NMR initiatives on understanding high-temperature superconductivity

Abstract We review a recent progress of NMR studies [H. Mukuda, et al., Phys. Rev. Lett 96 (2006) 087001; S. Shimizu, et al., submitted for publication.] on multi-layered cuprates. This work has shed new light to a generic phase diagram of high-temperature superconductivity (HTSC) which suggests a competition between antiferromagnetism (AFM) and superconductivity (SC). The multi-layered cuprates include two types of CuO 2 planes, an outer CuO 2 plane (OP) in a pyramidal coordination and an inner CuO 2 plane (IP) in a square one with no apical oxygen. Remarkable feature of the multi-layered systems is the presence of ideally flat CuO 2 planes that are homogeneously doped. Systematic Cu-NMR studies on the optimally-doped five-layered HgBa 2 Ca 4 Cu 5 O 12 + δ (Hg-1245(OPT)) and slightly overdoped Tl-1245(OVD) have revealed the coexistent phase of SC and AFM in a unit cell [H. Kotegawa, et al., Phys. Rev. B 64 (2001) 064515; H. Kotegawa, et al., Phys. Rev. B 69 (2004) 014501.]. The optimally doped two OPs are predominantly superconducting with T c = 108 and 100 K, whereas the under-doped three IPs show the AFM order below T N = 60 and 45 K for Hg-1245(OPT) and Tl-1245(OVD), respectively. Recently exciting is the finding of the uniform mixing of AFM and HTSC in a single CuO 2 layer in the under-doped Hg-1245(UD) and the heavily underdoped four-layered Ba 2 Ca 3 Cu 4 O 8 F 2 (0234F(2.0)) that has fluorine ions ( F 1 - ) as apical ions [H. Mukuda, et al., Phys. Rev. Lett 96 (2006) 087001; S. Shimizu, et al., submitted for publication.]. In Hg-1245(UD) with T c = 72 K and T N = 290 K , the OPs exhibit the uniform mixing of AFM and HTSC with AFM moment of M AFM ( OP ) = 0.1 μ B , whereas the IPs are possibly AFM insulators with a small doping [H. Mukuda, et al., Phys. Rev. Lett 96 (2006) 087001.]. In 0234F(2.0) with T c = 55 K and T N = 100 K , the uniform mixing of AFM and HTSC is demonstrated to take place in electron (n)- doped IPs [S. Shimizu, et al., submitted for publication.], thanks to insight from recent ARPES results [Y. Chen, et al., unpublished.]. The Cu-NMR studies at zero magnetic field have revealed that in the hole (p)-doped OPs, the uniform mixing of AFM with 0.06 μ B and HTSC realizes. The NMR works have demonstrated that the uniform mixing of AFM and HTSC takes place in both n- and p-underdoped regimes.