Influence of orbital character on the ground state electronic properties in the van Der Waals transition metal iodides VI3 and CrI3

This work was performed in the framework of the Nanoscience Foundry and Fine Analysis (NFFA-MUR Italy) facility and was supported by JST-CREST (No. JPMJCR18T1). A part of the computation in this work, using the VASP code (43) in the GGA approximation (44), was performed by using the facilities of the Supercomputer Center, the Institute for Solid State Physics, the University of Tokyo and MASAMUNE-IMR, Center for Computational Materials Science, Institute for Materials Research, Tohoku University (Project No. 20K0045).Two-dimensional van der Waals magnetic semiconductors display emergent chemical and physical properties and hold promise for novel optical, electronic and magnetic “few-layers” functionalities. Transition-metal iodides such as CrI3 and VI3 are relevant for future electronic and spintronic applications; however, detailed experimental information on their ground state electronic properties is lacking often due to their challenging chemical environment. By combining X-ray electron spectroscopies and first-principles calculations, we report a complete determination of CrI3 and VI3 electronic ground states. We show that the transition metal-induced orbital filling drives the stabilization of distinct electronic phases: a wide bandgap in CrI3 and a Mott insulating state in VI3. Comparison of surface-sensitive (angular-resolved photoemission spectroscopy) and bulk-sensitive (X-ray absorption spectroscopy) measurements in VI3 reveals a surface-only V2+ oxidation state, suggesting that ground state electronic properties are strongly influenced by dimensionality effects. Our results have direct implications in band engineering and layer-dependent properties of two-dimensional systems.Publisher PDFPeer reviewe

Observation of termination-dependent topological connectivity in a magnetic Weyl Kagome lattice

The research leading to these results has received funding from the European Union’s Horizon 2020 research and innovation program under Marie Skłodowska-Curie Grant Agreement 897276. The authors gratefully acknowledge the Gauss Centre for Supercomputing e.V. (https://www.gauss-centre.eu) for funding this project by providing computing time on the GCS Supercomputer SuperMUC-NG at Leibniz Supercomputing Centre (https://www.lrz.de). The authors are grateful for funding support from the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) under Germany’s Excellence Strategy through the Würzburg-Dresden Cluster of Excellence on Complexity and Topology in Quantum Matter ct.qmat (EXC 2147, Project 390858490), through FOR 5249-449872909 (Project P5), and through the Collaborative Research Center SFB 1170 ToCoTronics (Project 258499086). The authors greatly acknowledge the Diamond Light Source that supported the entire micro-ARPES experiment and corresponding costs. The Flatiron Institute is a division of the Simons Foundation. P.D.C.K. and C.B. gratefully acknowledge support from The Leverhulme Trust via Grant RL-2016-006.Engineering surfaces and interfaces of materials promises great potential in the field of heterostructures and quantum matter designers, with the opportunity to drive new many-body phases that are absent in the bulk compounds. Here, we focus on the magnetic Weyl kagome system Co3Sn2S2 and show how for the terminations of different samples the Weyl points connect differently, still preserving the bulk-boundary correspondence. Scanning tunneling microscopy has suggested such a scenario indirectly, and here, we probe the Fermiology of Co3Sn2S2 directly, by linking it to its real space surface distribution. By combining micro-ARPES and first-principles calculations, we measure the energy-momentum spectra and the Fermi surfaces of Co3Sn2S2 for different surface terminations and show the existence of topological features depending on the top-layer electronic environment. Our work helps to define a route for controlling bulk-derived topological properties by means of surface electrostatic potentials, offering a methodology for using Weyl kagome metals in responsive magnetic spintronics.Publisher PDFPeer reviewe

Templating effect of the substrate on the structure of Cu-phthalocyanine thin film

An experimental study of electronic properties, structure and morphology of Copper-phthalocyanine films deposited onto Al(100) and Au(110), as a function of thickness up to tens of nanometers, is presented. The monolayers grown on these two model substrates are already known to exhibit very different behavior for what concerns both the degree of interaction with the substrate and the formation of long range order; in this experiment, by means of low energy electron scattering and Electron Energy Loss Spectroscopy (EELS), remarkable differences are revealed also in the successive growth. Exploiting the link between the crystal structure and the lineshape of HOMOâLUMO transition in EELS spectrum, two different structural phases are identified, compatible with α and β phases, respectively in the case of the film grown on aluminum and on gold. Besides, the evolution of the specular reflection elastic peak indicates the formation of islands on the gold substrate and a more homogeneous growth on the aluminum one

Secondary electron generation mechanisms in carbon allotropes at low impact electron energies

More than a century after the discovery of the electron, there are still fundamental, yet unresolved, questions concerning the generation-ejection mechanism of the ubiquitous Secondary Electrons (SEs) from a solid surface. Broadness of the field of application for these SEs makes it desirable to be able to control this phenomenon, which requires the understanding of the elementary physical mechanism leading to their generation and emission. This paper reports on the dissection of such a tangled process operated by the help of spectroscopic tools of increasing finesse; measuring differential cross sections with an increasing degree of differentiation. These results demonstrate that single ionising scattering events, assisted by collective excitations, constitute the fundamental ingredient leading to SE-generation and -emission. To this end, the interaction of Low Energy Electrons (LEEs) with various Carbon allotropes has been investigated by means of Total Electron Yield (TEY) and (e,2e)-coincidence spectroscopy measurements. Carbon allotropes are chosen as targets since they are important in technological applications where both minimisation and maximisation of the SE-yield is a relevant issue. This is the first time that such complete set of benchmarks on the SE-yield from well characterised surfaces has been gathered, interpreted and is made available to the scientific community. This comprehensive investigation has led to the disentanglement of the elementary processes relevant for the understanding of the SE-generation probability, that fully take into account both energy and momentum conservation in the collision and the band structure of the solid as well as many-body effects

Lifetime of Photogenerated Positive Charges in Hybrid Cerium Oxide-Based Materials from Space and Mirror Charge Effects in Time-Resolved Photoemission Spectroscopy

Space and mirror charge effects in time-resolved photoemission spectroscopy can be modeled to obtain relevant information on the recombination dynamics of charge carriers. We successfully extracted from these phenomena the reneutralization characteristic time of positive charges generated by photoexcitation in CeO2-based films. For the above-band-gap excitation, a large fraction of positive carriers with a lifetime that exceeds 100 ps are generated. Otherwise, the sub-band-gap excitation induces the formation of a significantly smaller fraction of charges with lifetimes of tens of picoseconds, ascribed to the excitation of defect sites or to multiphoton absorption. When the oxide is combined with Ag nanoparticles, the sub-band-gap excitation of localized surface plasmon resonances leads to reneutralization times longer than 300 ps. This was interpreted by considering the electronic unbalance at the surface of the nanoparticles generated by the injection of electrons, via localized surface plasmon resonance (LSPR) decay, into CeO2. This study represents an example of how to exploit the space charge effect in gaining access to the surface carrier dynamics in CeO2 within the picosecond range of time, which is fundamental to describe the photocatalytic processes

Evidence of robust half-metallicity in strained manganite films

We investigated the relationship between ferromagnetism and metallicity in strained La0.67Ca0.33MnO3 films grown on lattice-mismatched NdGaO3 (001) by means of spectroscopic techniques directly sensitive to the ferromagnetic state, to the band structure, and to the chemical state of the atoms. In this system, the ferromagnetic metallic (FMM) phase spatially coexists with an insulating one in most of the phase diagram. First, the observation of an almost 100% spin polarization of the photoelectrons at the Fermi level in the fundamental state provides direct evidence of the half-metallicity of the FMM phase, a result that has been previously observed through direct probing of the valence band only on unstrained, phase-homogeneous La0.67Sr0.33MnO3. Second, the spin polarization results to be correlated with the occupancy at the Fermi level for all the investigated temperature regimes. These outcomes show that the half-metallic behavior predicted by a double-exchange model persists even in phase-separated manganites. Moreover, the correlation between metallicity and ferromagnetic alignment is confirmed by X-ray magnetic circular dichroism, a more bulk-sensitive technique, allowing one to explain transport properties in terms of the conduction through aligned FMM domains

Evidence of magnetism-induced topological protection in the axion insulator candidate EuSn2P2

We unravel the interplay of topological properties and the layered (anti)ferromagnetic ordering in EuSn2P2, using spin and chemical selective electron and X-ray spectroscopies supported by firstprinciple calculations. We reveal the presence of in-plane longrange ferromagnetic order triggering topological invariants and resulting in the multiple protection of topological Dirac states. We provide clear evidence that layer-dependent spin-momentum locking coexists with ferromagnetism in this material, a cohabitation that promotes EuSn2P2 as a prime candidate axion insulator for topological antiferromagnetic spintronics applications

Coherent narrowband light source for ultrafast photoelectron spectroscopy in the 17-31 eV photon energy range

Here, we report on a novel narrowband High Harmonic Generation (HHG) light source designed for ultrafast photoelectron spectroscopy (PES) on solids. Notably, at 16.9 eV photon energy, the harmonics bandwidth equals 19 meV. This result has been obtained by seeding the HHG process with 230 fs pulses at 515 nm. The ultimate energy resolution achieved on a polycrystalline Au sample at 40 K is similar to 22 meV at 16.9 eV. These parameters set a new benchmark for narrowband HHG sources and have been obtained by varying the repetition rate up to 200 kHz and, consequently, mitigating the space charge, operating with approximate to 3x10(7) electrons/s and approximate to 5x10(8) photons/s. By comparing the harmonics bandwidth and the ultimate energy resolution with a pulse duration of similar to 105 fs (as retrieved from time-resolved experiments on bismuth selenide), we demonstrate a new route for ultrafast space-charge-free PES experiments on solids close to transform-limit conditions