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Register a new userLayered-specific hole concentrations in Bi2Sr2(Y1-xCax)Cu2O8+d as probed by XANES spectroscopy and coulometric redox analysis
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Induction of holes not only in the superconductive CuO2 plane but also in the Bi2O2+d charge reservoir of the Bi2Sr2(Y1-xCax)Cu2O8+d superconductor upon CaII-for-YIII substitution is evidenced by means of two independent techniques, i.e., high-resolution x-ray-absorption near-edge structure (XANES) spectroscopy measurements and coulometric redox titrations. The absolute values derived for the CuO2-plane hole concentration from the Cu L2,3-edge XANES spectra are in good agreement with those obtained from the coulometric redox analysis. The CuO2-plane hole concentration is found to increase from 0.03 to 0.14 concomitantly with the increase in the BiO1+d/2-layer hole concentration from 0.00 to 0.13 as the Ca-substitution level, x, increases from 0 to 1. The threshold CuO2-plane hole concentration for the appearance of superconductivity is determined at 0.06, while the highest Tc is obtained at the hole concentration of 0.12. In the O K-edge XANES spectrum, the increases in the CuO2-plane and BiO1+d/2-layer hole concentrations with increasing x are seen as enhancement in the relative intensities of the pre-edge peaks at ~528.3 and ~530.5 eV, respectively.
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Bi2Sr2-xLaxCuO6+d and Bi2-yPbySr2-xLaxCuO6+d high-Tc superconductors in a wide doping range from overdoped to heavily underdoped were studied by X-ray absorption and photo-emission spectroscopy. The hole concentration p was determined by an analysis of the Cu L3-absorption edge. Besides the occupied density of states derived from photoemission, the un-occupied density of states was determined from the prepeak of the O K-absorption edge. Both, the occupied as well as the unoccupied density of states reveal the same dependence on hole doping, i.e. a continuous increase with increasing doping in the hole underdoped region and a constant density in the hole overdoped region. By comparing these results of single-layer BSLCO with previous results on single-layer LSCO it could be argued that besides the localized holes on Cu sites the CuO2-planes consist of two types of doped holes, from which the so-called mobile holes determine the intensity of the prepeak of the O 1s absorption edge.
We have studied the evolution of the structural modulation in epitaxial, c-axis oriented, Bi2Sr2-xLaxCuO6+d thin films when varying the La content x and for a given x as a function of oxygen content. A series of thin films with 0<x<0.8 have been prepared in-situ by rf magnetron sputtering and characterized by R(T) measurements and RBS, TEM and X-Ray diffraction techniques. The oxygen content of each individual film was varied by thermal annealing across the phase diagram. The evolution of the structural modulation has been thoroughly studied by X-Ray diffraction in determining the variation of the amplitude of satellite reflections in special 2 axes 2theta /theta-theta scans (reciprocal space scans). It is shown that the amplitude of the modulation along the c-axis decreases strongly when x increases from 0 to 0.2. It is demonstrated that this variation is essentially governed by La content x and that changing the oxygen content by thermal treatments has a much lower influence, even becoming negligible for x>0.2. Such study is important to understand the electronical properties of Bi2Sr2-xLaxCuO6+d thin films.
In the vortex core of slightly overdoped Bi2Sr2CaCu2Ox, the electron-like and hole-like states have been found to exhibit spatial modulations in anti-phase with each other along the Cu-O bonding direction. Some kind of one-dimensionality has been observed in the vortex core, and it is more clearly seen in differential conductance maps at lower biases below +-9 mV.
We argue that the method, which was used for the analysis of equilibrium magnetization data in the original publication, is not adequate to the experimental situation. As a result, the temperature dependencies of the upper critical field Hc2(T) and the magnetic field penetration depth lambda(T), obtained in this work, are incorrect. Using a different approach, we reanalyze the presented experimental data and demonstrate that the normalized Hc2(T) curves are rather different from those presented in the original publication and do not follow predictions of the Werthamer-Helfand-Hohenberg theory. Another interesting observation is that the Hc2(T) dependencies for two samples with close levels of doping are rather different.
The discovery of infinite-layer nickelate superconductors has spurred enormous interest. While the Ni$^{1+}$ cations possess nominally the same 3d$^9$ configuration as Cu$^{2+}$ in high-$T_C$ cuprates, the electronic structure consistencies and variances remain elusive, due to the lack of direct experimental probes. Here, we present a soft x-ray photoemission spectroscopy study on both parent and doped infinite-layer Pr-nickelate thin films with a doped perovskite reference. By identifying the Ni character with resonant photoemission and comparison to density function theory + U calculations, we estimate U ~ 5 eV, smaller than the charge transfer energy $Delta$ ~ 8 eV, in contrast to the cuprates being charge transfer insulators. Near the Fermi level (EF), we observe a signature of rare-earth spectral intensity in the parent compound, which is depleted upon doping. The parent compound, self-doped from rare-earth electrons, exhibits higher density of states at EF but manifests weaker superconducting instability than the Sr-doped case, demonstrating a complex interplay between the strongly-correlated Ni 3d and the weakly-interacting rare-earth 5d states in these oxide-intermetallic nickelates.
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