Transport properties of a and c axis oriented (Y1?xCax)Ba2Cu3Oy thin films

Anisotropy and Hall effect measurements have been performed in calcium-doped, i.e., overdoped YBa₂Cu₃O_y ((Y_1−xCa_x)Ba₂Cu₃O_y) thin films witha andc axis orientations. In highly overdoped films (x=0.4), the anisotropy of the normal resistivity decreases and a drastic change in Hall conductivity in the mixed state is observed. The change in Hall conductivity in the overdoped region is consistent with recent experimental results for La_2−xSr_xCuO₄ films and seems to be common in highT _c superconductors.     

Effect of the percentage of yttrium oxide on the phase composition of annealed samples of YzBa5Cu7Oy and YzBa3Cu5Oy composition

The phase composition of Y _z Ba₅Cu₇O _y (1) and Y _z Ba₃Cu₅O _y (2) samples with a variable percentage of yttrium up to the stoichiometric composition of the Y _n Ba _m Cu _{m + n} O _y series is investigated by X-ray phase and elemental analyses, electron diffraction, and high resolution imaging in a transmission electron microscope at a temperature of 930°C in the crystallization field of a matrix oxide (Ba: Cu) of 5: 6 composition on the phase diagram of the BaO-CuO _x system at P(O₂) = 21 kPa. The substantial effect of yttrium oxide’s presence on the phase composition of both objects is found, providing evidence of a complex ionic equilibrium within the melt. The fine-domain oxide structure of the YBa₂Cu₃O₆ tetragonal form, which is due to the coexistence of oxides of an Y _n Ba _m Cu _m+n O _y homologous series of (Y: Ba: Cu) 235, 123, and 257 composition is revealed. The domain size for these phases is 20–50 ?. The domains are joined coherently along axis c.     

SUPERCONDUCTING PROPERTIES AND STRUCTURAL PERFECTION OF EPITAXIAL YBa2Cu3O7-x THIN FILMS

High quality epitaxial YBa₂Cu₃O_7-x thin films have been succcessfully prepared by dc magnetron sputtering deposition, on (100) and (110) aligned SrTiO₃, LaAlO₃ and yttria-stabilized zirconia (YSZ) substrates. The films showed zero resistance around 90 K and had a J_c (at 77 K, H=0) over 10⁶A/cm². It was found that superconducting properties and structures of the films were strongly dependent on oxygen pressure and substrate temperature. The epitaxial structure of the films have been studied by X-ray diffraction. Rutherford backscattering and channeling spectroscopy, X-ray double-crystal diffraction and transmission election microscopy. The experimental results demonstrated that the epitaxial YBa₂Cu₃O_7-x films had excellent superconducting properties and quite perfect structure.     

Primary crystallization field of YnBamCum + nOy oxides with YBa2Cu3O6 tetragonal structure

The phase composition of specimens in the primary crystallization field of the Y _n Ba _m Cu _{m + n} O _y oxide series with compositions of (Y:Ba:Cu) 235, 123, and 257 was investigated by means of X-ray powder diffraction, X-ray microprobe analysis, electron diffraction, and high resolution imaging in a transmission electron microscope. Y _n Ba _m Cu _{m + n} O _y oxides have the YaBa₂Cu₃O₆ tetragonal structure and can be represented in the general form as 123 oxides. It was found that the crystallization field of 123 oxides corresponds to the crystallization fields of the Ba _m Cu _{m + n} O _y oxide series with the BaCuO₂ cubic structure. These crystallization fields are located in the matrix phase diagram of the BaO-CuO _x system at P(O₂) = 21 kPa in the range of 50–90 mol % CuO and 910–1000°C. The 123 oxides are formed according to an intercalation mechanism with the participation of barium-copper oxide matrices with the composition (Ba: Cu) 3: 5, 2: 3, and 5: 7     

Effect of pressure on the electrical resistivity and thermoelectric power of the compounds Ba(Pb1-xBi x )O3 and K y Ba1-yPb0.5Cu0.5O3-x

High pressure studies on the electrical resistivity and thermoelectric power of BaPb_1-xBi _x O₃ and K _y Ba_1-yPb_0.5Cu_0.5O_3-x show that many compounds in these systems exhibit qualitatively the same type of behaviour as observed in the superconducting oxides (La, Sr)CuO₄ and YBa₂Cu₃O_7-x. The compounds K _y Ba_1-yPb_0.5Cu_0.5O_3-x show temperature-independent resistivity over a wide range of temperatures, but are not found to be superconducting down to 77 K.     

On the existence of a homologous series of BamCum+nOy oxides with the cubic structure of the BaCuO2 oxide

Phase relations have been studied in the BaO–CuO_x system in the range of 42.0–83.0 mol.% CuO at P(O₂) = 21 kPa (air) by visual polythermal analysis (VPA), powder X-ray diffraction (XRD), differential thermal analysis (DTA), thermogravimetric analysis (TGA), chemical analysis (CA), and electron diffraction (ED) with simultaneous elemental analysis (EA) in a transmission electron microscope (TEM). The existence of discrete crystallization fields of barium–copper oxides of cation compositions Ba₄Cu₅O_y, Ba₅Cu₆O_y, Ba₇Cu₈O_y, Ba₁₂Cu₁₃O_y, and Ba₂₄Cu₂₅O_y, which have the cubic structure of the BaCuO₂ oxide, is revealed in the studied region of the system. The oxides may be represented as members of a Ba_mCu_m+nO_y homologous series. The BaCuO₂ oxide does not exist in the subsolidus region and does not have its own crystallization field. The oxygen-deficient oxide BaCuO_1.78 of the cation composition (Ba:Cu) 1:1 with the BaCuO₂ cubic structure is found in melted samples of the 50.0 mol.% CuO composition quenched at 1020–1060 °С.     

Superconducting properties of Sm0.33Eu0.33Gd0.33Ba2Cu3Oy films by metal-organic deposition using metal 2-ethylhexanates

We have prepared Sm_0.33Eu_0.33Gd_0.33Ba₂Cu₃O_y (SEGBCO) films on LaAlO₃ single-crystal substrates by metal-organic deposition using 2-ethylhexanate solutions which contain no fluorine. The SEGBCO film fired at 850 °C under a low oxygen partial pressure of 2 × 10⁻⁶ atm exhibited strong c-axis oriented X-ray diffraction peaks and weak a/b-axis oriented peaks of the SEGBCO phase, and had a granular surface. High resolution cross-sectional transmission electron microscope images indicated the epitaxial growth of the SEGBCO film on the substrate with overgrowth of a/b-axis oriented grains. The critical temperature and critical current density values of the SEGBCO film was 86.5 K and 1.29 MA/cm², respectively, at 77.3 K at the self-field.     

双掺杂BaxAgyCa3-x-yCo4O9氧化物的织构与电输运性能

采用柠檬酸溶胶凝胶和放电等离子烧结制备了Ca位双掺杂型的Ba_xAg_yCa_3-x-yCo₄O₉烧结体,利用X射线衍射仪、扫描电子显微镜、电阻率测试仪等研究了烧结体相组成、取向度、织构及电性能.结果表明：含Ag的掺杂试样中出现了偏离化学计量比分布的Ag单质,掺杂试样取向度随Ba与Ag掺杂量之比x/y的增大而提高,含Ag的掺杂试样取向度低于未掺杂试样,不含Ag的掺杂试样取向度高于未掺杂试样.x=y=01的试样导电机理发生变化.Ba,Ag掺杂量相等的试样保持较低取向度的同时具有较低的电阻率,在973 K时达到最低值（73 mΩcm）,而取向度最低的Ag单掺杂试样电阻率在所有试样中最低,在973 K时为63 mΩcm. 关键词： 3Co₄O₉')" href="#">Ca₃Co₄O₉ 双掺杂 织构 电输运性能     

Electronic structure of high-temperature superconductors I. Y-Ba-Cu-O systems

The electronic structure of high-T _c copper based superconductors has been studied using the quasirelativistic INDO molecular orbital method. Model clusters of YBa₂Cu₃O _y systems have been investigated fory ranging between 6·0 and 7·0. Monomeric units [Ba₈Cu₃O _x ] ^q and [Y₄Ba₈Cu₄O _x ] ^q , dimeric units [Ba₁₂Cu₆O _x ] ^q and [Y₆Ba₁₂Cu₈O _x ] ^q as well as tetrameric units [Ba₁₈Cu₁₂O _x ] ^q over 300 valence orbitals have been studied. From the set of occupied molecular orbitals the density of states (DOS) function has been calculated along with its d-orbital projection. The highest occupied molecular orbitals cover the oxygen p-band whereas the copper d-band lies at lower energies. Superconducting phase models (y=6·75 to 7·0) have a higher d-DOS function at higher energies with respect to the non-superconducting phase (y=6·0).     

Comparative analysis of the effect of La and Co on the superconductivity and energyband spectrum of YBa2Cu3Oy for different oxygen contents

Temperature dependences of the resistivity and Seebeck coefficient of Y(Ba_1−x La_x)₂Cu₃O_y and YBa₂Cu_3−x Co_xO_y samples (x=0–0.25) have been measured under maximum sample saturation with oxygen, as well as following their anneal in an oxygen-deficient atmosphere. The T _c (x) dependences for as-prepared samples were found to pass through a maximum at x=0.05, which persists after annealing for Y(Ba_1−xLa_x)₂Cu₃O_y and disappears for YBa₂Cu_3−x Co_xO_y. A phenomenological model of the band spectrum in normal phase has been used to determine the parameters of the conduction band and of the carrier system, and to analyze their variation with the dopant type and content, as well as with annealing. Despite the differences observed in the T _c (x) dependence, the critical temperatures for all the sample series studied were found to correlate with the conduction-band effective width. The mechanism of the effect of impurities on the band-structure parameters and the reasons for the different influence of annealing on the properties of Y(Ba_1−x La_x)₂Cu₃O_y and YBa₂Cu_3−x Co_xO_y are discussed. Fiz. Tverd. Tela (St. Petersburg) 41, 389–394 (March 1997)     

Preparation of Tl-Ba-Ca-Cu-O thin films by diffusion of Tl into laser evaporated Ba2Ca2Cu3Ox films

Preparation of superconducting Tl-Ba-Ca-Cu-O thin films by diffusion of Tl into laser evaporated Ba₂Ca₂Cu₃O_x thin films is reported. From a sintered Ba₂Ca₂Cu₃O_x bulk sample we prepared using a pulsed Nd:YAG laser, Ba-Ca-Cu-O thin films on sapphire and SrTiO₃ single-crystal substrates. Subsequently the films were loaded with Tl by simultaneously annealing the films together with a sintered Tl₂Ba₂Ca₂Cu₃O₁₀ sample both enclosed in a small stainless steel box; in our procedure Tl contamination was reduced to a minimum. T_c values near 100 K and critical currents of 5·10³ A/cm² at 77 K were obtained.     

Evidence of local structural transition during oxidation process of RBCO epitaxial films using time resolved reflexafs measurements

We report results of a time resolved X-ray Absorption Spectroscopy (XAS) experiment on the oxidation process of epitaxial Y₁(Nd_0.05Ba_1.95)Cu₃O_x superconducting thin films. For the first time Cu K-edge XAS technique has been used to explore local structural changes around the Cu ions during the oxidation process of a high critical temperature superconducting film. The results show that during the oxygenation of a tetragonal Y₁(Nd_0.05Ba_1.95)Cu₃O_x additional local transitions appear in the orthorhombic I phase, which are not linked to long range order transformations as shown by in situ X-ray diffraction experiment. New questions concerning the dynamic microscopic process leading to the oxygenation and eventually to superconductivity of the complex R_1+xBa_2-xCu₃O_x (R = Y or rare earth) compounds arose from these results. Received 21 December 2001 Published online 25 June 2002     

Competing energy scales in high‐temperature superconductors: Ultrafast pump–probe studies

We present a review of photoexcited quasiparticle dynamics of cuprate and pnictide high‐temperature superconductors in regimes (temperature, doping) where different phases such as superconductivity, spin‐density‐wave (SDW) and pseudogap phases coexist or compete with one another. We start with the overdoped cuprate superconductor Y_1–xCa_x Ba₂Cu₃O_7–δ, where the superconducting gap and pseudogap coexist in the superconducting state. In another cuprate Tl₂Ba₂Ca₂Cu₃O_y, we ob‐ serve a competition between SDW and superconducting orders deep in the superconducting state. Finally, in the underdoped iron pnictide superconductor (Ba,K)Fe₂As₂, SDW order forms at 85 K, followed by superconductivity at 28 K. We also find the emergence of a normal‐state order that suppresses SDW at a temperature T * ～ 60 K and argue that this normal‐state order is a precursor to superconductivity. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Analysis of the possible reasons for the suppression of superconductivity in the Y1−x PrxBa2Cu3Oy system on the basis of thermoelectric power data

The temperature dependence of the thermoelectric power in samples of Y_1−x Pr_xBa₂Cu₃O_y (x=0−0.6) is investigated. The principal parameters of the system of charge carriers are determined on the basis of an analysis of this dependence within a narrow-conduction-band model, and their variation with increasing praseodymium content is analyzed. It is found that an increase in x leads to a very weak increase in the electron filling and considerable broadening of the conduction band accompanied by strong localization of the charge carriers. It is concluded on the basis of a comparative analysis of the results obtained and data for the YBa₂Cu₃O_y system in the case of aliovalent substitutions in the barium and copper sites that the valence of praseodymium in Y_1−x Pr_xBa₂Cu₃O_y is very close to 3+. It is shown that there is a universal correlation between the effective width of the conduction band and the critical temperature following various cation substitutions in the Y-Ba-Cu-O system. It is concluded that the main reason for suppression of the superconducting properties of Y_1−x Pr_xBa₂Cu₃O_y is the strong modification of the band spectrum caused by praseodymium. Fiz. Tverd. Tela (St. Petersburg) 39, 1520–1525 (September 1997)     

Peak effect in surface resistance at microwave frequencies in Dy-123 thin films

A pronounced peak in the microwave (at frequency 9.55 GHz) surface resistance, R _s vs. T plot (where T is the temperature) has been observed in epitaxial DyBa₂Cu₃O_7−y superconducting thin films in magnetic fields (parallel to c-axis) in the range 2 to 8 kOe, and temperatures close to the superconducting transition temperature T _c(H). Our data suggest that the nature of peaks observed in the two films is different, thereby indicating different defect structures in the films.     

The possibility of introducing additional states in the conduction band of YBa2Cu3Oy by Ca doping

Measurements of the temperature dependences of the resistivity and Seebeck coefficient S, as well as of the Hall coefficient R _H at T=300 K are reported for two series of ceramic Y_1−x Ca_xBa₂Cu₃O_y samples with oxygen contents in the initial sample close to stoichiometric and then lowered by annealing. It has been found that an increase in calcium content in the first series results in a drop of the critical temperature T _c , a weak variation of S (300 K), and an increase of R _H (300 K), whereas in the second series T _c increases, S (300 K) decreases, and R _H (300 K) remains practically constant. The S (T) relations acquire additional features with increasing doping level, which are not typical of the YBa₂Cu₃O_y system for other types of substitution. An analysis of the data obtained made in terms of the phenomenological band model permits a conclusion that calcium is capable of introducing additional states in the conduction band. Based on this assumption, we have succeeded in providing a qualitative explanation for the unusual features in the behavior of the transport coefficients and critical temperature in Y_1−x Ca_xBa₂Cu₃O_y. Fiz. Tverd. Tela (St. Petersburg) 40, 2145–2152 (December 1998)     

Structural studies and T c dependence in La2−x Dy x Ca y Ba2Cu4+y O z type mixed oxide superconductors

A new series of mixed oxide superconductors with the stoichiometric composition La_2−x Dy _x Ca _y Ba₂Cu_4+y O _z (x=0.0 − 0.5, y=2x) has been studied for structural and superconductiong properties. Our earlier studies on La_2−x (Y/Er) _x Ca _y Ba₂Cu_4+y O _z series, show a strong dependence of T _c on hole concentration (p _sh). In the present work, the results of the analysis of the neutron diffraction measurements at room temprerature on x=0.3 and 0.5 samples are reported. It is interesting to know that Ca substitutes for both La and Ba site with concomitant displacement of La onto Ba site. Superconductivity studies show that maximum T _c is obtained for x=0.5, y=1.0 sample (T _c ∼ 75 K), for La_1.5Dy_0.5Ca₁Ba₂Cu₅O _z (La-2125).     

Superconductor/ferroelectric epitaxial heterostructures for tunable microwave devices

The formation of epitaxial heterostructures including YBa₂Cu₃O_7−δ and SrTiO₃ thin films on R-plane sapphire substrates has been studied. The thin CeO₂ layer and the CeO₂/SrBi₂Nb₂O₉ epitaxial bilayer served as buffers. The temperature dependence of the dielectric permittivity ɛ of the SrTiO₃ film behaved in essentially different ways when incorporated in the YBa₂Cu₃O_7−δ /SrTiO₃/YBa₂Cu₃O_7−δ and in Ag/SrTiO₃/YBa₂Cu₃O_7−δ capacitor structures. An external bias U _b=±2.5 V applied to a plane-parallel YBa₂Cu₃O_7−δ /SrTiO₃/YBa₂Cu₃O_7−δ capacitor structure (T<100 K) reduced the ɛ of the SrTiO₃ film to one half its original value. Fiz. Tverd. Tela (St. Petersburg) 39, 222–227 (February 1997)     

Epitaxial growth of yttria-stabilised zirconia buffer layers on X-cut LiNbO3 for superconducting electrodes

We report on the epitaxial growth of yttria-stabilised zirconia (YSZ) buffer layers on X-cut LiNbO₃ (LNO) single crystals by pulsed laser deposition. Despite the low chemical stability of the substrates at high temperature, high quality fully reproducible films were obtained over a relatively broad range of processing conditions. The films were (00h) out-of-plane single oriented and the in-plane edge of the YSZ unit cell was aligned with the polar axis of the LNO. However, the YSZ deposition also promoted the formation of the compound LiNb₃O₈. This compound is epitaxial and located at the interface. The homogeneous YSZ film presents a uniform surface, free of outgrowths and with a low roughness. These characteristics are suitable for the epitaxial growth of other oxides, as has been demonstrated with the preparation of YBa₂Cu₃O₇/CeO₂/YSZ/LNO heterostructures. The superconducting YBa₂Cu₃O₇ films were epitaxial, with the c axis perpendicular to the surface and single in-plane orientation, and presented good transport properties (critical temperatures around 86 K and critical current densities close to 10⁶ A/cm² at 77 K). Received: 5 April 2001 / Accepted: 30 July 2001 / Published online: 30 October 2001     

Dynamics and distribution of doped holes in the CuO2 plane of slightly doped antiferromagnetic YBa2(Cu1 ? zLiz)3O6 + x (x < 0.1) studied by Cu(1) NQR

Incidence of doped holes in the CuO₂ plane on the AF state was studied by Cu(1) nuclear quadrupole resonance (NQR) in slightly doped YBa₂(Cu_{1 − z} Li _z )₃O_{6 + x} compounds. Inhomogeneous distribution of doped holes in the plane was detected in the low temperature measurements of transverse (1/T ₂) and longitudinal (1/T ₁) relaxation rates. We establish that at lower T the holes motion slows down and we estimate that the holes localize finally in restricted regions (∼3 lattice constants) in the Coulomb potential of the Li+ ions. Also we compared the hole behavior in slightly doped YBa₂(Cu_{1 − z} Li _z )₃O_{6 + x} samples with that in slightly doped Y_{1 − y} Ca _y Ba₂Cu₃O₆. A stronger trapping potential of the in-plane Li⁺ impurities was concluded as compared to slightly doped Y_{1 − y} Ca _y Ba₂Cu₃O₆ compound with out-of-plane Ca²⁺ impurities.