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1. Cavity-Enhanced 2D Material Quantum Emitters Deterministically Integrated with Silicon Nitride Microresonators

   Parto, K.; Azzam, S. I.; Lewis, N.; Patel, S. D.; Umezawa, S.; Watanabe, K.; Taniguchi, T.; Moody, G. (June 2022, ArXivorg)

   Optically active defects in 2D materials, such as hexagonal boron nitride (hBN) and transition metal dichalcogenides (TMDs), are an attractive class of single-photon emitters with high brightness, room-temperature operation, site-specific engineering of emitter arrays, and tunability with external strain and electric fields. In this work, we demonstrate a novel approach to precisely align and embed hBN and TMDs within background-free silicon nitride microring resonators. Through the Purcell effect, high-purity hBN emitters exhibit a cavity-enhanced spectral coupling efficiency up to 46% at room temperature, which exceeds the theoretical limit for cavity-free waveguide-emitter coupling and previous demonstrations by nearly an order-of-magnitude. The devices are fabricated with a CMOS-compatible process and exhibit no degradation of the 2D material optical properties, robustness to thermal annealing, and 100 nm positioning accuracy of quantum emitters within single-mode waveguides, opening a path for scalable quantum photonic chips with on-demand single-photon sources. 

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2. Superconductivity from On-Chip Metallization on 2D Topological Chalcogenides

   https://doi.org/10.1103/PhysRevX.14.021051  

   Jia, Yanyu; Yu, Guo; Song, Tiancheng; Yuan, Fang; Uzan, Ayelet J; Tang, Yue; Wang, Pengjie; Singha, Ratnadwip; Onyszczak, Michael; Zheng, Zhaoyi Joy; et al (June 2024, Physical Review X)

   Two-dimensional (2D) transition metal dichalcogenides (TMDs) is a versatile class of quantum materials of interest to various fields including, e.g., nanoelectronics, optical devices, and topological and correlated quantum matter. Tailoring the electronic properties of TMDs is essential to their applications in many directions. Here, we report that a highly controllable and uniform on-chip 2D metallization process converts a class of atomically thin TMDs into robust superconductors, a property belonging to none of the starting materials. As examples, we demonstrate the introduction of superconductivity into a class of 2D air-sensitive topological TMDs, including monolayers of ${\mathrm{T}}_{\mathrm{d}}\text{−}{\mathrm{WTe}}_2$ , $1{\mathrm{T}}^{\prime }\text{−}{\mathrm{MoTe}}_2$ , and $2\mathrm{H}\text{−}{\mathrm{MoTe}}_2$ , as well as their natural and twisted bilayers, metallized with an ultrathin layer of palladium. This class of TMDs is known to exhibit intriguing topological phases ranging from topological insulator, Weyl semimetal to fractional Chern insulator. The unique, high-quality two-dimensional metallization process is based on our recent findings of the long-distance, non-Fickian in-plane mass transport and chemistry in 2D that occur at relatively low temperatures and in devices fully encapsulated with inert insulating layers. Highly compatible with existing nanofabrication techniques for van der Waals stacks, our results offer a route to designing and engineering superconductivity and topological phases in a class of correlated 2D materials. Published by the American Physical Society2024 

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3. Real-space visualization of a defect-mediated charge density wave transition

   https://doi.org/10.1073/pnas.2402129121  

   Hart, James L; Pan, Haining; Siddique, Saif; Schnitzer, Noah; Mallayya, Krishnanand; Xu, Shiyu; Kourkoutis, Lena F; Kim, Eun-ah; Cha, Judy J (August 2024, Proceedings of the National Academy of Sciences)

   We study the coupled charge density wave (CDW) and insulator-to-metal transitions in the 2D quantum material 1T-TaS₂. By applying in situ cryogenic 4D scanning transmission electron microscopy with in situ electrical resistance measurements, we directly visualize the CDW transition and establish that the transition is mediated by basal dislocations (stacking solitons). We find that dislocations can both nucleate and pin the transition and locally alter the transition temperatureT_cby nearly ~75 K. This finding was enabled by the application of unsupervised machine learning to cluster five-dimensional, terabyte scale datasets, which demonstrate a one-to-one correlation between resistance—a global property—and local CDW domain-dislocation dynamics, thereby linking the material microstructure to device properties. This work represents a major step toward defect-engineering of quantum materials, which will become increasingly important as we aim to utilize such materials in real devices. 

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4. Critical behavior of the specific heat in Ti-Si amorphous alloys at the metal-insulator transition

   Rogachev, Andrey; Ikuta, Hiroshi; Mizutani, Uichiro (August 2022, Physical review B Condensed matter and materials physics)

   In this paper, we report the measurements of specific heat of an amorphous Ti9.5Si90.5 alloy located very close to the critical point of the metal-insulator transition. In the presence of a magnetic field, the specific heat is dominated by the Schottky anomaly caused by magnetic moments associated with the dangling bonds in the matrix of amorphous Si. Subtraction of this contribution exposes the behavior of the electronic specific heat coefficient γ . The coefficient is temperature independent above 2 K and is, in order of magnitude, close to the value expected in the absence of electron-electron interactions. In the temperature range 0.4–1.5 K, the coefficient γ shows an anomalous downturn, which can be approximated by the dependence γ (T ) = γ0 ln(T/T0 ), with T0 ≈ 0.2 K. In a companion paper, we found that the Hall coefficient in Ti-Si alloys is affected by the electron-electron interaction up to much higher temperature of 150 K and also varies critically across the metal-insulator transition. We compare our results with theoretical predictions for three models, which can potentially explain the anomalous behavior of the specific heat: generalized nonlinear σ model, Coulomb glass, and many-body localization. 

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5. Two-dimensional charge order stabilized in clean polytype heterostructures

   https://doi.org/10.1038/s41467-021-27947-5  

   Sung, Suk Hyun; Schnitzer, Noah; Novakov, Steve; El Baggari, Ismail; Luo, Xiangpeng; Gim, Jiseok; Vu, Nguyen M.; Li, Zidong; Brintlinger, Todd H.; Liu, Yu; et al (December 2022, Nature Communications)

   Abstract Compelling evidence suggests distinct correlated electron behavior may exist only in clean 2D materials such as 1T-TaS 2 . Unfortunately, experiment and theory suggest that extrinsic disorder in free standing 2D layers disrupts correlation-driven quantum behavior. Here we demonstrate a route to realizing fragile 2D quantum states through endotaxial polytype engineering of van der Waals materials. The true isolation of 2D charge density waves (CDWs) between metallic layers stabilizes commensurate long-range order and lifts the coupling between neighboring CDW layers to restore mirror symmetries via interlayer CDW twinning. The twinned-commensurate charge density wave (tC-CDW) reported herein has a single metal–insulator phase transition at ~350 K as measured structurally and electronically. Fast in-situ transmission electron microscopy and scanned nanobeam diffraction map the formation of tC-CDWs. This work introduces endotaxial polytype engineering of van der Waals materials to access latent 2D ground states distinct from conventional 2D fabrication. 

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