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NMR Study of Electronic State in the Votex Core in HTSC

HTSC 中 Votex 核心电子态的核磁共振研究

基本信息

批准号：
14340096
负责人：
KUMAGAI Ken-ichi
金额：
$ 9.47万
依托单位：
Hokkaido University
依托单位国家：
日本
项目类别：
Grant-in-Aid for Scientific Research (B)
财政年份：
2002
资助国家：
日本
起止时间：
2002 至 2003
项目状态：
已结题

项目摘要

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

最近的实验结果表明，高T_c铜酸盐涡旋态下的准粒子激发与传统的s波超导体有很大不同。对涡核中的局域准粒子能级和从涡核延伸出的准粒子态等令人着迷的电子态进行了深入研究。此外，还提出了涡流可以积累有限电荷的可能性。尽管 NMR 和 NQR 测量是研究这些问题的有力探针，但直到最近才实现了高温超导体 NMR 和 NOR 的空间分辨测量。在这里，我们报告了关于涡核物质物理的空间分辨核磁共振和核磁共振研究。首先，我们通过核四极频率的高分辨率测量获得了实验证据，表明 HTSC 中的涡流捕获了有限的电荷。在稍微过量掺杂的 YBa_2Cu_3O_7 中，涡流带负电，而在欠掺杂的 YBa_2Cu_4O_8 中，涡流带正电。俘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，累积的电荷比普通超导体中的预期大得多。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，我们报告了与YBa_2Cu_3O_7和YBa_2Cu_4O_8的涡旋态相关的^<17>O-T_1的位点选择性依赖性。我们发现平面位点的 T_1^<-1> 表现出不寻常的非单调 NMR 频率依赖性。在涡核之外的区域，T_1^<-1>不能简单地用d波超导体中多普勒频移准粒子的态密度来解释。基于涡核区域中的T_1^<-1>，我们建立了强有力的证据，表明涡核内的局域态密度大大降低。第三，我们测量了空间分辨核磁共振 ^最佳掺杂的Tl2201化合物的205Tl位点。 ^<205>TI 原子核是观察反铁磁涨落的良好探针。我们在 Tl2201 涡核中获得了反铁磁序的明确证据。涡核区域的NMR弛豫率(T_1^-1)大大增强，这在超导区域中没有观察到。这种T_1^<-1>增强是由涡核中的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反铁磁涡旋核的存在。这些特征可归因于HTSC中涡旋的新颖电子结构。较少的