Shousheng Yan

Thermoelectric power in normal state of superconductor MgB2

Abstract Experimental results of thermoelectric power (TEP) measurements in MgB 2 from 80 to 320xa0K are presented. The TEP is positive and about 7.6xa0μV/K at room temperature. It varies with temperature linearly in temperature regions below 190xa0K and above 240xa0K, but with different slope. The results imply a multiple band contribution to the transport properties.

1f noise in epitaxial Nd0.7Sr0.3MnO3 films

Abstract 1 f noise of epitaxial Nd0.7Sr0.3MnO3 films, which show giant magnetoresistive effects, was measured in a temperature range of 77–250 K. Below 150 K, where the resistivity reaches its maximum, the noise level increases exponentially with decreasing temperature. Above that temperature, it is almost temperature independent. A possible mechanism to understand this behaviour is discussed.

Paramagnetic behavior in NiNaY zeolites

Abstract The temperature dependence of the DC magnetization of a series of NiNaY zeolite samples has been measured at 2 T from 1.5 K to 140 K. The results show typical paramagnetic behavior for all the samples. The obtained magnetic moment of the paramagnetic unit implies a ferromagnetic coupling between the nearest neighboring Ni2+ ions in a non-reduced sample. For the Ni0 cluster in a supercage of the reduced samples, the number of atoms in the cluster is close to the magic numbers 18 and 20 for alkali metal clusters. No magnetic coupling between the Ni2+ pairs and between the Ni0 clusters was found.

Spin-charge separation phenomena in CeNiSn

Abstract We have studied the magnetic properties and specific heat in CeNiSn. It is found that the peak of susceptibility in CeNiSn, which is not caused by the long distance antiferromagnetic correlation, shifts to low temperature with increasing the magnetic field. The V-shaped density of states can not explain the shift although it can explain many other experimental results. We for the first time explain the shift and magnetoresistance measurements by spin-charge separation. The critical field H c ≈6T, at which the spin gap itself closes, and the spin gap Δ s ≈18K in CeNiSn are obtained.

Observation of strong reduction of 1/f noise in Pr0.67Sr0.33MnO3 films

The resistance and 1/f noise in two representative Pr0.67Sr0.33MnO3 films were recorded continuously from liquid nitrogen temperature to room temperature. Film A has a sharp resistance peak near 160 K and a strong reduction of 1/f noise was observed at slightly below the peak temperature. Film B has a moderate resistance peak near 233 K and instead of noise reduction a noise peak was observed at a higher temperature. Bipolaron theory is used to discuss the order of the phase transition. Differences in the order of phase transition, a broadened transition width and crossing of energy levels are discussed as possible explanations for observed noise reduction and the difference between two samples.

Effect of non-uniform magnetic field produced by Sn film in intermediate state on weak localization

Weak localization magnetoresistance (MR) of a thin Ag film was measured in non-uniform magnetic field created by a superconducting Sn film in intermediate state. MR less than that in uniform field was observed, which is in contrary to the reported MR behavior in the case where the non-uniform field was produced by a superconductor in mixed state. However, the approximately linear behavior of the MR field dependence is in common. The result is discussed based on the different field non-uniformity between intermediate and mixed states. In addition, the detailed structure of the MR indicates a step-like change of the normal domains in intermediate state.

Study of 1f noise in the normal state of YBa2Cu3O7/PrBa2Cu3O7 superlattice

Abstract We have studied the 1 f noise behavior of an YBa2Cu3O7/PrBa2Cu3O7 superlattice sample in the normal state, and, measured, for comparison, the 1/f noise power of an YBa2Cu3O7 film and PrBa2Cu3O7 film. The noise power of superlattice sample is about 2 order of magnitude lower than that of YBa2Cu3O7 film, close to the reported noise level in YBa2Cu3O7 single crystal. The results are analysed by a simplified parallel noise sources model. The high level of 1 f noise in PrBa2Cu3O7 film prefers an origin of incipient localization for copper-oxide superconductors in the normal state.

Distribution of flux pinning energies in Powder-Melt-Textured-Grown YBa2Cu3O7−δ

Abstract Flux pinning energies and their distributions at 77 K in a Powder-Melt-Textured-Grown YBa 2 Cu 3 O 7−δ sample have been determined by 90° rotating sample magnetic measurements. These measurements are done in three different configurations in which the relative orientations between the trapped flux and the depinning fields vary. Four kinds of peaks are found in the distribution curves. Besides the core pinning with energy of 0.2–0.3 eV, the results suggest the existence of magnetic pinning due to the energy decrease of both the core and the surrounding supercurrents of a flux line. Twin boundaries show the strongest pinning. Interactions between flux lines might have make an additional contribution.

Studies of flux-pinning energies in high-Jc YBa2Cu3O7 prepared by melt-textured growth method

Abstract Flux pinning energies and their distribution in the high- J c YBa 2 Cu 3 O 7 superconductor prepared by the melt-textured growth (MTG) method were determined at liquid nitrogen temperature by using rotating sample magnetic measurements. Anisotropy was found in the distribution of the pinning energies in the ab -plane and along the c -axis. It is about 1.1 eV and 1.7 eV, respectively, which is one order of magnitude higher than that of ceramic and single crystal YBCO.

Upper limit of transition temperatures of strong-coupled superconductors

Abstract This paper found that the McMillan T c formula should be modified for higher T c superconductors, and it shows that the phonon mechanism of the high-T c superconductivity is impossible.