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NMR STUDY OF THE ELECTRONIC STRUCTURE OF VORTICES IN D-WAVE SUPERCONDUCTORS

D波超导体中涡旋电子结构的核磁共振研究

基本信息

批准号：
11640350
负责人：
ZHENG Guo-qing
金额：
$ 2.3万
依托单位：
OSAKA UNIVERSITY
依托单位国家：
日本
项目类别：
Grant-in-Aid for Scientific Research (C)
财政年份：
1999
资助国家：
日本
起止时间：
1999 至 2000
项目状态：
已结题

来源：
https://kaken.nii.ac.jp/grant/KAKENHI-PROJECT-11640350/
关键词：
high temperature superconductor vortex NMR spin susceptibility ナイトシフト

项目摘要

Magnetic field penetrates into a second-kind superconductor in a form of quantized vortex flux. In an s-wave superconductor with isotropic energy gap, the quasiparticle is localized within the vortex core whose radius is the superconducting coherence length. By contrary, the energy gap of d-wave superconductors, such as the high temperature superconductors, is anisotropic with line-nodes in the gap structure. Because of this gap anisotropy, the electronic state is expected to be quite different from that in an s-wave superconductor. In fact, recent theories predict that the quasiparticle state may be extended along the nodal directions to the outside of the core. However, this prediction is yet to be confirmed by experiments. In this project, we have studied the electronic structure around a vortex using the ^<63>Cu NMR method.In an overdoped high-T_c cuprate TlSr_2CaCu_2O_<6.8>(T_c=68 K), we have measured the Knight shift in the superconducting state as a function of magnetic field up to 28 Tesla. We find that the spin shift increases as increasing field. This gives an experimental evidence that there exist quasiparticle states outside the vortex core. Analysis shows that the spin shift increases as a square root of the applied field, which is in agreement with what is predicted by recent theories for a d-wave superconductor. Furthermore, we find that among the field-induced density of states, 85% of it are extended outside of the vortex core, and 15% is localized in the core.This study provides a quantitative account for the quasiparticle density of states associated with a vortex for the first time.

磁场以量子化涡旋通量的形式进入第二类超导体。在具有各向同性能隙的s波超导体中，准粒子定域在半径为超导相干长度的涡核内。而d波超导体（如高温超导体）的能隙是各向异性的，差距结构中存在线节点。由于这种带隙各向异性，电子状态预计与s波超导体中的电子状态截然不同。事实上，最近的理论预测准粒子态可能会沿着节点方向延伸到核的外部。然而，这一预测还有待实验证实。在本项目中，我们用~ 1HCu NMR方法研究了涡旋周围的电子结构<63>，在过掺杂的高T_c铜酸盐TlSr_2CaCu_2O_<6.8>（T_c=68 K）中，测量了在28 T磁场下超导态的奈特位移。我们发现自旋位移随电场的增加而增加。这为涡核外存在准粒子态提供了实验证据。分析表明，自旋位移随外加磁场的平方根增加，这与最近的d波超导体理论的预测是一致的。此外，我们还发现，在场致态密度中，有85%的场致态密度分布在涡旋核外，15%的场致态密度分布在涡旋核内，这一结果首次对与涡旋相关的准粒子态密度提供了定量的解释.