铁磁超导体UGe2中量子临界行为的中子散射研究

The research on heavy fermion materials has been being the central issue in the field of condensed matter physics, due to the fact which possesses rich phase transitions and novel physical properties. Uranium-based heavy fermion UGe2 has metallic ferromagnetism ground state at low temperatures. At enhanced pressures the non-conventional superconductivity would arise on the border of itinerant ferromagnetism. The research on the ferromagnetic transitions, the non-conventional superconductivity and their relationship would significantly promote the understanding of interaction and quantum critical phenomenon in strongly correlated materials. Neutron scattering is a kind of key experimental technique during the detection of both static structure (crystal structure and magnetic structure) and dynamics (phonon spectra and magnetic excitation). In this project, we would carry out the research on quantum critical behavior in UGe2 by employing the neutron scattering technique. Through combining research from neutron diffraction, inelastic neutron scattering and band structure calculation, the knowledge and understanding of the ground state of UGe2 at ambient pressures will be obtained. The characteristic of the ferromagnetic transition’s order parameters will be analyzed based on the neutron diffraction, transport and magnetism measurement under high pressures. The phase diagram on temperature and pressure will be built. The ferromagnetic transition, non-conventional superconductivity and their relationship will be correctly understood.

重费米子体材料因为具有非常丰富的相变行为和新奇的物理特性而成为凝聚态物理研究中的热点。U基重费米子体UGe2是一类巡游铁磁性材料，加压下会诱导出非常规超导电性。对UGe2中的铁磁相变、非常规超导电性及其关系的研究将显著加深人们对于物质内部相互作用和量子临界行为的认识。中子散射是研究材料结构（晶格结构、磁结构）和动力学行为（声子谱、磁激发）的关键实验技术。本项目拟采用中子散射实验技术研究U基重费米子体材料UGe2中的量子临界特性。结合中子衍射技术、中子非弹性散射测量和能带结构计算的研究，建立对UGe2常压下系统基态的正确认识和理解。结合高压下的中子衍射和常规的输运和磁性测量，获得加压下铁磁相变序参量的变化特征。综合上述研究，获得UGe2的温度-压力二元相图并建立对UGe2中铁磁相变、非常规超导电性及其相互关系的正确认识和理解。

f电子体系材料具有丰富多彩的相变行为和复杂多样的有序态，如铁磁性、反铁磁性、超导电性、电荷密度波、自旋密度波、重费米子行为、隐藏序等。f电子由于相互作用在低温下往往会形成磁有序态，而这种磁有序行为同超导电性、重费米子行为等相互竞争和共存，其磁性性质是决定和理解f电子体系材料奇异量子特性的重要基础。本项目基于中子散射和输运、红外光谱、理论计算等手段相结合的方法，研究了U基材料UGe2、USb2、UAu0.8Sb2中5f电子和Ce基材料CeSb2、Ce、CeAuAl3中4f电子的低温磁性和晶体场行为。1）研究获得了UGe2的磁结构和序参量信息，UGe2在52 K经历铁磁相变，但30 K附近未发现与FM2态相关的转变行为；2）USb2在210 K经历反铁磁相变，同时费米面上打开两个能隙44 meV和66 meV，f电子表现出巡游性，该反铁磁相变为自旋密度波相变；3）UAu0.8Sb2在71 K经历反铁磁相变，其反铁磁结构与USb2相同。低温下其反铁磁性保持不变，不存在其他类型的磁性相变；4）CeSb2低温下具有四个磁性相变，分别为磁有序、反铁磁1、反铁磁2和铁磁；5）测量获得了CeDHCP相和FCC相的4f电子晶体场能级4 meV；6）CeAuAl3非中心对称的四方相中，4f电子能级劈裂为三个，其最低的能级与晶体声子谱耦合形成反交叉，形成杂化的能带。本项目研究获得了对U基5f电子和Ce基4f电子低温下磁性相变行为的认识，测量得到了Ce基4f电子的晶体场激发行为，为深入理解f电子体系材料的复杂物理性质提供了重要的支持和依据。