PROBING ELECTRON CORRELATION DYNAMICS: A MULTI-TECHNIQUE STUDY APPLIED TO THE HALF-METALLIC OXIDE LA1-XSRXMNO3

This thesis completes my work as doctoral student of the Scuola di Dottorato in Fisica, Astrofisica e Fisica Applicata at the Universit\ue0 degli Studi di Milano that has been carried out, starting in November 2014, mostly at the Laboratorio TASC of IOM-CNR in the premises of the Elettra - Sincrotrone Trieste and FERMI@Elettra infrastructures, in the framework of the NFFA and APE-beamline facilites, as well as by accessing international large scale infrastructures and laboratories. The activity has addressed the development of experimental methodologies and novel instrumentation oriented to the study of the dynamical properties of highly correlated materials after high energy excitation. The science programme has been carried out by exploiting ultrafast femtosecond probes from the optical regime (Ti-Sa lasers, fibre laser oscillators) to the extreme UV-soft X rays at FERMI, to the picosecond hard X-rays from the SPring-8 and Diamond synchrotron radiation source. The sample synthesis of correlated oxides and its characterization has been performed within the NFFA facility and APE-group collaboration in Trieste as well as the design and construction of the all new laser High Harmonic Generation beam line NFFA-SPRINT and its end station for time resolved vectorial electron spin polarimetry. This report concentrates on the main scientific concern of my work that has been the relaxation of external perturbations in a correlated electron material both in the time and space domain. I have employed Photoelectron Spectroscopy (PES) mostly in the Hard X-ray regime (HAXPES), pushing the boundaries of its application to achieve a coherent perspective. The material I have mainly focused on is La0.67Sr0.33MnO3 (LSMO), of high interest for spintronics. This system is prototypical, yielding the highest simplicity in the class of transition metal oxides. In the spatial investigation, I have controlled with high precision the PES probing depth and I have observed the evolution of one spectral feature. I have identified it as probe of electronic hybridization and long-range ordering. I have studied LSMO films of 40 nm in three substrate-induced strain states (1% tensile in-plane, relaxed, 1% compressive in-plane) and a 18 nm film of (Ga,Mn)As (GMA), a well-studied diluted magnetic semiconductor. I have found that the electronic properties to be modified at significant distances from the surface, 4 nm for LSMO and 1.2 nm for GMA, while strain had no detectable effects. In the temporal study, I have employed HAXPES in pump-probe mode (TR-HAXPES) to observe the evolution of the electronic structure after intense optical excitation. A detailed dynamical characterization with optical techniques has allowed me to identify the characteristic time of the collapse of long-range magnetic order to be significantly longer than the one of elemental transition metals. I have ascribed this effect to the half-metallic character of LSMO. With TR-HAXPES I have observed that the whole electronic band-structure evolution is bottlenecked by the slow response of the magnetization, proceeding on hundreds of picoseconds timescales. Finally, I have described the techniques and the instrumentation that can be used to push these investigations to shorter spatial and temporal scales. This has been realized in the form of the NFFA-SPRINT laboratory, a facility open to users, which I participated in designing and developing

Wnt/CTNNB1 signal transduction pathway inhibits the expression of ZFP36 in squamous cell carcinoma, by inducing transcriptional repressors SNAI1, SLUG and TWIST

The Wnt/CTNNB1 pathway is often deregulated in epithelial tumors. The ZFP36 gene, encoding the mRNA binding protein Tristetraprolin (TTP), is downregulated in several cancers, where it has been described to behave as a tumor suppressor. By this report, we show that Wnt/CTNNB1 pathway is constitutively activated, and ZFP36 expression is downregulated in Squamous Cell Carcinoma (SCC) cell lines compared to normal keratinocytes. Moreover, we suggest that the decrease of ZFP36 expression might depend on the activity of transcriptional repressors SNAI1, SLUG and TWIST, whose expression is induced by Wnt/CTNNB1, highlighting a potential regulatory mechanism underlying ZFP36 downregulation in epithelial cancers

A quantitative comparison of time-of-flight momentum microscopes and hemispherical analyzers for time- and angle-resolved photoemission spectroscopy experiments

Time-of-flight-based momentum microscopy has a growing presence in photoemission studies, as it enables parallel energy- and momentum-resolved acquisition of the full photoelectron distribution. Here, we report table-top extreme ultraviolet (XUV) time- and angle-resolved photoemission spectroscopy (trARPES) featuring both a hemispherical analyzer and a momentum microscope within the same setup. We present a systematic comparison of the two detection schemes and quantify experimentally relevant parameters, including pump- and probe-induced space-charge effects, detection efficiency, photoelectron count rates, and depth of focus. We highlight the advantages and limitations of both instruments based on exemplary trARPES measurements of bulk WSe2. Our analysis demonstrates the complementary nature of the two spectrometers for time-resolved ARPES experiments. Their combination in a single experimental apparatus allows us to address a broad range of scientific questions with trARPES.Comment: 19 pages, 9 figures. The following article has been submitted to Review of Scientific Instruments / AIP Publishing. After it is published, it will be found at https://aip.scitation.org/journal/rs

Promoter Methylation Leads to Decreased ZFP36 Expression and Deregulated NLRP3 Inflammasome Activation in Psoriatic Fibroblasts

The mRNA-destabilizing protein tristetraprolin (TTP), encoded by the ZFP36 gene, is known to be able to end inflammatory responses by directly targeting and destabilizing mRNAs encoding pro-inflammatory cytokines. We analyzed its role in psoriasis, a disease characterized by chronic inflammation. We observed that TTP is downregulated in fibroblasts deriving from psoriasis patients compared to those deriving from healthy individuals and that psoriatic fibroblasts exhibit abnormal inflammasome activity compared to their physiological counterpart. This phenomenon depends on TTP downregulation. In fact, following restoration, TTP is capable of directly targeting for degradation NLRP3 mRNA, thereby drastically decreasing inflammasome activation. Moreover, we provide evidence that ZFP36 undergoes methylation in psoriasis, by virtue of the presence of long stretches of CpG dinucleotides both in the promoter and the coding region. Besides confirming that a perturbation of TTP expression might underlie the pathogenesis of psoriasis, we suggest that deregulated inflammasome activity might play a role in the disease alongside deregulated cytokine expression

Ultrafast Momentum-resolved Hot Electron Dynamics in the Two-dimensional Topological Insulator Bismuthene

Two-dimensional quantum spin Hall (QSH) insulators are a promising material class for spintronic applications based on topologically-protected spin currents in their edges. Yet, they have not lived up to their technological potential, as experimental realizations are scarce and limited to cryogenic temperatures. These constraints have also severely restricted characterization of their dynamical properties. Here, we report on the electron dynamics of the novel room-temperature QSH candidate bismuthene after photoexcitation using time- and angle-resolved photoemission spectroscopy. We map the transiently occupied conduction band and track the full relaxation pathway of hot photocarriers. Intriguingly, we observe photocarrier lifetimes much shorter than in \red{conventional} semiconductors. This is ascribed to the presence of topological in-gap states already established by local probes. Indeed, we find spectral signatures consistent with these earlier findings. Demonstration of the large band gap and the view into photoelectron dynamics mark a critical step toward optical control of QSH functionalities.Comment: 13 pages, 11 figure

Ultrafast Light-Induced Lifshitz Transition

Fermi surface is at the heart of our understanding of metals and strongly correlated many-body systems. An abrupt change in the Fermi surface topology, also called Lifshitz transition, can lead to the emergence of fascinating phenomena like colossal magnetoresistance and superconductivity. While Lifshitz transitions have been demonstrated for a broad range of materials and using different types of static external perturbations such as strain, doping, pressure and temperature, a non-equilibrium route toward ultrafast and transient modification of the Fermi surface topology has not been experimentally demonstrated. Combining time-resolved multidimensional photoemission spectroscopy with state-of-the-art TDDFT+U simulations, we introduce a scheme for driving an ultrafast Lifshitz transition in the correlated Weyl semimetal Td-MoTe2. We demonstrate that this non-equilibrium topological electronic transition finds its microscopic origin in the dynamical modification of the effective electronic correlations. These results shed light on a novel ultrafast and all-optical scheme for controlling the Fermi surface topology in correlated quantum materials

Identification of hidden orbital contributions in the La_{0.65} Sr_{0.35} MnO_{3} valence band

Hybridization of electronic states and orbital symmetry in transition metal oxides are generally considered key ingredients in the description of both their electronic and magnetic properties. In the prototypical case of La_{0.65} Sr_{0.35} MnO_{3} (LSMO), a landmark system for spintronics applications, a description based solely on Mn 3d and O 2p electronic states is reductive. We thus analyzed elemental and orbital distributions in the LSMO valence band through a comparison between density functional theory calculations and experimental photoelectron spectra in a photon energy range from soft to hard x rays. We reveal a number of hidden contributions, arising specifically from La 5p, Mn 4s, O 2s orbitals, considered negligible in previous analyses; our results demonstrate that all these contributions are significant for a correct description of the valence band of LSMO and of transition metal oxides in general

Distinct behaviour of localized and delocalized carriers in anatase TiO2 (001) during reaction with O2

Two-dimensional (2D) metallic states induced by oxygen vacancies (VOs) at oxide surfaces and interfaces provide opportunities for the development of advanced applications, but the ability to control the behavior of these states is still limited. We used angle resolved photoelectron spectroscopy combined with density-functional theory (DFT) to study the reactivity of VO-induced states at the (001) surface of anatase TiO2, where both 2D metallic and deeper lying in-gap states (IGs) are observed. The 2D and IG states exhibit remarkably different evolutions when the surface is exposed to molecular O2: while IGs are almost completely quenched, the metallic states are only weakly affected. DFT calculations indeed show that the IGs originate from surface VOs and remain localized at the surface, where they can promptly react with O2. In contrast, the metallic states originate from subsurface vacancies whose migration to the surface for recombination with O2 is kinetically hindered on anatase TiO2 (001), thus making them much less sensitive to oxygen dosing.PostprintPeer reviewe

Distinct behavior of localized and delocalized carriers in anatase TiO2 (001) during reaction with O-2

Two-dimensional (2D) metallic states induced by oxygen vacancies ( V O s ) at oxide surfaces and interfaces provide opportunities for the development of advanced applications, but the ability to control the behavior of these states is still limited. We used angle resolved photoelectron spectroscopy combined with density-functional theory (DFT) to study the reactivity of V O -induced states at the (001) surface of anatase TiO 2 , where both 2D metallic and deeper lying in-gap states (IGs) are observed. The 2D and IG states exhibit remarkably different evolutions when the surface is exposed to molecular O 2 : while IGs are almost completely quenched, the metallic states are only weakly affected. DFT calculations indeed show that the IGs originate from surface V O s and remain localized at the surface, where they can promptly react with O 2 . In contrast, the metallic states originate from subsurface vacancies whose migration to the surface for recombination with O 2 is kinetically hindered on anatase TiO 2 (001), thus making them much less sensitive to oxygen dosing

Direct measurement of key exciton properties: Energy, dynamics, and spatial distribution of the wave function

Excitons, Coulomb-bound electron-hole pairs, are the fundamental excitations governing optoelectronic properties of semiconductors. While optical signatures of excitons have been studied extensively, experimental access to the excitonic wave function itself has been elusive. Using multidimensional photoemission spectroscopy, we present a momentum-, energy- and time-resolved perspective on excitons in the layered semiconductor WSe2. By tuning the excitation wavelength, we determine the energy-momentum signature of bright exciton formation and its difference from conventional single-particle excited states. The multidimensional data allows to retrieve fundamental exciton properties like the binding energy and the exciton-lattice coupling and to reconstruct the real-space excitonic wave function via Fourier transform. All quantities are in excellent agreement with microscopic calculations. Our approach provides a full characterization of the exciton wave function and is applicable to bright and dark excitons in semiconducting materials, heterostructures, and devices