The rare earth nickel borocarbides, with the generic formula RNi2B2C, have recently been shown to display a rich variety of phenomena. Most striking has been the competition between, and even coexistence of, antiferromagnetism and superconductivity. We have measured the Fermi surface (FS) of LuNi2B2C, and shown that it possesses nesting features capable of explaining some of the phenomena experimentally observed. In particular, it had previously been conjectured that a particular sheet of FS is responsible for the modulated magnetic structures manifest in some of the series. We report the first direct experimental observation of this sheet.

The recent discovery of superconductivity coexisting with weak itinerant ferromagnetism in the d-electron intermetallic compound ZrZn2 strongly suggests spin-fluctuation mediated superconductivity. Ab initio electronic structure calculations of the Fermi surface and generalized susceptibilities are performed to investigate the viability of longitudinal spinfluctuation-induced spin-triplet superconductivity in the ferromagnetic state. The critical temperature is estimated to be of the order of 1 K. Additionally, it is shown that in spite of a strong electron-phonon coupling (λ ph = 0.7), conventional swave superconductivity is inhibited by the presence of strong spin-fluctuations.The generalization of the Bardeen-Cooper-Schrieffer (BCS) theory to electron-electron interactions by Kohn and Luttinger [1], paved the way for speculation about the possibility of non-s-wave, or "unconventional" superconductivity. Following the suggestion that a magnetically mediated interaction plays an important role in the superfluidity of liquid 3 He [2], the search began for superconductivity in nearly magnetic metals where strong spin fluctuations might provide the pairing mechanism [3,4]. Recent experiments on Sr 2 RuO 4 [5] have made it a strong candidate for exhibiting spin-triplet, possibly pwave superconductivity. For a spin-singlet Cooper pair, where the electrons have anti-parallel spins, the presence of ferromagnetic spin-fluctuations will be antagonistic towards the development of such a superconducting state. However, the recent reports of the coexistence of ferromagnetism with superconductivity in UGe 2 [6] and ZrZn 2[7] suggest a spin-triplet Cooper pairing, probably driven by such spin-fluctuations. Moreover, in ZrZn 2 , the disappearance of superconductivity at the same point as magnetism, and the sensitivity of its occurrence to sample purity [7,8] are perhaps the strongest indications yet that the superconductivity is intimately connected with the magnetism in this material.Unlike other "magnetic" superconductors (e.g. borocarbides [9] , RuSr 2 GdCu 2 O 8 [10]) where the magnetism and superconductivity occur in different parts of the unit cell, in both UGe 2 and ZrZn 2 it is the same itinerant electrons that are thought to form the Cooper pairs as well as produce ferromagnetism. Moreover, whereas some questions regarding the itineracy of 5f electrons and the roles of the strong magnetocrystalline anisotropy and quasitwo-dimensional electronic structure can be raised with respect to UGe 2 , ZrZn 2 is a three-dimensional intermetallic compound free of such effects. Discovered by Matthias and Bozorth [11] in the 1950s, it was initially of interest because of the presence of weak ferromagnetism, in spite of the fact that neither constituent was ferromagnetic. It has the C15 cubic Laves crystal structure, with a lattice constant of 7.393Å (13.97 a.u.) [7]. The possibility of there being triplet pairing in high-purity C15 compounds like TiBe 2 and ZrZn 2 was first suggested by Fay and Appel [4]. In this Letter we investi...

Positron lifetime spectroscopy has been applied to estimate the free volume hole size distribution in polycarbonate (PC) and in polystyrene (PS) at room temperature. The hole radius density distributions are determined from the ortho-positronium lifetime distributions obtained via a Laplace inversion of the measured positron lifetime spectra. The hole volume density distributions and the number density distributions of holes are estimated from the hole radius density distributions. All of the distributions may be well approximated by single Gaussians. The hole radius and the hole number density distributions have centres (q) and (qn) at 0.29 nm and 0.1 nm3 in PC, and 0.28 nm and 0.09 nm3 in PS. The FWHM of the corresponding distributions are 0.042 nm and 0.040 nm3 (PC), and 0.039 nm and 0.34 nm3 (PS), respectively. Both, the shape and the width of the distributions correlate well with the free volume theory of Bueche. The possible influence of different effects, such as a nonspherical shape of holes and a scanning of several holes by positronium before annihilation, on the mean hole size and the hole size distribution as detected by the positron lifetime method is finally discussed.