Spin Current Density Research Articles

Mathematical modeling of semiconductor quantum dots based on the nonparabolic effective-mass approximation

Within the eective mass and nonparabolic band theory, a general framework of mathematical models and numerical methods is developed for theoretical studies of semiconductor quantum dots. It includes single-electron models and many-electron models of Hartree-Fock, configuration interaction, and current-spin density functional theory approaches. These models result in nonlinear eigenvalue problems from a suitable discretization. Cubic and quintic Jacobi-Davidson methods of block or nonblock version are then presented for calculating the wanted eigenvalues that are clustered in the interior of the spectrum and may have small gaps and degeneracy. These are challenging issues arising from modeling a great variety of semiconductor nanostructures fabricated by advanced technology in semiconductor industry and science. Generic algorithms for many-electron simulations under this framework are also provided. Numerical results obtained within this framework are summarized to three eminent aspects, namely, accuracy of models, physical novelty, and eectivity of nonlinear eigensolvers. Concerning numerical accuracy, important details related to experimental data are also addressed.

Surface acoustic wave driven ferromagnetic resonance in nickel thin films: Theory and experiment

We present an extensive experimental and theoretical study of surface acoustic wave-driven ferromagnetic resonance. In a first modeling approach based on the Landau-Lifshitz-Gilbert equation, we derive expressions for the magnetization dynamics upon magnetoelastic driving that are used to calculate the absorbed microwave power upon magnetic resonance as well as the spin current density generated by the precessing magnetization in the vicinity of a ferromagnet/normal metal interface. In a second modeling approach, we deal with the backaction of the magnetization dynamics on the elastic wave by solving the elastic wave equation and the Landau-Lifshitz-Gilbert equation selfconsistently, obtaining analytical solutions for the acoustic wave phase shift and attenuation. We compare both modeling approaches with the complex forward transmission of a LiNbO$_3$/Ni surface acoustic wave hybrid device recorded experimentally as a function of the external magnetic field orientation and magnitude, rotating the field within three different planes and employing three different surface acoustic wave frequencies. We find quantitative agreement of the experimentally observed power absorption and surface acoustic wave phase shift with our modeling predictions using one set of parameters for all field configurations and frequencies.

Experimental Verification ofPbBi2Te4as a 3D Topological Insulator

The experimental evidence is presented of the topological insulator state in PbBi2Te4. A single surface Dirac cone is observed by angle-resolved photoemission spectroscopy with synchrotron radiation. Topological invariants Z2 are calculated from the ab initio band structure to be 1;(111). The observed two-dimensional isoenergy contours in the bulk energy gap are found to be the largest among the known three-dimensional topological insulators. This opens a pathway to achieving a sufficiently large spin current density in future spintronic devices.

Few electron limit of n-type metal oxide semiconductor single electron transistors

We report the electronic transport on n-type silicon single electron transistors (SETs) fabricated in complementary metal oxide semiconductor (CMOS) technology. The n-type metal oxide silicon SETs (n-MOSSETs) are built within a pre-industrial fully depleted silicon on insulator (FDSOI) technology with a silicon thickness down to 10 nm on 200 mm wafers. The nominal channel size of 20 × 20 nm2 is obtained by employing electron beam lithography for active and gate level patterning. The Coulomb blockade stability diagram is precisely resolved at 4.2 K and it exhibits large addition energies of tens of meV. The confinement of the electrons in the quantum dot has been modeled by using a current spin density functional theory (CS-DFT) method. CMOS technology enables massive production of SETs for ultimate nanoelectronic and quantum variable based devices.

Geometry dependence on inverse spin Hall effect induced by spin pumping in Ni81Fe19/Pt films

Geometric effects on the inverse spin Hall effect (ISHE) induced by the spin pumping driven by the ferromagnetic resonance (FMR) have been investigated quantitatively. We measured the FMR spectra and the electromotive force induced by the ISHE with changing the size and the thickness in Ni${}_{81}$Fe${}_{19}$/Pt films. The intensity of generated charge currents due to the ISHE changes systematically with changing the film geometry, which is consistent with the prediction of the ISHE. The experimental results show a clear difference between Ni${}_{81}$Fe${}_{19}$ and Pt thickness dependence of the ISHE induced by the spin pumping. With a constant Pt thickness, the intensity of the generated charge current is kept proportional to that of the injected spin-current density, which decreases with increased spin relaxation in the Ni${}_{81}$Fe${}_{19}$ layer due to interfacial effect, inversely proportional to its film thickness. On the other hand, reflecting the spin diffusion mechanism in the Pt layer, the charge current decreases significantly with the decrease of the Pt thickness, while injected spin-current density is almost kept constant.

Electric Field Induced by the Spin Current in a Semiconductor Dresselhaus Nanowire

Using a new spin current definition the spin current and spin-current-induced electric field in a weak semiconductor Dresselhaus spin-orbit coupling nanowire (quantum wire) have been investigated theoretically. It is found that two nonzero linear spin current density elements take on oscillation peaks at the center of nanowire and their strengths can be changed by the number of propagation modes and Dresselhaus constant, respectively. Moreover, the spin-current-induced electric field has been calculated and its strength is measurable with today's technology which can be used to spin current detection. Our consequences also show that the strengths of spin current and spin-current-induced electric field for the Dresselhaus case are smaller than that for the Rashba case.

Transverse spin current in the s-wave/p-wave Josephson junction

We report a theoretical study on spin transport in the hybrid Josephson junction composed of singlet s-wave and triplet p-wave superconductor. The node of the triplet pair potential is considered perpendicular to the interface of the junction. Based on a symmetry analysis, we predict that there is no net spin density at the interface of the junction but instead a transverse mode-resolved spin density can exist and a nonzero spin current can flow transversely along the interface of the junction. The predictions are numerically demonstrated by means of the lattice Matsubara Green’s function method. It is also shown that, when a normal metal is sandwiched in between two superconductors, both spin current and transverse mode-resolved spin density are only residing at two interfaces due to the smearing effect of the multimode transport. Our findings are useful for identifying the pairing symmetry of the p-wave superconductor and generating spin current.

Dynamics of domain wall driven by spin-transfer torque

Spin-torque switching of magnetic devices offers new technological possibilities for data storage and integrated circuits. We have investigated domain-wall motion in a ferromagnetic thin film driven by a spin-polarized current using an atomistic spin model with a modified Landau-Lifshitz-Gilbert equation including the effect of the spin-transfer torque. The presence of the spin-transfer torque is shown to create an out-of-plane domain wall, in contrast to the external-field-driven case where an in-plane wall is found. We have investigated the effect of the spin torque on domain-wall displacement, domain-wall velocity, and domain-wall width, as well as the equilibration time in the presence of the spin-transfer torque. We have shown that the minimum spin-current density, regarded as the critical value for domain-wall motion, decreases with increasing temperature.

An Electron Paramagnetic Resonance Study of Defects in Interlayer Dielectrics

The electronic properties of thin film low-κ interlayer dielectric (ILD) and etch stop layers (ESL) are important issues in present day ULSI development (1-6). Low-κ ILD and ESLs with dielectric constants significantly less then those of SiO2 and SiN are utilized to reduce capacitance induced RC delays in ULSI circuits. Leakage currents, time dependent dielectric breakdown (TDDM) and stress induced leakage currents (SILC) are critical problems that are not yet well understood in ILD. A topic of current interest is ultraviolet light (UV curing) of low-k materials (5,6). We have made electron spin resonance (ESR) and current density versus voltage measurements on a moderately extensive set of dielectric/silicon structures involving materials of importance to low-k interconnect systems. Most of the dielectrics studied involve various compo-sitions of SiOC:H. In addition we have also made measurements on other dielectrics including SiO2, SiCN:H and SiN:H.

Effect of disorder on spin-transfer torque in magnetic tunnel junctions

We have generalized the nonequilibrium Green’s functions Keldysh formalism to study the effect of interfacial disorder on the average spin transfer torque, 〈T∥〉, in magnetic tunnel junctions (MTJs). We find a sinusoidal angular behavior of the average 〈T∥〉 as in ideal MTJs. We demonstrate for the first time that the general expression of the bias behavior of the average 〈T∥〉 in terms of the interplay of average spin current densities in collinear configurations is valid even in the presence of disorder. This explains the strong enhancement and sign reversal of 〈T∥〉 in the positive bias region, due to the disorder-induced resonance states at interface which selectively assist the transmission of right-coming electrons.

Calculation of spin-current densities using gauge-including atomic orbitals

The gauge-including magnetically induced current method for calculating the components of the current-density tensor using gauge-including atomic orbitals has been extended to treating open-shell molecules. The applicability of the method is demonstrated by calculations of first-order induced current densities on cyclobutadiene, Al(3), and B(3) at correlated ab initio levels of theory. For comparison, current-density calculations were also performed on the lowest closed-shell singlet state of cyclobutadiene as well on the closed-shell Al(3)(-) and B(3)(-) anions. The ring-current susceptibilities of the open-shell species are computed at the Hartree-Fock self-consistent-field, second-order Møller-Plesset perturbation theory, and coupled-cluster singles and doubles levels, whereas for the closed-shell systems also density functional theory calculations were employed. Explicit values for the current strengths caused by α and β electrons as well as the difference, representing the spin current, were obtained by numerical integration of the current-density contributions passing a plane perpendicular to the molecular ring. Comparisons of the present results to those recently obtained for the lowest triplet state of biphenyl emphasize that electron correlation effects must be considered for obtaining an accurate description of spin-current densities.

Extrinsic and Intrinsic Contributions to the Spin Hall Effect of Alloys

A fully relativistic description of the spin-orbit induced spin Hall effect is presented that is based on Kubo's linear response formalism. Using an appropriate operator for the spin-current density a Kubo-Středa-like equation for the spin Hall conductivity (SHC) is obtained. An implementation using the Korringa-Kohn-Rostoker band structure method in combination with the coherent potential approximation allow detailed investigations on various alloy systems. A decomposition of the SHC into intrinsic and extrinsic contributions is suggested. Accompanying calculations for the skew-scattering contribution of the SHC using the Boltzmann equation demonstrate the equivalence to the Kubo formalism in the dilute alloy regime and support the suggested decomposition scheme.

Spin current pumped by a rotating magnetic field in zigzag graphene nanoribbons

We study electron spin resonance in zigzag graphene nanoribbons by applying a rotatingmagnetic field on the system without any bias. By using the nonequilibrium Green’sfunction technique, the spin-resolved pumped current is explicitly derived in a rotatingreference frame. The pumped spin current density increases with the system size and theintensity of the transverse rotating magnetic field. For graphene nanoribbonswith an even number of zigzag chains, there is a nonzero pumped charge currentin addition to the pumped spin current owing to the broken spatial inversionsymmetry of the system, but its magnitude is much smaller than the spin current.The short-ranged static disorder from either impurities or defects in the ribboncan depress the spin current greatly due to the localization effect, whereas thelong-ranged disorder from charge impurities can avoid inter-valley scattering so thatthe spin current can survive in the strong disorder for the single-energy mode.

Spin projection and spin current density within relativistic electronic-transport calculations

A spin projection scheme is presented which allows the decomposition of the electric conductivity into two different spin channels within fully relativistic $ab$ $initio$ transport calculations that account for the impact of spin-orbit coupling. This is demonstrated by calculations of the spin-resolved conductivity of Fe$_{1-x}$Cr$_x$ and Co$_{1-x}$Pt$_x$ disordered alloys on the basis of the corresponding Kubo-Greenwood equation implemented using the Korringa-Kohn-Rostoker coherent potential approximation (KKR-CPA) band structure method. In addition, results for the residual resistivity of diluted Ni-based alloys are presented that are compared to theoretical and experimental ones that rely on Mott's two-current model for spin-polarized systems. The application of the scheme to deal with the spin-orbit induced spin Hall effect is discussed in addition.

Domain-Wall Depinning Assisted by Pure Spin Currents

We study the depinning of domain walls by pure diffusive spin currents in a nonlocal spin valve structure based on two ferromagnetic Permalloy elements with copper as the nonmagnetic spin conduit. The injected spin current is absorbed by the second Permalloy structure with a domain wall, and from the dependence of the wall depinning field on the spin current density we find an efficiency of 6×10{-14} T/(A/m{2}), which is more than an order of magnitude larger than for conventional current induced domain-wall motion. Theoretically we find that this high efficiency arises from the surface torques exerted by the absorbed spin current that lead to efficient depinning.

Spin transport in a tubular two-dimensional electron gas with Rashba spin-orbit coupling

We investigate the properties of local spin current and local spin density in a tubular two-dimensional electron gas system with Rashba spin-orbit coupling. To evaluate the spatial distribution of the spin-related quantities in both equilibrium and nonequilibrium cases, we derive the tubular version of tight-binding model and then employ the Landauer–Keldysh formalism. Our work reveals that the tubular geometry effect can cause spin-orbit-induced phenomena very different from those in a planar sample. In particular, a persistent spin current circulating in the tubular system is found to be robust against the disorder, thus suggesting its observability. In spite of the absence of spin accumulation, the spin Hall current can still arise and even circulate in the tubular sample. Moreover, a spin-independent impurity will induce distinctive spin accumulation patterns around the impurity, which may serve as a novel mechanism to control electron spins by arranging the impurities for spintronic devices.

Generation of electric field by spin-currents in the [formula omitted] gauge invariant Pauli–Schrödinger non-relativistic theory

The non-relativistic Pauli–Schrödinger theory has a richer gauge structure than usually expected, being invariant under the U ( 1 ) × SU ( 2 ) gauge group, which allows to define spin-current density vectors and obtains the relevant conserved quantities from Noether's theorem. The electromagnetic fields E and B play the role of the gauge potentials for the SU ( 2 ) sector of the gauge group and can possibly contribute with a corresponding invariant curvature self-energy term in the Lagrangian density. From the dynamics of the U ( 1 ) and SU ( 2 ) gauge fields we show that electric fields can be induced by spin-currents originated from the SU ( 2 ) gauge symmetry.

SPIN CURRENT INDUCED ELECTRIC FIELD IN A RASHBA QUANTUM WIRE

We investigate theoretically the spin current and spin current induced electric field in a weak Rashba spin-orbit coupling quantum wire (QW) using a definition for spin current by means of scattering matrix. It is found that there exists two non-zero linear spin current density elements which have oscillation peaks at the center of QW and their strengths can be changed by the number of propagation modes and Rashba constant, respectively. Moreover, the spin current induced electric field has also been calculated and its strength is measurable with present technology with which can be used to detect spin current.

Effect of electrical current pulses on domain walls in Pt/Co/Pt nanotracks with out-of-plane anisotropy: Spin transfer torque versus Joule heating

The effect of electrical current pulses on magnetic domain walls is studied in a nanometer-scale magnetic track defined in a ${\text{He}}^{+}$-irradiated Pt/Co(0.5 nm)/Pt film with out-of-plane anisotropy. Current pulses in a wide range of intensities and durations are shown to result either in track demagnetization, or in polarity-independent domain-wall propagation, due to Joule heating during the current pulse. None of the expected spin transfer effects is shown to occur. To explain this, the spin-dependent current-density distribution in the stack is evaluated, in the frame of an adapted Fuchs-Sondheimer model. The spin-polarized current density in the cobalt layer is shown to be unexpectedly low as compared to the charge current in the whole stack, which causes Joule heating. This is explained by a low electron transmission at Co/Pt interfaces, as is deduced from resistance measurements on a series of samples with different cobalt thicknesses. This leads us to emphasize that the balance between spin and total charge current densities should be carefully considered when addressing spin transfer torque effects.

Tensor part of the Skyrme energy density functional. II. Deformation properties of magic and semi-magic nuclei

We study systematically the impact of the time-even tensor terms of the Skyrme energy density functional, i.e., terms bilinear in the spin-current tensor density, on deformation properties of closed-shell nuclei corresponding to $20,28,40,50,82$, and $126$ neutron or proton shell closures. We compare results obtained with three different families of Skyrme parametrizations whose tensor terms have been adjusted on properties of spherical nuclei: (i) $\mathrm{T}\mathit{IJ}$ interactions proposed in the first article of this series [T. Lesinski et al., Phys. Rev. C 76, 014312 (2007)] that were constructed through a complete readjustment of the rest of the functional and (ii) parametrizations whose tensor terms have been added perturbatively to existing Skyrme interactions, with or without readjusting the spin-orbit coupling constant. We analyze in detail the mechanisms at play behind the impact of tensor terms on deformation properties and how studying the latter can help screen out unrealistic parametrizations. It is expected that findings of the present article are to a large extent independent of remaining deficiencies of the central and spin-orbit interactions and will be of great value for the construction of future, improved energy functionals.