D.N. Zheng

Pressure effects, anisotropic transport properties and upper critical fields of Bi2Sr2CaCu2O8+x single crystals - characterization of needle-like BiSrCaCuO crystals

Abstract We study the physical properties of Bi 2 Sr 2 CaCu 2 O 8+ x platelet-like single crystals with a zero resistance superconducting transition T c of ≅ 80 K. Normal state electrical resistivity and upper critical fields are more anisotropic than in the YBa 2 Cu 3 O 7 single crystals. The normal state resistivity along the a-b plane linearly increases with the temperature as in the YBa 2 Cu 3 O 7 compounds. Along the c -axis, the resistivity is very high and exhibits a metallic behaviour which can be induced by defects instead to be intrinsic. The superconducting transition temperature, T c , increases under pressure at a rate of ≅ 0.2 K/kbar. Superconducting needle-like crystals are also studied. They are not single crystals but are composed of two intergrowing phases. One of them is insulating, the other one superconducting. This last phase could be different from the “2212” one. Similarities between the Biue5f8Srue5f8Caue5f8Cuue5f8O compounds and La 2 CuO 4 compounds are underlined.

Effects of Al doping on the superconducting and structural properties of MgB2

We have investigated the structural and superconducting properties of a series of Al doped MgB2 samples. The results show that the superconductivity of MgB2 is destroyed when Al is alloyed into MgB2 as reported previously. A structural instability was observed from the data of X-ray diffraction. Further study by TEM shows that the Al substitution leads to an ordering occupation on Mg sites. This corresponds to a superstructure with a repeat period of 2c along the c-axis. The superstructure was identified to be a form of MgAlB4 structure which intergrows with MgB2. Measurements on the low doping level samples showed that the upper critical field B-c2, irreversible field B-irr and the critical current density J(c) of these samples were reduced as the All content was increased. Analysis of the upper critical field data suggests that the density of states near the Fermi surface N(E-F) is reduced by Al doping. This is consistent with the decrease in the superconducting transition temperature T-c. The field dependence of pinning force is also discussed for these samples

Reproducible low-voltage resistive switching in a low-initial-resistance Pr0.7Ca0.3MnO3 junction

Current?voltage (I?V) characteristics are investigated in a low-initial-resistance Ag/Pr0.7Ca0.3MnO3/Pt sandwich structure. It is found that the junction can show stable low and high resistance states in ?0.3?V voltage sweeping cycles. The set and reset voltage values are, respectively, +0.1?V and ?0.2?V, which are very low as compared with those reported previously. Furthermore, the I?V curves in both resistance states exhibit rather linear behaviour, without any signature of metal/insulator interface effects. This implies that the Schottky interface mechanism might not be an indispensable factor for the colossal electroresistance effect. The origin of low switching voltages is attributed to the reduced effective distance for electric field action due to the sufficient oxygen content of the PCMO layer. The underlying physics is discussed in terms of the filament network model together with the field-induced oxygen vacancy motion model.

Single crystal growth of superconducting compound Bi-Ca-Sr-Cu-O

We have prepared two kinds of single crystals in Bi-Sr-Ca-Cu oxides by flux method;plate-like single crystals which exhibit metal to superconductor at 93 K, being the high-Tclayered compound BiSr (Ca0.35Sr0.25Bi0.40)2Cu2Oy with orthorhombic structure with a=5.42 A, b=5.44 A and c=30.78 A respectively, and another kind of bar-shapedbismuth-poor single crystals which is superconducting at the temperature below 82K whilebeing semiconducting above this temperature.

Intrinsic percolative superconductivity in heavily overdoped high-temperature superconductors

Magnetic measurements on heavily overdoped La 2 − x Sr x CuO 4, Tl2Ba2CuO6, Bi2Sr2CuO6, Y 1 − x (Ca x)Ba2Cu3O7 − δ and Bi2Sr2CaCu2O8 single crystals reveal a new type of magnetization hysteresis loops characterized by the vanishing of the usual central peak near zero field. Since this effect has been observed in various systems with very different structural details, it probably reflects a generic behavior for all high-temperature superconductors. This easy penetration of magnetic flux can be understood in the picture of percolative superconductivity due to the inhomogeneous electronic state in heavily overdoped regime.

Structural and superconducting properties of Ca-doped MgB2 superconductors

A series of Ca-doped MgB2 superconducting compounds has been synthesized under high pressure and high temperature with a calcium source of Ca3B2N4. Their structure and superconducting properties have been investigated, revealing that the lattice parameters of a and c for the Ca-doped MgB2 are enlarged and the superconducting transition temperature Tc is suppressed from 37.8xa0K for pure MgB2 to 36.5xa0K for Mg0.92Ca0.08B2 by the Ca doping. In addition to the main phase of the Ca-doped MgB2, the MgO and CaB6 phases have been observed with the MgO and CaB6 grains being of the size of several hundred nanometres and being embedded randomly in the MgB2 matrix. In comparison to the undoped MgB2, the rapid decrease in Jc observed in the Ca-doped samples is mainly due to the relatively large grain sizes of the second phases, which result in the poor connectivity between grain boundaries.

High-Tc superconductivity above 110 K in the Tl-Ba-Ca-Cu-O system

High-Tc superconductivity above 110 K has been observed in Tl-Ba-Ca-Cu oxides. The dependence of electrical resistivity and AC magnetic susceptibility on temperature show that the zero-resistance transition temperature is about 114 K and a dramatic magnetic susceptibility drop appears at 117 K. The superconductivity remains stable after several cooling and heating cycles. The observation of superconductivity in this system was carried out at least eight times.

MEASUREMENT OF ANISOTROPIC THERMOPOWER IN SINGLE CRYSTALS Bi2Sr2CaCu2Oy

The thermopower in single crystals of the high Tc superconductor Bi2Sr2CaCu2Oy has been measured in both a-b plane (Sab) and along c axis (Sc) respectively. The temperature dependences are distinctly anisotropic. Sc increases monotonously with temperature and is positive from Tc to room temperature, while Sab increases rapidly near Tc and then decreases with temperature. The maximum of Sab is at 115 K and Sab changes its sign at 250 K.

Characterization of Ba0.1Sr0.9TiO3/YBa2Cu3O7-δ heterostructures on vicinal LaAlO3 substrates

Abstract Ba0.1Sr0.9TiO3/YBa2Cu3O7−δ heterostructure films have been fabricated by pulsed-laser deposition on LaAlO3 substrates. During the fabrication, a c-axis oriented YBa2Cu3O7−δ thin film with atomically smooth surfaces was epitaxially grown in the step-flow mode on a vicinal LaAlO3 substrate. The superconducting transition temperature Tc was above 88xa0K. Then a single-crystalline Ba0.1Sr0.9TiO3 layer was deposited on top of the YBa2Cu3O7−δ layer. A parallel-plate capacitor configuration was used to measure the capacitance–voltage properties of the Ba0.1Sr0.9TiO3 thin films at 300xa0K and 77xa0K, respectively. The dielectric constant and loss tangent measured at 1xa0MHz and 77xa0K are 1200 and 0.0045, respectively. A tunability of 60% has been achieved under ±30xa0V DC bias, confirming their usefulness for frequency-agile microwave devices.

Preparation of highly crystalline a-axis-oriented YBa2Cu3O7-δ superconducting films by a self-template technique

We have prepared high-quality a-axis-oriented superconducting YBa2Cu3O7-delta (YBCO) films using a self-template method on (100) SrTiO3 (STO) substrates by de sputtering. X-ray diffraction and transmission electron microscopy analysis showed that the films were grown epitaxially on the STO substrates with the a-axis perpendicular to the surface. The full-width at half-maximum value of 0.081 was measured through the rocking curve of the (200) diffraction peak. The effect of the deposition temperature and the thickness of the template layer and the upper layer on the quality of the films was investigated. We found evidence suggesting that structural strain was present at the interface between the template layer and the upper layer. For the film deposited under optimal conditions, the zero-resistance T-c was 85 K and the transition width was less than 2 K. The anisotropy of normal state resistance and critical current density was also studied.