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1. Low-Temperature 2D/2D Ohmic Contacts in WSe ₂ Field-Effect Transistors as a Platform for the 2D Metal–Insulator Transition

   https://doi.org/10.1021/acsami.0c21440  

   Stanley, Lily J.; Chuang, Hsun-Jen; Zhou, Zhixian; Koehler, Michael R.; Yan, Jiaqiang; Mandrus, David G.; Popović, Dragana (February 2021, ACS Applied Materials & Interfaces)

   null (Ed.)

   We report the fabrication of hexagonal-boron-nitride (hBN) encapsulated multi-terminal WSe_2 Hall bars with 2D/2D low-temperature Ohmic contacts as a platform for investigating the two-dimensional (2D) metal-insulator transition. We demonstrate that the WSe_2 devices exhibit Ohmic behavior down to 0.25 K and at low enough excitation voltages to avoid current-heating effects. Additionally, the high-quality hBN-encapsulated WSe_2 devices in ideal Hall-bar geometry enable us to accurately determine the carrier density. Measurements of the temperature (T) and density (n_s) dependence of the conductivity \sigma(T,n_s) demonstrate scaling behavior consistent with a metal-insulator quantum phase transition driven by electron-electron interactions, but where disorder-induced local magnetic moments are also present. Our findings pave the way for further studies of the fundamental quantum mechanical properties of 2D transition metal dichalcogenides using the same contact engineering. 

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2. Dielectric catastrophe at the Wigner-Mott transition in a moiré superlattice

   https://doi.org/10.1038/s41467-022-32037-1  

   Tang, Yanhao; Gu, Jie; Liu, Song; Watanabe, Kenji; Taniguchi, Takashi; Hone, James C.; Mak, Kin Fai; Shan, Jie (December 2022, Nature Communications)

   Abstract The bandwidth-tuned Wigner-Mott transition is an interaction-driven phase transition from a generalized Wigner crystal to a Fermi liquid. Because the transition is generally accompanied by both magnetic and charge-order instabilities, it remains unclear if a continuous Wigner-Mott transition exists. Here, we demonstrate bandwidth-tuned metal-insulator transitions at fixed fractional fillings of a MoSe 2 /WS 2 moiré superlattice. The bandwidth is controlled by an out-of-plane electric field. The dielectric response is probed optically with the 2s exciton in a remote WSe 2 sensor layer. The exciton spectral weight is negligible for the metallic state with a large negative dielectric constant. It continuously vanishes when the transition is approached from the insulating side, corresponding to a diverging dielectric constant or a ‘dielectric catastrophe’ driven by the critical charge dynamics near the transition. Our results support the scenario of continuous Wigner-Mott transitions in two-dimensional triangular lattices and stimulate future explorations of exotic quantum phases in their vicinities. 

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3. Colossal negative magnetoresistance from hopping in insulating ferromagnetic semiconductors

   https://doi.org/10.1088/1674-4926/43/11/112502  

   Liu, Xinyu; Riney, Logan; Guerra, Josue; Powers, William; Wang, Jiashu; Furdyna, Jacek K.; Assaf, Badih A. (November 2022, Journal of Semiconductors)

   Abstract Ferromagnetic semiconductor Ga 1– x Mn x As 1– y P y thin films go through a metal–insulator transition at low temperature where electrical conduction becomes driven by hopping of charge carriers. In this regime, we report a colossal negative magnetoresistance (CNMR) coexisting with a saturated magnetic moment, unlike in the traditional magnetic semiconductor Ga 1– x Mn x As. By analyzing the temperature dependence of the resistivity at fixed magnetic field, we demonstrate that the CNMR can be consistently described by the field dependence of the localization length, which relates to a field dependent mobility edge. This dependence is likely due to the random environment of Mn atoms in Ga 1– x Mn x As 1– y P y which causes a random spatial distribution of the mobility that is suppressed by an increasing magnetic field. 

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4. Anomalous Hall effect and perpendicular magnetic anisotropy in ultrathin ferrimagnetic NiCo ₂ O ₄ films

   https://doi.org/10.1063/5.0097869  

   Chen, Xuegang; Wu, Qiuchen; Zhang, Le; Hao, Yifei; Han, Myung-Geun; Zhu, Yimei; Hong, Xia (June 2022, Applied Physics Letters)

   The inverse spinel ferrimagnetic NiCo₂O₄possesses high magnetic Curie temperature T_C, high spin polarization, and strain-tunable magnetic anisotropy. Understanding the thickness scaling limit of these intriguing magnetic properties in NiCo₂O₄thin films is critical for their implementation in nanoscale spintronic applications. In this work, we report the unconventional magnetotransport properties of epitaxial (001) NiCo₂O₄films on MgAl₂O₄substrates in the ultrathin limit. Anomalous Hall effect measurements reveal strong perpendicular magnetic anisotropy for films down to 1.5 unit cell (1.2 nm), while T_Cfor 3 unit cell and thicker films remains above 300 K. The sign change in the anomalous Hall conductivity [Formula: see text] and its scaling relation with the longitudinal conductivity ([Formula: see text]) can be attributed to the competing effects between impurity scattering and band intrinsic Berry curvature, with the latter vanishing upon the thickness driven metal–insulator transition. Our study reveals the critical role of film thickness in tuning the relative strength of charge correlation, Berry phase effect, spin–orbit interaction, and impurity scattering, providing important material information for designing scalable epitaxial magnetic tunnel junctions and sensing devices using NiCo₂O₄. 

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5. Physical Origin of Negative Differential Resistance in V ₃ O ₅ and Its Application as a Solid‐State Oscillator

   https://doi.org/10.1002/adma.202208477  

   Das, Sujan_Kumar; Nandi, Sanjoy_Kumar; Marquez, Camilo_Verbel; Rúa, Armando; Uenuma, Mutsunori; Puyoo, Etienne; Nath, Shimul_Kanti; Albertini, David; Baboux, Nicolas; Lu, Teng; et al (December 2022, Advanced Materials)

   Abstract Oxides that exhibit an insulator–metal transition can be used to fabricate energy‐efficient relaxation oscillators for use in hardware‐based neural networks but there are very few oxides with transition temperatures above room temperature. Here the structural, electrical, and thermal properties of V₃O₅thin films and their application as the functional oxide in metal/oxide/metal relaxation oscillators are reported. The V₃O₅devices show electroforming‐free volatile threshold switching and negative differential resistance (NDR) with stable (<3% variation) cycle‐to‐cycle operation. The physical mechanisms underpinning these characteristics are investigated using a combination of electrical measurements, in situ thermal imaging, and device modeling. This shows that conduction is confined to a narrow filamentary path due to self‐confinement of the current distribution and that the NDR response is initiated at temperatures well below the insulator–metal transition temperature where it is dominated by the temperature‐dependent conductivity of the insulating phase. Finally, the dynamics of individual and coupled V₃O₅‐based relaxation oscillators is reported, showing that capacitively coupled devices exhibit rich non‐linear dynamics, including frequency and phase synchronization. These results establish V₃O₅as a new functional material for volatile threshold switching and advance the development of robust solid‐state neurons for neuromorphic computing. 

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