arXiv subjects

Z. X. Shi

Publications and source records attributed to Z. X. Shi.

29 records · Page 2Linked to original sources

arXiv2013

M dwarf catalog of LAMOST pilot survey

We present a spectroscopic catalog of 67082 M dwarfs from the LAMOST pilot survey. For each spectrum of the catalog, spectral subtype, radial velocity, equivalent width of H$α$, a number of prominent molecular band indices and the metal sensitive parameter $ζ$ are provided . Spectral subtype have been estimated by a remedied Hammer program (Original Hammer: Covey et al. 2007), in which indices are reselected to obtain more accurate auto-classified spectral subtypes. All spectra in this catalog have been visually inspected to confirm the spectral subtypes. Radial velocities have been well measured by our developed program which uses cross-correlation method and estimates uncertainty of radial velocity as well. We also examine the magnetic activity properties of M dwarfs traced by H$α$ emission line. The molecular band indices included in this catalog are temperature or metallicity sensitive and can be used for future analysis of the physical properties of M dwarfs. The catalog is available on the website \url{http://sciwiki.lamost.org/MCatalogPilot/}.

astro-ph.SR↗

arXiv2012

Annealing, acid, and alcoholic beverage effects on Fe1+yTe0.6Se0.4

We have systematically investigated and compared different methods to induce superconductivity in iron chalcogenide Fe1+yTe0.6Se0.4 including annealing in vacuum, N2, O2, I2 atmosphere, and immersing samples into acid and alcoholic beverages. Vacuum and N2 annealing are proved to be ineffective to induce superconductivity in Fe1+yTe0.6Se0.4 single crystal. O2 and I2 annealing, acid and alcoholic beverages can induce superconductivity by oxidizing the excess Fe in the sample. Superconductivity in O2 annealed sample is in bulk nature, while I2, acid and alcoholic beverages can only induce superconductivity near the surface. By comparing different effects of O2, I2, acid and alcoholic beverages, we propose a scenario to explain how the superconductivity is induced in the non-superconducting as-grown Fe1+yTe0.6Se0.4.

cond-mat.supr-con↗

arXiv2011

Density effect on critical current density and flux pinning properties of polycrystalline SmFeAsO1-xFx superconductor

A series of polycrystalline SmFeAs1-xOx bulks was prepared to systematically investigate the influence of sample density on flux pinning properties. Different sample densities were achieved by controlling the pelletizing pressure. The superconducting volume fraction, the critical current densities Jcm and the flux pinning force densities Fp were estimated from the magnetization measurements. Experimental results manifest that: (1) the superconducting volume fraction decreases with the decreasing of sample density. (2) The Jcm values have the similar trend except for the sample with very high density may due to different connectivity and pinning mechanism. Moreover, The Jcm(B) curve develops a peak effect at approximately the same field at which the high-density sample shows a kink. (3) The Fp(B) curve of the high-density sample shows a low-field peak and a high-field peak at several temperatures, which can be explained by improved intergranular current, while only one peak can be observed in Fp(B) of the low-density samples. Based on the scaling behaviour of flux pinning force densities, the main intragranular pinning is normal point pinning.

cond-mat.supr-con↗

arXiv2011

Anomalous Tail Effect on Resistivity Transition and Weak-link Behavior of Iron Based Superconductor

Temperature dependent resistivity of the iron-based superconductor NdFeAsO0.88F0.12 was measured under different applied fields and excitation currents. Arrhenius plot shows an anomalous tail effect, which contains obvious two resistivity dropping stages. The first is caused by the normal superconducting transition, and the second is supposed to be related to the weak-link between the grains. A model for the resistivity dropping related to the weak-link behavior is proposed, which is based on the Josephson junctions formed by the impurities in grain boundaries like FeAs, Sm2O3 and cracks together with the adjacent grains. These Josephson junctions can be easily broken by the applied fields and the excitations currents, leading to the anomalous resistivity tail in many polycrystalline iron-based superconductors. The calculated resistivity dropping agrees well with the experimental data, which manifests the correctness of the explanation of the obtained anomalous tail effect.

cond-mat.supr-con↗

arXiv2011

Electronic Behavior of Superconducting SmFeAsO0.75

High-quality polycrystalline SmFeAsO0.75 was synthesized with a superconducting transition width less than 1 K, and the electronic behavior was systematically studied by transport and specific heat measurements. An obvious superconducting jump was witnessed, together with a very small normalized superconducting jump, which is much smaller than expected by the BCS theory. A strong temperature dependent Hall coefficient was found and attributed to the partial gapping of the Fermi surface up to the temperature of 160 K which was predicted and supported by the emergence of the pseudogap. The charge-carrier density as well as the effective mass were also obtained and discussed in detail.

cond-mat.supr-con↗

arXiv2011

Doping Effect and Flux Pinning Mechanism of Nano-SiC Additions in MgB2 Strands

Superconducting MgB2 strands with nanometer-scale SiC additions have been investigated systematically using transport and magnetic measurements. A comparative study of MgB2 strands with different nano-SiC addition levels has shown C-doping-enhanced critical current density Jc through enhancements in the upper critical field, Hc2, and decreased anisotropy. The critical current density and flux pinning force density obtained from magnetic measurements were found to greatly differ from the values obtained through transport measurements, particularly with regards to magnetic field dependence. The differences in magnetic and transport results are largely attributed to connectivity related effects. On the other hand, based on the scaling behavior of flux pinning force, there may be other effective pinning centers in MgB2 strands in addition to grain boundary pinning.

cond-mat.supr-con↗

arXiv2011

Effects of the sintering atmosphere on the superconductivity of SmFeAsO1-xFx compounds

A series of SmFeAsO1-xFx samples were sintered in quartz tubes filled with air of different pressures. The effects of the sintering atmosphere on the superconductivity were systematically investigated. The SmFeAsO1-xFx system maintains a transition temperature (Tc) near 50 K until the concentration of oxygen in quartz tubes increases to a certain threshold, after which Tc decreases dramatically. Fluorine losses, whether due to vaporization, reactions with starting materials, and reactions with oxygen, proved to be detrimental to the superconductivity of this material. The deleterious effects of the oxygen in the sintering atmosphere were also discussed in detail.

cond-mat.supr-con↗

arXiv2010

Peak Effect of Flux Pinning in Sc5Ir4Si10 Superconductors

Magnetic hysteresis loops (MHLs) have been comparatively measured on both textured and single crystalline Sc5Ir4Si10 superconductors. Critical current densities and flux pinning forces are calculated from MHLs by Bean model. Three kinds of peaks of the flux pinning force are found at low fields near zero, intermediated fields, and high fields near the upper critical field, respectively. The characters and origins of these peaks are studied in detail.

cond-mat.supr-con↗

arXiv2010

Comparative study of transport and magnetic Jc in the polycrystalline iron-based superconductor NdFeAsO{0.88}F{0.12}

Polycrystalline NdFeAsO0.88F0.12 superconductors prepared by high pressure (HP) and ambient pressure (AP) method were comparatively studied by magnetization and transport measurements. Upper critical field (Hc2), irreversibility field (Hirr) and the anisotropy parameter were estimated from resistance transition curves. The broadening of transition width was observed, and ascribed to both Hc2 anisotropy and superconductivity inhomogeneity of samples. Magnetic hysteresis loops (MHLs) in low fields were measured to detect the trace of weak-link behavior. The reclosed hysteresis loops in low fields manifest that there are weak-links in both samples. Magnetization critical current density Jcm were derived from MHLs. High-pressure synthesized sample shows higher Jcm. However, by means of direct transport I-V measurements, transport critical current density Jct was very low. The Jct values for two samples are comparable. Large discrepancies between Jcm and Jct also indicate that there are weak-links in both samples. The relative mechanism is discussed in detail.

cond-mat.supr-con↗

arXiv2010

Influence of magnetism on the superconducting properties of iron-based superconductor NdFeAsO0.88F0.12

Both DC and AC magnetization measurements were performed on the NdFeAsO0.88F0.12 superconductor to investigate the influence of magnetism on the superconducting properties of this system. The crossover of the ZFC and FC magnetic susceptibility curves under 7.5KOe was observed. The imaginary component of the first harmonics of the AC magnetic susceptibility, increases with the increasing DC field below 10K and shows frequency dependency under 7.5KOe at low temperature. The paramagnetism of Nd3+ ions tilts the magnetic hysteresis loops and broadens the hysteresis width. After correction for the paramagnetism, the field and temperature dependence of intrinsic Jcm was obtained and compared with the experimentally obtained total Jcm. The origin of the abnormal behavior of magnetization was investigated and attributed to a magnetic background, which was speculated to be caused by the spin-glass state. However, this magnetic background does not affect the flux pinning properties in this sample. The related mechanism was discussed.

cond-mat.supr-con↗

arXiv2010

Anisotropic Connectivity and its Influence on Critical Current Densities, Irreversibility Fields, and Flux Creep in In-Situ-Processed MgB2 Strands

The anisotropy of the critical current density (Jc) and its influence on measurement of irreversibility field (Birr) has been investigated for high quality, in-situ MgB2 strands. Comparison of transport and magnetization measurements has revealed the onset of a regime where large differences exist between transport and magnetically measured values of the critical current density and Birr. These effects, initially unexpected due to the lack of crystalline texture in these in-situ processed strands, appear to be due to a fibrous microstructure, connected with the details of the wire fabrication and MgB2 formation reactions. Scanning electron micrographs of in-situ-processed MgB2 monocore strands have revealed a fibrous microstructure. Grains (~100 nm) are randomly oriented, and there is no apparent local texture of the grains. However, this randomly oriented polycrystalline material has a fibrous texture at a larger length scale, with stringers of MgB2 (~ 60 μm long and ~5 μm in diameter) partially separated by elongated pores -- the spaces previously occupied by stringers of elemental Mg. This leads to an interpretation of the differences observed in transport and magnetically determined critical currents, in particular a large deviation between the two at higher fields, in terms of different transverse and longitudinal connectivities within the strand. The different values of connectivity also lead to different resistive transition widths, and thus irreversibility field values, as measured by transport and magnetic techniques. Finally, these considerations are seen to influence estimated pinning potentials for the strands.

cond-mat.supr-con↗