Transport Properties and Fluctuations in Underdoped GdBa2Cu3O x Thin Films

We have investigated the transport properties, resistivity and Hall effect, in a series of underdoped GdBa ₂ Cu ₃ O _x thin films grown by off-axis magnetron sputtering. We find a systematic correlation between the critical temperature T _c and the inverse Hall constant R _H ^-1 , related, in simple models, to the carrier concentration n. Our experimental thin film T _c (n) data are in good agreement with the temperature-doping phase diagram obtained for YBa ₂ Cu ₃ O _x single crystals. By measuring the activation energies in the liquid vortex phase, and by using a 2-dimensional model for vortex dynamics, we have extracted the penetration depth of these samples, and studied the relation between the carrier concentration and the superfluid density to probe the role of phase fluctuations on superconductivity.     

Highly Dissipative State of YBa2Cu3O7 Thin Films at Very Large Current Densities

Flux flow instabilities at high current densities have been recently reported to occur in high temperature superconductors. Such flux flow instabilities were already predicted by Larkin and Ovchinnikov. However, other mechanisms may possibly also account for this behaviour, as for instance hot spots or thermal instabilities. To investigate the mechanism at the origin of these instabilities, we have carried out detailed I- V experiments, using short current pulses, on YBCO thin films grown on sapphire substrates. When the current density exceeds 2-3 times the conventional critical current density, we observe an extremely fast transition from a low dissipative state into a highly dissipative state. We present some indications that this transition does not originate from temperature instabilities but is driven by the current.     

Recrystallization of YBa2Cu3Ox superconductors in vacuum

YBa₂Cu₃O _x ceramics were recrystallized in vacuum at high temperatures. Recrystallized layers consisting of small grains were observed near the surfaces of the original large-grain 1 2 3 ceramics. The small grains consisted of transformation twins and were identified to orthorhombic 1 2 3 using X-ray diffraction. As vacuum annealing time increased, the thickness of the recrystallized layer increased. The relationship between the thickness and the annealing time showed a linear relationship and an effective diffusion coefficient of 6.25 × 10^–10cm²s^–1. The recrystallized layer showed a critical temperature of 90 K.     

Synthesis of a phase-pure orthorhombic YBa2Cu3Ox under low oxygen pressure

Reaction of Y₂O₃, BaCo₃ and CuO for 4 h at 800 °C in flowing O₂ with a total pressure of about 2.7 × 10² Pa, followed by cooling in O₂ at ambient pressure, has produced phase-pure orthorhombic YBa₂Cu₃O_x. Keeping the ratio of O₂ to evolved CO₂ above 50 was necessary to ensure phase purity. The resultant powder yielded pressed and inserted pellets with improved superconducting properties.     

Microstructure of plasma-sprayed YBa2Cu3Ox high-temperature superconductors

The arc plasma spraying process has been applied to deposit YBaCuO superconductors on to steel substrates. Detached, as-sprayed coatings were heat treated at 950 °C in air in order to restore superconductivity. Oxygen annealing at temperatures 400–500 °C has been used as the final stage in material preparation. The changes in the microstructure of the material during heat treatment have been followed with the aid of scanning electron and optical microscopy. The phase transformations resulting from heat treatment have been studied by differential thermal analysis and X-ray diffraction.     

Preparation and densification of superconducting YBa2Cu3Ox ceramics

The solid-state reaction method to form the superconducting oxide YBa₂Cu₃O _x was studied. It was found that the starting cupric and yttrium components accelerated the decomposition of the BaCO₃ component. At a constant heating rate of 10 ° Cmin^–1 in thermogravimetric analysis, the temperature of complete decomposition,T _f, was lowered from greater than 1000 ° C in pure BaCO₃ to between 915 and 985 ° C. The effectiveness in decreasingT _f can be ranked in the order of oxalate, carbonate and oxide. The highest sintered density achieved in this study was 6.03 g cm^–3 (/_th = 95%) at 990 ° C and 5.85 g cm^–3 (/_th = 92%) at 960 ° C. The source of cupric ion had the largest effect on densification. The use of cupric carbonate resulted in a consistently high Archimedes density of about 6.00gcm^–3 and large dimensional shrinkage of about 20% at 990 ° C for 12h. In contrast, the use of cupric oxide gave the lowest density and smallest shrinkage. Within the same powder lot, higher sintered density and smaller dimensional shrinkage were observed in samples with higher initial green density and compaction pressure. However, the data suggested that the enhanced densification and higher density achieved by the use of cupric carbonate and oxalate cannot be accounted for by the different physical characteristics of the powders and the mechanics of powder compaction, measured collectively by the green density.     

Variations of internal friction in YBa2Cu3Ox superconductors

The internal friction (Q ^–1) spectrum of YBa₂Cu₃O_x superconducting ceramic exhibits several peaks. It has been confirmed that the high-temperature peak (around 240 K) depends on structural changes and varies during subsequent cycles of cooling and heating.Q ^–1 conductivity, X-ray spectra and the shielding effect have been measured on several samples having different superconducting properties obtained by various thermal treatments. Splitting is a characteristic feature of the high-temperature internal friction peak of the sample which exhibits good superconducting properties. In the case of the specimen exhibiting the worst properties the peaks decrease and overlap. In both cases an increase can be observed of this peak with the number of thermal cycles. After ageing at 470 K, the high-temperature peak disappears. Subsequent thermal cycles slightly recover it. Hysteresis of the Young modulus is also observed. The results are interpreted as transition of the 04 oxygen atom between two energy minima in the O4-Cu-O4 chain.     

MOCVD-grown homoepitaxial YBa2Cu3Ox thin films and its multilayers

YBa₂Cu₃O _x (YBCO) films, Zn-doped YBCO (YBCO : Zn) films, and their bilayers have been epitaxially grown on SrTiO₃(100) and single-crystal YBCO(001) substrates by metalorganic chemical vapour deposition. The YBCO(001) films homoepitaxially grown on YBCO(001) substrates have flat surfaces on an atomic scale, and interfaces free from crystalline defects. We can systematically reduce the superconducting transition temperature (T _c) of YBCO : Zn films from 90 K to 37 K by increasing Zn concentration. The bilayers have a sharp distribution of Zn as evaluated fromT _c measurements of the upper YBCO films and depth profiles of secondary ion mass spectrometer, suggesting the possibility to form the homoepitaxial SNS (S, superconductor; N, normal metal) junction operatable between 40 K and 90 K.     

Microstructural Studies of Tl2Ba2Ca2Cu3Ox Thin Films on LaAlO3 and MgO Substrates.

Tl₂Ba₂Ca₂Cu₃O_x thin films have been fabricated on (001) LaAlO₃ and (001) MgO substrates. Films grown on LaAlO₃ have T_c=112K and R_S(80K, 10GHz)=0.2mΩ, while films on MgO have T_c=117K and R_s(80K, 10GHz)=0.7mΩ. The grain size and alignment of the films has been investigated using X-ray diffraction, Scanning Electron Microscopy and Electron Backscattered Diffraction. We show evidence for a markedly higher in-plane angular spread for films on MgO and believe that for films grown on this substrate the lowest achievable values of R_s are limited by disorder in the in-plane alignment of the TBCCO film caused by the large lattice mismatch between the materials.     

Thermodynamics of oxygen uptake for the YBa2Cu3Ox superconductor

Thermogravimetric analysis and pressure (p) monitoring in known volume systems are used to give phase diagram information (e.g. vant Hoff plots) for roughly 6<x<7 in YBa₂Cu₃O_x. As an example we find for O₂ desorption for the important composition x = 6.8, beyond which superconductivity with transition temperature, T_s > 90 K becomes essentially independent of x, ΔH^o = 43.4 ±1 kcal/mo (O₂) and ΔS^o = 48.6 ±1 cal/Kmo (O₂). ΔS^o stays roughly constant for x > 6.7 but decreases for x < 6.7 indicating increased disorder on the O substructure. p (x) curves do not show phase separation which corresponds to above critical behavior, but given indications for limiting compositions near x = 6 and x = 7. Accordingly a model is proposed in which these compositions correspond to valencies of Cu⁺ and Cu³⁺ respectively for the symmetric Cu site (Ba coordinated) while the asymmetric Cu (Ba and Y coordinated) is taken to be 2+ over this whole range in x.     

Hydrostatic and uniaxial pressure effect on Tc of YBa2Cu3Ox

The Variation of the transition temperature T_c of YBa₂Cu₃O_x with hydrostatic He-gas pressure depends on the oxygen content x. The pressure effect dT_c/dp increases from small negative values at x=7 to dT_c/dp=7.4 K/GPa at x=6.7. For oxygen contents below x=6.7 dT_c/dp drops to 3 K/GPa and remains nearly constant¹. The charge transfer model² cannot explain the drop at x=6.7.Thermal expansion measurements on YBa₂Cu₃O_x indicated that the uniaxial pressure effects along the three crystal axes are different^3,4 To investigate the uniaxial pressure effects inductively an experimental setup was constructed. The T_c-change of several YBa₂Cu₃O_x single crystals with different oxygen contents has been investigated under pressure along the c-axis. To avoid oxygen ordering processes the samples were held below 105 K during the measurements. The results of uniaxial pressure measurements in c-axis direction fit to former uniaxial pressure data^3,4,5 and are explained within the charge transfer model. Hydrostatic pressure data¹² of overdoped samples fit to the same curve. However, this is not the case for underdoped samples. From this we conclude that only a part of the hydrostatic pressure effect can be explained by charge transfer in the underdoped region. The remaining part can be ascribed to uniaxial pressure effects along the a- and b-axis.     

Understanding High Critical Currents in YBa2Cu3O7 Thin Films and Coated Conductors

The main challenges for the success of high temperature superconducting wires, the YBa₂Cu₃O₇ (YBCO) coated conductors (CC), are to avoid the the weak-link problem through the production of biaxially textured films, and to increase the critical current density (J _c) through the introduction of large densities of appropriate defects. To that end, it is essential to understand the pinning mechanisms and their correlation with the microstructure of the CC. We first present a brief overview of the main methods currently used to produce YBCO CC, and we describe the architecture of the YBCO on IBAD fabricated at Los Alamos, summarizing the recent improvements of their structural and superconducting properties. Then, we analyze some aspects of the J _c dependence on temperature and magnetic field (orientation and intensity) for the best CC available, and we compare and contrast the results with those of YBCO thin films on single crystal substrates, in order to determine if the defects controlling the pinning mechanisms are the same in both cases. Our results indicate that over large field and angular ranges J _c on CC is higher than J _c in thin films on SCS.     

Magnetic Fieid-lnduced Recovery of Superconductivity in Epitaxial YBa2Cu3O7-x Films at Microwave Frequencies

An anomalous magnetic field-effect has been observed in the microwave surface impedance of epitaxial YBa₂Cu₃O_7−x films on LaAlo₃ substrates. The application of dc and microwave magnetic fields up to 20 mT caused a significant decrease of microwave surface reactanceX _s (≤3%) and resistanceR _s (≤20%) at 8.5 GHz, 77 K and at 87 GHz, 4.2 K. The field effects onX _s andR _s s were found to correlate within the framework of the two-fluid model. The observed effect is attributed to the presence of magnetic impurities in the superconducting films, giving rise to pair breaking. Alignment of the impurity spins suppresses the spin-flip scattering rate and, as a result, leads to a recovery of microwave superconductivity.     

a轴取向YBa2Cu3O7-x薄膜的制备及其Raman光谱研究

采用直流磁控溅射法在SrTiO3(100)衬底上制备a轴取向的YBa2Cu3O7-x薄膜,用四引线法测量R-T曲线Tc0=86K.对于YBa2Cu3O7-x薄膜进行喇曼光谱测量,发现0(4)振动的声子峰(～500cm-1)强度远远大于0(2)-0(3)反相振动的声子峰(～340cm-1)强度,应用群论分析证明薄膜主要是沿a轴生长的.在较低的衬底温度下(<780℃),YBa2Cu3O7-x薄膜沿a轴生长,随着衬底温度的升高,薄膜中沿c轴生长的组分逐渐增加.     

Compositional variations in the bulk of YBa2Cu3Ox thick films

Thick films of YBa₂Cu₃O_x (123) prepared on a number of substrate materials, namely Si, Al₂O₃, SrTiO₃, MgO and YSZ, have been investigated for the compositional variations/impurity phases present, microstructure, grain orientation etc. Energy-dispersive X-ray and X-ray diffraction analyses for as-prepared films of two different thicknesses, 2–3 m and 10–15 m, and after their partial and complete etching, are reported. The results show that the film-substrate reaction dominates up to 2–3 m thickness of a film. The decomposition of 123 material at the processing temperature and the substrate-film reaction together are seen to govern the crystallization behaviour, compositional variations and the superconducting properties of the films. In various cases, the films have been found to undergo superconducting phase transitions at temperatures between 66 and 89.5 K.     

Preparation of YBa2Cu3O7 − δ Thin Films by the Self-Template Sputtering Method

Using the self-template technique, c-axis-oriented epitaxial YBa₂Cu₃O_{7 ? δ} thin films have been prepared in situ on LaAlO₃ substrate by the d.c. magnetron sputtering method. The properties of thin film dependence on the deposition conditions of the two-step self-template method have been systematically investigated. By optimizing the parameters, high-quality YBCO thin films with T _c0 ≥ 90 K, ΔT _c ≤ 1 K, R _s (77 K, 10 GHz)～500 μΩ were reproducibly obtained. The best sample grown under optimal conditions gave a low R _s of 330 μΩ at 77 K, 10 GHz, which can be used in a microwave field.     

SrTiO3斜切基片上外延生长YBa2Cu3O7-δ薄膜的扫描探针显微镜研究

采用激光脉冲沉积法在钛酸锶SrTiO3 (0 0 1)斜切基片上外延生长YBa2 Cu3 O7 δ薄膜 ,在大气环境下采用扫描探针显微镜对YBa2 Cu3 O7 δ薄膜的表面纳米形貌进行直接观察。发现YBa2 Cu3 O7 δ薄膜具有相对光滑的表面形貌 ,薄膜表面由沿SrTiO3台阶趋向外延生长的纳米台阶组成 ,薄膜生长模式主要以台阶媒体生长为主。     

The Underdoped Region of the Phase Diagram of YBa2Cu3O6+ x

Here we present a reviewed phase diagram of the high-T _c superconducting YBa₂Cu₃O_{6+ x} compound, finely mapped in the strongly underdoped region (0?<?x?<?0.5), from the pure antiferromagnetic state to the superconducting regime. The Neèl and spin freezing temperatures have been measured by μSR experiments while the hole density per Cu atom in the CuO₂ planes has been determined from the resistive T _c and from Seebeck coefficients at 290 K. The phase diagram is discussed in comparison to those of La_{2? x} Sr _x CuO₄ and Y_{1? x} Ca _x Ba₂Cu₃O₆ cuprate systems.     

Cu_2ZnSnS_4薄膜的研究进展

Cu2ZnSnS4(CZTS)薄膜太阳能电池具有低成本、高效率、安全无毒等优点,是最具发展前景的太阳能电池之一,近几年来开始受到广泛关注。简要介绍了国内外几种制备Cu2ZnSnS4薄膜的方法,包括蒸发法、溅射法、脉冲激光沉积法、电化学沉积法、喷涂热解法、Sol-gel法、丝网印刷法,并阐述了这几种方法的优点及存在的问题,展望了今后CZTS薄膜的研究方向,认为通过溶剂热或热注入法制备出CZTS纳米晶体后,再通过丝网印刷法或旋涂等法制成CZTS薄膜能降低生产成本,在电池的工业化生产中具有很广阔的应用前景。     

Temperature-dependent change of Cu-O bond in La2CuO4 and YBa2Cu3O7

Valence-band spectra of La₂CuO₄ and YBa₂Cu₃O₇ were obtained by using X-ray photoemission varying the temperature of the measurement (35, 260, 300, and 500 C). In La₂CuO₄ the broad band centred around 4eV splits into two peaks at 260 C. At both 35 and 500 C the spectra show almost similar shape. In YBa₂Cu₃O₇ the broad peak centred around 4eV splits into two peaks at 500 C. Below 300 C the spectra show almost similar shape. These splittings of the valence-band spectra may be due to the rearrangement of the crystal structure accompanying the phase transition from the orthorhombic to tetragonal symmetry.