Beyond the absence of long-range magnetic orders, the most prominent feature of the elusive quantum spin liquid (QSL) state is the existence of fractionalized spin excitations, i.e., spinons. When the system orders, the spin-wave excitation appears as the bound state of the spinon-antispinon pair. Although scarcely reported, a direct comparison between similar compounds illustrates the evolution from spinon to magnon. Here, we perform the Raman scattering on single crystals of two quantum kagome antiferromagnets, of which one is the kagome QSL candidate Cu3Zn(OH)6FBr, and another is an antiferromagnetically ordered compound EuCu3(OH)6Cl3. In Cu3Zn(OH)6FBr, we identify a unique one spinon-antispinon pair component in the E2g magnetic Raman continuum, providing strong evidence for deconfined spinon excitations. In contrast, a sharp magnon peak emerges from the one-pair spinon continuum in the Eg magnetic Raman response once EuCu3(OH)6Cl3 undergoes the antiferromagnetic order transition. From the comparative Raman studies, we can regard the magnon mode as the spinon-antispinon bound state, and the spinon confinement drives the magnetic ordering.
Structural symmetry of crystals plays important roles regarding the physical properties of two-dimensional (2D) materials, particularly in the nonlinear optics regime. There has been a long-term exploration of the physical properties in 2D materials with various stacking structures, which correspond to different structural symmetries. Usually, the manipulation of rotational alignment between layers in 2D heterostructures is realized at the synthetic stage through artificial stacking like assembling building blocks. However, the reconfigurable control of translational symmetry of crystalline structure is still challenging. High pressure, as a powerful external control knob, provides a very promising route to circumvent this constraint. Here, a pressure-controlled symmetry transition in layered InSe is experimentally demonstrated. The continuous and reversible evolution of structural symmetries can be in situ monitored by using polarization-resolved second-harmonic-generation (SHG) spectroscopy. As pressure changes, the reconfigurable symmetry transition of the SHG pattern from threefold rotational symmetry to mirror symmetry is experimentally observed in layered InSe samples and successfully explained by the proposed interlayer-translation model. This opens new routes toward potential applications of manipulating crystal symmetry of 2D materials.
Ambipolar field-effect transistor (FET) devices based on two-dimensional (2D) materials have been attracted much attention due to potential applications in integrated circuits, flexible electronics and optical sensors. However, it is difficult to tune Fermi level between conduction and valence bands using a traditional SiO2 as dielectric layer. Here, we employed the lithium-ion conductive glass ceramic (LICGC) as the back-gate electrode in a monolayer WS2 FET. The effective accumulation and dissipation of Li+ ions in the interface induce a wide tune of Fermi level in the conducting channel by electron and hole doping, which show an ambipolar transport characteristics with threshold voltages at 0.9 V and −1.3 V, respectively. Our results provide an opportunity for fabricating ultra-thin ambipolar FET based on 2D materials.
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