Thermal boundary between normal conductivity and fluctuation superconductivity in YBa2Cu3O7 − δ

The absence of a linear behaviour of the dependence of the resistance() over a temperature interval 77–300 K was observed for ceramic superconductors YBa₂Cu₃O_7 − δand thick polycrystallin films of the same composition with the addition of silver. A comparative analysis of the excess conductivity for HTSC and metal samples, characterizing the deviationR(T) from a linear dependence, allows one to determine the onset temperature of fluctuation superconductivity: within 105–110 K. The analysis of fluctuation superconductivity is performed using the Aslamazov–Larkin model. A smooth transition from the one-dimensional to the two- and three-dimensional nature of superconducting fluctuations in the order parameter was observed for ceramic samples as the temperature decreased.     

Hole superconductivity in Nd2 − zCezCuO4

The reclassification of Nd_2 − zCe_zCuO₄from an electron superconductor to a hole superconductor is suggested by simple valence and electrostatics arguments: In Nd_2 − zCe_zCuO₄, isolated Ce substituting for Nd is not an electron donor; the proposed actual dopant is a defect-complex of Ce and interstitial oxygen which provides holes, making Nd_2 − zCe_zCuO₄a hole superconductor.     

Predictions of the bonding properties inCd1 − xZnxTe

The electronic, optical and elastic properties of the ternary II–VI semiconductor alloys Cd_1 − x Zn _xTe are calculated by thesp³s ^*  semi-empirical tight-binding theory and the bond-orbital model. We found a nonlinear decrease of the transverse effective charge and refractive index and a nonlinear increase of the bandgap and elastic constants with increasing Zn composition x. For all these behaviours, the corresponding bowing factors are predicted. The results are compared with previous theoretical estimates and experiments.     

Modification of B-site doping of perovskite LaxSr1 − xFe1 − y−zCoyCrzO3 − δ oxide by Mg2+ ion

Co-doping B-site of perovskite oxide La_xSr_1 ? xCo_yFe_1 ? yO_3 ? δ (LSCFO) with Cr⁶⁺ and Mg²⁺ ions has been attempted in this research for revamping chemical stability and oxygen ionic conductivity of this mixed conducting oxide. It is known that partial substitution for B-site cations of LSCFO by Cr gives rise to a significant improvement on chemical and thermal stability of the perovskite oxide. On the basis of this doped structure, introduction of an immaterial dose of Mg²⁺ ion into its B-site results in a microstructure consisting of smaller grains with higher density than its precursor. Furthermore, the resulting perovskite oxide La_0.19Sr_0.8Fe_0.69Co_0.1Cr_0.2 Mg_0.01O_3 ? δ (LSFCCMO) displays higher O^2? conductivity than the solely Cr-doped LSCFO besides the improved chemical stability against reduction in 5% CH₄/He stream at 850 °C. A detailed examination of the oxidation states of B-site transition metal ions by XPS has also been conducted as a part of structural characterizations of LSFCCMO. The assessment of relative O^2? conductivity shows that the grain boundary area plays a more important role than the bulk phase in facilitating ion transport, but with comparable boundary areas the higher densification level is favorable.     

Calculation of the optical and electronic properties of the zinc-blende alloy Zn1 − xMgxSe

In order to clarify the electronic and optical properties of wide-energy gap zinc-blende structures ZnSe, MgSe and their alloys (ZnSe)_1 − x(MgSe)_x, a simple pseudo-potential scheme (EPM) within an effective potential, the virtual crystal approximation (VCA) which incorporates compositional disorder as an effective potential, is presented. Various quantities, including the fundamental band gap, the energies of several optical gaps, charge densities, ionicity character, transverse effective charge, and refractive index are obtained for this alloy.     

Structural and electrochemical properties of layered lithium nitridocuprates Li3 − xCuxN

We report a systematic study of the layered lithium nitridocuprates Li_3 ? xCu_xN with 0.1 ≤ x ≤ 0.39. The structural data obtained from experimental XRD patterns, Rietveld refinements and unit cell parameters calculation vs x, indicate that copper (I) substitute interlayer lithium ions in the parent nitride Li₃N to form the Li_3 ? xCu_xN compound without any Li vacancy in the Li₂N^? layer. Electrochemical results report Li insertion into the corresponding layered structures cannot take place in the 1.2/0.02 V voltage range as in the case of lithium into nitridonickelates and nitridocobaltates. However, in the initial charge process of Li_3 ? xCu_xN at 1.4 V leading to a specific capacity higher than 1000 mA h/g, the oxidation of copper and nitride ions is probably involved inducing a strong structural disordering process. As a consequence a new rechargeable electrochemical system characterized by discharge–charge potential of ≈ 0.3 V/1.2 V appears from the second cycle. Cycling experiments 0.02 V voltage/0.02 V range induce a complete destruction of the layered host lattice and the presence of Cu₃N in the charge state suggests a conversion reaction. The capacity recovered in the 1.4/0.02 V range practically stabilizes around 500 mA h/g after 20 cycles.     

Magnetic properties of the high Tc superconductor YBa2Cu3O7−δ

Magnetic measurements have been performed on the YBa₂Cu₆O_7−δ superconducting system in the temperature range 4.2–320 K. A hystereris loop indicating a coexistence of the superconducting and magnetic ordering below H_c1 at 4.2 K has been recorded.     

Thermal and magnetic properties of some YBa2Cu3O7−δ samples

Thermal and magnetic measurements have been performed on several YBa₂Cu₃O_7−δ compounds, some ones showing a large content of high T_c (93 K) superconducting phase. A jump in the specific heat ΔCp, is well evidenced at the transition allowing a determination of the ratio ΔCp/T_c ≅ 23 ± 5. mJ/ (mole Cu)K². In addition, an estimation of the γ value (≅ 11 mJ/(mole Cu). K²) has been drawn from the determination of the electronic entropy at T_c. The samples have been characterized by susceptibility, magnetization and resistivity experiments. The critical field slopes at T_c were found to be dHc₁/dT ≅ 17 Oe/K and dHc₂/dT ≅ 20 kOe/K. The results are discussed in the framework of the Ginzburg-Landau theory.     

On the structure of non-superconducting YBa2Cu3O6+ɛ

The crystal structure of non-superconducting, tetragonal YBa₂Cu₃O₆₊ ( close to 0) (s.g.P4/mmm) was studied at room temperature by means of single crystal X-ray diffraction, electron microscopy and electron diffraction. Crystals may easily be obtained by heating YBa₂Cu₃O₇ samples under vacuum. For 0, the tetragonal single crystals are chemically and crystallographically homogeneous — without any twin domains. The only defects immediately apparent are stacking faults alongc. The structure is very closely related to that of the high-T _c superconductor YBa₂Cu₃O₇. The most important difference is, that the oxygen on theb-axis (O(1)), which together with Cu(1) forms Cu–O–Cu–O-chains alongb in YBa₂Cu₃O₇, is vacant in YBa₂Cu₃O₆. Some crystals show a superstructure with superstructure reflections which are sharp in thea ^*–a ^*-plane but form diffuse streaks alongc ^*. We propose a model which relates these superstructure reflections to a 2-dimensional ordering of residual oxygen atoms on the O(1) site (corresponding to >0 in the chemical formula YBa₂Cu₃O₆₊).     

Temperature dependence of the Cu(2) NQR line width in YBa2Cu3O7−y

Precision X-ray structural studies were carried out for LiNbO₃:Zn_x single crystals with x=0.0, 2.87, 5.20, and 7.60 at. %. It was found that the insertion of the Zn atoms into the Li position was accompanied by a decrease in the concentration of intrinsic Nb_Li defects. At x=7.6%, the Zn atoms change their locations in the lattice and partially occupy the Nb positions. This clarifies the structural nature of the “threshold” Zn concentration, which manifests itself as singularities in the concentration dependences of various optical properties. The structural origin of the threshold concentration is likely a common feature of all nonphotorefractive impurities (Mg, Zn, In, and Sc) in LiNbO₃. A change in the intrinsic defect structure of the LiNbO₃ crystals with different Zn concentrations is discussed.     

Coexistence of magnetism and superconductivity in R1.4Ce0.6RuSr2Cu2O10 − δ(R = Eu,Sm and Gd)

The superconducting R_1.4Ce_0.6RuSr₂Cu₂O_10 − δ(R = Sm, Eu and Gd) withT_c ∼ 28, 32 and 42 K are also magnetically ordered atT_N ∼ 220, 122 and 180 K, respectively, thus,T_N ⪢ T_c. This is in contrast to intermetallic magnetic superconductors (such as RNi₂B₂C) in whichT_c ⪢  T_N. Magnetic susceptibility and Mossbauer spectroscopy show that superconductivity is confined to the CuO₂planes, whereas magnetism is due to the Ru sublattice. Irreversibility phenomena and magnetic anomalies, observed at low magnetic fields originate from antisymmetric exchange coupling of the Dzyaloshinsky–Moria type, and from spin reorientation of the Ru moments. The shielding fraction is about 100%, supporting the conclusion that the materials consist of a single phase, manifesting both magnetism and superconductivity at once.     

The effect of copper concentration on structural,optical and dielectric properties of CuxZn1 − xS thin films

Cu_xZn_1 ? xS (x = 0, 0.25, 0.50, 0.75, 1) thin films were deposited on glass substrates using Successive Ionic Layer Adsorption and Reaction (SILAR) method at room temperature and ambient pressure. The copper concentration (x) effect on the structural, morphological and optical properties of Cu_xZn_1 ? xS thin films was investigated. The X-ray diffraction (XRD) and scanning electron microscopy (SEM) studies showed that all the films exhibit polycrystalline nature and are covered well with glass substrates. The crystalline and surface properties of the films improved with increasing copper concentration. The energy bandgap values were changed from 2.07 to 3.67 eV depending on the copper concentration. The refractive index (n), optical static and high frequency dielectric constants (ε_o, ε_∞) values were calculated by using the energy bandgap values as a function of the copper concentration.     

Oxygen ordering in YBa2Cu3O7−y in terms of the Landau theory

Oxygen ordering in nonstoichiometric compositions of YBa₂Cu₃O_7?y is considered in terms of the Landau theory of second-order phase transitions. It is shown that there can only be ten types of homogeneous long-range order, of which two types correspond to the OI and OII phases. The Landau theory predicts that the filling of vacant positions 2(f)D _4h ¹ by oxygen must follow a scenario that is far from forming chains but compatible with known facts. Such a scenario of filling vacancies with oxygen was not considered earlier. It is demonstrated that the predicted structures can be experimentally identified from the spectra of copper ions. In the course of identification of diffraction patterns, the symmetry-allowed displacement of copper ions from centrosymmetric positions in a Cu(1)(O_x□_1?x )₂ layer should be taken into account.     

On Magnetometric Determination of the Radiation Defect Concentration in a Superconducting GdBa2Cu3O7 – x Film

Physics of the Solid State - An express magnetometric technique which enables the determination of inhomogeneities of changing the superconducting properties of a thin semiconductor layer due to a...     

Calculation of electronic and optical properties of zinc blende GaP1 − xNx

In order to clarify the electronic properties of the ternary compound semiconductor GaPN, in a zinc-blende structure, a simple pseudopotential scheme (EPM), within an effective potential (VCA), is proposed. The effects of disorder and spin–orbit coupling are neglected. Various quantities, such as energy levels, charge densities, ionicity character, transverse effective charge, and refractive index are obtained for this alloy. Moreover, the crossover of the direct and indirect band gaps is predicted.     

Local structure,magnetism, and superconductivity in Sr analogs of Fe-doped YBa2Cu3−y Fe y O6+x

The local structure, magnetism, and superconductivity of Y_1–z Ca _z Sr₂Cu_3–y Fe _y O_6+x have been investigated by magnetic susceptibility,⁵⁷Fe Mössbauer absorption spectroscopy, X-ray powder diffraction, and iodometric titration. By modification of the hole distribution between the Cu(1)O _x chains and Cu(2)O₂ sheets, we induced bulk superconductivity with a maximum superconducting critical temperatureT _c near 20 K in Y–zCa _z Sr₂Cu_2.5Fe_0.5O_6+x (0.05z0.18). We interpret the development of a broad maximum inT _c and the Meissner fractionV _f versusz nearz=0.12 to be due to an increasing transfer of holes from the Cu(1)O _x chains to the Cu(2)O₂ sheets induced by a shorterc-axis and reduced Coloumb repulsion of the positively charged Y_1–z Ca _z plane. The increase in hole transfer from the Cu(1)O _x chains to the Cu(2)O₂ sheets also modulates the magnetic ordering temperatureT _m. We show thatT _m of YSr₂Cu₂FeO_7.26 is approximately 65 K, which is much lower than that observed in YBa₂Cu_2.31Fe_0.69O_7.21 (T _m400 K).     

Electrostatic tuning of the electrical properties of YBa2Cu3O7−x using an ionic liquid

Ultrathin YBa₂Cu₃O_7?x (YBCO) films were grown on SrTiO₃ (STO) substrates using the technique of high-pressure oxygen sputtering. Films were then incorporated in a field effect transistor configuration, which facilitated the control of superconductivity by electrostatic charging. While devices using STO as both the substrate and gate dielectric have produced only relatively small shifts in film electrical properties, very large changes can be realized using an electric double layer transistor configuration employing the ionic liquid DEME-TFSI as the dielectric. By depleting holes an electrostatically tuned superconductor insulator transition was studied using a finite size scaling analysis. The breakdown of scaling at the lowest temperatures suggests the presence of a mixed insulator/superconductor phase separating the two ground states. Further depletion of holes resulted in a change of the majority carriers from holes to electrons and the emergence of what appeared to be very weak re-entrant superconductivity. Also by accumulating holes an underdoped film was tuned into the overdoped regime. A two-step mechanism for electrostatic doping was revealed. Hall effect measurements suggested the presence of an electronic phase transition or a change in the Fermi surface as a function of doping near optimal doping.     

First principle study of structural,phase stabilization and oxygen-ion diffusion properties of β-La2 − xLxMo2O9 (L = Gd,Sm, Nd and Bi) and β-La2Mo2 − yMyO9 (M = Cr,W)

We have performed a first principle study of structural and phase stabilization of β-La_2 ? xL_xMo₂O₉ (L = Gd, Sm, Nd and Bi) and β-La₂Mo_2 ? yM_yO₉ (M = Cr, W). The substitutional-site properties were discussed in terms of the empirical parameter, bond valence sums (BVS), which characterizes the interactions between atoms and its nearest-neighbor atoms and correlates well with the stability of the structure. We found that Gd, Sm and Nd atoms prefer the crystallographic sites with largest BVS values. The nonlinear dependence of cell parameter on W content in W-doped systems results from the nonlinear change in Mo/W–O bond length with W content. The decrease of cohesive energy and the deviation of BVS values from the expected values upon the Gd, Sm, Nd and W-doped concentration help us understand the experimentally observed stabilization of the β phase to lower temperatures in these doped system. The O ion diffusion properties in W-doped systems have been studied using the nudged elastic band method and the dimer method. We found that, W-doping leads to the obvious increase in the energy barriers of O ion concerted diffusion. In addition, there is a remarkable decrease in the difference of energy barriers between two diffusion channels involving O(1) ion, which sheds light on only one relaxation peak in the mechanical relaxation measurement in W-doped system, compared to undoped system.     

Effect of Plane Deformation on Superconducting and structural Properties of YBa2Cu3–yFeyO7–x Ceramics

Russian Physics Journal -     

Influence of various lanthanides on the properties of Sr0.8Ce0.1Ln0.1MnO3 − δ and Sr0.9Ce0.05Ln0.05CoO3 − δ ceramics and thick film electrodes

In this work nonstoichiometric strontium cerium manganites and cobaltites Sr_0.8Ce_0.1Ln_0.1MnO_3 ? δ and Sr_0.9Ce_0.05Ln_0.05CoO_3 ? δ with low content of various lanthanides (Ln = La, Ce, Nd, Sm, Gd, Dy, Yb) were investigated as potential electrode materials for solid state fuel cells and gas sensors. Synthesis and sintering conditions as well as dc resistivities and thermal expansion coefficients of the developed ceramics in the range 20–900 °C were studied. On the basis of complex impedance spectroscopic studies of yttria stabilized zirconia solid electrolyte samples with thick film perovskite electrodes four contributions to the dielectric response were found, attributed to the grains and the grain boundaries of the solid electrolyte, to the electrode–electrolyte interface and to the electrodes, in the descending order of frequency. Dc conductivity of the ceramic samples and thick films exhibited semiconducting, semimetallic or metallic character, depending on the composition and the examined temperature range. In the temperature range of 600–900 °C, a very good agreement was found between the experimental and the theoretical values of electromotive force of concentration oxygen cells based on zirconia solid electrolyte with perovskite electrodes.