Yz Orbitals Research Articles

Electronic and optical properties of ternary kagome Rb2Ni3S4: a density functional study

The application of semiconductors with optical properties has grown significantly in the development of semiconductor photovoltaics. Here, we explore the electronic and optical properties of ternary transition metal sulfide Rb2Ni3S4 by means of density functional theory. From the structural perspective, Ni atoms are found to form a kagome-like lattice in a two-dimensional plane of Rb2Ni3S4. From our calculations, Rb2Ni3S4 is found to be a semiconductor with an indirect band gap of ∼0.67 eV. Strong hybridization was observed between the S-3p with the Ni-3d xz and Ni-3d yz orbitals. Interestingly, a flat band was noticed below the Fermi level demonstrating one significant feature of kagome lattice. From the optical calculations, Rb2Ni3S4 is found to exhibit optical activity in both the visible and lower ultraviolet regions of the incident photon energies. The optical response suggests this material may be a potential candidate for opto-electronic device, given its ability to interact with light across a broad range of wavelengths. This work is expected to motivate the experimental group for transport measurements and may provide a new foundation in optics.

Semimetallic spin-density wave state in iron pnictides

We examine the existence of semimetallic spin-density wave (SDW) states in iron pnictides. In the experimentally observed metallic SDW state, the symmetry-protected Dirac cones are located away from the Fermi surface giving rise to tiny pockets and there are also additional Fermi pockets such as one around Γ. We find that the location of a pair of Dirac points with respect to the Fermi surface exhibits significant sensitivity to the orbital splitting between the d xz and d yz orbitals. Besides, in the presence of orbital splitting, the Fermi pockets not associated with the Dirac cones, can be suppressed so that a semimetallic SDW state can be realized. We explain these findings in terms of difference in the slopes and orbital contents of the bands which form the Dirac cone, and obtain the necessary conditions dependent on these two and other parameters for the coexisting Dirac semimetallic and SDW states. Additionally, the topologically protected edge states are studied in the ribbon geometry when the same are oriented either along x or y axes.

Giant and strain-tunable interfacial magnetic anisotropy in MgO-based magnetic heterostructures with heavy atoms insertion

MgO-based magnetic heterostructures with interfacial magnetic anisotropy has attracted increasing attention due to its application in building high-density magnetic random access memories. A large and tunable interfacial magnetic anisotropy constant (Ki) is required for high thermal stability and flexible data writability. In this study, the Ki of Fe/MgO, Fe/Pt/MgO, and Fe/Ir/MgO heterostructures with strains from −4.5% to 4.5% were calculated by ab initio electronic structure calculations. It has been found that the Fe/Pt/MgO and Fe/Ir/MgO where the Pt and Ir monolayers are inserted in the interface possess Ki of 2.415 mJ m−2 and −4.468 mJ m−2, which are much larger by several times than that (0.840 mJ m−2) of the Fe/MgO. In particular, the out-of-plane Ki from the interfacial Pt atoms in Fe/Pt/MgO is as high as 5.978 mJ m−2. The magnetic anisotropy of these structures can be significantly manipulated by strain. Combining second-order perturbation theory, the origin of these behaviors has been analyzed by layer-resolved, orbital-resolved, and k-resolved Ki. The spin-flip terms of d z 2/d yz orbitals in the interfacial layer are mainly responsible for the out-of-plane Ki and its variation with strain. This work provides a useful guide for the design of high and tunable magnetic anisotropy in the MgO-based magnetic heterostructures.

Manipulation of intrinsic quantum anomalous Hall effect in two-dimensional MoYN2CSCl MXene

Quantum anomalous Hall effect (QAHE) is an innovative topological spintronic phenomenon with dissipationless chiral edge states and attracts rapidly increasing attention. However, it has only been observed in few materials in experiments. Here, according to the first-principles calculations, we report that the MXene MoYN2CSCl shows a topologically nontrivial band gap of 37.3 meV, possessing QAHE with a Chern number of C = 1, which is induced by band inversion between d xz and d yz orbitals. Also, the topological phase transition for the MoYN2CSCl can be realized via strain or by turning the magnetization direction. Remarkably, MoYN2CSCl shows the nodal-line semimetal state dependent on the electron correlation U. Our findings add an experimentally accessible and tunable member to the QAHE family, which stands a chance of enriching the applications in spintronics.

Study of the transport properties of cobalt atomic contact under mechanical strain in a nitrogen atmosphere

Achieving an efficient spin-filter effect and large magneto-resistance in molecular junctions is very important in spintronics. By employing the non-equilibrium Green's function formalism combined with the density functional theory, this study investigated the atomic structures and spin-resolved transport properties of molecular junctions consisting of nitrogen (N2) sandwiched between two cobalt (Co) electrodes with different crystal lattices as a function of stretching distance. In the case of face-centred cubic Co electrodes, our calculations showed that the transport properties depend strongly on the stretching process, which changes the contact between the N2 and Co electrodes. Specifically, the maximum spin polarization is due to the configuration of N2 molecules sitting in parallel with the protruding Co atoms. Furthermore, the Co dxz,yz orbitals interact with the N px,y orbitals, which contribute to the high transmission coefficient at the Fermi level for spin-down electrons. Meanwhile, the Co s-dz2 hybridization orbitals, which interact weakly with the N p orbitals, contribute to the small transmission coefficient at the Fermi level. This can also be verified by the molecular junction model adopting simple monatomic Co chain electrodes, which also show a perfect spin-filter effect and achieve an ideal magnetoresistance. In contrast, in the case of hexagonal Co electrodes, the spin-filter effect disappears. However, the effect appears again by adding one more Co atom to the central point contact. Our findings show that the spin-resolved electronic transport of molecular junctions depends on the stretching process and the nature of the electrodes, which can help in the future design of molecular devices based on the spin degree of freedom.

Structural, electronic and magnetic properties of weakly correlated metal Sr2CrTiO6: a first principles study

Structural, electronic and magnetic behaviour of a less-explored material Sr2CrTiO6 has been investigated using ab initio calculations with generalized gradient approximation (GGA) and GGA + U methods, where U is the Hubbard parameter. For each of the three possible Cr/Ti cationic arrangements in the unit cell, considered in this work, the non-magnetic band structure shows distinct traits with significant flat-band regions leading to large t 2g density of states around the Fermi energy. The Cr4+ ion in Sr2CrTiO6, which is a d 2 system, shows a reverse splitting of the t 2g orbitals. The calculated hopping matrix contains non-zero off-diagonal elements between the d xz and d yz orbitals, while the d xy orbitals remain largely unmixed. A minimal tight binding model successfully reproduces the six t 2g bands around the Fermi energy. The Fermi surface shows a two-dimensional nesting feature for the layered arrangement of Cr and Ti atoms. Fixed spin moment studies suggest that the magnetism in this compound cannot be explained by Stoner’s criterion of an itinerant band ferromagnet. In the absence of Hubbard U term, the ground state is a half-metallic ferromagnet. Calculations for the anti-ferromagnetic spin arrangement show re-arrangement of orbital character resulting in (a) narrow d xz /d yz bands and sharp peaks in the density of states at the Fermi energy and (b) highly dispersive d xy bands with a broader density of states around the Fermi energy. The metallicity persists even with increasing U for both the spin arrangements, thus suggesting that Sr2CrTiO6 belongs to the class of weakly correlated metals, while the shifting of O 2p states towards the Fermi energy with U indicates a negative charge-transfer character in Sr2CrTiO6.

Revelation of Mott insulating state in layered honeycomb lattice Li2RuO3

We investigate the role of electron correlation in the electronic structure of honeycomb lattice Li2RuO3 using photoemission spectroscopy and band structure calculations. Monoclinic Li2RuO3 having Ru network as honeycomb lattice undergoes magneto-structural transition at T c ∼ 540 K from high temperature phase C2/m to low temperature dimerized phase P21/m. Room temperature valence band photoemission spectra reveal an insulating ground state with no intensity at Fermi level (E F). Ru 4d band extracted from high and low photon energy valence band photoemission spectra reveal that the surface and bulk electronic structures are very similar in this system. Band structure calculations using generalized gradient approximation leads to metallic ground state while screened hybrid (YS-PBE0) functional reveals opening up of a gap in almost degenerate d zx /d yz orbitals, whereas d xy orbital is already gapped. Ru 3d core level spectra with prominent unscreened feature provides direct evidence of strong electron correlation among Ru 4d electrons which is also manifested by |E − E F|2 dependence of spectral density of states in the vicinity of E F in the high-resolution spectra, establishing Li2RuO3 as Mott insulator.

Orbital selective dynamics in Fe-pnictides triggered by polarized pump pulse excitations

Quantum materials display exotic behaviours related to the interplay between temperature-driven phase transitions. Here, we study the electron dynamics in one such material, CaFe2As2, a parent Fe-based superconductor, employing time- and angle-resolved photoemission spectroscopy. CaFe2As2 exhibits concomitant transition to spin density wave state and tetragonal to orthorhombic structure below 170 K. The Fermi surface of this material consists of three hole pockets , β and around the Γ-point and two electron pockets around the X-point. The hole pockets have d xy , d yz and d zx orbital symmetries. The β band constituted by d xz /d yz orbitals exhibits a gap across the magnetic phase transition. We discover that polarized pump pulses can induce excitations of electrons of a selected symmetry. More specifically, while s-polarized light (polarization vector perpendicular to the xz plane) excites electrons corresponding to all the three hole bands, p-polarized light excites electrons essentially from , bands which are responsible for magnetic order. Interestingly, within the magnetically ordered phase, the excitation due to the p-polarized pump pulses occur at a time scale of 50 fs, which is significantly faster than the excitation induced by s-polarized light (∼200 fs). These results suggest that the relaxation of different ordered phases occurs at different time scales and this method can be used to achieve selective excitations to disentangle complexity in the study of quantum materials.

Mössbauer spectroscopy study of nematicity in Ba(Fe0.962Cu0.038)2As2 single crystal: enhanced orbital effect

The origin of the nematic order remains unclear due to the strong coupling between orbital, spin and lattice degrees of freedom in iron-based superconductors. Although the driving force of hole-doped BeFe2As2 is still controversial, the nematic fluctuation of electron-doped compounds is generally believed to be spin fluctuation driven. Here, we present a comprehensive study of the nematic phase transition in Ba(Fe0.962Cu0.038)2As2 single crystal by using Mössbauer spectroscopy. The electric field gradient and its in-plane asymmetry on Fe nucleus, which are directly determined by the occupation of individual t 2g orbital, are sensitive to the local nematicity of Fe ions. The nematic phase transition happens at T nem ≈ 73.8 K in the compound while the band splitting between d xz /d yz orbitals begins far above T nem and reaches 18.8 meV at 30 K. The temperature evolution of the hyperfine parameters proves the existence of electron–phonon interaction and non-Fermi-liquid behaviour near T nem. However, the spin–lattice relaxation signal is only evident below T nem. These observations show that the role of orbital degrees of freedom is more active in driving nematicity than in Co- or Ni-doped BaFe2As2 compounds, and can be attributed to enhanced electronic localization caused by Cu doping.

Spin resonances in iron selenide high- Tc superconductors by proximity to a hidden spin density wave

Recent inelastic neutron scattering studies by Pan et al., Nature Communications 8, 123 (2017), find evidence for spin excitations at energies above the quasi-particle gap in an iron-selenide high-Tc superconductor. The momenta of the spin excitations form a diamond around the checkerboard wavevector, Q_AF, that is associated with the square lattice of iron atoms that makes up the system. It has been suggested that such a "hollowed-out" spin-excitation spectrum is due to hidden Neel order. We study such a hidden spin-density wave (hSDW) state that results from nested Fermi surfaces at the center and at the corner of the unfolded Brillouin zone. It emerges within mean field theory from an extended Hubbard model over a square lattice of iron atoms that contain the minimal d_xz and d_yz orbitals. Opposing Neel order exists over the isotropic d+ = d_xz + i d_yz and d- = d_xz - i d_yz orbitals. The dynamical spin susceptibility of the hSDW is computed within the random phase approximation, at perfect nesting. Unobservable Goldstone modes that disperse acoustically are found at Q_AF. A threshold is found in the spectrum of observable spin excitations that forms a "floating ring" at Q_AF also. The ring threshold moves down in energy toward zero with increasing Hund's Rule coupling, while it moves up in energy with increasing magnetic frustration. Comparison with the normal-state features of the spin-excitation spectrum shown by electron-doped iron selenide is made. Also, recent predictions of a Lifshitz transition from the nested Fermi surfaces to Fermi surface pockets at the corner of the folded Brillouin zone will be discussed.

Crystalline anisotropy associated with a second antiferromagnetic phase in the absence of spin density wave in the heavily hydrogen-doped LaFeAsO1−xHx (x∼0.5)

Electronic and magnetic properties of the heavily H-doped LaFeAsO1−xHx (x∼0.5) were studied in the framework of the density functional theory combined with the dynamical mean field theory. We found a stripe-like-ordered structure of hydrogen and oxygen atoms, as the ground state, in a 2 a×2b×c supercell, with the same configuration as the antiferromagnetic order. The new configuration could explain the existing experimental results related to the heavily H-doped LaFeAsO1−xHx, such as the in-plane electronic anisotropy and the nonuniform magnetic behavior. A significant anisotropy was observed between Fe-3dxz (xz) and Fe-3dyz (yz) orbitals in the ground state in the absence of the pseudogap resulting from the spin density wave phase, which was found to originate from the crystalline anisotropy. Magnetic moments were not spatially uniform and were sensitive to the crystal configuration. We found that the nonuniform magnetic behavior is associated with the As–Fe–As bond angle in the structure. Our findings would clarify the importance of crystal details and orbital degrees of freedom in iron-based superconductors.

Electric field induced modulation to the magnetic anisotropy of Fe/silicene heterostructures: First-principles study

Abstract In view of the importance of inducing magnetism and large perpendicular magnetic anisotropy in silicene, the influence of an external electric field on the magnetism and the magnetic anisotropy of the Fe(15, 18, 21 monolayers)/silicene heterostructures are investigated by using first-principles calculations. The results show that the magnetic moments and the magnetic anisotropy of the Fe/silicene heterostructures are mainly from Fe films. With increasing the thickness of Fe film, the easy magnetization axis of the Fe/silicene heterostructure changes from out-of-plane to in-plane direction. In addition, the magnetic moments in all the Fe/silicene heterostructures are insensitive to the electric field. Particularly, under the positive electric field, the perpendicular magnetic anisotropy of the Fe(15 monolayers)/silicene can be enhanced, the magnetic anisotropy of the Fe(18 monolayers)/silicene increases from in-plane magnetic anisotropy to perpendicular magnetic anisotropy, and the magnetic anisotropy of the Fe(21 monolayers)/silicene also increases although its easy magnetization axis is still along in-plane direction. The mechanism of the electric-field-driven modulation of magnetic anisotropy is mainly from the increase of the positive contribution from spin-orbit coupling interaction between opposite spin d z 2 and d yz orbitals of the second Fe layer from the Fe/silicene interface. Our study reveals that the perpendicular magnetic anisotropy can be achieved or enhanced by applying positive electric field if the thickness of Fe film in Fe/silicene heterostructure is suitable.

Orbital Selectivity Enhanced by Nematic Order in FeSe.

Motivated by the recent low-temperature experiments on bulk FeSe, we study the electron correlation effects in a multiorbital model for this compound in the nematic phase using the U(1) slave-spin theory. We find that a finite nematic order helps to stabilize an orbital selective Mott phase. Moreover, we propose that when the d- and s-wave bond nematic orders are combined with the ferro-orbital order, there exists a surprisingly large orbital selectivity between the xz and yz orbitals even though the associated band splitting is relatively small. Our results explain the seemingly unusual observation of strong orbital selectivity in the nematic phase of FeSe, uncover new clues on the nature of the nematic order, and set the stage to elucidate the interplay between superconductivity and nematicity in iron-based superconductors.

Spin-orbit coupling induced magnetic anisotropy and large spin wave gap in NaOsO3

The role of spin–orbit coupling and Hund’s rule coupling on magnetic ordering, anisotropy, and excitations are investigated within a minimal three-orbital model for the 5d3 compound NaOsO3. Small asymmetry between the magnetic moments for the xy and xz, yz orbitals, arising from the hopping asymmetry generated by OsO6 octahedral tilting and rotation, together with the weak correlation effect, are shown to be crucial for the large SOC induced magnetic anisotropy and spin wave gap observed in this compound. Due to the intrinsic SOC-induced changes in the electronic densities under rotation of the staggered field, their coupling with the orbital energy offset is also found to contribute significantly to the magnetic anisotropy energy.

Determination of d-Orbital Populations in a Cobalt(II) Single-Molecule Magnet Using Single-Crystal X-ray Diffraction.

The tetrahedral cobalt(II) compound (Ph4P)2[Co(SPh)4] was the first mononuclear transition-metal complex shown to exhibit slow relaxation of the magnetization in zero external magnetic field. Because the relative populations of the d orbitals play a vital role in dictating the magnitude of the magnetic anisotropy, the magnetic behavior of this complex is directly related to its electronic structure, yet the exact role of the soft, thiophenolate ligands in influencing the d-electron configuration has previously only been investigated via theoretical methods. To provide detailed experimental insight into the effect of this ligand field, the electron density distribution in this compound was determined from low-temperature, single-crystal X-ray diffraction data and subsequent multipole modeling. Topological analysis of the electron density indicates significant covalent contributions to the cobalt-sulfur bonds. The derived d-orbital populations further reveal a fully occupied d z2 orbital, minor d xz orbital population, and nearly equal population of the d xy, d x2- y2, and d yz orbitals. Notably, we find that an electrostatic interaction between Co(II) and one hydrogen atom from a thiophenolate group in the xz plane increases the energy of the d x2- y2 orbital, leading to the nearly equal population with d xy and strong magnetic anisotropy.

Nematicity, magnetism, and superconductivity in FeSe under pressure: Unified explanation based on the self-consistent vertex correction theory

To understand the rich electronic phase diagram in FeSe under pressure that vividly demonstrates the strong interplay between the nematicity, magnetism and superconductivity, we analyze the electronic states by including the higher-order many-body effects called the vertex correction (VC). We predict the pressure-induced emergence of xy-orbital hole-pocket based on the first-principles analysis. Due to this pressure-induced Lifshitz transition, the spin fluctuations on the xy orbital are enhanced, whereas those on xz,yz orbitals are gradually reduced. For this reason, nonmagnetic orbital order $O = n_{xz} - n_{yz}$, which is driven by the spin fluctuations on xz,yz orbitals through the intra-orbital VCs, is suppressed, and it is replaced with the magnetism of xy-orbital d-electrons. The nodal s-wave state at ambient pressure ($O > 0$) and the enhancement of Tc under pressure are driven by the cooperation between the spin and orbital fluctuations.

Model of orbital populations for voltage-controlled magnetic anisotropy in transition-metal thin films

Voltage controlled magnetic anisotropy (VCMA) is an efficient way to manipulate the magnetization states in nanomagnets, promising for low-power spintronic applications. The underlying physical mechanism for VCMA is known to involve a change in the d-orbital occupation on the transition metal interface atoms with an applied electric field. However, a simple qualitative picture of how this occupation controls the magnetocrystalline anisotropy (MCA) and even why in certain cases the MCA has opposite sign still remains elusive. In this paper, we exploit a simple model of orbital populations to elucidate a number of features typical for the interface MCA and the effect of electric field on it, for 3d transition metal thin films used in magnetic tunnel junctions. We find that in all considered cases including the Fe (001) surface, clean Fe1-xCox(001)/MgO interface and oxidized Fe(001)/MgO interface, the effects of alloying and electric field enhance the MCA energy with electron depletion which is largely explained by the occupancy of the minority-spin dxz,yz orbitals. On the other hand, the hole doped Fe(001) exhibits an inverse VCMA, where the MCA enhancement is achieved when electrons are accumulated at the Fe (001)/MgO interface with applied electric field. In this regime we predict a significantly enhanced VCMA which exceeds 1pJ/Vm. Realizing this regime experimentally may be favorable for a practical purpose of voltage driven magnetization reversal.

Two-dimensional electron systems in ATiO3 perovskites ( A=Ca , Ba, Sr): Control of orbital hybridization and energy order

We report the existence of a two-dimensional electron system (2DES) at the (001) surface of CaTiO3. Using angle-resolved photoemission spectroscopy, we find a hybridization between the d_xz and d_yz orbitals, not observed in the 2DESs at the surfaces of other ATiO3 perovskites, e.g. SrTiO3 or BaTiO3. Based on a comparison of the 2DES properties in these three materials, we show how the electronic structure of the 2DES (bandwidth, orbital order and electron density) is coupled to different typical lattice distortions in perovskites. The orbital hybridization in orthorhombic CaTiO3 results from the rotation of the oxygen octahedra, which can also occur at the interface of oxide heterostructures to compensate strain. More generally, the control of the orbital order in 2DES by choosing different A-site cations in perovskites offers a new gateway towards 2DESs in oxide heterostructures beyond SrTiO3.

Nematicity at the Hund's metal crossover in iron superconductors

The theoretical understanding of the nematic state of iron-based superconductors and especially of FeSe is still a puzzling problem. Although a number of experiments calls for a prominent role of local correlations and place iron superconductors at the entrance of a Hund metal state, the effect of the electronic correlations on the nematic state has been theoretically poorly investigated. In this work we study the nematic phase of iron superconductors accounting for local correlations, including the effect of the Hund's coupling. We show that Hund's physics strongly affects the nematic properties of the system. It severely constraints the precise nature of the feasible orbital-ordered state and induces a differentiation in the effective masses of the zx=yz orbitals in the nematic phase. The latter effect leads to distinctive signatures in different experimental probes, so far overlooked in the interpretation of experiments. As notable examples the splittings between zx and yz bands at Gamma and M points are modified, with important consequences for ARPES measurements.

Isotropic Cooper pairs with emergent sign changes in a single-layer iron superconductor

We model a single layer of heavily electron-doped FeSe by spin-1/2 moments over a square lattice of iron atoms that include the 3d xz and 3d yz orbitals, at strong on-site Coulomb repulsion. Above half filling, we find emergent hole bands below the Fermi level at the center of the one-iron Brillouin zone in a half metal state characterized by hidden magnetic order and by electron-type Fermi surface pockets at wavenumbers that double the unit cell along the principal axes. "Replicas" of the emergent hole bands exist at lower energy in the two-iron Brillouin zone. Exact calculations with two mobile electrons find evidence for isotropic Cooper pairs that alternate in sign between the electron bands and the emergent hole bands.