Spin-up Electrons Research Articles

Photonic spin Hall effect: fundamentals and emergent applications

The photonic spin Hall effect (SHE) refers to the transverse spin separation of photons with opposite spin angular momentum, after the beam passes through an optical interface or inhomogeneous medium, manifested as the spin-dependent splitting. It can be considered as an analogue of the SHE in electronic systems: the light’s right-circularly polarized and left-circularly polarized components play the role of the spin-up and spin-down electrons, and the refractive index gradient replaces the electronic potential gradient. Remarkably, the photonic SHE originates from the spin-orbit interaction of the photons and is mainly attributed to two different geometric phases, i.e., the spin-redirection Rytov-Vlasimirskii-Berry in momentum space and the Pancharatnam-Berry phase in Stokes parameter space. The unique properties of the photonic SHE and its powerful ability to manipulate the photon spin, gradually, make it a useful tool in precision metrology, analog optical computing and quantum imaging, etc. In this review, we provide a brief framework to describe the fundamentals and advances of photonic SHE, and give an overview on the emergent applications of this phenomenon in different scenes.

Interface layer DFT study of first transition series XO oxides (X = Ti, Cr, Mn, Fe, Co, Ni, Cu and Zn)

The whole universe is governed by energy in many forms. Solar cells use light energy and convert it into electrical energy. Nowadays, most of the applications depend on electrical activities. The interface layers are critical to the efficient and high-quality operation of solar cells. Solar cells, also known as photovoltaic devices, are a prospective choice for solar energy generation because of their compelling characteristics such as environmental friendliness, low cost, and lightweight [1]. We have explored new materials to design more efficient organic or inorganic solar cells through enhancement in structural and electronic properties. In this work, a Density Functional Theory (DFT) is implemented in the Spanish Initiative for Electronic Simulations with Thousands of Atoms (SIESTA), which has been performed intending to explore the interface layer of solar cells. A theoretical study is conducted to explore the structural, electronic, magnetic and optical properties of eight different compounds of d-block oxide compounds. The first transition series of oxide compounds in 2-dimensional sheet arrangements, such as TiO, CrO, MnO, FeO, CoO, NiO, CuO, and ZnO, are explored in the fabrication of solar cells. In the magnetic properties, ZnO has 0 magnetic moments or spin polarization means there is equal number of spin-up and spin-down electrons are paired in each orbitals. However, the bandgap fluctuates from the properties of semiconductors to metals. Because the first transition series oxides exhibit essentially magnetic behaviour, they have two separate band gaps for spin-up and spin-down flucation, ranging from 0 to 1 eV and 0 to 4 eV, respectively. Among all the d-block transition series oxides, CoO has the highest optical qualities, with 4.7e+05 cm−1 absorption coefficient (α), 1.2e+05 S cm-1 conductivity (σ), and 0.125 reflectivity (ρ).

Strain-tunable magnetic and electronic properties of a CuCl3 monolayer.

Recently, theoretical search has found that a two-dimensional CuCl3 monolayer is a ferromagnetic semiconductor. Here, we apply density functional theory to study its geometrical structure, magnetic and electronic properties under the influence of a biaxial strain ε. It is found that the CuCl3 monolayer exhibits ferromagnetic ordering at the ground state with ε = 0 and its Curie temperature increases monotonously with respect to the biaxial strain, which can be increased to about 100 K at 10% tensile strain. When a compressive strain of about 6.8% is applied, a transition from the ferromagnetic to the antiferromagnetic state occurs. In addition to the transition of the magnetic ground state, the electronic band gaps of spin-up and spin-down electrons undergo direct-indirect and indirect-direct-indirect transitions at the tensile strains, respectively. The tunable magnetic and electronic properties investigated in this work are helpful in understanding the magnetism in the CuCl3 monolayer, which is useful for the design of spintronic devices based on ferromagnetic semiconductors.

Electrically controllable spin filtering in antiferromagnetic topological insulator tunneling junctions

We study the surface-state band structure and transport property for an antiferromagnetic MnBi2Te4 film with an external electric field vertical to its top surface. The electric field can reduce the gap of surface states induced by a finite-size effect, and restore the chirality of surface states, which may be beneficial to the realization of an axion state. We also demonstrate that an adjustable electric field can not only switch between the spin-up and spin-down electron channels to realize spin filtering, but also close all the electronic channels to realize on-off current in an antiferromagnetic MnBi2Te4 thin film with two junctions.

The electronic, half-metallic, elastic, and magnetic properties of new PtWZ (Z = In, Tl, Sn, and Pb) half-Heusler alloys via GGA and GGA+mBJ methods

The first-principle calculations of PtWZ (Z = In, Tl, Sn, and Pb) half-Heusler alloys were calculated by WIEN2k for GGA and GGA+mBJ methods. First, the ferromagnetic (FM) phases were obtained more energetically stable than non-magnetic (NM) and antiferromagnetic (AFM) phases in each alloy. The Curie temperatures of PtWIn, PtWTl, PtWSn, and PtWPb alloys were obtained as 286.98 K, 467.14 K, 721.98 K, and 1114.31 K, respectively, by utilizing the energy differences of the AFM and FM phases. In each method and alloy used, spin-up electrons showed metallic character. In the GGA method, PtW(In, Tl) alloys have direct band gaps of 0.72044 eV and 0.91488 eV in spin-down electrons, while PtW(Sn, Pb) alloys have indirect band gaps of 1.2558 eV and 1.11892 eV, respectively. In the GGA+mBJ method, the bandgap directions in all compounds remained the same. Here, band gaps in PtW(In, Tl, Sn, and Pb) alloys were obtained as 0.99918 eV, 1.15385 eV, 1.42676 eV, and 1.17497 eV, respectively. While the total magnetic moment values of PtW(In, Tl) half-Heusler alloys were obtained as 1.00 μ B/f.u., the total magnetic moments of PtW(Sn, Pb) alloys were obtained as 2.00 μ B/f.u. These results are in full agreement with the Slater-Pauling rule. According to elastic calculations, PtWIn, PtWTl, PtWSn, and PtWPb half-Heusler alloys are elastically stable and ductile.

Relativistic Landau quantization in non-uniform magnetic field and its applications to white dwarfs and quantum information

We investigate the two-dimensional motion of relativistic cold electrons in the presence of `strictly’ spatially varying magnetic fields satisfying, however, no magnetic monopole condition. We find that the degeneracy of Landau levels, which arises in the case of the constant magnetic field, lifts out when the field is variable and the energy levels of spin-up and spin-down electrons align in an interesting way depending on the nature of change of field. Also, the varying magnetic field splits Landau levels of electrons with zero angular momentum from positive angular momentum, unlike the constant field which only can split the levels between positive and negative angular momenta. Exploring Landau quantization in non-uniform magnetic fields is a unique venture on its own and has interdisciplinary implications in the fields ranging from condensed matter to astrophysics to quantum information. As examples, we show magnetized white dwarfs, with varying magnetic fields, involved simultaneously with Lorentz force and Landau quantization affecting the underlying degenerate electron gas, exhibiting a significant violation of the Chandrasekhar mass-limit; and an increase in quantum speed of electrons in the presence of a spatially growing magnetic field.

Electronic, magnetic and elastic calculations on half-metallic Heusler Ti2RuTl compound

ABSTRACT Electronic, magnetic and elastic calculations of a new Ti2RuTl full Heusler compound have been made using the Vienna Ab-initio Simulation Package (VASP). The computations were carried out for the ferromagnetic (FM) state in the F-43 m (No.216) space group. Ground-state values of the lattice parameter, bulk modulus, volume and energy were obtained. The lattice parameter of the cubic structure was 6.478 Å. According to the electronic calculations, the spin-up electrons were found to have metallic properties while the spin-down electrons were found to have a semiconductor nature. Therefore, these metallic and semiconductor spin states showed that the Ti2RuTl full Heusler compound is a half-metallic (HM) ferromagnet with 1.00 µB/f.u. magnetic moment. Finally, the elastic calculations showed that the compound is an elastically stable, ductile material and could be a new candidate material for using in spintronics.

First-principles study of two-dimensional electron gas on a layered Gd2C electride surface

Electrides are ionic compounds in which electrons behave as anions in the interior of a positively charged framework. As a layered electride, ${\mathrm{Gd}}_{2}\mathrm{C}$ receives attention because of its ferromagnetism. Although previous research has focused on the bulk properties of ${\mathrm{Gd}}_{2}\mathrm{C}$, few studies have focused on ultrathin layers or surfaces for two-dimensional (2D) characteristics. Here, we report a first-principles study of the electronic properties of few-layer ${\mathrm{Gd}}_{2}\mathrm{C}$ structures. ${\mathrm{Gd}}_{2}\mathrm{C}$ has a work function of 3.35 eV. When a layered electride is exfoliated, the interstitial layer becomes a surface and may be exposed to the outside. Because the interlayer region has changed to the surface, the properties of the electron gases once located in the interlayer in the past will also change. We found that the surface anionic electrons accounted for about 25% of the number of electrons in the interlayer region in the absence of an external electric field. When we applied an external electric field, the number of surface electrons increased, and the increase was proportional to the square of the field intensity. Since the electronic properties of 2D materials can be understood through scanning tunneling spectroscopy (STS), we also performed the STS simulations. At $\ensuremath{-}0.9\phantom{\rule{0.28em}{0ex}}\mathrm{eV}$, the STS image was blurred because of surface anionic electrons. In contrast to the spin-up electron, an interlayer band of the spin-down electron crossed the Fermi level in the ultrathin ${\mathrm{Gd}}_{2}\mathrm{C}$ layers. Our findings open a possibility that the spin-polarized electronic gas in the few-layer electride could be used for spintronics.

First-principles magnetic treatment of the uranium nitride (100) surface and effect on corrosion initiation

The magnetic properties of uranium nitride (UN) surfaces are not well understood experimentally or computationally but they have a significant effect on UN performance as a nuclear fuel. We investigated ferromagnetic (FM), antiferromagnetic (AFM), nonmagnetic (NM), and three hybrid magnetic structures of the most stable UN surface (100). To account for electron correlation and metastability, a U-ramp was performed to an effective Hubbard U-term of 2.0 eV. FM was found to be the most energetically favorable magnetic structure. Type 1 AFM slab was optimized to a new magnetic structure consisting of (100) planes with either all spin-up electrons, all spin-down electrons, or half spin-up and half spin-down electrons on uranium atoms. After OH adsorption to simulate corrosion initiation, the AFM, FM, and NM structures yield relatively similar bond lengths but varying bond angles, adsorption energies, and electronic profiles. Partial charge density maps show varying degradation mechanisms across magnetic structures. Electron localization function reveals more charge localized to AFM uranium atoms with spin-down electrons than uranium atoms with spin-up electrons. This leads to different properties depending on if an adsorbate interacts with a spin-up or spin-down terminated AFM surface. This work supports the physical accuracy of future computational studies toward corroborating with experiments and addressing UN fuel corrosion.

Magnetic properties of Mn-doped monolayer MoS2

We use first-principles method to investigate the crystal structure, electronic structure and magnetic properties of Mn-doped MoS2 monolayer (Mo0.75Mn0.25S2). The spin-up electron states exhibit metallic properties, while the spin-down electron states exhibit insulating properties, which indicates the half-metallic properties of Mo0.75Mn0.25S2 monolayer. The doping of Mn atoms leads to the strong ferromagnetism of the system with the coupling between dopant Mn 3d orbitals via Mo 4d and S 3p states. Under the generalized Bloch condition, we explore the dispersion relation of Mo0.75Mn0.25S2 between the spin-spiral energy E(q⇀) and the wave vector q⇀. The exchange coupling coefficient Ji (i=1,2,3,4) is obtained through the Heisenberg interaction (HBI) model. It is found that although the first nearest-neighbor J1 plays an important role, the second-nearest-neighbor interaction exchange coupling coefficient J2 is non-negligible.

Double refraction of electron spin across a metal-semiconductor junction with Rashba and Dresselhaus spin-orbit interactions: A stronger spin-filtering effect

The reflection and refraction of electrons across a two-dimensional metal-semiconductor tunnel junction are studied incorporating both Rashba and Dresselhaus spin-orbit interactions for the semiconductor region. The effects of incident electron energy , angle of incidence and spin-orbit interactions on the angles of refraction of the spin-up and spin-down electrons are calculated exactly. It is shown that the Dresselhaus interaction decreases the angle of refraction for the spin-down electrons more than the spin-up electrons causing a larger splitting between the spin-up and spin-down electrons. Also an increase in the incident energy is shown to increase the spin-filtering effect. The refraction coefficients, spin-up and spin-down current densities and tunnelling conductance are also studied with respect to spin-orbit interactions. Finally, the spin polarization current is determined in the semiconductor region in the presence of Rashba interaction alone and in the presence of both Rashba and Dresselhaus interactions. • Tunneling of electrons across a 2D metal-semiconductor junction is considered. • Rashba and Dresselhaus interactions are incorporated only for semiconductor region. • Reflection and refraction of spin-up and spin-down electrons are calculated exactly. • Dresselhaus interaction enhances the splitting between up- and down-spin electrons. • Spin polarization current is also determined in semiconductor region.

Shot noise of spin-polarized electrons in a single-channel magnetic tunnel junctions

Based on the free electron approximation and Egues’ shot noise theory, the shot noise of spin-polarized electrons tunneling in ferromagnetic/semiconductor/ferromagnetic tunnel junctions is studied. Considering the matching of conduction band between ferromagnetic and semiconductor layers, our results show that the Fano factors of spin-polarized electrons have resonant tunneling characteristics when the semiconductor thickness and Rashba spin–orbit coupling strength are increased. When the magnetic moments in two ferromagnetic layers are parallel, with the increase of the molecular field in the ferromagnets, the Fano factor for spin-up electron decreases to zero and then increases exponentially and the Fano factor for spin-down electron is always linear. But when the magnetic moments are antiparallel, the Fano factors for different spin directions tend to be the same. In addition, the Fano factors for different spin directions are almost zero when the incident electron energy is located in the low energy region, but exhibit irregular oscillation when the incident electron energy is located in the high energy region. At the same time, with the variations of the angle of the magnetic moments in two ferromagnetic layers, the electrons Fano factors for different spin orientations show obvious separation characteristics. On the other hand, the conduction band mismatch between ferromagnetic and semiconductor layers is considered, the Fano factors of electrons with different spin directions show obvious difference compared with the results of conduction band matching.

Unconventional domain wall magnetoresistance of patterned Ni/Nb bilayer structures below superconducting transition temperature of Nb

Scattering of spin-up and spin-down electrons while passing through a ferromagnetic domain wall (DW) leads to an additional resistance for transport current, usually observed prominently in constricted magnetic structures. In this report we use the resistance of the DW as a probe to find an indirect signatures of the theoretically predicted spin-singlet supercurrent to spin-triplet supercurrent conversion effect of ferromagnetic DWs. Here we examine the DW induced resistance in Ni stripe in a bilayer Ni/Nb geometry in the normal state and in the superconducting state of Nb. By making a 3 μm wide gap in the top Nb layer we routed the transport current through the Ni layer in the normal state and in the superconducting state of Nb. In the normal state of Nb, in-field transport measurements showed a clear domain wall magneto-resistance (DWMR) peak of amplitude ∼5.9 mΩ near the coercive field, where the DW density is expected to be maximum. Interestingly, however, below the superconducting transition temperature of Nb, the DWMR peak of the Ni layer showed a sharp drop in the field range where the number of DWs become maximum. This observation may be a possible signature of magnetic DW induced spin-triplet correlations in the Ni layer due to the direct injection of spin-singlet Cooper pairs from Nb into the magnetic DWs.

Stress-Driven Evolution on Mismatched Ca2Co2O5 Oxide Material: From Geometry to the Electronic States

The geometrical structures, phase stabilities, electron energy band structures, electron density of states, and atom recombination together with the electron conduction behaviors of the sandwiched Ca2Co2O5 with external stress of 1 GPa are intensively studied by the density functional theory method. The studying results show that the symmetry remains undisturbed; the strain to the stress response is anisotropic. The strain of microarchitecture induced by external stress is also anisotropic. There is stronger covalent binding between Co and O. The binding between Co and O within CdI2 like CoO2 is very much even covalent, and it is weakened under external stress. But the covalent Co-O binding within the rock salt like CaCoO layer is enhanced. The Ca-O binding strength is insensitive to external stress. An energy gap of 0.1 eV below Fermi level for the spin-up electron band disappears, and the two energy gaps are narrowed for the spin-down electron bands. The p orbital electrons form primarily the bands below Fermi level and the d orbital electrons form primarily the bands above Fermi level. The transitions from p orbital electrons to d orbital electrons produce the conduction. The CdI2 like CoO2 layer has been enhanced in terms of participating in the conduction properties with external stress of 1 GPa, and the capability of Co is enhanced while the capability of O is decreased.

Entangled electron–photon pair production by channel-exchange in high-energy Compton scattering

The present work theoretically investigates the probability of generation of entangled electron-photon pair in high-energy Compton scattering of unpolarized electrons and photons due to scattering-channel-exchange mechanism. The study suggests that the scattering of unpolarized electrons and photons with nearly equal energy of the order of MeV at cross-channel can create entangled pair of up-spin electron and RCP photon or down-spin electron and LCP photon. The entanglement is quite strong exhibiting jump-concurrence for particular directions of scattering however not limited by the Kapitza-Dirac restriction. Such strong electron-photon entanglement is observed to be highly selective along the directions of scattering on the surface of two opposite cones at 45 degree around the electron-detector axis and on the plane perpendicular to it, the latter one exhibiting maximal entanglement. The work predicts that spin-polarization entangled electron-photon pairs are highly possible to be found in practical experiments of high-energy Compton scattering with specific directional selectivity.

Spin-polarized plasmon in ferromagnetic metals

We exploit the spinor Boltzmann equation associated with Poisson equation to study the excitation of plasmons induced by electron–electron interactions in a ferromagnetic metal. Besides the collective excitation of spin-polarized charge density, we demonstrate the decoherence of such spin-polarized plasmons. By applying a rotating resonant magnetic field to the system, we obtain the long-lived coherent superposition state of spin-up and spin-down electrons, which can be used to construct qubits. We also investigate the interference of several spin-polarized plasmons controlled by their corresponding rotating magnetic fields, and find the entanglement between qubits. Our results show that a new quantum information processing route based on spin-polarized plasmons may be feasible.

Modulation instability of ion-acoustic waves and exchange interaction in magnetized quantum plasma with relative density effects of spin-up and spin-down electrons

The separate-spin evolution quantum hydrodynamic (SSE-QHD) model is used to study Ion-Acoustic Waves (IAWs) in a magnetized quantum plasma having electrons with different spin populations in the presence of the spin particle exchange interactions (Coulomb type) in spin-down electrons only. The plasma consists of inertia-less separated spin-up and spin-down electrons states, while ions are inertial and classical. Besides, a uniform magnetic field is assumed to be directed in the Z-direction i.e. . An interesting interplay between exchange interaction and density polarization ratio κ has been observed on dispersive properties, effective Debye screening length and effective ion-acoustic speed. The non-linear Schrödinger equation is derived through a reductive perturbation approach. Moreover, the modulation instability condition and resulting envelope waves are analyzed. The parametric effects of density polarization ratio κ and Coulomb exchange interaction on dispersion, effective screening length, effective ion-acoustic speed, instability growth rate, and on the solitons profile have been discussed in detail. This model can be helpful to study properties of the dilute magnetic semiconductors and degenerate 2D & 3D Fermi electron gas, where exchange interactions are significant.

Estimation of sp–d exchange constants revisited

We present a new computational method for estimating the sp–d exchange constant, , applicable to transition metal doped diluted magnetic semiconductors, transition metal oxides, and 2D- and 3D- dichalcogenides. The proposed method is based on results describing the variation of the magnetic features of a doped system with the variation of its magnetization density (M). The results for obtained with the proposed method are compared with the corresponding results, , obtained from estimations of the spin electron orbital splitting, ΔE VBM, at the valence band maximum (VBM). The latter is estimated in two ways; either directly from plots of the band structure calculations or by calculating the energy difference between the band-centers of the spin-up and spin-down electron density of states of the doped systems. Despite the inherent drawbacks in these two estimation methods for ΔE VBM, they lead to equivalent results and the corresponding are in good agreement with the ones. Ab initio results obtained for the 2D-MoS2 doped with 3d-series transition metals are presented to demonstrate the validity and applicability of the proposed computational schemes for obtaining . The proposed methods can be utilized as useful tools in the search of new materials for spintronics and valleytronics applications.

Nonlinear excitations of magnetosonic solitary waves and their chaotic behavior in spin-polarized degenerate quantum magnetoplasma.

The quantum hydrodynamic model is used to study the nonlinear propagation of small amplitude magnetosonic solitons and their chaotic motions in quantum plasma with degenerate inertialess spin-up electrons, spin-down electrons, and classical inertial ions. Spin effects are considered via spin pressure and macroscopic spin magnetization current, whereas the exchange effects are considered via adiabatic local density approximation. By applying the reductive perturbation method, the Korteweg-de Vries type equation is derived for small amplitude magnetosonic solitary waves. We present the numerical predictions about the conservative system's total energy in spin-polarized and usual electron-ion plasma and observed low energy in spin-polarized plasma. We also observe numerically that the soliton characteristics are significantly affected by different plasma parameters such as soliton phase velocity increases by increasing quantum statistics, magnetization energy, exchange effects, and spin polarization density ratio. Moreover, it is independent of the quantum diffraction effects. We have analyzed the dynamic system numerically and found that the magnetosonic solitary wave amplitude and width are getting larger as the quantum statistics and spin magnetization energy increase, whereas their amplitude and width decrease with increasing spin concentration. The wave width increases for high values of quantum statistic and exchange effects, while their amplitude remains constant. Most importantly, in the presence of external periodic perturbations, the periodic solitonic behavior is transformed to quasiperiodic and chaotic oscillations. It is found that a weakly chaotic system is transformed to heavy chaos by a small variation in plasma parameters of the perturbed spin magnetosonic solitary waves. The work presented is related to studying collective phenomena related to magnetosonic solitary waves, vital in dense astrophysical environments such as pulsar magnetosphere and neutron stars.

Magnetic Field and Hydrostatic Pressure Effects on Electron Transport in Heterostructure Based on InAs/GaAs Triple Barriers with Dresselhaus Interaction

This paper theoretically examines the spin-dependent transport of electrons through InAs/GaAs symmetric triple barriers by solving the spin-dependent Schrodinger equation. The Transfer Matrix Method (TMM) was used to investigate the impact of the hydrostatic pressure, temperature, and magnetic field on the transmission coefficient and spin polarization of electrons while taking into account the Rashba and Dresselhaus spin orbit interactions. The results indicate that when both the hydrostatic pressure and temperature increase, the two resonant states of spin-up and down electrons move toward lower energies (blue shift). In addition, when the magnitude of the applied magnetic field increases, the resonant states of spin-up electrons shift toward lower energies, whereas the spin-down ones shift toward higher energies, which leads to perfect spin separation. This important spin-separation behavior could be very useful in the design of spin filters, which are strongly required in spintronics applications.