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.
We investigate the electronic structure of CaFe$_2$As$_2$ using high resolution photoemission spectroscopy. Experimental results exhibit three energy bands crossing the Fermi level making hole pockets around the $Gamma$-point. Temperature variation r
eveal a gradual shift of an energy band away from the Fermi level with the decrease in temperature in addition to the spin density wave (SDW) transition induced Fermi surface reconstruction of the second energy band across SDW transition temperature. The hole pocket in the former case eventually disappears at lower temperatures while the hole Fermi surface of the third energy band possessing finite $p$ orbital character survives till the lowest temperature studied. These results reveal signature of a complex charge redistribution among various energy bands as a function of temperature that might be associated to the exotic properties of this system.
We studied the electronic structure of a Shastry-Sutherland lattice system, HoB4 employing high resolution photoemission spectroscopy and ab initio band structure calculations. The surface and bulk borons exhibit subtle differences, and loss of boron
compared to the stoichiometric bulk. However, the surface and bulk conduction bands near Fermi level are found to be similar. Evolution of the electronic structure with temperature is found to be similar to that observed in a typical charge-disordered system. A sharp dip is observed at the Fermi level in the low temperature spectra revealing signature of antiferromagnetic gap. Asymmetric spectral weight transfer with temperature manifests particle-hole asymmetry that may be related to the exotic properties of these systems.
