Direct Visualization of Surface Phase of Oxygen Molecules Physisorbed on the Ag(111) Surface: A Two-dimensional Quantum Spin System

Oxygen molecule (O$_{\mathrm{2}})$ is one of the smallest molecular magnets with an $S=$1 quantum spin. This makes O$_{\mathrm{2}}$ attractive as a building block of low-dimensional (LD) quantum spin systems. Recently, the existence of a spin in physisorbed O$_{\mathrm{2}}$ on Ag(111) was confirmed by the ortho-para conversion of molecular hydrogen. Therefore, there is a strong need for STM-based techniques with single-molecule resolution in order to verify the potential of the O$_{\mathrm{2}}$/Ag(111) for LD quantum spin systems. Here we report the real-space observation of oxygen molecules physisorbed on an Ag(111) surface by using low-temperature scanning tunneling microscopy and spectroscopy. A well-ordered O$_{\mathrm{2}}$ structure was observed, and the lattice was distorted from an isosceles triangular lattice. The distortion can be explained by the competition between the magnetic and elastic instabilities of the O$_{\mathrm{2}}$ lattice. In differential tunneling conductance spectra, we found no feature of the Kondo resonance at 4.7 K; in contrast, the physisorbed O$_{\mathrm{2}}$ on Ag(110) showed a clear Kondo resonance at 18 K. Based on these observations, we discuss the realization of an $S=$1 two-dimensional antiferromagnetic quantum spin system.