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 (μ-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.

We study the temperature evolution of quasiparticles in the correlated metal Sr$_2$RuO$_4$. Our angle resolved photoemission data show that quasiparticles persist up to temperatures above 200~K, far beyond the Fermi liquid regime. Extracting the quasiparticle self-energy we demonstrate that the quasiparticle residue $Z$ increases with increasing temperature. Quasiparticles eventually disappear on approaching the bad metal state of Sr$_2$RuO$_4$ not by losing weight but via excessive broadening from super-Planckian scattering. We further show that the Fermi surface of Sr$_2$RuO$_4$ - defined as the loci where the spectral function peaks - deflates with increasing temperature. These findings are in semi-quantitative agreement with dynamical mean field theory calculations.

We study the temperature evolution of quasiparticles in the correlated metal Sr_{2}RuO_{4}. Our angle resolved photoemission data show that quasiparticles persist up to temperatures above 200 K, far beyond the Fermi liquid regime. Extracting the quasiparticle self-energy, we demonstrate that the quasiparticle residue Z increases with increasing temperature. Quasiparticles eventually disappear on approaching the bad metal state of Sr_{2}RuO_{4} not by losing weight but via excessive broadening from super-Planckian scattering. We further show that the Fermi surface of Sr_{2}RuO_{4}-defined as the loci where the spectral function peaks-deflates with increasing temperature. These findings are in semiquantitative agreement with dynamical mean field theory calculations.

$V_2O_3$ has long been studied as a prototypical strongly correlated material. The difficulty in obtaining clean, well ordered surfaces, however, hindered the use of surface sensitive techniques to study its electronic structure. Here we show by mean of X-ray diffraction and electrical transport that thin films prepared by pulsed laser deposition can reproduce the functionality of bulk $V_2O_3$. The same films, when transferred in-situ, show an excellent surface quality as indicated by scanning tunnelling microscopy and low energy electron diffraction, representing a viable approach to study the metal-insulator transition (MIT) in $V_2O_3$ by means of angle-resolved photoemission spectroscopy. Combined, these two aspects pave the way for the use of $V_2O_3$ thin films in device-oriented heterostructures.

We use angle-resolved photoemission to map the Fermi surface and quasiparticle dispersion of bulk-like thin films of SrMoO$_3$ grown by pulsed laser deposition. The electronic self-energy deduced from our data reveals weak to moderate correlations in SrMoO$_3$, consistent with our observation of well-defined electronic states over the entire occupied band width. We further introduce spectral function calculations that combine dynamical mean-field theory with an unfolding procedure of density functional calculations and demonstrate good agreement of this approach with our experiments.

We report the synthesis and characterization of polycrystalline Na2RuO3, a layered material in which the Ru4+ (4d(4) configuration) form a honeycomb lattice. The optimal synthesis condition was found to produce a nearly ordered Na2RuO3 (C2/c phase), as assessed from the refinement of the time-of-flight neutron powder diffraction. Magnetic susceptibility measurements reveal a large temperature-independent Pauli paramagnetism [chi(0) similar to 1.42(2) x 10(-3) emu/mol Oe] with no evidence of magnetic ordering down to 1.5 K, and with an absence of dynamic magnetic correlations, as evidenced by neutron scattering spectroscopy. The intrinsic susceptibility (chi(0)) together with the Sommerfeld coefficient of gamma = 11.7(2) mJ/Ru mol K-2 estimated from heat capacity measurements gives an enhanced Wilson ratio of R-w approximate to 8.9(1), suggesting that magnetic correlations may be present in this material. While transport measurements on pressed pellets show nonmetallic behavior, photoemission spectroscopy indicates a small but finite density of states at the Fermi energy, suggesting that the bulk material is metallic. Except for resistivity measurements, which may have been compromised by near-surface and interface effects, all other probes indicate that Na2RuO3 is a moderately correlated electron metal. Our results thus stand in contrast to earlier reports that Na2RuO3 is an antiferromagnetic insulator at low temperatures.

L. S. I. Veiga ,1,2 M. Etter,3 E. Cappelli,4 H. Jacobsen ,5,6 J. G. Vale,1 C. D. Dashwood,1 D. Le ,7 F. Baumberger,4,8 D. F. McMorrow,1 and R. S. Perry9 1London Centre for Nanotechnology and Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, United Kingdom 2Diamond Light Source Ltd., Harwell Science & Innovation Campus, Didcot, Oxfordshire OX11 0DE, United Kingdom 3Deutsches Elektronen-Synchrotron (DESY), Hamburg 22607, Germany 4Department of Quantum Matter Physics, University of Geneva, 24 Quai Ernest-Ansermet, 1211 Geneva 4, Switzerland 5Clarendon Laboratory, Department of Physics, University of Oxford, Oxford OX1 3PU, United Kingdom 6Paul Scherrer Institute, Laboratory for Neutron Scattering and Imaging, 5232 Villigen, Switzerland 7ISIS Facility, Rutherford Appleton Laboratory, STFC, Chilton, Didcot OX11 0QX, United Kingdom 8Swiss Light Source, Paul Scherrer Institute, CH-5232 Villigen, Switzerland 9London Centre for Nanotechnology and Institute for Materials Discovery, University College London, Gower Street, London WC1E 6BT, United Kingdom

Thin films of the correlated transition-metal oxide LaNiO3 undergo a metal–insulator transition when their thickness is reduced to a few unit cells. Here, we use angle-resolved photoemission spectroscopy to study the evolution of the electronic structure across this transition in a series of epitaxial LaNiO3 films of thicknesses ranging from 19 u.c. to 2 u.c. grown in situ by RF magnetron sputtering. Our data show a strong reduction in the electronic mean free path as the thickness is reduced below 5 u.c. This prevents the system from becoming electronically two-dimensional, as confirmed by the largely unchanged Fermi surface seen in our experiments. In the insulating state, we observe a strong suppression of the coherent quasiparticle peak, but no clear gap. These features resemble previous observations of the insulating state of NdNiO3.

We report high-resolution angle resolved photoemission measurements on single crystals of Pt2HgSe3 grown by high-pressure synthesis. Our data reveal a gapped Dirac nodal line whose (001)-projection separates the surface Brillouin zone in topological and trivial areas. In the non-trivial $k$-space range we find surface states with multiple saddle-points in the dispersion resulting in two van Hove singularities in the surface density of states. Based on density functional theory calculations, we identify these surface states as signatures of a topological crystalline state which coexists with a weak topological phase.

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.

Funding: The experimental work has been supported by the European Research Council (ERC), the Scottish Funding Council, the UK EPSRC and the Swiss National Science Foundation (SNSF). Theoretical work was supported by the ERC grant ERC-319286-QMAC and by the SNSF (NCCR MARVEL).

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.

Two-dimensional crystals of semi-metallic van der Waals materials hold much potential for the realization of novel phases, as exemplified by the recent discoveries of a polar metal in few-layer 1T'-WTe2 and of a quantum spin Hall state in monolayers of the same material. Understanding these phases is particularly challenging because little is known from experiments about the momentum space electronic structure of ultrathin crystals. Here, we report direct electronic structure measurements of exfoliated mono-, bi-, and few-layer 1T'-WTe2 by laser-based microfocus angle-resolved photoemission. This is achieved by encapsulating with monolayer graphene a flake of WTe2 comprising regions of different thickness. Our data support the recent identification of a quantum spin Hall state in monolayer 1T'-WTe2 and reveal strong signatures of the broken inversion symmetry in the bilayer. We finally discuss the sensitivity of encapsulated samples to contaminants following exposure to ambient atmosphere.