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

Interfacial Electron-Phonon Coupling Constants Extracted from Intrinsic Replica Bands in Monolayer FeSe/SrTiO3_3

The observation of replica bands by angle-resolved photoemission spectroscopy has ignited interest in the study of electron-phonon coupling at low carrier densities, particularly in monolayer FeSe/SrTiO$_3$, where the appearance of replica bands has motivated theoretical work suggesting that the interfacial coupling of electrons in the FeSe layer to optical phonons in the SrTiO$_3$ substrate might contribute to the enhanced superconducting pairing temperature. Alternatively, it has also been recently proposed that such replica bands might instead originate from extrinsic final state losses associated with the photoemission process. Here, we perform a quantitative examination of replica bands in monolayer FeSe/SrTiO$_3$, where we are able to conclusively demonstrate that the replica bands are indeed signatures of intrinsic electron-boson coupling, and not associated with final state effects. A detailed analysis of the energy splittings between the higher-order replicas, as well as other self-energy effects, allow us to determine that the interfacial electron-phonon coupling in the system corresponds to a value of $\lambda = 0.19 \pm 0.02$.Comment: 5 pages, 4 figure

Strong interlayer interactions in bilayer and trilayer moiré superlattices

Moiré superlattices constructed from transition metal dichalcogenides have demonstrated a series of emergent phenomena, including moiré excitons, flat bands, and correlated insulating states. All of these phenomena depend crucially on the presence of strong moiré potentials, yet the properties of these moiré potentials, and the mechanisms by which they can be generated, remain largely open questions. Here, we use angle-resolved photoemission spectroscopy with submicron spatial resolution to investigate an aligned WS2/WSe2 moiré superlattice and graphene/WS2/WSe2 trilayer heterostructure. Our experiments reveal that the hybridization between moiré bands in WS2/WSe2 exhibits an unusually large momentum dependence, with the splitting between moiré bands at the Γ point more than an order of magnitude larger than that at K point. In addition, we discover that the same WS2/WSe2 superlattice can imprint an unexpectedly large moiré potential on a third, separate layer of graphene (g/WS2/WSe2), suggesting new avenues for engineering two-dimensional moiré superlattices

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

Absence of 3a03a_0 Charge Density Wave Order in the Infinite Layer Nickelates

A hallmark of many unconventional superconductors is the presence of many-body interactions which give rise to broken symmetry states intertwined with superconductivity. Recent resonant soft x-ray scattering experiments report commensurate $3a_0$ charge density wave order in the infinite layer nickelates, which has important implications regarding the universal interplay between charge order and superconductivity in both the cuprates and nickelates. Here, we present x-ray scattering and spectroscopy measurements on a series of NdNiO$_{2+x}$ samples which reveal that the signatures of charge density wave order are absent in fully reduced, single-phase NdNiO$_2$. The $3a_0$ superlattice peak instead originates from a partially reduced impurity phase where excess apical oxygens form ordered rows with 3 unit cell periodicity. The absence of any observable charge density wave order in NdNiO$_2$ highlights a crucial difference between the phase diagrams of the cuprate and nickelate superconductors.Comment: Main Text: 8 pages, 4 figures. Supplemental: 12 pages, 12 figure

Atomically smooth films of CsSb: a chemically robust visible light photocathode

Alkali antimonide semiconductor photocathodes provide a promising platform for the generation of high brightness electron beams, which are necessary for the development of cutting-edge probes including x-ray free electron lasers and ultrafast electron diffraction. However, to harness the intrinsic brightness limits in these compounds, extrinsic degrading factors, including surface roughness and contamination, must be overcome. By exploring the growth of CsxSb thin films monitored by in situ electron diffraction, the conditions to reproducibly synthesize atomically smooth films of CsSb on 3C-SiC (100) and graphene coated TiO2 (110) substrates are identified, and detailed structural, morphological, and electronic characterization is presented. These films combine high quantum efficiency in the visible (up to 1.2% at 400 nm), an easily accessible photoemission threshold of 550 nm, low surface roughness (down to 600 pm on a 1 um scale), and a robustness against oxidation up to 15 times greater then Cs3Sb. These properties suggest that CsSb has the potential to operate as an alternative to Cs$_3$Sb in electron source applications where the demands of the vacuum environment might otherwise preclude the use of traditional alkali antimonides.Comment: 11 pages, 6 figures, 1 tabl

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

TOXICITY OF CADMIUM IN PREGNANT RATS FED A ZINC-DEFICIENT DIET.

Abstract not availabl

Synthesis, Characterization, and Scattering Measurements of the Perovskite and Infinite Layer Nickelates

227 pagesMaterials hosting unconventional and high Tc superconductivity demonstrate phase diagrams of extreme complexity where superconductivity cooperates and competes with other ground states involving entangled charge, lattice, and spin degrees of freedom. Joining the Fe-based and Cu-based families of superconductors, the newly realized infinite layer nickelates, RENiO2, RE=La, Nd, Pr, offer a novel platform to study the nature of high Tc superconductivity and the normal state from which it emerges. However, this class of materials poses significant synthesis challenges which must be overcome to investigate their intrinsic ground state properties. In this thesis we discuss the synthesis of perovskite and infinite layer nickelates by reactive oxide molecular-beam epitaxy and activated hydrogen reduction. First, we examine the growth of the perovskite nickelate NdNiO3 by ozone assisted molecular-beam epitaxy and describe a novel reduction procedure, using atomic hydrogen, for transforming the perovskite to the infinite layer phase. We illustrate the utility of atomic hydrogen reduction for producing highly crystalline undoped NdNiO2 by a variety of experimental probes. Second, we explore the properties of high quality NdNiO3 thin films in the ultrathin limit, revealing a previously undetected thickness driven structural transformation and suppression of the metal-to-insulator transition in films under tensile strain. Finally, we re-examine the presence of charge density wave order in NdNiO2, arguing that the previously reported 3a0 ordering can be attributed to interstitial oxygen ordering rather than a correlation driven density wave. These results serve to clarify some of the essential features of the phase diagrams of both the perovskite and infinite layer nickelates