Yang Sen-Zu

Phonon Characteristics of Polycrystalline Cubic SrTiO3 Thin Films

Polycrystalline SrTiO3 thin films with the cubic perovskite structure were grown on quartz substrates by pulsed laser deposition. The first-order Raman scattering processes, which were forbidden in SrTiO3 single crystals, were observed in the films at 300 K and lower temperatures due to the structural distortion causing by strain effect and oxygen vacancies. The polar TO2 phonon showed a typical Fano asymmetry in the entire temperature region from 95 K to 300 K. In contrast, the nonpolar TO3 phonon and polar TO4 phonons were mostly symmetric.

Barrier Formation on a YBa2Cu3Oy Thin Film Using CF4 Plasma Fluorination

We investigate the surface structure and composition of a YBa2Cu3Oy (YBCO) thin film modified by CF4 plasma fluorination. In addition to the absorption of hydrocarbons, chemical reactions of the YBCO surface take place during CF4 plasma treatment. Various x-ray photoelectron spectroscopic data are reported and discussed. The existence of a thin barrier is confirmed, which homogeneously covers the edge of the base YBCO film in our interface engineering Josephson junction. Measurements of Auger electron spectroscopic data and the resistance versus temperature indicate that the barrier is a controllable-insulating layer.

C-Axis Current-voltage Characteristics of Mesa Structures on Bi2Sr2CaCu2O8+δ Single Crystals Fabricated by a Simple Technique Without Photolithography

A simple technique is reported for fabricating the mesa structure on Bi2Sr2CaCu2O8+δ single crystal. In the patterning process, metal masks are used instead of photolithography and argon ion milling is applied to form the small mesa on the Bi2Sr2CaCu2O8+δ crystal surface. Real four-probe transport measurements are made on the mesa structure and typical c-axis current-voltage (I - V) characteristics of the intrinsic Josephson effect have been observed. The superconducting gap parameter can be extracted from the multi-branch structure in the I - V characteristics. Additionally, from the strong hysteresis in the I - V characteristics, the capacitance CJ of the unit intrinsic Josephson junction has been estimated to be 2.3 pF, which is in good agreement with that evaluated from the geometric parameters of the unit junction between the two copper oxide layers.

Injection Efficiency of Spin-Polarized Quasi-particles in Y-Ba-Cu-O Thin Film

The injection efficiency of spin-polarized quasi-particles in a superconducting Y-Ba-Cu-O (YBCO) film is investigated by means of current injection into the Nd0.7Sr0.3MnO3/SrTiO3/YBa2Cu3O7-δ heterostructures. We have successfully prepared six injection junction windows in the YBCO thin film strip, which are all 56 µm wide, but 80 µm, 40 µm, 20 µm, 10 µm, 5 µm, and 2 µm long, respectively. At 16 K, our YBCO thin film strip (80 nm thick) has Jc = 2×105A/cm2. Under the spin-polarized injection current of Iinj = 0.5 mA, with decreasing injection junction length L, injection efficiency η (η≡ΔJc/ΔJinj; ΔJc: the suppression of critical current density due to finite ΔJinj) increases gradually, until L is not more than 20 µm. In that case, η hardly varies with the change of L, reaching up to about 6. We think that these results are related to the spin diffusion length of spin-polarized quasi-particles in the superconducting thin film. In our heterostructures, the YBCO thin film keeps good superconductivity and the injection junction windows are of proper size, therefore, we have obtained relatively high injection efficiency of spin-polarized quasi-particles.

High-Tc Superconductor Detection Coils for a Magnetic Resonance System

Considering that in a magnetic resonance system, if the detection coil contributes dominantly to the system noise, the performance of the whole system can certainly be improved by switching to a detection coil made of a high-temperature superconductor, and using YBa2Cu3O7 thin films on 25?25?mm2 LaAlO3 substrates, we have prepared two kinds of detection coils: single-coil and two-coil. Encouragingly, their quality factors are measured to be Q>2500 for two-coil (at 22.566?MHz and 77?K) and Q>5500 for single-coil (at 92.3?MHz and 77?K), respectively. Here, we describe the details of the design, fabrication and testing of the coils.

Modification of a YBa2Cu3O7-δ Thin Film Using an Atomic Force Microscope

A YBa2Cu3O7-δ thin film is modified by a probe electric field of an atomic force microscope to form a ridge with the width of only a grain cell. The modification varies with the operation parameters of the bias voltage, the moving velocity of the probe and the ambient humidity. Energy dispersive spectroscopy analysis shows only oxygen deficiency in the modified YBCO thin film. As a result, the suppressed superconductivity was found in the junction crossing the ridge.

Experimental Study of the Plasma Fluorination of Y-Ba-Cu-O Thin Films

We have experimentally studied the surface modifications of Y-Ba-Cu-O (YBCO) thin films using CF4 plasma. The intensity of the plasma fluorination was controlled by changing the biasing voltage and the time of the plasma treatment. Microstructural analyses reveal that the oxygen content of the YBCO thin films was changed. Transport measurements of sufficient fluorinated YBCO films imply that the films changed totally into an oxygen-deficient semi-conducting state. From these experimental results, we believe that plasma fluorination is quite a useful method to form controllable a thin barrier layer in fabricating interface engineered junctions and to form a stable narrow weak-link region in fabricating planar superconductor-normal-superconductor junctions.

Flux creep, flux pinning and critical current density of Y-, Gd-, and Tl-based superconductive thin films

Magnetic hysteresis loops have been measured for YBa2Cu3O6+x, GdBa2Cu3O6+x and Tl2Ba2Ca2Cu3O10-y films at different temperatures with a vibrating sample magnetometer. Based on Bean model, magnetic critical current density Jcm has been derived approximately. The field and temperature dependence of Jcm for all samples can be written as: Jcm = Jc0(T) × f(B,T), where f(B,T) = 1 - Aln(B/B0(T)), which is similar to the transport critical density strongly affected by flux creep: Jct = Jc0(B,T)(1 - (kBT/U0)ln(BΩd/Ec)). The extraordinary similarity suggests that Jcm is determined not only by flux pinning but also by flux creep. In the first formula, f(B,T) may be correlated with the effect of flux creep on Jcm, and Jc0(T) is determined by flux pinning. Jc0(T) is independent of magnetic field and is proportional to (Tc - T). Similar results have also been found for other samples. It may be the common characteristic of high-Tc superconductors. Magnetic relaxation of YBa2Cu3O6+x and GdBa2Cu3O6+x films has been measured at LN2 temperature. Using the equation of Hagen et al., effective activation energy U0 has been deduced, which is about 0.3 eV, one order bigger than the value obtained by A = kBT/U0. U0 is almost independent of the field in the field range selected.