NMR Study of Electronic State in the Votex Core in HTSC

最近的实验结果表明，高t_c铜酸盐涡旋态的准粒子激发与传统的s波超导体有很大的不同。对涡旋核中的局域准粒子能级和从涡旋核延伸出来的准粒子态等引人入胜的电子态进行了深入的研究。此外，还提出了涡旋可以积累有限电荷的可能性。虽然核磁共振和NQR的测量是研究这些问题的有力探针，但直到最近才实现了高t_c超导体的核磁共振和NOR的空间分辨测量。在这里，我们报告了涡旋核物质物理的空间分辨核磁共振和NQR研究。首先，我们从核四极子频率的高分辨率测量中获得了HTSC中涡旋捕获有限电荷的实验证据。在轻度掺杂的YBa_2Cu_3O_7中，涡旋带负电，而在欠掺杂的YBa_2Cu_4O_8中，涡旋带正电。捕获电荷的符号与传统BCS理论预测的符号相反。此外，在这两种材料中，累积电荷都比普通超导体中预期的要大得多。这些意想不到的特征可以归因于HTSC中涡旋的新颖电子结构。其次，我们报道了^<17>O-T_1与YBa_2Cu_3O_7和YBa_2Cu_4O_8涡旋状态的位点选择性依赖关系。我们发现在平面位置T_1^<-1>表现出不寻常的非单调核磁共振频率依赖性。在涡旋核外的区域，T_1^<-1>不能简单地用d波超导体中多普勒位移准粒子的态密度来解释。基于涡旋核区域的T_1^<-1>，我们建立了涡旋核内局部态密度被强烈降低的有力证据。第三，我们测量了最佳掺杂tl2201化合物^<205>Tl位点的空间分辨核磁共振。^<205>的TI核是观测反铁磁波动的良好探针。我们获得了Tl2201涡旋核反铁磁序的明确证据。涡旋核区的核磁共振弛豫率（T_1^<-1>）大大增强，这在超导区是没有观察到的。这种T_1^<-1>增强是由涡旋核中反铁磁矩产生的磁波动引起的。T_1^<-1>的温度依赖性和谱展宽也支持了反铁磁涡旋核的存在。这些特征可归因于HTSC中涡的新颖电子结构。少

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