Superconductivity in just four pairs of (BETS)2GaCl4 molecules

How small can a sample of superconducting material be and still display superconductivity? This question is relevant to our fundamental understanding of superconductivity, and also to applications in nanoscale electronics, because Joule heating of interconnecting wires is a major problem in nanoscale devices. It has been shown that ultrathin layers of metal can display superconductivity1,2,3, but any limits on the size of superconducting systems remain a mystery. (BETS)2GaCl4, where BETS is bis(ethylenedithio)tetraselenafulvalene, is an organic superconductor, and in bulk it has a superconducting transition temperature Tc of ∼8 K (ref. 4) and a two-dimensional layered structure5,6,7 that is reminiscent of the high-Tc cuprate superconductors8,9. Here, we use scanning tunnelling spectroscopy to show that a single layer of (BETS)2GaCl4 molecules on an Ag(111) surface displays a superconducting gap that increases exponentially with the length of the molecular chain. Moreover, we show that a superconducting gap can still be detected for just four pairs of (BETS)2GaCl4 molecules. Real-space spectroscopic images directly visualize the chains of BETS molecules as the origin of the superconductivity. Real-space scanning tunnelling spectroscopy has provided new insights into the properties of a well-known two-dimensional organic superconductor, including a superconducting gap that increases exponentially with the length of the molecular chain.