R.K. Singhal

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.

Study of some Ce intermetallics by core-level photoemission

Ce intermetallics CeAlNi, CeAlCu and CeNi2Si2 have been studied by core-level photoemission spectroscopy to investigate their electronic structure. The Ce 3d XPS data clearly indicate that the final state is a quantum mechanical admixture of 4f0, 4f1 and 4f2 configurations in these systems with an increasing 4f0 contribution as Cu is replaced by Ni. The two Ni-based systems are found to be mixed-valence systems with nf<1 while the ground state of the Cu-containing system appears to retain a purely trivalent character with nf approximately=1. These results are in agreement with the authors earlier reported X-ray absorption and resonant photoemission data. The present study highlights the fact that the replacing of Ni by Cu seems to be propitious for driving the Ce from an intermediate-valence (IV) to a non-IV ground state. The results Ce 3d and Ce 4d XPS are discussed.

Study of anomalous temperature dependence of itinerant holes in under- and overdoped La2-xSrxCuO4 single crystals using polarised soft X-ray absorption spectroscopy

Abstract Some experiments have recently shown that in the YBCO detwinned system charge aggregation takes place in the Cu–O2 plane along the b-axis at T≫Tc followed by formation of Cooper pairs, again in the normal state. Polarised X-ray absorption measurements at the O K and Cu L3 absorption edges in E∥b orientation have been carried out on underdoped and overdoped single crystals of La2−xSrxCuO4 through a temperature range of 10–300 K to study the dependence of the itinerant hole density. Both the crystals do show an anomalous temperature dependence but there is a wide divergence in the earlier and our results. This paper discusses the results and possible causes for the difference.

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.

An electronic structure study of c-axis oriented NdBCO (123) thin films using polarized soft x-ray absorption spectroscopy on Cu L3 and O K edges

Polarized x-ray absorption spectroscopic (XAS) measurements have been made on Cu L3 and O K edges on four superconducting thin films of NdBCO with different Tc values to study the relative importance of the in-plane (Cu 3dx2-y2 and O 2px,y orbitals) and the out-of-plane (Cu 3dz2 and O2pz) orbitals in the mechanism of superconductivity. Our results clearly show that a significant number of out-of plane O 2pz and Cu 3dz2-r2 do exist in these films, still hardly affecting their Tc. Tc is found to depend more upon number of itinerant holes rather than the orientation or substrate of the film. However the out-of-plane holes perhaps do affect the superconducting fraction in the film. O K-edge spectra recorded for E∥ab to measure the density of itinerant holes in plane show a good agreement with the conclusion drawn from Cu L3 data. The results have been compared with the XRD and the magnetic susceptibility measurements reported on these films earlier and a good qualitative agreement found. From the present study we can confidently state that no direct correlation exists between Tc and out-of-plane covalent and doping hole densities and the models based on the premise that out-of-plane orbitals, if present, will destroy superconductivity also do not seem to be valid in these systems.

POLARISED TEMPERATURE DEPENDENT XANES STUDY OF SUPERCONDUCTING La2-xSrxCuO4-y UNDER- AND OVER-DOPED SINGLE CRYSTALS

High resolution XANES measurements have been made at the CuK-absorption edge on two La2-xSrxCuO4-y single crystals; one under-doped (x = 0.13) and the other over-doped (x = 0.19). These measurements have been made in E//ab and E//c directions at several temperatures to compare the doping, orientation and temperature dependence in their behaviour. Broadly speaking, their XANES spectra resemble the typical one from a crystal with square-planar geometry and the various fine structure features assigned to the possible transitions they arise due to, particularly in terms of the fact that the 4p band in Cu splits into 4p(π) and 4p(σ) components in these square-planar systems. However, the temperature dependence of the various features in the spectra from the two crystals appear to be opposite in nature and exhibit some characteristic temperatures at which the trend seems to reverse. All the experimental results and their implication are discussed and a possibility of phase transition of these systems at lower temperatures is hinted at.

STUDY OF ITINERANT HOLES IN PLANAR AND APICAL OXYGEN IN TWO DIFFERENT BSCCO (2212) SINGLE CRYSTALS USING POLARISED X-RAY ABSORPTION

X-ray absorption measurements have been made on CuL3 and O K-edge on BSCCO (2212) single crystals annealed under two different conditions at 1 bar O2 (under-doped) and another annealed in 1:4 ratio of O2: Ar (optimally doped) using the F. Y. detection technique. Our results clearly indicate that hole density decreases while going towards c-axis from the ab plane. Our results shows that the doping holes have predominantly x, y symmetry compared to those having pz symmetry. The Tc for both the crystals does not depend on the density of doping holes in the apical oxygen. However, the matter needs to be further explored.

STUDY OF LOCAL STRUCTURE IN UNDER-DOPED La2-xSrxCuO4-y BY POLARIZED EXAFS

Temperature dependence of local structure of the CuO2 plane has been studied by in plane polarized (E//ab) Cu K-edge extended X-ray absorption fine structure (EXAFS) experiments on an under-doped single crystal of superconducting La1.87Sr0.13CuO4-y system with Tc=32 K. The temperature dependence of the Debye–Waller factors for the Cu-O and Cu-La/Sr pairs are determined. We find that the motion of Cu and O atoms in the Cu-O bond is partially correlated at higher temperatures, going over to an uncorrelated motion at lower temperatures. The results are discussed in terms of the local lattice instabilities and stripes as reported in other systems.

Polarization dependent EXAFS study of Bi2Sr2Ca0.4Pr0.6Cu2O8−δ insulating single crystal

Pr doping studies occupy a special place for determining the role played by different cations in evolution of superconductivity in the cuprate perovskites. It is known to quench superconductivity in different systems through different mechanisms that still remain to be firmly established. Reporting of a pure Pr123 superconducting crystal has served to further heighten this debate. Polarized EXAFS measurements have been made on the Pr LIII edge in a Bi2Sr2Ca0.4Pr0.6Cu2O8−Y insulating single crystal to examine the site geometry of the Pr cations and the mechanism of quenching superconductivity in this system. Measurements have been confined to within the ab-plane in E∥a, E∥â 45 and E∥b polarizations. Results show that Pr has a valence of 3+ only, occupies only the Ca sites and hence quenches the itinerant holes rendering the CuO2 planes insulating. No evidence is found of presence of any distortion in the Pr/Ca plane.