Angle Resolved Photoemission Spectroscopy (ARPES) allows to measure directly the electronic band structure of crystalline solids. It is based on the photoelectric effect: the electron inside the solid absorbs energy and (negligible) momentum from the incoming photon and is ejected in vacuum where it is detected retrieving information about its initial state energy, momentum and spin. State of the art electron energy and momentum analyzers, and high energy resolution and polarization control of the exciting light are needed for resolving the fine electronic structure. Adding efficient spin-polarization measurement to ARPES, i.e. measuring Spin Polarized-ARPES at uncompromised energy and momentum resolution will enable addressing the study of the magnetic properties of surfaces, interfaces and nanostructures, as well al the spin-orbit coupling effects that determine the spin texture of the surface bands in complex materials of interest for their potential in spintronics.
The APE LE end station has been completely renewed with a novel mu-metal UHV chamber optimally designed for hosting a VG Scienta DA30 analyzer and a vectorial spin analysis apparatus developed by us on the VLEED principle. The D30 can operate in the deflection in the direction perpendicular to the analyzer slit therefore reducing the need of sample rotations as electronic bands over 30° angular range are collected for a given alignment. Beyond higher time efficiency in data acquisition this features make easier to control the effective experimental geometry and do not imply changes of light incidence angle on the surface greatly simplifying the exploitation of dichroic effects in ARPES and make it easier to measure on very small samples or small surface domains.
The exit plane of the D30 analyser is also fitted with two channels of variable diameter for extracting energy and momentum selected electrons and transport them to the spin analysis. The spin detection is based on very low energy electron diffraction (VLEED) from magnetic surface oxide targets. Two orthogonal VLEED scattering chambers receive the photoelectrons from the corresponding channels. These custom-designed chambers host identical detector surfaces that can be permanently magnetized in plane in two perpendicular directions forming, altogether, a 3D vectorial spin polarization detection system. Both VLEED 2D detectors are connected with a dedicated target preparation chamber where the magnetic targets are grown by e-beam deposition and controlled surface treatments, developed on our own project at IOM-CNR.
SP-ARPES at APE-LE is accessible through NFFA-Trieste proposal system.