More Like this

1. 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.
2. Emergent helical edge states in a hybridized three-dimensional topological insulator

   https://doi.org/10.1038/s41467-022-33643-9  

   Chong, Su Kong ; Liu, Lizhe ; Watanabe, Kenji ; Taniguchi, Takashi ; Sparks, Taylor D. ; Liu, Feng ; Deshpande, Vikram V. ( October 2022 , Nature Communications)

   As the thickness of a three-dimensional (3D) topological insulator (TI) becomes comparable to the penetration depth of surface states, quantum tunneling between surfaces turns their gapless Dirac electronic structure into a gapped spectrum. Whether the surface hybridization gap can host topological edge states is still an open question. Herein, we provide transport evidence of 2D topological states in the quantum tunneling regime of a bulk insulating 3D TI BiSbTeSe₂. Different from its trivial insulating phase, this 2D topological state exhibits a finite longitudinal conductance at ~2e²/h when the Fermi level is aligned within the surface gap, indicating an emergent quantum spin Hall (QSH) state. The transition from the QSH to quantum Hall (QH) state in a transverse magnetic field further supports the existence of this distinguished 2D topological phase. In addition, we demonstrate a second route to realize the 2D topological state via surface gap-closing and topological phase transition mechanism mediated by a transverse electric field. The experimental realization of the 2D topological phase in a 3D TI enriches its phase diagram and marks an important step toward functionalized topological quantum devices.
3. Ferroelectric nonlinear anomalous Hall effect in few-layer WTe2

   https://doi.org/10.1038/s41524-019-0257-1  

   Wang, Hua ; Qian, Xiaofeng ( December 2019 , npj Computational Materials)

   Under broken time reversal symmetry such as in the presence of external magnetic field or internal magnetization, a transverse voltage can be established in materials perpendicular to both longitudinal current and applied magnetic field, known as classical Hall effect. However, this symmetry constraint can be relaxed in the nonlinear regime, thereby enabling nonlinear anomalous Hall current in time-reversal invariant materials – an underexplored realm with exciting new opportunities beyond classical linear Hall effect. Here, using group theory and first-principles theory, we demonstrate a remarkable ferroelectric nonlinear anomalous Hall effect in time-reversal invariant few-layer WTe₂where nonlinear anomalous Hall current switches in odd-layer WTe₂except 1T′ monolayer while remaining invariant in even-layer WTe₂upon ferroelectric transition. This even-odd oscillation of ferroelectric nonlinear anomalous Hall effect was found to originate from the absence and presence of Berry curvature dipole reversal and shift dipole reversal due to distinct ferroelectric transformation in even and odd-layer WTe₂. Our work not only treats Berry curvature dipole and shift dipole on an equal footing to account for intraband and interband contributions to nonlinear anomalous Hall effect, but also establishes Berry curvature dipole and shift dipole as new order parameters for noncentrosymmetric materials. The present findings suggest that ferroelectric metalsmore »and Weyl semimetals may offer unprecedented opportunities for the development of nonlinear quantum electronics.

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4. Observation of topologically protected states at crystalline phase boundaries in single-layer WSe2

   https://doi.org/10.1038/s41467-018-05672-w  

   Ugeda, Miguel M. ; Pulkin, Artem ; Tang, Shujie ; Ryu, Hyejin ; Wu, Quansheng ; Zhang, Yi ; Wong, Dillon ; Pedramrazi, Zahra ; Martín-Recio, Ana ; Chen, Yi ; et al ( August 2018 , Nature Communications)

   Transition metal dichalcogenide materials are unique in the wide variety of structural and electronic phases they exhibit in the two-dimensional limit. Here we show how such polymorphic flexibility can be used to achieve topological states at highly ordered phase boundaries in a new quantum spin Hall insulator (QSHI), 1T′-WSe₂. We observe edge states at the crystallographically aligned interface between a quantum spin Hall insulating domain of 1T′-WSe₂and a semiconducting domain of 1H-WSe₂in contiguous single layers. The QSHI nature of single-layer 1T′-WSe₂is verified using angle-resolved photoemission spectroscopy to determine band inversion around a 120 meV energy gap, as well as scanning tunneling spectroscopy to directly image edge-state formation. Using this edge-state geometry we confirm the predicted penetration depth of one-dimensional interface states into the two-dimensional bulk of a QSHI for a well-specified crystallographic direction. These interfaces create opportunities for testing predictions of the microscopic behavior of topologically protected boundary states.
5. Emergent long-range magnetic order in ultrathin (111)-oriented LaNiO3 films

   https://doi.org/10.1038/s41535-021-00345-2  

   Kane, Margaret M. ; Vailionis, Arturas ; Riddiford, Lauren J. ; Mehta, Apurva ; N’Diaye, Alpha T. ; Klewe, Christoph ; Shafer, Padraic ; Arenholz, Elke ; Suzuki, Yuri ( May 2021 , npj Quantum Materials)

   The emergence of ferromagnetism in materials where the bulk phase does not show any magnetic order demonstrates that atomically precise films can stabilize distinct ground states and expands the phase space for the discovery of materials. Here, the emergence of long-range magnetic order is reported in ultrathin (111) LaNiO₃(LNO) films, where bulk LNO is paramagnetic, and the origins of this phase are explained. Transport and structural studies of LNO(111) films indicate that NiO₆octahedral distortions stabilize a magnetic insulating phase at the film/substrate interface and result in a thickness-dependent metal–insulator transition att = 8 unit cells. Away from this interface, distortions relax and bulk-like conduction is regained. Synchrotron x-ray diffraction and dynamical x-ray diffraction simulations confirm a corresponding out-of-plane unit-cell expansion at the interface of all films. X-ray absorption spectroscopy reveals that distortion stabilizes an increased concentration of Ni²⁺ions. Evidence of long-range magnetic order is found in anomalous Hall effect and magnetoresistance measurements, likely due to ferromagnetic superexchange interactions among Ni²⁺–Ni³⁺ions. Together, these results indicate that long-range magnetic ordering and metallicity in LNO(111) films emerges from a balance among the spin, charge, lattice, and orbital degrees of freedom.