Pronounced −Log T Divergence in Specific Heat of Nonmetallic CeOBiS2: A Mother Phase of BiS2-Based Superconductor

The low-temperature properties of CeOBiS2 single crystals are studied by electrical resistivity, magnetization, and specific heat measurements. Ce 4f-electrons are found to be in a well-localized state split by crystalline-electric-field (CEF) effects. The CEF ground state is a pure Jz = ±1/2 doublet, and excited doublets are located far above. At low temperatures in zero field, we observe pronounced −log T divergence in the specific heat, revealing the presence of quantum critical fluctuations of 4f magnetic moments near a quantum critical point (QCP). Considering that CeOBiS2 is a nonmetal, this phenomenon cannot be attributed to the competition between Kondo and the Ruderman–Kittel–Kasuya–Yosida (RKKY) interactions as in numerous f-electron-based strongly correlated metals, indicating an unconventional mechanism. We suggest that CeOBiS2 is the first material found to be located at a QCP among geometrically frustrated nonmetallic magnets.