Chen Xianhui

Pseudogap and Superconducting Gap in Sm FeAs(O1-xFx) Superconductor from Photoemission Spectroscopy

High resolution photoemission measurements are carried out on non-superconducting SmOFeAs parent compound and superconducting SmFeAs(O1-xFx) (x = 0.12, and 0.15) compounds. The momentum-integrated spectra exhibit a clear Fermi cutoff that shows little leading-edge shift in the superconducting state. A robust feature at 13meV is identified in all these samples. Spectral weight suppression near Ef with decreasing temperature is observed in both undoped and doped samples that points to a possible existence of a pseudogap in these Fe-based compounds.

Effects of Ag doping on the critical current density of YBa2Cu3O7−y

Abstract A series of (YBa 2 Cu 3 O 7−y ) 1−x Ag x samples were prepared using solid state reaction method. On the basis of X-ray diffraction analysis, scanning electron microscopic observation, measurements of the resistivity; a.c. magnetic susceptibility as well as the critical current density, we find that enhancing the intergranular coupling by improving the properties of grainboundaries, while the properties of grains are kept unchanged, helps to increase the critical current density. The dependences of the critical current density on the Ag content, the temperature and the magnetic field are discussed. Our results suggest that eliminating grainboundaries together with improving the properties of grains may be an effective way in raising the critical current density in bulk materials.

Preparation and superconductivity of the high-Tc phase of Pb-doped Bi-Sr-Ca-Cu-O superconductor

Abstract With the aim of preparing a single high-Tc phase in the BiPbSrCaCuO system, optimization of the composition and heat treatment have been studied. It has been found that the temperature range at which the high-Tc phase forms is very narrow (850–880°C). The high-Tc single phase Bi2.5Pb0.5Sr2Ca2Cu3Oy has been prepared. The zero-resistance temperature (Tce) is as high as 105K. There are no steps in the resistivity and the susceptibility. The structure is tetragonal with a=5.414 A and c=37.106 A . It is suggested that the phase with c=37.106 A is responsible for the high temperature (110K) superconductor.

Common Features in Electronic Structure of the Oxypnictide Superconductors from Photoemission Spectroscopy

High resolution photoemission measurements have been carried out on non-superconducting LaOFeAs parent compound and various superconducting R(O1-xFx)FeAs (R=La, Ce and Pr) compounds. We found that the parent LaOFeAs compound shows a metallic character. Through extensive measurements, we have identified several common features in the electronic structure of these Fe-based compounds: (1). 0.2 eV feature in the valence band; (2). A universal 13~16 meV feature; (3). A clear Fermi cutoff showing zero leading-edge shift in the superconducting state;(4). Lack of superconducting coherence peak(s); (5). Near EF spectral weight suppression with decreasing temperature. These universal features can provide important information about band structure, superconducting gap and pseudogap in these Fe-based materials.High resolution photoemission measurements are carried out on non-superconducting LaFeAsO parent compound and various superconducting RFeAs(O1-xFx) (R=La, Ce and Pr) compounds. It is found that the parent LaFeAsO compound shows a metallic character. By extensive measurements, several common features are identified in the electronic structure of these Fe-based compounds: (1) 0.2 eV feature in the valence band, (2) a universal 13-16 meV feature, (3) near Ef spectral weight suppression with decreasing temperature. These universal features can provide important information about band structure, superconducting gap and pseudogap in these Fe-based materials.

Angle Integrated Photoemission Study of SmO0.85F0.15FeAs

The electronic structure of the new superconductor SmO1-xFxFeAs (x = 0.15) is studied by angle-integrated photoemission spectroscopy. Our data show a sharp feature very close to the Fermi energy, and a relative flat distribution of the density of states between 0.5eV and 3eV binding energy, which agrees well with the band structure calculations considering an antiferromagnetic ground state. No noticeable gap opening is observed at 12K below the superconducting transition temperature, indicating the existence of large ungapped regions in the Brillouin zone.

Superconductivity in the (Bi,Pb)CaSrBaCuO system

Abstract In the (Bi,Pb)CaSrBaCuO system, a rather large orthorhombic cell with a=5.4A, b=26.9A and c=30.7A have been found. The ac susceptibility measurement shows that there exists only one superconducting transition at 110K, the zero-resistance temperature is as high as 106K.

The role of oxygen content in the (Bi,Pb)2Sr2Ca2Cu3Oy system

Abstract The 110K single phase sample (Bi,Pb)2Sr2Ca2Cu3Oy was prepared by solid state reaction method. The samples were characterized using X-ray powder diffraction, the measurements of resistivity and dc. susceptibility, and the X-ray photoemission spectrum and specific heat measurement and the electron diffraction analysis. The results show that the superconductivity of (Bi,Pb)SrCaCuO system is sensitive to oxygen content like the 1-2-3 phase material. We suppose that different oxygen content leads to the changes of modulated structure and electronic structure and the ratio of Cu3+, and so to great difference in the superconductivity.

The influence of Pb on the stability of the Bi-type modulated structure

Abstract The mechanism of the Bi-type modulated structure was studied in Bi-based high-temperature superconductors with nominal compositions of Bi2Sr2CaCu2Oy, Bi1.85Pb0.15Sr2CaCu2Oy and Bi1.8Pb0.35Sr2Ca2Cu3O y. It was found that the three samples annealed in oxygen flow showed the presence of Bi-type modulation in electron diffraction patterns; after the samples were annealed in nitrogen atmosphere the Bi-type modulation still existed for the sample Bi2Sr2CaCu2Oy but disappeared completely for the Pb- doped samples Bi1.85Pb0.15Sr2CaCu2Oy and Bi1.8Pb0.35Sr2Ca2Cu3Oy. This suggests that doping Pb in the Bi-Sr-Ca-Cu-O system strongly changes the stability of the Bi-type modulation. It is confirmed that the driving force of Bi-type modulation is not the consequence of extra oxygen for the charge balance, but the adjustment of the BiO layer to the lattice mismatch.

The synthesis and structure of a new Bi-based copper oxide with (Bi, M)O monolayers: (Bi, M)Sr2(R, Ce)3Cu2O11−δ

Abstract A new layered cuprate, (Bi, M)Sr 2 (R, Ce) 3 Cu 2 O 11−δ (M+Cd, Cu; R: rare earth elements) (Bi-based “1232”), has been synthesized. The structure of the compounds has been determined by the powder X-ray diffraction method. It is found that the cell parameters are approximately equal to a = 3.85 A and c = 17 A . Comparison of the calculated and observed diffraction intensities shows that the structures are similar to that of (TI, Cu)Sr 2 (R, Ce) 3 Cu 2 O 11 . The behavior of the resistivity dependence on temperature is directly related to that heat treatment.

Thermal effects on the microstructure of superconducting (Bi1.8Pb0.35)Sr2Ca2Cu3Oy

Thermal effects on the microstructure of Bi2Sr2CaCu2Oy and (Bi, Pb)2Sr2Ca2Cu3Oy superconductors are studied by electron diffraction and differential thermal analysis (DTA). The DTA results show that there exist two endothermal peaks and one exothermal peak at about 530, 680 and 740 degrees C respectively. The electron diffraction indicates that a 2a, 2b, c superstructure is formed at about 450 degrees C. The superstructure begins to disappear at about 680 degrees C and the collapse of the crystal structure occurs at about 700 degrees C, which is consistent with the results of DTA. The superstructure originates in oxygen-vacancy ordering as for similar superstructures occurring in Bi2Sr2YCu2Oy and YBa2Cu3O7-x.