Strongly correlated electron materials: A core level photoelectron spectroscopy investigation

Abstract

This thesis work is dedicated to the investigation of electron correlation in 3d and 4d transition metal systems by means of core level photoelectron spectroscopy (PES). Although conventional PES is widely recognised as a standard tool for the study of the electronic structure of materials, it is somewhat limited by its intrinsic surface sensitivity. In fact due to e ects such as symmetry breaking and contamination the properties of surfaces of the samples are not always representative of the properties of the bulk. While this has allowed to establish PES as one of the leading techniques in the eld of surface science, it has also become an obstacle for applications where it is necessary to probe the real electronic properties of the bulk of the materials, not to mention study the complicated interplay between degrees of freedom which is at the base of electron correlation. These di culties have however been overcome by performing PES with photon energies in the so-called hard X-ray regime ( h! > 2 keV), which allows to probe several hundreds of angstr om of the materials from the surface, to be compared to the few tens of angstr om of standard PES. This technique has been dubbed hard X-ray PES (HAXPES), and its development has been made possible thanks to the buildout of dedicated undulator based beamlines at third generation synchrotron radiation facilities. When applied to strongly correlated materials such as 3d transition metal oxides, HAXPES has provided a wealth of information about bulk electron correlation, which is mainly manifested by the identi cation of a class of low binding energy satellites which was known only in the soft X-ray spectra of 4d transition metal elements. In the case of 3d materials, these peaks are particularly intense and resolved in the 2p core level spectra of the transition metal element in the compound. These features have been extensively studied on transition metal oxides and more recently on diluted magnetic semiconductors: it has been proved that they are related to the conduction and magnetic properties and undergo signi cant changes in intensity when crossing transition temperatures (metal-insulator transition and/or magnetic order transition). Since these peaks seem to be particularly sensitive to the macroscopic conditions of the materials (temperature, conduction state, magnetic order, ...), people have started to wonder whether the presence (or absence) of these peaks in the measured spectra could be used as a sort of spectroscopic \ ngerprint" of metallic character in systems close to the metal-insulator transition and/or to evaluate the quality of samples produced for device implementation. However, since it has also been proved that these peaks are present in spectra measured with su ciently high photon energy, which in turn means only when a certain thickness of material is probed, it is important to put some constraints on the values of this \critical thickness" on di erent class of strongly correlated materials. This thesis work addresses such an issue: by exploiting the broad photon energy range (800 eV h! 6000 eV) available at the I09 beamline of the Diamond synchrotron radiation facility (Didcot, United Kingdom), we have been able to study the photon energy dependence of a low binding energy peak in the photoemission spectrum of Mn 2p on Ga1􀀀xMnxAs and La1􀀀xSrxMnO3 samples. In both systems a well resolved and intense extra-peak can be observed in the Mn 2p core level spectrum measured at the highest available photon energy. We are able to demonstrate that the extra-peak contribution to the total amplitude of the Mn 2p spectrum decreases monotonically when the energy of the probing photons is reduced. This behaviour has been interpreted by assuming the existence of a layer of material close to the surface where the screening channel associated to the extra-peak is suppressed with respect to the bulk. This slab in which the extra-peak is absent attenuates the signal coming from the \substrate": by considering a simple exponential attenuation of the intensity of the satellite, an estimation of the thickness of this layer can be obtained. This type of analysis has revealed that in manganites the attenuating layer is signi cantly thicker with respect to Ga1􀀀xMnxAs. A similar experiment has been carried out also on Ru 3d spectrum of rutile RuO2, a 4d transition metal oxide. In this case the low binding energy feature is the main feature of the spectrum already at photon energies in the soft X-ray regime, although our measurements show clearly that the relative intensity with respect to the rest of the lineshape evolves as a function of the photon energy. Finally, we present what is to our knowledge the rst example of infrared pump-hard X-ray probe experiment on the extra-peak of the Mn 2p3=2 spectrum from La1􀀀xSrxMnO3, performed at the BL19LXU beamline of the SPring-8 light source (Hyougo, Japan). The measurements show a clear reduction of the contribution of the extra-peak to the total area of the spectrum on the time scale of several hundreds of ps. Such experiment can rekindle the interest towards time-resolved HAXPES, which could bene t of the unique characteristics of the light produced at X-ray free electron lasers (XFELs), both in terms of photon ux (HAXPES experiments are particularly photon \hungry") and pulse duration (as small as few tens of fs) to extract dynamical information at the time scales typical of electronic processes. In order to perform such experiments at XFEL facilities, a new class of electron spectrometers will be needed to be designed and built, instruments capable of detecting in a pulse-by-pulse fashion, high intensity photoelectron beams with good time (< 1 ns) and energy ( 100 meV at several keV kinetic energies) resolution. In this thesis, we have addressed this issue by studying through electron trajectory simulations the behaviour of two di erent types of time-of- ight (TOF) electron spectrometers suitable for time-resolved HAXPES at the European X-ray Free Electron Laser (EXFEL), a fourth generation synchrotron radiation light source which is currently under construction at Hamburg, Germany. A cylindrical lens with six separately biased electrodes and a spherical re ector based on tow concentric hemispheres and ten guard rings have been simulated with the SIMION R software. Our results showed that the cylindrical lens can o er a wider angular acceptance with respect to the spherical mirror, while the latter can suppress undesired energies more e ciently and displays resolving power one order of magnitude larger than the former.