The existence of a quantum spin liquid (QSL) in which quantum fluctuations of spins are sufficiently strong to preclude spin ordering down to zero temperature was originally proposed theoretically more than 40 years ago, but its experimental realisation turned out to be very elusive. In 1973 Anderson introduced a resonating valence bond (RVB) state (1) as a new kind of insulator that was proposed to be the ground state of the triangular-lattice S = 1/2 Heisenberg antiferromagnet instead of a more conventional Neel state. The proposal was put forward to account for the unusual magnetic properties of a perfect triangular atomic lattice of Ta atoms in the layered transition metal dichalcogenide 1T-TaS2. Since then, the list of materials with triangular lattice and with properties indicating the existence of a quantum spin-liquid (QSL), i.e., a state without spontaneously broken triangular lattice symmetry and whose behaviour is dominated by emergent fractional excitations, is still remarkably short: it includes YbMgGaO4 (2) and some organic molecular solids, e.g., K-(ET)2Cu2(CN)3 (3). Compared to these compounds, layered dichalcogenides have perfect triangular lattice geometry and a weaker spin-orbit coupling, offering a possibility for obtaining a unique insight into the competition between antagonistic QSL and Neel states, however, no signatures of QSL behaviour have been observed with spins on atomic lattice sites.
Here we report (4) on an almost ideal spin liquid state that appears to be realized by atomic-cluster spins on the triangular lattice of a charge-density wave (CDW) state of 1T-TaS2. In this system, the charge excitations have a well-defined gap of about ~0.3 eV, while nuclear magnetic quadrupole resonance and muon spin relaxation experiments reveal that the spins show gapless quantum spin liquid dynamics and no long range magnetic order down to 70 mK. Canonical T2 power-law temperature dependence of the spin relaxation dynamics characteristic of a QSL is observed from 200 K to Tf = 55 K. Below this temperature we observe a new gapless state with reduced density of spin excitations and high degree of local disorder signifying new quantum spin order emerging from the QSL.
(1) P. Anderson, Materials Research Bulletin 8, 153 (1973).
(2) Y. Shen, et al., Nature 540, 559 (2016).
(3) T. Itou, A. Oyamada, M. S., R. Kato, Nat. Phys. 6, 673 (2010).
(4) M. Klanjšek et al., arXiv:1704.06450.