Ganesh Adhikary

Tunable orbital angular momentum in high-harmonic generation

Optical vortices are currently one of the most intensively studied topics in optics. These light beams, which carry orbital angular momentum (OAM), have been successfully utilized in the visible and infrared in a wide variety of applications. Moving to shorter wavelengths may open up completely new research directions in the areas of optical physics and material characterization. Here, we report on the generation of extreme-ultraviolet optical vortices with femtosecond duration carrying a controllable amount of OAM. From a basic physics viewpoint, our results help to resolve key questions such as the conservation of angular momentum in highly nonlinear light–matter interactions, and the disentanglement and independent control of the intrinsic and extrinsic components of the photons angular momentum at short-wavelengths. The methods developed here will allow testing some of the recently proposed concepts such as OAM-induced dichroism, magnetic switching in organic molecules and violation of dipolar selection rules in atoms.

Influence of 4f electronic states on the surface states of rare-earth hexaborides

We study the surface electronic structure of a series of rare-earth hexaborides using state-of-the-art high resolution photoemission spectroscopy. Experimental results reveal a surface state around 1.8 eV binding energy in all the hexaborides indicating its generic nature in this class of compounds. The surface and bulk electronic structures near the Fermi level, ϵF are almost similar in each of the compounds. This suggests an interesting possibility of fabricating new materials possessing low work function like LaB6 where the behavior of mobile electrons can be tuned by rare-earth substitutions.

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.

Unusual spectral renormalization in hexaborides.

Employing high resolution photoemission spectroscopy, we studied the evolution of the spectral features in rare earth hexaboride single crystals as a function of temperature and 4f binding energy, where the variation of the 4f binding energy is obtained by changing the rare earth element. High energy resolution helped to reveal the distinct features corresponding to the various photoemission final states. Experimental results of CeB(6), a dense Kondo system, exhibit the growth of the features near the Fermi level with the decrease in temperature relative to the uncompensated local moment contributions. The valence band spectra of the antiferromagnetic compounds, PrB(6) and NdB(6), exhibit multiple features-the 4f ionization peaks (poorly screened features) appear at higher binding energies and the features in the vicinity of the Fermi level possessing significant 4f character are due to the well-screened photoemission final states. These results indicate finite hybridization between the 4f and B 2s2p conduction electronic states. Interestingly, the well-screened features in PrB(6) and NdB(6) exhibit unusual enhancement in intensity at low temperature.

Complex evolution of the electronic structure of Cr with temperature

Employing state-of-the-art high resolution photoemission spectroscopy, we studied the electronic structure evolution of Cr with temperature. Experimental results reveal signature of a pseudogap much below the spin density wave transition temperature. A sharp peak appears near the Fermi level at low temperatures presumably related to the orbital Kondo effect. These results provide possible origin of the complex electronic properties observed in this system.

Spectral evolution in an insulator exhibiting linear specific heat

We investigate the spectral evolution of an antiferromagnetic insulator, La0.2Sr0.8MnO3, exhibiting linear specific heat, using state-of-the-art high resolution photoemission spectroscopy. Experimental spectral functions exhibit Fermi liquid-like energy dependence at all the temperatures studied. Room temperature spectra possess finite density of states at the Fermi level, which vanishes, generating a soft gap, at about 260 K (the magnetic transition temperature). High-resolution spectra reveal a hard gap in the magnetically ordered phase (C-type antiferromagnet). These results indicate an amorphous phase coexisting with the long-range ordered phase in 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 studied the temperature evolution of the electronic structure of Chromium across the spin density wave‐type antiferromagnetic transition using high resolution photoemission spectroscopy. The spectral density of states (SDOS) remain almost unchanged across the magnetic transition. Signature of a pseudogap appears at low temperatures. In addition, SDOS exhibits growth of a peak just above the Fermi level at low temperatures similar to that found in various Kondo lattice systems.

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.