Nishaina Sahadev

Complex spectral evolution in a BCS superconductor, ZrB12

We investigate the electronic structure of a complex conventional superconductor, ZrB12 employing high resolution photoemission spectroscopy and ab initio band structure calculations. The experimental valence band spectra could be described reasonably well within the local density approximation. Energy bands close to the Fermi level possess t2g symmetry and the Fermi level is found to be in the proximity of quantum fluctuation regime. The spectral lineshape in the high resolution spectra is complex exhibiting signature of a deviation from Fermi liquid behavior. A dip at the Fermi level emerges above the superconducting transition temperature that gradually grows with the decrease in temperature. The spectral simulation of the dip and spectral lineshape based on a phenomenological self energy suggests finite electron pair lifetime and a pseudogap above the superconducting transition temperature.

Surface bulk differences in a conventional superconductor, ZrB12

We studied the electronic structure of a conventional superconductor, ZrB(12) using high resolution x-photoemission spectroscopy and single crystalline samples. Experimental results with different bulk sensitivity reveals boron deficiency and different valence states of Zr at the surface relative to the bulk. Signature of a satellite features is observed in the Zr core level spectra corresponding to the bulk of the material suggesting importance of electron correlation among the conduction electrons in the bulk while the surface appears to be uncorrelated. These results provide an insight in fabricating devices based on such superconductors.

Importance of ligands in the electronic properties of FeTe0.6Se0.4

We investigate the electronic structure of FeTe0.6Se0.4 employing high resolution photoemission spectroscopy and ab initio band structure calculations. Fe 2p core level and the valence band spectra exhibit signature of strong electron correlation in the electronic structure. The electronic states near the Fermi level reduces in intensity with the decrease in temperature in conformity with the insulating transport observed near 300 K. An insulator to metal transition around 150 K could be related to the spectral lineshape change in the vicinity of the Fermi level. The spectral features near Fermi level exhibit significant p orbital character due to the correlation induced Fe d spectral weight transfer. The experimental spectra reveal dominant temperature dependence of the spectral functions possessing large p-character. While the origin of the anomalous electronic properties in the normal phase could be revealed in the electronic structure of this material, these results emphasizes the importance of ligand...We investigate the electronic structure of FeTe(0.6)Se(0.4) employing high resolution photoemission spectroscopy and ab initio band structure calculations. Fe 2p core level and the valence band spectra exhibit signature of strong electron correlation in the electronic structure. The electronic states near the Fermi level reduces in intensity with the decrease in temperature in conformity with the insulating transport observed near 300 K. The observation of an insulator to metal transition around 150 K in the transport properties may be related to the spectral lineshape change in the vicinity of the Fermi level observed in this study. The spectral features near Fermi level exhibit significant p orbital character due to the correlation induced Fe d spectral weight transfer. The experimental spectra reveal dominant temperature dependence of the spectral functions possessing large p-character. These results demonstrate significant renormalization of the character of the conduction electrons due to electron correlation and emphasizes the importance of ligand states in the superconductivity of these materials.

Complex temperature evolution of the electronic structure of CaFe2As2

Employing high resolution photoemission spectroscopy, we investigate the temperature evolution of the electronic structure of CaFe2As2, which is a parent compound of high temperature superconductors—CaFe2As2 exhibits superconductivity under pressure as well as doping of charge carriers. Photoemission results of CaFe2As2 in this study reveal a gradual shift of an energy band, α away from the chemical potential with decreasing temperature in addition to the spin density wave (SDW) transition induced Fermi surface reconstruction across SDW transition temperature. The corresponding hole pocket eventually disappears at lower temperatures, while the hole Fermi surface of the β band possessing finite p orbital character survives till the lowest temperature studied. These results, thus, reveal signature of complex charge redistribution among various energy bands as a function of temperature.

Composition dependence of M

We investigated Ag and Pd MNN Auger transitions in AgPdx alloys with low Pd concentrations using high-resolution x-ray excited Auger electron spectroscopy. The Ag MNN Auger profile exhibits composition-dependent kinetic energy shifts and also broadening. The Auger kinetic energy shift varies as the square root of the Pd concentration in the alloy, and the highest value of the shift observed for AgPd is about 0.27 eV. The fine structure observed in the Ag MNN Auger transition is independent of composition, but the Pd MNN Auger transition exhibits dramatic changes in intensities of fine features. The Ag MNN Auger transition exhibits a predominantly atomic character, whereas the Pd MNN Auger profile changes from quasi-atomic to band behavior with an increase in Pd concentration.

_{4,5}

We have investigated the electronic structure of CeB6 using photoemission spectroscopy as a function of surface sensitivity of the technique. Experimental results provide evidence that Kondo effect is a bulk phenomenon. The Madelung potential at the surface is found to be different from that of the bulk.

N

Fe-based superconductors are studied extensively during past decade to understand the interplay of superconductivity and magnetism. We studied the electronic structure of some of these fascinating systems exhibiting antiferromagnetic ordering and superconductivity, employing high resolution photoemission spectroscopy. We observed signature of finite hybridization of the electronic states corresponding to the local moment and the conduction electrons. The electronic states near Fermi level exhibit significant pnictogen/chalcogen p character. Signature of Kondo like features are observed near M-point in correlated Fe-compound.

_{4,5}

We studied the electronic structure of CaFe2As2 using photoemission spectroscopy. The features in the x‐ray photoemission spectra could be described using band structure results. The electronic states close to the Fermi level are found to be dominated by the Fe 3d contributions. The valence band spectra exhibit significant dependence on the photon energy used for the experiments. A comparison of the spectra at different photon energies suggests that the temperature dependence of the bulk and surface electronic states are significantly different–the bulk Fe 3d states seem to be strongly temperature dependent while the surface states appear to be almost insensitive. These results have significant implication in the understanding of superconductivity in this class of materials.

N

We report on a giant photothermal effect in few-layer Reduced Graphene Oxide (RGO) in powder form. Graphite oxide synthesized following modified Hummers method was thermally exfoliated and reduced to obtain RGO consisting of ∼8–10 layers. Upon irradiation with an incoherent, broad-band light source (wavelengths ranging from 250 to 450 nm), an enormous photothermal effect was observed. The heat generated by RGO determined from the isothermal differential photocalorimetric technique is as high as ∼319 W/g resulting from the dominant non-radiative de-excitation of photoexcited electrons due to the absence of a radiative pathway. A practical applicability was demonstrated using a commercial thermoelectric generator wherein upon illumination from a solar-simulator, an open voltage in the mV range was developed, giving a direct proof of the exothermic effect in powder RGO upon light illumination. Herewith, we have demonstrated a proof-of-concept of photothermal effects in solid-state RGO.

_{4,5}

Employing high resolution photoemission spectroscopy, we investigate the temperature evolution of the electronic structure of CaFe2As2, which is a parent compound of high temperature superconductors—CaFe2As2 exhibits superconductivity under pressure as well as doping of charge carriers. Photoemission results of CaFe2As2 in this study reveal a gradual shift of an energy band, α away from the chemical potential with decreasing temperature in addition to the spin density wave (SDW) transition induced Fermi surface reconstruction across SDW transition temperature. The corresponding hole pocket eventually disappears at lower temperatures, while the hole Fermi surface of the β band possessing finite p orbital character survives till the lowest temperature studied. These results, thus, reveal signature of complex charge redistribution among various energy bands as a function of temperature.