Ba 3d Research Articles

High-Pressure Synthesis and Superconductivity of a New Binary Barium Germanide BaGe3

A new binary barium germanide BaGe(3) was prepared by high-pressure and high-temperature reactions using a Kawai type multi-anvil press. It crystallizes in a hexagonal unit cell with a = 6.814(1) Å, c = 5.027(8) Å, and V = 202.2(5) Å(3) (the space group P6(3)/mmc, No. 194). The unit cell contains two layers along the c axis composed of Ba atoms and Ge(3) triangular units. The triangular units stack along the c axis to form 1D columns in which the adjacent Ge(3) units turn to opposite directions. The columns, therefore, can be described as the face-sharing stacking of elongated Ge(6) octahedra. Each Ba atom is surrounded by six columns. BaGe(3) is metallic and shows superconductivity at 4.0 K. The band structure calculations revealed that there are four conduction bands mainly composed of Ge 4p and Ba 5d orbitals. From Fermi surface analysis, we confirmed that three of them have a large contribution of Ge 4pz orbitals in the vicinity of the Fermi level and show a simple 1D appearance. The remaining one contains Ge 4px, 4py, and Ba 5d contributions and shows a 2D property.

2D degenerate electron gas at Ba/n-AlGaN and Ba/n-GaN interfaces

Ba/n-GaN(0001) and Ba/n-AlGaN(0001) interfaces were investigated for the first time by means of ultraviolet photoelectron spectroscopy. The spectra of the photoemission from a valence band along with the spectra of the core levels of Ga 3d, Al 2p, and Ba 4d were studied. The formation of a 2D degenerate electron gas (an accumulated layer on the n-GaN and n-AlGaN surfaces during adsorption of Ba atoms) was revealed.

Valence Band Studies of p- and n-Type Ba8Ga16Ge30 Using High-Resolution Photoelectron Spectroscopy

The electronic states of p- and n-type Ba8Ga16Ge30 (BGG) are studied by high-resolution x-ray photoelectron spectroscopy. In BGG, three bands are resolved in the valence band region. Theoretical calculations show that the three band structures in the valence band are mainly constructed by the Ge/Ga 4s and 4p wavefunctions with little contribution from Ba 5s, 5p, and 5d. The valence band around the Fermi level region of n-type BGG is sensitive to temperature, while that of p-BGG is stable when the temperature changes. The data indicate that the endohedral Ba in p- and n-type BGG rattle with different modes due to the different hybridization with the orbitals of the framework polyhedra.

Syntheses and characterization of two new selenides Ba 5Al 2Se 8 and Ba 5Ga 2Se 8

Two new barium selenides Ba 5Al 2Se 8 and Ba 5Ga 2Se 8 have been synthesized by solid-state reactions. The structures of Ba 5Ga 2Se 8 and Ba 5Al 2Se 8 were determined by single-crystal X-ray diffraction method and the Rietveld method, respectively. The two isostructural compounds crystallize in space group Cmca of the orthorhombic system with isolated MSe 4 (M = Al, Ga) tetrahedra separated by Ba atoms. The optical band gap of 2.51(2) eV for Ba 5Ga 2Se 8 was deduced from the diffuse reflectance spectrum. Band structure calculation indicates that Ba 5Ga 2Se 8 is a direct-gap semiconductor. The valence band maximum is dominated by Se 4p orbitals, while the Ba 5d orbitals have the largest contribution to bottom of the conduction band.

Influence of the ferroelectric polarization on the electronic structure of BaTiO3 thin films

AbstractMicron scale ferroelectric (FE) domains have been written into a 20‐nm‐thick epitaxial thin film of BaTiO3 (001) [BTO(001)] on a Nb‐doped SrTiO3 substrate using a piezoforce microscope (PFM). The domain‐dependent electronic structure has been studied using fully energy‐filtered photoemission electron microscopy (PEEM) and synchrotron radiation. Shifts, induced by FE polarization, of up to 300 meV were observed in the work function of the sample. The surface is Ba‐O terminated. Polarization‐induced distortion of the electronic structure was observed in the valence band and on the Ba 3d, Ti 2p and O 1s core levels of BTO. Polarization‐dependent surface adsorption was observed. A simple electrostatic model based on net surface charge is not sufficient to explain the observed modifications in the electronic levels. Copyright © 2010 John Wiley & Sons, Ltd.

First-principles study on the electronic properties of BaCu 2S 2

Using the pseudopotential plane-wave method under the density functional theory (DFT), the electronic properties of BaCu 2S 2 in α and β phases are calculated. The result shows that the bottom conduction band is mainly made of Ba 5d state, while the upper valence band is formed by S 3p and Cu 4d states, but both of them for β-BaCu 2S 2 are wider than those for α-BaCu 2S 2. The electron and hole effective masses for BaCu 2S 2 are also analyzed in detail. It is found that β-BaCu 2S 2 has very small hole effective masses, which implies that β-BaCu 2S 2 may possess a high hole mobility. Moreover, its hole effective masses are extremely anisotropic along ab plane and c axis. These properties are helpful to enhance its Seebeck coefficient, and means that β-BaCu 2S 2 could be a good candidate of thermoelectric materials.

Electronic structure of a Ba/n-AlGaN(0001) interface and the formation of a degenerate 2D electron gas

The electronic structure of ultrathin Ba/n-AlGaN(0001) interfaces has been investigated in situ in an ultra-high vacuum by the ultraviolet photoelectron spectroscopy method. The photoemission spectra from the n-AlGaN valence band and the spectra of the core levels Ga 3d, Al 2p, and Ba 4d have been studied under synchrotron excitation with photon energies of 60–400 eV. The modification of the spectra in the process of the formation of the Ba/n-AlGaN interface in the mode of the Ba submonolayer coverages has been revealed. It has been found that a decrease in the intensity in some spectral regions of the valence band is attributed to the interaction of the surface states of the AlGaN substrate with the Ba adatoms. It has been revealed that the interface formation results in the appearance of a new photoemission peak of the quasimetallic character at the Fermi level in the AlGaN bandgap. It has been established that the peak is due to the formation of the degenerate electron gas in the accumulation nanolayer induced by adsorption near the n-AlGaN surface.

Accurate calculations of the ground state and low-lying excited states of the (RbBa)+ molecular ion: a proposed system for ultracold reactive collisions

Collisions of ultracold Ba+ ions on a Rb Bose–Einstein condensate have been suggested as a possible benchmark system for ultracold ion-neutral collision experiments. However, a priori knowledge of the possible processes is desirable. For this purpose, we here present high-level four-component coupled cluster and multi-reference configuration interaction calculations of potential energy curves, dipole moments and spectroscopic constants of the experimentally interesting low-lying electronic states of the (RbBa)+ molecule. Our results show significant avoided crossings between the Rb + Ba+ entrance channels and low-lying charge transfer states of the Rb+ and Ba(3D) atomic channels, indicating that a fast non-radiative charge transfer is possible. Population analysis shows that a partially covalent polar bond is formed in the ground state, which thus deviates significantly from a pure Rb+ + Ba interaction. This finding is corroborated by the electric dipole moment which is found only to be 4.5 D at the equilibrium bond distance, compared with the 14 D for a pure Rb+ + Ba interaction, thereby supporting the view of a partial charge transfer between the two atoms.

Study on the electronic structures and the optical properties of the Mg-doped LiBaF 3 crystal

The most possible substituting positions of impurity magnesium ions in LiBaF 3 crystal are studied using the general utility lattice program (GULP). The electronic structures of Mg-doped LiBaF 3 crystal are studied within the framework of the fully relativistic self-consistent Dirac–Slater theory using DV-X α software. It is predicted that the most possible defect model is the [ Mg Li · + Li ′ Ba ] in the Mg: LiBaF 3 crystal, which exhibits donor level in the forbidden band. The optical transition energy from the Mg 3s state to the Ba 5d state is 2.975 eV, which is very close to the experimental result. It is indicated that the 420 nm absorption band originates from [ Mg Li · + Li ′ Ba ] and the presence of the 510 nm luminescence band is related to the defect cluster [ Mg Li · + Li ′ Ba ] in the Mg:LiBaF 3 crystal too.

First-principles calculations on the electronic structure and optical properties of BaSi2

The electronic structure, densities of states and optical properties of the stable orthorhombic BaSi2 have been calculated using the first-principle density function theory and pseudopotential method. The results show that BaSi2 is an indirect semiconductor with the band gap of 1.086 eV, the valence bands of BaSi2 are mainly composed of Si 3p, 3s and Ba 5d, and the conduction bands are mainly composed of Ba 6s, 5d as well as Si 3p. The static dielectric function ɛ 1(0) is 11.17, the reflectivity n 0 is 3.35, and the biggest peak of the absorption coefficient is 2.15×105 cm−1.

Soft x-ray photoelectron spectroscopy study of type-I clathrates

Extensive soft x-ray photoelectron spectroscopy studies are performed on Ba8Ga16Ge30 (BGG) and Sr8Ga16Ge30 (SGG) single crystals ranging from Fermi to core levels, at a high-energy facility. Valence band x-ray photoelectron spectroscopy (XPS) experiments with theoretical calculations revealed that the valence band is mainly constructed by the Ge/Ga 4s and 4p wave functions with little contribution of the Ba/Sr atomic orbitals. Surprisingly, unexpected features evidencing the different shift for the 2a- and 6d- sites between Ba 4d and Sr 3d are observed. The detailed analyses including theoretical support by first-principles band-structure calculations lead to the conclusion that the component distributions of the larger tetrakaidecahedral cage are different depending on the endohedral atoms, which contrasts with the past consensus that BGG and SGG have the same framework structure. This may give thorough reconsiderations on earlier interpretations of experimental data.

Characterization of electronic structure of superconducting (Cu, C)–Ba–O films by in situphotoemission spectroscopy

Change of electronic structure of superconducting (Cu, C)-1201 thin filmsgrown by pulsed laser deposition with superconducting critical temperatureTc has been characterized by means of in situ photoemission spectroscopy. The finite spectralweight at the Fermi level in the valence band of (Cu, C)-1201 superconducting thin films and arigid-band-like approach of the main peak of the valence band toward the Fermi level with a rise ofTc has been observed, which reveals hole-doping-induced superconductivity ofthis system. The obtained Ba 4d and Cu 2p signals with an increase ofTc suggest excess oxygen should be introduced around Ba ions in the earlystage of the emergence of superconductivity. Then, they should bepreferentially introduced around Cu ions. The changes of core signals withTc and comparisons of the obtained data with infinite-layer and other cupratesuperconductor compounds suggest that the (Cu, C)–O charge reservoir in thissystem is in the heavily hole-doped state, similar to that of the Cu–O chain inYBa2Cu3O7−δ.

The specific oxidation mechanism in the initial stages of O 2 adsorption on submonolayer Ba covered W(1 1 0) surface

The initial oxidation of a polarized 0.35-ML Ba-covered W(1 1 0) surface has been investigated in detail by high-resolution photoemission spectroscopy using synchrotron radiation. Upon small exposures (⩽0.15 L) to O 2, both interfacial W 4f (Ba-coordinated W) and Ba 4d core levels shift simultaneously toward smaller binding energies and then fix during 0.15–0.25 L, finally disappear at 0.6 L. The concurrent negative shifts of both interfacial W 4f and Ba 4d peaks can be attributed to be a consequence of the increased occupancy of 5d states. This explanation is supported by the variations of 5d stats in valence band spectra upon the initial oxidation. Our results clearly demonstrate that charge rearrangement is the dominant effect in the surface reactivity of our oxidized surface. We also found that this charge redistribution is associated with a structural change during oxidation, which is in contrary to previous studies of alkali-metal promotion. That is, oxygen is first chemisorbed atop the polarized Ba adlayer, then incorporates beneath Ba adatoms to form a covalent Ba–O–W complex upon further dosage, and reacts strongly with the Ba overlayer to become an ionic Ba +2O −2 surface dipole layer at 0.6 L.

Etching characteristics and plasma-induced damage of Ba 0.65Sr 0.35TiO 3 thin films etched in CF 4/Ar/O 2 plasma

Sol–gel-derived Ba 0.65Sr 0.35TiO 3 (BST) thin films were etched in CF 4/Ar/O 2 plasma using magnetically enhanced reactive ion etching technology. The maximum etch rate of BST film is 8.47 nm/min when CF 4/Ar/O 2 gas mixing ratio is equal to 9/36/5. X-ray photoelectron spectroscopy analysis indicates the accumulation of fluorine-containing by-products on the etched surface due to their poor volatility, resulting in (Ba,Sr)-rich and (Ti,O)-deficient etched surface. Compared to the unetched counterparts, the etched Ba 3d 5/2, Ba 3d 3/2, Sr 3d 5/2, Sr 3d 3/2, Ti 2p 3/2, Ti 2p 1/2 and O 1s photoelectron peaks shift towards higher binding energy regions by amounts of 1.31, 1.30, 0.60, 0.79, 0.09, 0.46 and 0.50 eV, respectively. X-ray diffraction (XRD) analysis reveals that intensities of the etched BST (1 0 0), (1 1 0), (2 0 0) and (2 1 1) peaks are lowered and broadened. Raman spectra confirm that the Raman peaks of the etched film shift towards lower wave number regions with the values of 7, 6, 4 and 4 cm −1, and the corresponding phonon lifetimes are longer than those of the unetched film because of the plasma-induced damage. When the etched films are postannealed at 650 °C for 20 min under an O 2 ambience, the chemical shifts of Ba 3d, Sr 3d, Ti 2p and O 1s peaks, the variations for atomic concentrations of Ba, Sr, Ti and O, and the Raman redshifts are reduced, while the corresponding XRD peak intensities increase. It is conceivable that the plasma-induced damage of the etched film could be partially recovered during the postannealing process.

Effect of oxygen gas and annealing treatment for magnetically enhanced reactive ion etched (Ba0.65,Sr0.35)TiO3 thin films

Sol-gel-derived (Ba0.65,Sr0.35)TiO3 (BST) thin films were etched in CF4∕Ar and CF4∕Ar∕O2 plasmas using magnetically enhanced reactive ion etching technology. Experimental results show that adding appropriate O2 to CF4∕Ar can better the etching effects of BST films for the increase of etching rate and decrease of etched residues. The maximum etching rate is 8.47nm∕min when CF4∕Ar∕O2 gas-mixing ratio is equal to 9∕36∕5. X-ray photoelectron spectroscopy (XPS) data confirm accumulation of reaction products on the etched surface due to low volatility of reaction products such as Ba and Sr fluorides, and these residues could be removed by annealing treatment. The exact peak positions and chemical shifts of the interested elements were deduced by fitting XPS narrow-scan spectra with symmetrical Gaussian-Lorentzian product function for Ba 3d, Sr 3d, and O 1s peaks, meanwhile asymmetrical Gaussian-Lorentzian sum function was used to fit Ti 2p doublet to adjust the multiple splitting and/or shake-up process of transition-metal Ti cations. Compared to the unetched counterparts, the etched Ba 3d5∕2, Ba 3d3∕2, Sr 3d5∕2, Sr 3d3∕2, Ti 2p3∕2, Ti 2p1∕2, and O 1s peaks shift towards higher binding energy regions by amounts of 1.31, 1.30, 0.60, 0.79, 0.09, 0.46, and 0.50eV, respectively. While the etched Ti 2p3∕2 and Ti 2p1∕2 peaks have small chemical shifts for two reasons. One is that Ti fluoride (TiFz) is mostly removed from the etched surface because of its higher volatility in the process of thermal desorption. The other is that there is a shift compensation between TiFz and the etched BST matrix in which Ti4+ cations are partially reduced to form Tix+ (0<x<4) cations in the presence of adequate oxygen vacancies. The simulated formula of each BST surface is obtained to be (Ba0.65,Sr0.35)Ti0.97O2.86, (Ba0.70,Sr0.30)Ti0.24O1.39, and (Ba0.68,Sr0.32)Ti0.95O2.74, and then the average valence of Ti cations is estimated to be +3.84, +3.25, and +3.66 with respect to the electroneutrality principle, respectively. It is inferred that electrical properties of the etched BST film may be partially recovered by postannealed after etched.

Quasiparticle band structures of BaO and BaS

We calculate the band structures of BaO and BaS using the local density approximation and the GW approximation (GWA). The Ba 4d states are treated as valence states. We find that BaO is an indirect band-gap semiconductor and BaS is a direct band-gap semiconductor. The results show that the band gaps of GWA agree excellently with the experimental results.

X-ray photo-emission studies of Cu 1− xTl xBa 2Ca 3Cu 4O 12− y superconductor thin films

X-ray photo-emission spectroscopy (XPS) studies of Cu 1− x Tl x Ba 2Ca 3Cu 4O 12− y superconductor thin films have been carried out for understanding the mechanism of superconductivity and to find out the reasons for the increase of zero resistivity critical temperature T c( R = 0) with post-annealing in a nitrogen atmosphere. It is observed from these studies that reduction of charge state of thallium is a source of doping of carriers to the CuO 2 planes. The reduced charge state of thallium (i.e. Tl 1+) promotes lower oxygen concentration in the charge reservoir layer, which possibly results in movement of electrons to the conducting CuO 2 planes. The higher density of electrons in the CuO 2 planes optimizes the hole concentration “ n p” in these planes. The reduced charge state of thallium in the Cu 1− x Tl x Ba 2O 4− δ charge reservoir layer is also supported by a shift of the Ba 3d 5/2 and Ba 3d 3/2 XPS lines to lower binding energies with post-annealing in nitrogen atmosphere. Moreover, the movement of the valance band spectrum to lower binding energies suggested that the electronic density of states changes in the valance band with the post-annealing in nitrogen, which possibly becomes a source of doping of carriers to the CuO 2 planes. The increased doping of electrons to the CuO 2 planes optimizes the Fermi-vector K F and Fermi-velocity V F of the carriers and increases the T c( R = 0) of final compound.

Generalized oscillator strengths for the3delectrons of Cs, Ba, and Xe: Effects of spin-orbit-activated interchannel coupling

This paper investigates for the first time ever the use of the spin-orbit interaction to probe correlation effects in the generalized oscillator strength (GOS) of the 3d subshells of Cs, Ba, and Xe, viz. we study how the GOSs of the Cs, Ba, and Xe 3d{sub 3/2} and 3d{sub 5/2} levels are affected by the intradoublet correlations. The calculations are carried out using the Hartree-Fock approximation and within the framework of a modified version of the spin-polarized random phase approximation with exchange, which takes into account multielectron correlation effects. The effects of relaxation of the excited electrons due to the creation of the 3d vacancy are also accounted for. Our GOS results for Cs, Ba, and Xe, obtained for values of the momentum transfer q from 0 to 4 a.u. and energy transfer {omega} from 0.1 to 8 Ry, demonstrate the strong interaction between the components of the spin-orbit doublet of the 3d electrons in Cs, Ba, and Xe. This leads to the appearance of an additional maximum in the GOS for the 3d{sub 5/2} subshell, due to the action of the 3d{sub 3/2} electrons. Additionally, for the dipole transition in Cs and Ba, the 3d{sub 5/2} subshell perturbs the more » 3d{sub 3/2} strongly for q=4 a.u. only, while in Xe, the perturbation is at the {omega} value of approximately 58 Ry for all considered q values. It is concluded that the intradoublet correlations are generally very important in the dipole, monopole, and quadrupole transitions, but particularly in the dipole transition. « less

Electronic and optical properties of bulk crystals of semiconducting orthorhombic BaSi 2 prepared by the vertical Bridgman method

BaSi 2 crystals were grown from a near-stoichiometric melt containing mixtures of Ba (99.9%) and Si (99.99999%) with Ba:Si at.% ratios of 36:64 (Ba-rich) and 30:70 (Si-rich) by the vertical Bridgman growth method. X-ray diffraction analysis revealed that the grown crystals were semiconducting orthorhombic BaSi 2. The samples produced were polycrystalline consisting of single crystal grains. Optical absorption measurements were carried out to identify the different types of optical transition at room temperature. All the bulk orthorhombic BaSi 2 grown exhibited an indirect transition with an indirect band-gap energy ( E g indir ) of 1.13 eV. Emissions from photoluminescence measurements at 15 K, that seem to be due to indirect transitions, revealed that there were impurities and crystal defects. To understand the experimental results, we calculated the electronic-band structure from first principles using a local density approximation (LDA) in the all-electron full-potential linearized augmented plane wave (FLAPW) method. The calculation is consistent with the experimental indirect band gap results giving an indirect band gap of 0.72 eV and a direct gap of 0.81 eV. The band gap arises from the Si 3p orbitals, while Ba 5d orbitals are strongly hybridized with the Si 3p orbitals in the conduction band.

Quasiparticle Band Structure of BaS

We calculate the band structure of BaS using the local densityapproximation and the GW approximation (GWA), i.e. incombination of the Green function G and the screenedCoulomb interaction W. The Ba 4d states are treated asvalence states. We find that BaS is a direct band-gap semiconductor.The result shows that the GWA band gap (Eg−GW = 3.921 eV)agrees excellently with the experimental result (Eg−EXPT = 3.88 eV or 3.9 eV).