Spin Subbands Research Articles

Optical signatures of suppressed carrier localization in encapsulated WSe2 monolayer

Low carrier mobility, closely associated with the formation of localized states, is the major bottleneck of utilizing the unique quantum transport properties in transition metal dichalcogenides (TMDCs). Here, we demonstrate an effective method to quantify the localization energy based on the temperature-dependent spectral variation of photoluminescence (PL) in pristine and hexagonal boron nitride (h-BN) encapsulated monolayer (ML) WSe2. Considering the protecting capability of h-BN against contamination and degradation, while not affecting the electronic structure as an insulating dielectric, the localization energy was comparatively extracted out of PL spectra in pristine and encapsulated ML WSe2. In pristine ML WSe2, two distinctive energy traces were resolved with an energy difference of about 17 meV, which was associated with the localized state revealed below 200 K. Clear evidence for the carrier localization was also evident in the integrated PL intensity trace with temperature as the trace from pristine ML clearly deviates from the dark-exciton-like behavior of ML WSe2, violating the spin selection rule of the lowest exciton state. In clear contrast, the temperature dependency of the h-BN encapsulated ML WSe2 in PL spectra matches well with the typical Varshni formula of free excitonic peaks and the integrated intensity trace of thermally populated spin subbands. Our study suggests that the h-BN encapsulation could suppress the carrier localization channels by avoiding surface oxidation due to air exposure and could provide insights into how one could preserve the excitonic features in TMDC materials and devices.

Itinerant magnetism, electronic properties and half-metallicity of Co2ZrSn and Co2HfSn Heusler alloys

Half-metallic ferromagnetic alloys are attracting considerable interest for their potential applications in spintronic devices. Co-based Heusler alloys are considered to be promising half-metallic compounds as they combine suitable magnetic, electronic and transport properties with compositional versatility and high thermal stability. In this work, Co 2 ZrSn and Co 2 HfSn Heusler alloys were studied by combining experimental and ab-initio investigations in order to accurately estimate their electronic density of states in proximity of the Fermi level and to determine their magnetic and electronic properties. Magnetization measurements, performed between 2 K and 550 K, suggest for both alloys a gradual transition from localized ferromagnetism to weak itinerant ferromagnetism with increasing temperature, well described by a simple mean field model up to the Curie temperature (454 K in Co 2 ZrSn and 432 K in Co 2 HfSn) and beyond. Ab-initio calculations were performed using two exchange-correlational functionals, PBE and PBE optimized for solids (PBEsol), in order to assess the reproducibility of theoretical results. Overall, band structures and density of states (DOS) diagrams indicated, for both compounds, the presence of a half-metallic band gap in the minority spin sub-band. The dependence of magnetic moments and band gap width on cell parameters is discussed in detail. Calculated magnetic moments and cell parameters satisfactorily match the experimental results obtained in this work and previously reported in the literature. • Itinerant weak ferromagnetic behaviour is confirmed for both alloys as they follow the Edwards-Wohlfarth model. • The mean field model indicates that the density of states profiles at the Fermi level are smooth. • Ab-initio calculation performed with PBE and PBEsol functionals show half-metallic behaviour at 0 K. • A crossover from localized magnetism to itinerant magnetism as function of temperature was found for both alloys. • The M/T crossover can be attributed to a change in spin-flip mechanism at low temperature.

Identification of a state of persistent spin helix in a parallel magnetic field, and exploration of its transport properties

The tensors of conductivity and spin susceptibility of a two-dimensional electron gas (2DEG) with equal Rashba and Dresselhaus spin-orbit interaction constants [the persistent spin helix (PSH) state] in a parallel magnetic field are calculated. Applying a parallel magnetic field to the PSH state leads to the appearance of a saddle point in the spectrum and, accordingly, to a Van Hove singularity (VHS), whose amplitude increases indefinitely as the magnetic field strength $(\mathbf{b})$ approaches to some critical value ${b}_{cr}$. The presence of a VHS in the density of states is an important factor determining the conductivity and spin susceptibility tensors. When only the lower spin subband is filled, the off-diagonal elements of the conductivity tensor are nonzero; that is, a Hall voltage arises due to the anisotropy of the Fermi surface and scattering. In the region where two spin subbands are filled, the diagonal and off-diagonal components of the spin susceptibility tensor are equal, and the off-diagonal terms of the conductivity tensor vanish. A sharp decrease in the conductivity and spin susceptibility at the beginning of the filling of the second spin subband, the ratio of the diagonal component of the spin susceptibility to the off-diagonal one, and also the number of critical points in the spectrum make it possible to establish the PSH 2DEG state.

Emission of electromagnetic radiation due to spin-flip transitions in a ferromagnet

We theoretically analyze the probability of electromagnetic wave emission due to electron transitions between spin subbands in a ferromagnet. Different mechanisms of such spin-flip transitions are considered. One mechanism is electron transitions caused by the magnetic field of the wave. Another mechanism is due to the Rashba spin–orbit interaction. While the two mentioned mechanisms contribute in a homogeneously magnetized ferromagnet, there are two other mechanisms that occur for a non-collinearly magnetized medium. The first mechanism is known and is due to the dependence of the exchange interaction constant on the quasimomentum of the conduction electrons. The second one is due to the minimal coupling. It follows from the connection of spin and spatial degrees of freedom in any non-collinearly magnetized medium. We study these mechanisms in a non-collinear ferromagnet with a helicoidal magnetization distribution. Estimations of the probabilities of electron transitions due to different mechanisms are made for realistic parameters, and we compare the mechanisms. We also estimate the radiation power and the threshold current in a simple model in which spin is injected into the ferromagnet by a spin-polarized electric current through a tunnel barrier.

Silicene dynamic optical response in the presence of external electric and exchange fields

We investigate the dynamic optical transition of monolayer silicene in the presence of external electric and exchange fields within the low-energy tight-binding model. Applying external electric and exchange fields breaks the silicene band structure spin and valley degeneracies. Three phases of silicene corresponding to different strengths of perpendicular electric field with respect to the spin–orbit coupling (Δ z < Δso, Δ z = Δso and Δ z > Δso) are considered. We obtain the spin-valley-dependent optical responses to the incoming circularly polarized light using the Kubo formula. We show and discuss how the magnitude and direction of the transverse and longitudinal optical responses of such a system change with the electric and exchange fields. Our calculations suggest that the intraband part of the longitudinal optical response as well as the initial point of the interband part have strong dependencies on the exchange field. Furthermore, we show that one of the spin subbands plays a dominant role in the response to polarized light. Depending on the type of incident light polarization, the dominant subband may change. Our results shed light on the relation between silicene dynamic optical responses and externally applied fields.

Фотогальванический эффект в ферромагнетике со спин-орбитальным взаимодействием

The effect of the appearance of an electric current induced by the electromagnetic radiation at the interface of a ferromagnet and a non-magnetic material is calculated theoretically, taking into account the Rashba spin-orbit coupling. It is shown that the electric dipole transitions between the spin subbands of the conduction electrons of a ferromagnet due to the Rashba interaction lead to a photocurrent. This current has a resonance at a frequency corresponding to the energy of the exchange splitting of spin subbands. The resonance width is determined by the spin-orbit interaction constant. The estimates made show the possibility of experimental observation of this effect in specially prepared multilayer systems.

Photovoltaic effect in a ferromagnet with spin-orbit coupling

The effect of the appearance of an electric current induced by the electromagnetic radiation at the interface of a ferromagnet and a non-magnetic material is calculated theoretically, taking into account the Rashba spin-orbit coupling. It is shown that the electric dipole transitions between the spin subbands of the conduction electrons of a ferromagnet due to the Rashba interaction lead to a photocurrent. This current has a resonance at a frequency corresponding to the energy of the exchange splitting of spin subbands. The resonance width is determined by the spin-orbit interaction constant. The estimates show the possibility of experimental observation of this effect in specially prepared multilayer systems. Keywords: Ferromagnet, exchange coupling, Rashba spin-orbit coupling, photovoltaic effect.

Shear-strain-mediated photoluminescence manipulation in two-dimensional transition metal dichalcogenides

In two-dimensional transition metal dichalcogenides, normal strain can modulate electronic band structures, yet leaving the optical selection rules intact. In contrast, a shear strain can perturb the spin-valley locked band structures and possibly induce mixing of the spin subbands which in turn can transfer oscillator strength between spin-allowed bright and spin-forbidden dark excitons. Here, we report a novel scheme to manipulate photoluminescence (PL) in a monolayer WSe2-MoSe2 lateral heterostructures, controlled by an external bending method in which strong out-of-plane shear strain (OSS) of up to 5.6% accompanies weak in-plane normal strain up to 0.72%. The spectra revealed a striking dependence on the bending direction that is stagnant in the negative (compressive) strain region and then rapidly changes with increasing positive (tensile) strain. The dependency of the PL signal under tensile bending was represented not only by the large energy shift (40 meV) of the lowest excited states of both the WSe2 and MoSe2 monolayers, but also by the tendency to violate the optical selection rules that brightens (darkens) the excitons of the WSe2 (MoSe2) side. The analyses on the observed energy shifts and PL intensity changes confirm the different origins in compressive bending compared with tensile bending. The well-established band-anticrossing is identified to be affecting only the compressive deformation region. The spectral changes in the tensile region, on the other hand, originates mainly from the generation of an off-diagonal perturbation to a spin-specific Hamiltonian induced by OSS. The degree of spin-state mixing, which correlates precisely with the spin-flip coefficient of the theoretical model, is further represented by the OSS matrix elements, the spin splitting energy, and the shear deformation potential.

THz radiation spectra in magnetic Fe/Mo and Fe/Co2FeAl junctions

An investigation was made of THz radiation spectra in the Fe/Mo and Fe/Co2FeAl magnetic junctions arising from the pumping by a high-density current of the upper spin subband at the Fe/Mo interface. The tensor character of the exchange interaction constant at the Fe/Mo interface, associated with the presence of the DMI-like interaction, promotes radiative relaxation.

Specific features of the conductivity and spin susceptibility tensors of a two-dimensional electron gas with Rashba and Dresselhaus spin-orbit interactions

The two-dimensional electron gas with spin-orbit interactions (SOIs) of Rashba and Dresselhaus types is known to form an anisotropic system with a Van Hove singularity in the density of states. Moreover, the ``amplitude'' and the energy position of this singularity depend on the ratio of the constants of Rashba $\ensuremath{\alpha}$ and Dresselhaus $\ensuremath{\beta}$ SOIs, $\ensuremath{\gamma}=\ensuremath{\beta}/\ensuremath{\alpha}$. Here the dependencies of the conductivity and spin susceptibility tensors on the position of the Fermi level are calculated for a wide range of $\ensuremath{\gamma}$. It is shown that if only the lower spin subband is filled the diagonal elements of the conductivity tensor have dips that appear when the Fermi level passes the singularity point both for $\ensuremath{\gamma}&lt;1$ and $\ensuremath{\gamma}&gt;1$. The amplitude of these features and their energy position depend on $\ensuremath{\gamma}$ and, in particular, for the state of persistent spin helix $\ensuremath{\gamma}=1$ they disappear. When only the lower spin subband is filled, the off-diagonal elements of the conductivity tensor are nonzero, that is, there is a Hall voltage due to the anisotropy of the Fermi surface and the scattering. In the region of filling of two spin subbands the ratio of diagonal and nondiagonal components of the spin susceptibility tensor is equal to $\ensuremath{\gamma}$ and in the conductivity tensor, the off-diagonal terms vanish.

Interplay between magnetic moment and magnetocrystalline anisotropy in tetragonally distorted galfenol films

Magnetic properties of Fe100−xGax (x = 15 and 30) epitaxial thin films on GaAs(001) substrates were studied experimentally and theoretically. The samples grown by molecular beam epitaxy under certain conditions adopt body-centered structures with tetragonal distortion along the film normal with a c/a ratio varying from 1.014 to 1.036. The density functional theory (DFT) calculations were performed for different (c/a) ratio values and Ga contents. DFT results showed that Fe atoms placed at the first neighborhood of Ga positions present a much stronger distortion in the local exchange-correlation field than second neighbor Fe atoms. The formation of non-bonding Fe 3d states in the minority spin sub-band and the predominance of hybridization of Ga 4p states with Fe 4p states due to Ga content cause this magnetic behavior. The in-plane magnetic anisotropies are explained in terms of the exchange-correlation field isosurfaces. As a result of that, the global magnetic anisotropy can be described as cubic magnetocrystalline mixed with sixfold anisotropy.

Experimental and theoretical studies on induced ferromagnetism of new (1 \u2212 x)Na0.5Bi0.5TiO3\u2009+\u2009xBaFeO3\u2212\u03b4 solid solution

New solid solution of Na0.5Bi0.5TiO3 with BaFeO3−δ materials were fabricated by sol–gel method. Analysis of X-ray diffraction patterns indicated that BaFeO3−δ materials existed as a well solid solution and resulted in distortion the structure of host Na0.5Bi0.5TiO3 materials. The randomly incorporated Fe and Ba cations in the host Na0.5Bi0.5TiO3 crystal decreased the optical band gap from 3.11 to 2.48 eV, and induced the room-temperature ferromagnetism. Our density-functional theory calculations further suggested that both Ba for Bi/Na-site and Fe dopant, regardless of the substitutional sites, in Na0.5Bi0.5TiO3 lead to the induced magnetism, which is illustrated in terms of the exchange splitting between spin subbands through the crystal field theory and Jahn–Teller distortion effects. Our work proposes a simple method for fabricating lead-free ferroelectric materials with ferromagnetism property for multifunctional applications in smart electronic devices.

THz-излучение в магнитном переходе Fe/Mo

An investigation was made of THz radiation in the Fe/Mo magnetic junction arising from the pumping by a high-density current of the upper spin subband in the induced ferromagnetism layer in Mo. The tensor character of the exchange interaction constant at the Fe / Mo interface, associated with the presence of the Dzyaloshinsky-Moriya interaction, promotes radiative relaxation.

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.

Injection of a Nonequilibrium Spin into a Helicoidal Ferromagnet

The features of spin injection into a helicoidal ferromagnet are investigated. Two methods of spin injection are considered: injection of a spin-polarized current and spin pumping. For the case when the axis of the helicoid is perpendicular to the interface through which the spin injection occurs, the conditions are determined under which electrons fill the specified spin subband. For the case when the axis of the helicoid is parallel to the interface, the occurrence of an effect similar to the topological Hall effect is demonstrated. In the second geometry of spin pumping, we discovered the phenomenon of conversion of the injected spin current into an electric current, flowing along the axis of the helicoid, due to the exchange interaction in a ferromagnet.

Exchange-Induced Generation of Electromagnetic Radiation in a Helical Magnetic Structure

It has been predicted that electromagnetic radiation can be generated owing to the transition of conduction electrons between spin subbands in a noncollinear ferromagnet with a helical magnetic structure. The frequency dependence of the power density of spontaneous radiation has been obtained from the calculation of the probability of such transitions in the electric dipole approximation. For experimentally known parameters of holmium, the threshold current of generation and radiation power have been estimated for a helical structure placed in a resonator. Two cases have been considered. In the first case, the frequency of the resonator corresponds to the resonance frequency of the system (it is determined by the splitting energy of spin subbands in holmium and is approximately 89 THz). In the second case, the frequency of the resonator is strongly shifted from the resonance frequency by about 1 THz. The results indicate that the effect can possibly be detected in experiment.

Anisotropy of spin-transfer torques and Gilbert damping induced by Rashba coupling

Spin-transfer torques (STT), Gilbert damping (GD), and effective spin renormalization (ESR) are investigated microscopically in a 2D Rashba ferromagnet with spin-independent Gaussian white-noise disorder. Rashba spin-orbit coupling induced anisotropy of these phenomena is thoroughly analysed. For the case of two partly filled spin subbands, a remarkable relation between the anisotropic STT, GD, and ESR is established. In the absence of magnetic field and other torques on magnetization, this relation corresponds to a current-induced motion of a magnetic texture with the classical drift velocity of conduction electrons. Finally, we compute spin susceptibility of the system and generalize the notion of spin-polarized current.

Many-body effects due to the electron–electron interaction in silicene under an applied exchange field: The case of valley–spin coupling

We investigate the many-body effects induced by the electron–electron interaction in a valley–spin-polarized silicene under a perpendicularly applied exchange field. We calculate the real and imaginary parts of the self-energy within the leading order dynamical screening approximation where the screened interaction is obtained from the random phase approximation. Our study on the valley- and spin-dependent real and imaginary parts of the self-energy indicates that the different coupled valley–spin subbands may exhibit distinct characteristics. Moreover, we obtain the corresponding spectral functions and find that the plasmaron and quasiparticle peaks have different spectral weights and broadenings in all states. Interestingly, it seems that there are clear dependencies for the position and broadening of the peaks on valley–spin indexes. In addition, we study the effect of the electron–electron interaction on the renormalized velocity in the on-shell approximation and show that the renormalized velocity in gapped states becomes greater, and in gapless states, it becomes smaller as the wave vector grows.

Competition between BCS and FFLO States in Magnetic Superconductors in a Cryptoferromagnetic Phase

The possibility of appearance of inhomogeneous superconducting Fulde—Ferrell—Larkin—Ovchinnikov (FFLO) states in magnetic superconductors in a cryptoferromagnetic phase with helical magnetic ordering has been analyzed. The dependence of the critical temperature on the angle between the wave vectors of the spatial modulation of the FFLO state and helical magnetic structure has been calculated within the proposed model. It has been shown that their mutually perpendicular orientation corresponds to the most energetically favorable state. The numerical calculations have also shown the existence of a tricritical point on a line separating the Bardeen—Cooper—Schrieffer and FFLO phases on the phase diagram of states. Furthermore, FFLO states can appear in a magnetic superconductor even at fairly strong exchange fields because of the difference between the effective masses of conduction electrons in different spin subbands and the anisotropy of the Fermi surface.

Spin Polarization of Nonequilibrium Conduction Electrons in Magnetic Junctions

An original equation for nonequilibrium spin polarization in magnetic junctions based on dynamic equations for magnetic moment was obtained. Spatial nonuniformity of the distribution of carriers is taken into account in the equations, and the magnetic moment is averaged over the ensemble of nonequilibrium spin-injected electrons. The solution to the dynamic equations for the magnetic moment is used to estimate the probability of spin-flip electron transitions upon spin injection, and the solution to the equation for the nonequilibrium spin polarization in magnetic junctions is used to calculate frequencies of photon emission or absorption at energies corresponding to the energy of effective exchange splitting of spin subbands.