X.S. Wu

Microstructural variations of YBa2Cu3Oy doped with Ca at high doping level

Abstract We have prepared the Y 1− x Ca x Ba 2 Cu 3 O y with x ≤ compounds by the solid state reaction technique. The superconducting critical temperature was measured. The structural parameters as a function of Ca substitution were obtained through the detailed analysis of the X-ray diffraction for these Ca-doped samples. The variation of the lattice parameters, some meaningful bond lengths, and the microstrain in lattice change with the Ca content have been discussed.

Structure and superconductivity in YBa2Cu3Oy with additives of NaNO3 and NaCl

Abstract YBa 2 Cu 3 O y (YBCO) cuprates with the additives of NaNO 3 and NaCl were synthesized by the standard solid-state reaction technique. For the samples with addition of NaNO 3 , the superconducting transition temperature T c0 remains almost unchanged although the lattice constants of the unit cell decrease monotonically with an increase in the additive of NaNO 3 . The superconducting transition temperature T c0 for NaCl doping in YBCO, on the other hand, decreases with an increase in the content of NaCl. The lattice constants of a , b , and c increase slightly for NaCl doping in YBCO. We compared these results with that of the KNO 3 added in YBCO and believe that the effects of lattice strain and chlorine replacing for Ba on superconductivity and structure should be considered in these system.

Transport mechanism of nanocrystalline-silicon film tunnelling diodes

Based on the research into the conduction mechanism of the hydrogenated nanocrystalline silicon (nc-Si:H) films, we have fabricated nanocrystalline-silicon film tunnelling diodes. The structure of the diode is Al/c-Si/nc-Si:H/Al, in which the nc-Si:H thin film is the active layer. A characteristic of local ordered structures was detected in the nc-Si:H film, which indicated that both ordered micro-zones and disordered regions existed in the film. In some diodes quantum staircases and quantum oscillation phenomena were observed in the I-V, -V and C-V curves at liquid nitrogen temperatures or below (<100 K). On the basis of energy level calculation, we consider that the current through the diode comes from two parts: in the ordered micro-zones of the active layer electrons transport via resonant tunnelling, while in the disordered regions resonant tunnelling will be smeared and the tunnelling current increases continually with increasing bias.

High-temperature characteristics of strain in AlGaN/GaN heterostructures

The high-temperature characteristics of strain in fully strained and partially strain-relaxed Al0.2Ga0.8N/GaN heterostructures were investigated at temperatures from room temperature to 800 K by means of the high-resolution X-ray diffraction technique. The results show that an obvious temperature-dependent strain relaxation occurs in both 50-nm- and 100-nm-thick AlGaN layers when the temperature exceeds 523 K. The degree of strain relaxation increases by about 12% and 15% in 50-nm- and 100-nm-thick AlGaN layers, respectively, over the whole temperature range in our measurements. The temperature-dependent strain relaxation results in a theoretical 2DEG reduction of about 5 and 6% for the fully strained and partially strain-relaxed Al0.22Ga0.78N/GaN heterostructures, respectively. When the temperature is under 523 K, the in-plane strain in the 50-nm-thick AlGaN layer is stabler than that in the 100-nm-thick AlGaN layer due to a lower density of dislocations in the 50-nm-thick AlGaN layer.

Microstrains in La1.85Sr0.15Cu1−yMyO4 (M=Zn, Ni, Mg) cuprates with y≤0.30

Abstract La 1.85 Sr 0.15 Cu 1− y M y O 4 ceramic cuprates with M=Zn, Ni, Mg, and y ≤0.30 have been synthesized. The superconducting critical temperatures and the room temperature resistivities for M doping have been determined by a standard four-probe method. The width of each reflection of X-ray diffraction for each sample is analyzed by Rietveld refinements which is related to the average grain size and the average lattice microstrain. In this way we attempt to explain the stronger suppression for non-magnetic doping such as Zn or Mg than that for magnetic doping such as Ni.

A neutron powder diffraction study of the superconductor Hg-1223 via Pb substitution

The mercury-based superconductor Hg-1223 with Pb substituted for Hg has a transition temperature (, the zero-resistance temperature, ; , the diamagnetic onset temperature, = 143 K) higher than that for pure Hg-1223 compound. The structure of this Pb-doped sample has been investigated via neutron powder diffraction. Some selected bond distances are compared to those of Pb-free Hg-1223 cuprate. The contents of Pb and oxygen have been refined. The estimated valence of copper is about 2.30, which is consistent with that for YBCO superconductor.

Structural anomalies in LaBa2Cu3Ox cuprates with iron substitution

Abstract Ceramic LaBa 2 Cu 3− y Fe y O x compounds with y =0.002∼2.8125 were synthesized by the solid state reaction technique. Rietveld analysis for X-ray diffraction (XRD) was performed on these iron-doped samples. The content of the impurity BaCuO 2 phase can be subtracted by iron doping as y >0.60. The anomalous structural phase transitions of orthorhombic-to-tetragonal-to-orthorhombic-to-tetragonal occur within the doping range of 0.00≤ y ≤2.25. The lattice parameter of a varies with y anomalously, while b increases with y monotonously as y ≥0.60. The lattice parameter of c decreases firstly when y ≤0.15, and then increases as y ≥0.25, with increasing y . The occupancy of oxygen at O(4) site, which is in the La plane at z =1/2, increases with increasing iron content. These results may correlate to the iron-preferred occupancy at Cu(1) and/or Cu(2) sites. More complex impurity phases were detected in samples with y =2.625 and 2.8125.

Optimization and enhancement of transition temperature for HgBa2Ca2Cu3O8+δ ceramic superconductors

Abstract To control and optimize the superconductivity critical temperature T c of Hg-based superconductor is not only important for applications, but also useful for providing possible explanations for the mechanism of high T c superconductivity. In the present work, we successfully obtained a T c R=0 (zero-resistance temperature) = 135 K, T c dia (diamagnetic onset temperature) = 143 K, an optimum hole concentration p 0 = 0.22 and the T c -p phase diagram for the HgBa 2 Ca 2 Cu 3 O 8+δ superconductors with substitution of Hg by Pb.

Synthesis and properties of the Hg-1223 superconducting thin films prepared by ion implantation

Using the pulse laser ablation and ion implantation techniques, HgBa2Ca2Cu3O8+δ superconducting thin films of about 2500–3500 A in thickness have been successfully synthesized. The structure was characterized by synchrotron X-ray diffraction, which was carried out at the diffraction station located at Beijing Synchrotron Radiation Laboratory (BSRL). The formation of the superconducting thin films was examined by X-ray diffraction (XRD) and a.c. magnetic susceptibility measurement. By varying the annealing conditions, the optimum annealing conditions were found. The thin films annealed at the optimum conditions display a tetragonal structure with a P4mmm symmetry and have the lattice parameters a = 3.851(4) A and c = 15.8511(6) A, and a magnetically determined superconducting transition at 118 K. Similar to those for bulk Hg-1223 compounds, the main impurities in the above described thin film are BaCuO2, CaHgO2 and CuO.

Spin Gap Characteristic of Y(Ba1−xGdx)2Cu3O7−δ

Y(Ba1−xGdx)2Cu3O7−δ compounds with x = 0 ∼ 0.15 are prepared using the solid reaction technique. With structure analysis by Rietveld refinement of x-ray diffraction, we find that Gd3+ ions prefer to occupy Y sites within lighter doping x ≤ 0.08 due to ion size effects, then begin partially to occupy Ba sites with doping content increasing, which gives vital influence on superconductivity and spin-gap properties. The magnetic doping effects of Gd3+ ions on spin-gap properties are investigated in detail by contrast of the distinguished behaviors between T* and Tc, indicating that spin-gap temperature is not completely determined by the carrier density, but strongly dependent on the strength of interplane antiferromagnetic coupling. Finally, we propose an expression of in-plane resistivity dependent on the maximal width of spin-gap Δ0 to derive their values for different samples, which almost keep constant with the increase of Gd doping contents.