B.S. Cao

Effect of Li doping on structure and superconducting transition temperature of Mg1-xLixB2

Abstract We report the preparation of Mg 1− x Li x B 2 compounds. Nearly single phase samples were obtained for x ⩽0.3. The in-plane lattice parameter a decreases with Li doping, while the lattice parameter c does not show obvious change. The superconducting transition temperature of Mg 1− x Li x B 2 decreases with Li doping and loss of superconductivity occurs for x =0.5 sample. The results were discussed in terms of the T c dependence on hole doping and contraction of the lattice parameter a .

Influence of the starting composition Mg1−xB2 on the structural and superconducting properties of the MgB2 phase

We report the preparation of samples with the nominal composition Mg1−xB2 (0⩽x⩽0.5). Nearly single-phase MgB2 was obtained for x=0 with a minor amount of MgO. For 0<x⩽0.5, MgB4 coexists with MgB2 and some minor impurity phases. The amount of MgB4 increases with x. With increasing x, the lattice parameter c of MgB2 increases, the lattice parameter a decreases, and correspondingly Tc decreases. The results are discussed in terms of the presence of Mg vacancies or B interstitials in the MgB2 structure. This work is helpful to understand MgB2 films with different Tcs, as well as the Mg site doping effect for MgB2.

Preparation and magnetic properties of Re-doped HgBa2Ca2Cu3O8+δ superconductors

Abstract Nearly phase-pure (Hg 1- x Re 0.1 )Ba 2 Ca 2 Cu 3 O 8+δ superconducting specimens were prepared by a conventional powder-mixing and encapsulation technique. The magnetization M was measured for sintered bulk samples and for grain-aligned powders, and M is much larger for H ‖ c -axis than for H ‖ a - b plane. | M | decreases sharply just above the field H p in the field-increasing branch. At high and moderate temperatures ( T ≥ 40 K), the remanent magnetization at H = 0 is small and the M - H curves are completely asymmetric about the central line of M = 0. These features clearly show that the magnetization originates from the Bean-Livingston surface barrier. Substantial bulk pinning exists only at low temperatures, T ≤ 20 K. The magnetic hysteresis curves of Re-doped Hg-oxide samples also exhibited paramagnetism, in which a Curie-Weiss behavior occurs above T c . This paramagnetism is probably mainly carried by doped Re ions.

Study of the reactions between MgB2 and Ag at high temperature

We report the reactions of MgB2 with Ag at high temperature and its dependence on the ratio of MgB2:Ag. It was found that Ag would react with MgB2 at high temperature to form the binary compound Mg0.5Ag0.5 and MgB4. As long as the sample was still superconducting, T-c remained at about 39 K, indicating that the superconducting substance was still MgB2. The critical current density of the samples dropped with Ag addition and this drop could be explained by considering the reaction between MgB2 and Ag

Effect of Oxygen Content on the Electrical Transport Properties of La0.4Ca0.6MnO3—y

The effect of oxygen content on the electrical transport properties of La 0.4 Ca 0.6 MnO 3-y was studied. Upon decreasing oxygen content, the lattice parameters a, b and c of the samples increase and the electrical resistivity of the samples also increases abruptly, contradicting what is expected in terms of Mn 4+ /Mn 3+ ratio. This work suggests that other factors rather than the ratio of Mn 4+ /Mn 3+ may play a role in determining the properties of La 0.4 Ca 0.6 MnO 3-y with less oxygen contents.

Investigation into the flange problem of resist along the edge of substrate caused by spin coating

The flange phenomena of the photoresist (PR) along the edge of the small substrate, caused by the spin coating process in lithography, were observed and analyzed. A facile and valid way to diminish largely the thickness of the PR flange was introduced and the corresponding experiment was carried out to prove its feasibility.

Investigation on the Structure and Superconducting Properties of Mg1 − xMxB2 (M = deficiency or Ca)

We report the preparation of Mg1 − xMxB2 (M = deficiency or Ca) compounds and their structure and superconducting properties. For Mg1 − xB2, although nearly single-phase samples can be obtained for x = 0, MgB4 coexists with the MgB2 phase and some minor impurity phases, and the amount of MgB4 increases with x for 0 < x ≤ 0.5. The lattice parameters a and c of MgB2 decreases and increases, respectively, with the increase of x, and Tc also decreases. While for Mg1 − xCaxB2, the superconducting transition temperature remains unchanged for x ≤ 0.3 and loss of superconductivity occurs for x > 0.3. X-ray diffraction patterns for x ≤ 0.3 samples show that MgB2 phase coexists with CaB6, Mg, and MgO. With increasing x, the amount of CaB6, Mg, and MgO increases, while the amount of MgB2 decreases. The lattice parameters of MgB2 phase do not show any obvious change in contrast to Mg1 − xB2. The results were discussed by considering some possible contributions.

An HTS 20-Pole Microstrip Filter for GSM-1800 Base Stations

In order to resolve the problems that the Chinese mobile communication was facing (high dropped call rate, not so good voice quality, etc.), we developed a 20-pole high-temperature superconductor thin-film microstrip filter with steep band edges. This filter which had a quasi-elliptic type frequency response was designed for GSM-1800 base station receiver system. Its frequency ranged from 1711 MHz to 1789 MHz. It was fabricated using double-sided YBCO thin films on LaAlO3 substrate. The substrate size was 0.52 mm × 21 mm × 40 mm. Experiments were performed on a Stirling cooler at a temperature of 60 K. The measured insertion loss in the passband is below 0.35 dB, return loss is bigger than 13.5 dB, and the steepness of the band-edge is up to 18 dB/MHz. Then we assembled it with an LNA (low noise amplifier) into a vacuum chamber to get a base station receiver front end and measured it. Its measured gain is 22.5 dB, and noise figure is below 0.6 dB in the passband which is very important to improve the sensitivity of the base station system.

Influence of Preparation on Structure and Properties of La0.67Ca0.33MnO3 Bulk Material

During growth of La0.67Ca0.33MnO3 in an usual solid state reaction process, the diffusion of La-site metal ions is a rate-limiting factor. In the initial stage of the growth process, La-deficient manganite with rhombohedral structure was easily formed. With the repetition of the grinding– pressing–sintering process, the metal content on the La-site was gradually increased, and the phase was changed from rhombohedral to cubic structure. For slightly La-deficient (La,Ca)1–xMnO3 manganites, the ferromagnetic–paramagnetic (F–P) transition temperature TM is less sensitive to the La-site deficiency, but their electric transport properties are greatly affected. The slightly La-site deficient compound was characterized by double bumps on its resistivity versus temperature (R–T) curve, one of which was located at higher temperature very near to its temperature TM. For the La0.67Ca0.33MnO3 single phase, only one resistance peak was observed at TR on its R–T curve. For the sample sintered at lower temperatures (<1100 °C) in air TR is much less than TM, and no magnetoresistance (MR) peak was observed within the temperature range of 80 K < T < 300 K. But for the sample sintered at 1200 °C in air TR is well in agreement with TM and its MR is strongly peaked very close to TM. All the bulk samples showed substantial MR in a magnetic field of 0.6 T at low temperatures that increased almost linearly with decreasing temperature.

Structural, transport and magnetic properties of PrBa2−xSrxCu3O7-δ

Abstract Structural, electrical transport and magnetic properties of polycrystalline samples of PrBa 2−x Sr x Cu 3 O 7-δ were investigated. The results were discussed based on the variation of hybridization between Pr ions and O2p of the CuO 2 planes induced by Sr doping.