Spin-dependent Transmission Probability Research Articles

Spin-dependent shot noise in 8-Pmmn borophene based-superlattice

The effect of the Rashba spin-orbit interaction on the shot noise, spin/valley polarization in 8-Pmmn borophene-based superlattice are studied theoretically, by applying the transfer-matrix approach. The spin-dependent transmission probability can be easily controlled by the number of superlattice barriers, the strength of the Rashba, and the incident electron angle. The Fano factor without spin rotation as well as with spin rotation shows an oscillatory pattern versus the strength of the Rashba, and the oscillatory behavior becomes more pronounced as the number of barriers increases. Also, the Fano factor shows a strong dependence on the superlattice direction and number of barriers. The spin and valley polarizations can be easily controlled by the number of barriers and the direction of superlattice. Our findings in this work could provide useful for valleytronics/spintronics systems based on the 8-Pmmn borophene.

Quantum transport of massless Dirac fermions through wormhole-shaped curved graphene in presence of constant axial magnetic flux

In this work, we have studied the spin-dependent quantum transport of charged fermion on (2+1)\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$(2+1)$$\\end{document}-dimensional spacetime, whose spatial part is described by a wormhole-type geometry in the presence of constant axial magnetic flux. Choosing the solutions of the Dirac equation associated with real energy and momentum, we explored the spin-dependent transmission probabilities and giant magnetoresistance (GMR) through a single layer of wormhole graphene with an external magnetic field, using the transition matrix (T-Matrix) approach. The spin-up and spin-down components within the A and B sublattices of graphene in the matrix of 4×1\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$4\ imes 1$$\\end{document} wave function are coupled to each other due to the wormhole structure and the magnetic field. We have found that transport properties strongly depend on the magnetic field, incident energy, and geometric parameters of the system. We observed that the transmission probability increases as the radius of the wormhole increases, and the length of the wormhole decreases. The higher energies lead to a decrease in the transmission probabilities of particles. Furthermore, we observed that the probability of the spin-flip effect is almost larger than that of the non-spin-flip effect, illustrating that electrons lose their spins during transmission. These findings highlight the complex and interesting behavior of wormhole graphene in the presence of external magnetic fields and suggest that these nano structures can have potential applications in electronic and spintronic devices.

Effect of iron thicknesses on spin transport in a Fe/Au bilayer system

This paper is concerned with a theoretical analysis of the behavior of optically excited spin currents in bilayer and multilayer systems of ferromagnetic and normal metals. As the propagation, control, and manipulation of the spin currents created in ferromagnets by femtosecond optical pulses is of particular interest, we examine the influence of different thicknesses of the constituent layers for the case of electrons excited several electronvolts above the Fermi level. Using a Monte-Carlo simulation framework for such highly excited electrons, we first examine the spatiotemporal characteristics of the spin current density driven in a Fe layer, where the absorption profile of the light pulse plays an important role. Further, we examine how the combination of light absorption profile, spin-dependent transmission probabilities, and iron layer thickness affects spin current density in a Fe/Au bilayer system. For high-energy electrons studied here, the interface and secondary electron generation have a small influence on spin transport in the bilayer system. However, we find that spin injection from one layer to another is most effective within a certain range of iron layer thicknesses.

Giant chirality-induced spin selectivity of polarons

The chirality-induced spin selectivity (CISS) effect gives rise to strongly spin-dependent transport through many organic molecules and structures. Its discovery raises fascinating fundamental questions as well as the prospect of possible applications. The basic phenomenology, a strongly asymmetric magnetoresistance despite the absence of magnetism, is now understood to result from the combination of spin-orbit coupling and chiral geometry. However, experimental signatures of electronic helicity were observed at room temperature, i.e., at an energy scale that exceeds the typical spin-orbit coupling in organic systems by several orders of magnitude. This work shows that a new energy scale for CISS emerges for currents carried by polarons, i.e., in the presence of strong electron-phonon coupling. In particular, we found that polaron fluctuations play a crucial role in the two manifestations of CISS in transport measurements -- the spin-dependent transmission probability through the system and asymmetric magnetoresistance.

Spintronics in double stranded magnetic helix: role of non-uniform disorder

The spin dependent transport phenomena are investigated in a double stranded (ds) magnetic helix (MH) structure. Two different helical systems, short-range hopping helix and long range hopping (LRH) helix, are taken into account. We explore the role of these two kinds of geometries on spin dependent transport phenomena. Using Green’s function formalism within a tight-binding framework we compute transport quantities which include spin dependent transmission probabilities, junction currents and spin polarization (SP) coefficient. High degree of SP is obtained for the LRH MH. The SP can be tuned by changing the inter-strand hopping and the direction of magnetic moments at different lattice sites. We find atypical features when we include impurities in one strand of the MH, keeping the other strand free. Unlike uniform disordered systems, SP gets increased with impurity strength beyond a critical value. The effect of temperature on SP and experimental possibilities of our proposed quantum system are also discussed, to make the present communication a self-contained one. Our analysis may provide a new route to explore interesting spintronic properties using similar kind of fascinating helical geometries, possessing higher order electron hopping and subjected to non-uniform disorder.

Spin polarization in an ac-driven magnetic material with vanishing net magnetization: a new proposal

In this work, we address the fundamental question of whether a magnetic material having zero net magnetization can produce polarized spin current from a completely unpolarized one. Common wisdom suggests that this is not possible, but if we break the symmetry in hopping integrals in different segments of the magnetic sample, then a finite possibility of getting polarized spin current is established. To substantiate this fact, we consider a one-dimensional magnetic chain with vanishing net magnetization where one part of the chain is subjected to an ac electric field, keeping the other part free. The ac field, introduced through Peierls substitution, modulates the hopping integrals yielding a misalignment of up and down spin channels, which is the primary requirement to get finite spin polarization. Simulating the system within a tight-binding framework, we compute spin-dependent transmission probabilities using the well-known Green’s function prescriptionand determine junction currents following the Landauer–Büttiker formalism. Our analysis may shed some light on designing spin-polarized devices using driven magnetic materials with vanishing net magnetization.

A new prescription to achieve a high degree of spin polarization in a spin–orbit coupled quantum ring: efficient engineering by irradiation

The present work discusses the possibility to achieve a high degree of spin polarization in a three-terminal quantum system. Irradiating the system, subjected to Rashba spin–orbit (SO) interaction, we find high degree of spin polarization under a suitable input condition along with different magnitudes and phases at the two output leads. The system is described within a tight-binding (TB) framework and the effect of irradiation is incorporated following the Floquet–Bloch (FB) ansatz. All the spin-dependent transmission probabilities are evaluated through Green’s function technique using Landauer–Büttiker formalism. Several possible aspects are included to make the system more realistic and examined rigorously in the present work. To name a few, the effects of irradiation, SO interaction, interface sensitivity, system size, system temperature are investigated, and finally, the role of correlated impurities are studied. Despite having numerous proposals available to generate and manipulate spin-selective transmissions, such a prescription exploiting the irradiation effect is relatively new to the best of our concern.

Conformational effect on spin filtration through a multi-terminal magnetic helix

We investigate spin dependent electron transport through a magnetic helix (MH) which is coupled to one input and two output leads. Suitably adjusting the physical parameters and lead positions we can achieve high degree of spin polarizations with opposite phase at the output terminals. We consider both long-range and short-range hopping magnetic helices and critically examine the interplay between these hopping integrals on spin polarization. From our results we find that the long-range hopping MH is much superior than the short-range one. Simulating the quantum system within a tight-binding framework we compute spin dependent transmission probabilities and spin polarization co-efficient using the well known Green’s function formalism. Our proposition can provide a deeper impact in realizing efficient spin based devices in near future.

Engineering spin polarization in a driven multistranded magnetic quantum network

Possible engineering of spin polarization through a multistranded magnetic quantum network in the presence of light irradiation is reported. Each site of the network is associated with a net magnetic moment which provides a spin-dependent scattering via spin-spin exchange interaction, yielding a finite spin polarization across the network. The quantum system is described within a tight-binding framework where the irradiation effect is incorporated through Floquet-Bloch prescription. The spin-polarization coefficient, measured by determining spin-dependent transmission probabilities following the Green's function formalism, shows several atypical features with irradiation parameters. Apart from getting a high degree of spin polarization, a complete phase reversal can be achieved. These two issues are extremely important in designing efficient spin-based electronic devices. A detailed mathematical description is given to decouple the Hamiltonian of a generalized multistranded magnetic ladder into distinct one-dimensional lattices in separate spin subspaces for any arbitrary orientation of local spin, which clearly illustrates the allowed energy spin subbands. Effects of system size and uncorrelated disorder on spin polarization are critically examined. Finally, we discuss the possible experimental realization of our magnetic quantum systemto regulate the degree and phase of spin polarization by irradiating a magnetic sample, and thus we believe that the present analysis may provide a route of engineering spin-dependent electron transfer through a spin-polarized device.

Spin-dependent transport properties and Seebeck effects for a crossed graphene superlattice p-n junction with armchair edge

Using the nonequilibrium Green’s function method combined with the tight-binding Hamiltonian, we theoretically investigate the spin-dependent transmission probability and spin Seebeck coefficient of a crossed armchair-edge graphene nanoribbon (AGNR) superlattice p-n junction under a perpendicular magnetic field with a ferromagnetic insulator, where junction widths W1 of 40 and 41 are considered to exemplify the effect of semiconducting and metallic AGNRs, respectively. A pristine AGNR system is metallic when the transverse layer m = 3j + 2 with a positive integer j and an insulator otherwise. When stubs are present, a semiconducting AGNR junction with width W1 = 40 always shows metallic behavior regardless of the potential drop magnitude, magnetization strength, stub length, and perpendicular magnetic field strength. However, metallic or semiconducting behavior can be obtained from a metallic AGNR junction with W1 = 41 by adjusting these physical parameters. Furthermore, a metal-to-semiconductor transition can be obtained for both superlattice p-n junctions by adjusting the number of periods of the superlattice. In addition, the spin-dependent Seebeck coefficient and spin Seebeck coefficient of the two systems are of the same order of magnitude owing to the appearance of a transmission gap, and the maximum absolute value of the spin Seebeck coefficient reaches 370 µV/K when the optimized parameters are used. The calculated results offer new possibilities for designing electronic or heat-spintronic nanodevices based on the graphene superlattice p-n junction.

Dwell time, Hartman effect and transport properties in a ferromagnetic phosphorene monolayer

In this paper, spin-dependent dwell time, spin Hartman effect and spin-dependent conductance were theoretically investigated through a rectangular barrier in the presence of an exchange field by depositing a ferromagnetic insulator on the phosphorene layer in the barrier region. The existence of the spin Hartman effect was shown for all energies (energies lower than barrier height) and all incident angles in phosphorene. We also compared our results of the dwell time in the phosphorene structure with similar research performed on graphene. We reported a significant difference between the tunneling time values of incident quasiparticles with spin-up and spin-down. We found that the barrier was almost transparent for incident quasiparticles with a wide range of incident angles and energies higher than the barrier height in phosphorene. We also found that the maximum spin-dependent transmission probability for energies higher than barrier height does not necessarily occur in the zero incident angle. In addition, we showed that the spin conductance for energies higher (lower) than barrier height fluctuates (decays) in terms of barrier thickness. We discovered that, in contrast to graphene, the Klein paradox does not occur in the normal incident in the phosphorene structure. Furthermore, the results demonstrated the achievement of good total conductance at certain thicknesses of the barrier for energies higher than the barrier height. This study could serve as a basis for investigations of the basic physics of tunneling mechanisms and also for using phosphorene as a spin polarizer in designing nanoelectronic devices.

Spin-dependent Seebeck effects in a graphene superlattice p–n junction with different shapes

We theoretically calculate the spin-dependent transmission probability and spin Seebeck coefficient for a zigzag-edge graphene nanoribbon p–n junction with periodically attached stubs under a perpendicular magnetic field and a ferromagnetic insulator. By using the nonequilibrium Green’s function method combining with the tight-binding Hamiltonian, it is demonstrated that the spin-dependent transmission probability and spin Seebeck coefficient for two types of superlattices can be modulated by the potential drop, the magnetization strength, the number of periods of the superlattice, the strength of the perpendicular magnetic field, and the Anderson disorder strength. Interestingly, a metal to semiconductor transition occurs as the number of the superlattice for a crossed superlattice p–n junction increases, and its spin Seebeck coefficient is much larger than that for the T-shaped one around the zero Fermi energy. Furthermore, the spin Seebeck coefficient for crossed systems can be much pronounced and their maximum absolute value can reach 528 μV by choosing optimized parameters. Besides, the spin Seebeck coefficient for crossed p–n junction is strongly enhanced around the zero Fermi energy for a weak magnetic field. Our results provide theoretical references for modulating the thermoelectric properties of a graphene superlattice p–n junction by tuning its geometric structure and physical parameters.

Selective spin transport through a quantum heterostructure: Transfer matrix method

In the present work, we propose that a one-dimensional quantum heterostructure composed of magnetic and non-magnetic (NM) atomic sites can be utilized as a spin filter for a wide range of applied bias voltage. A simple tight-binding framework is given to describe the conducting junction where the heterostructure is coupled to two semi-infinite one-dimensional NM electrodes. Based on transfer matrix method, all the calculations are performed numerically which describe two-terminal spin-dependent transmission probability along with junction current through the wire. Our detailed analysis may provide fundamental aspects of selective spin transport phenomena in one-dimensional heterostructures at nanoscale level.

The Next Nearest Neighbor Effect on the Spin-Dependent Transport Properties of DNA Molecule Based Spin Valve

Based on tight-binding model and a generalized Green’s function method in Landauer-Buttiker formalism, the next nearest-neighbor (NNN) effects on the spin-dependent transport properties of a poly(G)-poly(C) DNA molecule sandwiched between ferromagnetic 3-dimensional electrodes are numerically investigated. Our results show that the spin-dependent transport characteristics, including the spin-dependent transmission probability, current-voltage characteristics, noise power, Fano factor (F), and tunnel magnetoresistance (TMR) are strongly influenced by NNN effect. Results show that the inclusion of NNN effect can broaden transmission spectra and leads to semiconductor-metal transition in the system. Therefore, the spin-dependent current and noise power amplitudes increase. The Poisson limit (F=1) region decreases due to the presence of NNN effect, where the inclusion of diagonal hopping integrals between the DNA sites increases electron correlations. Furthermore, with increasing diagonal hopping integrals strength between the DNA sites, the first sharp decrease of TMR happens in lower bias voltage and the region of the maximum value of TMR decreases.

Electron scattering in a graphene nanoribbon in the presence of ferromagnetic layer and Rashba interaction

We study the possibility to control the spin polarization and spin-dependent transport in a graphene sheet by considering a ferromagnetic layer in the presence of the Rashba spin–orbit interaction. Studying the scattering problem with the help of the Green function (which was found explicitly), we obtained simple analytical expressions for the spin dependent transmission probability. Using the small exchange parameter and Rashba coupling constant, we can obtain any degree of spin polarization, but in the case of a small interaction region, only for slow electrons.

Spin-dependent and photon-assisted transmission enhancement and suppression in a magnetic-field tunable ZnSe/Zn1–x Mn x Se heterostructure

Using the effective-mass approximation and Floquet theory, we theoretically investigate the terahertz photon-assisted transport through a ZnSe/Zn1−xMnxSe heterostructure under an external magnetic field, an electric field, and a spatially homogeneous oscillatory field. The results show that both amplitude and frequency of the oscillatory field can accurately manipulate the magnitude of the spin-dependent transmission probability and the positions of the Fano-type resonance due to photon absorption and emission processes. Transmission resonances can be enhanced to optimal resonances or drastically suppressed for spin-down electrons tunneling through the heterostructure and for spin-up ones tunneling through the same structure, resonances can also be enhanced or suppressed, but the intensity is less than the spin-down ones. Furthermore, it is important to note that transmission suppression can be clearly seen from both the spin-down component and the spin-up component of the current density at low magnetic field; at the larger magnetic field, however, the spin-down component is suppressed, and the spin-up component is enhanced. These interesting properties may provide an alternative method to develop multi-parameter modulation electron-polarized devices.

Effect of spin-orbit coupling on the wave vector and spin dependent transmission probability for the GaN/AlGaN/GaN heterostructure

The effect of the Rashba and Dresselhaus spin–orbit coupling (SOC) on the transmission of electrons through the GaN/AlGaN/GaN heterostructure is studied. It is found that the Dresselhaus SOC causes the evident dependence of the transmission probability on the spin polarization and the in-plane wave vector of electrons, and also induces evident spin splitting of the resonant peaks in the (Ez-k) plane. Because the magnitude of the Rashba SOC is relatively small, its effect on the transmission of electrons is much less. As k increases, the peaks of transmission probability for spin-up electrons (T+) shift to a higher energy region and increase in magnitude, while the peaks of transmission probability for spin-down electrons (T−) shift to a lower energy region and decrease in magnitude. The polarization efficiency (P) is found to peak at the resonant energies and increases with the in-plane wave vector. Moreover, the built-in electric field caused by the spontaneous and piezoelectric polarization can increase the amplitude of P. Results obtained here are helpful for the efficient spin injection into the III-nitride heterostructures by nonmagnetic means from the device point of view.

Effects of interface structure on spin filter tunnel junctions

In this article, the spin filter tunneling in the low biases has been investigated by considering the interfacial barrier roughness. In spite of the approaches that use an artificial periodic interface to find the effects of roughness on transport calculation, our method is based on implementing the height deviation from a mean surface as the roughness characteristics in the transfer matrix method. Moreover the spin-dependent transport properties have been calculated by using the height of a simulated interface in which the rough interface has been grown by ballistic deposition. The results show that the surface roughness suppresses the temperature dependence of spin polarization and reduces the maximum achievable tunneling magnetoresistance and spin-dependent transmission probability.

Enhancement of the spin conductance of a spin filter

For a spin device application of a magnetic superlattice, we investigate the optimal condition to obtain a fully spin-polarized current with sizable ballistic conductance. We consider a quasi-one-dimensional magnetic superlattice, formed by a periodic magnetic field and a split gate technique on a two-dimensional electron gas system. We obtain such a condition by analyzing an energy miniband dispersion and a spin-dependent transmission probability for each channel. The transfer matrix theory and Bloch's theorem are used in the detailed calculation. From the results, we propose an optimized aspect ratio of size parameters of a quasi-one-dimensional magnetic superlattice as a spin filter to generate currents having simultaneous full spin polarization and sizable ballistic conductance.

The study of transport properties in rough ferromagnetic semiconductor rough junctions

The effect of self-affine interface roughness on spin filter tunnelling as a function of interface roughness parameters and the temperature dependent barrier parameters at low biases is studied. The interface roughness is described by the k-correlation model and in terms of self-affine scaling by root-mean-square (rms) deviation Δ, correlation length ξ, and the roughness exponent H(0≤H≤1). In general, the spin-dependent transmission probability is strongly sensitive to roughness and reduced for the case of spin up and down directions in all configurations. Our numerical results show that the roughness affects spin polarization (SP) and tunnelling magnetoresistance (TMR) at different temperatures and decreases the maximum achievable TMR and SP.