Observation of piezoelectricity in free-standing monolayer MoS 2. Piezoelectricity of single-atomic-layer MoS 2 for energy conversion and piezotronics. Experimental discovery of Weyl semimetal TaAs. Signatures of a time-reversal symmetric Weyl semimetal with only four Weyl points. Spectroscopic evidence for a type II Weyl semimetallic state in MoTe 2. Imaging quantum spin Hall edges in monolayer WTe 2. Coexistence of large conventional and planar spin Hall effect with long spin diffusion length in a low-symmetry semimetal at room temperature. Observation of the quantum spin Hall effect up to 100 kelvin in a monolayer crystal. The valley Hall effect in MoS 2 transistors. Lightwave valleytronics in a monolayer of tungsten diselenide. Nonlinear anomalous Hall effect in few-layer WTe 2. Two-dimensional gas of massless Dirac fermions in graphene. Quantum anomalous Hall effect in intrinsic magnetic topological insulator MnBi 2Te 4. This work represents one of the earliest experimental studies of nonlinear Hall effect enabled by the combination of inversion symmetry breaking and C 3 rotational symmetry breaking.ĭeng, Y. Observation of the nonlinear Hall effect under time-reversal-symmetric conditions. Tuning inelastic light scattering via symmetry control in the two-dimensional magnet CrI 3. Electrical control of second-harmonic generation in a WSe 2 monolayer transistor. Nonlinear optics with 2D layered materials. Spontaneous gyrotropic electronic order in a transition-metal dichalcogenide. This paper demonstrated the nonreciprocal second-order nonlinear optical effect in 2D materials enabled by the inversion symmetry breaking of the magnetic point group. Giant nonreciprocal second-harmonic generation from antiferromagnetic bilayer CrI 3. Two-dimensional magnetic crystals and emergent heterostructure devices. Light–valley interactions in 2D semiconductors. Optical modulators with 2D layered materials. 2D materials and van der Waals heterostructures. Novoselov, K., Mishchenko, A., Carvalho, A. Electric field effect in atomically thin carbon films. Through engineering spontaneous symmetry breaking in magic-angle moiré superlattices, semimetallic graphene can be turned into a series of quantum states (such as a band insulator, Mott-like correlated insulator, quantum anomalous Hall insulator or superconductor), potentially offering new insights into strongly correlated physics, such as high-temperature unconventional superconductors and quantum spin liquids. This opens the possibilities to manipulate the internal quantum degrees of freedom (such as spin, valley and layer pseudospin) for the emerging fields of spintronics, valleytronics and twistronics.Įngineering symmetry breaking in 2D materials can create unique opportunities to integrate different broken symmetries within one system, providing an unprecedented path to underpin new physics and transform the landscape of technological innovations. The symmetry breaking in 2D materials can be engineered by a wide variety of physical and chemical approaches. Most of the fascinating physical phenomena in 2D materials are dictated by their underlying symmetry breaking, namely, the breaking of inversion, rotational, time-reversal and gauge symmetries. In this Technical Review, we focus on the recent progress on engineering the breaking of inversion, rotational, time-reversal and gauge symmetries in 2D layered materials, and present our perspectives on how these may lead to new physics and applications. Indeed, over the past 15 years, a wide variety of physical, structural and chemical approaches have been developed to engineer the symmetry breaking of 2D layered materials. Engineering symmetry breaking of 2D layered materials not only offers extraordinary opportunities to tune their physical properties but also provides unprecedented possibilities to introduce completely new physics and technological innovations in electronics, photonics and optoelectronics. Symmetry breaking in 2D layered materials plays a significant role in their macroscopic electrical, optical, magnetic and topological properties, including, but not limited to, spin-polarization effects, valley-contrasting physics, nonlinear Hall effects, nematic order, ferroelectricity, Bose–Einstein condensation and unconventional superconductivity.
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