Zhou Yueliang

Laser molecular beam epitaxy system and its key technologies

The laser molecular beam epitaxy system and its key technologies have been successfully developed in China. Atomically regulated unit-cell by unit-cell homoepitaxial SrTiO3 (STO) and heteroepitaxial BaTiO3 (BTO) films and STO/BTO superlattices were fabricated on STO (100) substrates by the laser molecular beam epitaxy. The fine streak patterns and more than 1000 cycles of undamping intensity oscillation were obtained by using in-situ reflection high-energy electron diffraction. The surfaces of films are atomically smooth.

Investigation the positive moments on the M-T curve of YBCO films measured by using zero-field cooling

The superconducting transition of a YBCO film was measured by a MPMS-5 superconducting quantum interference device magnetometer, using a zero-field cooling process. The experimental results have shown that there are positive magnetic moment and positive peak on the M-T curve. We have proven that these anomalous behaviours are due to measurement error, but not phase transition. We have proposed a simple formula to explain and to calculate quantitatively these anomalous behaviours. It was found that, provided dH > 0.59H(p) (dH is the inhomogeneous field of the remnant field, H-p is the fully penetrated field of the measured sample), the experimental results will be positive, not negative. If dH greater than or equal to 2H(p), the experimental results will be symmetrical to their real negative values. From the M-T curve, which has positive moment and positive peak below T-c (superconducting transition temperature), we found a new possible method to obtain Hp of the measured sample.

Quantum well infrared detector with grating enhancement

Abstract We have investigated theoretically enhanced quantum well IR detector. The field enhancement expressions are given by means of the perturbation method. The quantum efficiency and enhancement ratio have been obtained for a doped GaAs/AlGaAs quantum well structure at λ = 10 μm for different grating periods. We explained why Goossen and Lyon only observed a small enhancement.

Study of resonant excitation of infrared surface second harmonic at the GaAs-Al interface.

By using a tunable TEA CO(2) laser and by grating coupling at the interface of GaAs-Al, we have obtained, for the first time to our knowledge, second-harmonic generation from a resonantly excited infrared surface polariton. The experimental results provided a dispersion relation that compares well with theoretical calculations. The deduced value of chi(2), 5.9 x 10(-7) esu, agrees well with other measurements.

Effects of DC magnetic field upon the properties of a HTS microstrip antenna at 4.2 K

Abstract A circular polarized microstrip antenna with a working frequency of 6.26 GHz was fabricated from YBCO HTS films. The transmission coefficient and circular polarization ratio of the antenna at different external magnetic field B were measured at 4.2 K. The experimental results showed that its transmission coefficient and circular polarization ratio hardly changed with B up to 1 T. It is concluded that the field-induced increase of surface resistance of YBCO films would not influence the microwave properties of this antenna at B ⩽ 1T and T = 4.2 K.

Growth mode of Cu-rich particles on laser-ablated YBa2Cu3O7 thin films

The microstructure of the laser-ablated YBa2Cu3O7 (YBCO) thin film deposited on heated (100) SrTiO3 substrate was examined by transmission electron microscope. The particles on the film mainly consist of CuO and few CuYO2. Most of these non-superconducting particles nucleate on or near the surface of the film and protrude about 100-400 nm in height. A large amount of a-axis YBCO grains also exist in the film, which nucleate at the substrate surface and grow perpendicularly above the c-axis YBCO film. The YBCO thin film deposited under low oxygen pressure has very different microstructure compared with YBCO thin film deposited under high oxygen pressure.

Study of the properties of infrared intersubband transition in doped GaAs/AlxGa1-xAs multiple quantum wells

The properties of intersubband transition of GaAs/AlxGa1-xAs multiple quantum wells with various well widths and doped-well concentrations have been studied. Both theoretical and experimental results are in good agreement. For the appropriate well width and higher doping concentration, we directly observed two intersubband absorption peaks from E1 → E2 and E2 → E3 transitions in well. The experimental results and theoretical analysis are given.

Fabrication of MgB2 superconducting films by different methods

Abstract MgB2 superconducting films have been successfully fabricated on single crystal MgO (111) and c-AL2O3 substrates by different methods. The film deposited by pulsed laser deposition is c-axis oriented with zero resistance transistion temperature of 38.4K, while the other two films fabricated by chemical vapor deposition and electrophoresis are c-axis textured with zero resistance transistion temperature of 38 K and 39 K, respectively. Magnetization hysteresis measurements yield critical current density J c of 107 A/ cm2 at 15K in zero field for the thin film and of 105 A/ cm2 for the thick film. For the thin film deposited by chemical vapor deposition, the microwave surface resistance at 10 K is found to be as low as 100µΩ, which is comparable with that of a high-quality high-temperature superconducting thin film of YBCO. * Supported by the Major State Basic Research Development Program of China (Grant No. 1999064604)

YBa2Cu3O7 Bicrystal Josephson Junctions and dc SQUIDs

Bicrystal Josephson junctions and direct current superconducting quantum interference devices (dc SQUIDs) were prepared on YBa(a)2Cu(3)O(7) (YBCO) thin films deposited on yttrium stabilized ZrO2 bicrystal substrates. The critical current densities j(c) similar to 2 x 10(6) A/cm(2) for YBCO thin film and (1-3) x 10(3) A/cm(2) at boundaries were obtained at 77K. For YBCO Josephson junction made on 28 degrees misorientated boundary, the Shapiro steps were observed. The dc SQUID was made on the 32 degrees misorientated boundary junctions, which consisted of 40 x 40 mu m of loop and 2000 x 2000 mu m planar superconductive flux focuser. The voltage modulation depth of this SQUID was 6 mu V, and the magnetic field sensitivity to be similar to 2 pT/root Hz from the measured noise level.

EFFECTS OF THE QUALITY OF SUBSTRATE ON THE STRUCTURE AND SUPERCONDUCTING PROPERTIES OF Yba 2 Cu 3 O 7-x THIN FILM

We have systematically investigated the structures, morphologies, microwave surface resistance, DC performances and the substrate qualities of three YBa2Cu3O7-x(YBCO) thin films 30 mm in diameter. The three samples deposited by the same process have been proved of different properties. It has been shown that the structure and superconducting properties of the thin film depend strongly on the quality of substrate.