Spin-down Channel Research Articles

Structural, electronic, magnetic, and optical properties of new TiXY (X = F and Cl; Y = S, Se and Te) Janus monolayers: A first-principles study

Extensive investigations have been carried out to search for new multifunctional two-dimensional (2D) materials with intriguing electronic and magnetic properties. In this paper we investigate novel Janus monolayers using first-principles calculations, which may be created by halogenation of the titanium dichalcogenide (TiCh2) single layers. We employ the full-potential linearized augmented plane wave (FP-LAPW) method to solve the self-consistent Kohn–Sham equations. The exchange–correlation potentials are described with the Wu–Cohen scheme. Our simulations assert that the TiFSe, TiClS, and TiClSe monolayers show the half-metallicity, resulting from the metallic spin-up channel and semiconductor spin-down channel. In contrast the TiFS, TiFTe, and TiClTe layers exhibit metallic nature in both spin channels. Significant magnetization with total spin magnetic moments ranging from 0.812 to 1.010 μB may be obtained, which are produced mainly by the spin-polarization of the Ti-3d orbital. The absorption coefficient spectra suggest a wide band absorption from infrared to ultraviolet regimes, with large values of the order of 105/cm. Results obtained herein suggest that the TiXCh (X = F and Cl; Ch = S, Se and Te) may be prospective candidates for applications in nano spintronic and optoelectronic devices.

Effect of Edge Fluorination on Electronic and Transport Properties of Aarmchair Boron Nitride Nanoribbons: A First-Principles Study

We have systematically investigated the effect of edge fluorination on structural stability, electronic and transport properties of armchair boron nitride nanoribbons (ABNNRs), using first-principles calculations within the frame work of local spin-density approximation (LSDA). ABNNRs with F-passivation on only edge B atoms, regardless of their width, are found to be half-metallic and energetically most stable. For these ribbons, completely spin polarized charge transport is predicted across the Fermi level as a result of giant spin splitting. Transmission spectrum analysis also confirms the separation of spin-up and spin-down electronic channels. It is revealed that F-passivation of only edge N atoms transforms nonmagnetic bare ribbons into magnetic semiconductors whereas fluorination of both the edges does not affect the electronic and magnetic state of bare ribbons significantly. Predicted Half-metallicity projects these ABNNRs as potential candidates for inorganic spintronic devices.

Strain drived band aligment transition of the ferromagnetic VS2/C3N van derWaals heterostructure* *Project supported by the National Key Research and Development Program of China (Grant No. 2017YFA0207500), Natural Science Foundation of Henan Province, China (Grant No. 202300410507), and Key Research & Development and Promotion Projects in Henan Province, China (Grant No. 212102210134).

Exploring two-dimensional (2D) magnetic heterostructures is essential for future spintronic and optoelectronic devices. Herein, using first-principle calculations, stable ferromagnetic ordering and colorful electronic properties are established by constructing the VS2/C3N van der Waals (vdW) heterostructure. Unlike the semiconductive properties with indirect band gaps in both the VS2 and C3N monolayers, our results indicate that a direct band gap with type-II band alignment and p-doping characters are realized in the spin-up channel of the VS2/C3N heterostructure, and a typical type-III band alignment with a broken-gap in the spin-down channel. Furthermore, the band alignments in the two spin channels can be effectively tuned by applying tensile strain. An interchangement between the type-II and type-III band alignments occurs in the two spin channels, as the tensile strain increases to 4%. The attractive magnetic properties and the unique band alignments could be useful for prospective applications in the next-generation tunneling devices and spintronic devices.

Nonlocal Interactions in the Double Perovskite Sr2FeMoO6 from Core-Level X-ray Spectroscopy

The valence electronic structure of the half-metallic double perovskite Sr2FeMoO6 forms from a strongly hybridized band in the spin-down channel of Fe 3d and Mo 4d states that provides metallic conductivity and a gapped spin-up channel. The ground-state description has previously been explored in terms of many-body interactions where local and nonlocal interactions produce states with a combination of a charge-transfer configuration and intersite charge fluctuations. Here, we provide a qualitative understanding on nonlocal effects in Sr2FeMoO6 using a combination of core-level X-ray spectroscopies, specifically X-ray absorption, emission, and photoelectron spectroscopies. Our spectroscopic data indicate intersite Fe 4p–O 2p–Mo 4d interactions to be the origin of these nonlocalized transitions. Close to the Fermi level, this interaction is dominated by Mo 4d–O 2p character. When our data are compared against first-principles electronic structure calculations, we conclude that a full understanding of the nature of these states requires a spin-resolved description of the hybridization functions and that the nonlocal screening occurs predominantly through hybridization in the minority spin channel of the Mo 4d bands.

Ferromagnetism and intrinsic half-metallicity of two-dimensional MnN monolayer with square–octagonal structure

The MnN monolayer with square–octagonal structure (so-MnN) is explored using density functional calculations. The results show that the so-MnN monolayer is energetically, dynamically, thermally and mechanically stable, and exhibits the ferromagnetism and intrinsic half-metallicity. The total magnetic moment is 16 μ B in unit cell (Mn4N4). The energy band of spin-up crosses the Fermi energy level (E F), while the spin-down channel has semiconductor characteristic with a direct band gap of 3.0 eV at Γ-point. By applying the biaxial strain, the band gap in spin-down channel can be tuned, and the so-MnN monolayer still possesses the characteristic of ferromagnetism and intrinsic half-metallicity. Finally, the Curie temperature T C increases gradually under biaxial strains from 0 to +3%, while the T C has a decreasing trend under the biaxial strains from 0 to −3%.

Comment on “Spin-orbit interaction and spin selectivity for tunneling electron transfer in DNA”

The observation of chiral-induced spin selectivity (CISS) in biological molecules still awaits a full theoretical explanation. In a recent Rapid Communication, Varela et al. [Phys. Rev. B 101, 241410(R) (2020)] presented a model for electron transport in biological molecules by tunneling in the presence of spin-orbit interactions. They then claimed that their model produces a strong spin asymmetry due to the intrinsic atomic spin-orbit strength. As their Hamiltonian is time-reversal symmetric, this result contradicts a theorem by Bardarson [J. Phys. A: Math. Theor. 41, 405203 (2008)], which states that such a Hamiltonian cannot generate a spin asymmetry for tunneling between two terminals (in which there are only a spin-up and a spin-down channels). Here we solve the model proposed by Varela et al. and show that it does not yield any spin asymmetry, and therefore cannot explain the observed CISS effect.

First-Principles study of thermoelectric properties of Half-Metallic Non-Magnetic Half-Heusler alloy Mn2Ge

Research about Heusler alloys have been massively conducted through experimental and theoretical studies due to their fascinating properties. Most of Heusler alloy compounds exhibit half-metallic behavior, in which one of the spin channels of such a material behaves as a metal and the other behaves as an insulator because there is an open energy gap at the Fermi level. On the other hand, the thermoelectric properties of the materials usually work well in the insulator or semiconductor phase rather than in the metal phase. Motivated by such interesting phenomena, we conduct first-principles study to explore the thermoelectric properties of Mn2Ge, one of Heusler compounds that exhibits half-metallic behavior. Our calculation results show that Mn2Ge of the spin-down channel has a high Seebeck coefficient and ZT value (electron part) at room temperature around 892 MV/K and 0.97, respectively. The positive sign of the Seebeck coefficient on spin-up and spin down indicate that the majority charge carrier in Mn2Ge are positive charges (holes).

Stability, electronic structures, magnetic and optical properties of Fe and non-metal (NM=B, C, N) co-doped monolayer 2H-WSe2

The effects of Fe–NM (NM=B, C, N) co-doping on the stability, electronic structures, magnetic and optical properties of 2H-WSe2 monolayer are investigated in detail by spin-polarized density functional theory (DFT) calculations. The results show that the Fe-doped WSe2 monolayer exhibits magnetic half-metallicity (HM) with a 100% spin polarization due to the electrons partially occupied Fe [Formula: see text] and [Formula: see text] bonding states at the Femi level [Formula: see text], and eventually induces a magnetic moment of 2 [Formula: see text]. The Fe–B and Fe–N co-doped WSe2 monolayers show obvious spin-polarization and retain semiconductor character with an indirect bandgap of 0.034 eV and 0.220 eV, respectively, which can be attributed to the strong hybridization between the Fe [Formula: see text], [Formula: see text] states and [Formula: see text], [Formula: see text] orbitals at the [Formula: see text]. Interestingly, the Fe–C co-doped WSe2 monolayer exhibits a typical non-magnetic semiconductor due to the effective charge compensation between Fe and C atoms, leading to completely symmetrical spin-up and spin-down channel of [Formula: see text], [Formula: see text], [Formula: see text] and [Formula: see text] states near [Formula: see text]. Moreover, the optical properties of WSe2 monolayer can be effectively tuned by Fe–NM co-doping, which can attribute to the introduction of impurity state. The excellent magnetic HM, tunable magnetic, optical properties of Fe–NM co-doped WSe2 monolayers are of great significance for further application in the fields of spintronics, opto-electronics and magneto-optics devices.

Two-dimensional FeTe2 and predicted Janus FeXS (X: Te and Se) monolayers with intrinsic half-metallic character: tunable electronic and magnetic properties via strain and electric field.

Driven by the fabrication of bulk and monolayer FeTe2 (ACS Nano, 2020, 14, 11473-11481), we explore the lattice, dynamic stability, electronic and magnetic properties of FeTeS and FeSeS Janus monolayers using density functional theory calculations. The obtained results validate the dynamic and thermal stability of the FeTeS and FeSeS Janus monolayers examined. The electronic structure shows that the FeTe2 bulk yields a total magnetization higher than the FeTe2 monolayer. FeTeS and FeSeS are categorized as ferromagnetic metals due to their bands crossing the Fermi level. So, they can be a good candidate material for spin filter applications. The biaxial compressive strain on the FeTe2 monolayer tunes the bandgap of the spin-down channel in the half-metal phase. By contrast, for FeTeS, the biaxial strain transforms the ferromagnetic metal into a half-metal. The electric field applied to the FeSeS monolayer in a parallel direction transforms the half-metal to a ferromagnetic metal by closing the gap in the spin-down channel.

Strain-controlled electronic and magnetic properties of tVS2/hVS2 van der Waals heterostructures.

The structural, electronic and magnetic properties of the T-phase and H-phase of the VS2 monolayer and their heterobilayers are studied by means of first-principles calculations. We find that the two phases of the VS2 monolayer are both ferromagnetic (FM) semiconductors and that these two monolayers form a typical van der Waals (vdW) heterostructure with a weak interlayer interaction. By comparing the energy of different magnetic configurations, the FM state of the tVS2/hVS2 heterostructure is found to be in the ground state under normal conditions or biaxial strains. Under compressive strains, the anti-FM (AFM) and FM states degenerate. Based on the band structure obtained and the work function, it is found that the T-phase and H-phase are capable of forming an efficient p-n heterostructure. Due to spontaneous charge transfer at the interface, a gapless semiconductor is formed in our HSE06 calculations. We also find that the twist angle between the monolayers has a negligible impact on the band structure of the heterostructure in its spin-down channel. Moreover, the tVS2/hVS2 heterostructure is found to switch from a gapless semiconductor to a metal or a half-metal under some given biaxial or uniaxial strains. Therefore, the heterostructure could be a half-metallic property with strains, realizing 100% polarization at the Fermi level. Our study provides the possibility of realizing 100% spin-polarization at the Fermi level in these FM vdW heterostructures, which is significant for further spin transport exploration.

Band structure regulation in Fe-doped MgZnO by initial magnetic moments.

ZnO-based diluted magnetic semiconductors have high prospects in spintronics applications. In this study, the electronic and magnetic properties of Fe-doped MgZnO are studied by density functional theory calculations. The investigations of the band structure, total density of states, and projected density of states revealed a strong correlation between Mg and O atoms in addition to the magnetism and impurity level generated by the Fe atoms. In the spin charge density and band structure of 2.78% Fe-doped MgZnO, Fe atoms always cause paramagnetic coupling with oxygen atoms bonded around them, and when the initial magnetic moments were parallel, the band gap is broadened in the opposite channel. On the contrary, when the initial magnetic moments are anti-parallel, the band gap is narrowed in both the spin-up and spin-down channels. This shows that the initial magnetic moments have a great influence on the band structure, giving another way to tune the gap dynamically.

Electronic band structure, phase stability, magnetic and thermoelectric characteristics of the quaternary Heusler alloys CoCuZrAs and CoRhMoAl: Insights from DFT computations

The search for new materials with stable structures and unique functional properties is important for the development of future technologies. Herein, by performing first-principles calculations based on density functional theory (DFT), we have explored two new quaternary Heusler alloys, CoCuZrAs and CoRhMoAl. The total energy calculations reveal that CoCuZrAs (CoRhMoAl) is stable in the Y2 (Y1)-type structure among the three possible atomic configurations and that both alloys are ferromagnetic in the ground state. Electronic properties computed at the GGA level show that the present alloys have metallic behavior, whereas CoRhMoAl is a half-metal at GGA + U (U ≥ 1 eV) due to the opening of a band gap at the Fermi level in the spin-down channel. Furthermore, CoRhMoAl retains its half-metallicity when pressure is applied (in the range of 0–25 GPa). The magnetic data reveal that CoCuZrAs has a total spin moment of 2.34 µB, whereas CoRhMoAl has an integral spin moment of 3.0 µB, which is in line with the Slater-Pauling formula Mt = Zt-24 (where Mt = total spin moment/cell, Zt = total valence electrons/cell). In CoCuZrAs, the Co, Cu, Zr and As atoms contribute individual spin moments of 2.16 µB, −0.03 µB, 0.18 µB and 0.04, respectively, whereas in CoRhMoAl, the Co, Rh, Mo, and Al atoms carry spin moments of 1.81, 0.17, 1.14 and −0.09 µB, respectively. These results are also supported by the corresponding spin density plots, where the spin density is mainly located at Co atoms. The thermoelectric properties calculated up to 1000 K exhibit interesting features that might be useful in energy production.

First principles calculations of 3d-4d transition metal based LiMgPdSn–type FeCrRuZ (Z = Al, Ga, In, Si) equiatomic quaternary Heusler alloys

We have investigated the structural, electronic and magnetic properties of the equiatomic quaternary Heusler (EQH) alloys FeCrRuZ (Z = Al, Ga, In, Si) using density functional theory. The structural study reveals that all the FeCrRuZ (Z = Al, Ga, In, Si) alloys stabilized to Y1 type structural configuration with ferrimagnetic ordering. The spin-polarized density of states (DOS) calculation demonstrates that FeCrRuAl alloy exhibits unusual spin-gapless half-metallic (SG-HM) behavior with majority spin-up channel possessing metallic characteristic and minority spin-down channel showing spin-gapless nature. The FeCrRuSi, FeCrRuGa and FeCrRuIn alloys display half-metallic, nearly half-metallic and metallic nature, respectively. The FeCrRuAl and FeCrRuSi alloys greatly follow the Slater-Pauling rule and give 100% spin-polarization at the Fermi level with total integer magnetic moment of 1 and 2 μB, respectively. In all, the total magnetic moment of FeCrRuZ (Z = Al, Ga, In, Si) alloys is contributed mainly by Cr atoms. The origin of the half-metallic band gap is elucidated through a schematic molecular orbital (MO) diagram. Further, the role of main group elements on structural, electronic, magnetic and half-metallic properties are explored in greater detail.

Fully spin-polarized double-Weyl fermions with type-III dispersion in the quasi-one-dimensional materials X2RhF6 ( X=K , Rb, Cs)

Double-Weyl fermions, as novel topological states of matter, have been mostly discussed in nonmagnetic materials. Here, based on density-functional theory and symmetry analysis, we propose the realization of fully spin-polarized double-Weyl fermions in a family ferromagnetic materials ${\mathrm{X}}_{2}{\mathrm{RhF}}_{6}$ ($X=\text{K}$, Rb, Cs). These materials have the half-metal ground states, where only the bands from the spin-down channel present near the Fermi energy. The spin-down bands form a pair of triply degenerate nodal points (TDNPs) if spin-orbit coupling (SOC) is not included. Under SOC, one TDNP splits into two double-Weyl points featuring quadratic dispersion along two momentum directions with the chiral charge of $\ifmmode\pm\else\textpm\fi{}2$, and they are protected by the three-fold rotation (${C}_{3}$) symmetry. Unlike most double-Weyl semimetals, the Weyl points proposed here have the type-III dispersion with one of the crossing bands being saddle-shaped. An effective model is constructed, which describes well the nature of the Weyl points. These Weyl points are fully spin-polarized, and are characterized with double Fermi arcs on the surface spectrum. Breaking ${C}_{3}$ symmetry by lattice strain could shift one double-Weyl point into a pair of type-II single-Weyl points. The ${\mathrm{X}}_{2}{\mathrm{RhF}}_{6}$ materials proposed here are excellent candidates to investigate the novel properties of type-III double-Weyl fermions in ferromagnetic system, as well as generate potential applications in spintronics.

Two-dimensional Weyl semimetal with coexisting fully spin-polarized type-I and type-II Weyl points

As novel topological states of matter, Weyl semimetals (WSM) have been well studied in three-dimensional (3D) system. However, two-dimensional (2D) WSMs, especially type-II ones, are rarely reported. Here, we propose that monolayer Cr2C is a 2D WSM with coexisting fully spin-polarized type-I and type-II Weyl points. The material is ferromagnetic and has a high Curie temperature of 562 K. It is metallic in the spin-up channel but insulating in the spin-down channel, showing a half metal band structure. The low-energy bands in the spin-up channel form one pair type-I Weyl points and three pairs of type-II Weyl points, which are protected by the rotation symmetry in high-symmetry path. The Weyl points can survive under spin-orbit coupling with magnetization along y direction. The Weyl points show fully spin-polarized Fermi arc of edge states within the projected Weyl points. Most interestingly, we find the 2D type-II Weyl points show unusual Magnetoresponse with complete Landau levels (LLs) collapse independent with the magnetic-field. Such LLs collapse is different with type-II Weyl points in 3D system. The material provides a promising platform to study the fundamental physics of 2D type-I and type-II Weyl points, as well potential entanglements between them.

Quadruple perovskite oxide LaCu3Co2Re2O12: A ferrimagnetic half metal with nearly 100% B-site degree of order

An A- and B-site ordered quadruple perovskite oxide LaCu3Co2Re2O12 was synthesized at 9 GPa and 1323 K. The compound possesses a Pn-3 space group, where both A and B sites are orderly occupied by different cations with a nearly 100% degree of order. Bond valence sum calculations and x-ray absorption spectroscopy confirm the charge distribution to be LaCu2+3Co2+2Re5.5+2O12. A ferrimagnetic phase transition is found to occur around 150 K due to the Cu2+(↑)-Co2+(↑)-Re5.5+(↓) spin coupling found by x-ray magnetic circular dichroism at Cu-, Co-, and Re-L2,3 edges. The magnetoresistance effects as well as the first-principle calculations indicate the half-metallic nature for LaCu3Co2Re2O12 with a wider energy gap at the up-spin channel and a conducting band at the down-spin channel.

Formation Energetics, Electronic Structure and Ferromagnetic Properties of C-, Si- and Ge-Doped Zinc Blende Cadmium Chalcogenides

In the present study, we have investigated the structural, energetic, electronic and magnetic properties of zinc blende cadmium chalcogenides CdCh (Ch = S, Se and Te) compounds doped with group 14 elements, M = C, Si and Ge, at the anion site, using spin-polarized density functional theory calculations. The stability of all the M-doped CdCh systems is examined by computing formation energy at selected values of the chemical potentials for the atomic species participating in substitutional doping. In addition, the thermal stability of pristine as well as doped systems is examined by employing ab-initio molecular dynamics simulations. The effect of structural relaxation on the bond lengths together with the magnetic moments of M-doped CdCh systems indicate that the difference in the electronegativities and atomic radii of the dopant and chalcogen atoms determines the magnetic behaviour in these materials. Our results reveal that the spin-polarized electronic properties in M-doped cadmium chalcogenides are mainly caused by the occupancy of p orbitals of the dopant tetrels. Asymmetric insulating and p-type nature in spin-up and spin-down channels, respectively, is obtained in all M-doped CdS and CdSe compounds and Ge-doped CdTe, which results in a total magnetic moment of 2.0 μB per supercell. The magnetic moments of silicon and carbon doped CdTe, on the other hand, are found to be 1.88 μB and 0.07 μB, respectively, where the vanishing magnetic moments is caused by decreasing assymteric distribution of electrons in the spin-up and spin-down states of Si-3p and C-2p states.

TB-mBJ calculations of optical and thermoelectric properties of half-Heusler FeCrAs alloy

The optical and thermoelectric properties of FeCrAs half-Heusler are carried out by employing full potential linearized augmented plane wave method. In order to have, a precise band gap value, the TB-mBJ potential is employed. The density of states and spin-polarized band structures analysis, confirm the ferromagnetic half-metallic behavior and reveal that the FeCrAs has an indirect bandgap in spin-down (↓) channel, while the spin-up (↑) channel exhibits a metallic behavior. The important optical coefficients such as dielectric function, reflectivity, absorption coefficient and refractive index are given in the ultraviolet, visible and near-infrared range. The FeCrAs can be employed as effective ultraviolet absorber due to its high absorption coefficient of 139.104/cm and a wide absorption band. The properties, which describes the thermoelectric characteristics such as Seebeck coefficient (S), thermal and electrical conductivity and Merit factor (ZT), were obtained via the BoltzTraP code. A high ZT of about 0.99 in the p- and n-type region was obtained, what makes this compound very useful for thermoelectric applications.

Spin–flip manipulation in HgTe/CdTe heterostructure interface states with Rashba and Dresselhuas spin–orbit interactions

We investigate theoretically the electron transport through (001) HgTe/CdTe quantum well heterostructures with inverted-band in planar geometry, where Dirac-like spin–momentum locked interface states have been predicted to be formed and topologically protected against perturbations preserving time-reversal symmetry. In this work, k.p theory and scattering matrix method are used to characterize the interface states of the (001) HgTe/CdTe heterostructure as well as to understand the influence of external electric fields. We find distinct features of the transmission and conductance by considering both Rashba and Dresselhuas spin–orbit interactions. The efficient spin–flip conversion between spin-up and spin-down channels are found by tuning the strength of Rashba and Dresselhuas spin–orbit interactions. It is shown that ideal spin–flip efficiency, i.e., the complete spin–flip, can be achieved with specific spin–orbit coupling strengths. This spin–flip mechanism provides us a realistic way to manipulate the spin orientation which can be constructed low-power information processing device. Furthermore, we address the topological edge states induced unique spin manipulation capability by comparing with the conventional electron transport properties in bulk states. In addition, we analyze the transport behavior when the system in the presence of Zeeman term which may play an important role for a high magnetic field in a realistic experimental setup.

Functionalization of monolayer-CdS by metal and non-metal elemental substitution: First-principle understanding

Herein, monolayer-CdS substitutional elemental doping with metal (X; Fe) and non-metal (X; H, N, F) atoms has been investigated by the first-principle calculations. In non-magnetic monolayer-CdS, magnetism can be induced by metal and non-metal atom substitution at possible dopant sites except for FS (F doped at the chalcogen site). In particular, Fe doping at Cd-Site (FeCd) results in a maximum magnetic moment of 4.0 μB, and half-metallic character with a small dispersion at Γ point. Among all the substitutional dopants at the hole-site, Fh is ferromagnetic, being most energetically favorable under both atomic and molecular phases. The Hh in monolayer-CdS retained the semiconducting nature with reduced band gaps for spin-up and spin-down channels. Nh exhibits half-metallic character with a small dispersion of 0.13 eV at K-Γ point. The Fh and Feh show metallic behavior. To determine the exchange-field strength ferromagnetic (FM) and anti-ferromagnetic (AFM) coupling between the two dopants at far positions (1, 3) and near positions (1, 2) have been considered. The effect of dopants concentration on FM and AFM coupling were also investigated. It was found NS (Θ = 4 %) at near positions (1, 2) shows FM state with Curie temperature (Tc) of 106 K. As the biaxial strain of −5 % is applied the maximum FM state was achieved withTc of 164 K. The functionalization of monolayer-CdS with the proposed elements provided in this work can help tailoring two-dimensional (2D) materials applied in spintronics and electronics devices.