arXiv subjects

Luke Holtzman

Publications and source records attributed to Luke Holtzman.

10 recordsLinked to original sources

arXiv2026

Long-Range Fermi-Polaron Blockade in Monolayer MoSe$_2$

Strong optical nonlinearities at the few-photon level are a central goal for quantum photonics, yet they remain difficult to realize in solid-state systems. In doped two-dimensional semiconductors, coupling between excitons and a degenerate Fermi sea gives rise to exciton-Fermi polarons, many-body quasiparticles whose optical response is governed by fermionic correlations. Here, using femtosecond pump-probe transient absorption microscopy, we directly image the spatially resolved nonlinear optical response of exciton-Fermi polarons in monolayer MoSe$_2$. We observe a pronounced spatial suppression of resonant absorption associated with the attractive Fermi polaron, from which we extract an optical blockade radius that is more than ten times larger than that of the neutral exciton. Microscopic analysis indicates that this extended nonlinearity arises primarily from fermion-mediated interactions between exciton-Fermi polarons. Our results establish exciton-Fermi polarons in two-dimensional semiconductors as electrically tunable, strongly interacting optical quasiparticles, and identify them as a promising platform for ultralow-power nonlinear optical devices.

cond-mat.mes-hall↗

arXiv2026

Pre-Patterned Superconducting Contacts for Clean Superconductor-Topological Material Interfaces Enabling Long-Range Josephson Coupling

Phase-coherent superconducting proximity in topological materials requires clean superconductor-topological material (SC-TM) interfaces, yet conventional top-contact fabrication often degrades them through oxidation, polymer residue, and process-induced disorder. Here we introduce a pre-patterned superconducting bottom-contact architecture in which MoRe/Au electrodes are defined before van der Waals crystal transfer, thereby avoiding on-flake lithography after transfer. In WTe2- and Bi1.5Sb0.5Te1.7Se1.3-based Josephson junctions, this architecture yields systematically larger I_c R_N and longer-ranged coupling than conventional top contacts. Cross-sectional STEM/EDS reveals atomically abrupt, chemically well-separated interfaces. These results establish pre-patterned SC-TM contacts as a practical route to reproducible, micrometer-scale Josephson platforms in van der Waals topological materials.

cond-mat.mes-hall↗

arXiv2025

Angle evolution of the superconducting phase diagram in twisted bilayer WSe2

Recent observations of superconductivity in twisted bilayer WSe$_2$ have extended the family of moir\'e superconductors beyond twisted graphene. In WSe$_2$ two different twist angles were studied, 3.65{\deg} and 5.0{\deg}, 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{\deg} and 5{\deg}, 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.

cond-mat.mes-hall↗

arXiv2025

Ultrafast Charge-Doping via Photo-Thermionic Injection in van der Waals Devices

Van der Waals (vdW) heterostructures of two-dimensional (2D) materials have become a rich playground for the exploration of correlated quantum phases, and recent studies have begun to probe their non-equilibrium dynamics under femtosecond laser excitation. In a time-resolved experiment, optical excitation of the multilayer structure can lead not only to rich dynamic responses from the target layers, such as moir\'e interfaces, but also to additional device functionality from the layer degree of freedom. Here, we investigate optical excitation in a prototypical moir\'e device of dual-gated twisted WSe2 bilayers, with few-layer graphite gates and hexagonal boron nitride (hBN) spacers. We establish an ultrafast photodoping mechanism in the moir\'e bilayer from photo-thermionic emission of the graphite gates. Using transient reflectance experiments, we reveal photo-induced hole injection evidenced by: (i) a shift of gate voltages at which optical signatures of correlated insulators are observed, (ii) a persistent optical signature indicative of charge diffusion at microsecond timescales and local charge buildup from pulse-to-pulse accumulation, and (iii) photoinduced absorption due likely to transient formation of correlated insulators. We further demonstrate that the injected holes can be selectively controlled by tuning the excitation energy, fluence, and gate bias.

cond-mat.str-el↗

arXiv2025

Photoinduced Metal-to-Insulator Transitions in 2D Moir\'e Devices

Photoexcitation has been utilized to control quantum matter and to uncover metastable phases far from equilibrium. Among demonstrations to date, the most common is the photo-induced transition from correlated insulators to metallic states; however, the reverse process without initial orders has not been observed. Here, we show ultrafast metal-to-insulator transition in gate-doped WS2/WSe2 and WSe2/WSe2 moir\'e devices using photo-thermionic hole injection from graphite gates. The resulting correlated insulators are metastable, with lifetimes exceeding microseconds. These findings establish an effective mechanism for the ultrafast control of correlated electronic phases in van der Waals heterostructures.

cond-mat.str-el↗

arXiv2024

Real-Space Imaging of the Band Topology of Transition Metal Dichalcogenides

The topological properties of Bloch bands are intimately tied to the structure of their electronic wavefunctions within the unit cell of a crystal. Here, we show that scanning tunneling microscopy (STM) measurements on the prototypical transition metal dichalcogenide (TMD) semiconductor WSe$_2$ can be used to unambiguously fix the location of the Wannier center of the valence band. Using site-specific substitutional doping, we first determine the position of the atomic sites within STM images, establishing that the maximum electronic density of states at the $K$-point lies between the atoms. In contrast, the maximum density of states at the $\Gamma$ point is at the atomic sites. This signifies that WSe$_2$ is a topologically obstructed atomic insulator, which cannot be adiabatically transformed to the trivial atomic insulator limit.

cond-mat.mtrl-sci↗

arXiv2024

Superconductivity in twisted bilayer WSe$_2$

The discovery of superconductivity in twisted bilayer and twisted trilayer graphene has generated tremendous interest. The key feature of these systems is an interplay between interlayer coupling and a moir\'e superlattice that gives rise to low-energy flat bands with strong correlations. Flat bands can also be induced by moir\'e patterns in lattice-mismatched and or twisted heterostructures of other two-dimensional materials such as transition metal dichalcogenides (TMDs). Although a wide range of correlated phenomenon have indeed been observed in the moir\'e TMDs, robust demonstration of superconductivity has remained absent. Here we report superconductivity in 5 degree twisted bilayer WSe$_2$ (tWSe$_2$) with a maximum critical temperature of 426 mK. The superconducting state appears in a limited region of displacement field and density that is adjacent to a metallic state with Fermi surface reconstruction believed to arise from antiferromagnetic order. A sharp boundary is observed between the superconducting and magnetic phases at low temperature, reminiscent of spin-fluctuation mediated superconductivity. Our results establish that moir\'e flat-band superconductivity extends beyond graphene structures. Material properties that are absent in graphene but intrinsic among the TMDs such as a native band gap, large spin-orbit coupling, spin-valley locking, and magnetism offer the possibility to access a broader superconducting parameter space than graphene-only structures.

cond-mat.mes-hall↗

arXiv2023

Detecting Atomic Scale Surface Defects in STM of TMDs with Ensemble Deep Learning

Atomic-scale defect detection is shown in scanning tunneling microscopy images of single crystal WSe2 using an ensemble of U-Net-like convolutional neural networks. Standard deep learning test metrics indicated good detection performance with an average F1 score of 0.66 and demonstrated ensemble generalization to C-AFM images of WSe2 and STM images of MoSe2. Defect coordinates were automatically extracted from defect detections maps showing that STM image analysis enhanced by machine learning can be used to dramatically increase sample characterization throughput.

cond-mat.mtrl-sci↗

arXiv2023

Charge-transfer Contact to a High-Mobility Monolayer Semiconductor

Two-dimensional (2D) semiconductors, such as the transition metal dichalcogenides, have demonstrated tremendous promise for the development of highly tunable quantum devices. Realizing this potential requires low-resistance electrical contacts that perform well at low temperatures and low densities where quantum properties are relevant. Here we present a new device architecture for 2D semiconductors that utilizes a charge-transfer layer to achieve large hole doping in the contact region, and implement this technique to measure magneto-transport properties of high-purity monolayer WSe$_2$. We measure a record-high hole mobility of 80,000 cm$^2$/Vs and access channel carrier densities as low as $1.6\times10^{11}$ cm$^{-2}$, an order of magnitude lower than previously achievable. Our ability to realize transparent contact to high-mobility devices at low density enables transport measurement of correlation-driven quantum phases including observation of a low temperature metal-insulator transition in a density and temperature regime where Wigner crystal formation is expected, and observation of the fractional quantum Hall effect under large magnetic fields. The charge transfer contact scheme paves the way for discovery and manipulation of new quantum phenomena in 2D semiconductors and their heterostructures.

cond-mat.mes-hall↗

arXiv2022

P-type Ohmic contact to monolayer WSe2 field-effect transistors using high electron affinity amorphous MoO3

1 School of Physics and Astronomy, Monash University, Clayton, Victoria 3800, Australia 2 Australian Research Council Centre of Excellence in Future Low-Energy Electronics Technologies (FLEET), Monash University, Clayton, Victoria 3800, Australia 3 Department of Mechanical Engineering, Columbia University, New York, New York 10027, United States 4 Department of Applied Physics and Applied Mathematics, Columbia University, New York, New York 10027, United States 5 School of Physics, the University of Melbourne, Melbourne, VIC 3010, Australia 6 Research Center for Functional Materials, National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan 7 International Center for Materials Nanoarchitectonics, National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan 8 School of Physics, University of New South Wales, 2052 Sydney, Australia 9 Center of Condensed Matter Sciences and Center of Atomic Initiative for New Material, National Taiwan University, Taipei 106, Taiwan 10 Monash Centre for Atomically Thin Materials, Monash University, Clayton, 3800, VIC, Australia

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