Orbit Coupling Effects Research Articles

Magneto-transport and electronic structures in MoSi2 bulks and thin films with different orientations

We report a comprehensive study of magneto-transport properties in MoSi2 bulk and thin films. Textured MoSi2 thin films of around 70 nm were deposited on silicon substrates with different orientations. Giant magnetoresistance of 1000% was observed in sintered bulk samples while MoSi2 single crystals exhibit a magnetoresistance (MR) value of 800% at low temperatures. At the low temperatures, the MR of the textured thin films show weak anti-localization behaviour owing to the spin orbit coupling effects. Our first principle calculation show the presence of surface states in this material. The resistivity of all the MoSi2 thin films is significantly low and nearly independent of the temperature, which is important for electronic devices.

Electronic structure and spectroscopy of the low-lying electronic states of antimony hydride

The electronic structure and spectroscopic properties of antimony hydride(SbH) have been investigated based on high-level ab initio calculations by utilizing the internally contracted multi-reference configuration interaction plus the Davidson correction(icMRCI+Q) method. The core-valence correlation (CV) and spin–orbit coupling (SOC) effect are considered. The potential energy curves of the 12 Λ-S states as well as the 25 Ω states that split from them via SOC effect are obtained. The spectroscopic constants of bound states are determined and well reproduced the results of the available experimental and theoretical studies. The SOC induced predissociation mechanisms of the A3Π state have been analyzed with the aid of the spin−orbit matrix element. The transition dipole moments, Franck-Condon factors, and the radiative lifetimes of the a2-X21 and b0+- X3Σ− transitions have been reported. Our study is expected to be helpful for understanding the spectroscopy and dynamics of electronic states of the SbH molecule.

Promising room temperature thermoelectric conversion efficiency of zinc-blende AgI from first principles

The theoretical investigation on structural, vibrational, and electronic properties of zinc-blende (ZB) AgI were carried out employing first principles density functional theory calculations. Thermoelectric properties then were predicted through semi-classical Boltzmann transport equations within the constant relaxation time approximation. Equilibrium lattice parameter, bulk modulus, elastic constants, and vibrational properties were calculated by using generalized gradient approximation. Calculated properties are in good agreement with available experimental values. Electronic and thermoelectric properties were investigated both with and without considering spin–orbit coupling (SOC) effect which is found to have a strong influence on p-type Seebeck coefficient as well as the power factor of the ZB–AgI. By inclusion of SOC, a reduction of the band-gap and p-type Seebeck coefficients as well as the power factor was found which is the indication of that spin–orbit interaction cannot be ignored for p-type thermoelectric properties of the ZB–AgI. By using deformation potential theory for electronic relaxation time and experimentally predicted lattice thermal conductivity, we obtained a ZT value 1.69 (0.89) at 400 K for n-type (p-type) carrier concentration of 1.5 × 1018 (4.6 ×1019) cm−3 that makes ZB–AgI as a promising room temperature thermoelectric material.

Observation of “Outlaw” Dual Aromaticity in Unexpectedly Stable Open-Shell Metal Clusters Caused by Near-Degenerate Molecular Orbital Coupling

The Huckel’s rule, Baird’s rule, and electronic shell closure model are classical and well-established concepts in chemistry, which have long been employed in rationalizing the aromaticity/antiarom...

Unraveling crystal symmetry and strain effects on electronic band structures of SiC polytypes

The modulations of the electronic band structures of hexagonal (2H, 4H, and 6H) and cubic (3C) SiC under biaxial (0001) and (111) in-plane strain are investigated by using first-principles calculations including spin–orbit coupling effects. We have clarified that the strain dependency of the valence bands is closely related to the crystal symmetry and hexagonality. Specifically, tensile strain induces hybridization and crossover between the heavy-hole and light-hole bands in the hexagonal polytypes. On the other hand, the degeneracy between the heavy-hole and light-hole bands breaks in the cubic polytype under tensile strain. Consequently, the hole effective masses change significantly under certain tensile strain in all four polytypes. The values of the critical tensile strain are approximately proportional to the energy differences between the heavy-hole and crystal-field splitting bands under no strain and, in turn, show a correlation with the hexagonality. In contrast to the case of the valence bands, the band structures around the conduction band minima and, therefore, the electron effective masses are insensitive to the strain, except for the ML direction in 6H–SiC. The present study provides principles for elucidating and designing the crystal structure and strain dependency of the electronic band structures and transport properties of SiC.

Pseudospin versus magnetic dipole moment ordering in the isosceles triangular lattice material K3Er(VO4)2

Spin-1/2 antiferromagnetic triangular lattice models are paradigms of geometrical frustration, revealing very different ground states and quantum effects depending on the nature of anisotropies in the model. Due to strong spin orbit coupling and crystal field effects, rare-earth ions can form pseudo-spin-1/2 magnetic moments with anisotropic single-ion and exchange properties. Thus, rare-earth based triangular lattices enable the exploration of this interplay between frustration and anisotropy. Here we study one such case, the rare-earth double vanadate glaserite material K$_3$Er(VO$_4$)$_2$, which is a quasi-2D isosceles triangular antiferromagnet. Our specific heat and neutron powder diffraction data from K$_3$Er(VO$_4$)$_2$ reveal a transition to long range magnetic order at 155 $\pm$ 5 mK which accounts for all R$\ln$2 entropy. The quasi-2D magnetic order leads to anisotropic Warren-like Bragg peak profiles, and is best described by alternating layers of b-axis aligned antiferromagnetism and zero moment layers. Our magnetic susceptibility data reveal that Er$^{3+}$ takes on a strong XY single-ion anisotropy in K$_3$Er(VO$_4$)$_2$, leading to vanishing moments when pseudo-spins are oriented along c. Thus, the magnetic structure, when considered from the pseudo-spin point of view comprises alternating layers of b-axis and c-axis aligned antiferromagnetism.

Spin–orbit coupling effect on energy level splitting and band structure inversion in CsPbBr3

The band structures and density of states (DOS) of all the three structural configurations of CsPbBr3 without spin–orbit coupling (SOC = 0) and with the addition of spin–orbit coupling (SOC ≠ 0) effects were calculated, using density functional theory. Upon the inclusion of the spin–orbit coupling, the bandgaps exhibit reductions of 1.27 eV, 1.16 eV and 1.08 eV for the cubic, tetragonal and orthorhombic phases, respectively. These calculations provide a positive split-off energy value of Δso = 1.69 eV for the simple cubic phase. For the lower symmetry phases, the p-like fourfold degenerate $$\varGamma_{8v}^{(4)}$$ band has been observed to split to form two bands, in addition to the $$\varGamma_{6v}^{(2)}$$ split-off band. The calculated splitting energies between these bands are found to be in close agreement with previous experimentally measured values. The calculated electronic band structures show that CsPbBr3 has a negative ‘inversion energy’ (Δi < 0). The magnitude of the inversion energy for the cubic phase is 2.36 eV for SOC = 0, which increased by 0.4–2.76 eV with the addition of the spin–orbit coupling. The arrangement of Bloch levels in the band structure of CsPbBr3 has been found to resemble that of a typical topological semimetal, but with a nonzero bandgap opening, due to the presence of the inversion asymmetry within its molecular structure.

First-principle study of electronic and magnetic properties in cobalt-niobium alloys

The electronic structure and magnetic properties of the binary alloys CoxNb1-x have been investigated by means of the density functional theory (DFT), implemented in the WIEN2k code. The exchange and correlation potential was processed by different approximations GGA and GGA + U including the spin orbit coupling (SOC) effect. The inclusion of SOC shows a very weak effect on the cobalt magnetic moments, However, Hubbard's correction induces a considerable improvement in the magnetic moment of the Co element. The calculation shows that the density of state (DOS) of the Co element is dominated by the 3d band state contribution, whereas the Nb state density is dominated by the 4d band states. Hybridization between the 3d-(Co) and 4d-(Nb) states in Co-Nb alloys reduces the spin polarization of 3d-(Co) states, resulting in a strong decrease in magnetic moment. The magnetic moment of the studied alloys is not only influenced by cobalt atom concentration but it is also influenced by the crystalline phase.

Exchange-bias controlled correlations in magnetically encapsulated twisted van der Waals dichalcogenides

Twisted van der Waals materials have become a paradigmatic platform to realize exotic correlated states of matter. Here, we show that a twisted dichalcogenide bilayer (WSe2) encapsulated between a magnetic van der Waals material (CrBr3) features flat bands with tunable valley and spin flavors. We demonstrate that, when electron–electron interactions are included, spin-ferromagnetic and valley-ferromagnetic states emerge in the flat bands, stemming from the interplay between correlations, intrinsic spin–orbit coupling (SOC) and exchange proximity effects. We show that the specific symmetry broken state is controlled by the relative alignment of the magnetization of the encapsulation, demonstrating the emergence of correlated states controlled by exchange bias. Our results put forward a new van der Waals heterostructure where symmetry broken states emerge from a genuine interplay between twist engineering, SOC and exchange proximity, providing a powerful starting point to explore exotic collective states of matter.

Electron correlation effect versus spin–orbit coupling for tungsten and impurities

The electron correlation and spin–orbit coupling (SOC) effects are investigated for body-centered-cubic tungsten and intrinsic & irradiative impurities using first principles calculations based upon the density functional theory. It is found that the electron correlation between the localized 5d electrons and the SOC effect are significant in modifying the band structures and the formation energies of defects. For the latter one, the involving of electron correlation always makes the defects stabler than the Perdew–Burke–Ernzerhof results, while the SOC contributes diversely for different defects. Moreover, the migration barrier of single tungsten vacancy moving in ⟨111⟩ direction is explored, where the inclusion of electron correlations remarkably decreases the migration barrier, while the influence of SOC is almost negligible. This study can help to validate the previous studies on irradiative defects in tungsten and improve the further investigations.

Simulated field-modulated x-ray absorption in titania.

In this paper, we present a method to compute the x-ray absorption near-edge structure (XANES) spectra of solid-state transition metal oxides using real-time time-dependent density functional theory, including spin-orbit coupling effects. This was performed on bulk-mimicking anatase titania (TiO2) clusters, which allows for the use of hybrid functionals and atom-centered all electron basis sets. Furthermore, this method was employed to calculate the shifts in the XANES spectra of the Ti L-edge in the presence of applied electric fields to understand how external fields can modify the electronic structure, and how this can be probed using x-ray absorption spectroscopy. Specifically, the onset of t2g peaks in the Ti L-edge was observed to red shift and the eg peaks were observed to blue shift with increasing fields, attributed to changes in the hybridization of the conduction band (3d) orbitals.

Stabilization of Plutonium(V) Within a Crown Ether Inclusion Complex

Crystalline coordination complexes of actinides, especially in atypical oxidation states, are not only fundamentally important for expanding the notably limited knowledge on the bonding nature of a...

Elastic anisotropy, electronic and magnetic behaviours of ferromagnetic Europium Niobate EuNbO3 in orthorhombic structure: DFT + U, MFA and QTAIM studies

ABSTRACT Recently, previous studies have shown that at room temperature, EuNbO3 adopts an orthorhombic structure of Imma space group with a ferromagnetic behaviour. The relevant idea of ⁣this work is to provide a new insight into some electronic and magnetic behaviours of EuNbO3 in orthorhombic structure, which are still unknown, based on the results already found previously. The structural properties have been determined using two functionals (GGA-PBE and GGA-PBEsol) and the found results are in good agreement with the previous ones. Mechanical behaviour and elastic anisotropy have also been studied in detail from which, it has been confirmed that EuNbO3 is mechanically stable in Imma orthorhombic perovskite structure. The directional analysis of Young’s modulus has shown that it is elastically anisotropic. For the electronic properties, many approaches have been used in order to take all the considerations that are not taken into account by GGA semilocal functional, hence, spin–orbit coupling effect (SOC), Hubbard correction (GGA + U) and Modified Becke–Johnson exchange potential (mBJ) have been adopted. The two adopted mBJ parameterizations gave different results, hence, for that of TB-mBJ, the studied compound is almost halfmetallic, while that of Radi A. Jishi et al. showed a semiconductor behaviour. To estimate Curie temperature, we have employed the mean field approximation (MFA), hence, its estimated value is higher than that of transition to the tetragonal I4/mcm structure, which has been determined previously. The ionic character of Eu-O and Nb-O bonds has been confirmed by The Quantum Theory of Atoms in Molecules (QTAIM).

First-principles investigation of two-dimensional topological insulators BiX (X=H, F, O)

In order to exploit the nontrivial topological phase of matter for realizing room temperature transport applications, two-dimensional (2D) topological insulators (TIs) with large bulk band gaps are necessary. Based on first-principles calculations, the electronic and topological properties of 2D large gap TIs BiX (X = H, F, O) have been thoroughly investigated. We confirmed their nontrivial topological nature by calculating topological invariant (using Fu-Kane method) and observed odd number pairs of topologically protected gapless edge states from band structures of their zigzag nanoribbons. The strong spin–orbit coupling effect arising from heavy bismuth atoms not only affects the electronic structures, but also obviously influences the structural parameters such as lattice constant, bond length and buckling height. The key role of orbital filtering effect of X atoms in the electronic and topological properties has been illustrated by employing atomic-orbital-resolved band structures. Furthermore, we proposed a large band gap substrate, hexagonal boron nitride (h-BN), in a BiF/h-BN heterostructure, in which the h-BN substrate preserves the nontrivial topological behavior of BiF. To the best of our knowledge, BiF/h-BN with the gap of 1.041 eV has the largest band gap among all previously studied TI/substrate heterostructures. These findings make this structure a promising platform for high temperature low-dissipation spintronic nanodevices.

Fluctuation-driven superconductivity in Sr2RuO4 from weak repulsive interactions

We provide results for the leading superconducting instabilities for a model pertaining to [Formula: see text] obtained within spin-fluctuation mediated superconductivity in the very weak-coupling limit. The theory incorporates spin–orbit coupling (SOC) effects both in the band structure and in the pairing kernel in the form of associated magnetic anisotropies. The leading superconducting phase is found to be [Formula: see text] and a nodal [Formula: see text]-wave state. However, the odd-parity helical solution can become leading either for small SOC and Hund’s coupling [Formula: see text] in the weak [Formula: see text]-limit, or in the opposite limit with large SOC and [Formula: see text] at larger values of the Hubbard-[Formula: see text]. The odd-parity chiral solution is never found to be leading. Finally we discuss the form of the resulting superconducting spectral gaps in the different explored parameter regimes.

Topological phase transition from T-carbon to bct-C16

The realization of nontrivial fermions in three-dimensional carbon allotropes greatly facilitates topological applications in carbon based-materials. In this work, we find a topological phase transition from T-carbon to bct-C16 based on first principles calculations. After carrying out a heating or a doping process on T-carbon, a new carbon phase termed bct-C16 is obtained. The potential energy and the crystal orbital Hamilton population confirm the phase transition process, respectively. Importantly, we also investigate the quantized Berry phase and drumhead surfaces states, confirming the topological nodal line semimetallic features of bct-C16. Due to the extremely weak spin–orbital coupling effect in carbon, the nodal ring in bct-C16 can be guaranteed to be nearly intact. This work not only provides two methods to obtain the carbon phase bct-C16, but also an avenue for bct-C16 to observe nontrivial fermions.

Nodal ring spin gapless semiconductor: New member of spintronic materials

IntroductionSpin gapless semiconductors (SGSs) and nodal ring states (NRSs) have aroused great scientific interest in recent years due to their unique electronic properties and high application potential in the field of spintronics and magnetoelectronics. ObjectivesSince their advent, all SGSs and NRSs have been predicted in independent materials. In this work, we proposed a novel type of material, nodal ring spin gapless semiconductor (NRSGS), which combines both states of the SGSs and NRSs. MethodsThe synthesized material Mg2VO4 has been detailed with band structure analysis based on first principle calculations. ResultsObtained results revealed that there are gapless crossings in the spin-up direction, which are from multiple topological nodal rings located exactly at the Fermi energy level. Mg2VO4 combines the advantages inherited from both NRSs and SGSs in terms of the innumerable gapless points along multiple nodal rings with all linear dispersions and direct contacts. In addition, Mg2VO4 also shows strong robustness against both the spin orbit coupling effect and strain conditions. ConclusionFor the first time, we propose the concept of an NRSGS, and the first such material candidate Mg2VO4 can immediately advance corresponding experimental measurements and even facilitate real applications.

Spin orbit coupling effects on the non-collinear magnetism of structurally relaxed Fe/Cu (001) thin films: First principles calculations

The effect of the spin–orbit coupling interaction and atomic relaxation, on the ground state non-collinear magnetism of deposited Fe thin films, on a Cu (001) surface, is studied by using first principles density functional theory calculations. The total energy values and spin and orbital magnetic local moments are calculated for a Fe film of N = 6 atomic layers deposited on a 12 layer thick Cu film playing the role of substrate. As a first step, we allow the thin film and surface layers to atomically relax. We found important contractions of about 20% between the first and second, the third and fourth, and firth and sixth Fe layers. Then, we obtain collinear and non-collinear magnetic arrangements which agree with the main experimental resultssatte that show a non-collinear configuration at low temperatures (around 40 K). The magnetic properties of the first four Fe layers are well reproduced. The agreement is better than the results obtained ignoring the spin–orbit coupling and the structure relaxation. Although both calculations predict non-collinear configurations with energy values slightly above that of a collinear ground state. In some cases, the energy differences between two collinear (or non-collinear) solutions with different spin orientation axis is only under the value of one meV. The calculated orbital local moment arrangements are clearly similar to the spin magnetic local moment ones and show a correct dependence on the dimensionality aspects of the system when changing the spin axis quantization direction.

Pnma metal hydride system LiBH: a superior topological semimetal with the coexistence of twofold and quadruple degenerate topological nodal lines

To date, a handful of topological semimetals (TMs) with multiple types of topological nodal line (TNL) states have been theoretically predicted in novel materials. However, their TNLs are often affected by many factors, such as spin–orbit coupling (SOC) effect, strain, and the extraneous bands near the band crossing points, and therefore, the TNL states cannot be easily verified by experiments. Here, by using first-principles calculations, we report that the Pnma LiBH is a potential TM with twofold and quadruple degenerate topological nodal lines. These TNLs situate very close to the Fermi level, and do not coexist with other extraneous bands. More importantly, the TNLs of this material are very robust to the effect of SOC, uniform strain, and biaxial strain. The nontrivial band structure in LiBH produces drum-head-like surface states in the (001) surface projection. Our result reveals that LiBH material is an excellent candidate to study the multiple kinds of TNLs.

A novel antiferromagnetic semiconductor hidden in pyrite

In this paper, a new two-dimensional antiferromagnetic material, 2D pyrite, was reported, which exhibited a completely planar pentagonal structure. Pentagonal FeS2 were revealed by the first-principles calculation with GGA + U (Ueff = 2 eV) method and proved excellent mechanically stable, dynamically stable and thermally stable. Intriguingly, this single-layer structure was perfectly in line with the geometric characteristic of “Cairo pentagon”. Completely different from the bulk pyrite, the two-dimensional FeS2 was proved an amazing magnetic semiconductor and the ground state configuration of the magnetic coupling is antiferromagnetic with a 0.31 eV band gap. Considering the spin–orbit coupling effect, the magnetocrystalline anisotropy energy (MAE) in different orientation was calculated and the easy axis was proved along the c axis. The highest MAE in our GGA + U calculation could reach 454.77 μeV per Fe atom. Furthermore, biaxial strain was applied to regulate the electronic structure of 2D FeS2 and the results suggested that 2D pyrite would be converted into ferromagnetic configuration when the strain ranging from 2.2% to 5.0%. Therefore, our research predicted a new two-dimensional spintronic materials and proved a new thought for the application of pyrite.