Experimental Evidence for Spin Liquid States

Journal Club talk by Andrea Lupascu
on May 20, 2010 (click here for ppt file)
Summary by R. Ganesh


References
Yamashita et al.: Nature Physics, 4, 459 (2008)
Nakatsuji et al.: Science, 309, 1697 (2005)
Helton et al., PRL 98, 107204 (2007)

Andrea presented a crisp and topical journal club talk on Quantum Spin Liquids. After a broad defintion, she listed essential features of the QSL states. While there is no conclusive experimental signature, there are many experimental hints. She discussed candidate QSL states which all have antiferromagnetic correlations seen from negative Curie Weiss temperatures. All candidates are also Mott insulators (the candidates from the organic family are weakly Mott
insulating).

Taking two prominent examples - Herbertsmithite with a Kagome lattice structure and an organic salt with a triangular lattice structure, Andrea discussed experimental pointers to a spin liquid ground state. In particular, she showed susceptibility and heat capacity data as a function of temperature. These quantities show no sign of ordering and point to the existence of a Fermi surface, even though the systems are Mott insulators as seen from resistivity. This Fermi surface is conjectured to be associated with spinon excitations of a spin liquid ground state.

With this background, she presented her own results on an organic salt in which Mn ions form a 'star' lattice. Its large value of the \gamma coefficient makes it a promising candidate for a QSL. This led to copious and enjoyable discussion on issues such as the lattice structure, the phonon contribution to specific heat, other probes and possibility of ordering at lower temperatures.