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 “hidden” 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 Sr2_2RuO4_4

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 Sr$_2$RuO$_4$. We then constrain a low energy model of the VHs of the surface layer of Sr$_2$RuO$_4$ 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 $L$-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 θ-2θ 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

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 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. 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 μm at 4 K) among all known metals.1–3 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 electron–phonon scattering mainly occurring within a single, closed, highly dispersive band of primarily palladium character at the Fermi level (EF).1,4–8 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.10–14 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

Charge doping into spin minority states mediates doubling of TCT_\mathrm{C} in ferromagnetic CrGeTe3_3

The recent discovery of the persistence of long-range magnetic order when van der Waals layered magnets are thinned towards the monolayer limit has provided a tunable platform for the engineering of novel magnetic structures and devices. Here, we study the evolution of the electronic structure of CrGeTe$_3$ as a function of electron doping in the surface layer. From angle-resolved photoemission spectroscopy, we observe spectroscopic fingerprints that this electron doping drives a marked increase in $T_\mathrm{C}$, reaching values more than double that of the undoped material, in agreement with recent studies using electrostatic gating. Together with density functional theory calculations and Monte Carlo simulations, we show that, surprisingly, the increased $T_\mathrm{C}$ is mediated by the population of spin-minority Cr $t_{2g}$ states, forming a half-metallic 2D electron gas at the surface. We show how this promotes a novel variant of double exchange, and unlocks a significant influence of the Ge -- which was previously thought to be electronically inert in this system -- in mediating Cr-Cr exchange.Comment: 10 pages including supplementary informatio

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$_2$ layer of the delafossite oxide PdCrO$_2$, 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 $(\sqrt{7}\times\sqrt{7})\mathrm{R}\pm 19.1^\circ$ 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

Surface reconstructions and electronic structure of metallic delafossite thin films

Funding: This paper was primarily supported by the U.S. Department of Energy, Office of Basic Sciences, Division of Materials Science and Engineering under Award No. DE-SC0002334. This research was funded in part by the Gordon and Betty Moore Foundation’s EPiQS Initiative (Grant Nos. GBMF3850 and GBMF9073 to Cornell University). This paper made use of the Cornell Center for Materials Research shared facilities, which are supported through the NSF Materials Research Science and Engineering Centers Program (Grant No. DMR-1719875). B.D.F., M.R.B., and B.P. acknowledge support from the National Science Foundation Platform for the Accelerated Realization, Analysis, and Discovery of Interface Materials (PARADIM) under Cooperative Agreement No. DMR-2039380. This paper also made use of the Cornell Energy Systems Institute Shared Facilities partly sponsored by the NSF (Grant No. MRI DMR-1338010) and the Kavli Institute at Cornell. Substrate preparation was performed, in part, at the Cornell NanoScale Facility, a member of the National Nanotechnology Coordinated Infrastructure, which is supported by the NSF (Grant No. NNCI-2025233). P.K. acknowledges support from the European Research Council (through the QUESTDO project, 714193) and The Leverhulme Trust (grant No. RPG-2023-256).The growing interest in the growth and study of thin films of low-dimensional metallic delafossites, with the general formula ABO2, is driven by their potential to exhibit electronic and magnetic characteristics that are not accessible in bulk systems. The layered structure of these compounds introduces unique surface states as well as electronic and structural reconstructions, making the investigation of their surface behavior pivotal to understanding their intrinsic electronic structure. In this work, we study the surface phenomena of epitaxially grown PtCoO2, PdCoO2, and PdCrO2 films, utilizing a combination of molecular-beam epitaxy and angle-resolved photoemission spectroscopy. Through precise control of surface termination and treatment, we discover a pronounced √3 x √3 surface reconstruction in PtCoO2 films and PdCoO2 films, alongside a 2 × 2 surface reconstruction observed in PdCrO2 films. These reconstructions have not been reported in prior studies of delafossites. Furthermore, our computational investigations demonstrate the BO2 surface’s relative stability compared to the A-terminated surface and the significant reduction in surface energy facilitated by the reconstruction of the A-terminated surface. These experimental and theoretical insights illuminate the complex surface dynamics in metallic delafossites, paving the way for future explorations of their distinctive properties in low-dimensional studies.Peer reviewe