Degree Of Spin Polarization Research Articles

Room-temperature electric field control of spin filtering efficiency for enhanced modulation of optical spin polarization in a defect-functional 0D-2D hybrid nanostructure.

In order to accomplish spin-based photoelectric information processing, it is necessary to modulate electron spin polarization in III-V semiconductor quantum dots (QDs) using an electric field. However, there is a principal limitation to the spin polarization degree and its control range, as the electron spin polarization is rapidly lost during injection into the QDs at room temperature (RT). Here, electric field control of optical spin polarization in the range of 15-40% is demonstrated at RT using InAs QDs tunnel-coupled with a defect-functional GaNAs quantum well (QW) spin filter. This compares with an electric field control of 1-4% for InAs QDs tunnel-coupled with an InGaAs QW. Transient polarization in the range of 30-60% is also obtained in the ultrafast time domain of less than 100 ps, the degree of polarization depending on the electric field. The enhanced polarization control is achieved by tuning the amplified spin polarization of electrons tunnel-injected from the GaNAs QW into QDs via the electric-field-dependent spin-filtering efficiency of GaNAs. These findings will provide a new way to extensively modulate the electron spin polarization in opto-semiconductors, by electric-field-induced on/off switching of spin amplification.

Orbital Occupancy and Spin Polarization: From Mechanistic Study to Rational Design of Transition Metal-Based Electrocatalysts toward Energy Applications.

Over the past few decades, development of electrocatalysts for energy applications has extensively transitioned from trial-and-error methodologies to more rational and directed designs at the atomic levels via either nanogeometric optimization or modulating electronic properties of active sites. Regarding the modulation of electronic properties, nonprecious transition metal-based materials have been attracting large interest due to the capability of versatile tuning d-electron configurations expressed through the flexible orbital occupancy and various possible degrees of spin polarization. Herein, recent advances in tailoring electronic properties of the transition-metal atoms for intrinsically enhanced electrocatalytic performances are reviewed. We start with discussions on how orbital occupancy and spin polarization can govern the essential atomic level processes, including the transport of electron charge and spin in bulk, reactive species adsorption on the catalytic surface, and the electron transfer between catalytic centers and adsorbed species as well as reaction mechanisms. Subsequently, different techniques currently adopted in tuning electronic structures are discussed with particular emphasis on theoretical rationale and recent practical achievements. We also highlight the promises of the recently established computational design approaches in developing electrocatalysts for energy applications. Lastly, the discussion is concluded with perspectives on current challenges and future opportunities. We hope this review will present the beauty of the structure-activity relationships in catalysis sciences and contribute to advance the rational development of electrocatalysts for energy conversion applications.

Antiferromagnetic helix as an efficient spin polarizer: Interplay between electric field and higher-order hopping

We report spin filtration operation considering an antiferromagnetic helix system, possessing zero net magnetization. Common wisdom suggests that for such a system, a spin-polarized current is no longer available from a beam of unpolarized electrons. But, once we apply an electric field perpendicular to the helix axis, a large separation between up and down spin energy channels takes place which yields a high degree of spin polarization. Such a prescription has not been reported so far to the best of our concern. Employing a tight-binding framework to illustrate the antiferromagnetic helix, we compute spin filtration efficiency by determining spin selective currents using Landauer-B\"{u}ttiker formalism. Geometrical conformation plays an important role in spin channel separation, and here we critically investigate the effects of short-range and long-range hoppings of electrons in presence of the electric field. We find that the filtration performance gets improved with increasing the range of hopping of electrons. Moreover, the phase of spin polarization can be altered selectively by changing the strength and direction of the electric field, and also by regulating the physical parameters that describe the antiferromagnetic helix. Finally, we explore the specific role of dephasing, to make the system more realistic and to make the present communication a self-contained one. Our analysis may provide a new route of getting conformation-dependent spin polarization possessing longer range hopping of electrons, and can be generalized further to different kinds of other fascinating antiferromagnetic systems.

Efficient Room‐Temperature Voltage Control of Picosecond Optical Spin Orientation Using a III‐V Semiconductor Nanostructure

AbstractManipulation of the optical spin orientation in semiconductors is a key technology for realizing spin‐based photoelectric information processing. Application of magnetic field is a simple method to control the spin polarization degree through Zeeman splitting. However, this effect can only be achieved at cryogenic temperatures and in a strong magnetic field. Here, room‐temperature voltage control of optical polarization in the range of 3–15%, corresponding to a 12–60% change in relative spin polarization is demonstrated. For this, a III‐V semiconductor quantum dot tunnel coupled with a quantum well spin reservoir is used. The spin‐flip scattering rate within quantum dots is electric‐field‐controlled in the time domain of several tens of picoseconds. This is achieved by precise control of the tunnel injection efficiency of electrons and holes via coupled potential modification. The findings will pave the way for the generation of ultrafast spin‐modulated optical signals by the electric field effect.

Anomalous Ferromagnetism of quasiparticle doped holes in cuprate heterostructures revealed using resonant soft X-ray magnetic scattering

We report strong ferromagnetism of quasiparticle doped holes both within the ab-plane and along the c-axis of Cu-O planes in low-dimensional Au/d-La1.8Ba0.2CuO4/LaAlO3(001) heterostructures (d = 4, 8 and 12 unit-cells) using resonant soft X-ray and magnetic scattering together with X-ray magnetic circular dichroism. Interestingly, ferromagnetism is stronger at a hole doped peak and at an upper Hubbard band of O with spin-polarization degree as high as 40%, revealing strong ferromagnetism of Mottness. For in-ab-plane spin-polarizations, the spin of doped holes in O2p–Cu3d–O2p is a triplet state yielding strong ferromagnetism. For out-of-ab-plane spin-polarization, while the spins of doped holes in both O2p–O2p and Cu3d–Cu3d are triplet states, the spin of doped holes in Cu3d–O2p is a singlet state yielding ferrimagnetism. A ferromagnetic-(002) Bragg-peak of the doped holes is observed and enhanced as a function of d revealing strong ferromagnetism coupling between Cu-O layers along the c-axis.

Spin-dependent transport in a driven non-collinear antiferromagnetic fractal network

Non-collinear magnetic texture breaks the spin-sublattice symmetry which gives rise to a spin-splitting effect. Inspired by this, we study the spin-dependent transport properties in a non-collinear antiferromagnetic fractal structure, namely, the Sierpinski Gasket (SPG) triangle. We find that though the spin-up and spin-down currents are different, the degree of spin polarization is too weak. Finally, we come up with a proposal, where the degree of spin polarization can be enhanced significantly in the presence of a time-periodic driving field. Such a prescription of getting spin-filtering effect from an unpolarized source in a fractal network is completely new to the best of our knowledge. Starting from a higher generation of SPG to smaller ones, the precise dependencies of driving field parameters, spin-dependent scattering strength, interface sensitivity on spin polarization are critically investigated. The spatial distribution of spin-resolved bond current density is also explored. Interestingly, our proposed setup exhibits finite spin polarization for different spin-quantization axes. Arbitrarily polarized light is considered and its effect is incorporated through Floquet–Bloch ansatz. All the spin-resolved transport quantities are computed using Green’s function formalism following the Landauer–Büttiker prescription. In light of the experimental feasibility of such fractal structures and manipulation of magnetic textures, the present work brings forth new insights into spintronic properties of non-collinear antiferromagnetic SPG. This should also entice the AFM spintronic community to explore other fractal structures with the possibility of unconventional features.

Electronic band structure and magnetism of CoFeV0.5Mn0.5Si

Half-metallic Heusler alloys have attracted significant attention due to their potential application in spin-transport-based devices. We have synthesized one such alloy, CoFeV0.5Mn0.5Si, using arc melting and high-vacuum annealing at 600 °C for 24 hours. First principles calculation indicates that CoFeV0.5Mn0.5Si shows a nearly half-metallic band structure with a degree of spin polarization of about 93%. In addition, this value can be enhanced by the application of tensile strain. The room temperature x-ray diffraction patterns are indexed with the cubic crystal structure without secondary phases. The annealed sample shows ferromagnetic order with the Curie temperature well above room temperature (Tc = 657 K) and a saturation magnetization of about 92 emu/g. Our results indicate that CoFeV0.5Mn0.5Si has a potential for room temperature spin-transport-based devices.

Optimization of a quantum control sequence for initializing a nitrogen-vacancy spin register

Implementation of many quantum information protocols requires an efficient initialization of the quantum register. In the present paper, we optimize a population trapping protocol for initializing a hybrid spin register associated with a single nitrogen-vacancy (NV) center in diamond. We initialize the quantum register by polarizing the electronic and nuclear spins of the NV with a sequence of microwave, radio-frequency, and optical pulses. We use a rate equation model to explain the distribution of population under the effect of the optical pulses. The model is compared to the experimental data obtained by performing partial quantum state tomography. To further increase the spin polarization, we propose a recursive protocol with optimized optical pulses. We also discuss the role of the relative values of the nuclear- and electronic-spin pumping rate in achieving the maximum degree of spin polarization.

Worm-graphene: A two-dimensional orthorhombic carbon semimetal with massless Dirac fermion

Low energy Dirac material of carbon other than graphene has always enthralled researchers for their unique properties. Based on first-principles calculation we propose a carbon polymorph identified by the predominance of a Dirac cone in the contiguity of the Fermi level in the Two-dimensional domain. Moreover, the stable pristine structure known as Worm-graphene involves six non-equivalent carbon atoms to effectuate an atomic density 0.372 atoms/Å2 which is close to graphene. The Dirac cone exhibits anisotropic Fermi velocity with 6.85×105 m/s being its highest value. Besides, simple tight-binding Hamiltonian can be formulated to express the origin of the Dirac cone. Interestingly, strain resilient semimetallic behavior with robust Dirac cone of the system can be validated within 10% compressive to 10% tensile strain. Further, the material also possesses anisotropic In-plane Young’s modulus unlike graphene. Appreciable degree of spin-polarization due to monovacancy defect can be established from induced magnetic moment. Additionally, nanotubes rolled-up from this sheet can be both semimetallic or semiconducting depending on the rolling direction. Free standing Worm-graphene sheet shows strikingly different optical response compared to graphene. Two collective oscillations of the system within visible range happen to be the distinguishing trait.

Close-to-ideal spin polarization in zinc-doped Fe–Mo double perovskites at the nanoscale

A high degree of spin polarization in half-metallic double perovskites is a prerequisite for several applications in spintronics, which depends crucially on the cationic order of the systems. This paper reports a study on tailoring the structure and morphology of nano-sized Sr2Fe1-xZnxMoO6 (x = 0.05, 0.1, 0.15) materials to improve their spin polarization. The combined analysis of synchrotron X-ray diffraction and magnetization data shows that Zn replaces Fe in the B sites. Although the majority of particles have lateral dimensions in the range 30–60 nm as observed by scanning electron microscope, the samples with x = 0.1 and 0.15 show finite-size effects with superparamagnetism below room temperature and a reduced Curie temperature (from 410 K for x = 0.05–390 K for x = 0.15). The results are due to the formation of networks of insulating Mo–O–Zn–O–Mo linkages and anti-phase boundaries, which divide the particles into smaller domains with a mean diameter of ∼11 nm as determined via a Langevin fit. The almost perfectly ordered structure in the nanodomains is responsible for a high magnetoresistance ratio. A value of -42% at 5 K in 50 kOe is recorded for the sample x = 0.15. Via fitting the magnetoresistance curve using the Inoue-Mekagawa theory, the spin polarization of 99% is determined.

Competing correlated states around the zero-field Wigner crystallization transition of electrons in two dimensions.

The competition between kinetic energy and Coulomb interactions in electronic systems leads to complex many-body ground states with competing orders. Here we present zinc oxide-based two-dimensional electron systems as a high-mobility system to study the low-temperature phases of strongly interacting electrons. An analysis of the electronic transport provides evidence for competing correlated metallic and insulating states with varying degrees of spin polarization. Some features bear quantitative resemblance to quantum Monte Carlo simulation results, including the transition point from the paramagnetic Fermi liquid to Wigner crystal and the absence of a Stoner transition. At very low temperatures, we resolve a non-monotonic spin polarizability of electrons across the phase transition, pointing towards a low spin phase of electrons, and a two-order-of-magnitude positive magnetoresistance that is challenging to understand within traditional metallic transport paradigms. This work establishes zinc oxide as a platform for studying strongly correlated electrons in two dimensions.

Electronic properties of semiconductor quantum wires for shallow symmetric and asymmetric confinements

Quantum wires (QWs) and quantum point contacts (QPCs) have been realized in GaAs/AlGaAs heterostructures in which a two-dimensional electron gas resides at the interface between GaAs and AlGaAs layered semiconductors. The electron transport in these structures has previously been studied experimentally and theoretically, and a 0.7 conductance anomaly has been discovered. The present paper is motivated by experiments with a QW in shallow symmetric and asymmetric confinements that have shown additional conductance anomalies at zero magnetic field. The proposed device consists of a QPC that is formed by split gates and a top gate between two large electron reservoirs. This paper is focussed on the theoretical study of electron transport through a wide top-gated QPC in a low-density regime and is based on density functional theory. The electron–electron interaction and shallow confinement make the splitting of the conduction channel into two channels possible. Each of them becomes spin-polarized at certain split and top gates voltages and may contribute to conductance giving rise to additional conductance anomalies. For symmetrically loaded split gates two conduction channels contribute equally to conductance. For the case of asymmetrically applied voltage between split gates conductance anomalies may occur between values of 0.25(2e 2/h) and 0.7(2e 2/h) depending on the increased asymmetry in split gates voltages. This corresponds to different degrees of spin-polarization in the two conduction channels that contribute differently to conductance. In the case of a strong asymmetry in split gates voltages one channel of conduction is pinched off and just the one remaining channel contributes to conductance. We have found that on the perimeter of the anti-dot there are spin-polarized states. These states may also contribute to conductance if the radius of the anti-dot is small enough and tunneling between these states may occur. The spin-polarized states in the QPC with shallow confinement tuned by electric means may be used for the purposes of quantum technology.

Effect of atomic disorder on electronic, magnetic and electron-transport properties of Ti2MnAl

We report the combined computational and experimental study on the effect of atomic disorder on electronic, magnetic, and electron-transport properties of Ti2MnAl Heusler alloy. Ti2MnAl is predicted to exhibit spin-gapless semiconducting behavior in the inverted cubic crystal structure, while the regular cubic structure is essentially non-spin-polarized and non-magnetic. Here, we analyze three types of atomic disorder, namely, A2, B2, and D03, which are commonly observed in Heusler alloys. Our first-principles calculations indicate that all three types of disorder have a relatively small effect on a non-spin-polarized nature of Ti2MnAl in the regular cubic crystal (prototype Cu2MnAl) structure. At the same time, the inverted cubic phase (prototype Hg2TiCu) retains a significant degree of spin-polarization in A2 and B2 disordered structures. In particular, A2 and B2 types of disorder of the inverted cubic phase result in a spin-polarization of about 50% and 74%, respectively. The D03 disordered inverted phase has a significantly smaller spin-polarization of less than 10%. All considered structures align ferrimagnetically, except the B2-disordered and non-disordered regular cubic phases, which are non-magnetic. Our experimental results are consistent with the predicted properties of Ti2MnAl with partial A2 and B2 disorder types. While atomic disorder in a considered system should be minimized or avoided for practical implementations, its type may play a decisive role for potential applications in spin-based electronics.

Spin selective electron transmission through a layered structure subjected to light irradiation: Efficient engineering

This work deals with engineering spin selective electron transmission through a layered structure where a ferromagnetic (FM) system is clamped between two non-magnetic (NM) regions. The site energies of NM layers are modulated in the well-known Aubry-Andre-Harper (AAH) form, and the FM layer is subjected to a light irradiation. The interplay between the cosine modulation and light irradiation brings several interesting features in spin-dependent transport which we investigate in detail within a tight-binding (TB) prescription following the Green's function formalism. The effect of irradiation is included within a minimal coupling scheme based on the Floquet ansatz. High degree of spin polarization together with its phase reversal can be obtained at multiple energy regions. The proposed prescription can be utilized to design efficient spintronic devices, and can be generalized to other similar kinds of fascinating systems.

Dynamic polarization of electron spins in indirect band gap (In,Al)As/AlAs quantum dots in a weak magnetic field: Experiment and theory

A novel spin orientation mechanism---dynamic electron spin polarization---has been recently suggested in Phys. Rev. Lett. 125, 156801 (2020). It takes place for unpolarized optical excitation in weak magnetic fields of the order of a few millitesla. In this paper we demonstrate experimentally and theoretically that the dynamic electron spin polarization degree changes sign as a function of time, strength of the applied magnetic field, and its direction. The studies are performed on indirect band-gap (In,Al)As/AlAs quantum dots and their results are explained in the framework of a theoretical model developed for our experimental setting.

Anomalous spin resonance around even fillings in the quantum Hall regime

The electron spin resonance (ESR) of two-dimensional electrons with large effective mass was studied experimentally near even filling factors of the integer quantum Hall effect. Surprisingly, the electron spin resonance did not vanish at the exact even fillings even in the case of large electron densities, where the ground state of the system was previously reported to be paramagnetic. Furthermore, the ESR amplitude was comparable between even and odd fillings. Such anomalous behavior suggests a substantial degree of spin polarization of the even fillings and was observed in two different material systems, namely AlAs quantum wells and ZnO/MgZnO heterojunctions. In AlAs quantum wells, spin resonance tended to split into two well-resolved lines near even fillings whereas it remained a single peak near odd ones. The evolution of ESR with varying filling factor around even fillings is studied in detail. The reported nontrivial findings suggest the modification of the ground state around even filling factors with the aid of strong $e\text{\ensuremath{-}}e$ interaction and may be viewed as a precursor for the Stoner instability.

Observation of current-induced spin polarization in the topological insulator Bi2Te3 via circularly polarized photoconductive differential current

Current-induced spin polarization (CISP) in the topological insulator (TI) ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$ has been observed via circularly polarized photoconductive differential current (CPDC). It is found that the CPDC induced by the CISP is proportional to the square of the longitudinal electric field when the electric field is weak. The intensity of the CPDC decreases and finally reverses the sign when the thickness of ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$ films is increased from 3 to 20 QL, owing to the switching of the dominant contribution to the CPDC from the top surface states to the bottom surface states. The spin polarization degree (SPD) of the photoconduction and the critical electric field ${E}_{c}$ at which the SPD becomes saturated are determined, and it is revealed that the SPD decreases while ${E}_{c}$ increases with the increasing of the thickness of ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$ films. In addition, it is demonstrated that the CPDC excited by 1064 nm is more than one order larger than that excited by 1342 nm light, which is due to the larger optical absorption coefficient when excited by 1064 nm light. The temperature dependence of the CPDC and SPD of ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$ films is also investigated, and it is found that the SPD induced by CISP first increases and then decreases with decreasing temperature. This work proposes a method using CPDC to investigate the CISP of TIs.

Bulk spin polarization of magnetite from spin-resolved hard x-ray photoelectron spectroscopy

There is broad consensus that magnetite (${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$) is a promising material for spintronics applications due to its high degree of spin polarization at the Fermi level. However, previous attempts to measure the spin polarization by spin-resolved photoemission spectroscopy have been hampered by the use of low photon energies resulting in high surface sensitivity. The surfaces of magnetite, though, tend to reconstruct due to their polar nature, and thus their magnetic and electronic properties may strongly deviate from each other and from the bulk, dependent on their orientation and specific preparation. In this study, we determine the intrinsic---i.e., bulk---spin polarization of magnetite by spin-resolved photoelectron spectroscopy on (111)-oriented thin films, epitaxially grown on ZnO(0001), with hard x-rays, making it a truly bulk-sensitive probe. This becomes possible by using a novel, specially adapted momentum microscope, featuring time-of-flight energy recording and an imaging spin-filter.

Pauli spin blockade with site-dependent g tensors and spin-polarized leads

Pauli spin blockade (PSB) in double quantum dots (DQDs) has matured into a prime technique for precise measurements of nanoscale system parameters. In this work we demonstrate that systems with site-dependent g-tensors and spin-polarized leads allow for a complete characterization of the g-tensors in the dots by magnetotransport experiments alone. Additionally, we show that special polarization configurations can enhance the often elusive magnetotransport signal, rendering the proposed technique robust against noise in the system, and inducing a giant magnetoresistance effect. Finally, we incorporate the effects of the spin-orbit interaction (SOI) and show that in this case the leakage current contains information about the degree of spin polarization in the leads.

Present and future trends in spin ARPES

Spin-orbit (SO) coupling combined with space inversion symmetry breaking is at the origin of the lifting of the spin degeneracy in the band structure of many materials. The momentum-dependent energy splitting and the degree of spin polarization are indicative of the strength and symmetry of the SO interaction, and enter in the figure of merit of essentially all conceptual designs aiming to control the electron spin degree of freedom in solids. Here we briefly review the role played by spin-and-angle–resolved photoemission (spin ARPES or SARPES) in revealing the spin polarization specifically in the topological states of 3-dimensional (3D) topological insulators (TIs), and we discuss how the progress in the technique can assist the rise of several active fields like spintronics, spin-orbitronics or the generation of spin-polarized beams.