高温超导体的核磁共振研究

NMR investigations of high-T_c superconductors

本文回顾了用核磁共振技术研究高温超导体的进展,着重介绍局域电荷分布,超导态性质以及赝能隙性质.除了CuO2面上的空穴密度总数外,Cu和O轨道上的空穴数的分布也是决定Tc的一个重要参数.核自旋晶格弛豫率以及奈特位移的实验结果显示,超导态的电子配对是d波单态配对.d波能隙函数里存在节点,使得准粒子从涡线中心“漏”到涡线外面.节点的存在也是非磁性杂质以及晶体无序会导致Tc明显降低的原因.45T高场下的实验发现,高温超导体的零温正常态是“费米弧”金属态.这说明赝能隙态和超导态是两个微观共存的物态.

This article reviews insights on high-Tc superconductors from the nuclear magnetic resonance （NMR） technique, with emphasis on local hole density, d-wave superconducting gap and its consequences, and the nature of the pseudogap. In addition to the total hole density in the CuO2 plane, the partition of the holes between Cu and O orbitals is an important parameter in determining Tc. The temperature dependence of the spin-lattice relaxation rate and the Knight shift indicate d-wave pairing, which brings about the extended quasiparticle states outside the vortex cores. The existence of nodes in the gap function also results in strong Tc reduction by non-magnetic impurity or crystal disorder. The zero-temperature normal state has been revealed by the application of strong magnetic fields up to 45T to be a ＂Fermi arc＂ metallic state, which suggests that the pseudogap and the superconducting states are coexisting states of matter.