Spin-dependent Chemical Potentials Research Articles

Spin polarization gate device based on the chirality-induced spin selectivity and robust nonlocal spin polarization.

Nonlocal spin polarization phenomena are thoroughly investigated in the devices made of chiral metallic single crystals of CrNb3S6 and NbSi2 as well as of polycrystalline NbSi2. We demonstrate that simultaneous injection of charge currents in the opposite ends of the device with the nonlocal setup induces the switching behavior of spin polarization in a controllable manner. Such a nonlocal spin polarization appears regardless of the difference in the materials and device dimensions, implying that the current injection in the nonlocal configuration splits spin-dependent chemical potentials throughout the chiral crystal even though the current is injected into only a part of the crystal. We show that the proposed model of the spin dependent chemical potentials explains the experimental data successfully. The nonlocal double-injection device may offer significant potential to control the spin polarization to large areas because of the nature of long-range nonlocal spin polarization in chiral materials.

Influence of diffusive transport on ultrafast magnetization dynamics

We study the influence of transport effects on time- and space-resolved magnetization dynamics in a laser-excited thick nickel film. We explicitly include diffusive heat transport and spin-resolved charge transport as well as Seebeck and Peltier effects and calculate the dynamics of spin-dependent electronic temperatures, chemical potentials, lattice temperatures, and magnetization. We find that transport has an influence on the magnetization dynamics closer to the excited surface as well as in regions deeper than the penetration depth of the laser. We reveal that, for higher absorbed fluences and in the presence of transport, thick magnetic films show a quenching time nearly independent of depth, though the magnitude of quenching is depth-dependent.

Non-integer spin-polarized conductance in multichannel magnetic point contacts

The ballistic conductance of a geometrically confined head-to-head 180° domain wall at a magnetic point contact in a two-dimensional (2D) metallic nanowire is studied theoretically. The ballistic conductance is computed by introducing the modified Landauer-Buttiker formalism in which the spin polarization effects are included. The analysis is based on coherent scattering of the carriers by the spin-dependent potential associated with the wall structure. The transmission properties of coherent states are obtained by introducing an algorithm to solve the coupled spin channels Schrodinger equation with mixed Dirichlet-Neumann boundary conditions applied far from the domain wall. Non-integer values for the conductance have been observed, due to the modified chemical potentials at the ferromagnetic contact probes. It has been shown that the dips in conductance arise when the electrons are partially polarized in the ferromagnetic leads, due to the formation of spin-dependent chemical potential.

Entropy production by thermodynamic currents in ambipolar conductors with identical spin dynamics characteristics between holes and electrons

We have evaluated the role of spin current in the energy dissipation mechanism in ambipolar conductors with identical spin-related characteristics between holes and electrons. Application of the Gibbs–Duhem relation to ambipolar conductors establishes a thermodynamic relation between the spin-dependent chemical potentials of holes and electrons, inducing an asymmetric spin splitting between the hole and electron chemical potentials. This yields two types of spin relaxation so as to allow the antiparallel spin current, where hole and electron spins flow in the opposite direction, to have a large spin diffusion length, but to retain that of the parallel spin current at a standard value.

Spin current generator in a single molecular magnet with spin bias

In this paper we study the spin- and charge-transport behaviors of a single molecular magnet (SMM) coupled to external reservoirs with spin-dependent chemical potentials. By using the Hubbard operator Green’s function method, we show that in the limit of a pinned macrospin the SMM can serve as an effective spin current filter or generator. Two configurations of spin bias are considered. For SMM coupled with symmetric dipolar spin bias, the system can generate spin-polarized current and pure spin current by adjusting the relative magnitude of spin bias and Coulomb interaction. It originates from the competition of two spin-opposite transport channels. Also due to this competition, one can observe a negative differential spin conductance. For SMM coupled with asymmetric dipolar spin bias, the system can only generate spin-polarized currents. The SMM as a spin current generator shows features different from the usual quantum dot system and these features can be useful in molecular spintronics.

Valley and spin polarization manipulated by electric field in magnetic silicene superlattice

Silicene is a close relative of graphene with a honeycomb lattice structure. However, silicene is unlike the strictly two-dimensional graphene and it has a buckled structure, i.e., the A and B atoms form two sublattice planes with a small vertical separation distance in between. Thus a perpendicular electric field applied to silicene can induce a staggered sublattice potential and different onsite energies in the A and B sublattices. As a result, silicene has a large spin-orbit gap compared with graphene. In addition, the mass of Dirac electrons in silicone is controllable by an external electric field, which leads to several controllable polarized transports in the silicene junction, including valley-, spin-and pseudospin-polarization transport. However, in a single silicone junction the manipulations of polarizations are not ideal. In this work, we consider several silicene-based superlattices in order to effectively control the properties of polarization transport. Using the transfer matrix method, we study valley-, spin-and pseudospin-polarization transport in silicene-based electrostatic potential, ferromagnetic and antiferromagnetic superlattices. The effects of ferromagnetic exchange field, antiferromagnetic exchange field and chemical potential on transport properties are analyzed and the roles of electrostatic field in regulating valley-, spin-and pseudospin-polarization are discussed. The ferromagnetic superlattices result in spin-dependent chemical potential in ferromagnetic regime, while Dirac-like mass depends on the antiferromagnetic exchange field and spin. For electrostatic potential superlattice, the pseudospin-polarization occurs and there is no spin-polarixation nor valley-polarization. The peaks of both the pseudospin conductances are completely separated from each other and the pseudospin is completely polarized in the wide range of the zero field for both sides. For ferromagnetic superlattice, the ferromagnetic exchange field and chemical potential lead to the concurrences of spin-and valley-polarizations. The spin-and valley-polarizations can realize a sudden reversal from -1 to +1 by adjusting the electric field. For antiferromagnetic superlattice, the similar properties of spin-and valley-polarizations are observed. Comparing with the ferromagnetic superlattice, only the polarization order is different when the same change is made in the electric field. These results indicate that when the number of lattices in the superlattice is more than 10, the valley-, spin-and pseudospin-polarization reach 100% easily in silicene-based superlattice. The polarization direction can be reversed by adjusting the electric field, which is helpful in manipulating the freedom degrees of valley, spin and pseudospin in silicene superlattice.

Spin-accumulation capacitance and its application to magnetoimpedance

It has been known that spin-dependent capacitances usually coexist with geometric capacitances in a magnetic multilayer. However, the charge and energy storage of the capacitance due to spin accumulation (SA) has not been fully understood. Here, we resolve this problem starting from the charge storage in the spin degree of freedom: spin accumulation manifests itself as an excess of electrons in one spin channel and an equal deficiency in the other under the quasi-neutrality condition. This enables us to model the two spin channels as the two plates of a capacitor. Taking a ferromagnet/nonmagnet junction as an example and using a method similar to that for treating quantum capacitance, we find that an SA capacitance can be introduced for each layer to measure its ability to store spins. A spatial charge storage is not essential for the SA capacitor and the energy stored in it is the splitting energy of the spin-dependent chemical potentials instead of the electrostatic energy. The SA capacitance is essentially a quantum capacitance due to spin accumulation on the scale of the spin-diffusion length. The SA capacitances can be used to reinterpret the imaginary part of the low-frequency magnetoimpedance.

Nonequilibrium spin-current detection with a single Kondo impurity

We present a theoretical study based on the Anderson model of the transport properties of a Kondo impurity (atom or quantum dot) connected to ferromagnetic leads, which can sustain a nonequilibrium spin current. We analyze the case where the spin current is injected by an external source and when it is generated by the voltage bias. Due to the presence of ferromagnetic contacts, a static exchange field is produced that eventually destroys the Kondo correlations. We find that such a field can be compensated by an appropriated combination of the spin-dependent chemical potentials leading to the restoration of the Kondo resonance. In this respect, a Kondo impurity may be regarded as a very sensitive sensor for nonequilibrium spin phenomena.

Transport across an Anderson quantum dot in the intermediate coupling regime

We describe linear and nonlinear transport across a single impurity Anderson model quantum dot with intermediate coupling to the leads, i.e., with tunnel coupling of the order of the thermal energy k_B T. The coupling is large enough that sequential tunneling processes alone do not suffice to properly describe the transport characteristics. Upon applying a density matrix approach, the current is expressed in terms of rates obtained by considering a very small class of diagrams which dress the sequential tunneling processes by charge fluctuations. We call this the "dressed second order" (DSO) approximation. One major achievement of the DSO is that, still in the Coulomb blockade regime, it can describe the crossover from thermally broadened to tunneling broadened conductance peaks. When the temperature is decreased even further, the DSO captures "Kondesque" behaviours of the Anderson quantum dot qualitatively: We find a zero bias anomaly of the differential conductance versus applied bias, an enhancement of the conductance with decreasing temperature as well as the onset of universality of the shape of the conductance as function of the temperature. We can address the case of a spin-degenerate level split energetically by a magnetic field and show that, if we assume in addition different capacitive couplings of the two spin-levels to the leads, one of the resonance peaks is vanishing. In case spin-dependent chemical potentials are introduced and only one of the four is varied, the DSO yields in principle only one resonance. This seems to be in agreement with experiments with pseudo-spin. Furthermore, we get qualitative agreement with experimental data showing a cross-over from the Kondo to the empty orbital regime.

Spin accumulation at ferromagnet/nonmagnetic material interfaces

Many proposed and realized spintronic devices involve spin injection and accumulation at an interface between a ferromagnet and a nonmagnetic material. We examine the electric field, voltage profile, charge distribution, spin fluxes, and spin accumulation at such an interface. We include the effects of both screening and spin scattering. We also include both the spin-dependent chemical potentials ${\ensuremath{\mu}}_{\ensuremath{\uparrow},\ensuremath{\downarrow}}$ and the effective magnetic field ${\stackrel{P\vec}{H}}^{*}$ that is zero in equilibrium. For a Co/Cu interface, we find that the spin accumulation in the copper is an order of magnitude larger when both chemical potential and effective magnetic field are included. We also show that screening contributes to the spin accumulation in the ferromagnet; this contribution can be significant.

Scattering theory of current-induced spin polarization

We construct a novel scattering theory to investigate magnetoelectrically inducedspin polarizations. Local spin polarizations generated by electric currents passingthrough a spin–orbit-coupled mesoscopic system are measured by an external probe.The electrochemical and spin-dependent chemical potentials on the probe arecontrollable and tuned to values ensuring that neither charge nor spin current flowbetween the system and the probe, on time average. For the relevant case of asingle-channel probe, we find that the resulting potentials are exactly independent of thetransparency of the contact between the probe and the system. Assuming thatspin relaxation processes are absent in the probe, we therefore identify the localspin-dependent potentials in the sample at the probe position, and hence thelocal current-induced spin polarization, with the spin-dependent potentials in theprobe itself. The statistics of these local chemical potentials is calculated withinrandom matrix theory. While they vanish on spatial and mesoscopic average,they exhibit large fluctuations, and we show that single systems typically havespin polarizations exceeding all known current-induced spin polarizations by aparametrically large factor. Our theory allows us to calculate quantum correlationsbetween spin polarizations inside the sample and spin currents flowing out of it. Weshow that these large polarizations correlate only weakly with spin currents inexternal leads, and that only a fraction of them can be converted into a spincurrent in the linear regime of transport, which is consistent with the mesoscopicuniversality of spin conductance fluctuations. We numerically confirm the theory.

Observation of Spin-Orbit Interaction Parameter Over a Wide Temperature Range Using Potentiometric Measurement

Spin-orbit interaction parameter (?) can be obtained by measuring the Shubnikov-de Haas oscillation, but this method is valid at only very low temperature. The current induced spin polarization in a 2-D electron gas layer is measured by potentiometric geometry which measures spin dependent chemical potential shifts. Subsequently, a spin-orbit interaction parameter can be extracted up to T = 250 K. In an inverted In0.52Al0.48As/In0.53Ga0.47As/In0.52Al0.48As quantum well system, a of 5.65 × 10-12 eVm and 3.85 × 10-12 eVm are obtained at T = 1.8 and 250 K, respectively.

Triple Quantum Dot Molecule as a Spin-Splitter

We propose a spin-splitter composed of triple quantum dots that works due to the Coulomb blockade effect and the charge and spin biases applied on external electron source and drains. The spin biases are applied only on the two drains and give their spin-dependent chemical potentials, which act as the driving forces for electron spin-polarized transport. By tuning the biases and the dots' levels, spin-up and spin-down electrons can be simultaneously split or separated from the source into two different drains. We show that such a tunneling process is detectable in terms of the spin accumulations on the dots or the currents flowing through the external leads. The present device is quite simple and realizable within currently existing technologies.

Resonant Charge Current in a Rashba Ring Induced by Spin-Dependent Potential

A one-dimensional ring subject to Rashba spin-orbit coupling is investigated. When it is attached to a lead with spin-dependent chemical potential, there will be charge current in the ring. The charge current response is resonantly maximized when the Fermi energy of the lead is equal to any energy level of the 1D ring. And if two probes are attached to the ring, the electric voltage between them creates sawtooth-like wave, which indicates the direction of the charge current. A ferromagnetic lead can also induce persistent charge current, which can be detected by magnetization intensity measurement.

Nonequilibrium Kondo Problem with Spin-Dependent Chemical Potentials: Exact Results

We give a brief review of the Kondo effect and exactly solve the nonequilibrium Kondo problem at the special point in the parameter space of the model with spin-dependent chemical potentials. Using the obtained solution, we compute several experimentally observable quantities: charge current, spin current and magnetic properties. Applications to the physically important cases such as the case where the impurity spin is placed in between two ferromagnetically polarized leads are discussed.

Charge Hall effect driven by spin-dependent chemical potential gradients and Onsager relations in mesoscopic systems

We study theoretically the spin-Hall effect as well as its reciprocal phenomenon (a transverse charge current driven by a spin-dependent chemical potential gradient) in electron and hole finite size mesoscopic systems. The Landauer-Buttiker-Keldysh formalism is used to model samples with mobilities and Rashba coupling strengths which are experimentally accessible and to demonstrate the appearance of measurable charge currents induced by the spin-dependent chemical potential gradient in the reciprocal spin-Hall effect. We also demonstrate that within the mesoscopic coherent transport regime the Onsager relations are fulfilled for the disorder averaged conductances for electron and hole mesoscopic systems.