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

The temperature dependence of the 195 Pt Knight shift, K, for the high quality single crystal UPt 3 has been measured down to T 28 mK in applied magnetic fields parallel and perpendicular to the hexagonal c axis. No change of K's has been found across the superconducting transition temperature T c down to 28 mK regardless of the crystal directions and independent of the superconducting multiphases. It is demonstrated that UPt 3 is the odd-parity superconductor with parallel spin pairing following the direction of the magnetic field in a range of 4.4 -15.6 kOe without an appreciable pinning of the order parameter to the lattice. [S0031-9007(96)

195 Pt Knight shift (KS) measurements covering the superconducting multiple phases for major field (H) orientations have been carried out on the high-quality single crystal UPt 3 . For H . 5 kOe, the KS does not change below the superconducting transition temperature T c down to 28 mK, regardless of major crystal orientations, which provides evidence that the odd-parity superconductivity with the parallel spin pairing is realized. By contrast, the KS decreases below T c for H b k b axis and H b , 5 kOe and for H c k c axis and H c , 2.3 kOe, whereas the KS for H a k a axis is T independent across T c down to H a ϳ 1.764 kOe. These novel findings entitle UPt 3 as the first spin-triplet oddparity superconductor including a nonunitary pairing characterized by the two-component d vector like d b 1 id c at low T and low H. [S0031-9007(98)

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We report on the basis of Cu-NQR measurements that the ground state in a series of Ce x Cu 2 Si 2 compounds evolves from a magnetically ordered phase at x 0.975 to a heavy-electron superconducting (SC) phase at x 1.025. We have found that the sample of x 0.99 does not exhibit any trace of magnetic phase transition down to 0.012 K. Slow magnetic fluctuations with low frequencies comparable to the NQR frequency develop rapidly below T m ϳ 1.2 K. This unusual "critical magnetic phase" can coexist with the SC phase. For the samples of x 1.00 and 1.025 such a state is expelled by the onset of the SC state. [S0031-9007(99)