We report a dramatic change in the intensity of a Raman mode with applied magnetic field, displaying a gigantic magnetooptical effect. Using the nonmagnetic layered material MoS 2 as a prototype system, we demonstrate that the application of a magnetic field perpendicular to the layers produces a dramatic change in intensity for the out-of-plane vibrations of S atoms, but no change for the in-plane breathing mode. The distinct intensity variation between these two modes results from the effect of field-induced broken symmetry on Raman scattering cross-section. A quantitative analysis on the field-dependent integrated Raman intensity provides a unique method to precisely determine optical mobility. Our analysis is symmetry-based and material-independent, and thus the observations should be general and inspire a new branch of inelastic light scattering and magneto-optical applications.Raman | layered | magneto-optical | broken symmetry | phonon

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Resonating valence bond ground state, the prototype of a quantum spin liquid (QSL), had been first proposed by P. W. Anderson back in 1973 on the triangular lattice. In such a 2D QSL, spinons created as unpaired spins can propagate by locally rearranging the uncorrelated valence bonds. However, the corresponding candidate materials are still rare to date. YbMgGaO4, first proposed in 2015 may fill this gap: the magnetic rare‐earth Yb3+ ions arrange on the triangular lattice with the perfect R‐3m symmetries and with a large interlayer distance. No conventional spin freezing is observed down to 40 mK without residual magnetic entropy, despite the significant antiferromagnetic coupling of ≈2 K. This report reviews and classifies the relevant experimental and theoretical progress on some (effective) spin‐1/2 triangular antiferromagnets, especially on YbMgGaO4, followed by discussion and outlook on some of the pending issues.

The Kondo effect, an eminent manifestation of many-body physics in condensed matter, is traditionally explained as exchange scattering of conduction electrons on a spinful impurity in a metal [1, 2]. The resulting screening of the impurity's local moment by the electron Fermi sea is characterized by a Kondo temperature T K , below which the system enters a non-perturbative strongly-coupled regime. In recent years, this effect has found its realizations beyond the bulkmetal paradigm in many other itinerant-electron systems, such as quantum dots in semiconductor heterostructures [3, 4] and in nanomaterials [5-7], quantum point contacts [8,9], and graphene [10]. Here we report on the first experimental observation of the Kondo screening by chargeless quasiparticles. This occurs in a charge-insulating quantum spin liquid, where spinon excitations forming a Fermi surface take the role of conduction electrons. The observed impurity behaviour therefore bears a strong resemblance to the conventional case in a metal. The discovered spinonbased Kondo effect provides a prominent platform for characterising and possibly manipulating enigmatic host spin liquids.The Kondo screening of a magnetic impurity embedded in a quantum spin liquid (Fig. 1), a highly-quantumentangled yet magnetically disordered state where charge degrees of freedom are frozen, has been the subject of theoretical investigations for more than two decades [11][12][13][14][15][16][17]. In this case, the itinerant electrons of the standard Kondo picture are effectively replaced by emergent fractional magnetic excitations, inherent to any spin liquid. Due to a large variety of essentially different low-energy excitations, spin liquids should be particularly versatile hosts of Kondo physics. In the case of spinon excitations with a Fermi surface -a spinon metal -a Kondolike effect similar to the one in an ordinary metal is expected [13,15]. However, emergent gauge fields mediating spinon-spinon interactions could cause deviations from the generic Fermi-liquid Kondo behaviour. As the spin-liquid Kondo effect has not yet been conclusively confirmed by experiment, these theoretical predictions remain to be verified.Zn-brochantite, ZnCu 3 (OH) 6 SO 4 , is a particularly well-suited compound for investigating the Kondo effect in a spin liquid. This quantum kagome antiferromagnet [2] is one of only a few examples where a spin-liquid state remains stable down to the lowest experimentally 0 1 2 3 0.0 0.5 1.0 1.5 T c weak Kondo coupling spin-polarized impurities T* T K strong Kondo coupling T (K) B (T) gapped spin liquidPhase diagram of Zn-brochantite. Kondo screening of impurity spins (large arrows) by spinons (small arrows) in the spin-liquid state of Zn-brochantite, with the Kondo temperature TK = 1.3 K. The intensity of the blue hue indicates the slope of the imaginary part of the dynamical impurity spin susceptibility χ (ω)/ω at frequency ω → 0. The dotted line marks the contour corresponding to the value at TK and B = 0. The temperature T * denotes the crossover ...

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