钾掺杂对三联苯的超导特性探寻

Searching superconductivity in potassium-doped p-terphenyl

新型超导材料的设计合成及其超导机理的探索是目前凝聚态物理学领域的重要研究方向.本文采用高真空热烧结方法制备了钾掺杂对三联苯粉末材料并表征了它们的晶体结构、分子振动、磁学及超导特性.X射线衍射图谱和拉曼光谱表明在烧结样品中除存在钾掺杂对三联苯和KH外,还含有苯环重组的C_(60)和石墨成分.拉曼光谱中部分峰位的红移进一步证实钾成功掺入对三联苯分子晶体中并将4 s电子转移到C原子上.零场冷却磁性测量结果表明:多数样品在整个温度测量区间表现为居里顺磁性,但少数样品呈现出抗磁性,而且在17.86,10.00和6.42 K三个温度点出现磁化率突降的反常行为,其中17.86 K处的突降很可能源于钾掺杂C60引起的超导转变,而后两者可能与钾掺杂对三联苯导致的超导相关.研究结果有助于理解金属掺杂芳香烃有机超导体这一新兴超导家族的晶体生长和物理特性,同时也提供了一种低温制备C_(60)和石墨的新方法.

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 ： 1in high-vacuum glass tube and then annealed at 170°C for 7 days or at 240 and 260°C 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 C_（60） 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 C_（60）. 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°C, Curie paramagnetic behaviors are observed in the whole temperature region. On the other hand, in one of the samples annealed at 260°C, 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 C_（60） 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 C_（60）, 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 C_（60） and graphite at low temperature.