UNDERSTANDING THE ELECTRONIC PROPERTIES OF QUANTUM MATERIALS BY MEANS OF PHOTOEMISSION WITH ANGULAR AND SPIN RESOLUTION

This thesis contains a selection of the results on the shallow electron states of quantum materials that I obtained as doctoral student of the Scuola di Dottorato in Fisica, Astrofisica e Fisica Applicata at the Università degli Studi di Milano. I carried out my doctoral research activity mostly at the TASC-IOM CNR laboratory, in the framework of the NFFA and APE-beamline facilities (Elettra Sincrotrone Trieste), as well in dedicated sessions at the I2; beamline of the Diamond light source, Harwell Campus, UK. To access the electronic properties of materials I specialised myself in photoemission spectroscopy techniques. High quality samples are a prerequisite for any attempt to study quantum materials so that a major effort in my PhD project has been to master the growth of novel quantum materials by means of Pulsed Laser Deposition (PLD). Given that the PLD is integrated in the suite of UHV facilities attached in-situ to the APE beamline, I directly characterised the electronic properties of the PLD grown samples exploiting both the spectroscopic techniques available at the beamline (ARPES, X-ray photoemission and absorption spectroscopies: XPS and XAS), either ex-situ structural characterisation tools (X-ray diffraction –XRD– and X-ray reflectivity, XRR). Photoemission spectroscopy is a powerful and versatile experimental tool for understanding the electronic properties of materials, providing deep insight into various physical and chemical phenomena, ranging from electronic correlations to surface reactions. Angle-resolved photoemission spectroscopy (ARPES) provides direct insight in the dispersion of extended valence electronic states. Its combination with spin selective detection (spin-ARPES) gives access to the spin polarisation of the photoelectrons at specific points in the Brillouin zone, i.e. to the spin texture of the corresponding band structure. The photoemission intensity is modulated by the "matrix element effects", which express the probability of photoelectron transition from the initial to the final state and link the specific experimental geometry to the symmetry properties of the electron states. This implies that a change in the light polarisation or incident/emission angle induce variations in the spectral intensity and can be misleading in the interpretation of ARPES and spin-ARPES results. In this scenario, a fruitful approach is the simultaneous detection of all quantum numbers of the final state photoelectrons together with the exploitation of light polarisation and photoionisation cross-section tuning. I directly addressed the matrix-element effects performing measurements on the spin polarised states of NbSe2 single crystals while varying the photon energy and polarisation at the highly efficient apparatus for vectorial spin-polarization analysis available at APE-LE beamline. The second part of my thesis work was dedicated to the investigation of the electronic properties of two oxide systems SrNbO3 and anatase TiO2, whose peculiar properties are exploited in catalysis. While TiO2 is a very well-known catalytic material, SrNbO3 has only recently been proposed for applications due to visible- light photo-catalytic properties driven by plasmonic resonances. Up to now, systematic experimental investigation of such a system with X-ray/UV electron spectroscopies was missing. Xray absorption and photoemission spectroscopies were used to probe the chemical states of the samples and the changes induced by different oxygen pressure during the growth. I further performed detailed ARPES investigation of the electronic band structure of SrNbO3. I found that the Fermi surface is made up by three bands mainly originating from t2g orbitals of Nb 4d, as reported for 3d based perovskite systems. The experimental results are consistent with the band dispersion predicted by bulk Density Functional Theory (DFT) calculations that I performed by means of the open source QuantumEspresso software. The narrower bandwidth observed in the ARPES spectra with respect to calculations suggests mass renormalisation arising from electronic correlations. Anatase TiO2 is utilised in a number of applications ranging from photocatalytic devices to sensors as well as solar cells. To efficiently tailor high performing devices, the understanding and control of carrier concentration in the material is a key aspect. I therefore investigate the role of oxygen vacancy defects at the (001) anatase surface that are known to induce extended metallic states on the surface of this nominally insulating material, by means of ARPES and Resonant-ARPES. I observed both localised and metallic delocalised electronic states and investigated the evolution of the spectral intensity as a function of varying oxygen vacancies. I found that the excess oxygen, provided by O2 dosing at the surface, significantly quenches the localised states, whereas O2 reaction has weak impact on the delocalised electronic states: the number of free carriers is reduced but could not be suppressed. These results are very promising for future applications as they may be exploited to tune the excess carriers’ concentration in novel anatase-based devices.