NMR Study of Electronic State in the Votex Core in HTSC

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Recent experimental results have revealed that the quasiparticle excitations in the vortex state of high-T_c cuprates are very different from those of conventional s-wave superconductors. The fascinating electronic states such as the localized quasipaticle levels in the vortex core and the quasiparticle states extended from the vortex core are studied intensively. Moreover, the possibility that a vortex can accumulate a finite electric charge has been proposed. Although measurements of NMR and NQR are powerful probes for studying those issues, it is only very recent that the spatially-resolved measurements of NMR and NOR of high-T_c superconductors have come to be realized. Here we report spatially resolved NMR and NQR studies on the physics of vortex core matters.First, we obtain an experimental evidence that a vortex in HTSC traps a finite electric charge from the high resolution measurements of the nuclear quadrupole frequencies. In slightly overdoped YBa_2Cu_3O_7, the vortex is neg … More atively charged, while in underdoped YBa_2Cu_4O_8, it is positively charged. The sign of the trapped charge is opposite to the sign predicted by the conventional BCS theory. Moreover, in both materials, the accumulated charge is much larger than expected in the ordinary superconductors These unexpected features can he attributed to the novel electronic structure of the vortex in HTSC.Second, we report the site-selective dependence of ^<17>O-T_1 associated with the vortex state of YBa_2Cu_3O_7 and, YBa_2Cu_4O_8. We found that T_1^<-1> at the planar sites exhibits an unusual nonmonotonic NMR frequency dependence. In the region well outside the vortex core, T_1^<-1> cannot be simply explained by the density of states of the Doppler-shifted quasiparticles in the d-wave superconductor Based on T_1^<-1> in the vortex core region, we establish strong evidence that the local density of states within the vortex core is strongly reduced.Third, we have measured spatially-resolved NMR on ^<205>Tl site of optimally-doped Tl2201compound. ^<205>TI nuclei are good probe to observe antiferromagnetic fluctuations. We obtained a clear evidence of antiferromagnetic order in the vortex core of Tl2201. NMR relaxation rate (T_1^<-1>) of the vortex core region is largely enhanced, which is not observed in the superconducting region. This T_1^<-1> enhancement is caused by magnetic fluctuations generated from antiferromagnetic moments in the vortex cores. Temperature dependences of T_1^<-1> and the spectrum broadening support also the existence of antiferromagnetic vortex core.These features can be attributed to the novel electronic structure of the vortex in HTSC. Less