Research and prospect of CrAs superconductor

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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 MnP-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 T N 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 T c and a T 3 dependence of 1/ T 1 below T c, 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.