Spectroscopy of a Tunable Moir\'e System with a Correlated and Topological Flat Band

Moir\'e superlattices created by the twisted stacking of two-dimensional crystalline monolayers can host electronic bands with flat energy dispersion in which interaction among electrons is strongly enhanced. These superlattices can also create non-trivial electronic band topologies making them a platform for study of many-body topological quantum states. Among the moir\'e systems realized to date, there are those predicted to have band structures and properties which can be controlled with a perpendicular electric field. The twisted double bilayer graphene (TDBG), where two Bernal bilayer graphene are stacked with a twist angle, is such a tunable moir\'e system, for which partial filling of its flat band, transport studies have found correlated insulating states. Here we use gate-tuned scanning tunneling spectroscopy (GT-STS) to directly demonstrate the tunability of the band structure of TDBG with an electric field and to show spectroscopic signatures of both electronic correlations and topology for its flat band. Our spectroscopic experiments show excellent agreement with a continuum model of TDBG band structure and reveal signatures of a correlated insulator gap at partial filling of its isolated flat band. The topological properties of this flat band are probed with the application of a magnetic field, which leads to valley polarization and the splitting of Chern bands that respond strongly to the field with a large effective g-factor. Our experiments advance our understanding of the properties of TDBG and set the stage for further investigations of correlation and topology in such tunable moir\'e systems.Comment: 13 pages, 5 figures and supplementary informatio

High mobility in a van der Waals layered antiferromagnetic metal

Magnetic van der Waals (vdW) materials have been heavily pursued for fundamental physics as well as for device design. Despite the rapid advances, so far magnetic vdW materials are mainly insulating or semiconducting, and none of them possesses a high electronic mobility - a property that is rare in layered vdW materials in general. The realization of a magnetic high-mobility vdW material would open the possibility for novel magnetic twistronic or spintronic devices. Here we report very high carrier mobility in the layered vdW antiferromagnet GdTe3. The electron mobility is beyond 60,000 cm2 V-1 s-1, which is the highest among all known layered magnetic materials, to the best of our knowledge. Among all known vdW materials, the mobility of bulk GdTe3 is comparable to that of black phosphorus, and is only surpassed by graphite. By mechanical exfoliation, we further demonstrate that GdTe3 can be exfoliated to ultrathin flakes of three monolayers, and that the magnetic order and relatively high mobility is retained in approximately 20-nm-thin flakes

Designing optoelectronic properties by on-surface synthesis: formation and electronic structure of an iron-terpyridine macromolecular complex

Supramolecular chemistry protocols applied on surfaces offer compelling avenues for atomic scale control over organic-inorganic interface structures. In this approach, adsorbate-surface interactions and two-dimensional confinement can lead to morphologies and properties that differ dramatically from those achieved via conventional synthetic approaches. Here, we describe the bottom-up, on-surface synthesis of one-dimensional coordination nanostructures based on an iron (Fe)-terpyridine (tpy) interaction borrowed from functional metal-organic complexes used in photovoltaic and catalytic applications. Thermally activated diffusion of sequentially deposited ligands and metal atoms, and intra-ligand conformational changes, lead to Fe-tpy coordination and formation of these nanochains. Low-temperature Scanning Tunneling Microscopy and Density Functional Theory were used to elucidate the atomic-scale morphology of the system, providing evidence of a linear tri-Fe linkage between facing, coplanar tpy groups. Scanning Tunneling Spectroscopy reveals highest occupied orbitals with dominant contributions from states located at the Fe node, and ligand states that mostly contribute to the lowest unoccupied orbitals. This electronic structure yields potential for hosting photo-induced metal-to-ligand charge transfer in the visible/near-infrared. The formation of this unusual tpy/tri-Fe/tpy coordination motif has not been observed for wet chemistry synthesis methods, and is mediated by the bottom-up on-surface approach used here

Evidence for a Monolayer Excitonic Insulator

The interplay between topology and correlations can generate a variety of unusual quantum phases, many of which remain to be explored. Recent advances have identified monolayer WTe2 as a promising material for exploring such interplay in a highly tunable fashion. The ground state of this two-dimensional (2D) crystal can be electrostatically tuned from a quantum spin Hall insulator (QSHI) to a superconductor. However, much remains unknown about the nature of these ground states, including the gap-opening mechanism of the insulating state. Here we report systematic studies of the insulating phase in WTe2 monolayer and uncover evidence supporting that the QSHI is also an excitonic insulator (EI). An EI, arising from the spontaneous formation of electron-hole bound states (excitons), is a largely unexplored quantum phase to date, especially when it is topological. Our experiments on high-quality transport devices reveal the presence of an intrinsic insulating state at the charge neutrality point (CNP) in clean samples. The state exhibits both a strong sensitivity to the electric displacement field and a Hall anomaly that are consistent with the excitonic pairing. We further confirm the correlated nature of this charge-neutral insulator by tunneling spectroscopy. Our results support the existence of an EI phase in the clean limit and rule out alternative scenarios of a band insulator or a localized insulator. These observations lay the foundation for understanding a new class of correlated insulators with nontrivial topology and identify monolayer WTe2 as a promising candidate for exploring quantum phases of ground-state excitons.Comment: 37 pages, 4 Main Figures + 15 SI Figur

Probing many-body scattering in Cu(111) via FT-STS : understanding local perturbations from the collective signatures of a 2D electron gas

Surface states of close-packed noble metals form an approximate two-dimensional electron gas whose many-body signatures can be locally probed using a scanning tunneling microscope (STM). In this work I present a study of the Cu(111) surface state with high-resolution Fourier Transform Scanning Tunneling Spectroscopy (FT-STS), and for the first time demonstrate that the energy dispersion and quasi-particle lifetime of the surface states can be accurately quantified in both the occupied and unoccupied states. The scat-tering phase-shift imposed by defect potentials is then extracted in Fourier space, which is consistent with previous real space analyses. This result is later used in the T-matrix simulation of the density of states that gives an accurate description of our data. Finally, I report that in dilute Co/Cu(111) where the absence of time-reversal symmetry allows for spin-flip scattering, spin-conserving scattering dominates the FT-STS signal.Science, Faculty ofPhysics and Astronomy, Department ofGraduat

Probing spontaneous symmetry-breaking in graphene quantum Hall wavefunctions with a Scanning Tunneling Microscope

In quantum materials, electrons confined in two dimensions (2D) under high magnetic fields reside in discrete energy levels known as Landau levels. Enhanced Coulomb repulsion in Landau levels beget a myriad of exotic phases with spontaneous broken-symmetry which favor ordering of the electron’s flavors, such as valley or spin, also known as quantum Hall ferromagnetism (QHFM). These systems are typically studied with bulk measurements unable to probe the microscopic order of electrons. Graphene’s zeroth Landau level (ZLL) with four-fold valley-spin degeneracy is a model for studying these phases, and yet after more than a decade of experiments the nature of its broken-symmetry states is still not understood. In this work, we used a scanning tunneling microscope (STM) to image the electronic wavefunction of the ZLL in ultra-clean graphene devices. Specifically, we discovered that near charge-neutrality the ZLL is in a coherent superposition of the two valleys, and detected a topological defect in the electron excitation spectrum. Moreover, we investigate lifting of the orbital degeneracy of the ZLL by charged impurities and compare it to perturbative models. By preparing non-invasive STM probes, we quantify the energy landscape of the fractional quantum Hall states, and detect first and second-order transitions in the valley phase diagram of the ZLL. These results set a benchmark for imaging other interacting 2D systems that harbor broken-symmetry states, including non-Abelian anyons in bilayer graphene or the broken-symmetry states recently discovered in moire materials

Spectroscopy of a tunable moiré system with a correlated and topological flat band.

Moiré superlattices created by the twisted stacking of two-dimensional crystals can host electronic bands with flat energy dispersion in which enhanced interactions promote correlated electron states. The twisted double bilayer graphene (TDBG), where two Bernal bilayer graphene are stacked with a twist angle, is such a moiré system with tunable flat bands. Here, we use gate-tuned scanning tunneling spectroscopy to directly demonstrate the tunability of the band structure of TDBG with an electric field and to show spectroscopic signatures of electronic correlations and topology for its flat band. Our spectroscopic experiments are in agreement with a continuum model of TDBG band structure and reveal signatures of a correlated insulator gap at partial filling of its isolated flat band. The topological properties of this flat band are probed with the application of a magnetic field, which leads to valley polarization and the splitting of Chern bands with a large effective g-factor