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Elements in the alkali metal series are regarded as unfavourable for superconductivity due to their monovalent character. 1,2 The superconducting transition at temperatures as high as 20 K recently found in compressed lithium, 3-6 the lightest alkali element, is considered to occur due to pressure induced changes in the conduction-electron band structure. 6-12 The condition at the ambient pressure in lithium had remained unresolved, both theoretically and experimentally. 11-16 Here we report that lithium is a superconductor also at zero pressure at extremely low temperatures below 0.4 mK. This is the lowest superconducting transition temperature for any pure metal ever observed. Lithium, as a particularly simple host for the conduction electron system, represents an important case for any attempts to classify the superconductors and transition temperatures, especially in judging if any nonmagnetic configuration can be assumed to exclude superconductivity down to zero temperature. Such a fundamental system provides a stringent test case for already highly developed computational methods in predicting the transition temperatures from first principles. Furthermore, the combination of extremely weak superconductivity and relatively strong nuclear magnetism in lithium would evidently lead to mutual
The effective interaction between 3 He quasiparticles in dilute liquid 3 He-4 He mixtures affects many of its physical properties. The interaction potential is determined here from the saturation solubility and osmotic pressure data reported recently. The interactions are examined consistently over the entire pressure range of liquid-helium mixtures at very low temperatures, that is, from 0 to 25.6 bar, where the solid phase appears. To reproduce all experimental data, it was necessary to include a concentration dependence in the potential. This dependence, however, turned out to be almost the opposite to what has been proposed earlier. The deduced potential can be used to calculate, among other things, an estimate for the superfluid transition temperature of 3 He-4 He mixtures. In addition, we also find values for some less well-established parameters for helium mixtures, such as the binding energy of a 3 He atom in superfluid 4 He.
We have studied the spontaneous antiferromagnetic (AF) order in the nuclear spin system of copper by use of neutron-diffraction experiments at nanokelvin temperatures. Copper is an ideal model system as a nearest-neighbor-dominated spin-2 fcc antiferromagnet. The phase diagram has been investigated by measuring the magnetic-field dependence of the (100) reflection, characteristic of a type-I AF structure, and of a Bragg peak at {0 3 3 ). The results suggest the presence of high-field (100) phases at 0. 12 (8 (B,=0.26 mT, for B along either the [100] or [011]crystalline axes, intermediate-field (0 -, -', )structures around 8 =0.09 mT for all field directions, and a zero-field (100) phase. No reflection corresponding to a high-field phase for B~i [111]has been found. The phase transition between the high-field phase and the intermediate-field structure is of first order. The change from {0 3 3 ) at intermediate fields to (100) at zero field is associated with a large region (0.02~B &0.06 mT) of coexisting-(100) and ( 0 3 3 )-type Bragg peaks, and can be interpreted as either a two-phase region with a first-order transition at -0.06 mT and huge hysteresis effects or as a single multiple-k phase which continuously transforms from being determined by (0 3 3 ) at -0.1 mT and (100) at zero field. The neutron-diffraction data have been compared with results of earlier susceptibility measurements in order to identify the translational periods of the three previously found antiferromagnetic phases for B~~ [100]. Recent theoretical work has yielded results in agreement with our experimental data.allows an accurate modeling and analysis of the spin assemblies. Antiferromagnetic order in a fcc lattice is a phenomenon characterized by intrinsic frustration. The requirement of oppositely aligned magnetic moments in a network of equilateral triangles is seemingly difficult to achieve. The structure of this system has puzzled theorists for decades but still remains unsolved. ' Nuclear magnets in certain metals, most notably copper, silver, and gold, are model systems for studies of the frustration problem. These three elements constitute a series with increasing strength of the antiferromagnetic coupling between adjacent nuclei.In noble metals, nuclear spins interact via the ordinary dipolar force, which is known accurately, and via the indirect, conduction-electron-mediated Ruderman-Kittel (RK) interaction, which can be calculated from electronic wave functions and the band structure. An external magnetic field also causes a Zeeman term, but quadrupolar forces are quenched by the cubic symmetry. Therefore, the Hamiltonian consists of only three terms, viz. , di +~RK+~ZThe anisotropic dipolar force favors ferromagnetism whereas the spin-rotation-invariant Ruderman-Kittel interaction tends to align neighboring spins antiparallel. In copper these competing interactions are of comparable strength. Experimentally, the parameter which is related to the average molecular field, can be measured in order to compare the strengths of the s...
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