CrAs超导体的研究和展望

Research and prospect of CrAs superconductor

重费米子超导体、铜基高温超导体和铁基高温超导体等新超导体的发现及非常规超导电性的研究一直推动着凝聚态物理学的发展.这类超导体的一个最基本特征是当长程反铁磁序被压制时会出现超导现象,而要破坏长程磁有序,除了掺杂不同元素以引入电荷载流子或者化学压力外,加压也是一种有效的调控手段.本工作在综述第一个Cr基化合物超导体CrAs的结构和基本物理性质基础上,重点介绍了通过加压CrAs的双螺旋磁性被压制,出现超导电性的衍化过程.此外也综述了CrAs的结构、磁性和超导电性等方面的后续研究工作以及其他相关新型超导体.在CrAs体系中,超导临近于双螺旋反铁磁序及出现的量子临界行为等多种实验证据表明CrAs具有非常规超导电性.CrAs超导电性的发现为探索新的非常规超导体打开了一条新路.

The study of superconductivity in unconventional superconductors such as heavy-fermions, high transition-tem-perature cuprates and iron pnictides has been promoting the development of condensed matter physics in recent decades. The superconductivity in these materials usually emerges in the vicinity of long-range antiferromagnetically ordered state. In addition to doping charge carriers, the application of external pressure is an effective way to induce unconventional superconductivity near a magnetic quantum critical point. In this paper, we summarize the structure and basic physical properties of CrAs, and the discovery of superconductivity in the vicinity of double helical antiferromagnetic order in CrAs via the application of external pressure. In addition, we also summarize several follow-up studies on the structure, magnetic and superconducting mechanism of CrAs. CrAs crystallize in the orthorhombic Mn P-type structure. A first order double helical magnetic transition occurs at about 270 K accompanied with discontinuous changes of lattice parameters. The antiferromagnetic fluctuations seem to be abundant above TN in that the magnetic susceptibility of CrAs keeps increasing with temperature up to at least 700 K under ambient pressure. As a matter of fact, such an increasing behaviour of magnetic susceptibility has been found to be universal in the recently discovered iron-based superconductors, and regarded as an indication for antiferromagnetic fluctuations. This magnetic transition can be readily suppressed under external pressure, and superconductivity is found with a maximum transition temperature of 2 K when the helical magnetic transition is completely suppressed at about 0.8 GPa. A number of experimental evidences suggest an unconventional pairing mechanism for CrAs. The observations of non-Fermi-liquid behavior and a dramatic enhancement of the effective mass near critical pressure in the transport measurements, which are characteristics of antiferromagnetic quantum critical point, signal the important role of critical spin fluctuations for superconductivity of CrAs. The superconducting volume and transition temperature in CrAs is very sensitive to sample quality, which is similar to Sr2RuO4 and some heavy fermion superconductors. The NMR（nuclear magnetic resonance） measurements indicate that there is no coherence effect near Tc and a T-3 dependence of 1/T1 below Tc, which are in support of the non BCS（Bardeen-Cooper-Schrieffer） mechanism and existence of line-node in superconducting gap function. In addition, the experimental results from NMR and neutron scattering under pressure also indicate the importance of magnetism and its possible interplay with the observed superconductivity. Controlling superconductivity by tunable quantum critical points by pressure in CrAs is a good example to find other new superconductor in the future. The existence of a QCP（quantum critical point） may offer a route to understand the origin of non-Fermi liquid properties, the microscopic coexistence between unconventional superconductivity and magnetic or some exotic order, and ultimately the mechanism of superconductivity itself, and we can deeply understand the critical spin fluctuations associated with the quantum criticality could act as an important glue medium for Cooper pairing. In this regard, further theoretical and experimental studies are needed to elucidate the pairing symmetry and the dominant mechanism, especially the role of magnetism, for the observed superconductivity in CrAs. The present finding opens a new avenue for searching novel unconventional superconductors in the Cr and other transition metal-based systems.