Direct visualization of surface phase of oxygen molecules physisorbed on Ag(111) surface: A two-dimensional quantum spin system

We report on the real-space observation of the two-dimensional distorted triangular lattice of physisorbed oxygen (O-2) molecules on an Ag(111) surface by low-temperature scanning tunneling microscopy. The physisorbed state of the O-2 monolayers was confirmed by measuring their thermal stability, which showed good agreement with previous thermal desorption spectroscopy. The distortion of the observed lattice was reproduced quantitatively by considering the intermolecular exchange interaction in Monte Carlo calculations, indicating a critical role of antiferromagnetic ordering of O-2 spins. In tunneling spectra, the Kondo resonance was not observed on the O-2 layer at 4.7 K unlike the case of physisorbed O-2 on Ag(110). These results indicate that an intrinsic S = 1 spin of the O-2 molecules was preserved to form a two-dimensional antiferromagnetic quantum spin system on the surface.