| Research Abstract |
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.
|
