Highly Mobile Gapless Excitations in a Two-Dimensional Candidate Quantum Spin Liquid

Quantum Spin Liquid In antiferromagnets, the lowest-energy state is reached when neighboring spins on the underlying lattice point in opposite directions. Because of geometric constraints on some lattices (such as the two-dimensional triangular lattice), this magnetic ordering cannot be achieved even at temperatures close to absolute zero, and these compounds are predicted to be in a quantum spin liquid state. Yamashita et al. (p. 1246) measured the thermal conductivity of a recently identified candidate quantum spin liquid, the organic compound EtMe3Sb[Pd(dmit)2]2, and characterized its lowest-lying excitations. Two types of excitations were observed: ballistically propagating gapless excitations and excitations associated with a finite spin gap. These results contribute to our understanding of this unusual state of matter, which is potentially relevant to other two-dimensional quantum systems. Thermal conductivity measurements on a novel organic insulator help describe its microscopic electronic structure. The nature of quantum spin liquids, a novel state of matter where strong quantum fluctuations destroy the long-range magnetic order even at zero temperature, is a long-standing issue in physics. We measured the low-temperature thermal conductivity of the recently discovered quantum spin liquid candidate, the organic insulator EtMe3Sb[Pd(dmit)2]2. A sizable linear temperature dependence term is clearly resolved in the zero-temperature limit, indicating the presence of gapless excitations with an extremely long mean free path, analogous to excitations near the Fermi surface in pure metals. Its magnetic field dependence suggests a concomitant appearance of spin-gap–like excitations at low temperatures. These findings expose a highly unusual dichotomy that characterizes the low-energy physics of this quantum system.