Spin-polarized Energy Bands Research Articles

Optical properties of twisted bilayer graphene with magnetic defects

Even when fabricated under ideal conditions twisted bilayer graphene (TBG) inevitably contains various defects which may significantly affect its physical properties. Here we comprehensively analyze the impact of typical point defects, represented by adsorbed hydrogen atoms, on the electronic and optical properties of TBG. It is shown using self-consistent tight-binding Hamiltonians that such point defects make TBG ferromagnetic, and that its ground state comprises a pair of nearly dispersionless spin-polarized energy bands around the Fermi level. Transitions to and from these bands strongly modify the infrared absorption of TBG and result in a sharp low-energy peak in its spectrum. It is also revealed that the adsorption of hydrogen atoms suppresses the circular dichroism of TBG due to the weakening of the electronic coupling between the graphene layers. Our findings will guide future experimental studies on the optical properties of TBG in realistic, impurity-rich environments.

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.

CO2 gas sensor based on Pt-, Ag-, Au- and Pd-doped InSe monolayer: a first-principles study

Through a first-principles study based on density functional theory, a physical model of CO2 gas molecule adsorption on an InSe monolayer was designed and built. The geometric structures of the InSe monolayer doped with different transition metal elements were optimized, and the spin-polarized energy band structure and magnetoelectric properties, such as the density of states, semimetallicity and magnetic moment of nanosheets with stable adsorbed phases, were calculated. The microscopic mechanism of these properties was analyzed by crystal field theory, and it was found that InSe monolayers are typical semiconductors, but transition metal–InSe monolayers can conduct electricity and are typical semi-metallic nanosheets. Second, the adsorption mechanism of CO2 gas molecule adsorption on InSe monolayers is studied. The calculation results show that when the O atom of the CO2 gas molecule is adsorbed on the surface of an InSe monolayer, the adsorption structure is relatively stable. The surface adsorption is mainly due to the transition of electrons on the InSe monolayer surface to the CO2 gas molecule. The doping of metal atoms gives CO2 gas molecules strong adsorption energy, promotes the magnetic properties of the adsorption system, and completes the transition from semiconductor to metal. The change of the work function of the adsorption system before and after doping indicates that this doping method can also enhance the sensitivity of the InSe monolayer to CO2 gas molecules.

Effect of Strong Correlations on the Spin-polarized Electronic Energy Bands of the CdMnTe Solid Solution

Effect of Strong Correlations on the Spin-polarized Electronic Energy Bands of the CdMnTe Solid Solution

First-principles calculations of electronic structures and ferromagnetism of Fe3Si(001)//MgO(001) films

The first-principles calculations based on density functional theory (DFT) were carried out in investigating electronic structures and ferromagnetism of Fe3Si films epitaxial on MgO(001). Firstly, the various geometric structures of Fe3Si(001)//MgO(001) constructed near lattice constant c = 3.995 Å were optimized to gain the most steady equilibrium state at c = 3.980 Å. Then, the calculated cohesive energy and negative heat of formation indicate that Fe3Si(001)//MgO(001) formed in this manner obtain high structural stability. The calculated results of spin-polarized energy band structures and density of states show that Fe3Si(001)//MgO(001) exhibit the metallic feature whose bonding orbitals are constituted by covalent bond and metallic bond. Two peaks located in both the sides of the Fermi level and the total density of states (TDOS) in this energy range are all due to the Fe 3d states, which implies that the pseudo energy gap exists in the Fermi level and covalent electron orbit hybridization takes place. Ferromagnetism of Fe3Si(001)//MgO(001) are determined by the 3d states of Fe atoms. There are two occupied sites for Fe atoms with different local magnetic moments, which is 1.34 [Formula: see text]/atom for Fe[A, C] atoms and a value of 2.68 [Formula: see text]/atom for Fe[B] atoms, likewise indicating Fe3Si films epitaxial on MgO(001) are ferromagnetic.

Electronic structures and magnetic properties of Fe3Si films epitaxial on Si(001)

The electronic structures and magnetic properties of Fe3Si films epitaxial on Si(001) were systematically investigated by using the first-principle calculations on plane-wave pseudo-potential theory. The calculated results show that Fe3Si films epitaxial on Si(001) have the most stable equilibrium state at the lattice constant c = 5.63 Å. The negative heat of formation and cohesive energy of Fe3Si(001)//Si(001) imply that Fe3Si films epitaxial on Si(001) formed in this manner have high structural stability. The calculated spin polarized energy band structures and density of states indicate that Fe3Si films epitaxial on Si(001) have characteristic of metal, whose bonding modes are covalent bond and metallic bond. The band through Fermi level is mainly due to the Fe 3d states and the Si 3p states. Ferromagnetic properties of Fe3Si(001)//Si(001) are attributed to 3d states of the Fe atoms. The atomic magnetic moments of Fe[A,C] and Fe[B] are different from each other, likewise implying Fe3Si films epitaxial on Si(001) is ferromagnetic.

Anomalous Hall effect in 2H -phase MX2 transition-metal dichalcogenide monolayers on ferromagnetic substrates ( M = Mo, W, and X = S, Se, Te)

We study the anomalous Hall effect in monolayers of transition-metal dichalcogenides 2H-$MX_2$ ($M$=Mo, W, and $X$=S, Se, Te) under a proximity effect of ferromagnetic substrate. If a proximity-induced exchange field is introduced, the spin-polarized energy bands in $K$ and $K'$ valleys are shifted in the opposite directions, and it causes the Hall effect by breaking time-reversal symmetry. The induced Hall effect is the most prominent in the valence band which has a large intrinsic spin splitting. Moreover, we find that tilting the magnetization from the perpendicular direction gives rise to a sensitive change in the Hall conductivity only in the electron side, and it is attributed to the mixing of the Berry phase by the in-plane field in the nearly degenerate conduction bands.

High energy Compton spectroscopy and electronic structure of Laves phase ZrFe 2

Abstract We present the first-ever experimental Compton profile of Laves phase ZrFe2 using indigenous 20 Ci 137Cs Compton spectrometer. To annotate the experimental electron momentum density, we have calculated the theoretical Compton profiles using density functional theory (DFT) and hybridization of Hartree–Fock and DFT within linear combination of atomic orbitals (LCAO) method. The spin-polarized energy bands and density of states are computed using LCAO and full potential-linearized augmented plane wave methods. The revised Perdew-Burke-Ernzerhof functional (for solids) based theoretical profile gives a marginally better agreement with the experimental profile as compared to other approximations considered in the present work. The Fermi surface topology of ZrFe2 is explained in terms of majority- and minority-spin energy bands.

Spin-polarized currents generated by magnetic Fe atomic chains

Fe-based devices are widely used in spintronics because of high spin-polarization and magnetism. In this work, freestanding Fe atomic chains, the thinnest wires, were used to generate spin-polarized currents due to the spin-polarized energy bands. By ab initio calculations, the zigzag structure was found to be more stable than the wide-angle zigzag structure and had a higher ratio of spin-up and spin-down currents. By our theoretical prediction, Fe atomic chains have a sufficiently long thermal lifetime only at T ≦̸ 150 K, while C atomic chains are very stable even at T = 1000 K. This means that the spintronic devices based on Fe chains could work only at low temperatures. A system constructed by a short Fe chain sandwiched between two graphene electrodes could be used as a spin-polarized current generator, while a C chain could not be used in this way. The present work may be instructive and meaningful to further practical applications based on recent technical developments on the preparation of metal atomic chains (Proc. Natl. Acad. Sci. USA 107 9055 (2010)).

First principles study on half-metallic ferromagnetismof ternary compounds ZnVSe

We have calculated the electronic structure of ZnVSe2 by the method of first-principles plane wave pseudopotential (PWP) with density functional theory (DFT) combined with generalized gradient approximation (GGA), such as spin polarized energy band structure, density of states, Mulliken population, magnetic moments and so on. The calculation results show that ternary compound ZnVSe2 is a half-metallic ferromagnet that shows significant ferromagnetism, its half-metallic energy band gap reaches 0.443 eV. The electron spin polarization is as high as almost 100%, and therefore, ZnVSe2 may be a useful material for spintronics.

Temperature dependent spin momentum densities in Ni–Mn–In alloys

The spin-dependent electron momentum densities inNi2MnIn andNi2Mn1.4In0.6 shape memory alloy using magnetic Compton scattering with 182.2 keV circularly polarizedsynchrotron radiation are reported. The magnetic Compton profiles were measured atdifferent temperatures ranging between 10 and 300 K. The profiles have been analyzedmainly in terms of Mn 3d electrons to determine their role in the formation of the totalspin moment. We have also computed the spin polarized energy bands, partial and totaldensity of states, Fermi surfaces and spin moments using full potential linearizedaugmented plane wave and spin polarized relativistic Korringa–Kohn–Rostokermethods. The total spin moments obtained from our magnetic Compton profiledata are explained using both the band structure models. The present Comptonscattering investigations are also compared with magnetization measurements.

Spin Filter of Graphene Nanoribbon Based Structure

Spin-dependent transport properties of the zigzag graphene nano-ribbon (zGNR) based structure Al-zGNR-Al are investigated by ab initio technique where density functional calculation is carried out within the Keldysh non-equilibrium Green's function formalism. The energy band structure of the infinite zigzag ribbon is sensitive to the dangling bonds of carbon atoms on both edge sides. For the three-circle-width zigzag ribbon with one edge monohydrogenated and the other edge dihydrogenated (zGNR(H-H2)), strongly spin-polarized energy bands are found. A spin-down branch is obtained just below the Fermi level while a spin up band appears above it. For the structure Al-zGNR(H-H2)-Al, where three-circle-width and seven-circle-length (3 × 7) zGNR(H-H2) is coupled by two (100) aluminium electrodes, an obvious spin filter property is found as the bias voltage changes. When the length of the sandwiched zGNR(H-H2) ribbon increases, the spin-up current is strongly restrained especially under higher bias voltage.

A study of the electronic structure of the first-row transition-metal compounds

A study has been made of the electronic structure of the first-row transition-metal compounds. In this study, particular attention has been given to the properties of the first-row transition-metal mono-oxides and a brief discussion of these properties is presented. The inability of either conventional energy band theory or of the ligand or crystal field theories to adequately describe the electronic structure of these compounds is brought out in this discussion. It is found that a model based upon the spin polarized energy band theory of solids, which allows for the localized or crystal field-like transitions that are characteristic of many of these compounds, is capable of adequately describing their electronic structure. The spin polarized energy band structures of MnO, α-MnS, and NiO are presented and discussed. These calculations lead to the prediction that MnO and α-MnS should be insulators and that NiO should be a semiconductor. It is shown that this nonmetallic behavior should continue after the loss of the antiferromagnetic ordering that characterizes their low temperature ground states. The importance of the intra-atomic exchange contributions to these band structures is discussed. A discussion is also given of the form of the exchange potential that is being used in conducting these studies.

Spin-polarized energy bands in antiferromagnetic MnO

The energy band structure of MnO has been calculated using a spin-polarized augmented-plane-wave procedure. It is a well-known fact that this compound is both an insulator and antiferromagnetic for T < 122°K. The calculated bands confirm this. The energy gap between the valence and conduction bands is about 0.2 Ry or 2.7 eV. The separation between the filled and empty d bands is about 0.6 Ry, which is approximately the same as the exchange splitting between the spin-up and spin-down d levels that resulted when a spin-polarized Hartree-Fock-Slater calculation was made on the Mn2+ ion. It is felt that this procedure will provide a promising starting point from which a fundamental understanding of the energy band structure of some of the other insulating antiferromagnets, in particular NiO, can be achieved.

Spin-polarized energy bands in sodium

The augmented-plane-wave (APW) method was used with a spin-dependent potential to calculate the one-electron energies in solid sodium. The charge density from which this potential was obtained was derived from spin-polarized Hartree-Fock-Slater atomic wave functions with fractional occupation of the valence spin orbitals. The atoms were placed on a cesium chloride Bravais lattice with alternating spin density. This “antiferromagnetic” arrangement was maintained with a decreasing moment as the calculation was carried to self-consistency. The possibility of this state having a lower total energy than the “paramagnetic” state is discussed. A “spin-density-wave” potential was introduced into the Hamiltonian and a perturbation calculation was performed. The unperturbed functions used in this calculation were self-consistent solutions of the “paramagnetic” state. By fitting the bands so obtained to the above “antiferromagnetic” bands the amplitude of the “spin-density-wave” in that configuration was determined. The possibility of obtaining a state with a lower total energy by using a “spin-density-wave” with this amplitude and varying wavelength is discussed.

Electric and magnetic properties of new rare-earth half-metal LiPr 2O 4

A kind of new rare-earth half-metal LiPr 2O 4 is found based on the first-principle pseudo-potentials. The calculated and analysed electronic and magnetic properties include the spin-polarized energy band structures and the state densities, the charge distribution and the molecular magnetic moments of LiPr 2O 4. Then these properties are compared with those of Fe 3O 4. The electronic structures of Li–ions and Pr–ions and the mechanism of magnetic and electric properties of LiPr 2O 4 are both analyzed in system. From our calculation, LiPr 2O 4 is a kind of new II-A half-metal materials, which is different from II-B Fe 3O 4. The molecular magnetic moment 2.98 μ B of LiPr 2O 4 is lower than 4.0 μ B of Fe 3O 4. There are hybridized orbits near the Fermi level caused by strong crystal field. The molecular magnetic moment comes from Pr4f electrons that are up-spin. Hybridized orbits cause the charge distribution and spin-polarization of the new half-metal LiPr 2O 4.

First principle calculation on electronic and magnetic properties of new half-metal TiFe2O4

The new half-metal TiFe2O4 is designed and optimized based on first-principle pseudo-potentials. The electronic and magnetic properties including the spin-polarized energy band structures and state densities, the charges, and the molecular magnetic moments of TiFe2O4 are calculated and compared with those of Fe3O4. It is found that TiFe2O4 is a new ferromagnetic II-B half-metal, which is different from ferrimagnetic Fe3O4. The conductivity of TiFe2O4 is a little higher than that of TiFe2O4. The molecular magnetic moment of 6.68μB of TiFe2O4 is remarkably higher than 4.0μB of Fe3O4. Doping of Ti-atoms into Fe3O4 is helpful in improving the room temperature magnetoresistance. Localization of Fe 3d or Ti 3d electrons caused by hybridization of O 2p and Fe 3d or Ti 3d orbits is the mechanism by which TiFe2O4 has higher molecular magnetic moment and conductivity, and hence higher room magnetoresistance.

Magnetic Compton scattering study ofNi2+xMn1−xGaferromagnetic shape-memory alloys

In this paper, we report the spin-polarized momentum densities of ${\mathrm{Ni}}_{2+x}{\mathrm{Mn}}_{1\ensuremath{-}x}\mathrm{Ga}$ ($x=0.03$, 0.26, and 0.35) Heusler alloys at various temperatures and magnetic fields using magnetic Compton scattering technique. Magnetization studies are also performed for comparison. It is seen that the variation of magnetic effect (ratio of magnetic to charge intensities) is consistent with the martensitic transition, as shown by the differential scanning calorimetry data. The magnetic Compton profiles have been analyzed mainly in terms of the contributions from the $3d$ electrons of Mn to determine their role in the formation of total spin moment. The full potential linearized augmented plane-wave method has been used to calculate the spin-polarized energy bands and the spin moments of ${\mathrm{Ni}}_{2}\mathrm{Mn}\mathrm{G}\mathrm{a}$ and ${\mathrm{Ni}}_{2.25}{\mathrm{Mn}}_{0.75}\mathrm{Ga}$. ${\mathrm{Ni}}_{2}\mathrm{Mn}\mathrm{G}\mathrm{a}$ exhibits half metallicity along certain high-symmetry directions of the Brillouin zone. For ${\mathrm{Ni}}_{2}\mathrm{Mn}\mathrm{Ga}$, the total and Mn local moments obtained from Compton scattering are in excellent agreement with theory.

The change in magnetic properties of Fe3Al compound due to substitution of Fe by Co

The magnetic moment using self-consistent spin-polarized energy band calculations of Fe 3 Al and Fe 2 CoAl Heusler phases are presented. These results are compared with the experimental values obtained from the magnetization curves of these materials.

The change in magnetic properties of Fe3Al compound due to substitution of Fe by Co

The magnetic moment using self-consistent spin-polarized energy band calculations of Fe3Al and Fe2CoAl Heusler phases are presented. These results are compared with the experimental values obtained from the magnetization curves of these materials.