We have discovered spontaneous ordering of nuclear spins in lithium metal by NMR measurements at very low temperatures. In low magnetic fields, B < 0:2 mT, the NMR spectra show a pronounced low-frequency anomaly. Also, nonadiabatic response to a slowly varying magnetic field was observed. A rich phase diagram with three different nonparamagnetic regions is proposed. We estimate a critical spin temperature T c 350 nK at B 0. We also report the absence of superconductivity in lithium at normal pressure down to T e 100 K (B < 10 nT). DOI: 10.1103/PhysRevLett.93.157201 PACS numbers: 75.30.Kz, 74.10.+v, 75.30.Cr, 76.60.Jx Lithium is often regarded as the prototype metal due to its apparent simplicity. However, to begin with, its lowtemperature lattice assembly is not that trivial, as competing close-packed structures have been found to coexist The occurrence of superconductivity in lithium at reasonable temperatures has been predicted by many theoretical studies [2 -6]. The estimates for T c vary from over 1 K to a few microkelvins, most predictions suggesting a T c at the millikelvin range. Prior experiments, however, did not indicate any transition down to 4 mK 7 Li, 92.5% abundant with a spin 3=2, is an excellent target for NMR studies due to its reasonably large magnetic moment 7 3:26 N . The remaining 7.5% of 6 Li, with I 6 1 and 6 0:82 N , do not contribute much to the magnetic behavior, but can be used as a probe in analyzing the interactions between the nuclear spins Lithium is dominantly a dipolar-coupled nuclear magnet, more so than any other investigated metal Nuclear-spin order in insulators has been possible due to extremely weak coupling of the spins to the lattice. Even in metals, the nuclear-spin temperature can be reduced by several orders of magnitude below that of the lattice and electrons. While the intermediate microKelvin regime is the practical limit for refrigerating the body of well-prepared specimens, their nuclear spins may be polarized by a high magnetic field and by subsequent adiabatic demagnetization be cooled to the nanokelvin regime and even below By such procedures we were able to prepare a magnetically ordered state of nuclear spins in lithium. On the basis of measurements of the spectral resonance shapes, observation of nonadiabatic effects, and determination of the spin temperature, we propose a nontrivial phase diagram with several distinct regions of different characteristics. We obtained the critical temperature of 350 nK in zero magnetic field. The sample material was purchased from Alfa Aesar [17], whose analysis indicated 99.97% purity. The content of magnetic impurities (iron) was at most 4 ppm, and we measured the residual resistivity ratio 900 100. Lithium reacts eagerly with air, whereby the samples were protected by copper capsules The best conditions for observing the two phenomena under discussion are somewhat contradictory, so that we prepared two different samples to satisfy both. A low transition temperature of superconductivity is associated with a very low critical magnetic field and potential supercooling of the normal state in any finite field. Therefore, the superconductivity (SC) sample was placed inside an efficient magnetic shield with a cylindrical layered high-permeability-superconducting (Pb) -highpermeability structure
