We study Na2IrO3 by angle-resolved photoemission spectroscopy, optics, and band structure calculations in the local-density approximation (LDA). The weak dispersion of the Ir 5d-t(2g) manifold highlights the importance of structural distortions and spin-orbit (SO) coupling in driving the system closer to a Mott transition. We detect an insulating gap Δ(gap)≃340  meV which, at variance with a Slater-type description, is already open at 300 K and does not show significant temperature dependence even across T(N)≃15  K. An LDA analysis with the inclusion of SO and Coulomb repulsion U reveals that, while the prodromes of an underlying insulating state are already found in LDA+SO, the correct gap magnitude can only be reproduced by LDA+SO+U, with U=3  eV. This establishes Na2IrO3 as a novel type of Mott-like correlated insulator in which Coulomb and relativistic effects have to be treated on an equal footing.

The electronic structure of Bi(2)Se(3) is studied by angle-resolved photoemission and density functional theory. We show that the instability of the surface electronic properties, observed even in ultrahigh-vacuum conditions, can be overcome via in situ potassium deposition. In addition to accurately setting the carrier concentration, new Rashba-like spin-polarized states are induced, with a tunable, reversible, and highly stable spin splitting. Ab initio slab calculations reveal that these Rashba states are derived from 5-quintuple-layer quantum-well states. While the K-induced potential gradient enhances the spin splitting, this may be present on pristine surfaces due to the symmetry breaking of the vacuum-solid interface.

Arguably the most intriguing aspect of the physics of cuprates is the close proximity between the record high-T c superconductivity (HTSC) and the antiferromagnetic charge-transfer insulating state driven by Mott-like electron correlations. These are responsible for the intimate connection between high and low-energy scale physics [1][2][3], and their key role in the mechanism of HTSC was conjectured very early on [4]. More recently, the detection of quantum oscillations in high-magnetic field experiments on YBa 2 Cu 3 O 6+x (YBCO) has suggested the existence of a Fermi surface of well-defined quasiparticles in underdoped cuprates [5,6], lending support to the alternative proposal that HTSC might emerge from a Fermi liquid across the whole cuprate phase diagram [7,8]. Discriminating between these orthogonal scenarios hinges on the quantitative determination of the elusive quasiparticle weight Z, over a wide range of hole-doping p. By means of angle-resolved photoemission spectroscopy (ARPES) on in situ doped YBCO [9], and following the evolution of bilayer bandsplitting, we show that the overdoped metal electronic structure (0.25 p 0.37) is in remarkable agreement with density functional theory [10][11][12] and the Z = 2p/(p+1) mean-field prediction [13,14]. Below p 0.10-0.15, we observe the vanishing of the nodal quasiparticle weight Z N ; this marks a clear departure from Fermi liquid behaviour and -consistent with dynamical mean-field theory [15] -is even a more rapid crossover to the Mott physics than expected for the doped resonating valence bond (RVB) spin liquid [13,14].Formally, the degree of quasiparticle integrity is revealed by Z k ≡ A coh (k, ω) dω, i.e. the integrated spectral weight of the coherent part of the single-particle spectral function A(k, ω) ≡ A coh (k, ω) + A incoh (k, ω) probed by ARPES [16,17]. Experimentally, while in the optimally-to-overdoped regime Z k is believed to be finite -yet quantitatively undetermined -at all momenta both above and below T c , the situation is much more controversial in the optimally-to-underdoped regime [16,17]. Although it has been conjectured that the T = 0 extrapolation of the pseudogap state is a nodal (N) liquid [18,19], a determination of Z N has not been possible. As for the antinodal (AN) quasiparticle spectral weight, the ARPES spectra are characterized by a dramatic temperature dependence, with broad incoherent features in the pseudogap state and quasiparticle-like excitations emerging below T c [16,17] Supplementary Information, where we present an analysis of the quasiparticle spectral weight across the whole YBCO phase diagram, this is particularly challenging on the underdoped side where Z k vanishes; and even in the overdoped regime, this allows at best a relativerather than absolute -determination of Z k .As an alternative, potentially more quantitative approach, in materials with CuO 2 bilayers within the unit cell such as Bi 2 Sr 2 CaCu 2 O 8+δ (Bi2212) and YBCO, the quasiparticle strength Z k might be estimated from the bonding (B) and a...

We present a detailed study of vortex core spectroscopy in slightly overdoped Bi2Sr2CaCu2O 8+δ using a low temperature scanning tunneling microscope. Inside the vortex core we observe a fourfold symmetric modulation of the local density of states with an energy-independent period of (4.3 ± 0.3)a0. Furthermore we demonstrate that this square modulation is related to the vortex core states which are located at ±6 meV. Since the core-state energy is proportional to the superconducting gap magnitude ∆p, our results strongly suggest the existence of a direct relation between the superconducting state and the local electronic modulations in the vortex core.

Ultrafast spectroscopies have become an important tool for elucidating the microscopic description and dynamical properties of quantum materials. In particular, by tracking the dynamics of non-thermal electrons, a material's dominant scattering processes -and thus the many-body interactions between electrons and collective excitations -can be revealed. Here we present a new method for extracting the electron-phonon coupling strength in the time domain, by means of time and angle-resolved photoemission spectroscopy (TR-ARPES). This method is demonstrated in graphite, where we investigate the 1 arXiv:1902.05572v3 [cond-mat.str-el] 1 Aug 2019 dynamics of photo-injected electrons at the K point, detecting quantized energyloss processes that correspond to the emission of strongly-coupled optical phonons.We show that the observed characteristic timescale for spectral-weight-transfer mediated by phonon-scattering processes allows for the direct quantitative extraction of electron-phonon matrix elements, for specific modes, and with unprecedented sensitivity.The concept of the electronic quasiparticle as proposed by Landau, i.e. the dressing of an electron with many-body and collective excitations (1), is essential to the modern understanding of condensed matter physics. Among the plethora of interactions relevant to solid-state systems, electron-phonon coupling (EPC) has been a persistent subject of great interest, related as it is to the emergence of disparate physical phenomena, from resistivity in normal metals to conventional (BCS) superconductivity and charge-ordered phases (2,3). While strong EPC is desirable in systems like BCS superconductors (4, 5), it is deleterious for conductivity in normal metals, curtailing the application of several compounds as room-temperature electronic devices (6).Given the important role of the electron-phonon interaction in relation to both conventional and quantum materials, extensive theoretical and experimental efforts have been devoted towards determining the strength and anisotropy of EPC. While ab-initio calculations are powerful, they rely on complex approximations which require precise experimental data to benchmark their validity (7). Inelastic scattering experiments -such as Raman spectroscopy (8), electron energy loss spectroscopy (9), inelastic x-ray (10), and neutron scattering (11) -are able to access EPC for specific phonon modes, yet integrated over all electronic states. Angle-resolved photoemission spectroscopy (ARPES), on the converse, can access the strength of EPC via phononmediated renormalization effects for specific momentum-resolved electronic states, as revealed by "kinks" in the electronic band dispersion (12-16). However, extraction of EPC strength from these kinks requires accurate modelling of the bare band dispersion and of the electronic

We observe apparent hole pockets in the Fermi surfaces of single-layer Bi-based cuprate superconductors from angle-resolved photoemission. From detailed low-energy electron diffraction measurements and an analysis of the angle-resolved photoemission polarization dependence, we show that these pockets are not intrinsic but arise from multiple overlapping superstructure replicas of the main and shadow bands. We further demonstrate that the hole pockets reported recently from angle-resolved photoemission [Meng et al., Nature (London) 462, 335 (2009)] have a similar structural origin and are inconsistent with an intrinsic hole pocket associated with the electronic structure of a doped CuO₂ plane.

The role of Co substitution in the low-energy electronic structure of Ca(Fe(0.944)Co(0.056))(2)As(2) is investigated by resonant photoemission spectroscopy and density-functional theory. The Co 3d state center of mass is observed at 250 meV higher binding energy than that of Fe, indicating that Co possesses one extra valence electron and that Fe and Co are in the same oxidation state. Yet, significant Co character is detected for the Bloch wave functions at the chemical potential, revealing that the Co 3d electrons are part of the Fermi sea determining the Fermi surface. This establishes the complex role of Co substitution in CaFe(2)As(2) and the inadequacy of a rigid-band shift description.

We revisit the normal-state electronic structure of Sr(2)RuO(4) by angle-resolved photoemission spectroscopy with improved data quality, as well as ab initio band structure calculations in the local-density approximation with the inclusion of spin-orbit coupling. We find that the current model of a single surface layer (√2×√2)R45° reconstruction does not explain all detected features. The observed depth-dependent signal degradation, together with the close quantitative agreement with the slab calculations based on the surface crystal structure as determined by low-energy electron diffraction, reveal that-at a minimum-the subsurface layer also undergoes a similar although weaker reconstruction. This model accounts for all features-a key step in understanding the electronic structure-and indicates a surface-to-bulk progression of the electronic states driven by structural instabilities. Finally, we find no evidence for other phases stemming from either topological bulk properties or, alternatively, the interplay between spin-orbit coupling and the broken symmetry of the surface.