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1. Endotaxial stabilization of 2D charge density waves with long-range order

   https://doi.org/10.1038/s41467-024-45711-3  

   Sung, Suk Hyun; Agarwal, Nishkarsh; El_Baggari, Ismail; Kezer, Patrick; Goh, Yin Min; Schnitzer, Noah; Shen, Jeremy M; Chiang, Tony; Liu, Yu; Lu, Wenjian; et al (December 2024, Nature Communications)

   Abstract Charge density waves are emergent quantum states that spontaneously reduce crystal symmetry, drive metal-insulator transitions, and precede superconductivity. In low-dimensions, distinct quantum states arise, however, thermal fluctuations and external disorder destroy long-range order. Here we stabilize ordered two-dimensional (2D) charge density waves through endotaxial synthesis of confined monolayers of 1T-TaS₂. Specifically, an ordered incommensurate charge density wave (oIC-CDW) is realized in 2D with dramatically enhanced amplitude and resistivity. By enhancing CDW order, the hexatic nature of charge density waves becomes observable. Upon heating via in-situ TEM, the CDW continuously melts in a reversible hexatic process wherein topological defects form in the charge density wave. From these results, new regimes of the CDW phase diagram for 1T-TaS₂are derived and consistent with the predicted emergence of vestigial quantum order. 

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2. Encoding multistate charge order and chirality in endotaxial heterostructures

   https://doi.org/10.1038/s41467-023-41780-y  

   Husremović, Samra; Goodge, Berit H.; Erodici, Matthew P.; Inzani, Katherine; Mier, Alberto; Ribet, Stephanie M.; Bustillo, Karen C.; Taniguchi, Takashi; Watanabe, Kenji; Ophus, Colin; et al (December 2023, Nature Communications)

   Abstract High-density phase change memory (PCM) storage is proposed for materials with multiple intermediate resistance states, which have been observed in 1T-TaS₂due to charge density wave (CDW) phase transitions. However, the metastability responsible for this behavior makes the presence of multistate switching unpredictable in TaS₂devices. Here, we demonstrate the fabrication of nanothick verti-lateralH-TaS₂/1T-TaS₂heterostructures in which the number of endotaxial metallicH-TaS₂monolayers dictates the number of resistance transitions in 1T-TaS₂lamellae near room temperature. Further, we also observe optically active heterochirality in the CDW superlattice structure, which is modulated in concert with the resistivity steps, and we show how strain engineering can be used to nucleate these polytype conversions. This work positions the principle of endotaxial heterostructures as a promising conceptual framework for reliable, non-volatile, and multi-level switching of structure, chirality, and resistance. 

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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. In operando cryo-STEM of pulse-induced charge density wave switching in TaS2

   https://doi.org/10.1038/s41467-023-44093-2  

   Hart, James L.; Siddique, Saif; Schnitzer, Noah; Funni, Stephen D.; Kourkoutis, Lena F.; Cha, Judy J. (December 2023, Nature Communications)

   Abstract The charge density wave material 1T-TaS₂exhibits a pulse-induced insulator-to-metal transition, which shows promise for next-generation electronics such as memristive memory and neuromorphic hardware. However, the rational design of TaS₂devices is hindered by a poor understanding of the switching mechanism, the pulse-induced phase, and the influence of material defects. Here, we operate a 2-terminal TaS₂device within a scanning transmission electron microscope at cryogenic temperature, and directly visualize the changing charge density wave structure with nanoscale spatial resolution and down to 300 μs temporal resolution. We show that the pulse-induced transition is driven by Joule heating, and that the pulse-induced state corresponds to the nearly commensurate and incommensurate charge density wave phases, depending on the applied voltage amplitude. With our in operando cryogenic electron microscopy experiments, we directly correlate the charge density wave structure with the device resistance, and show that dislocations significantly impact device performance. This work resolves fundamental questions of resistive switching in TaS₂devices, critical for engineering reliable and scalable TaS₂electronics. 

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5. Charge‐Density‐Wave Resistive Switching and Voltage Oscillations in Ternary Chalcogenide BaTiS ₃

   https://doi.org/10.1002/aelm.202300461  

   Chen, Huandong; Wang, Nan; Liu, Hefei; Wang, Han; Ravichandran, Jayakanth (September 2023, Advanced Electronic Materials)

   Abstract Phase change materials, which show different electrical characteristics across the phase transitions, have attracted considerable research attention for their potential electronic device applications. Materials with metal‐to‐insulator or charge density wave (CDW) transitions such as VO₂and 1T‐TaS₂have demonstrated voltage oscillations due to their robust bi‐state resistive switching behavior with some basic neuronal characteristics. BaTiS₃is a small bandgap ternary chalcogenide that has recently reported the emergence of CDW order below 245 K. Here, the discovery of DC voltage / current‐induced reversible threshold switching in BaTiS₃devices between a CDW phase and a room temperature semiconducting phase is reported. The resistive switching behavior is consistent with a Joule heating scheme and sustained voltage oscillations with a frequency of up to 1 kHz are demonstrated by leveraging the CDW phase transition and the associated negative differential resistance. Strategies of reducing channel sizes and improving thermal management may further improve the device's performance. The findings establish BaTiS₃as a promising CDW material for future electronic device applications, especially for energy‐efficient neuromorphic computing. 

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