Hye-Gyong Lee

Raman studies on single-phasic HgBa2CuO4+δ superconductors

Abstract Polycrystalline samples of HgBa 2 CuO 4+δ superconductors were prepared by solid state reaction of HgO and Ba 2 CuO 3+ x precursor in an evacuated silica tube. The HgBa 2 CuO 4+ δ has a T c of 92 K and has a tetragonal cell (P 4/mmm) with lattice parameters a and c with the values of 3.8868 (2) A and 9.4886 (1) A, respectively. Raman spectra of polycrystalline HgBa 2 CuO 4+δ show four peaks at 108 cm −1 , 158 cm −1 , 570 cm −1 and 592 cm −1 . Using a factor group analysis and Raman spectral data of the cuprate superconductors, these modes are assigned as E g and A 1g modes of Ba atoms and O(2) oxygens. The higher peaks at 570 cm −1 and 592 cm −1 are assigned to E g and A 1g modes of O(2) oxygen, respectively. The lower two modes at 108 cm −1 and 158 cm −1 are ascribed to vibrations of Ba atoms. Details of mode assignments are discussed based on symmetry and local structure.

Raman spectra of HgBa2Ca2Cu3O8+d superconductors

Abstract We have measured Raman spectra of HgBa2Ca2Cu3O8+d (Hg-1223) superconductors. The spectra show peaks at 92, 127, 138, 230, 479, 538, 574, and 585 cm-1. From a factor group analysis and the structural information about Hg-1223, we assign the modes at 92, 574, and 585 cm-1 to be A1g modes of Ba, O(2) and O(3) atoms, respectively. The modes at 479 and 538 cm-1 are proposed to be Eg modes of O(3) bending and O(2) stretching vibrations, respectively. Details of mode assignments are given in the discussion.

Raman mode assignments of HgBa2CuO4+δ crystallites

Abstract We have studied the Raman modes in HgBa 2 CuO 4+ δ crystallites through polarized Raman measurements. The Raman peaks at 158 and 592 cm -1 are assigned to be the Ba A 1g mode and the O(2) A 1g mode respectively. This is in reasonable agreement with the results of LDA total-energy phonon calculations of HgBa 2 CuO 4+ δ . To clarify symmetry and site assignment for the mode near 570 cm -1 , we studied its behavior at elevated temperatures. Together with a thermogravimetric analysis (TGA), it is found that the 570 cm -1 mode is not related to the O(3) ‘interstitial’ oxygen as suggested by some authors. However, the polarization dependence of the 570 cm -1 mode shows no conclusive sign of E g symmetry contrary to our previous speculation.

TWO-GAP NATURE OF THE BULK SUPERCONDUCTOR (Mg1-xAlx)B2 WITH LOW (x ≤ 0.07)Al DOPING

We have investigated the two-gap nature of bulk (Mg1-xAlx)B2 for 0 ≤ x ≤ 0.07 by studying the upper critical field Hc2(T). The Hc2(T) is analyzed based on the dirty-limit two-gap model. While the effective diffusivities of the π-band decreased gradually, those of the σ-band were relatively unchanged, which indicated that unlike the 2 dimensional σ-band, the isotropic π-band is seriously affected by the Al doping. The upward curvature of Hc2(T) near Tc is also explained by the dirty-limit two-gap model.

Pressure induced Raman spectra of HgBa2Ca2Cu3O8+δ superconductors

Abstract We assigned Raman modes and observed the effects of pressure on the Raman frequencies of the apical oxygen O A A 1g modes of mercury based superconductor. We found that the change of Raman frequency due to pressure is related to the charge transfer between the CuO 2 planes and Hg-O layers.

Superconducting properties versus oxygen content in a Sr-doped HgBa2Ca1−xSrxCu2O6+δ

Abstract Polycrystalline samples of HgBa2Ca1−xSrxCu2O6+δ were prepared by solid state reaction of HgO, Ba2CuO3+δ and Ca1−xSrCuO2 (x=0.14) in an evacuated silica tube. The as-synthesized sample has a Tc of 120 K and has tetragonal symmetry ( P4 mmm ) with lattice parameters of a =3.8584(1) A and c =12.6646(5) A . After annealing under O2 at 250 °C for 8 h, the Tc was enhanced to 124 K. The content of oxygen uptakes is about 0.11, which was estimated by a TGA analysis. However, the X-ray powder diffraction data and the Raman spectra of both as-synthesized and oxygen-annealed samples did not show any substantial difference.