Systematic study on thermal conductivity of organic triangular lattice systems ??-X [Pd(dmit)2]2

Systematic variation of the low-temperature heat transport properties of the quasi-two-dimensional quantum spin liquid (QSL) candidate ??'-EtMe3Sb[Pd(dmit)2]2 and its analog compounds showing antiferromagnetic (AFM) ordering is discussed using thermal conductivity ?? measurements. All compounds with the QSL ground state show a monotonic decrease in ?? with decreasing temperature, which is a typical feature of glass or amorphous materials, caused by their low-energy phonon structures. In contrast, the AFM compounds exhibit a peak structure of ?? around 10 K, which is the characteristic temperature dependence of normal crystals with coherent phonons. Our results indicate that the anomalous glassy behavior in ??'-EtMe3Sb[Pd(dmit)2]2 is an intrinsic and universal feature of the organic QSL phase and is not caused by extrinsic factors, such as the quality of the samples and the microcracks occurring during the cooling process. We propose a scenario for the origin of the glassy behavior of QSL compounds in which the charge fluctuation within the Pd(dmit)2 dimers changes the phonon properties from crystalline to glasslike.