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Approaching a Two-Dimensional (2D) Metallic State on the Surface of the Organic Semiconductor Rubrene

Whether metallic behavior can exist in 2D materials is a question that has troubled condensed matter physics for decades. Although originally thought impossible, evidence for such in ultra-clean high-purity doped inorganic semiconductor heterostructures based on materials such as Si and GaAs eventually changed the prevailing view. Research performed in IRG-1 using an approach to doping known as electrolyte gating has now shown that highly conductive (close to metallic) behavior can also be seen in 2D in an organic semiconductor, rubrene. This was enabled by techniques, based on the use of ionic liquids, that increase the density of holes on the surface by a thousand times over prior work. The mobility of the holes in rubrene remains far lower than inorganic semiconductors, however, raising perplexing questions about the fundamental origin of the conductive state.