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1. Moiré synaptic transistor with room-temperature neuromorphic functionality

   https://doi.org/10.1038/s41586-023-06791-1  

   Yan, Xiaodong ; Zheng, Zhiren ; Sangwan, Vinod K ; Qian, Justin H ; Wang, Xueqiao ; Liu, Stephanie E ; Watanabe, Kenji ; Taniguchi, Takashi ; Xu, Su-Yang ; Jarillo-Herrero, Pablo ; et al ( December 2023 , Nature)

   The convergence of topology and correlations represents a highly coveted realm in the pursuit of novel quantum states of matter [1, 2]. Introducing electron correlations to a quantum spin Hall (QSH) insulator can lead to the emergence of a fractional topological insulator and other exotic time-reversal-symmetric topological order [3– 10], not possible in quantum Hall and Chern insulator systems. However, the QSH insulator with quantized edge conductance remains rare, let alone that with significant correlations. In this work, we report a novel dual QSH insulator within the intrinsic monolayer crystal of TaIrTe4, arising from the interplay of its single-particle topology and density-tuned electron correlations. At charge neutrality, monolayer TaIrTe4 demonstrates the QSH insulator that aligns with single-particle band structure calculations, manifesting enhanced nonlocal transport and quantized helical edge conductance. Interestingly, upon introducing electrons from charge neutrality, TaIrTe4 only shows metallic behavior in a small range of charge densities but quickly goes into a new insulating state, entirely unexpected based on TaIrTe4’s single-particle band structure. This insulating state could arise from a strong electronic instability near the van Hove singularities (VHS), likely leading to a charge density wave (CDW). Remarkably, within this correlated insulating gap, we observe a resurgence of the QSH state, marked by the revival of nonlocal transport and quantized helical edge conduction. Our observation of helical edge conduction in a CDW gap could bridge spin physics and charge orders. The discovery of a dual QSH insulator introduces a new method for creating topological flat minibands via CDW superlattices, which offer a promising platform for exploring time-reversal-symmetric fractional phases and electromagnetism [3–5, 11, 12]. 

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2. Quasiparticle and excitonic properties of monolayer 1T′ WTe2 within many-body perturbation theory

   https://doi.org/10.1103/PhysRevB.110.075133  

   Wu, Jinyuan ; Hou, Bowen ; Li, Wenxin ; He, Yu ; Qiu, Diana Y ( August 2024 , Physical Review B)

   In the monolayer limit, 1⁢𝑇′ WTe2 is a two-dimensional topological insulator exhibiting the quantum spin Hall effect and is believed to host an excitonic insulator ground state. However, theoretical analysis of this system is complicated by the difficulty of obtaining descriptions of the single-quasiparticle band structure consistent with experimental measurement within conventional first-principles techniques. Previous band-structure calculations using the Perdew-Burke-Ernzerhof functional and a one-shot 𝐺⁢𝑊 approximation result in a semimetallic band structure, while calculations with hybrid functionals appear to open a band gap. Here, we demonstrate that self-consistently updating wave functions within a static 𝐺⁢𝑊 approximation (static COHSEX) can reproduce the insulating band structure experimentally observed by angle-resolved photoemission spectroscopy without resorting to mechanisms beyond the quasiparticle picture. Finally, a finite-momentum Bethe-Salpeter equation calculation on top of self-consistent 𝐺⁢𝑊 results in negative exciton excitation energies, leaving open the possibility of excitonic instability in 1⁢𝑇′ monolayer WTe2. 

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3. A highly correlated topological bubble phase of composite fermions

   https://doi.org/10.1038/s41567-023-01939-2  

   Shingla, Vidhi ; Huang, Haoyun ; Kumar, Ashwani ; Pfeiffer, Loren N. ; West, Kenneth W. ; Baldwin, Kirk W. ; Csáthy, Gábor A. ( May 2023 , Nature Physics)

   Strong interactions and topology drive a wide variety of correlated ground states. Some of the most interesting of these ground states, such as fractional quantum Hall states and fractional Chern insulators, have fractionally charged quasiparticles. Correlations in these phases are captured by the binding of electrons and vortices into emergent particles called composite fermions. Composite fermion quasiparticles are randomly localized at high levels of disorder and may exhibit charge order when there is not too much disorder in the system. However, more complex correlations are predicted when composite fermion quasiparticles cluster into a bubble, and then these bubbles order on a lattice. Such a highly correlated ground state is termed the bubble phase of composite fermions. Here we report the observation of such a bubble phase of composite fermions, evidenced by the re-entrance of the fractional quantum Hall effect. We associate this re-entrance with a bubble phase with two composite fermion quasiparticles per bubble. Our results demonstrate the existence of a new class of strongly correlated topological phases driven by clustering and charge ordering of emergent quasiparticles. 

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4. Electrically tunable low-density superconductivity in a monolayer topological insulator

   https://doi.org/10.1126/science.aar4642  

   Fatemi, Valla ; Wu, Sanfeng ; Cao, Yuan ; Bretheau, Landry ; Gibson, Quinn D. ; Watanabe, Kenji ; Taniguchi, Takashi ; Cava, Robert J. ; Jarillo-Herrero, Pablo ( October 2018 , Science)

   Turning on superconductivity in a topologically nontrivial insulator may provide a route to search for non-Abelian topological states. However, existing demonstrations of superconductor-insulator switches have involved only topologically trivial systems. Here we report reversible, in situ electrostatic on-off switching of superconductivity in the recently established quantum spin Hall insulator monolayer tungsten ditelluride (WTe₂). Fabricated into a van der Waals field-effect transistor, the monolayer’s ground state can be continuously gate-tuned from the topological insulating to the superconducting state, with critical temperaturesT_cup to ~1 kelvin. Our results establish monolayer WTe₂as a material platform for engineering nanodevices that combine superconducting and topological phases of matter.

    

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5. Competing correlated states and abundant orbital magnetism in twisted monolayer-bilayer graphene

   https://doi.org/10.1038/s41467-021-25044-1  

   He, Minhao ; Zhang, Ya-Hui ; Li, Yuhao ; Fei, Zaiyao ; Watanabe, Kenji ; Taniguchi, Takashi ; Xu, Xiaodong ; Yankowitz, Matthew ( August 2021 , Nature Communications)

   Flat band moiré superlattices have recently emerged as unique platforms for investigating the interplay between strong electronic correlations, nontrivial band topology, and multiple isospin ‘flavor’ symmetries. Twisted monolayer-bilayer graphene (tMBG) is an especially rich system owing to its low crystal symmetry and the tunability of its bandwidth and topology with an external electric field. Here, we find that orbital magnetism is abundant within the correlated phase diagram of tMBG, giving rise to the anomalous Hall effect in correlated metallic states nearby most odd integer fillings of the flat conduction band, as well as correlated Chern insulator states stabilized in an external magnetic field. The behavior of the states at zero field appears to be inconsistent with simple spin and valley polarization for the specific range of twist angles we investigate, and instead may plausibly result from an intervalley coherent (IVC) state with an order parameter that breaks time reversal symmetry. The application of a magnetic field further tunes the competition between correlated states, in some cases driving first-order topological phase transitions. Our results underscore the rich interplay between closely competing correlated ground states in tMBG, with possible implications for probing exotic IVC ordering.

    

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