Spin-polarized Electronic States Research Articles

Visualization of spin-polarized electronic states by imaging-type spin-resolved photoemission microscopy

Visualization of spin-polarized electronic states by imaging-type spin-resolved photoemission microscopy

Excellent spin diode and spin-dependent Seebeck effects in hetero-junctions based on ferromagnetic Cr3M4 (M = Se, Te)

Ferromagnetic (FM) half-metals have been regarded as the most promising candidates for spin injection into semiconductors due to their completely spin-polarized electronic states around the Fermi level. To explore the potential application of Cr3M4 (M = Se, Te) in spintronic devices, which are experimentally synthesized FM bulks as well as half-metallic monolayers with high Curie temperature, we propose to design a Bi2Se3/Cr3Se4 van der Waals hetero-junction and a 2D Cr3S4/Cr3Te4 hetero-junction motivated by the recent report on controlling synthesis of Cr x M y based heterostructures. First-principles calculations combined with non-equilibrium Green’s function uncover the perfect spin filtering and spin diode effect in Bi2Se3/Cr3Se4 van der Waals hetero-junction. More interestingly, the Cr3S4/Cr3Te4 hetero-junction shows an excellent spin-dependent Seebeck effect, in which spin-dependent currents with opposite spin orientation can be driven by a temperature gradient to flow in the opposite transport direction. These effects can be understood from the calculated spin-dependent band structures and transmission spectrum. Our results put forward a promising designing and application of spintronic devices based on Cr3M4 (M = Se, Te) FM materials.

Exploring spin-polarization in Bi-based high-Tc cuprates

The role of spin–orbit interaction has been recently reconsidered in high-T_{c} cuprates, stimulated by the recent experimental observations of spin-polarized electronic states. However, due to the complexity of the spin texture reported, the origin of the spin polarization in high-T_{c} cuprates remains unclear. Here, we present the spin- and angle-resolved photoemission spectroscopy (ARPES) data on the facing momentum points that are symmetric with respect to the Gamma point, to ensure the intrinsic spin nature related to the initial state. We consistently found the very weak spin polarization only along the nodal direction, with no indication of spin-splitting of the band. Our findings thus call for a revision of the simple application of the spin–orbit interaction, which has been treated within the standard framework of the Rashba interaction in high-T_{c} cuprates.

Comparative theoretical study of x-ray magnetic circularly polarized emission from ferromagnetic Fe, Co, and Ni

X-ray magnetic circularly polarized emission (XMCPE) is a novel magneto-optical phenomenon in which characteristic x-rays emitted from a magnetized material are circularly polarized. In this letter, we report a comparative theoretical study of XMCPE spectra of the K α emission from ferromagnetic Fe, Co, and Ni. Calculated XMCPE spectra have characteristic tail structures on the low-energy side of the K α emission peaks. Comparison of the spectra shows that Fe, Co, and Ni have broad, intermediate, and narrow tail structures, respectively. Since these tail structures originate from electron excitations in the 3d conduction bands, this difference in tail structure width can be explained by the difference in the spin-polarized 3d electron states among the three metals.

Tunable spin injection and detection across a van der Waals interface.

Van der Waals heterostructures with two-dimensional magnets offer a magnetic junction with an atomically sharp and clean interface. This attribute ensures that the magnetic layers maintain their intrinsic spin-polarized electronic states and spin-flipping scattering processes at a minimum level, a trait that can expand spintronic device functionalities. Here, using a van der Waals assembly of ferromagnetic Fe3GeTe2 with non-magnetic hexagonal boron nitride and WSe2 layers, we demonstrate electrically tunable, highly transparent spin injection and detection across the van der Waals interfaces. By varying an electrical bias, the net spin polarization of the injected carriers can be modulated and reversed in polarity, which leads to sign changes of the tunnelling magnetoresistance. We attribute the spin polarization reversals to sizable contributions from high-energy localized spin states in the metallic ferromagnet, so far inaccessible in conventional magnetic junctions. Such tunability of the spin-valve operations opens a promising route for the electronic control of next-generation low-dimensional spintronic device applications.

Tunable spin polarization and electronic structure of bottom-up synthesized MoSi2N4 materials

Manipulation of spin-polarized electronic states of two-dimensional (2D) materials under ambient conditions is necessary for developing new quantum devices with small physical dimensions. Here, we explore spin-dependent electronic structures of ultra-thin films of recently introduced 2D synthetic materials MSi$_2$Z$_4$ (M = Mo or W and Z = N or As) using first-principles modeling. Stacking of MSi$_2$Z$_4$ monolayers is found to generate dynamically stable bilayer and bulk materials with thickness-dependent properties. When spin-orbit coupling (SOC) is included in the computations, MSi$_2$N$_4$ monolayers display indirect bandgaps and large spin-split states at the $K$ and $K'$ symmetry points at the corners of the Brillouin zone with nearly 100\% spin polarization. The spins are locked in opposite directions along an out-of-the-plane direction at $K$ and $K'$, leading to spin-valley coupling effects. As expected, spin polarization is absent in the pristine bilayers due to the presence of inversion symmetry, but it can be induced via an external out-of-plane electric field much like the case of Mo(W)S$_2$ bilayers. A transition from an indirect to a direct bandgap can be driven by replacing N by As in MSi$_2$(N, As)$_4$ monolayers. Our study indicates that the MSi$_2$Z$_4$ materials can provide a viable alternative to the MoS$_2$ class of 2D materials for valleytronics and optoelectronics applications.

Vacancy-Induced Magnetism in Fluorographene: The Effect of Midgap State.

Based on density functional theory, we have systematically investigated the geometric, magnetic, and electronic properties of fluorographene with three types of vacancy defects. With uneven sublattice, the partial defect structures are significantly spin-polarized and present midgap electronic states. The magnetic moment is mainly contributed by the adjacent C atoms of vacancy defects. Furthermore, the strain dependence of the bandgap is analyzed and shows a linear trend with applied strain. This defect-induced tunable narrow bandgap material has great potential in electronic devices and spintronics applications.

1D topological systems for next-generation electronics

1D topological systems for next-generation electronics

Spin-polarized electronic states and atomic reconstructions at antiperovskite Sr3SnO(001) polar surfaces

We report a first-principles investigation of the atomic and electronic properties at the perfect and defective (001) surfaces of the antiperovskite ${\mathrm{Sr}}_{3}\mathrm{Sn}\mathrm{O}$. We first performed a thermodynamical study of the atomic structure terminations and demonstrated that SrSn-terminated surfaces should be the most stable one, either with a perfect $(1\ifmmode\times\else\texttimes\fi{}1$) structure or with a $(2\ifmmode\times\else\texttimes\fi{}1)$ reconstruction induced by the formation of Sn vacancies. We detailed the surface gap states obtained for these surfaces, which we compare with those of other surface terminations, also having relatively low energies. These gap states, located near the Fermi level, could have a major contribution to the transport properties. Due to the lack of inversion symmetry associated with the surface, we predict that they also experience spin splittings, an important property for spin-orbitronic applications. Finally, we found that ${\mathrm{Sr}}_{2}\mathrm{O}$-terminated surfaces could display a ferromagnetic ordering resulting from the population of $4d$ orbitals of Sr atoms at the surface and that this could lead to the formation of a spin-polarized two-dimensional electron gas.

Measuring spin-polarized electronic states of quantum materials: 2H−NbSe2

Probing the energy and spin electron properties of materials by means of photoemission spectroscopy gives insights into the low-energy phenomena of matter driven by spin orbit coupling or exchange interaction. The information that can be derived from complete photoelectron spectroscopy experiments, beyond $E$(k), is contained in the photoemission transition matrix elements that determine peak intensities. We present here a complete photoemission study of the spin-polarized bands of $2H\text{\ensuremath{-}}{\mathrm{NbSe}}_{2}$, a material that presents a surface spin-texture. Circular dichroism in angular-resolved photoemission spectroscopy (CD-ARPES) data are compared with spin-polarized angular-resolved spectra (SARPES) as measured with linearly polarized radiation in a well-characterized experimental chirality, at selected photon energy values. CD-ARPES is due to a matrix element effect that depends strongly on photon energy and experimental geometry: we show that it cannot be used to infer intrinsic spin properties in $2H\text{\ensuremath{-}}{\mathrm{NbSe}}_{2}$. On the other hand, SARPES data provide reliable direct information on the spin properties of the electron states. The results on $2H\text{\ensuremath{-}}{\mathrm{NbSe}}_{2}$ are discussed, and general methodological conclusions are drawn on the best experimental approach to the determination of the spin texture of quantum materials.

Spin-Polarized Positronium Time-of-Flight Spectroscopy for Probing Spin-Polarized Surface Electronic States.

The energy spectrum of positronium atoms generated at a solid surface reflects the electron density of states (DOS) associated solely with the first surface layer. Using spin-polarized positrons, the spin-dependent surface DOS can be studied. For this purpose, we have developed a spin-polarized positronium time-of-flight spectroscopy apparatus based on a ^{22}Na positron source and an electrostatic beam transportation system, which enables the sampling of topmost surface electrons around the Γ point and near the Fermi level. We applied this technique to nonmagnetic Pt(111) and W(001), ferromagnetic Ni(111), Co(0001) and graphene on them, Co_{2}FeGa_{0.5}Ge_{0.5} (CFGG) and Co_{2}MnSi (CMS). The results showed that the electrons of Ni(111) and Co(0001) surfaces have characteristic negative spin polarizations, while these spin polarizations vanished upon graphene deposition, suggesting that the spin polarizations of graphene on Ni(111) and Co(0001) were mainly induced at the Dirac points that were out of range in the present measurement. The CFGG and CMS surfaces also exhibited only weak spin polarizations suggesting that the half-metallicity expected for these bulk states was not maintained at the surfaces.

Tuning the Spin Density of Cobalt Single-Atom Catalysts for Efficient Oxygen Evolution.

Single-atom catalysts (SACs) with magnetic elements as the active center have been widely exploited for efficient electrochemical conversions. Understanding the catalytic role of spin, and thus modulating the spin density of a single-atom center, is of profound fundamental interest and technological impact. Here, we synthesized ferromagnetic single Co atom catalysts on TaS2 monolayers (Co1/TaS2) as a model system to explore the spin-activity correlation for the oxygen evolution reaction (OER). A single Co atom adsorbed at the hollow site (CoHS) with spin-polarized electronic states serves as the active site for OER, whose spin density can be regulated by its neighboring single Co site via tuning the Co loading. Both experimental and theoretical results reveal the spin density-dependent OER activity that an optimal spin density of CoHS can be achieved with a neighboring hetero-single CoTa site (substitution of Ta by Co) for a superior OER performance, in contrast to a homo-single CoHS site, which creates an excessive spin density over vicinal CoHS. An optimized spin density of CoHS results in an optimal binding energy of oxygen species for the OER. Establishing the spin-activity correlation in SACs may create a descriptor for designing efficient magnetic SACs for renewable energy conversions.

On-Surface Synthesis and Characterization of [7]Triangulene Quantum Ring.

The ability to engineer geometrically well-defined antidots in large triangulene homologues allows for creating an entire family of triangulene quantum rings (TQRs) with tunable high-spin ground state, crucial for next-generation molecular spintronic devices. Herein, we report the synthesis of an open-shell [7]triangulene quantum ring ([7]TQR) molecule on Au(111) through the surface-assisted cyclodehydrogenation of a rationally designed kekulene derivative. Bond-resolved scanning tunneling microscopy (BR-STM) unambiguously imaged the molecular backbone of a single [7]TQR with a triangular zigzag edge topology, which can be viewed as [7]triangulene decorated with a coronene-like antidot in the center. Additionally, dI/dV mapping reveals that both inner and outer zigzag edges contribute to the edge-localized and spin-polarized electronic states of [7]TQR. Both experimental results and spin-polarized density functional theory calculations indicate that [7]TQR retains its open-shell septuple ground state (S = 3) on Au(111). This work demonstrates a new route for the design of high-spin graphene quantum rings for future quantum devices.

Orbital Angular Momentum Induced Spin Polarization of 2D Metallic Bands.

The electrons in 2D systems with broken inversion symmetry are spin-polarized due to spin-orbit coupling and provide perfect targets for observing exotic spin-related fundamental phenomena. We observe a Fermi surface with a novel spin texture in the 2D metallic system formed by indium double layers on Si(111) and find that the primary origin of the spin-polarized electronic states of this system is the orbital angular momentum and not the so-called Rashba effect. The present results deepen the understanding of the physics arising from spin-orbit coupling in atomic-layered materials with consequences for spintronic devices and the physics of the superconducting state.

Application of generalized Bloch method: Strain engineering of low dimensional materials through inhomogeneous strains

Due to their special structural morphologies, low dimensional materials are in nanometer scale in zero, one or two dimensions. This feature offers them superior structural flexibility. This characteristic, together with their excellent electronic property response to mechanical deformation, makes strain engineering an important way to optimize the physical properties of materials. Because in low dimension, structural deformations are usually inhomogeneous, the standard electronic and phonon calculation methods formulated via translation under the periodic boundary conditions encounter fundamental difficulties. A fresh approach in this aspect is called for rapidly. In this paper, the essential idea of the generalized Bloch method is introduced, in the context of a tight-binding treatment of electrons. This special method makes use of the helical symmetry and rotational symmetry to deal with the two fundamental non-uniform structural deformations, i.e., torsion and bending. In this way, the atomistic simulation of the deformed structures can still be carried out through a relatively small repeating cell. Such treatment enables the realistic atomistic quantum mechanical simulations in a viable manner. In practice, this new method is implemented into the so-called density-functional tight-binding (DFTB) calculation, an approach widely used in the material computation today. We note that the utility of DFTB brings us several benefits. First of all, the self-consistent field improves the calculation accuracy to be compatible with first-principles calculations. Next, the capability to deal with spin freedoms of DFTB enables us to study the strain tunable properties of spin-polarized electronic states. We illustrate the application of the generalized Bloch method with several examples that can be categorized into “strain tunable electronic properties”, “spatial manipulation of dopants in semiconductor nanowires”, and “strain engineering of phonon properties”. We also have a vision that the generalized Bloch method shall have important application in more areas. Studies of twist bilayers of two-dimensional materials are one of the focuses of the current condensed matter physics research. However, due to the twist induced lattice mismatch, a twisted bilayer is often of a unit cell containing a huge number of atoms. This makes direct realistic atomistic simulations difficult. We indicate that the calculation can be simplified by using the generalized Bloch method where the helical symmetry and possible rotational symmetry of the twisted bilayer would be used. Exploring giant flexoelectricity is a growing area of materials science. Different from piezoelectricity, flexoelectricity is by definition the linear response of polarization to the strain gradient. Usually, in the actual experimental studies, the strain gradient of materials is generated by applying the bending deformation to the slab. This fact indicates that the generalized Bloch method can be used to simulate such systems. In today’s semiconductor technology, the design of flexible electronic devices is an important development field. The low dimensional materials are natural candidates of flexible systems. Obviously, the generalized Bloch method will also play an important role in this aspect. Our recent work has realized the phonon calculation for low-dimensional structures of helical and rotational symmetries based on the electronic structure calculation with the generalized Bloch method. This functionality will expand the application scope of the generalized Bloch method, making it possible to study the strain modulation of thermal properties in low-dimensional systems with inhomogeneous strain patterns.

Anisotropic current induced in topological surface states due to spin-polarized tunneling from a ferromagnet

In clean tunneling junctions (TJs), the momentum parallel to the barrier is conserved in the tunneling process. We predict that this phenomenon can be observed in a TJ between a ferromagnet (FM) and a topological insulator (TI). The FM possesses spin-polarized electron states, while the TI has spin-momentum locked electron states. The tunneling probability is determined by the spin-matrix element between these electron states and leads to the injection of electrons into the TI with finite momentum parallel to the topological surface state. Due to the spin polarization of the electrons in the FM, the resulting current in the TI is not isotropic. The magnitude of the current is calculated quantitatively in terms of the ferromagnetic polarization and should be experimentally detectable using only an electrical setup.

Giant Rashba system on a semiconductor substrate with tunable Fermi level: Bi/GaSb(110)–(2×1)

We fabricated spin-polarized surface electronic states with tunable Fermi level from semiconductor to low-dimensional metal in the Bi/GaSb(110)-(2$\times$1) surface using angle-resolved photoelectron spectroscopy (ARPES) and spin-resolved ARPES. The spin-polarized surface band of Bi/GaSb(110) exhibits quasi-one-dimensional character with the Rashba parameter $\alpha _{\rm R}$ of 4.1 and 2.6 eV\AA \ at the $\bar{\Gamma}$ and $\bar{\rm Y}$ points of the surface Brillouin zone, respectively. The Fermi level of the surface electronic state is tuned in situ by element-selective Ar-ion sputtering on the GaSb substrate. The giant Rashba-type spin splitting with switchable metallic/semiconducting character on semiconductor substrate makes this system a promising candidate for future researches in low-dimensional spintronic phenomena.

Spin-polarized electronic surface states of Re(0001): An ab-initio investigation

We study the electronic structure of the Re(0001) surface by means of ab-initio techniques based on the Fully Relativistic (FR) Density Functional Theory (DFT) and the Projector Augmented-Wave (PAW) method. We identify the main surface states and resonances and study in detail their energy dispersion along the main symmetry lines of the SBZ. Moreover, we discuss the effect of spin-orbit coupling on the energy splittings and the spin-polarization of the main surface states and resonances. Whenever possible, we compare the results with previously studied heavy metals surfaces. We find empty resonances, located below a gap similar to the L-gap of the (111) fcc surfaces, that have a downward dispersion and cross the Fermi level, similarly to the recently studied Os(0001) surface. Their spin polarization at the Fermi level is similar to that predicted by the Rashba model, but the usual level crossing at Γ¯ is not found with our slab thickness. Moreover, for selected states, we follow the spin polarization along the high symmetry lines, discussing its behavior with respect to k∥, the wave-vector parallel to the surface.

Edge State Engineering of Graphene Nanoribbons.

Zigzag edges of graphene nanoribbons, which are predicted to host spin-polarized electronic states, hold great promise for future spintronic device applications. The ability to precisely engineer the zigzag edge state is of crucial importance for realizing its full potential functionalities in nanotechnology. By combining scanning tunneling microscopy and atomic force microscopy, we demonstrate the zigzag edge states have energy splitting upon fusing manganese the phthalocyanine molecule with the short armchair graphene nanoribbon termini. Moreover, the edge state splitting can be reversibly switched by adsorption and desorption of a hydrogen atom on the magnetic core of manganese phthalocyanine. These observations can be explained by tuning the zigzag edge local doping through the charge transfer process, which provides a new route to functionalize graphene-based molecular devices.

Electronic, Magnetic, and Transport Properties of Polyacrylonitrile-Based Carbon Nanofibers of Various Widths: Density-Functional Theory Calculations

Spintronic devices require channels to efficiently conduct spin current. Carbon nanofibers based on polyacrylonitrile (PAN) are already of keen interest in nanotechnology and energy science; could they be useful in spintronics as well? Modeling PAN nanofibers as N-terminated zigzag graphene nanoribbons, the authors study the electronic structures and quantum transport properties of these quasi-one-dimensional systems as a function of width. They demonstrate that narrow ribbons bear finite magnetic moments, with spin-polarized electronic states exhibiting similar spin configurations on both edges, resulting in spin-dependent transport channels.