碳基超导体的单晶生长及其超快电子态和超快晶格动力学的研究

Recently, the application of ultrafast laser in materials to produce novel phases and even transient room temperature superconductivity has attracted wide attention, and it is playing an important role in understanding the superconducting mechanism and also searching new high-temperature superconductors. This new phenomenon is usually induced by coherent photon-phonon excitation, which needs comparable photon energy with the phonon energy. However, because of the limitation of laser technology, it is difficult to obtain far infrared ultrashort laser pulse with high intense and repetition rate in such studies. A new approach is to study materials with high lattice vibration energy, and it is necessary to search materials to meet the required phonon energy in such studies. Because of the light weight of carbon atom, carbon-based solid materials usually have higher vibration energy, and moreover, carbon-based compounds have rich physics such as the superconductivity in CaC6 and K3C60, which provides an opportunity for such research. This project will focus on the single crystal growth of CaC6 and K3C60, and using time- and angle-resolved photoemission spectroscopy and MeV ultrafast electron diffraction to study their ultrafast electronic and lattice dynamic after pumping by far-infrared laser pulses. The study will provide important and new information in understanding the mechanism of high-temperature superconductivity and great guidance in searching high-temperature superconductors.

近年来利用超快激光控制材料产生新奇物性甚至瞬态室温超导现象引起了研究者广泛关注，对研究高温超导机制和探索新的高温超导材料具有重要指导作用。这种新奇超快现象是由超快激光共振激发晶格振动或原子错位引起的。在此类研究中由于激光技术的限制，难以获得高强度、高重频的远红外超短激光脉冲。另一可行途径是在晶格振动能量较高的材料中开展此类研究。由于碳原子质量较轻，基于碳的化合物一般具有较高晶格振动能量，此外碳基化合物如CaC6、K3C60等具有新奇超导电性，这为开展此类研究提供了契机。本项目将采用化学方法合成高质量CaC6和K3C60碳基超导单晶，并利用时间分辨角分辨光电子能谱和兆电子伏特超快电子衍射实验研究它们在超快远红外激光控制下的非平衡电子态和超快晶格动力学。本项目将给出激光控制下非平衡室温超导的光电子能谱证据并结合超快晶格动力学理解其非平衡态超导机制，为研究高温超导机制和探索高温超导材料提供依据。

近年来利用超快激光控制材料产生新奇物性甚至瞬态室温超导现象引起了研究者广泛关注，对研究高温超导机制和探索新的量子材料具有重要指导作用。这种新奇超快现象是由超快激光共振激发晶格振动或原子错位引起的。在此类研究中由于激光技术的限制，难以获得高强度、高重频的远红外超短激光脉冲。另一可行途径是在晶格振动能量较高的材料中开展此类研究。由于碳原子质量较轻，基于碳的化合物一般具有较高晶格振动能量，此外碳基化合物如CaC6等具有新奇超导电性，这为开展此类研究提供了契机。激子绝缘体Ta2NiSe5具有与传统BCS超导体库珀对类似基态的电子空穴对，由于通常在临界温度电子空穴对会诱导电荷密度波和结构相变，其纯电子相变的物理机制缺乏实验证据。本项目首先将采用化学方法合成CaC6碳基超导单晶，并利用时间分辨角分辨光电子能谱研究CaC6和Ta2NiSe5在超快远红外激光控制下的非平衡电子态。本项目实验结果揭示了Ta2NiSe5中低温能隙是由于强电子空穴相互作用以及超快相变的起源是电子相变。同时实验结果也表明仅考虑电子空穴的库伦力是不足以来诱导如此强电子空穴相互作用，电子与其他自由度（如晶格、轨道）相互作用的多体效应应当考虑在内。本项目给出激光控制下非平衡量子材料的光电子能谱证据，为研究高温超导机制和探索量子材料提供依据。