Roland J. Koch
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View article: Discovery of interlayer plasmon polaron in graphene/WS$_2$ heterostructures
Discovery of interlayer plasmon polaron in graphene/WS$_2$ heterostructures Open
Harnessing electronic excitations involving coherent coupling to bosonic modes is essential for the design and control of emergent phenomena in quantum materials [1]. In situations where charge carriers induce a lattice distortion due to t…
View article: Nature of the current-induced insulator-to-metal transition in Ca$_2$RuO$_4$ as revealed by transport-ARPES
Nature of the current-induced insulator-to-metal transition in Ca$_2$RuO$_4$ as revealed by transport-ARPES Open
The Mott insulator Ca$_2$RuO$_4$ exhibits a rare insulator-to-metal transition (IMT) induced by DC current. While structural changes associated with this transition have been tracked by neutron diffraction, Raman scattering, and x-ray spec…
View article: Layer-Dependent Interaction Effects in the Electronic Structure of Twisted Bilayer Graphene Devices
Layer-Dependent Interaction Effects in the Electronic Structure of Twisted Bilayer Graphene Devices Open
Near the magic angle, strong correlations drive many intriguing phases in twisted bilayer graphene (tBG) including unconventional superconductivity and chern insulation. Whether correlations can tune symmetry breaking phases in tBG at inte…
View article: Nanoscale View of Engineered Massive Dirac Quasiparticles in Lithographic Superstructures
Nanoscale View of Engineered Massive Dirac Quasiparticles in Lithographic Superstructures Open
Massive Dirac fermions are low-energy electronic excitations characterized by a hyperbolic band dispersion. They play a central role in several emerging physical phenomena such as topological phase transitions, anomalous Hall effects, and …
View article: Large-Area Intercalated 2D-Pb/Graphene Heterostructure as a Platform for Generating Spin-Orbit Torque
Large-Area Intercalated 2D-Pb/Graphene Heterostructure as a Platform for Generating Spin-Orbit Torque Open
A scalable platform to synthesize ultrathin heavy metals may enable high efficiency charge-to-spin conversion for next-generation spintronics. Here we report the synthesis of air-stable, epitaxially registered monolayer Pb underneath bilay…
View article: Visualizing electron localization of WS <sub>2</sub> /WSe <sub>2</sub> moiré superlattices in momentum space
Visualizing electron localization of WS <sub>2</sub> /WSe <sub>2</sub> moiré superlattices in momentum space Open
Nano-ARPES reveals localization in the valence band of moiré tungsten disulfide/tungsten diselenide bilayers.
View article: Visualizing band structure hybridization and superlattice effects in twisted MoS$_2$/WS$_2$ heterobilayers
Visualizing band structure hybridization and superlattice effects in twisted MoS$_2$/WS$_2$ heterobilayers Open
A mismatch of atomic registries between single-layer transition metal dichalcogenides (TMDs) in a two dimensional van der Waals heterostructure produces a moiré superlattice with a periodic potential, which can be fine-tuned by introducing…
View article: Tunable Two-Dimensional Group-III Metal Alloys
Tunable Two-Dimensional Group-III Metal Alloys Open
Chemically stable quantum-confined 2D metals are of interest in next-generation nanoscale quantum devices. Bottom-up design and synthesis of such metals could enable the creation of materials with tailored, on-demand, electronic and optica…
View article: Photon-counting MCP/Timepix detectors for soft X-ray imaging and spectroscopic applications
Photon-counting MCP/Timepix detectors for soft X-ray imaging and spectroscopic applications Open
Detectors with microchannel plates (MCPs) provide unique capabilities to detect single photons with high spatial (<10 µm) and timing (<25 ps) resolution. Although this detection technology was originally developed for applications with low…
View article: Switchable X-Ray Orbital Angular Momentum from an Artificial Spin Ice
Switchable X-Ray Orbital Angular Momentum from an Artificial Spin Ice Open
The orbital angular momentum (OAM) of visible and infrared photons has been widely studied and practically applied. Now, there is rapidly growing interest in X-ray OAM for imaging and probing materials at the nanoscale. Recently, a team of…
View article: Photophysics\nand Electronic Structure of Lateral Graphene/MoS<sub>2</sub> and Metal/MoS<sub>2</sub> Junctions
Photophysics\nand Electronic Structure of Lateral Graphene/MoS<sub>2</sub> and Metal/MoS<sub>2</sub> Junctions Open
Integration\nof semiconducting transition metal dichalcogenides\n(TMDs) into functional optoelectronic circuitries requires an understanding\nof the charge transfer across the interface between the TMD and the\ncontacting material. Here, w…
View article: Photophysics and Electronic Structure of Lateral Graphene/MoS<sub>2</sub> and Metal/MoS<sub>2</sub> Junctions
Photophysics and Electronic Structure of Lateral Graphene/MoS<sub>2</sub> and Metal/MoS<sub>2</sub> Junctions Open
Integration of semiconducting transition metal dichalcogenides (TMDs) into functional optoelectronic circuitries requires an understanding of the charge transfer across the interface between the TMD and the contacting material. Here, we us…
View article: K-means-driven Gaussian Process data collection for angle-resolved photoemission spectroscopy
K-means-driven Gaussian Process data collection for angle-resolved photoemission spectroscopy Open
We propose the combination of k-means clustering with Gaussian Process (GP) regression in the analysis and exploration of 4D angle-resolved photoemission spectroscopy (ARPES) data. Using cluster labels as the driving metric on which the GP…
View article: Photo-physics and electronic structure of lateral graphene/MoS2 and metal/MoS2 junctions
Photo-physics and electronic structure of lateral graphene/MoS2 and metal/MoS2 junctions Open
Integration of semiconducting transition metal dichalcogenides (TMDs) into functional optoelectronic circuitries requires an understanding of the charge transfer across the interface between the TMD and the contacting material. Here, we us…
View article: Momentum-resolved view of highly tunable many-body effects in a graphene/hBN field-effect device
Momentum-resolved view of highly tunable many-body effects in a graphene/hBN field-effect device Open
Integrating the carrier tunability of a functional two-dimensional material electronic device with a direct probe of energy- and momentum-resolved electronic excitations is essential to gain insights on how many-body interactions are influ…
View article: Visualization of the flat electronic band in twisted bilayer graphene near the magic angle twist
Visualization of the flat electronic band in twisted bilayer graphene near the magic angle twist Open
Bilayer graphene was theorized to host a moire miniband with flat dispersion if the layers are stacked at specific twist angles known as the magic angles. Recently, such twisted bilayer graphene (tBLG) with the first magic angle twist was …
View article: Rigid Band Shifts in Two-Dimensional Semiconductors through External Dielectric Screening
Rigid Band Shifts in Two-Dimensional Semiconductors through External Dielectric Screening Open
We investigate the effects of external dielectric screening on the electronic dispersion and the band gap in the atomically thin, quasi-two-dimensional (2D) semiconductor WS_{2} using angle-resolved photoemission and optical spectroscopies…
View article: Tunable electronic structure in gallium chalcogenide van der Waals compounds
Tunable electronic structure in gallium chalcogenide van der Waals compounds Open
Transition-metal monochalcogenides comprise a class of two-dimensional materials with electronic band gaps that are highly sensitive to material thickness and chemical composition. Here, we explore the tunability of the electronic excitati…
View article: The Itinerant 2D Electron Gas of the Indium Oxide (111) Surface: Implications for Carbon‐ and Energy‐Conversion Applications
The Itinerant 2D Electron Gas of the Indium Oxide (111) Surface: Implications for Carbon‐ and Energy‐Conversion Applications Open
Transparent conducting oxides (TCO) have integral and emerging roles in photovoltaic, thermoelectric energy conversion, and more recently, photocatalytic systems. The functional properties of TCOs, and thus their role in these applications…
View article: How Substitutional Point Defects in Two-Dimensional WS<sub>2</sub> Induce Charge Localization, Spin–Orbit Splitting, and Strain
How Substitutional Point Defects in Two-Dimensional WS<sub>2</sub> Induce Charge Localization, Spin–Orbit Splitting, and Strain Open
Control of impurity concentrations in semiconducting materials is essential to device technology. Because of their intrinsic confinement, the properties of two-dimensional semiconductors such as transition metal dichalcogenides (TMDs) are …
View article: The Dirac nodal line network in non-symmorphic rutile semimetal RuO$_2$
The Dirac nodal line network in non-symmorphic rutile semimetal RuO$_2$ Open
We employ angle resolved photoemission spectroscopy (ARPES) to investigate the Fermi surface of RuO$_2$. We find a network of two Dirac nodal lines (DNL) as previously predicted in theory, where the valence- and conduction bands touch alon…
View article: Confinement Heteroepitaxy: Realizing Atomically Thin, Half-van der Waals Materials
Confinement Heteroepitaxy: Realizing Atomically Thin, Half-van der Waals Materials Open
Three-dimensional epitaxial heterostructures are based on covalently-bonded interfaces, whereas those from 2-dimensional (2D) materials exhibit van der Waals interactions. Under the right conditions, however, material structures with mixed…
View article: Orbital Fingerprint of Topological Fermi Arcs in the Weyl Semimetal TaP
Orbital Fingerprint of Topological Fermi Arcs in the Weyl Semimetal TaP Open
The monopnictides TaAs and TaP are well-established Weyl semimetals. Yet, a precise assignment of Fermi arcs, accommodating the predicted chiral charge of the bulk Weyl points, has been difficult in these systems, and the topological chara…
View article: Effects of Defects on Band Structure and Excitons in WS<sub>2</sub> Revealed by Nanoscale Photoemission Spectroscopy
Effects of Defects on Band Structure and Excitons in WS<sub>2</sub> Revealed by Nanoscale Photoemission Spectroscopy Open
Two-dimensional materials with engineered composition and structure will provide designer materials beyond conventional semiconductors. However, the potentials of defect engineering remain largely untapped, because it hinges on a precise u…
View article: Electronic structure of exfoliated and epitaxial hexagonal boron nitride
Electronic structure of exfoliated and epitaxial hexagonal boron nitride Open
Hexagonal boron nitride (hBN) is an essential component in van der Waals\nheterostructures as it provides high quality and weakly interacting interfaces\nthat preserve the electronic properties of adjacent materials. While exfoliated\nflak…
View article: Multimodal spectromicroscopy of monolayer WS <sub>2</sub> enabled by ultra-clean van der Waals epitaxy
Multimodal spectromicroscopy of monolayer WS <sub>2</sub> enabled by ultra-clean van der Waals epitaxy Open
Van der Waals epitaxy enables the integration of 2D transition metal dichalcogenides with other layered materials to form heterostructures with atomically sharp interfaces. However, the ability to fully utilize and understand these materia…