Searching superconductivity in potassium-doped p-terphenyl

Searching new superconducting materials and understanding their superconducting mechanisms are the important research directions in the condensed matter physics study. The recent discovery of aromatic hydrocarbon superconductors, including potassium-doped picene, phenanthrene and dibenzopentacene, has aroused considerable research interest of physicists and materials scientists. In this work, potassium-doped p-terphenyl is grown by sealing potassium and p-terphenyl with a mole ratio of 3 : 1 in high-vacuum glass tube and then annealed at 170 ℃ for 7 days or at 240 and 260 ℃ for 24 h. The crystal structure, molecular vibration, and magnetic property are characterized by using X-ray diffraction, Raman scattering, and superconducting quantum interference device. The combination of X-ray diffraction spectrum and Raman spectrum shows that besides potassium-doped p-terphenyl and KH, there exist C60 and graphite in annealed sample, which are found for the first time in the metal-doped aromatic hydrocarbon. Owing to the presence of potassium with high chemical activity, the C-H bond can be broken, resulting in dehydrogenated p-terphenyl with dangling bonds. Consequently, the recombination of dehydrogenated p-terphenyl will form graphite and C60. In addition, the red-shifts of partial peaks of p-terphenyl in Raman spectrum demonstrate that 4 s electron of doped potassium is transferred to C atom. For the samples annealed at 170 and 240 ℃, Curie paramagnetic behaviors are observed in the whole temperature region. On the other hand, in one of the samples annealed at 260 ℃, there exist three anomalous sharp decreases respectively at 17.86, 10.00 and 6.42 K in the zero-field cooling magnetic measurement. Previous studies indicated that the superconducting transition temperatures of potassium-doped C60 and potassium-doped graphite are about 18 K and 3 K. Therefore, it is reasonable to attribute the anomalous sharp decrease at 17.86 K to being produced by potassium-doped C60, while the anomalous sharp decreases at 10.00 and 6.42 K, which have not been reported yet, may be produced by potassium-doped p-terphenyl. The first principles calculations show that potassium-doped p-terphenyl lies in the metallic state, which can form superconductivity due to the electron-phonon interaction. Our results are useful for understanding the crystal growth and physical properties of metal-doped aromatic hydrocarbon organic superconductors. Furthermore, our findings provide a new routine to synthesizing C60 and graphite at low temperature.