we show that the ISHC is in general unsymmetrical (σ(z)(xy) ≠ -σ(z)(yx)), and it is governed by the asymmetric response function Δβ̃, which is the difference in band-splitting along two directions: those of the applied electric field and the spin-Hall current. The obtained non-vanishing asymmetric response function also implies that the ISHC can be larger than e/8π, but has an upper bound value of e/4π. We will show that the unsymmetrical properties of the ISHC can also be deduced from the manifestation of the Berry curvature in the nearly degenerate area. On the other hand, by investigating the equilibrium spin current, we find that detβ̃ determines the field strength of the SU(2) non-Abelian gauge field.

Microwave magnetoelectric fields and their role in the matter-field interaction

We show that in a source-free subwavelength region of microwave fields, there can exist field structures with a local coupling between the time-varying electric and magnetic fields differing from the electric-magnetic coupling in regular-propagating free-space electromagnetic waves. To distinguish such field structures from regular electromagnetic (EM) field structures, we term them as magnetoelectric (ME) fields. We study a structure and conservation laws of microwave ME near fields. We show that there exist sources of microwave ME near fields-the ME particles. These particles are represented by small quasi-two-dimensional ferrite disks with magnetic-dipolar-oscillation spectra. The near fields originating from such particles are characterized by topologically distinctive power-flow vortices, nonzero helicity, and a torsion degree of freedom. The paper consists of two main parts. In the first one, we give a theoretical background of properties of the electric and magnetic fields inside and outside of a ferrite particle with magnetic-dipolar-oscillation spectra resulting in the appearance of microwave ME near fields. In the second main part, we represent numerical and experimental studies of the microwave ME near fields and their interactions with matter. Based on the obtained properties of the ME near fields, we discuss possibilities for effective microwave sensing of natural and artificial chiral structures.

Spin-polarized transport induced by spin-pumping in a Rashba ring

The Keldysh Green's function method is employed to study spin-dependent electron transport through a Rashba ring with a quantum dot (QD) embedded in one of its arms. Zero charge bias is applied on the system while a rotating magnetic field is considered in the QD to pump pure spin current. The Rashba spin-orbital coupling (RSOC) can cause a spin precession phase of the electron passing through the ring, so that the quantum interference in the ring can lead to a spin-polarized charge current flowing in the leads and the arm without a QD, whereas only pure spin current is flowing in the other arm with a QD. It is shown that for low frequency ω of the rotating magnetic field, the pumped charge current is proportional to ω unlike the charge current produced by mono-parametric quantum charge pumping, which is usually proportional to ω(2). Moreover, the magnitude, the direction, as well as the spin-polarization of the charge current can be controlled by tuning the device parameters such as the QD energy level, the RSOC strength, and the strength of the electron tunneling between the leads and the QD. Hence the studied device may serve as a generating source for tunable spin-polarized current in the spintronics field.

Generation and detection of spin current in the three-terminal quantum dot

We propose a novel device composed of a quantum dot tunneling coupled to ferromagnetic, superconducting, and normal-metal leads. This device can generate, manipulate, and detect pure spin current through the interplay between the spin-polarized quantum transport and the Andreev reflection. The spin current in this device is a well-defined conserved current since there is no need for any spin-orbit coupling. The proposed device is realizable using present nanotechnology.

Spin polarization of electron current through a potential barrier in two-dimensional structures with spin-orbit interaction

We show that an initially unpolarized electron flow acquires spin polarization after passing through a lateral barrier in a two-dimensional (2D) system with spin-orbit interaction (SOI) even if the current is directed normally to the barrier. The generated spin current depends on the distance from the barrier. It oscillates with the distance in the vicinity of the barrier and asymptotically reaches a constant value. The most efficient generation of the spin current (with polarization above 50%) occurs when the Fermi energy is near the potential barrier maximum. Since the spin current in the SOI medium is not unambiguously defined, we propose to pass this current from the SOI region into a contacting region without SOI and show that the spin polarization loss under such transmission can be negligible.

Proposal for measuring mechanically equilibrium spin currents in the rashba medium

We demonstrate that an equilibrium spin current in a 2D electron gas with Rashba spin-orbit interaction (Rashba medium) results in a mechanical torque on a substrate near an edge of the medium. If the substrate is a cantilever, the mechanical torque displaces the free end of the cantilever. The effect can be enhanced and tuned by a magnetic field. Observation of this displacement would be an effective method to prove the existence of equilibrium spin currents. The analysis of edges of the Rashba medium demonstrates the existence of localized edge states. They form a 1D continuum of states. This suggests a new type of quantum wire: spin-orbit quantum wire.