3d Electronic States Research Articles

Vertical transport in type-II heterojunctions with InAs/GaSb/AlSb composite quantum wells in a high magnetic field

Vertical transport in type-II heterojunctions with a two-barrier AlSb/InAs/GaSb/AlSb quantum well (QW) grown by MOVPE on an n-InAs (100) substrate is investigated in quantizing magnetic fields up to B = 14 T at low temperatures T = 1.5 and 4.2 K. The width of the QWs is selected from the formation condition of the inverted band structure. Shubnikov–de Haas oscillations are measured at two orientations of the magnetic field (perpendicular and parallel) relative to the structure plane. It is established that conduction in the structure under study is occurs via both three-dimensional (3D) substrate electrons and two-dimensional 2D QW electrons under quantum limit conditions for bulk electrons (B > 5 T). The electron concentrations in the substrate and InAs QW are determined. The g-factor for 3D carriers is determined by spin splitting of the zero Landau level. It is shown that the conductance maxima in a magnetic field perpendicular to the structure plane and parallel to the current across the structure in fields B > 9 T correspond to the resonant tunneling of 3D electrons from the emitter substrate into the InAs QW through the 2D electron states of the Landau levels.

Intrinsic Ferromagnetism in Mn-Substituted MoS2 Nanosheets Achieved by Supercritical Hydrothermal Reaction.

Doping atomically thick nanosheets is a great challenge due to the self-purification effect that drives the precipitation of dopants. Here, a breakthrough is made to dope Mn atoms substitutionally into MoS2 nanosheets in a sulfur-rich supercritical hydrothermal reaction environment, where the formation energy of Mn substituting for Mo sites in MoS2 is significantly reduced to overcome the self-purification effect. The substitutional Mn doping is convincingly evidenced by high-angle annular dark-field scanning transmission electron microscopy and X-ray absorption fine spectroscopy characterizations. The Mn-doped MoS2 nanosheets show robust intrinsic ferromagnetic response with a saturation magnetic moment of 0.05 µB Mn-1 at room temperature. The intrinsic ferromagnetism is further confirmed by the reversibility of the magnetic behavior during the cycle of incorporating/removing Li codopants, showing the critical role of Mn 3d electronic states in mediating the magnetic interactions in MoS2 nanosheets.

Branching ratio study of resonant X-ray scattering intensities of GdB4

Resonant X-ray scattering measurements for a GdB4 single crystal have been carried out at Gd L 3- and L 2-edges. Branching ratios between x-ray scattering intensities at two x-ray energies are different for resonance peaks. Their analysis shows different anisotropic characters of 5d electron states of Gd ions corresponding to the peaks.

Local 3dElectronic Structures of Co-Based Complexes with Medicinal Molecules Probed by Soft X-ray Absorption

We have examined the local 3d electronic structures of Co-Au multinuclear complexes with the medicinal molecules D-penicillaminate (D-pen) [Co{Au(PPh3)(D-pen)}2]ClO4 and [Co3{Au3(tdme)(D-pen)3}2] by Co L_2,3-edge soft X-ray absorption (XAS) spectroscopy, where PPh3 denotes triphenylphosphine and tdme stands for 1,1,1-tris[(diphenylphosphino)methyl]ethane. The Co L_2,3-edge XAS spectra indicate the localized ionic 3d electronic states in both materials. The experimental spectra are well explained by spectral simulation for a localized Co ion under ligand fields with the full multiplet theory, which verifies that the ions are in the low-spin Co3+ state in the former compound and in the high-spin Co2+ state in the latter.

Enabling accurate first-principle calculations of electronic properties with a corrected [formula omitted] scheme

A computationally inexpensive k·p-based interpolation scheme is developed that can extend the eigenvalues and momentum matrix elements of a sparsely sampled k-point grid into a densely sampled one. Dense sampling, often required to accurately describe transport and optical properties of bulk materials, can be demanding to compute, for instance, in combination with hybrid functionals in density functional theory (DFT) or with perturbative expansions beyond DFT such as the GW method. The scheme is based on solving the k·p method and extrapolating from multiple reference k-points. It includes a correction term that reduces the number of empty bands needed and ameliorates band discontinuities. We show how the scheme can be used to generate accurate band structures, density of states, and dielectric functions. Several examples are given, using traditional and hybrid functionals, with Si, TiNiSn, and Cu as model materials. We illustrate that d-electron and semi-core states, which are particular challenging for the k·p method, can be handled with the correction scheme if the sparse grid is not too sparse.

Multiferroic properties of Ba/Ni co-doped KNbO3 with narrow band-gap

In this paper, the influence of Ba/Ni co-doping on the multiferroic properties of (K1−xBax)(Nb1−x/2Nix/2)O3−δ was investigated. A gradual transition from orthorhombic to tetragonal, and to cubic phase with increasing the doping ratio x was observed in the single phase perovskite structure at room temperature. The weakened antiferromagnetism and high ferroelectric polarization with narrow optical band-gaps (1.18–1.84 eV) for x = 0.1–0.3 were observed. Based on the first-principles calculations with GGA+U, we confirm that the Ni: 3d electronic states have a major contribution to the variation of band-gap size and the antiferromagnetic properties. Our results may shed some light on the understanding of the band-gap tuning and multiferroic mechanism in perovskite-type materials.

A new-type inorganic [KNbO3]0.9[BaCo1/2Nb1/2O3-δ]0.1perovskite oxide as sensitizer for photovoltaic cell

An electrode with [KNbO3]0.9[BaCo1/2Nb1/2O3–δ]0.1 (KBCNO) perovskite oxide as sensitizer and well-aligned TiO2 nanorod arrays is prepared via conventional standard solid-state synthesis method, followed by pulsed laser deposition technique for the first time. Enhanced absorption in visible light region is observed for KBCNO@TiO2 NRs photoelectrode, which is mainly because the inserted Co 3d electronic states can hybridize with O 2p states in the gap of KBCNO, reducing the band gap to 1.90 eV. Additionally, the oxygen vacancies existing in KBCNO can not only serve as photoinduced charge traps and adsorption sites but also prevent recombination of photoinduced electron–hole, giving rise to enhanced separation of photogenerated electron–hole pair and leading to an improved performance in solar energy conversion. The KBCNO@TiO2 NRs photoelectrode exhibits an energy conversion efficiency of 0.183% under one sun illumination. This work provide further insight for improving the efficiency of utilization of solar energy by using a new composite perovskite oxides material, which may be promising rational categories of material for solar conversion and storage devices.

Composition dependence of dielectric and optical properties of Hf-Zr-silicate thin films grown on Si(100) by atomic layer deposition

Composition dependence of the dielectric and optical properties of (HfZrO4)(1−x) (SiO2)(x) (0≤x≤0.2) gate dielectric thin films, grown on Si(100) by the atomic layer deposition method, was investigated by means of reflection electron energy loss spectroscopy (REELS). The quantitative analysis of REELS spectra was carried out by using the QUASES-XS-REELS and QUEELS-ε(k,ω)-REELS software, which determine the dielectric function and optical properties through an analysis of experimental REELS in terms of a simulated energy loss function (ELF). For HfZrO4, the ELF showed peaks in the vicinity of 10.5, 15.6, 18, 21.5, 26.7, 34.5, 39.5, 46.5 and 57eV. For HfZr-silicates with low SiO2 concentration (x=0.10 and 0.15), the peak positions were similar to those of HfZrO4, but for x=0.20, the number of peaks were reduced and are at 14.8, 23.5, 34.5, 40.5, and 46.5eV. These peaks originate from the superposition among d electron states of Zr, f electron states of Hf and p electron states of Si. The strength of the peak at 46.5eV decreased as we increased the amount of SiO2 in the compounds, which indicates that it is due to excitation between Hf 4f and Zr 3d electron states. Changes in the complex dielectric function, optical properties and band gap related to the SiO2 concentration in the films were discussed systematically. In addition, the inelastic mean free path (IMFP) was also calculated from the determined dielectric function. The IMFP of the HfZr-silicates increased with increasing SiO2 content and with increasing primary energy. This method is of high importance in terms of determining the dielectric and optical properties and inelastic mean free path from REELS spectra. The advantage of the method applied here is that no information on the morphology and components of the sample is required. This means that this method presented here turned out to be a convenient and efficient tool for investigation of optical properties and inelastic mean free path of ultrathin high-k alloy materials.

Induced magnetism at the interfaces of a Fe/V superlattice investigated by resonant magnetic x-ray scattering

The induced magnetic moments in the V 3d electronic states of interface atomic layers in a Fe(6ML)/V(7ML) superlattice was investigated by x-ray resonant magnetic scattering. The first V atomic layer next to Fe was found to be strongly antiferromagnetically polarized relatively to Fe and the magnetic moments of the next few atomic layers in the interior V region decay exponentially with increasing distance from the interface, while the magnetic moments of the Fe atomic layers largely remain bulk-like. The induced V moments decay more rapidly as observed by x-ray magnetic scattering than in standard x-ray magnetic circular dichroism. The theoretical description of the induced magnetic atomic layer profile in V was found to strongly rely on the interface roughness within the superlattice period. These results provide new insight into interface magnetism by taking advantage of the enhanced depth sensitivity to the magnetic profile over a certain resonant energy bandwidth in the vicinity of the Bragg angles.

Band gap tuning of amorphous Al oxides by Zr alloying

The optical band gap and electronic structure of amorphous Al-Zr mixed oxides with Zr content ranging from 4.8 to 21.9% were determined using vacuum ultraviolet and X-ray absorption spectroscopy. The light scattering by the nano-porous structure of alumina at low wavelengths was estimated based on the Mie scattering theory. The dependence of the optical band gap of the Al-Zr mixed oxides on the Zr content deviates from linearity and decreases from 7.3 eV for pure anodized Al2O3 to 6.45 eV for Al-Zr mixed oxides with a Zr content of 21.9%. With increasing Zr content, the conduction band minimum changes non-linearly as well. Fitting of the energy band gap values resulted in a bowing parameter of ∼2 eV. The band gap bowing of the mixed oxides is assigned to the presence of the Zr d-electron states localized below the conduction band minimum of anodized Al2O3.

Tunable Plasmonic Reflection by Bound 1D Electron States in a 2D Dirac Metal.

We show that the surface plasmons of a two-dimensional Dirac metal such as graphene can be reflected by linelike perturbations hosting one-dimensional electron states. The reflection originates from a strong enhancement of the local optical conductivity caused by optical transitions involving these bound states. We propose that the bound states can be systematically created, controlled, and liquidated by an ultranarrow electrostatic gate. Using infrared nanoimaging, we obtain experimental evidence for the locally enhanced conductivity of graphene induced by a carbon nanotube gate, which supports this theoretical concept.

Absence of low energy magnetic spin-fluctuations in isovalently and aliovalently doped LaCo2B2 superconducting compounds

Magnetization, resistivity and 11B, 59Co NMR measurements have been performed on the Pauli paramagnet , and the superconductors ( K) and ( K). The site selective NMR experiment reveals the multiband nature of the Fermi surface in these systems. The temperature independent Knight shift and 1/T1T clearly indicate the absence of correlated low energy magnetic spin-fluctuations in the normal state, which is in contrast to other Fe-based pnictides. The density of states (DOS) of Co 3d electrons has been enhanced in superconducting and with respect to the non superconducting reference compound . The occurrence of superconductivity is related to the DOS enhancement.

Electronic-Structure Analyses of Li-Ion-Battery Electrodes By Soft X-Ray Absorption/Emission Spectroscopy Including Operando experiments

Improving the energy density and power density of electrode materials for Li-ion batteries (LIBs) is highly important to further develop electric and hybrid-electric vehicles. For the improvements, understanding the charge-discharge mechanisms from a viewpoint of the electronic structure is indispensable. Recently, electronic-structure analyses of the electrode materials using soft x-ray spectroscopy have been of particular importance.In this study we demonstrate operando Mn L 3 soft x-ray emission spectroscopy (XES) for LiMn2O4 [1] which is a typical cathode material of LIB. We developed an operando cell consisting of the LiMn2O4 cathode, a counter electrode and an electrolyte solution [2] by modifying the in situ cell for fuel cell catalysts [3]. The charge-discharge experiments were performed by cyclic voltammetry. The operando XES experiments were carried out using ultrahigh-resolution x-ray emission spectrometer [4] at BL07LSU of SPring-8. The XES spectra were analyzed by theoretical analyses based on the configuration-interaction full-multiplet (CIFM) calculation [5-7].In the operando Mn L 3 XES study for LiMn2O4, we revealed that the open-circuit voltage (OCV) state is almost the same as the initial state consisting of the Mn3+ and Mn4+ states. For the charged state, the Mn L 3 XES spectrum largely changed corresponding to the oxidation of Mn3+ to Mn4+ state at the Mn3+ site for the OCV. The spectrum for the discharged state almost returned to the spectrum for the OCV, while the small difference indicates that the Mn3+ state is slightly enhanced for the discharged state. Thus, the Mn 3d electronic states were reversibly changed for the charge-discharge process [2]. Moreover, charge-transfer (CT) effects between the Mn 3d and O 2p orbitals for each valence state were clarified. For the Mn4+ state, a negative CT energy was determined by the CIFM calculation, suggesting a very strong CT effect from the O 2p to Mn 3d orbitals. In the presentation, XES and soft X-ray absorption spectroscopy studies for other electrode materials will also be reported, and the relationship between the electronic structure and electrochemical performance will be discussed.

Quasi 2D electronic states with high spin-polarization in centrosymmetric MoS2 bulk crystals.

Time reversal dictates that nonmagnetic, centrosymmetric crystals cannot be spin-polarized as a whole. However, it has been recently shown that the electronic structure in these crystals can in fact show regions of high spin-polarization, as long as it is probed locally in real and in reciprocal space. In this article we present the first observation of this type of compensated polarization in MoS2 bulk crystals. Using spin- and angle-resolved photoemission spectroscopy (ARPES), we directly observed a spin-polarization of more than 65% for distinct valleys in the electronic band structure. By additionally evaluating the probing depth of our method, we find that these valence band states at the point in the Brillouin zone are close to fully polarized for the individual atomic trilayers of MoS2, which is confirmed by our density functional theory calculations. Furthermore, we show that this spin-layer locking leads to the observation of highly spin-polarized bands in ARPES since these states are almost completely confined within two dimensions. Our findings prove that these highly desired properties of MoS2 can be accessed without thinning it down to the monolayer limit.

Magnetism, electronic structure and optical properties of TbNiGe2

TbNiGe2 compound crystallizes in the CeNiSi2 type orthorhombic crystal structure and shows antiferromagnetic ordering below 39 K. Magnetization isotherms show linear dependence for field up to 50 kOe. It shows inverse magnetocaloric effect, which is attributed to antiferromagnetic interactions between the moments. The resistivity data was fitted in high and low temperature regimes. The low temperature fit confirms the antiferromagnetic ordering, while the high temperature data follow Bloch-Grüneisen-Mott relation. Ab initio LSDA + U calculations show no spin polarization from Ni 3d electronic states and reproduce the antiferromagnetic long-range ordering of the Tb magnetic moments and their values in good agreement with the previous neutron diffraction data. Optical conductivity data supports the electronic structure calculations and show that it is mostly provided by the transitions involving Ni 3d and Tb 4f electronic states.

Crystallographic, Electronic, and Magnetic Properties of Rock Salt Superstructure Sulfide Lu2CrS4 with Jahn–Teller Distortion

A new chromium(II) sulfide, Lu2CrS4, with a novel structure was prepared by a solid-state reaction. The powder X-ray diffraction pattern could be indexed as a tetragonal system, with a = 7.46373(2) Å, c = 22.6338(2) Å, and space group I4̅2d (No. 122). Rietveld analysis of the pattern provided the crystal structure consisting of CrS6 and LuS6 octahedra sharing edges and apexes and revealed a rock salt superstructure with new cation (vacancy) arrangements. The electrical resistivity indicates semiconducting behavior. The magnetic susceptibility and specific heat measurements showed that the Cr ions are in the high-spin d(4) configuration and that their magnetic moments ordered antiferromagnetically at 55 K. The basic antiferromagnetic structure was determined using powder neutron diffraction data at 10 K. The band structure calculations demonstrate that the densities of states of Cr 3d electrons split into two spin-up eg bands because of Jahn-Teller distortion.

Theoretical investigation of the magnetic and structural transitions of Ni-Co-Mn-Sn metamagnetic shape-memory alloys

The composition-dependent crystal structure, elastic modulus, phase stability, and magnetic property of Ni2-xCoxMn1.60Sn0.40 (0 <= x <= 0.50) are studied by using first-principles calculations in combination with atomistic spin dynamics method. It is shown that the present lattice parameters and Curie temperature (T-C) are in agreement with the available experimental data. The martensitic phase transformation (MPT) occurs for x < 0.43, where the austenite is in the ferromagnetic (FM) state whereas the martensite is in the antiferromagnetic (AFM) one at 0 K. The x dependence of the lattice parameter, elastic modulus, and energy difference between the FM austenite and the AFM martensite well accounts for the decrease of the MPT temperature (TM) with the Co addition. With increasing x, the increase of the magnetic excitation energy between the paramagnetic and FM austenite of these alloys is in line with the T-C similar to x. The Ni 3d as well as the Co 3d electronic states near the Fermi level are confirmed mainly dominating the phase stability of the studied alloys.

Soft X-ray photoemission study of Co2(Cr1−xFex)Ga Heusler compounds

We have performed soft X-ray photoemission spectroscopy (SXPES) and X-ray absorption spectroscopy (XAS) of the Co-based Heusler compounds Co2(Cr1−xFex)Ga (x = 0.0, 0.4, and 1.0) in order to study their electronic structures. Band-structure calculation was carried out and compared with the experimental results. SXPES spectra show hν-dependence, revealing the contributions of the Co, Cr, and Fe 3d electronic states in the valence band. The band width observed by the SXPES seems to be narrower than that predicted by the band-structure calculation. XAS spectra depend strongly on the the value of x in Co2(Cr1−xFex)Ga. The electron correlation effects are found to be stronger as x changes from 0.0 to 1.0.

Specific heat of the Ce(Cu1−xNix)4Ga alloys

The influence of the modification of the 3d electronic states in the heavy fermion compound CeCu4Ga by substitution of Cu by Ni is reported. It has been verified that the crystallographic structure is hexagonal of the CaCu5 type and is preserved for all the x values in the alloys Ce(Cu1−xNix)4Ga. The measurements of the specific heat Cp(T) have revealed that the huge electronic specific coefficient γ = 3.3 J mol−1 K−2 for CeCu4Ga is reduced with the progressive Ni addition and follows the behavior of the hybridization parameter of the Ce f states with the conduction electrons. The analysis of the specific heat of the non‐magnetic reference compound LaCu4Ga has shown that including the mass correction corresponding to the change of Ce by La and Cu by Ni does not provide a correct estimation of the phonon term in the Ce(Cu1−xNix)4Ga alloys.

First-principles molecular dynamics study of water dissociation on the γ-U(1 0 0) surface

Based on first-principles molecular dynamics simulations at finite temperatures, we systematically study the adsorption and dissociation of water molecules on the γ-U(1 0 0) surface. We predict that water molecules spontaneously dissociate upon approaching the native γ-U(1 0 0) surface. The dissociation results from electronic interactions between surface uranium 6d states and 1b2, 3a1, and 1b1 molecular orbitals of water. With segregated Nb atoms existing on the surface, adsorbing water molecules also dissociate spontaneously because Nb 3d electronic states can also interact with the molecular orbitals similarly. After dissociation, the isolated hydrogen atoms are found to diffuse fast on both the γ-U surface and that with a surface substitutional Nb atom, which is very similar to the ‘Hot-Atom’ dissociation of oxygen molecules on the Al(1 1 1) surface. From a series of consecutive molecular dynamics simulations, we further reveal that on both the γ-U surface and that with a surface substitutional Nb atom, one surface U atom will be pulled out to form the U–O–U structure after dissociative adsorption of 0.44 ML water molecules. This result indicates that oxide nucleus can form at low coverage of water adsorption on the two surfaces.