Abstract:

The controlled coupling of spin centers is essential in the construction of molecular spin-based quantum information processing architectures. A major challenge is to induce the requisite coupling between two adjacent spins, while protecting them from neighboring spins and other environmental interactions. Owing to their native spin properties, endohedral fullerenes are attractive for use as elements in quantum information processing architectures. N@C₆₀ is an endohedral fullerene molecule with a highly reactive nitrogen atom at the center of the carbon cage. The endohedral nitrogen is atomic and not covalently bound to the cage atoms; therefore, the nitrogen atom is chemically inert toward the outer environment. Owing to its remarkably long electron-spin lifetimes and sharp resonances, N@C₆₀ has exceptional properties for quantum computing. The thermal stability and molecular structure of N@C₆₀ make it a useful embodiment of a quantum bit — a fundamental element for a quantum computer. Several future quantum computer architectures based on N@C₆₀ have been proposed, one of which is a two-dimensional, quantum-bit array on specific substrates. However, a challenging yet important task is to understand the effect of various substrates on the spin properties of the endohedral fullerene, since the interaction between the endohedral fullerene and the substrate may largely affect the spin characters of the endohedral N atom. The fabrication of an endohedral fullerene molecular array on substrates is also a challenge because high-temperature methods such as evaporation will cause decomposition of N@C₆₀. Here we report our investigation on the electron spin resonance (ESR) of N@C₆₀ molecules on various substrates such as Au(111), Si(111), and SiO₂. In this study, N@C₆₀ was prepared using the ion implantation method, and enrichment was performed using a multistep and recycling high-performance liquid chromatography (HPLC) system with a Cosmosil Buckyprep column. N@C₆₀ molecular films on Au(111) substrates were prepared at room temperature. In addition, scanning tunneling microscope (STM) topography of the N@C₆₀/C₆₀ monolayer on Au(111) was obtained at a sample temperature of 5 K in ultra high vacuum (UHV). We found that the ESR signal of the N@C₆₀ molecules decreases rapidly and disappeared approximately 360 min after the deposition of N@C₆₀ on Au(111). In comparison, the ESR signal was maintained for a longer time on the Si(111) and SiO₂ substrates. We propose that coupling between the Au(111) substrate and the endohedral N atoms quenches the ESR signal of the endohedral N atom, while the Si(111) and SiO₂ substrates have a smaller effect on the ESR signal. This result offers useful information for the design of basic quantum computer architectures.

Key words: Au(111), Endohedral fullerene, Scanning tunneling microscopy, Electron spin Resonance, Decohence, Dipolar coupling