Abhay Narayan Pasupathy

Stacking van der Waals materials provides a powerful route to engineer emergent electronic and magnetic behaviors through proximity-driven interactions. The graphene/CrSBr heterostructure has emerged as a compelling platform in this frontier, exhibiting exotic macroscopic responses (including uniaxial surface-plasmon-polariton propagation and an unconventional quantum Hall effect) indicative of strong interfacial electronic and magnetic coupling. However, a microscopic electronic landscape governing these phenomena has remained elusive. Here, we provide a comprehensive spectroscopic characterization of the graphene/CrSBr interface using a combination of angle-resolved photoemission spectroscopy (ARPES), low-energy electron microscopy (LEEM), and density functional theory. We resolve a massive redistribution of interfacial charge that concurrently hole-dopes graphene and populates the quasi-one-dimensional spin-polarized conduction band of CrSBr, leading to interfacial metallization. Furthermore, electronic structure of CrSBr exhibits strong momentum-dependent renormalization distinct from conventional charge doping, and theoretical modeling supported by Raman spectroscopy points to additional interfacial compressive strain. In addition, ARPES reveals a splitting-like feature in the graphene Dirac cone consistent with spin degeneracy lifting, providing possible evidence of magnetic proximity coupling. These findings provide crucial microscopic insight into the system’s optical and transport responses and establish graphene/CrSBr as a versatile platform for charge-transfer control, strain-driven band engineering, magnetic-proximity coupling, and directionally confined excitations for next-generation spintronic and nanophotonic devices.

Bound states in the continuum (BICs) are quantum states that remain localized despite lying within a continuum of extended, delocalized states. Whereas optical BICs have been realized in photonic systems, achieving electronic bound states embedded in a metallic background remains an ongoing challenge. In this study, we used scanning tunneling microscopy to identify two defect-induced electronic states that remain localized within the metallic continuum of Pd ₅ AlI ₂ , a two-dimensional van der Waals metal. Our calculations suggest that electron-hopping interference on the decorated checkerboard lattice of Pd ₅ AlI ₂ gives rise to a (quasi) flat band built from spatially localized eigenstates that are orthogonal to the dispersive continuum. Atomic defects within this frustrated lattice shift the associated flat-band localized states into the metallic continuum, forming robust, symmetry-protected BICs.

Many recent advances in our understanding of two-dimensional electron systems stem from van der Waals (vdW) heterostructures. The assembly process relies on the weak bonding across interfaces between layered vdW compounds, making it possible to construct exceptionally clean heterostructures from chemically and structurally distinct materials. Here we demonstrate an additional, dynamic degree of freedom afforded by vdW interfaces, wherein we use microstructured polymer stamps to disassemble and reconfigure vdW heterostructures by deterministic, reversible, and nondestructive sliding. We apply this technique to alter the dielectric environment of monolayer graphene, expose an encapsulated H-WSe2 monolayer characterized by electronic transport for scanning tunneling microscopy measurements, and facilitate the preparation of air-sensitive monolayer 1T′-WTe2 devices. We also show that sliding preserves underlying moiré patterns, and, moreover, can be used to produce twisted vdW interfaces. Together these demonstrations suggest a new paradigm for assembling and dynamically modifying vdW heterostructures.

We report the design and fabrication of guiding patterns for topography‐based searching of two‐dimensional (2D) devices for scanning tunneling microscopy (STM) measurements. Sub‐micron geometric coordinate markers were etched into SiO ₂ /Si wafers, serving as both substrates for 2D device integration and guiding maps for sample navigation. Here, we used a monolayer graphene/h‐BN device with an active area of smaller than 20 × 20 µm as a model system and demonstrated that the device could be reliably located in STM solely through topographic imaging of the guiding patterns and in situ stage calibration, without reliance on optical viewports or capacitive sensing. Atomically resolved topographic imaging and tunneling spectroscopy were also obtained. Our proposed navigation strategy is fully compatible with standard device fabrication procedures and requires no hardware modification to existing STM setups. As such, it offers a practical alternative for locating miniaturized devices in STM, shedding light on studying emerging quantum phenomena in 2D systems.

A cost-effective and robust image-processing pipeline is presented for the detection and characterization of exfoliated two-dimensional (2D) material flakes in optical microscope images, designed to facilitate automation in van der Waals heterostructure assembly. The system combines shallow machine learning (ML)–based material classification with a precision-first flake detection algorithm driven by edge morphology and color discontinuity. Step edges are resolved when supported by optical contrast, while spurious features such as dust and background texture are reliably rejected. Each identified flake is exported in a structured format that includes centroid coordinates, bounding geometries, average RGB color, and estimated optical thickness, enabling seamless integration into automated pick-up and stacking workflows. The pipeline is hardware-light and operates without reliance on large-scale deep-learning segmentation models or nanoscale ground-truth labels, making it practical for scalable front-end wafer processing at a hardware cost of under $30,000. In contrast to prior approaches that focus solely on detection accuracy, the proposed system unifies flake segmentation with indexing, filtering, and blueprint-driven stacking, forming a closed-loop workflow from image acquisition to device planning. Its low annotation requirement and flexible implementation enable rapid deployment across diverse 2D material systems and imaging conditions.

Recent observations of superconductivity in twisted bilayer WSe₂ (refs. ^1,2) have extended the family of moiré superconductors beyond twisted graphene^{3,4,5,6,7,8,9,10,11,12,13,14,15}. In WSe₂, two different twist angles were studied, 3.65° (ref. ¹) and 5.0° (ref. ²), and two seemingly distinct superconducting phase diagrams were reported, raising the question of whether the superconducting phases in the two devices share a similar origin. Here we address the question by experimentally mapping the evolution of the phase diagram across devices with twist angles spanning the range defined by the initial reports and comparing the results to twist angle-dependent theory. We find that the superconducting state evolves smoothly with twist angle and at all twist angles is proximal to a Fermi surface reconstruction with, presumably, antiferromagnetic ordering, but is neither necessarily tied to the Van Hove singularity nor to the half-band insulator. Our results connect the previously distinct phase diagrams at 3.65° and 5°, and offer new insight into the origin of the superconductivity in this system and its evolution as the correlation strength increases. More broadly, the smooth phase diagram evolution, repeatability between different devices and dynamic gate tunability within each device establish twisted transition metal dichalcogenides as a unique platform for the study of correlated phases as the ratio of interaction strength to bandwidth is varied.

Bistable switching typically arises from ferroic orders, such as ferroelectricity and ferromagnetism, in which the bistable states are encoded in charge or spin degrees of freedom^1,2. Here we report the observation of bistable superlattice switching in monolayer TaIrTe₄, a dual quantum spin Hall insulator^3-5. Switching occurs between two lattice configurations with sharply contrasting periodicities. In particular, in a pristine monolayer, we observe the spontaneous emergence of a long-period superlattice that can be programmed on and off in a non-volatile manner by electrostatic tuning of low-energy electronic states. This switching toggles the system between two structural configurations with unit cell areas differing by two orders of magnitude. Mechanistically, our results reveal two independent and distinct instabilities, one in the lattice and the other in the quantum spin Hall electrons. These instabilities are coupled, leading to electrostatic control of lattice configurations with non-volatile memory. This finding is enabled by combining linear and nonlinear transport measurements^6-13, Raman spectroscopy and scanning tunnelling microscopy, which probe complementary aspects of the underlying orders. Notably, this non-volatile memory stabilizes a spontaneous superlattice with a periodicity on the few-nanometre scale that remains robust across a wide doping range, persists over days and survives above 70 K. Our preliminary data also show the emergence of new insulating states at fractional superlattice fillings, which can be switched on and off together with the superlattice.

Anderson localization is anticipated to play a pivotal role in the manifestation of superconducting phases, optimal thermoelectric properties, and the quantum anomalous Hall effect. Dimensionality plays a central role in the physics of Anderson localization. Previous studies of localization in 2D conductors have relied on thin metallic films produced by vapor deposition, but challenges such as domain size effects, grain boundaries, and strain continue to complicate interpretation. Here, we overcome these challenges by investigating localization physics in Pd_{5}AlI_{2}, a van der Waals metal that can be exfoliated to the 2D limit. Using scanning tunneling spectroscopy, we demonstrate Anderson localization of electrons in monolayer Pd_{5}AlI_{2} from the presence of a Coulomb gap at the Fermi level and a unique density of state distribution characteristic of wave function localization. Moreover, we show evidence of phonon localization in Pd_{5}AlI_{2} from the layer-dependent broadening and softening of phonon modes. These signatures of electron and phonon localization, absent in multilayer samples, provide spectroscopic evidence for the theoretical predictions that localization is enhanced at reduced dimensionality.