34 research outputs found
Angle-resolved and core-level photoemission study of interfacing the topological insulator Bi1.5Sb0.5Te1.7Se1.3 with Ag, Nb and Fe
Interfaces between a bulk-insulating topological insulator (TI) and metallic
adatoms have been studied using high-resolution, angle-resolved and core-level
photoemission. Fe, Nb and Ag were evaporated onto Bi1.5Sb0.5Te1.7Se1.3 (BSTS)
surfaces both at room temperature and 38K. The coverage- and
temperature-dependence of the adsorption and interfacial formation process have
been investigated, highlighting the effects of the overlayer growth on the
occupied electronic structure of the TI. For all coverages at room temperature
and for those equivalent to less than 0.1 monolayer at low temperature all
three metals lead to a downward shift of the TI's bands with respect to the
Fermi level. At room temperature Ag appears to intercalate efficiently into the
van der Waals gap of BSTS, accompanied by low-level substitution of the Te/Se
atoms of the termination layer of the crystal. This Te/Se substitution with
silver increases significantly for low temperature adsorption, and can even
dominate the electrostatic environment of the Bi/Sb atoms in the BSTS
near-surface region. On the other hand, Fe and Nb evaporants remain close to
the termination layer of the crystal. On room temperature deposition, they
initially substitute isoelectronically for Bi as a function of coverage, before
substituting for Te/Se atoms. For low temperature deposition, Fe and Nb are too
immobile for substitution processes and show a behaviour consistent with
clustering on the surface. For both Ag and Fe/Nb, these differing adsorption
pathways leads to the qualitatively similar and remarkable behavior for low
temperature deposition that the chemical potential first moves upward (n-type
dopant behavior) and then downward (p-type behavior) on increasing coverage.Comment: 10 pages, 4 figures. In our Phys. Rev. B manuscript an error was made
in formulating the last sentence of the abstract that, unfortunately, was
missed in the page proofs. Version 2 on arxiv has the correct formulation of
this sentenc
Kondo hybridisation and the origin of metallic states at the (001) surface of SmB6
SmB6, a well-known Kondo insulator, has been proposed to be an ideal
topological insulator with states of topological character located in a clean,
bulk electronic gap, namely the Kondo hybridisation gap. Seeing as the Kondo
gap arises from many body electronic correlations, this would place SmB6 at the
head of a new material class: topological Kondo insulators. Here, for the first
time, we show that the k-space characteristics of the Kondo hybridisation
process is the key to unravelling the origin of the two types of metallic
states observed directly by ARPES in the electronic band structure of
SmB6(001). One group of these states is essentially of bulk origin, and cuts
the Fermi level due to the position of the chemical potential 20 meV above the
lowest lying 5d-4f hybridisation zone. The other metallic state is more
enigmatic, being weak in intensity, but represents a good candidate for a
topological surface state. However, before this claim can be substantiated by
an unequivocal measurement of its massless dispersion relation, our data raises
the bar in terms of the ARPES resolution required, as we show there to be a
strong renormalisation of the hybridisation gaps by a factor 2-3 compared to
theory, following from the knowledge of the true position of the chemical
potential and a careful comparison with the predictions from recent
LDA+Gutzwiler calculations. All in all, these key pieces of evidence act as
triangulation markers, providing a detailed description of the electronic
landscape in SmB6, pointing the way for future, ultrahigh resolution ARPES
experiments to achieve a direct measurement of the Dirac cones in the first
topological Kondo insulator.Comment: 9 pages, 4 Figures and supplementary material (including Movies and
CORPES13 "best prize" poster
Emergence of pseudogap from short-range spin-correlations in electron doped cuprates
Electron interactions are pivotal for defining the electronic structure of
quantum materials. In particular, the strong electron Coulomb repulsion is
considered the keystone for describing the emergence of exotic and/or ordered
phases of quantum matter as disparate as high-temperature superconductivity and
charge- or magnetic-order. However, a comprehensive understanding of
fundamental electronic properties of quantum materials is often complicated by
the appearance of an enigmatic partial suppression of low-energy electronic
states, known as the pseudogap. Here we take advantage of ultrafast
angle-resolved photoemission spectroscopy to unveil the temperature evolution
of the low-energy density of states in the electron-doped cuprate
NdCeCuO, an emblematic system where
the pseudogap intertwines with magnetic degrees of freedom. By photoexciting
the electronic system across the pseudogap onset temperature T*, we report the
direct relation between the momentum-resolved pseudogap spectral features and
the spin-correlation length with an unprecedented sensitivity. This transient
approach, corroborated by mean field model calculations, allows us to establish
the pseudogap in electron-doped cuprates as a precursor to the incipient
antiferromagnetic order even when long-range antiferromagnetic correlations are
not established, as in the case of optimal doping.Comment: 17 pages, 3 figure
Influence of Spin Orbit Coupling in the Iron-Based Superconductors
We report on the influence of spin-orbit coupling (SOC) in the Fe-based
superconductors (FeSCs) via application of circularly-polarized spin and
angle-resolved photoemission spectroscopy. We combine this technique in
representative members of both the Fe-pnictides and Fe-chalcogenides with ab
initio density functional theory and tight-binding calculations to establish an
ubiquitous modification of the electronic structure in these materials imbued
by SOC. The influence of SOC is found to be concentrated on the hole pockets
where the superconducting gap is generally found to be largest. This result
contests descriptions of superconductivity in these materials in terms of pure
spin-singlet eigenstates, raising questions regarding the possible pairing
mechanisms and role of SOC therein.Comment: For supplementary information, see
http://qmlab.ubc.ca/ARPES/PUBLICATIONS/articles.htm
Stable Weyl points, trivial surface states and particle-hole compensation in WP2
A possible connection between extremely large magneto-resistance and the
presence of Weyl points has garnered much attention in the study of topological
semimetals. Exploration of these concepts in transition metal phosphide WP2 has
been complicated by conflicting experimental reports. Here we combine
angle-resolved photoemission spectroscopy (ARPES) and density functional theory
(DFT) calculations to disentangle surface and bulk contributions to the ARPES
intensity, the superposition of which has plagued the determination of the
electronic structure in WP2. Our results show that while the hole- and
electron-like Fermi surface sheets originating from surface states have
different areas, the bulk-band structure of WP2 is electron-hole-compensated in
agreement with DFT. Furthermore, the detailed band structure is compatible with
the presence of at least 4 temperature-independent Weyl points, confirming the
topological nature of WP2 and its stability against lattice distortions.Comment: 6 pages, 4 figure
Collapse of superconductivity in cuprates via ultrafast quenching of phase coherence
The possibility of driving phase transitions in low-density condensates
through the loss of phase coherence alone has far-reaching implications for the
study of quantum phases of matter. This has inspired the development of tools
to control and explore the collective properties of condensate phases via phase
fluctuations. Electrically-gated oxide interfaces, ultracold Fermi atoms, and
cuprate superconductors, which are characterized by an intrinsically small
phase-stiffness, are paradigmatic examples where these tools are having a
dramatic impact. Here we use light pulses shorter than the internal
thermalization time to drive and probe the phase fragility of the
BiSrCaCuO cuprate superconductor, completely melting
the superconducting condensate without affecting the pairing strength. The
resulting ultrafast dynamics of phase fluctuations and charge excitations are
captured and disentangled by time-resolved photoemission spectroscopy. This
work demonstrates the dominant role of phase coherence in the
superconductor-to-normal state phase transition and offers a benchmark for
non-equilibrium spectroscopic investigations of the cuprate phase diagram.Comment: 24 pages, 9 figures, Main Text and Supplementary Informatio