Shouyu Dai

Highly conductive Nb doped SrTiO3 epitaxial thin films grown by laser molecular beam epitaxy

Highly conductive Nb doped SrTiO3 (Nb:STO) thin films with good crystallinity and smooth surfaces were epitaxially grown on SrTiO3 (STO) (1 0 0) substrates by laser molecular beam epitaxy. The resistivity, carrier concentration and mobility of the Nb:STO thin film are 3.6 x 10(-4) Omega cm, 2.8 x 10(21) cm(-3) and 12.7 cm(2)/V s, respectively, which are the best values in Nb:STO thin films reported so far. The root-mean-square surface roughness of the deposited thin him is measured to be 0.240 nm by atomic force microscopy. The chemical composition and its uniform distribution in the film were confirmed by angle-resolved X-ray photoelectron spectroscopy and Rutherford backscattering spectrometry. The difference of electronic structure in the band gap region between Nb : STO and STO thin films was studied by X-ray photoelectron spectroscopy

Development and Test of 10.5 kV/1.5 kA HTS Fault Current Limiter

Superconducting Fault Current Limiter (SFCL) is an attractive appliance for modern electrical power system. A 10.5kV/1.5 kA three-phase HTS fault current limiter was developed by IEE, CAS. This improved rectifier-type SFCL with HTS coil of 6.25 mH is going on a demonstrated long-term reliable operation in a real 10.5 kV substation located in Hunan, China. In a three-phase-to-ground short circuit test of grid, the prospective fault current of 3.5 kA was limited to 635 A at the pre-setup short-circuit point successfully

In-doped SrTiO3 ceramic thin films

We report the characterization of the ceramic SrIn0.1Ti0.9O3 thin film grown by laser molecular-beam epitaxy. The lattice constant is determined to be 0.3948 nm, slightly larger than that of the SrTiO3 substrate. Hall measurement confirms that this film is a p-type semiconductor either below 92 K or above 158 K. X-ray photoemission spectroscopy study shows that the width of the valence band of the p-type SrIn0.1Ti0.9O3 film is narrower than that of the n-type SrNb0.1Ti0.9O3 film. There is a 0.35 eV difference in the Fermi energy level of the two films. The electronic state of the surface layer of the SrIn0.1Ti0.9O3 film is found to be different from that of its interior.

Colossal magnetoresistance effect of the electron-doped manganese oxide La1?xSbxMnO3 (x = 0.05, 0.1)

La–Sb–Mn–O compounds have been prepared by the solid-state reaction of La2O3, Sb2O3 and MnCO3. The materials have a perovskite structure and exhibit the colossal magnetoresistance (CMR) effect, and the maximum magnetoresistance ratio could reach as high as 60% (at 225 K under 5 T) for La0.9Sb0.1MnO3. The valence states of manganese and antimony have been identified as Mn3+, Mn2+ and Sb5+. The temperature dependences of electrical transport and magnetic properties have been studied. The results suggest that the mechanism of CMR for La1−xSbxMnO3 is probably due to the double exchange interaction between Mn3+ and Mn2+ ions.

Optical properties of p-type In-doped SrTiO3 thin films

We report the optical properties of p-type SrInxTi1−xO3 (x=0.1 and 0.2) films prepared by laser molecular beam epitaxy under different oxygen pressures. The results of x-ray diffraction and near-field scanning optical microscopy indicate that the films have high crystallinity, smoothness, and uniformity. Hall measurement confirms that the films are p-type semiconductors. The optical transmittance spectra reveal that the films are highly transparent in most of the visible region. A blueshift of the absorption edge with an increase in oxygen pressure during deposition has been observed and discussed. Moreover, the first-order Raman scattering peaks of the optical phonons in these In-doped SrTiO3 films are presented, and they are Raman inactive in bulk SrTiO3 single crystal.

Colossal magnetoresistance effect in perovskite-type La-Sn-Mn-O epitaxial films

La–Sn–Mn–O (LSnMO) thin films epitaxially grown on single-crystal substrates by pulsed-laser deposition are reported. The films have a perovskite structure and perform the colossal magnetoresistance effect with the maximum magnetoresistance (MR) ratio of 103% (at 233 K and 6 T). The dependence of electrical transport and magnetic properties on the film thickness has been studied. The analyses reveal that the electrical transport, in contrast with the magnetic phase transition, is more sensitive to the thickness of the films.

Effects of donor concentration on the electrical properties of Nb-doped BaTiO3 thin films

Thin films of BaNbxTi1−xO3(0<x⩽0.5) were epitaxially grown on MgO substrates by laser molecular beam epitaxy. The thin films undergo tetragonal to cubic and semiconductor to metal transitions with Nb concentrations as shown by x-ray diffraction and electrical resistivity measurements. Room temperature resistivities are found to decrease monotonically with increasing Nb concentration and range from 101 to 10−4Ωcm. The fact that the temperature dependence of resistivity of the thin films can be fitted well using a small polaron model reveals the polaronic nature of the charge carriers in the thin films. This conclusion is further confirmed by the existence of localized states within the band gap of BaTiO3 as revealed by synchrotron radiation-based ultraviolet photoelectron spectroscopy.

La0.7Pr0.3MnO3 ceramic: An electron-doped colossal magnetoresistive manganite

We report a study on the synthesis, electrical transport, and magnetic properties of Pr-doped LaMnO3 ceramic material. We have found that La1−xPrxMnO3+δ (x=0.3) synthesized using solid-state reaction shows semiconductor behavior, and no colossal magnetoresistance (CMR) effect; while it shows CMR behavior when it is annealed in a flowing argon at certain temperature (about 873 K), which suggests that La0.7Pr0.3MnO3+δ has been transferred to La0.7Pr0.3MnO3. The x-ray photoemission spectroscopy reveals that Pr ions are in a mixed-valence state of Pr4+ and Pr3+ in this compound. Therefore, La1−xPrxMnO3 (x=0.3) could be an electron-doped CMR manganite.

Fabrication of MgB2 superconducting thick films by electrophoresis technique

MgB2 superconducting thick films of about 40 μm were fabricated successfully by a simple and low-cost approach. Boron films were prepared on Ag/Al2O3 substrates using the electrophoresis technique followed by a post-annealing process with excessive magnesium. Resistance measurements of the MgB2 thick films show Tcon of 39 K and ΔTc of 0.6 K. The X-ray diffraction analysis reveals that most part of the films consist of c-oriented crystalline MgB2. The images of surface morphology and cross-section profiles by scanning electron microscopy show well structured grain boundary MgB2 films and a well-defined compact layer.

Development of HTS AC Power Transmission Cables

Along with the rapid growth of the national economy in China, the electric utility is faced with an ever rising demand for electricity and the problem to solve large capacity power transmission. Under the support of the High Technology Research & Development Program of China, a 1.5 kA/10.5 kV 10-m long 3-phases HTS transmission power cable and a 75-m long, 3-phase, 10.5 kV/1.5 kA HTS power cable have been developed successfully by the Institute of Electrical Engineering, Chinese Academy of Sciences. The 75-m long HTS power cable operates now at a local live distribution grid of 6.6 kV to supply electric power for Changtongs factory in Baiyin, Gansu Province. The cable operates stably and reliably over 7000 hours since December 2004.