Spin-polarized Band Research Articles

Structural, mechanical and dynamical stabilities of K2NaMCl6 (M: Cr, Fe) halide perovskites along with electronic and thermal properties

The properties of K2NaMCl6 (M: Cr, Fe) are simulated using density functional theory. The exchange-correlation potential of halide perovskites is projected by generalized gradient approximation and integration of modified Becke–Johnson potential. The structure's stability is defined by optimizing total energy and using the tolerance factor. In contrast, the enthalpy of formation and elastic constants are used to predict the thermodynamical and mechanical stabilities, while phonon calculations for dynamical stability, respectively. The conventional lattice constant obtained is consistent with those reported for similar halide perovskites. Spin-polarized band structure and energy state distribution plots portray these materials' semiconducting behavior in both spin states. The magnetic moment of 3µB and 5µB is obtained for Cr and Fe-based materials, respectively. The magnetic moment is mainly from transition-metal-based constituents. The thermoelectric figure of merit suggests that these materials could be potential candidates for waste heat recovery.

Electronic, optical and magnetic properties of Gd2FeCrO6 double perovskite using DFT

The double perovskite oxide materials have enormous applications in electronics, solid oxide fuel cells, photochemistry, magnetocaloric devices, optical technologies, etc. In this paper, we calculated spin-polarized electronic band structure, optical properties, magnetic moments and density of states of Gd2FeCrO6 (GFCO) by applying generalized gradient approximation (GGA) and GGA + U theories. The energy bands in spin-up and down channels are reported corresponding to Hubbard parameter, Ueff = 2 and 4 eV applied on Fe and Cr-3d orbitals within GGA + U approach to examine the influence of on-site Coulomb interaction energy on various physical properties of GFCO. In contrast to GGA theory, wherein a small band gap was observed, the GGA + U computations resulted in enhanced band gap values in both channels in accordance with earlier reported literature. The dielectric function, refractive index, absorption coefficient and energy loss functions are calculated. The magnetic moments are also calculated by using GGA and GGA + U theories.

Magnetocrystalline anisotropy energy and Gilbert damping of two-dimensional half-metallic RhX2 (X = I, Br, Cl) ferromagnets: Density functional theory study

This work studies the monolayer rhodium dihalides family, RhX2 (where X = I, Br, Cl), using density functional theory. We first calculate the spin-polarized electronic band structure, revealing a wide intrinsic half-metallic gap (>1.1 eV) in the down spin bands of RhX2 monolayers. We then calculate the magnetocrystalline anisotropy energy (EMCA) and Gilbert damping (α), which originate from the spin–orbit coupling (SOC) phenomenon. We use the force theorem for EMCA calculation that results in substantial in-plane anisotropy in RhI2 (−2.31 meV/unit cell) and RhBr2 (−0.52 meV/unit cell), whereas small perpendicular anisotropy in RhCl2 (0.04 meV/unit cell) monolayers. To calculate α, we employ the Kambersky’s torque–torque correlation model and it comes out relatively low (i.e., 0.0212, 0.0079, and 0.0040 for RhI2, RhBr2, and RhCl2, respectively). The Curie temperature of these crystals is calculated using the Ising model and spin-wave theory. This work highlights the importance of 2D RhX2 half-metallic ferromagnets in the fabrication of future nanoscale spintronic devices.

First principles study on electronic, magnetic and optical properties of Gd2NiMnO6

The double perovskite oxide materials have various applications in light absorber, solid oxide fuel cells, electrode materials for electrochemical energy devices, solid state thermoelectric devices, etc. The calculations of electronic, optical and magnetic properties of Gd2NiMnO6 (GNMO) double perovskite are aimed in the present work. GNMO crystallizes in monoclinic structure with P21/n (14) space group. Implementing first principles density functional theory, we have reported the spin-polarised electronic band structure, density of states (DOS), optical absorption property and magnetic moment of GNMO double perovskite. Furthermore, the effect of on-site d-d Coulomb interaction energy (Ueff) on the electronic and optical properties were investigated by applying a range of Hubbard parameters from 0 to 6 eV on Ni-3d and Mn-3d orbitals within the local spin density approximation (LSDA) and generalised gradient approximation (GGA). Interestingly, on applying Ueff in this range, band gap of GNMO enhanced progressively in both majority and minority spins and also for Ueff = 4 and 6 eV implemented on Ni-3d and Mn-3d orbits, respectively, we observe the band gap value 1.16 and 2.12 eV for the spin-up and spin-down states which is in good agreement with the previously reported experimental band gap value (1.5 eV). In this regard, the electronic structure and light absorption of GNMO have been analysed for Ueff = 4 and 6 eV implemented on Ni-3d and Mn-3d orbits, respectively. The calculations revealed suitable narrow band gap and large visible light absorption coefficient in GNMO, which are extremely desired for the high photovoltaic performance.

Dual Topology of Dirac Electron Transport and Photo-galvanic Effect in Low-dimensional Topological Insulator Superlattices.

Dual topological insulators, simultaneously protected by time-reversal symmetry and crystalline symmetry, open great opportunities to explore different symmetry-protected metallic surface states. However, the conventional dual topological states located on different facets hinders integration into planar opto-electronic/spintronic devices. Here, we construct dual topological superlattices (TSLs) Bi2 Se3 -(Bi2 /Bi2 Se3 )N with limited stacking layer number N. Angle-resolved photoelectron emission spectra of the TSLs identify the coexistence and adjustion of dual topological surface states on Bi2 Se3 facet. The existence and tunability of spin-polarized dual-topological bands with N on Bi2 Se3 facet result in an unconventionally weak antilocalization effect (WAL) with variable WAL coefficient α (maximum close to 3/2) from quantum transport experiments. Most importantly, we identify the spin-polarized surface electrons from dual topological bands exhibit circularly and linearly polarized photo-galvanic effect (CPGE and LPGE). We anticipate that the stacked dual-topology and stacking layer number controlled bands evolution provide a platform for realizing intrinsic CPGE and LPGE. Our results show that the surface electronic structure of the dual TSLs is highly tunable and well-regulated for quantum transport and photoexcitation, which shed light on engineering for opto-electronic/spintronic applications. This article is protected by copyright. All rights reserved.

Electronic properties of two-dimensional kagome lattice based on transition metal phthalocyanine heterojunctions

Transition metal phthalocyanine molecules serve as building blocks for two-dimensional (2D) metal-organic frameworks with potential applications in optics, electronics, and spintronics. Previous theoretical studies predicted that a two-dimensional transition metal phthalocyanine framework with kagome lattice (kag-TMPc) has stable magnetically ordered properties, which are promising for spintronics and optoelectronics. However, there is a lack of studies on their heterojunctions, which can effectively tune the properties through interlayer coupling despite its weak nature. Here we use the density functional theory (DFT) to calculate the electronic properties of eight representative 2D kag-TMPc vertical heterojunctions with two different stackings (AA and AB) and interlayer distances. We find that most of the kag-MnPc-based heterojunctions can maintain the electronic properties of monolayer materials with low bandgap. The kag-MnPc/ZnPc is a ferromagnetic semiconductor with magnetic exchange energy above 40 meV, regardless of stacking sequences; the electronic properties of kag-MnPc/MnPc heterojunctions change from magnetic half-metal to magnetic semiconductor during the transition from AA stacking to AB stacking. Interestingly, the AB stacked kag-CuPc/CoPc heterojunction is a ferromagnetic semiconductor, and the spin-polarized energy band arrangement changes with the layer spacing: when the layer spacing is as long as the equilibrium distance, the spin-up and spin-down energy bands are aligned as type II; when the layer spacing increases by 0.2 Å, the spin-up energy bands are aligned as type-I energy bands, while the spin-down energy bands are aligned as type-II energy bands. This distance-dependent spin properties can realize magnetic optoelectronic “switching” and has potential applications in new magnetic field modulated electromagnetic and optoelectronic devices.

Enhancing thermoelectric performance via relaxed spin polarization upon magnetic impurity doping

Here, a significant enhancement of the Seebeck coefficient is reported in magnetic-impurity-doped higher-manganese silicides, which is attributed to a magnetic-doping-induced relaxation of the spin-polarized band structure.

Investigation of semimetallic properties of GdSb and TbSb compounds: A first-principle and optical study

The electronic and optical properties of binary semimetals GdSb and TbSb in the rocksalt cubic structure have been investigated. The spin-polarized band structure calculations were performed within GGA + U method accounting for strong electron correlations in rare-earth ions. The effect of spin-orbit interaction for the band structure is investigated. The resulting band features near the Fermi energy are found in a good agreement with the previous ARPES experimental data. Our first-principle calculations have shown that isostructural compounds GdSb and TbSb are electron-hole compensated semimetals with qualitatively similar electronic structures. A small pseudogap is observed in their densities of states. The energy dependencies of the real and imaginary parts of the complex dielectric permittivity of both materials are investigated by the spectroscopic ellipsometry in the range 0.078–5.6 eV. The experimental optical conductivity spectra are compared with the calculated ones. There is a reasonable agreement between experiment and theory on a number of spectral features formed by quantum transitions and related to the specifics of the electronic structures. The nature of the occurrence of intense absorption bands in the optical conductivity spectra of both compounds has been identified. The anomalous behavior of optical characteristics in the infrared region is associated with the semi-metallic properties of these materials.

Structural, electronic, magnetic, and optical properties of Fe-doped Na2ZnP2O7 host: ab-initio calculation

Recent experiments on the optical characterization of transition metal ions doped Na2ZnP2O7 host lattice, show promise as luminescent materials. A detailed study using ab-initio DFT-based calculations to understand how the properties of the Na2ZnP2O7 host lattice are affected by the Fe dopants is carried out. The GGA and GGA + U functionals were used to determine the electronic and optical properties of the host lattice and Fe ion doped Na2ZnP2O7.. Full structural geometric optimization was performed for pristine Na2ZnP2O7 host lattice, super-cell and Fe doped super-cell. The computed electronic band structure, density of states, and dielectric functions for the pristine and doped crystal structure reveal changes due to the Fe dopant ion, which induces various states within the band gap of Na2ZnP2O7 lattice. The total magnetic moment of the host lattice is found to be zero, which implies nonmagnetic behavior as shown by the spin-polarized band structure, while the spin-polarized band structure of the Fe-doped Na2ZnP2O7 structure shows a ferromagnetic character. This study provides new clues about the diphosphate compounds with transition metal dopant ions.

Correlation driven near-flat band Stoner excitations in a Kagome magnet

Among condensed matter systems, Mott insulators exhibit diverse properties that emerge from electronic correlations. In itinerant metals, correlations are usually weak, but can also be enhanced via geometrical confinement of electrons, that manifest as ‘flat’ dispersionless electronic bands. In the fast developing field of topological materials, which includes Dirac and Weyl semimetals, flat bands are one of the important components that can result in unusual magnetic and transport behaviour. To date, characterisation of flat bands and their magnetism is scarce, hindering the design of novel materials. Here, we investigate the ferromagnetic Kagomé semimetal Co3Sn2S2 using resonant inelastic X-ray scattering. Remarkably, nearly non-dispersive Stoner spin excitation peaks are observed, sharply contrasting with the featureless Stoner continuum expected in conventional ferromagnetic metals. Our band structure and dynamic spin susceptibility calculations, and thermal evolution of the excitations, confirm the nearly non-dispersive Stoner excitations as unique signatures of correlations and spin-polarized electronic flat bands in Co3Sn2S2. These observations serve as a cornerstone for further exploration of band-induced symmetry-breaking orders in topological materials.

Theoretical Prediction of Structural Stability, Electronic, Elastic, and Magnetic Properties of the New Half Metallic Half-Heusler XSrC (X=Li and Na) Alloys

In this paper, structural, elastic, electronic and magnetic properties of half-Heusler XSrC ([Formula: see text] and Na) compounds have been investigated utilizing full potential linearized augmented plane wave (FP-LAPW) method based on density functional theory (DFT). For exchange-correlation potentials, the generalized gradient approximation (GGA) has been used. The calculated cohesive and formation energy values showed that these compounds can be experimentally synthesized. The elastic properties were analyzed in detail and reveal that LiSrC and NaSrC compounds are mechanically stable. The spin-polarized band structure and density of states illustrate that LisrC and NaSrC alloys have a half metallic character. The total magnetic moment is 1[Formula: see text][Formula: see text] per formula unit that confirms the half metallic behavior and follows the Slater-Pauling rule [Formula: see text]. For half-Heusler XSrC ([Formula: see text] and Na) compounds, there are no available experimental or theoretical studies, our calculations are considered as first predictions.

Transition Metal-Doped Boron Nitride Atomic Sheets with an Engineered Bandgap and Magnetization

Controlled doping of flat atomic sheets of boron nitride (BN) can in principle break charge and spin symmetries. In contrast to graphene that oxidizes at >500 °C, due to its high melting point (ca. 2700 K) and the electrically insulating nature (Eg ∼ 5.5 eV), BN presents strong material candidature for flexible magnetic memory chips that are functional even under fire. Here, we report the transition metal (Fe and Cr) doping of BN by microwave and solvothermal methods. X-ray photoelectron spectroscopy reveals ∼17% doping by microwave (800 W) and <11% doping by the solvothermal method. Cr or Fe doping brings the bandgap down to ∼3.6 or ∼2.0 eV, respectively, for the doped BN samples. Cr doping by microwave results in intrinsic ferromagnetic ordering evident from the high Curie point (∼380 K) in contrast to orbital-mediated magnetic ordering in solvothermal Cr-doped BN systems. Fe doping raises magnetization values in particular. Spin-polarized DFT band structure calculations suggest vivid spin asymmetry caused by doping, supporting our experimental findings on doped BN samples.

DFT analogue of prospecting the spin-polarised properties of layered perovskites Ba2ErNbO6 and Ba2TmNbO6 influenced by electronic structure

Since the unexpected accelerated discovery of half-metallic perovskites is continuously on the rise both from basic sciences and application-oriented sides. Herein, for the first time in this carried research work, we significantly delivered a detailed analysis on one of experimentally synthesized perovskite structure Ba2ErNbO6 and in related to Ba2TmNbO6 within the realm of unified density functional theory. Initially, the structural stability of two molecular perovskite structures were critically established interms of their total ground state and cohesive energies by the expendition of Brich Murnaghan equation of state. Also, the tolerance factor (τ) oversees the cubic structural stability without possessing any geometrical strains. More likely, the density functional perturbation theory (DFPT) has been calibrated to perceive the dynamical context of these layered structures. Also, from the understandings of second order elastic and mechanical parameters adresses their suitable ductile characteristics. The quantum mechanical refinement of their intrinsic electronic structures were systematically tuned by the exploitation of Generalised gradient approximation (GGA), on-site Hubbard scheme (GGA + U) selected to the strongly correlated electrons of particular angular momentum and modified Becke-Johnson (mBJ) potential. Moreover, the two-dimensional representation of asymmetric density of states (DOS) pinned around the Fermi-level (EF) and the interpretation linked to their corresponding spin-polarised band structures signatures the well-known half-metallic nature. Subsequently, the transport properties especially the value of figure of merit (ZT) equals to unity (1) along the selected chemical potential range at different temperatures. The summed-up properties and the overall tendency triggers the possibility of these materials to register their extending applications in spintronics, thermoelectrics, nanoengineering, and radioisotope generator perspectives.

New magnesium based half-metallic ferromagnetic chalcogenide MgFe2Z4 (Z = S, Se) spinels; a promising materials for spintronic applications

This article focuses on the physical properties of MgFe2Z4 (Z = S, Se) spinels investigated by employing density functional theory calculations. To explore the magnetic and electronic properties, WIEN2k code was executed whereas thermoelectric properties were studied using the BoltzTraP. The determined negative formation energies and positive phonon frequencies show that the system under investigation is stable. The lowest possible ground state energies clearly indicate that spinels lie in ferromagnetic state. Studied spinels illustrated half-metallic nature upon employing the calculations of density of states (DOS) and spin-polarized band structures (BS). Ferromagnetic (FM) state was found to be stable ground state. Observation of ferromagnetism in these compounds was ensured by exchange energies, Jahn-Teller energy and hybridization and is attributed to electron spin in place of Fe2+ clustering. Curie temperature and spin polarization of these compounds is also comprehensively investigated in this study. Analysis of the thermoelectric properties showed a good fit between the ratio of electrical (σ/τ) and thermal conductivity (κ e/τ). Thermoelectric efficiency of studied compounds is found to be appropriate as demonstrated by the thermoelectric power factors.

Functionalized boron–nitride nanotubes: First-principles calculations

With the use of density functional theory (DFT) calculations, we study the effect of functionalizing the surface of a boron nitride nanotube (BNNT) with hydroxyl (-OH) and carboxyl (-COOH) organic groups at the N and B sites. The results indicate that the -OH radical remains adsorbed in the B site whereas it is detached from the surface when added to the N site. Contrarily, the -COOH group remains attached for both adsorption sites. Both functionalizations induced a magnetic moment, produced spin-polarized electronic bands, created flat impurity-like bands, decreased the band gaps, and modified the electron density of the nanotubes. All of these changes can be used to control and modify the interaction of BNNTs with other molecules of interest like drugs, heavy metals, and greenhouse gasses, among other substances.

Type-II quantum spin Hall effect in two-dimensional metals

The quantum spin Hall (QSH) effect has been observed in topological insulators and long quantum wells using spin–orbit coupling as the probe, but it has not yet been observed in a metal. An experiment is proposed to measure the different Type-II QSH effect of an electron or hole in a two-dimensional (2D) metal by using the previously unexplored but relativistically gauge-invariant form of the generated 2D QSH Hamiltonian. Instead of using the electric field in the surface of the spin-polarized bands of a topological insulator or across the quantum well width as the probe, ones uses an applied azimuthal vector potential and an applied radial electric field as the tools to generate a spontaneously quantized spin current in an otherwise spin unpolarized 2D metal. A long cylindrical solenoid lies normally through the inner radius of a 2D metallic Corbino disk. The current surrounding the solenoid produces an azimuthal magnetic vector potential but no magnetic field in the disk. In addition, a radial electric field is generated across the disk by imposing either a potential difference or a radial charge current I across its inner and outer radii. Combined changes in and in either or I generate spontaneously quantized azimuthal charge and spin currents. The experiment is designed to measure these quantized azimuthal charge and spin currents in the disk consistently. The quantum Hamiltonians for both experiments are solved exactly. A method to control the Joule heating is presented, which could potentially allow the Type-II QSH measurements to be made at room temperature.

Ab initio prediction of half-metallicity in the NaMnZ2 (Z = S, Se, Te) ternary layered compounds

Motivated by recent theoretical studies predicting half-metallicity in some ternary manganese chalcogenides, first-principles calculations based on spin-polarized density functional theory (DFT) are applied to investigate the structural, elastic, electronic and magnetic properties of the ternary layered compounds NaMnZ2 (Z = S, Se, Te). We assume for all compounds the experimentally determined, in the case of NaMnSe2 and NaMnTe2 compounds, noncentrosymmetric trigonal structure (space group P3m1; no. 156). The analysis of spin-polarized band structures and of the diagrams of density of states reveals the half-metallic character of the considered materials. We find that the strong spin polarization of the d orbitals of the manganese atoms is at the origin of the ferromagnetism of these compounds with a total magnetic moment per formula unit of 4μB. The studied compounds show a Slater-Pauling behavior and the total spin magnetic moment per cell (Mt) evolves with the total number of valence electrons (Zt) according to the lawMt=(Zt−22)μB. The calculated values of the minority-spin energy gap (Eg) and of the half-metallic gap (EHM) for the explored materials show that they are promising compounds for spintronic applications.

Coexisting structural disorder and robust spin-polarization in half-metallic FeMnVAl

Half-metallic ferromagnets (HMF) are on one of the most promising materials in the field of spintronics due to their unique band structure consisting of one spin sub-band having metallic characteristics along with another sub-band with semiconductor-like behavior. In this work, we report the synthesis of a novel quaternary Heusler alloy FeMnVAl and have studied the structural, magnetic, transport, and electronic properties complemented with first-principles calculations. Among different possible structurally ordered arrangements, the optimal structure is identified by theoretical energy minimization. The corresponding spin-polarized band structure calculations indicates the presence of a half-metallic ferromagnetic ground state. A detailed and careful investigation of the x-ray diffraction data, M\"{o}ssbauer and nuclear magnetic resonance spectra suggest the presence of site-disorder between the Fe and Mn atoms in the stable ordered structure of the system. The magnetic susceptibility measurement clearly establishes a ferromagnetic-like transition below $\sim$213 K. The ${^{57}}$Fe M\"{o}ssbauer spectrometry measurements suggest only the Mn-spins could be responsible for the magnetic order, which is consistent with our theoretical calculation. Surprisingly, the density-functional-theory calculations reveal that the spin-polarization value is almost immunized (92.4\% ${\rightarrow}$ 90.4\%) from the Mn-Fe structural disorder, even when nonmagnetic Fe and moment carrying Mn sites are entangled inseparably. Robustness of spin polarization and half metallicity in the studied FeMnVAl compound comprising structural disorder is thus quite interesting and could provide a new direction to investigate and understand the exact role of disorders on spin polarization in these class of materials, over the available knowledge.

Electronic Behavior and Optical Properties of New Ferromagnetic Silver-Based Sulfo-spinel: AgV2S4

This study reports the intriguing properties of a novel ternary silver-based sulfo-spinel vanadium system (AgV2S4) having a face centered cubic structure (FCC). The magnetic nature, electronic behavior and optical properties of this system are revealed. The calculations were performed with spin-effect and by using generalized gradient approximation (GGA) under Density Functional Theory (DFT). After obtaining the optimized Wyckoff positions for the atoms in the crystal structure of this composition, it was decided that this spinel material has ferromagnetic nature in view of the energy-volume curves obtained for three different magnetic phases and of the calculated cohesive energies. Furthermore, the spin-polarized electronic band structure with the orbital projected density of electronic states was calculated within first principles to investigate its electronic behavior and bonding characteristic in detail. The observed small band gap in minority spin channel is Eg = 0.41 eV, so its electronic band structure imply that this system has half-metallic character. Finally, to evaluate some optical features, frequency dependent complex dielectric functions were calculated. Then, some optical properties were investigated by using the real and imaginary parts of the dielectric function.

Proximity-induced diversified magnetic states and electrically controllable spin polarization in bilayer graphene: Towards layered spintronics

Compared to monolayer graphene, electrons in Bernal-stacked bilayer graphene (BLG) have an additional layer degree of freedom, offering a platform for developing layered spintronics with the help of proximity-induced magnetism. Based on an effective phenomenological model, we systematically study the effect of this magnetism on the spin-dependent band structure near the Fermi energy and identify the magnetic phases induced in BLG by proximity with magnets. We show that spin polarization can develop in BLG due to this proximity effect. This spin polarization depends strongly on the layer distribution of magnetism, and can always be controlled by gate voltage which shifts spin-dependent band edges and modifies the total band gap. We further show that the band spin polarization can be modified by the proximity-induced staggered sublattice potential. By taking full advantage of layer-dependent magnetism in BLG, we propose that spintronic devices such as a spin filter, a giant magnetoresistance device, and a spin diode can operate under fully electric control, which is easier than the common magnetic field control.