Magnetic characterization of Mn5SiB2 and Mn5Si3 phases

In this work the Mn₅Si₃ and Mn₅SiB₂ phases were produced via arc melting and heat treatment at 1000 °C for 50 h under argon. A detailed microstructure characterization indicated the formation of single-phase Mn₅Si₃ and near single-phase Mn₅SiB₂ microstructures. The magnetic behavior of the Mn₅Si₃ phase was investigated and the results are in agreement with previous data from the literature, which indicates the existence of two anti-ferromagnetic structures for temperatures below 98 K. The Mn₅SiB₂ phase shows a ferromagnetic behavior presenting a saturation magnetization M_s of about 5.35×10⁵ A/m (0.67 T) at room temperature and an estimated Curie temperature between 470 and 490 K. In addition, AC susceptibility data indicates no evidence of any other magnetic ordering in 4-300 K temperature range. The magnetization values are smaller than that calculated using the magnetic moment from previous literature NMR results. This result suggests a probable ferrimagnetic arrangement of the Mn moments.     

Al NMR studies of NpPd5Al2

We present ²⁷Al NMR studies for a single crystal of the Np-based superconductor NpPd₅Al₂. We have observed a five-line ²⁷Al NMR spectrum with a center line and four satellite lines separated by first-order nuclear quadrupole splittings. The Knight shift clearly drops below T_c. The temperature dependence of the ²⁷Al nuclear spin-lattice relaxation rate shows no coherence peak below T_c, indicating that NpPd₅Al₂ is an unconventional superconductor with an anisotropic gap. The analysis of the present NMR data provides evidence for strong-coupling d-wave superconductivity in NpPd₅Al₂.     

STRUCTURE ANALYSIS OF NEW SUPERCONDUCTOR MgB2

The superconductivity of a sintered pellet of the newly discovered MgB₂ superconductor has been studied with magnetic measurements and its crystal structure was analysed using the Rietveld method of powder X-ray patterns. It has hexagonal symmetry (S.G. P6/mmm) with unit cell lattice parameters a=0.308136(14)nm and c=0.351782(17)nm.     

Phase stability and electronic structure of Si2Sb2Te5 phase-change material

On the basis of an ab initio computational study, the present work provide a full understanding on the atomic arrangements, phase stability as well as electronic structure of Si₂Sb₂Te₅, a newly synthesized phase-change material. The results show that Si₂Sb₂Te₅ tends to decompose into Si₁Sb₂Te₄ or Si₁Sb₄Te₇ or Sb₂Te₃, therefore, a nano-composite containing Si₁Sb₂Te₄, Si₁Sb₄Te₇ and Sb₂Te₃ may be self-generated from Si₂Sb₂Te₅. Hence Si₂Sb₂Te₅ based nano-composite is the real structure when Si₂Sb₂Te₅ is used in electronic memory applications. The present results agree well with the recent experimental work.     

Pressure dependence of electronic structure and magnetic properties in Fe16N2

The electronic structures and magnetic properties of Fe₁₆N₂ system and their pressure dependence were investigated by using first-principles calculations based on the density functional theory. It has been found that the total magnetic moment in Fe₁₆N₂ system decreases monotonically as increasing pressure from 0 to 14.6 GPa. A phase transition from ferromagnetic (FM) to non-magnetic (NM) occurs with a volume collapse of around 0.008  at 14.6 GPa, The lattice constants a and c for magnetic results decrease monotonically as pressure increasing from 0 to 14.6 GPa, at 14.6 GPa, the lattice constant a decreases sharply, on the contrary, the lattice constant c increases abruptly. We think that the change of microscopic structure of Fe₁₆N₂ is responsible for the phase transition from FM to NM.     

First-principles study of electronic properties and stability of Nb5SiB2（001） surface

The density functional calculations are performed to study the electronic structure and stability of Nb 5 SiB 2(001) surface with different terminations.The calculated cleavage energies along the(001) planes in Nb 5 SiB 2 are 5.015 J · m 2 and 6.593 J · m 2 with the break of Nb-Si and Nb-NbB bonds,respectively.There exists a close correlation between the surface relaxation including surface ripple and the cleavage energy:the larger the cleavage energy,the larger the surface relaxation.Moreover,the surface stability of the Nb 5 SiB 2(001) with different terminations has been investigated by the chemical potential phase diagram.From a thermodynamics point of view,the four terminations can be stabilized under different conditions.In chemical potential space,NbB(Nb) and Nb(Si) terminations are just stable in a small area,whereas Si(Nb) and Nb(NbB) terminations are stable in a large area(the letters in brackets represent the subsurface atoms).     

Large atomic disorder in nanostructured LaNi5 alloys: A La L3-edge extended X-ray absorption fine structure study

Local structure of the nanostructured LaNi₅ alloys, prepared by ball-milling, has been studied using La L₃-edge extended X-ray absorption fine structure spectroscopy. The near-neighbor distances tend to decrease with the ball-milling, and the mean square relative displacements (MSRD) show substantial increase suggesting an increased atomic disorder. High temperature annealing helps in partial recovery of atomic order in the ball-milled samples for milling times upto 20 h, however, the long-time ball-milled samples seems to gain only a local random order. The results suggest that reduced unit-cell volume together with large atomic-disorder might be causing a higher energy-barrier for the hydride-phase formation in the long time ball-milled LaNi₅ powders.     

Anomalous upper critical field in ternary iron-silicide superconductor Lu2Fe3Si5

We report the upper critical field H_c2 in a ternary iron-silicide superconductor Lu₂Fe₃Si₅ with T_c  6 K obtained by the resistivity measurements. We find that H_c2 increases linearly with decreasing temperature down to T_c/3, and H_c2(T = 0) exceeds the orbital depairing field described by the Werthamer–Helfand–Hohenberg theory. We also find that the anisotropy of H_c2 is nearly independent of temperature and the angular dependence of H_c2 is well-described by the anisotropic Ginzburg–Landau model. These results strongly indicate the presence of two distinct superconducting gaps in Lu₂Fe₃Si₅ although the behavior is slightly different from that of the typical two-gap superconductor MgB₂.     

Ag concentration dependent transport properties of LiF-MoO3-P2O5 glasses

LiF-MoO₃-P₂O₅ glasses mixed with different concentrations of Ag₂O (ranging from 0 to 1.0 mol%) was prepared. D.C. conductivity and dielectric properties over a range of temperature have been investigated. The analysis of the results of d.c. conductivity has indicated that T>θ_D/2, the small polaron hoping model seems to be fit and the conduction is adiabatic in nature. These results further indicated that there is a change over of conduction mechanism from electronic to ionic at about 0.4 mol% of Ag₂O. The low temperature part of a.c. conductivity is explained based on quantum mechanical tunneling model. The quantitative analysis of these results is further extended with the aid of the data on optical absorption, ESR and IR spectral studies.     

Emission features of Ho ion in Nb2O5, Ta2O5 and La2O3 mixed Li2O-ZrO2-SiO2 glasses

Li₂O-ZrO₂-SiO₂: Ho³⁺ glasses mixed with three interesting d-block elemental oxides, viz., Nb₂O₅, Ta₂O₅ and La₂O₃, were prepared. Optical absorption and photoluminescence spectra of these glasses have been recorded at room temperature. The luminescence spectra of Nb₂O₅ and Ta₂O₅ mixed Li₂O-ZrO₂-SiO₂ glasses (free of Ho³⁺ ions) have also exhibited broad emission band in the blue region. This band is attributed to radiative recombination of self-trapped excitons (STEs) localized on substitutionally positioned octahedral Ta⁵⁺ and Nb⁵⁺ ions in the glass network. The Judd-Ofelt theory was successfully applied to characterize Ho³⁺ spectra of all the three glasses. From this theory various radiative properties, like transition probability A, branching ratio β_r and the radiative lifetime τ_r, for ⁵S₂ emission levels in the spectra of these glasses have been evaluated. The radiative lifetime for ⁵S₂ level of Ho³⁺ ions has also been measured and quantum efficiencies were estimated. Among the three glasses studied the La₂O₃ mixed glass exhibited the highest quantum efficiency. The reasons for such higher value have been discussed based on the relationship between the structural modifications taking place around the Ho³⁺ ions.     

Large upper critical field in non-centrosymmetric superconductor Y2C3

We determine the upper critical field μ₀H_c2(T_c) of non-centrosymmetric superconductor Y₂C₃ using two distinct methods: the bulk magnetization M(T) and the tunnel-diode oscillator (TDO) based impedance measurements. It is found that the upper critical field reaches a value of 30 T at zero temperature which is above the weak-coupling Pauli paramagnetic limit. We argue that the observation of such a large μ₀H_c2(0) in Y₂C₃ could be attributed to the admixture of spin-singlet and spin-triplet pairing states as a result of broken inversion symmetry.     

Vibrational and dielectric properties of La2Hf2O7 : Experiment and theory

The physical origin of the static dielectric constant and its relationship with lattice dynamics of La₂Hf₂O₇are studied by combining infrared spectroscopy and density functional perturbation theory (DFPT). Both La and Hf show obvious effective charge anomaly which is attributed to the hybridization between 2p states of the oxygen and 5d states of the cations, indicating a mixed covalent-ionic bonding between the cations and the oxygen. The dielectric response is determined by seven infrared phonon modes and the static dielectric constant extracted from infrared reflection spectrum is in close agreement with DFPT calculation. Both experiment and theory reveal that most of the contributions to the static dielectric constant are dominated by three infrared phonon modes at 137, 172 and 297 cm⁻¹. Two of them (172 and 297 cm⁻¹) are from the displacements of oxygen atoms inside HfO₆ octahedra and the other one (137 cm⁻¹) is from the bending of La₂O′ chain. This result indicates that the origin of the static dielectric constant of La₂Hf₂O₇ is directly connected with the two interpenetrating sub-networks of pyrochlore structure (HfO₆ octahedra and La₂O′ chain).     

The magnetism for NN AFM ground state in Fe-based superconductor: Sr1−xKxFe2As2

The crystal structure, magnetism properties, and density of states for FeAs layered compound SrFe₂As₂ have been investigated by using the density functional theory (DFT) method. The magnetism under a checkerboard nearest neighbor anti-ferromagnetic (NN AFM) and ferromagnetic (FM) order ground-state have been analyzed with substitution for Sr with K ion in Sr_1−xK_xFe₂As₂. The results indicate that the distortion of FeAs tetrahedrons is sensitive to the electron doping concentration. The system magnetism was suppressed by K doping in NN-AFM ground state instead of FM. The density of states at Fermi level N(E_F) under NN AFM ground state would be regarded as a driving force for the increased T_c of Sr_1−xK_xFe₂As₂ system as observed experimentally. Our calculation reflects that NN AFM type spin fluctuation may still exist in the Sr_1−xK_xFe₂As₂ system and it may be an origin of strong spin fluctuation in this system besides the spin density wave (SDW) states.     

First-principles study on electronic structure of Sr2Bi2O5 crystal

The electronic structure of Sr₂Bi₂O₅ is calculated by the GGA approach. Both of the valence band maximum and the conduction band minimum are located at Γ-point. This means that Sr₂Bi₂O₅ is a direct band-gap material. The wide energy-band dispersions near the valence band maximum and the conduction band minimum predict that holes and electrons generated by band gap excitation have a high mobility. The conduction band is composed of Bi 6p, Sr 4d and O 2p energy states. On the other hand, the valence band can be divided into two energy regions ranging from −9.5 to −7.9 eV (lower valence band) and from −4.13 to 0 eV (upper valence band). The former mainly consists of Bi 6s states hybridizing with O 2s and O 2p states, and the latter is mainly constructed from O 2p states strongly interacting with Bi 6s and Bi 6p states.     

Mechanism of amorphous Ge2Sb2Te5 removal during chemical mechanical planarization in acidic H2O2 slurry

In this paper, chemical mechanical planarization (CMP) of amorphous Ge2Sb2Te5 (a-GST) in acidic H2O2 slurry is investigated. It was found that the removal rate of a-GST is strongly dependent on H2O2 concentration and gradually increases with the increase in H2O2 concentration, but the static etch rate first increases and then slowly decreases with the increase in H2O2 concentration. To understand the chemical reaction behavior of H2O2 on the a-GST surface, the potentiodynamic polarization curve, surface morphology and cross-section of a-GST immersed in acidic slurry are measured and the results reveal that a-GST exhibits a from active to passive behavior for from low to high concentration of H2O2 . Finally, a possible removal mechanism of a-GST in different concentrations of H2O2 in the acidic slurry is described.     

Bandlike and localized defect states in CuInSe2 solar cells

We used the deep-level transient spectroscopy (DLTS) to investigate the electronic properties of p-type CuInSe₂ (CIS) polycrystalline thin-film solar cells. We detected electron (or minority) traps with activation energies ranging from E_c−0.1 to E_c−0.22 eV (where E_c is the energy of electrons at the conduction band minimum). While varying the filling pulse duration, we observed the gradual increase in the amplitude of the DLTS signal for these states until it apparently saturates at a pulse duration ∼1 s. Increasing the duration of the filling pulse also results in broadening the DLTS signals and shifting the maximum of these signals towards lower temperature, whereas the high-temperature sides coincide. We also detected a hole (or majority) trap around a temperature of 190 K. Using a model that allows us to distinguish between bandlike states and localized ones based on the dependence of the shape of their DLTS-signal on the filling-pulse duration, we relate the electron trap to bandlike states and the hole trap to localized ones.     

Large room-temperature magnetoresistance and temperature-dependent magnetoresistance inversion in La0.67Sr0.33MnO3/Alq3 - Co nanocomposites/Co devices

Large room-temperature (RT) magnetoresistance (MR) and temperature-dependent MR inversion have been observed in tris (8-hydroxyquinoline) aluminum (Alq₃)-cobalt nanocomposites-based organic-inorganic hybrid devices. Negative MR-high resistance for parallel electrodes configuration — due to magnetization reversal of ferromagnetic (FM) electrodes has been observed at low temperatures. As the temperature increases, the MR undergoes a sign change. At room temperature, a positive MR of ∼9.7% with the resistivity dropping monotonously with increasing magnetic fields has been observed. The RT MR is about two orders of magnitude of that in organic-FM nanocomposites measured with nonmagnetic electrodes. The enhancement of RT MR is attributed to the injection of spin polarized carriers into Alq₃-Co nanocomposites.     

Characterization of CuAlO2 thin film prepared by rapid thermal annealing of an Al2O3/Cu2O/sapphire structure

CuAlO₂ thin film was successfully prepared by rapid thermal annealing of an Al₂O₃/Cu₂O/sapphire structure in air above 1000 °C. The film was mostly with single crystalline structure as verified by X-ray diffraction methods. We found that crystal quality and electrical conductivity of the films were affected by the cooling rate after annealing. The highest conductivity obtained in this work was 0.57 S/cm. Optical gap of this film was determined to be 3.75 eV.     

Microstructural changes in Fe-doped Gd5Si2Ge2

Gd₅Si₂Ge₂-based alloys can exhibit a giant magnetocaloric effect (GMCE); this gives them the potential for use in cooling technologies. It has also been reported that a small addition of iron reduces the hysteresis losses in Gd₅Si₂Ge₂-based alloys, thus increasing the net refrigerating capacity. In this investigation, we have been the first to look at the effect on the microstructure and magnetic properties of Gd₅Si₂Ge₂ resulting from a wide range of substitutions of Si by Fe. The macrostructures of the arc-melted buttons revealed some very unusual surface morphologies, and the analytical results revealed a gradual substitution of the Gd₅(Si,Ge,Fe)₄-type phase by a Gd₅(Si,Ge,Fe)₃-type phase and the presence of three grain-boundary phases, two of which contain substantial amounts of iron. The magnetic measurements indicated that larger amounts of iron reduced the hysteresis losses, but at the same time reduced the Curie temperatures to below lower values that would make the material useful in practice.     

XPS investigation of diffusion of two-layer ZrO2/SiO2 and SiO2/ZrO2 sol–gel films

Two-layer ZrO₂/SiO₂ and SiO₂/ZrO₂ films were deposited on K9 glass substrates by sol–gel dip coating method. X-ray photoelectron spectroscopy (XPS) technique was used to investigate the diffusion of ZrO₂/SiO₂ and SiO₂/ZrO₂ films. To explain the difference of diffusion between ZrO₂/SiO₂ and SiO₂/ZrO₂ films, porous ratio and surface morphology of monolayer SiO₂ and ZrO₂ films were analyzed by using ellipsometry and atomic force microscopy (AFM). We found that for the ZrO₂/SiO₂ films there was a diffusion layer with a certain thickness and the atomic concentrations of Si and Zr changed rapidly; for the SiO₂/ZrO₂ films, the atomic concentrations of Si and Zr changed relatively slowly, and the ZrO₂ layer had diffused through the entire SiO₂ layer. The difference of diffusion between ZrO₂/SiO₂ and SiO₂/ZrO₂ films was influenced by the microstructure of SiO₂ and ZrO₂.