Quantum spin liquids (QSLs) are a novel state of matter predicted to arise in quantum antiferromagnets where magnetic frustration or quantum fluctuations are strong enough to prevent magnetically ordered states even down to the lowest temperatures. QSLs are believed to exist in strongly correlated Mott insulators, and are thus related to unconventional superconductivity. Much work on QSLs has focused on triangular lattices where frustration is strong. An example is the bulk Mott insulator 1T-TaS2 which has attracted attention as a QSL candidate due to localized d-orbitals in the Ta atoms that form a triangular lattice in this material. This scenario, however, is complicated by interlayer coupling and possible different stacking orders in the bulk, thus motivating investigation into related single-layer materials.
I will discuss our recent studies on single-layer (SL) 1T-TaSe2 that provide evidence for 2D QSL behavior. We have characterized the electronic structure of SL 1T-TaSe2 (grown via molecular beam epitaxy) by means of scanning tunneling microscopy/spectroscopy (STM/STS), angle-resolved photoemission spectroscopy (ARPES), and first-principles calculations. We observe Mott insulating behavior in SL 1T-TaSe2, including novel orbital texture not seen in bulk samples. Vertical heterostructures formed by a single 1T-TaSe2 layer placed on top of metallic 1H-TaSe2 exhibit Kondo behavior, providing direct evidence for a triangular array of local spins in SL 1T-TaSe2, a prerequisite for the QSL behavior. Moreover, in SL 1T-TaSe2 we observe long-wavelength super-modulations that are explained quantitatively by a QSL-based spinon Fermi surface instability.