Nature © Macmillan Publishers Ltd 1998 8 letters to nature 658 NATURE | VOL 396 | 17 DECEMBER 1998 | www.nature.comT CO 317 K. Hence a conducting, magnetically ordered state is not found in Fe 2 OBO 3 as T C , T CO . The differences between the estimated activation energies for electron hopping in the chargedisordered (E a < 0 for T . T V ) and ordered (E a < 0:04 eV for T , T V ) 13 states of magnetite and those of Fe 2 OBO 3 (E a < 0:31 eV for E a < 0:35 eV for T . T CO are equal, showing that this small difference is essentially independent of spin alignment, although the ferromagnetic order in magnetite reduces both activation energies by 0.3 eV relative to those in paramagnetic Fe 2 OBO 3 .M

We present57 Fe-NMR measurements of the novel normal and superconducting-state characteristics of the iron-arsenide superconductor Ba 0:6 K 0:4 Fe 2 As 2 (T c ¼ 38 K). In the normal state, the measured Knight shift and nuclear spin-lattice relaxation rate (1=T 1 ) demonstrate the development of wave-number (q)-dependent spin fluctuations, except at q ¼ 0, which may originate from the nesting across the disconnected Fermi surfaces. In the superconducting state, the spin component in the 57 Fe-Knight shift decreases to almost zero at low temperatures, evidencing a spin-singlet superconducting state. The 57 Fe-1=T 1 results are totally consistent with a s AE -wave model with multiple full gaps in the strong coupling regime. We demonstrate that the respective 1=T 1 data for Ba 0:6 K 0:4 Fe 2 As 2 and LaFeAsO 0:7 , which seemingly follow a T 5 -and a T 3 -like behaviors below T c , are consistently explained in terms of this model only by changing the size of the superconducting gap. The recent discovery of superconductivity in the iron (Fe)-based oxypnictide LaFeAsO 1Àx F x at the superconducting (SC) transition temperature T c ¼ 26 K has provided a new route toward the realization of high-T c superconductivity.1) The mother material, LaFeAsO, exhibits a structural phase transition from tetragonal (P4=nmm) to orthorhombic (Cmma) form at T $ 155 K and then exhibits a striped antiferromagnetic (AFM) order with Q ¼ ð0; Þ or ð; 0Þ and T N $ 140 K.2) The calculated Fermi surfaces (FSs) for undoped LaFeAsO consist of two small electron cylinders around the tetragonal M point and two hole cylinders, plus a heavy 3D hole pocket, around the À point.3) Measurements of the nuclear spin-lattice relaxation rate (1=T 1 ) for the LaFeAsO system in the SC state revealed the lack of a coherence peak below T c and the presence of T 3 -like behavior, suggesting an unconventional SC nature. [4][5][6]

We report a site selective Cu-NMR study on underdoped Hg-based five-layered high-Tc cuprate HgBa2Ca4CU5O(12+delta) with a Tc = 72 K. Antiferromagnetism (AFM) has been found to take place at T(N) = 290 K, exhibiting a large antiferromagnetic moment of 0.67-0.69 microB at three inner planes (IP). This value is comparable to the values reported for nondoped cuprates, suggesting that the IP may be in a nearly nondoped regime. Most surprisingly, the AFM order is also detected with M(AFM)(OP) = 0.1 microB even at two outer planes (OP) that are responsible for the onset of superconductivity (SC). The high-Tc SC at Tc = 72 K can uniformly coexist on a microscopic level with the AFM at OP's. This is the first microscopic evidence for the uniform mixed phase of AFM and SC on a single CuO2 plane in a simple environment without any vortex lattice and/or stripe order.

We report a genuine phase diagram for a disorder-free CuO 2 plane based on the precise evaluation of the local hole density (N h ) by site-selective Cu-NMR studies on five-layered high-T c cuprates. It has been unraveled that (1) the antiferromagnetic metallic state (AFMM) is robust up to N h % 0:17, (2) the uniformly mixed phase of superconductivity (SC) and AFMM is realized at N h 0:17, (3) the tetracritical point for the AFMM/(AFMM+SC)/SC/PM (paramagnetism) phases may be present at N h % 0:15 and T % 75 K, (4) T c is maximum close to a quantum critical point (QCP) at which the AFM order collapses, suggesting the intimate relationship between the high-T c SC and the AFM order. The results presented here strongly suggest that the AFM interaction plays the vital role as the glue for the Cooper pairs, which will lead us to a genuine understanding of why the T c of cuprate superconductors is so high.

We have investigated a gap structure in the spin-triplet superconductor Sr2RuO4 through the measurement of the 101Ru nuclear spin-lattice relaxation rate (101)(1/T1) down to 0.09 K at zero magnetic field. In the superconducting state, 1/T1 in a high-quality sample with T(c) approximately 1.5 K exhibits a sharp decrease without the coherence peak, followed by a T3 behavior down to 0.15 K. This result is in marked contrast to the behavior observed below approximately 0.4 K in samples with lower T(c), where T1T is a constant. This behavior is demonstrated to be not intrinsic. We conclude that the gap structure in Sr2RuO4 is significantly anisotropic, consistent with line-node-like models.