用$^{63}$ Cu核磁共振和核四极共振研究S型单晶CeCu $_2$ Si $_2$的磁性和超导性质
Magnetic and superconducting properties on S-type single-crystal CeCu$_2$Si$_2$ probed by $^{63}$Cu nuclear magnetic resonance and nuclear quadrupole resonance
摘要：
我们进行了$^{63}$ Cu核磁共振/核四极共振测量，以研究CeCu $_2$ Si $_2$的“超导主导”（$S$型）单晶的磁性和超导（SC）性质。虽然反铁磁（AFM）涨落的发展下降到1~K表明AFM临界性接近，但在0.8~K以下观察到Korringa行为，而在$T_{\rm c} \sim$ 0.6 K以上没有观察到磁异常。这些行为在$S$型CeCu $_2$ Si $_2$中是预期的。零场下的核自旋-晶格弛豫速率$1/T_1$的温度依赖性在130 mK以下与以前的多晶样品几乎相同，但在120 mK以下则向下偏离。事实上，SC态的$1/T_1$可以用双能隙$s_{\pm}$ -波模型来拟合，而不是用双能隙$s_{++}$ -波模型来拟合。在磁场下，在两个方向上的自旋磁化率明显降低到低于$T_{\rm c}$，表明形成了自旋单重态配对。自旋磁化率的剩余部分由$1/T_1T$计算的场致剩余态密度来理解，这归因于涡核的影响。在上临界场$H_{c2}$以上没有观察到磁异常，但观察到AFM涨落的发展，表明在强AFM涨落中实现了超导。
英文摘要：
We have performed $^{63}$Cu nuclear magnetic resonance/nuclear quadrupole resonance measurements to investigate the magnetic and superconducting (SC) properties on a "superconductivity dominant" ($S$-type) single crystal of CeCu$_2$Si$_2$. Although the development of antiferromagnetic (AFM) fluctuations down to 1~K indicated that the AFM criticality was close, Korringa behavior was observed below 0.8~K, and no magnetic anomaly was observed above $T_{\rm c} \sim$ 0.6 K. These behaviors were expected in $S$-type CeCu$_2$Si$_2$. The temperature dependence of the nuclear spin-lattice relaxation rate $1/T_1$ at zero field was almost identical to that in the previous polycrystalline samples down to 130~mK, but the temperature dependence deviated downward below 120~mK. In fact, $1/T_1$ in the SC state could be fitted with the two-gap $s_{\pm}$-wave rather than the two-gap $s_{++}$-wave model down to 90~mK. Under magnetic fields, the spin susceptibility in both directions clearly decreased below $T_{\rm c}$, indicative of the formation of spin singlet pairing. The residual part of the spin susceptibility was understood by the field-induced residual density of states evaluated from $1/T_1T$, which was ascribed to the effect of the vortex cores. No magnetic anomaly was observed above the upper critical field $H_{c2}$, but the development of AFM fluctuations was observed, indicating that superconductivity was realized in strong AFM fluctuations.
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