Superconductivity in potassium-doped p-quaterphenyl

Exploring new materials with high temperature superconductivity has attracted great interests. By doping potassium into p-quaterphenyl with four phenyl rings connected by single C-C bond in para position, we find that this material can have two superconducting phases with the critical temperature of 4.3 K and 7.2 K, respectively. Their superconducting parameters and phase diagram are obtained and compared with other members of polyparaphenylene oligomers. The formation of bipolarons and the effect of dopant are determined by Raman scattering measurements.