164 research outputs found
Angle, spin, and depth resolved photoelectron spectroscopy on quantum materials
PK gratefully acknowledges The Royal Society for support.The role of X-ray based electron spectroscopies in determining chemical, electronic, and magnetic properties of solids has been well-known for several decades. A powerful approach is angle-resolved photoelectron spectroscopy, whereby the kinetic energy and angle of photoelectrons emitted from a sample surface are measured. This provides a direct measurement of the electronic band structure of crystalline solids. Moreover, it yields powerful insights into the electronic interactions at play within a material and into the control of spin, charge, and orbital degrees of freedom, central pillars of future solid state science. With strong recent focus on research of lower-dimensional materials and modified electronic behavior at surfaces and interfaces, angle-resolved photoelectron spectroscopy has become a core technique in the study of quantum materials. In this review, we provide an introduction to the technique. Through examples from several topical materials systems, including topological insulators, transition metal dichalcogenides, and transition metal oxides, we highlight the types of information which can be obtained. We show how the combination of angle, spin, time, and depth-resolved experiments are able to reveal âhiddenâ spectral features, connected to semiconducting, metallic and magnetic properties of solids, as well as underlining the importance of dimensional effects in quantum materials.PostprintPeer reviewe
Engineering higher order Van Hove singularities in two dimensions: the example of the surface layer of SrRuO
The properties of correlated electron materials are often intricately linked
to Van Hove singularities (VHs) in the vicinity of the Fermi energy. The class
of these VHs is of great importance, with higher order ones -- with power-law
divergence in the density of states -- leaving frequently distinct signatures
in physical properties. We use a new theoretical method to detect and analyse
higher order Van Hove singularities (HOVHs) in two-dimensional materials and
apply it to the electronic structure of the surface layer of SrRuO. We
then constrain a low energy model of the VHs of the surface layer of
SrRuO against angle-resolved photoemission spectroscopy and
quasiparticle interference data to analyse the VHs near the Fermi level. We
show how these VHs can be engineered into HOVHs.Comment: 8 pages including Supplemental Material, 5 figure
Ramifications of Optical Pumping on the Interpretation of Time-Resolved Photoemission Experiments on Graphene
In pump-probe time and angle-resolved photoemission spectroscopy (TR-ARPES)
experiments the presence of the pump pulse adds a new level of complexity to
the photoemission process in comparison to conventional ARPES. This is
evidenced by pump-induced vacuum space-charge effects and surface
photovoltages, as well as multiple pump excitations due to internal reflections
in the sample-substrate system. These processes can severely affect a correct
interpretation of the data by masking the out-of-equilibrium electron dynamics
intrinsic to the sample. In this study, we show that such effects indeed
influence TR-ARPES data of graphene on a silicon carbide (SiC) substrate. In
particular, we find a time- and laser fluence-dependent spectral shift and
broadening of the acquired spectra, and unambiguously show the presence of a
double pump excitation. The dynamics of these effects is slower than the
electron dynamics in the graphene sample, thereby permitting us to deconvolve
the signals in the time domain. Our results demonstrate that complex
pump-related processes should always be considered in the experimental setup
and data analysis.Comment: 9 pages, 4 figure
Changes of Fermi Surface Topology due to the Rhombohedral Distortion in SnTe
Stoichiometric SnTe is theoretically a small gap semiconductor that undergoes
a ferroelectric distortion on cooling. In reality however, crystals are always
non-stoichiometric and metallic; the ferroelectric transition is therefore more
accurately described as a polar structural transition. Here we study the Fermi
surface using quantum oscillations as a function of pressure. We find the
oscillation spectrum changes at high pressure, due to the suppression of the
polar transition and less than 10 kbar is sufficient to stabilize the
undistorted cubic lattice. This is accompanied by a large decrease in the Hall
and electrical resistivity. Combined with our density functional theory (DFT)
calculations and angle resolved photoemission spectroscopy (ARPES) measurements
this suggests the Fermi surface -pockets have lower mobility than the
tubular Fermi surfaces that connect them. Also captured in our DFT calculations
is a small widening of the band gap and shift in density of states for the
polar phase. Additionally we find the unusual phenomenon of a linear
magnetoresistance that exists irrespective of the distortion that we attribute
to regions of the Fermi surface with high curvature.Comment: 8 pages, 5 figure
Ultrafast Dynamics of Massive Dirac Fermions in Bilayer Graphene
Bilayer graphene is a highly promising material for electronic and
optoelectronic applications since it is supporting massive Dirac fermions with
a tuneable band gap. However, no consistent picture of the gap's effect on the
optical and transport behavior has emerged so far, and it has been proposed
that the insulating nature of the gap could be compromised by unavoidable
structural defects, by topological in-gap states, or that the electronic
structure could be altogether changed by many-body effects. Here we directly
follow the excited carriers in bilayer graphene on a femtosecond time scale,
using ultrafast time- and angle-resolved photoemission. We find a behavior
consistent with a single-particle band gap. Compared to monolayer graphene, the
existence of this band gap leads to an increased carrier lifetime in the
minimum of the lowest conduction band. This is in sharp contrast to the second
sub-state of the conduction band, in which the excited electrons decay through
fast, phonon-assisted inter-band transitions.Comment: 5 pages, 4 figure
Hybrid reflections from multiple x-ray scattering in epitaxial oxide films
E.H.S. and D.G.S. acknowledge support by the National Science Foundation (NSF) MRSEC program (DMR-1420620).In numerous symmetric Ξ-2Ξ scans of phase-pure epitaxial complex oxide thin films grown on single-crystal substrates, we observe x-ray diffraction peaks that correspond to neither the film nor the substrate crystal structure. These peaks are the result of multiple, sequential diffraction events that occur from both the film and the substrate. The occurrence of so-called "hybrid" reflections, while described in the literature, is not widely reported within the complex oxide thin-film community. We describe a simple method to predict and identify peaks resulting from hybrid reflections and show examples from epitaxial complex oxide films belonging to three distinct structural types.Publisher PDFPeer reviewe
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Growth of PdCoO2 films with controlled termination by molecular-beam epitaxy and determination of their electronic structure by angle-resolved photoemission spectroscopy
Funding: This paper is primarily supported by the U.S. Department of Energy, office of Basic Sciences, Division of Materials Science and Engineering under Award No. DE-SC0002334. C.P acknowledges support from Air Force Office of Scientific Research grant number FA9550-21-1-0168 and National Science Foundation DMR-2104427. P.K. gratefully acknowledges support from the European Research Council (through the QUESTDO project, 714193). Q.X. acknowledges support from the REU Site: Summer Research Program at PARADIM (Grant No. DMR-2150446).Utilizing the powerful combination of molecular-beam epitaxy (MBE) and angle-resolved photoemission spectroscopy (ARPES), we produce and study the effect of different terminating layers on the electronic structure of the metallic delafossite PdCoO2. Attempts to introduce unpaired electrons and synthesize new antiferromagnetic metals akin to the isostructural compound PdCrO2 have been made by replacing cobalt with iron in PdCoO2 films grown by MBE. Using ARPES, we observe similar bulk bands in these PdCoO2 films with Pd-, CoO2-, and FeO2-termination. Nevertheless, Pd- and CoO2-terminated films show a reduced intensity of surface states. Additionally, we are able to epitaxially stabilize PdFexCo1âxO2 films that show an anomaly in the derivative of the electrical resistance with respect to temperature at 20 K, but do not display pronounced magnetic order. Metallic oxides with the delafossite structure, shown in Fig. 1(a), have drawn significant attention in recent years due to their unique structural and electronic properties. Examples include PtCoO2, which has the highest conductivity per carrier of all materials, and PdCoO2, which has the longest mean free path (exceeding 20 ÎŒm at 4 K) among all known metals.1â3 The in-plane electrical conductivity of PdCoO2 at room temperature, which is about four times higher than that of palladium metal itself, has been argued to arise from electronâphonon scattering mainly occurring within a single, closed, highly dispersive band of primarily palladium character at the Fermi level (EF).1,4â8 The large spin-splitting of the surface state arising from the CoO2 termination, in combination with the layered structure of PdCoO2-based heterostructures makes this system ideal to study itinerant surface electrons driven by inversion-symmetry breaking.9 As for the magnetic properties of delafossites, PdCrO2 is the only highly conducting delafossite material that orders magnetically; it orders antiferromagnetically (AFM) at around 37 K. Focusing on the electronic structure, the single band at the Fermi level with palladium character forms a reconstruction driven by the AFM order from the adjacent CrO2 layer.10â14 Comparing AFM PdCrO2 with nonmagnetic PdCoO2, the spins from Cr3+ interacting inside the CrO2 layer with the palladium monolayers on either side of the CrO2 layer play a critical role in the magnetic state of PdCrO2.13Publisher PDFPeer reviewe
Avoided metallicity in a hole-doped Mott insulator on a triangular lattice
Charge carrier doping of a Mott insulator is known to give rise to a wide
variety of exotic emergent states, from high-temperature superconductivity to
various charge, spin, and orbital orders. The physics underpinning their
evolution is, however, poorly understood. A major challenge is the chemical
complexity associated with traditional routes to the addition or removal of
carriers. Here, we study the Mott insulating CrO layer of the delafossite
oxide PdCrO, where an intrinsic polar catastrophe provides a clean route to
induce substantial doping of the surface layer. Despite this, from scanning
tunneling microscopy and angle-resolved photoemission, we find that the surface
retains an insulating character, but with a modified electronic structure and
the development of a short-range ordered state with a distinct
periodicity. From density
functional theory, we demonstrate how this reflects the formation of an
intricate charge disproportionation that results in an insulating ground state
of the surface layer that is disparate from the hidden Mott insulator found in
the bulk. By applying voltage pulses to the surface layer, we induce
substantial local modifications to this state, which we find relax on a time
scale of tens of minutes, pointing to a glassy nature of the
charge-disproportionated insulator realised here.Comment: manuscript and supplementary, 37 pages in total, 4 figures in the
main text and 9 in the supplementar
Hierarchy of Lifshitz transitions in the surface electronic structure of Sr2RuO4 under uniaxial compression
Funding: We gratefully acknowledge support from the Engineering and Physical Sciences Research Council (Grant Nos. EP/T02108X/1 and EP/R031924/1), the European Research Council (through the QUESTDO project, 714193), and the Leverhulme Trust (Grant No. RL-2016-006). E.A.M., A.Z., and I.M. gratefully acknowledge studentship support from the International Max-Planck Research School for Chemistry and Physics of Quantum Materials. N.K. is supported by a KAKENHI Grants-in-Aids for Scientific Research (Grant Nos.18K04715, and 21H01033), and Core-to-Core Program (No. JPJSCCA20170002) from the Japan Society for the Promotion of Science (JSPS) and by a JST-Mirai Program (Grant No. JPMJMI18A3). APM and CWH acknowledge support from the Deutsche Forschungsgemeinschaft - TRR 435 288 - 422213477 (project A10). We thank Diamond Light Source for access to Beamline I05 (Proposals SI27471 and SI28412), which contributed to the results presented here.We report the evolution of the electronic structure at the surface of the layered perovskiteSr2RuO4 under large in-plane uniaxial compression, leading to anisotropic B1g strains of Δxx â Δyy = â0.9 ± 0.1%. From angle-resolved photoemission, we show how this drives a sequence of Lifshitz transitions, reshaping the low-energy electronic structure and the rich spectrum of van Hove singularities that the surface layer of Sr2RuO4 hosts. From comparison to tight-binding modelling, we find that the strain is accommodated predominantly by bond-length changes rather than modifications of octahedral tilt and rotation angles. Our study sheds new light on the nature of structural distortions at oxide surfaces, and how targeted control of these can be used to tune density of states singularities to the Fermi level, in turn paving the way to the possible realisation of rich collective states at the Sr2RuO4 surface.PostprintPeer reviewe
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Growth of PdCoO2 films with controlled termination by molecular-beam epitaxy and determination of their electronic structure by angle-resolved photoemission spectroscopy
Utilizing the powerful combination of molecular-beam epitaxy (MBE) and angle-resolved photoemission spectroscopy (ARPES), we produce and study the effect of different terminating layers on the electronic structure of the metallic delafossite PdCoO2. Attempts to introduce unpaired electrons and synthesize new antiferromagnetic metals akin to the isostructural compound PdCrO2 have been made by replacing cobalt with iron in PdCoO2 films grown by MBE. Using ARPES, we observe similar bulk bands in these PdCoO2 films with Pd-, CoO2-, and FeO2-termination. Nevertheless, Pd- and CoO2-terminated films show a reduced intensity of surface states. Additionally, we are able to epitaxially stabilize PdFexCo1-xO2 films that show an anomaly in the derivative of the electrical resistance with respect to temperature at 20 K, but do not display pronounced magnetic order
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