The structure and stability of Si@Al<Subscript>12</Subscript>H<Subscript>n</Subscript> (n=1−14) clusters

Density functional theory has been employed in order to investigate the structure and stability of Si@Al₁₂H_n () clusters. Hydrogenated Si@Al₁₂ clusters exhibit pronounced stability for even numbers of H atoms. Large binding energy, HOMO-LUMO gaps and increased ionization potentials imply that these clusters should be physically and chemically stable. The analysis of the charge density of the HOMO plot illustrates that a pair of hydrogen atoms prefer to occupy opposing on-top sites for clusters with an even n number. Studies of deformation charge density plots demonstrate that significant charge transfer occurs from the Si@Al₁₂ to the H atoms.     

Synthesis of bimetallic nanocompositions Au<Subscript>x</Subscript>Pd<Subscript>1-x</Subscript>/γ-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> for catalytic CO oxidation

Colloidal suspensions of Au_xPd_1-x nanoalloys were prepared via hydrazine co-reduction of [AuCl₄]^? and [PdCl₄]^2? complex anions in aqueous solution. High molecular weight polymeric compounds polyvinylpyrrolidone (PVP), polyvinyl alcohol (PVA), and cryptoionic surfactants (AF-6 and AF-12 neonols, Triton X-100) were used as surface capping agents. Nanoparticles prepared under different experimental conditions were immobilized on γ-Al₂O₃ supports. The removal of the capping agents from the surface of the active particles was achieved through calcination of samples in oxidative atmosphere (air, 500 °C). This pretreatment of the catalysts significantly enhances their performance. Powder XRD, TEM, and EDX were employed to characterize the structure, size, and composition of the Au_xPd_1-x/γ-Al₂O₃ catalysts. The immobilized particles consist of uniformly mixed alloys having multi-domain face-centered cubic structure with typical crystallite size of 3–6 nm. The activity of the prepared samples was examined with temperature-programmed CO oxidation reaction (TP-CO+O₂). Triton X-100 surfactant is superior in a number of parameters. Among all Au_xPd_1-x/γ-Al₂O₃ catalysts tested, the one stabilized with Triton X-100 (0.4%Au-0.2%Pd@Triton X-100) was found to have the highest activity for conversion of CO into CO₂.     

Field-induced valence transition in EuPtP<Subscript>1−x</Subscript>As<Subscript>x</Subscript>

The ternary compound EuPtP exhibits two valence transitions at T ₁ = 235 K and T ₂ = 190 K. In order to examine a field-induced valence transition of Eu, we synthesized EuPtP_1−x As _x compounds with 0.05 ≤ x ≤ 0.5 and studied the magnetic and valence behavior. The substitution of As for P increases the lattice volume linearly and decreases both valence transition temperatures, T ₁ and T ₂, in contrast to the behavior under external pressures. The magnetization process in a pulsed magnetic field revealed that EuPtP_0.5As_0.5 exhibits an onset of metamagnetic transition above 50 T with a large hysteresis, which evidences a first-order field-induced valence transition. The analysis of the magnetization curves of x = 0.5 at various temperatures has demonstrated that the field-induced transition is essentially the same as the transition induced by temperature at T ₁.     

Galvanomagnetic study of the quantum-well valence band of germanium in the Ge<Subscript>1−x</Subscript>Si<Subscript>x</Subscript>/Ge/Ge<Subscript>1−x</Subscript>Si<Subscript>x</Subscript> potential well

The structure of the quantum-well valence band in a Ge(111) two-dimensional layer is calculated by the self-consistent method. It is shown that the effective mass characterizing the motion of holes along the germanium layer is almost one order of magnitude smaller than the mass for the motion of heavy holes along the [111] direction in a bulk material (this mass is responsible for the formation of quantum-well levels). This creates a unique situation in which a large number of subbands appear to be populated at moderate values of the layer thickness d _w and the hole concentration p _s . The depopulation of two or more upper subbands in a 38-nm-thick germanium layer at a hole concentration p _s = 5 × 10¹⁵ m^?2 is revealed from the results of measuring the magnetoresistance in a strong magnetic field aligned parallel to the germanium layers. The destruction of the quantum Hall state at a filling factor ν = 1 indicates that the two lower subbands merge together in a self-formed potential profile of the double quantum well. It is demonstrated that, in a quasi-two-dimensional hole gas, the latter effect should be sensitive to the layer strain.     

First principles study on electronic and optical properties of Al<Subscript>x</Subscript>Ga<Subscript>1−x</Subscript>N and In<Subscript>y</Subscript>Ga<Subscript>1−y</Subscript>N

The band structure, density of states of Al_xGa_1?xN and In_yGa_1?yN was performed by the first-principles method within the local density approximation. The calculated energy gaps of the AlN, Al_0.5Ga_0.5N, GaN, In_0.5Ga_0.5N and InN were 5.48, 4.23, 3.137, 1.274 and 0.504 eV, which were in agreement with the experimental result. The dielectric functions, absorption coefficient and loss function were calculated based on Kramers–Kronig relations. Further more, the relationships between electronic structure and optical properties were investigated theoretically. For Al_xGa_1?xN and In_yGa_1?yN materials, the micromechanism of the optical properties were explained.     

Theoretical study on the electronic and magnetic properties of double perovskite La<Subscript>2−x</Subscript>Sr<Subscript>x</Subscript>MnCoO<Subscript>6</Subscript> (x = 0,1,2)

In this paper, the electronic and magnetic properties of double perovskite La_2−x Sr _x MnCoO₆ (x = 0,1,2) have been studied using the local-spin-density approximation + U method. For the three compositions investigated, the low symmetry P2₁/n structure yields consistently lower energy than that of the high symmetry \hbox{Fm[`3]mFm\bar{3}m} Fm3̅m structure. The strong electronic correlation and the orbital polarization of Co-d electrons play crucial roles. In agreement with experiments, we find that La<sub>2</sub>MnCoO<sub>6</sub> is a ferromagnetic insulator with both Mn and Co ions in their high-spin states. The tilting of oxygen octahedrons is most significant in this case and is responsible for its insulating behavior; for LaSrMnCoO<sub>6</sub>, the ground state remains a ferromagnetic insulator with Mn and Co ions in their high-spin states. The optimized P2<sub>1</sub>/n and \hbox{Fm[`3]mFm\bar{3}m}Fm3̅m crystal structures are nearly the same, and the P2₁/n structure is stabilized by the spontaneous layer-wise antiferro-orbital ordering of Co-d electrons. We also predict that Sr₂MnCoO₆ is a ferromagnetic metal, and its electronic structure can be viewed as a rigid band shifting from that of LaSrMnCoO₆. Due to the strong covalency between transition metal and oxygen ions, the valences of Mn and Co ions differ considerably from those derived from purely ionic model. Also, doping induced holes mainly go to oxygen sites though the density of states near the Fermi energy has strong mixed character. This feature, together with the orbital ordering phenomenon, should be observable via the X-ray near-edge absorption spectroscopy and the polarized X-ray diffraction spectra.     

Computational investigation of CO adsorbed on Au<Subscript>x</Subscript>, Ag<Subscript>x</Subscript> and (AuAg)<Subscript>x</Subscript> nanoclusters (x = 1 − 5, 147) and monometallic Au and Ag low-energy surfaces

Density functional theory calculations have been performed to investigate the use of CO as a probe molecule for the determination of the structure and composition of Au, Ag and AuAg nanoparticles. For very small nanoclusters (x = 1 ? 5), the CO vibrational frequencies can be directly correlated to CO adsorption strength, whereas larger 147-atom nanoparticles show a strong energetic preference for CO adsorption at a vertex position but the highest wavenumbers are for the bridge positions. We also studied CO adsorption on Au and Ag (100) and (111) surfaces, for a 1 monolayer coverage, which proves to be energetically favourable on atop only and bridge positions for Au (100) and atop for Ag (100); vibrational frequencies of the CO molecules red-shift to lower wavenumbers as a result of increased metal coordination. We conclude that CO vibrational frequencies cannot be solely relied upon in order to obtain accurate compositional analysis, but we do propose that elemental rearrangement in the core@shell nanoclusters, from Ag@Au (or Au@Ag) to an alloy, would result in a shift in the CO vibrational frequencies that indicate changes in the surface composition.     

Structural and magnetic phases of Bi<Subscript>1−x</Subscript>A<Subscript>x</Subscript>FeO<Subscript>3−</Subscript><Subscript>δ</Subscript> (A = Sr,Pb) perovskites

The Bi_1−xA_xFeO_{3− δ} (A = Sr, Pb) systems have been studied using the X-ray, neutron powder diffraction and magnetization measurements in a magnetic field up to 14 T. It was found that around x ∼ 0.06 the crystal symmetry changes from a rhombohedral (space group R3c) to pseudo-tetragonal. In the composition range 0.07 ≤ x ≤ 0.14 the phases with different symmetry of the unit cell coexist independent of synthesis conditions. The neutron powder diffraction shows that the iron ions have average oxidation state close to 3+. The magnetic structure for Bi_0.5Sr_0.5FeO_{3− δ} is found to be G-type antiferromagnetic with magnetic moment of about 3.8 μ_B/Fe³⁺. The weak ferromagnetic state due to magnetoelectric interactions was revealed in the lightly doped rhombohedrally distorted compositions. No evidence for a spontaneous magnetization was observed for the pseudo-tetragonal phases. These compositions show irreversible nonlinear magnetization vs. field behavior apparently due to small local deviations from the collinearity of the magnetic moments.     

Photoluminescence of TlCu<Subscript>1−x</Subscript>GaSe<Subscript>2</Subscript> single crystals

The results of the study of photoluminescence and its excitation spectra in Tl _x Cu_1−x GaSe₂ single crystals are presented. The crystals under study are layered and characterized by anisotropic optical properties. In this respect, it is important to investigate optical properties of the crystals under study.     

Magnetocaloric properties of manganites La<Subscript>1−x</Subscript>(Ag,K)<Subscript>x</Subscript>MnO<Subscript>3</Subscript>

The temperature and magnetic-field dependences of the magnetocaloric effect in manganites La_1−x Ag _x MnO₃ (x = 0.1; 0.15; 0.2) and La_1−x K _x MnO₃ (x = 0.1; 0.11; 0.13; 0.15; 0.175) were studied by a direct method. Large changes in the sample temperature were detected as a magnetic field changed by ΔH = 10 kOe. Temperatures of the magnetocaloric effect maxima are near room temperatures. Field dependences of the magnetocaloric effect show no signs of saturation in fields to 30 kOe.     

Computation of the electronic structure and direct-gap absorption spectra in Ge-rich Si<Subscript>1−x</Subscript> Ge<Subscript>x</Subscript>/Ge/Si<Subscript>1−x</Subscript>Ge<Subscript>x</Subscript> type-I quantum wells

This paper presents the results of conduction band discontinuities calculation for strained/relaxed Si_1?x Ge _x /Si_1?y Ge _y heterointerfaces in Γ _15C , Γ _2′C and L upper bands minima, as well as the room-temperature strained (vs. relaxed) band gaps deduced from the classical model-solid theory. Based upon the obtained data, we propose a type-I W-like Si_1?y Ge _y /Si_1?x Ge _x /Ge/Si_1?x Ge _x /Si_1?y Ge _y quantum wells heterostructure optimized in terms of compositions and thicknesses. Electronic states and wave functions are found by solving Schrödinger equation without and under applied bias voltage. An accurate investigation of the optical properties of this heterostructure is done by calculating the energies of the interband transitions and their oscillator strengths. Moreover, a detailed computation of the bias-voltage evolution of the absorption spectra is presented. These calculations prove the existence of type-I band alignment at Γ _2′C point in compressively strained Ge quantum wells grown on relaxed Ge-rich Si_1?y Ge _y buffers. The strong absorption coefficient (> 8 × 10³ cm^-1) and the large Stark effect (0.1 eV @ 2 V) of the Γ _2′C transitions thresholds open up perspectives for application of these heterostructures for near-infrared optical modulators.     

AC and DC properties of M-type SrCo<Subscript>x</Subscript>Ti<Subscript>x</Subscript>Fe<Subscript>(12−2x)</Subscript>O<Subscript>19</Subscript> hexagonal ferrite

Microwave characterization of SrCo _x Ti _x Fe_(12?2x)O₁₉ (x = 0.0, 0.2, 0.3, 0.5, 0.6, 0.7, 1.0) ferrites has been studied as a function of frequency, substitution and thickness, and static electrical current density-electric field characteristics have been investigated as a function of substitution. Microwave characteristics have been measured using power meter in the rectangular slotted waveguide and current density is measured using electrometer. The microwave absorption is evaluated using the standard available model. The results depict ?11.57 dB reflection loss at 10.38 GHz in composition x = 0.6. The electrical current density decreases at lower substitution and increases at higher substitution. The substitution of Co²⁺ and Ti⁴⁺ ions causes enhancement of electromagnetic and static electrical properties.     

Dynamic magnetic susceptibility of compounds in Gd<Subscript>5</Subscript>Si<Subscript>2−x</Subscript>Ge<Subscript>2−x</Subscript>Sn<Subscript>2x</Subscript> (2x = 0 − 0.1) system

The dynamic magnetic susceptibility (χ_ac) of magnetically ordered Gd₅Si_2?xGe_2?xSn_2x compounds with the partial substitution for silicon and germanium atoms by isovalent tin atoms (2x = 0 ? 0.1) has been investigated experimentally. From the temperature dependence of χ_ac the Curie temperatures of these alloys are determined. It is established that tin-doped alloys have higher Curie temperatures as compared to Gd₅Si₂Ge₂ (ΔT _C ≈ 15 K).     

Rotatable anisotropy of epitaxial Fe<Subscript>1−x</Subscript>Ga<Subscript>x</Subscript> thin films

We show by a combined magnetic force microscopy and synchrotron radiation spectroscopy study that stripe-like patterned magnetic domains are present in Fe_1?x Ga _x thin films. These stripes, whose origin is attributed to an out-of-plane magnetic component, can be rotated by an external magnetic field.     

Microwave characterization of Pb<Subscript>1−x</Subscript>Ca<Subscript>x</Subscript>Fe<Subscript>0.5</Subscript>Nb<Subscript>0.5</Subscript>O<Subscript>3</Subscript> multiferroics at X-band

The microwave characteristics of Pb_1?x Ca _x Fe_0.5Nb_0.5O₃ multiferroics (x = 0.0, 0.4, 0.45, 0.5, 0.55, 0.6), have been investigated as a function of frequency and substitution. The results depict ?13.99 dB reflection loss at 11.65 GHz in composition x = 0.6. Microwave absorption is enhanced with substitution of Ca²⁺ ions and undoped composition 0.0 behaves as electromagnetic shield. The model governing microwave absorption is discussed and different compositions for electromagnetic applications have been suggested.     

DFT study of CO adsorption on neutral and charged Pd<Subscript>n</Subscript>(n = 1–7) clusters

Density functional calculations have been performed to investigate CO adsorption on neutral, cationic and anionic Pd_n (n=1–7) clusters. From the results, it is observed that the binding of CO molecule to neutral and cationic palladium clusters takes place via 1-, 2- and 3-fold coordination. On the other hand, only terminal adsorption of CO molecule is possible in anionic clusters barring bridging adsorption in Pd₇ ^- cluster.     

Spin-polarized first-principles study of ferromagnetism in zinc-blende In<Subscript>1−x</Subscript>Mn<Subscript>x</Subscript>Sb

First-principles calculations based on the density functional theory are performed to study the structural properties, spin-polarized electronic band structures, density of states and magnetic properties of the zinc blende In_{1− x} Mn _x Sb (x = 0.125, 0.25, 0.50, 0.75, 1.0). The calculated lattice constants of In_{1− x} Mn _x Sb obey the Vegard’s law with a marginal upward bowing. With the increase of Mn concentration in In_{1− x} Mn _x Sb, a transition from the semi-metallic to the half-metallic behavior happens such that the majority-spin valence states crosses the Fermi level and the minority-spin states have a gap at the Fermi level. A large exchange splitting (∼ 4 eV) is observed between Mn 3d states of the majority-spins and the minority-spins. The total magnetic moment of In_{1− x} Mn _x Sb half-metallic ferromagnets per Mn atom basis is 4μ _B. The total magnetic moment per Mn atom indicate that Mn atoms act as acceptors in InSb and contribute to holes in the lattice of InSb. Due to p-d hybridization, the free space charge of Mn reduces that results a loss in its magnetic moment. The loss in the magnetic moment of the Mn atoms is converted into a small local magnetic moments on the In and Sb sites.     

Nernst effect of stripe ordering La<Subscript>1.8−x</Subscript>Eu<Subscript>0.2</Subscript>Sr<Subscript>x</Subscript>CuO<Subscript>4</Subscript>

We investigate the transport properties of La_1.8−xEu_0.2Sr_xCuO₄ (x = 0.04, 0.08, 0.125, 0.15, 0.2) with a special focus on the Nernst effect in the normal state. Various anomalous features are present in the data. For x = 0.125 and 0.15 a kink-like anomaly is present in the vicinity of the onset of charge stripe order in the LTT phase, suggestive of enhanced positive quasiparticle Nernst response in the stripe ordered phase. At higher temperature, all doping levels except x = 0.2 exhibit a further kink anomaly in the LTO phase which cannot unambiguously be related to stripe order. Moreover, a direct comparison between the Nernst coefficients of stripe ordering La_1.8−xEu_0.2Sr_xCuO₄ and superconducting La_2−xSr_xCuO₄ at the doping levels x = 0.125 and x = 0.15 reveals only weak differences. Our findings make high demands on any scenario interpreting the Nernst response in hole-doped cuprates.     

Structure and magnetic properties of La-doped Si<Subscript>n</Subscript> (n = 1–12, 24) clusters: a density functional theory investigation

In the framework of density functional theory (DFT), the structure and magnetic properties of La-doped Si_n clusters have been systematically studied. It is found that La atom always prefers locating on the surface site of the clusters and no cage-like structures are found up to n = 12. The La atom doping remarkably enhances the stabilities of silicon clusters. In contrast to some other clusters with the magnetic moment completely quenched, La doping is shown to invariable magnetism with the total spin magnetic moment of LaSi_n clusters 1 _¼B up to n = 24, as that of a free La atom.     

Strong electron-phonon coupling in Be<Subscript>1−x</Subscript>B<Subscript>2</Subscript>C<Subscript>2</Subscript>: ab initio studies

Several structures for off-stoichiometric beryllium diboride dicarbide Be_1−xB₂C₂ have been designed, and their properties studied from first-principles density functional methods. Among the most stable phases examined, the layered hexagonal structures are shown to exhibit various features in the electronic properties and in the lattice dynamics reminiscent of the superconducting magnesium diboride and alkaline earth-intercalated graphites. For substoichiometric composition x ∼ 1/3, the system is found metallic with a moderately strong electron-phonon coupling through a predominant contribution arising from high frequency streching modes modulating the σ-bonding of the B-C network, and a weaker contribution at medium frequency range of the phonon spectra, arising from the intercalent motion coupled to the π-bonding states. Further, anharmonicities emerging from the proximity of the Fermi level to the σ-band edge, contributes to reduce the phonon softening hence stabilizing the structure. All these effects appear to combine favourably to produce a high temperature phonon-superconductivity.