Electronic structure of few-layer black phosphorus from μ\mu-ARPES

Black phosphorus (BP) stands out among two-dimensional (2D) semiconductors because of its high mobility and thickness dependent direct band gap. However, the quasiparticle band structure of ultrathin BP has remained inaccessible to experiment thus far. Here we use a recently developed laser-based micro-focus angle resolved photoemission ($\mu$-ARPES) system to establish the electronic structure of 2-9 layer BP from experiment. Our measurements unveil ladders of anisotropic, quantized subbands at energies that deviate from the scaling observed in conventional semiconductor quantum wells. We quantify the anisotropy of the effective masses and determine universal tight-binding parameters which provide an accurate description of the electronic structure for all thicknesses.Comment: Supporting Information available upon reques

Direct evidence for flat bands in twisted bilayer graphene from nano-ARPES

Transport experiments in twisted bilayer graphene revealed multiple superconducting domes separated by correlated insulating states. These properties are generally associated with strongly correlated states in a flat mini-band of the hexagonal moir\'e superlattice as it was predicted by band structure calculations. Evidence for such a flat band comes from local tunneling spectroscopy and electronic compressibility measurements, reporting two or more sharp peaks in the density of states that may be associated with closely spaced van Hove singularities. Direct momentum resolved measurements proved difficult though. Here, we combine different imaging techniques and angle resolved photoemission with simultaneous real and momentum space resolution (nano-ARPES) to directly map the band dispersion in twisted bilayer graphene devices near charge neutrality. Our experiments reveal large areas with homogeneous twist angle that support a flat band with spectral weight that is highly localized in momentum space. The flat band is separated from the dispersive Dirac bands which show multiple moir\'e hybridization gaps. These data establish the salient features of the twisted bilayer graphene band structure.Comment: Submitted to Nature Materials. Nat. Phys. (2020

Topological electronic structure of few-layer and bulk van der Waals materials probed by angle-resolved photoemission

Van der Waals materials can be exfoliated down to atomically thin crystals, offering potential to explore physics in lower dimensionality. In this thesis, I investigated their topological electronic structure with ARPES. I studied mono-, bi- and few-layer Td-WTe2. The small size of exfoliated crystals and their air-sensitivity pose new challenges for standard ARPES setups, successfully overcome in the present study. The measurements showed the topological band gap in the monolayer, a strong Rashba splitting in the bilayer, and thickness-dependent quantum-well states up to thicknesses above 15 layers. I describe conventional measurements on the recently grown Pt2HgSe3. ARPES measurements on the bulk crystal were motivated by the prediction of a record-high topological band gap in the monolayer. This implies a weak topological insulator in the bulk with surface states only on lateral surfaces. However, ARPES measurements showed surface states on the top surface, revealing a richer topological picture

Long-term health and economic impacts of air pollution in Greater Geneva

International audienceIntroduction: We estimated the health and economic impacts of chronic exposure to air pollution for the Swiss part of the Greater Geneva area from 2016 to 2018. Materials and methods: We extracted from fine-scale modelled concentration maps for two pollutant indicators, particulate matter PM 2.5 and nitrogen dioxide. Then, we performed a quantitative health impact assessment of the health burden attributable to anthropogenic-origin air pollution, and estimated the benefits of compliance with the federal Ordinance on Air Pollution Control (OAPC) limit values. Finally, we computed the economic impacts of these health effects. Results: Exposure to fine particles of anthropogenic origin was responsible for 7.5% of annual mortality (280 deaths or 5,900 life years lost), for 14 lung cancers and for 68 strokes annually in the Canton of Geneva. Compliance with the OAPC limit value of 10 µg/m 3 as an annual average would reduce annual mortality by 1.5% (62 deaths avoided or 1,300 life years gained). Exposure to anthropogenic-origin NO 2 was associated with 5.3% of annual deaths (approximately 200 deaths per year). The estimated total negative economic impacts of anthropogenic-origin fine particles were at least CHF 2017 1.3 billion per year, whereas compliance with the OAPC limit values would result in annual economic benefits of at least CHF 2017 290 million. Conclusion: We confirmed that air quality remains a health issue on which stakeholder mobilisation is vital. Action plans should tackle emissions from freight and personal mobility, heating, industry and agriculture, while seeking to improve knowledge on health risks from air pollution exposure

Better accounting for long-term health effects in economic assessments: an illustration for air pollution in the Canton of Geneva

International audienceObjectives: We propose a general framework for estimating long-term health and economic effects that takes into account four time-related aspects.We apply it to a reduction in exposure to air pollution in theCanton of Geneva.Study design: Methodological developments on the evaluation of long-term economic and health benefits, with an empirical illustration.Methods: We propose a unified frameworkdthe comprehensive impact assessment (CIA)dto assess the long-term effects of morbidity and mortality in health and economic terms. This framework takes fullaccount of four time-related issues: cessation lag, policy/technical implementation timeframe, discounting and time horizon. We compare its results with those obtained from standard quantitativehealth impact assessment (QHIA) in an empirical illustration involving air pollution reduction in the canton of Geneva.Results: We find that by neglecting time issues, the QHIA estimates greater health and economic benefits than the CIA. The overestimation is about 50% under reasonable assumptions and increases ceterisparibus with the magnitude of the cessation lag and the discount factor. It decreases both with the time horizon and with the implementation timeframe.Conclusion: A proper evaluation of long-term health and economic effects is an important issue when they are to be used in cost-benefit analyses, particularly for mortality, which often represents the largestfraction. We recommend using the CIA to calculate more accurate values

Electronic structure of 2D van der Waals crystals and heterostructures investigated by spatially- and angle-resolved photoemission

Angle-resolved photoemission is a direct probe of the momentum-resolved electronic structure and proved influential in the study of bulk crystals with novel electronic properties. Thanks to recent technical advances, this technique can now be applied for the first time for the study of van der Waals heterostructures built by stacking two-dimensional crystals. In this article we will present the current state of the art in angle-resolved photoemission measurements on two-dimensional materials and review this still young field. We will focus in particular on devices similar to those used in transport and optics experiments, including the latest developments on magic-angle twisted bilayer graphene and on the in-operando characterization of gate tunable devices

Fermi Arcs and Their Topological Character in the Candidate Type-II Weyl Semimetal MoTe₂

We report a combined experimental and theoretical study of the candidate type-II Weyl semimetal MoTe_{2}. Using laser-based angle-resolved photoemission, we resolve multiple distinct Fermi arcs on the inequivalent top and bottom (001) surfaces. All surface states observed experimentally are reproduced by an electronic structure calculation for the experimental crystal structure that predicts a topological Weyl semimetal state with eight type-II Weyl points. We further use systematic electronic structure calculations simulating different Weyl point arrangements to discuss the robustness of the identified Weyl semimetal state and the topological character of Fermi arcs in MoTe_{2}

In situ strain tuning of the metal-insulator-transition of Ca₂RuO₄ in angle-resolved photoemission experiments

Pressure plays a key role in the study of quantum materials. Its application in angle resolved photoemission (ARPES) studies, however, has so far been limited. Here, we report the evolution of the k-space electronic structure of bulk Ca2RuO4, lightly doped with Pr, under uniaxial strain. Using ultrathin plate-like crystals, we achieve uniaxial strain levels up to −4.1%, sufficient to suppress the insulating Mott phase and access the previously unexplored electronic structure of the metallic state at low temperature. ARPES experiments performed while tuning the uniaxial strain reveal that metallicity emerges from a marked redistribution of charge within the Ru t2g shell, accompanied by a sudden collapse of the spectral weight in the lower Hubbard band and the emergence of a well-defined Fermi surface which is devoid of pseudogaps. Our results highlight the profound roles of lattice energetics and of the multiorbital nature of Ca2RuO4 in this archetypal Mott transition and open new perspectives for spectroscopic measurements