Enhancement of Zero Resistance Temperature above 90 K and Hole Distribution in (Hg_0.5Pb_0.5)Sr₂(Ca_0.7Y_0.3)Cu₂O_7-δ via Chemical Substitution of Ba into Sr Sites

Abstract

A significant enhancement of zero resistance temperature [T(c(zero))] from 78 K for x = 0 to 92.5 K for x = 0.2 in the series (Hg(0.5)Pb(0.5))(Sr(2-x)Ba(x))(Ca(0.7)Y(0.3))Cu(2)O(7-delta) compounds has been found. On the basis of the O K-edge X-ray-absorption near-edge structure (XANES) spectra for the series of (Hg(0.5)Pb(0.5))(Sr(2-x)Ba(x))(Ca(0.7)Y(0.3))Cu(2)O(7-delta) samples with x = 0-0.5, the chemical substitution of Ba(2+) for Sr(2+) gives rise to a decrease in the O 2p hole concentration within the out-of-plane oxygen sites, while that within the in-plane CuO(2) layers remains almost unchanged for 0 </= x </= 0.2. This indicates that the out-of-plane oxygen may play an important role in controlling the increase of T(c(zero)) up to the maximum value of 92.5 K.

Hole doping in high temperature superconductors using the XANES technique

“…It can be ascribed to the transition from 3d 9 L → O 1s3d 9 (L denotes a hole on the O 2p x,y orbital) states corresponding to creation of a core hole on the O 1s level and filling of O 2p x,y states admixed to the upper Hubbard band. This transition is strongly related to the hole concentration in CuO 2 and is identical to the corresponding peak discussed above for HgPb-1223/F x [34][35][36][37][38][39][40].…”

“…The pre-edge peak A at about 528.3 eV is attributed to the transition of O 1s electrons to O 2p holes in the CuO 2 planes from 3d 9 L → O 1s3d 9 (L denotes a hole on the O 2p x,y orbital) states corresponding to the creation of a core hole on the O 1s level and filling of O 2p x,y states located in the CuO 2 planes near the Fermi level. The assignment of this peak to the O 2p xy symmetry rather than the out-of-plane O 2p z symmetry is confirmed by several polarization studies [14,[34][35][36][37]. Chen et al, for example found that the intensity of this pre-edge peak in Tl-2223 decreases with increasing the polarized light's incident angle with the c axis [31].…”

“…The intensity of this peak increased on going from E1|ab to E1|c polarization, i.e. this peak has O 2P z symmetry [31,34].…”

“…Peak C is ascribed to O 2p holes in the Hg-O planes [17]. The spectral features above 535 eV reveal information about the local density of unoccupied states corresponding to O 2p states hybridized with Cu 4sp and Ba/Ca intermixing [34][35][36][37][38][39]. HgPb-1223 has four nonequivalent oxygen sites: O(1) and O(1) within the CuO 2 layers of the square planner and pyramidal arrangements, respectively, O(2) in the Ba-O and O(3) in the Hg-O planes [26,31,32,36].…”