Searching superconductivity in potassium-doped p-terphenyl

Abstract

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.