K.B. Garg

EXPERIMENTAL EVIDENCE OF ITINERANT Cu 3d9 - OXYGEN HOLE MANY BODY CONFIGURATION IN THE HIGH-TC SUPERCONDUCTOR YBa2Cu3O~7

Cu L3 x-ray absorption near edge structure (XANES) and Cu L3 x-ray photoelectron spectroscopy (XPS) of YBa2Cu3O6.5+x are compared. The breakdown of one-electron picture of its electronic structure is reported. The data are interpreted by mixing of Cu 3d9 and of (where is a hole in the oxygen derived band, ligand hole) many body configuration in the initial state. The localization of Cu 3d9 configuration is indicated by the bare Coulomb interaction Udd~6 eV. The conductivity is assigned to the itinerant configuration. The experimental evidence that the additional oxygen x, giving higher Tc, increases the weight of the configuration is reported. The presence of holes on the oxygen atoms is confirmed by the Ols XPS spectra. The Cu3+(Cu 3d8) configuration is not observed in L3 XANES in agreement with valence band XPS giving the energy of the 3d8 excited state at about 12 eV above the ground state. An energy scheme of the many body configurations in YBa2Cu3O~7 is obtained. These experiments give experimental evidence that the high Tc superconductivity is due to pairing of holes in the oxygen valence band interacting with localized electrons at the Cu sites.

Electronic structure of Mn-doped ZnO by?x-ray emission and absorption spectroscopy

We report an investigation of Mn-doped ZnO pellets with diluted Mn concentration by soft-x-ray emission and absorption spectroscopy. We have compared the electronic structure of two samples with different Mn concentration and different magnetic properties at room temperature: ferromagnetism in one case (Zn(0.98)Mn(0.02)O) and no magnetic order in the other (Zn(0.96)Mn(0.04)O). The results show that most of the Mn ions of the ferromagnetic sample are in the divalent state. For the nonmagnetic sample, a larger contribution of higher oxidation Mn states is present, which can be correlated to the suppressed ferromagnetism. The presence of oxygen atoms bonded to Mn ions and hybridized Mn 3d-O 2p states has been detected in both compounds. The partial density of states in the valence band has been measured with x-ray emission spectroscopy and the Mn 3d states have been found inside the bandgap of ZnO.

Core-level X-ray photoemission studies of Bi2Sr2Ca1−xEuitxCu2Oy

Abstract A well-characterized Eu-doped Bi 2 Sr 2 Ca 1− x Eu x Cu 2 O y ( x =0 to 1) system has been studied using core-level photoe mission. The trivalent Eu is found to be incorporated into the lattice, causing an increase in the valence states of Bi, Sr and Ca and a decrease in the Cu valence. The observed superconductor-insulator transition in this system is found to be caused by an enhanced insulating nature of the Bi-O planes and a decrease in the O 2p hole concentration in the Cu-O planes. The results of Sr 3d, Ca 2p, Cu 2p and Bi 4f core-level spectra are discussed in detail.

Comparative study of the electronic structure of natural and synthetic rubies using XAFS and EDAX analyses

We have studied the Cr-K-edge XANES and EXAFS in natural Indian rubies from two sources and a synthetic ruby at ESRF. Weight % of various constituents in them is determined using EDAX measurements. Taking the results from the three techniques together we are able to demonstrate their feasibility in quantitative study of precious stones.

A polarized x-ray absorption spectroscopy study of O K and Cu edges in a Tl(2212) thin film

A reliable estimate of the density of states having in-plane and out-of-plane symmetry in the O K-edge and the -edge polarized spectra is important for checking the validity of several theoretical models which predict an active role for these states in the mechanism of superconductivity. In the present contribution, we report on our fluorescence-yield O K-edge and -edge polarized spectra obtained for a well characterized and oriented overdoped Tl(2212) thin film as well as our simultaneous fluorescence- and total-yield measurements on an optimally doped Tl(2212) thin film. Our results show that a significant number of out-of-plane and holes also exist in thallium cuprates, more so in the overdoped case, and that use of the bulk-sensitive fluorescence technique is better for obtaining a reliable estimate of the density and symmetry of these holes. The spectra are compared with those reported earlier for other cuprate systems and are interpreted in terms of the available band-structure calculations. No direct correlation between superconductivity and the presence or absence of out-of-plane covalent and doping hole densities appears plausible, so theories based on the existence of a large density of such holes appear to be inconsistent with the experiment.

Polarized x-ray absorption spectroscopy study of the symmetry of unoccupied electronic states near the Fermi level in the Bi2Sr2CaCu2O8 system

A study of the electronic states near the Fermi level in high- superconductors, which is so important to understanding the mechanism of superconductivity, has been carried out on well characterized single crystals of superconducting . A large number of polarized x-ray absorption spectra have been recorded with the E vector parallel to the a - b plane and perpendicular to it at the Dragon beam line at the Brookhaven Light Source using the fluorescence mode of detection. These bulk spectra of and O K are then analysed to obtain the density of the unoccupied and orbitals from the spectra and the density of the and orbitals from the O K spectra. The results clearly show the presence of orbitals in a substantial amount although the same cannot be said of the orbitals. Tight-binding calculations of the holes are also reported and compared with experimental results.

Doped holes and Mn valence in manganites: a polarized soft x-ray absorption study of LaMnO3 and quasi-2D manganite systems

The question of Mn valence and symmetry and population of doped holes in La1.2Sr1.65Ca0.15Mn2O7 and LaSr2Mn2O7 bilayer single crystals has been studied with polarized soft x-ray absorption spectroscopy. The observed changes in the O K and Mn L spectra with polarization provide a strong indication for the existence of a competition between the charge dynamics and the lattice distortion that leads to transfer of some of the holes doped in the out-of-plane (3z2−r2) states to the in-plane (x2−y2) states. The changes observed in these with doping are shown not due to a decrease in the electron population in the dx2−y2 states but caused by an increase in density of holes in the dz2−x2 and dz2−y2 states, and the electrons predominantly occupy the corresponding orthogonal states, i.e., d3x2−r2 and d3y2−r2. No evidence is found for the existence of a formal Mn4+ valence state in these systems.

Polarized XANES study of the importance of inter-block vis-à-vis intra-block coupling in evolution of Tc in halide-molecule-intercalated Bi2Sr2CaCu2O8-δ single crystals

In addition to doping in the lattice that affects the intra-block coupling, intercalated molecules sit in between the consecutive basal planes, thereby increasing the effective length of the c-axis. This, in turn, must lead to a decrease in inter-block coupling. Both the doping and intercalation have been reported to affect the evolution of Tc in a system, implying the inherent importance of both types of coupling. In the latter case, the resulting depression of Tc is ascribed to transfer of charge between the intercalate and the host CuO2 plane. Most interesting studies in this regard pertain to use of I2, HgI2 and HgBr2 molecules as intercalates for the Bi2Sr2CaCu2O8+y system. Earlier reports mostly claim that the host CuO2 plane in Bi2Sr2CaCu2O8+y invariably becomes overdoped whichever the intercalate, thereby leading to a fall in Tc. In this paper, we examine these claims in the case of Bi2Sr2CaCu2O8+y single crystals by measuring the number of itinerant holes before and after intercalation by making polarization-dependent soft-x-ray absorption measurements at the O K and the Cu L3 edges. Our results do support the earlier claims on overdoing of the CuO2 plane in the case of iodine intercalate but are not in agreement with those in the case of the HgI2 intercalate.

X-ray absorption study of the system

X-ray absorption spectroscopy has been used to investigate the environment of all four cations in a series of six samples of (x = 0.0, 0.2, 0.4, 0.6, 0.8 and 1.0). Interatomic distances and their mean square variations, together with some coordination numbers, have been obtained from an analysis of the EXAFS using the multiple-scattering fast curved-wave theory. The environments of the Ba, Cu and Y atoms were found to be invariant with changing composition, the structural parameters being in good agreement with crystallographic data for the end-point materials. The environment of the Pr atom was found to change with composition, both the mean Pr-O distance and its mean square variation varying in a characteristic way. The variations with composition may be closely modelled by the forms and . These results have been interpreted in terms of a combination of two Pr-O distances, each invariant with composition, the proportions of these varying linearly with praseodymium content. The consequences of this variation, both for the praseodymium valence and the superconducting properties of the material, are considered.

X-ray adsorption study at Pr L3-edge in (Y, Br)Ba2Cu3O7−δ system

Abstract We have measured Pr L 3 edge X-ray absorption near edge structure spectra (XANES) on well characterized samples of Y 1− x Pr x Ba 2 Cu 3 O 7−δ ((Y,Pr)BCO) system for different Pr concentrations ( x = 0.4, 0.6, 0.8, 1.0). Additional peak at ∼14 eV above the main edge was observed. The variation of the relative intensities of the main edge and its satellite (∼14 eV) peak with Pr concentrations and broadening of the main edge is discussed.