Infrared Spectroscopy of Phase Transitions in the Lowest Landau Levels of Bilayer Graphene.

We perform infrared magnetospectroscopy of Landau level (LL) transitions in dual-gated bilayer graphene. At ν=4 when the zeroth LL (octet) is filled, two resonances are observed indicating the opening of a gap. At ν=0 when the octet is half-filled, multiple resonances disperse nonmonotonically with increasing displacement field, D, perpendicular to the sheet, showing a phase transition at modest displacement fields from a canted antiferromagnet (CAFM) to the layer-polarized state, with a gap that opens linearly in D.

Isotope engineering for spin defects in van der Waals materials.

Spin defects in van der Waals materials offer a promising platform for advancing quantum technologies. Here, we propose and demonstrate a powerful technique based on isotope engineering of host materials to significantly enhance the coherence properties of embedded spin defects. Focusing on the recently-discovered negatively charged boron vacancy center ([Formula: see text]) in hexagonal boron nitride (hBN), we grow isotopically purified hBN crystals.

Coherent dynamics of strongly interacting electronic spin defects in hexagonal boron nitride.

Optically active spin defects in van der Waals materials are promising platforms for modern quantum technologies. Here we investigate the coherent dynamics of strongly interacting ensembles of negatively charged boron-vacancy ([Formula: see text]) centers in hexagonal boron nitride (hBN) with varying defect density. By employing advanced dynamical decoupling sequences to selectively isolate different dephasing sources, we observe more than 5-fold improvement in the measured coherence times across all hBN samples.

Design and optimization of thin-film tungsten (W)-diamond target for multi-pixel X-ray sources.

Background: Emerging multi-pixel X-ray source technology enables new designs for X-ray imaging systems. The power of multi-pixel X-ray sources with a fixed anode is limited by focal spot power density.

Purpose: The purpose of this study is to optimize the W-diamond target and predict its performance in multi-pixel X-ray sources.

Ultrasharp Lateral p-n Junctions in Modulation-Doped Graphene.

We demonstrate ultrasharp (≲10 nm) lateral p-n junctions in graphene using electronic transport, scanning tunneling microscopy, and first-principles calculations. The p-n junction lies at the boundary between differentially doped regions of a graphene sheet, where one side is intrinsic and the other is charge-doped by proximity to a flake of α-RuCl across a thin insulating barrier. We extract the p-n junction contribution to the device resistance to place bounds on the junction width.

Modulation Doping via a Two-Dimensional Atomic Crystalline Acceptor.

Two-dimensional nanoelectronics, plasmonics, and emergent phases require clean and local charge control, calling for layered, crystalline acceptors or donors. Our Raman, photovoltage, and electrical conductance measurements combined with calculations establish the large work function and narrow bands of α-RuCl enable modulation doping of exfoliated single and bilayer graphene, chemical vapor deposition grown graphene and WSe, and molecular beam epitaxy grown EuS. We further demonstrate proof of principle photovoltage devices, control via twist angle, and charge transfer through hexagonal boron nitride.

Electronic transport properties of a lithium-decorated ZrTe thin film.

Through a combination of single crystal growth, experiments involving in situ deposition of surface adatoms, and complimentary modeling, we examine the electronic transport properties of lithium-decorated ZrTe thin films. We observe that the surface states in ZrTe are robust against Li adsorption. Both the surface electron density and the associated Berry phase are remarkably robust to adsorption of Li atoms.

Optically Driven Magnetic Phase Transition of Monolayer RuCl.

Strong light-matter interactions within nanoscale structures offer the possibility of optically controlling material properties. Motivated by the recent discovery of intrinsic long-range magnetic order in two-dimensional materials, which allow for the creation of novel magnetic devices of unprecedented small size, we predict that light can couple with magnetism and efficiently tune magnetic orders of monolayer ruthenium trichloride (RuCl). First-principles calculations show that both free carriers and optically excited electron-hole pairs can switch monolayer RuCl from a proximate spin-liquid phase to a stable ferromagnetic phase.

Many-Particle Effects in the Cyclotron Resonance of Encapsulated Monolayer Graphene.

We study the infrared cyclotron resonance of high-mobility monolayer graphene encapsulated in hexagonal boron nitride, and simultaneously observe several narrow resonance lines due to interband Landau-level transitions. By holding the magnetic field strength B constant while tuning the carrier density n, we find the transition energies show a pronounced nonmonotonic dependence on the Landau-level filling factor, ν∝n/B. This constitutes direct evidence that electron-electron interactions contribute to the Landau-level transition energies in graphene, beyond the single-particle picture.

Observation of anomalous phonon softening in bilayer graphene.

The interaction of electron-hole pairs with lattice vibrations exhibits a wealth of intriguing physical phenomena such as the renowned Kohn anomaly. Here we report the observation in bilayer graphene of an unusual phonon softening that provides the first experimental proof for another type of phonon anomaly. Similar to the Kohn anomaly, which is a logarithmic singularity in the phonon group velocity [W.