Indranil Sarkar

Probing bismuth ferrite nanoparticles by hard x-ray photoemission: Anomalous occurrence of metallic bismuth

We have investigated bismuth ferrite nanoparticles (∼75u2009nm and ∼155u2009nm) synthesized by a chemical method, using soft X-ray (1253.6u2009eV) and hard X-ray (3500, 5500, and 7500u2009eV) photoelectron spectroscopy. This provided an evidence for the variation of chemical state of bismuth in crystalline, phase pure nanoparticles. X-ray photoelectron spectroscopy analysis using Mg Kα (1253.6u2009eV) source showed that iron and bismuth were present in both Fe3+ and Bi3+ valence states as expected for bismuth ferrite. However, hard X-ray photoelectron spectroscopy analysis of the bismuth ferrite nanoparticles using variable photon energies unexpectedly showed the presence of Bi0 valence state below the surface region, indicating that bismuth ferrite nanoparticles are chemically inhomogeneous in the radial direction. Consistently, small-angle X-ray scattering reveals a core-shell structure for these radial inhomogeneous nanoparticles.

High photon energy spectroscopy of NiO: Experiment and theory

We have revisited the valence band electronic structure of NiO by means of hard x-ray photoemission spectroscopy (HAXPES) together with theoretical calculations using both the GW method and the local density approximation + dynamical mean-field theory (LDA+DMFT) approaches. The effective impurity problem in DMFT is solved through the exact diagonalization (ED) method. We show that the LDA+DMFT method in conjunction with the standard fully localized limit (FLL) and around mean field (AMF) double-counting alone cannot explain all the observed structures in the HAXPES spectra. GW corrections are required for the O bands and Ni-s and p derived states to properly position their binding energies. Our results establish that a combination of the GW and DMFT methods is necessary for correctly describing the electronic structure of NiO in a proper ab initio framework. We also demonstrate that the inclusion of photoionization cross section is crucial to interpret the HAXPES spectra of NiO. We argue that our conclusions are general and that the here suggested approach is appropriate for any complex transition metal oxide.

Evaporation temperature-tuned physical vapor deposition growth engineering of one-dimensional non-Fermi liquid tetrathiofulvalene tetracyanoquinodimethane thin films

We describe the growth of high quality tetrathiofulvalene tetracyanoquinodimethane (TTF-TCNQ) organic charge-transfer thin films which show a clear non-Fermi liquid behavior. Temperature dependent angle resolved photoemission spectroscopy and electronic structure calculations show that the growth of TTF-TCNQ films is accompanied by the unfavorable presence of neutral TTF and TCNQ molecules. The quality of the films can be controlled by tuning the evaporation temperature of the precursor in physical vapor deposition method.

Nature of electron correlation and hybridization in NixCu1−xMnSb Heusler alloys

The electronic structure of Heusler alloys having mixed magnetic phases, comprising of vicinal anti-ferromagnetic and ferromagnetic orders, is of great significance. We present the results of an electronic structure study on NixCu1−xMnSb Heusler alloys, using Mn-2p core-level photoemission spectroscopy. Room temperature data in the paramagnetic phase reveal a non-monotonic variation of both electron correlation strength and conduction-band hybridization such that the former enhances while the latter weakens for compositions showing a mixed phase relative to compositions at the phase boundaries to the ordered phases. The results suggest a possible electronic driving force for settling mixed-magnetic phases.

Core level photoemission studies on conducting polypyrrole polymer nanotubes showing switching transitions

Conducting polypyrrole (PPY) nanotubes is a classical model system for strongly correlated disordered materials showing intriguing switching transitions from low to high conductivity states at low temperature. This switching behaviour can be tuned by incorporating gold nanoclusters to form composite nanotubes (AuPPY). Here, we present core level electronic structure studies on PPY and AuPPY nanotubes with different diameters using hard X-ray photoemission spectroscopy at room temperature. The spectroscopic data provide information on the role of diameter as well as metal cluster incorporation for a modification of the electronic structure of this important class of nanotubes. Furthermore, electrical transport measurements were performed at low temperature to extract the change in localization length and doping level in these nanotubes. Based on these results, we explain the observed electrical transport behavior of these nanotubes as the interplay of disorder and carrier concentration.