Metal–insulator phase transition in hydrogenated thin films of V<Subscript>2</Subscript>O<Subscript>3</Subscript>

Temperature dependences of the electrical conductivity of thin vanadium sesquioxide V2O3 films obtained by using the laser sputtering technique have been studied. A significant decrease (by four–five orders of magnitude) in the electrical conductivity has been observed below 150 K as a result of a metal–insulator phase transition. It is shown that hydrogenation of films lowers the temperature of this phase transition.     

Anomalies in the magnetic and magnetoelastic properties of Nd<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Ca<Subscript>x</Subscript>MnO<Subscript>3</Subscript> single crystals in strong magnetic fields

Measurements of the magnetic and magnetoelastic properties of Nd_0.5Ca_0.5MnO₃, Nd_0.55Ca_0.45MnO₃, and Nd_0.6Ca_0.4MnO₃ single crystals have revealed correlations between the observed anomalies in magnetization and magnetostriction associated with suppression of the antiferromagnetic phase and charge ordering by a strong magnetic field followed by transition to the ferromagnetic state. H-T phase diagrams are constructed.     

First-order phase transition characteristic of the high temperature metal–semiconductor transition in [Ca<Subscript>2</Subscript>CoO<Subscript>3</Subscript>]<Subscript>0.62</Subscript>[CoO<Subscript>2</Subscript>]

Transportation and thermodynamic properties of misfit-layered polycrystalline [Ca₂CoO₃]_0.62[CoO₂] were measured in order to clarify the nature of metal– semiconductor transition (MST) at T _MS≈400 K, above which the simultaneous decrease of resistivity and increase of thermopower with temperature give rise to a great enhancement of thermoelectric power factor up to 1000 K. A first-order phase transition characteristic around T _MS was revealed by anomalies of resistivity, differential scanning calorimetry, and thermal expansion. The first-order characteristic of the MST can be rationalized from the Virial theorem at an itinerant to localized electron transition in the narrow e ^T band within the [CoO₂] plane. Above T _MS, the reduction of the retained delocalized states within the matrix of localized states and the enhancement of charge carrier effective mass with increasing temperature might account for the considerable enhancement of the thermopower.     

Phase transition in the vortex structure of granular YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7 − δ</Subscript> HTSCs in weak magnetic fields

This study aims at establishing the interrelation between the current-carrying capacity and peculiarities of magnetoresistance of granular YBa₂Cu₃O_{7 ? δ} HTSCs (T _c = 92.5 K). The transverse magnetoresistance of several batches of YBa₂Cu₃O_{7 ? δ} HTSC samples with noticeably different values of critical supercurrent density j _c is measured in magnetic fields H _ext up to H _ext ^max ≈ 500 Oe in a wide range of transport currents (5 mA ≤ I ≤ 1600 mA) at T = 77.4 K. Samples with relatively high values of j _c (H _ext = 0) ≥ 100 A/cm² do not exhibit any anomalies in their field dependences. Magnetoresistance jumps δρ_BG-VG/ρ_273K are observed for samples with low values of j _c ≥ 20 A/cm² in fields H _BG-VG ≈ 200–260 Oe. The width ΔH _BG-VG of the anomalous resistance region increases upon an increase in I. The magnetoresistance jumps decrease with increasing I in increasing field H _ext(0 → H _ext ^max ) and increase in decreasing field H _ext(H _ext ^max → 0). It is found that these peculiarities of the field dependences of magnetoresistance are associated with a first-order phase transition (in magnetic field) in the vortex structure of YBa₂Cu₃O_{7 ? δ} HTSCs of the Bragg glass-vortex glass type.     

Synthesis,structure, and optical and photocatalytic properties of quasi-one-dimensional ZnO doped with Со<Subscript>3</Subscript>O<Subscript>4</Subscript> and carbon

One-dimensional nanocomposites Zn_1–x Co _x O_1–y С _у :nCo₃O₄ and solid solutions Zn_1–x Co _x O_1–y С _у , which are promising photocatalysts for the oxidation of toxic organic compounds in visible light, are obtained via the thermolysis of Zn_1–x Co _x (HCOO)(OCH₂CH₂O)_1/2 (0.1 ≤ x ≤ 0.5) precursor in a controlled gaseous atmosphere.     

Investigation on the effects of addition of SiO<Subscript>2</Subscript> nanoparticles on ionic conductivity,FTIR, and thermal properties of nanocomposite PMMA–LiCF<Subscript>3</Subscript>SO<Subscript>3</Subscript>–SiO<Subscript>2</Subscript>

Composite polymer electrolyte systems composed of poly(methyl methacrylate) (PMMA) as the host polymer, lithium trifluoromethanesulphonate (also known as lithium triflate; LiCF₃SO₃) as dopant salt, and a variety of different concentrations of nano-sized fumed silica (SiO₂) as inorganic filler were studied. The effect upon addition of SiO₂ on the ionic conductivity of the composite polymer electrolytes was investigated, and it was proven that the ionic conductivity had been enhanced. In addition, the interfacial stability also showed improvement. Maximum conductivity was obtained upon addition of 2 wt.% SiO₂. The complexation of PMMA and LiCF₃SO₃ was verified through Fourier transform infrared studies. The thermal stability of the polymer electrolytes was also found to improve after dispersion of inorganic filler. This was proven in the thermogravimetric studies.     

Electronic structure of γ -Li<Subscript>3</Subscript>N

The results of ab initio calculations of electronic structure of the cubic phase of lithium nitride are presented, which were performed using the pseudo-atomic orbitals scheme. An equilibrium value of the lattice constant, bulk elasticity moduli and atomic charges are obtained. A detailed analysis of the band structure is presented, relying on the investigation of atomic state dispersion pattern, and an optical absorption spectrum is obtained.     

Synthesis and magnetic properties of CdS/α-Fe<Subscript>2</Subscript>O<Subscript>3</Subscript> hierarchical nanostructures

CdS/α-Fe₂O₃ hierarchical nanostructures, where the CdS nanorods grow irregularly on the side surface of α-Fe₂O₃ nanorods, were synthesized via a three-step process. The diameters and lengths of CdS nanorods can be tuned by changing the ethylenediamine (EDA) and Cd ion concentrations. The magnetic investigations by superconducting quantum interference device indicate that the hierarchical nanostructures have an Morin transition at lower temperature (230 K) than that of the single bulk α-Fe₂O₃ materials (263 K). Importantly, the hierarchical nanostructures exhibit weakly ferromagnetic characteristics at 300 K. A sharp peak assigned to the surface trap induced emission are observed in room temperature PL spectra. Combining with the optoelectronic properties of CdS, the CdS/α-Fe₂O₃ hierarchical nanostructures may be used as multi-functional materials for optoelectronic and magnetic devices. Supported by the National Natural Science Foundation of China (Grant Nos. 50772025 and 50872159), the Ministry of Science and Technology of China (Grant No. 2008DFR20420), the China Postdoctoral Science Foundation (Grant Nos. 20060400042 and 200801044), the Natural Science Foundation of Heilongjiang Province, China (Grant No. F200828), the Specialized Research Fund for the Doctoral Program of Higher Education of China (Grant No. 20070217002), and the Innovation Foundation of Harbin City (Grant No. RC2006QN017016)     

Electronic properties of A<Subscript>2</Subscript>B<Subscript>6</Subscript> quantum dots incorporated into Langmuir–Blodgett films

An experimental and theoretical study is performed for features of the electronic spectrum of CdSe/CdS/ZnS quantum dots incorporated into Langmuir–Blodgett films. Analysis of the investigated samples assesses the state of the first three levels of the electron spectrum for a quantum object. Good qualitative and quantitative agreement is achieved between the experimental results and theoretical estimates. It is shown that the mechanism of the observed field emission current through a quantum dot is described satisfactorily by the Morgulis–Stratton theory under the considered experimental conditions.     

Strong electron–phonon interaction and colossal magnetoresistance in EuTiO<Subscript>3</Subscript>

At low temperatures, EuTiO₃ system has very large resistivities and exhibits colossal magnetoresistance. Based on a first principle calculation and the dynamical mean-field theory for small polaron we have calculated the transport properties of EuTiO₃. It is found that due to electron–phonon interaction the conduction band may form a tiny polaronic subband which is close to the Fermi level. The tiny subband is responsible for the large resistivity. Besides, EuTiO₃ is a weak antiferromagnetic material and its magnetization would slightly shift the subband via exchange interaction between conduction electrons and magnetic atoms. Since the subband is close to the Fermi level, a slight shift of its position gives colossal magnetoresistance.     

Heat capacity and thermodynamic properties of HoMnO<Subscript>3</Subscript> in the range of 364–1046 K

The temperature dependence of the molar heat capacity of HoMnO₃ has been measured by differential scanning calorimetry. The experimental data have been used to calculate the thermodynamic properties of the oxide compound (changes in the enthalpy H°(T)–H°(364 K), entropy S°(T)–S°(364 K), and reduced Gibbs energy Φ°(T)). The data on the heat capacity of HoMnO₃ have been generalized in the range of 40–1000 K.     

Theoretical study of the structure of lead zirconate–titanate PbZr<Subscript>0.6</Subscript>Ti<Subscript>0.4</Subscript>O<Subscript>3</Subscript>

A model of the structure of the piezoelectric ceramic lead zirconate–titanate PbZr_1–x Ti _x O₃ (PZT) is proposed. The model is based on ab initio calculations for possible local structures using the density functional theory (DFT) approach. A comparison of the calculated neutron diffraction data for local structures and the measured diffraction data obtained for actual powder samples shows there is a partially established long-range crystalline order in the material, in the sublattice of Zr and Ti cations.     

Electrical switching in the MoO<Subscript>3</Subscript>–WO<Subscript>3</Subscript>–P<Subscript>2</Subscript>O<Subscript>5</Subscript> and MoO<Subscript>3</Subscript>–P<Subscript>2</Subscript>O<Subscript>5</Subscript> glasses

High field electrical switching on blown films of MoO₃(60%)–P₂O₅(40%), MoO₃(50%)–WO₃(10%)–P₂O₅(40%), and MoO₃(45%)–WO₃(15%)–P₂O₅(40%) having different thicknesses was studied and compared. Switching was observed using two terminal samples. S-type current–voltage characteristic (current-controlled negative resistance—CCNR) with memory was observed in molybdenum–phosphate glasses, but N-type characteristic (voltage-controlled negative resistance—VCNR) with threshold in tungsten–molybdenum–phosphate glasses was observed. The important observation was that with the addition of WO₃ to binary MoO₃–P₂O5 led to a change of I–V characteristic from CCNR with memory to VCNR with threshold. The measurements of density and molar volume showed linear relation between MoO₃ content and density which decreased with the increase of MoO₃ content. The samples’ thickness had no significant effect on threshold voltage. The attained results also indicated that the electrode material had no effect on switching property of devices. The switching behavior of the devices did not show any dependence on the polarity of the applied voltage. In terms of the effect of heat on the switching behavior of molybdenum–phosphate glasses, it was found that threshold voltage decreases with increasing of temperature. Finally, the switching phenomenon was explained by thermal (formation of crystalline filaments) and electronic models.     

Elastic properties of TeO<Subscript>2</Subscript>–B<Subscript>2</Subscript>O<Subscript>3</Subscript>–Ag<Subscript>2</Subscript>O glasses

A series of glasses [(TeO₂) _x (B₂O₃)_1−x ]_1−y [Ag₂O] _y with x = 70 and y = 10, 15, 20, 25 and 30 mol% were synthesised by rapid quenching. Longitudinal and shear ultrasonic velocity were measured at room temperature and at 5 MHz frequency. Elastic properties, Poisson's ratio, microhardness, softening temperature and Debye temperature have been calculated from the measured density and ultrasonic velocity at room temperature. The experimental results indicate that the elastic constants depend upon the composition of the glasses and the role of the Ag₂O inside the glass network is discussed. Estimated parameters based on Makishima–Mackenzie theory and bond compression model were calculated in order to analyse the experimental elastic moduli. Comparison between the experimental elastic moduli data obtained in the study and the calculated theoretically by the mentioned above models has been discussed.     

Electronic structure of α-Al<Subscript>2</Subscript>O<Subscript>3</Subscript>: Ab initio simulations and comparison with experiment

Al₂O₃ films 150 Å thick are deposited on silicon by the ALD technique, and their x-ray (XPS) and ultraviolet (UPS) photoelectron spectra of the valence band are investigated. The electronic band structure of corundum (α-Al₂O₃) is calculated by the ab initio density functional method and compared with experimental results. The α-Al₂O₃ valence band consists of two subbands separated with an ionic gap. The lower band is mainly formed by oxygen 2s states. The upper band is formed by oxygen 2p states with a contribution of aluminum 3s and 3p states. A strong anisotropy of the effective mass is observed for holes: m _h⊥ ^* ≈ 6.3m ₀ and m _h‖ ^* ≈ 0.36m ₀. The effective electron mass is independent of the direction m _e‖ ^* ≈ m _e⊥ ^* ≈ 0.4m ₀.     

Synthesis and characterization of CoFe<Subscript>2</Subscript>O<Subscript>4</Subscript>–Ba<Subscript>0.9</Subscript>Sr<Subscript>0.1</Subscript>TiO<Subscript>3</Subscript> magnetoelectric composites with dielectric and magnetic properties

Composite structures consisting of (001)-oriented SrTiO₃ (STO)/La_0.7Sr_0.3MnO₃ (LSMO) films of 30 nm thickness, grown on an (001) Pb(Mg_1/3Nb_2/3)TiO₃– 28 mol.% PbTiO₃ piezoelectric relaxor-ferroelectric single-crystalline wafer were investigated by means of Wide-Angle X-ray Diffraction (WAXRD) in situ under influence of a d.c. electric field with strength E up to ±18 kV/cm. The WAXRD measurements of the films and substrate reflection profiles resulted in a determination of the strain s in the films and the substrate separately. The strained state of the STO/LSMO films is effectively controlled by a huge converse piezoelectric effect of the PMN-PT substrate. The coefficients of coupling between electric-field-induced out-of-plane strain in the films and in the substrate for the composite system STO/LSMO/PMN-PT are obtained.     

Magnetic and magnetoresistive properties of Al<Subscript>2</Subscript>O<Subscript>3</Subscript>–Sr<Subscript>2</Subscript>FeMoO<Subscript>6–δ</Subscript>–Al<Subscript>2</Subscript>O<Subscript>3</Subscript> nanoheterostructures

A method has been developed for fabricating nanoporous matrices based on anodic aluminum oxide for the deposition of ferromagnetic nanoparticles in them. The modes of deposition of strontium ferromolybdate thin films prepared by the ion-plasma method have been worked out, and the magnetic and magnetoresistive properties, structure, and composition of the films have been investigated. It has been revealed that the microstructure and properties of the strontium ferromolybdate films deposited by ionplasma sputtering depend on the deposition rate and the temperature of the substrate. Based on the measurement of the electrical resistivity of nanoheterostructures in a magnetic field, it has been found that the magnetoresistance reaches 14% at T = 15 K and B = 8 T, which is due to the manifestation of tunneling magnetoresistance.     

Computational study on the half-metallicity in transition metal—oxide-incorporated 2D g-C<Subscript>3</Subscript>N<Subscript>4</Subscript> nanosheets

In this study, based on the first-principles calculations, we systematically investigated the electronic and magnetic properties of the transition metal–oxide-incorporated 2D g-C₃N₄ nanosheet (labeled C₃N₄–TM–O, TM = Sc–Mn). The results suggest that the TM–O binds to g-C₃N₄ nanosheets strongly for all systems. We found that the 2D C₃N₄–TM–O framework is ferromagnetic for TM = Sc, Ti, V, Cr, while it is antiferromagnetic for TM = Mn. All the ferromagnetic systems exhibit the half-metallic property. Furthermore, Monte Carlo simulations based on the Heisenberg model suggest that the Curie temperatures (T _c ) of the C₃N₄–TM–O (TM = Sc, Ti, V, Cr) framework are 169 K, 68 K, 203 K, and 190 K, respectively. Based on Bader charge analysis, we found that the origin of the half-metallicity at Fermi energy can be partially attributed to the transfer of electrons from TM atoms to the g-C₃N₄ nanosheet. In addition, we found that not only electrons but also holes can induce half-metallicity for 2D g-C₃N₄ nanosheets, which may help to understand the origin of half-metallicity for graphitic carbon nitride.     

Reciprocal and nonreciprocal magnetic linear birefringence in the γ-Dy<Subscript>2</Subscript>S<Subscript>3</Subscript> sesquisulfide

A study has been made of the spectral dependence of the Cotton-Mouton effect (CME) quadratic in magnetic field, nonreciprocal birefringence (NB) linear in magnetic field, and the Faraday effect (FE) in the cubic magnetic semiconductor γ-Dy₂S₃. Unlike the FE, the CME and the NB in this crystal are anisotropic, with the pattern of the anisotropy being dependent on the photon energy. The dependence of the CME and NB dispersion on the direction of the magnetic field B indicates contribution from a variety of electronic transitions and mechanisms to these phenomena. It is shown that the resonant contributions to the CME and NB in the transparency region originate from electronic transitions near E?3.4 eV (beyond the band edge E _g=2.8 eV), which are likely transitions from the localized ground state of the Dy³⁺ ion to states derived from mixing of the band and 4f ^N?1 5d states of the dysprosium ion. The character of the CME anisotropy in the transparency region and near the local electronic transition ⁶ H _15/2 → ⁶ F _3/2 connecting states of the unfilled 4f shell of the Dy³⁺ ion suggests the presence of a strong axial component of the crystal field acting on the rare earth ion.