Copyright (c) 2009 All rights reserved. http://works.bepress.com/sxdou Recent documents in Professor Shi Xue Dou en-us Sun, 31 May 2009 12:36:32 PDT 3600 http://works.bepress.com/sxdou/97 http://works.bepress.com/sxdou/97 Mon, 02 Feb 2009 14:46:11 PST Correlation of upper critical field (Hc2) and critical current density (Jc) with processing temperature of nano-C doped MgB2 has been studied in comparison to SiC and pure MgB2. SiC and C doped MgB2 exhibit opposite trends in the dependence of Jc and Hc2 on sintering temperature. This is explained by different reactivities of carbon available upon creation of MgB2 for the two types of doping. Nanocarbon doped MgB2 requires sintering temperatures in excess of 900 °C to obtain high boron substitution for carbon, enhancing the vortex pinning and impurity scattering of charge carriers. However, carbon substitution in nano-SiC doped MgB2 occurs at less than 650 °C, allowing lower sintering temperature and high degree of carbon substitution. Both pure and SiC doped MgB2 benefit from low sintering temperature, which results in more grain boundary defects. Substantial carbon substitution can compensate for the disadvantage of sintering at high temperature of nano-C doped MgB2, giving the best Jc of 4.8x103 A/cm2 at 4.5 K and 12 T. This is comparable to the low-temperature sintered nano-SiC doped MgB2 wires. W. K. Yeoh http://works.bepress.com/sxdou/96 http://works.bepress.com/sxdou/96 Mon, 02 Feb 2009 14:46:10 PST Synthesis and characterization of Fe and rare earth (RE) (RE=Eu,Nd) codoped bulk In2O3 oxide are reported. Composites consisting of Fe-doped In2O3 and InREO3 phases were formed, as determined by x-ray diffraction and transmission electron microscopy, with particle sizes of 50-120 nm. The composites exhibit superparamagnetic and semiconducting behavior. A giant positive magnetoresistance is observed at low temperatures, reaching 81% and 50% at 5 K in Eu-Fe and Nd-Fe codoped samples, respectively. G. Peleckis http://works.bepress.com/sxdou/95 http://works.bepress.com/sxdou/95 Mon, 02 Feb 2009 14:46:08 PST The authors investigated the doping effects of single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs) on the Tc, lattice parameters, Jc(B), microstructure, and Hc2 of MgB2 /Fe wire. These effects systematically showed the following sequence for Tc and the α axis: the SWCNT doped wire<the >MWshortCNT doped wire<the >MWlongCNT doped wire <undoped >wire, while Jc(B) followed the sequence of the SWCNT doped wire>the MWshortCNT doped wire>the MWlongCNT doped wire>undoped wire. A dominating mechanism behind all these findings is the level of C substitution for B in the lattice. J. H. Kim http://works.bepress.com/sxdou/94 http://works.bepress.com/sxdou/94 Mon, 02 Feb 2009 14:46:07 PST We report on the magnetoresistance (MR) and magnetic properties of In1.90−xMn0.1SnxO3 (0≤x ≤0.06) oxide. All samples were found to be ferromagnetic below TC=46 K. Sn doping changed In1.90Mn0.1O3 from an insulator to a highly conducting phase at 300 K. A positive MR effect was observed over a wide temperature range just below TC. Calculated MR values reached a maximum of 20% at 5 K. A change in the MR effect, from positive to negative, occurred under magnetic field H>4 T at 5 K. The results of x-ray absorption near-edge spectroscopy indicated that Mn ions are present both as Mn2+ and Mn4+. X. L. Wang http://works.bepress.com/sxdou/93 http://works.bepress.com/sxdou/93 Mon, 02 Feb 2009 14:46:06 PST We report the results on structural aspects, magnetic, and superconducting properties of the La1-xPrxBaCaCu3O7 system. Both x-ray and neutron diffraction studies reveal that Pr substitutes isostructurally in the tetragonal LaBaCaCu3O7 (La:1113) system until the complete replacement of La by Pr. The superconducting transition temperature, Tc , determined from ac susceptibility measurements decreases, with increasing x. The relative Tc depression due to Pr in the LaBaCaCu3O7 superconductor is less in comparison to that found for La-site Pr substituted LaBa2Cu3O7 (La:123). While the critical Pr concentration (xcr) to completely suppress the superconductivity in the former is around 70 at. % of Pr at La site, the same is reported as only 30 at. % for the latter. For nonsuperconducting samples, i.e., for x=0.70 and 1.0, possible antiferromagnetic ordering with Neel temperatures, TN , of 4 and 8 K, respectively, are observed from both dc magnetic and heat capacity measurements. Interestingly in the La1-xPrx :1113 system the xcr of Pr is 0.70, and TN of Pr:1113 is 8 K, while for the La1-xPrx :123 system xCr is 0.30 and TN of Pr:123 is 17 K. The results are explained on the basis of distribution of Pr at both RE and Ba sites in the RE:1113 structure. This leads to a lower TN and a less deleterious effect of Pr on superconductivity of La:1113 compared to La:123. V. P. S. Awana http://works.bepress.com/sxdou/92 http://works.bepress.com/sxdou/92 Mon, 02 Feb 2009 14:46:05 PST The influence of sintering temperature on the critical transition temperature Tc and critical current density Jc for the MgB2 superconductor was investigated systematically with the observation of Raman scattering measurement and flux pinning force Fpanalysis. The enhanced E2g mode in Raman spectra with increasing in situ sintering temperature shows gradual strengthening of the electron-phonon coupling in MgB2, which means that the crystals become more harmonic after higher temperature sintering. However, the crystal harmonicity is degraded for samples sintered at even higher temperature due to Mg deficiency. A possible explanation for the Jc(H) performance, which is in accordance with the Raman spectroscopy observation and Fp analysis, is the cooperation between the electron-phonon coupling in the E2g mode and the flux pinning centers, mainly originating from the lattice distortion due to the different sintering temperatures. W. X. Li http://works.bepress.com/sxdou/91 http://works.bepress.com/sxdou/91 Mon, 02 Feb 2009 14:46:04 PST The La0.7Ca0.3MnO3 thin film shows a metal-insulator transition at temperature around 150 K, about 100 K lower than that of the polycrystalline sample. It is found that the activation barrier in the thin film is lower than that in the bulk sample. The FIR reflectance of the thin film exhibits a new mode which is not present in the bulk sample. We propose that the new mode is the corresponding surface bending mode. F. Gao http://works.bepress.com/sxdou/90 http://works.bepress.com/sxdou/90 Mon, 02 Feb 2009 14:46:03 PST We have investigated the RSr2Fe3O9 (R=La, Y, Pr, and Gd) materials by several experimental techniques, including x-ray and neutron diffraction, magnetic susceptibility, and Mossbauer spectroscopy measurements. All materials studied are single phase and crystallize in the hexagonal perovskite structure. Magnetic susceptibility studies reveal that, for R=La and Pr, the Fe ions order antiferromagnetically at about TN=190 K. Short range magnetic correlations are observed up to 250 K. For R=La and Pr, 57Fe Mossbauer studies reveal two inequivalent magnetic sextets below 190 K, with the area ratio 2:1. Above TN, one singlet is observed with an isomer shift value typical to Fe4+. For R=Y and Gd, the magnetization curves do not lend themselves to easy determination of the magnetic transition due to an extra magnetic phase which exists up to 350 K. This phase is a result of deficiency of oxygen. The 57Mossbauer spectra at 300 K indicate two superimposed singlet lines, and contain a small magnetic sextet. There is no indication that the R sublattice in RSr2Fe3O9 (R=Pr and Gd) orders magnetically down to 1.5 K. V. P. S. Awana http://works.bepress.com/sxdou/89 http://works.bepress.com/sxdou/89 Mon, 02 Feb 2009 14:46:02 PST This work describes in detail the simultaneous enhancement of the upper critical field (Hc2) and the critical current density (Jc) of MgB2 bulk samples doped with nano-SiC particles, as well as single-walled and double-walled (dw) carbon nanotubes (CNTs). The magnetization properties were examined in a superconducting quantum interference device magnetometer, and four-probe transport measurements were performed using a 50 T pulsed magnet to determine Hc2(T).We found that the Jc enhancement is similar in all doped samples at 5 K but nano-SiC addition is more effective to improve the flux pinning in the high temperature range (T≥20 K); this improvement cannot solely be attributed to the C incorporation to the lattice but also to the presence of other types of defects (i.e., several kinds of nanoinclusions). CNTs produce a better C incorporation that is more effective to enhance Hc2 [i.e., dwCNT-doped samples reached a record Hc2(0)~44 T value for bulk MgB2]. All the Hc2(T) curves obtained for different types of doping can be successfully described using a model for a two-gap superconductor in the dirty limit. G. Serrano http://works.bepress.com/sxdou/88 http://works.bepress.com/sxdou/88 Thu, 29 May 2008 14:29:40 PDT Much smoother surfaces and significantly improved superconducting properties of relatively thick YBa2Cu3O7 (YBCO) films have been achieved by introducing a multilayered structure with alternating main YBCO and additional NdBCO layers. The surface of thick (1 µm) multilayers has almost no holes compared to YBCO films. Critical current density (Jc) has been drastically increased up to a factor >3 in 1 µm multilayered structures compared to YBCO films over entire temperature and applied magnetic field range. Moreover, Jc values measured in thick multilayers are even larger than in much thinner YBCO films. The Jc and surface improvement have been analyzed and attributed to growth conditions and corresponding structural peculiarities. A. V. Pan