Nonunitary triplet pairing in the centrosymmetric superconductor LaNiGa2

Muon spin rotation and relaxation experiments on the centrosymmetric intermetallic superconductor LaNiGa2 are reported. The appearance of spontaneous magnetic fields coincides with the onset of superconductivity, implying that the superconducting state breaks time reversal symmetry, similarly to noncentrosymmetric LaNiC2. Only four triplet states are compatible with this observation, all of which are nonunitary triplets. This suggests that LaNiGa2 is the centrosymmetric analogue of LaNiC2. We argue that these materials are representatives of a new family of paramagnetic nonunitary superconductors.

A neutron polarization analysis study of moment correlations in (Dy(0.4)Y(0.6))T(2) (T = Mn, Al)

We present a study of the magnetic moment correlations of two pseudo-binary C15 Laves phase compounds, (Dy(0.4)Y(0.6))Mn(2) and (Dy(0.4)Y(0.6))Al(2), both of which have spin-glass-like magnetic ground states at low temperature. We use neutron powder diffraction with polarization analysis to isolate the diffuse scattering associated with the correlated spin-glass ground state, and compare and contrast the two systems. Despite there being differences of correlation length scale, we discover that the moment-moment correlations of these two disordered magnets are quite similar over a short range, and hence conjecture that the Mn ions in (Dy(0.4)Y(0.6))Mn(2) have little influence on the ground-state magnetic properties.

Magnetic short-range order in β-Mn(1-x)Co(x)

We present a study of the magnetic ground state properties of β-Mn metal alloyed with Co, using neutron polarization analysis of the diffuse neutron scattering cross-section. We analyse the magnetic structure obtained using a reverse Monte Carlo procedure to extract the Mn-Mn spin correlations. The addition of Co leads to a static disordered magnetic structure with medium-range correlations. Our analysis of the spin correlations indicates that both 8c and 12d non-equivalent lattice sites present in the β-Mn structure contribute to the magnetic ground state, where previously it was thought that the 8c site was non-magnetic.

Evidence for time-reversal symmetry breaking in the noncentrosymmetric superconductor LaNiC2

Muon spin relaxation experiments on the noncentrosymmetric intermetallic superconductor LaNiC2 are reported. We find that the onset of superconductivity coincides with the appearance of spontaneous magnetic fields, implying that in the superconducting state time-reversal symmetry is broken. An analysis of the possible pairing symmetries suggests only four triplet states compatible with this observation, all of them nonunitary. They include the intriguing possibility of triplet pairing with the full point group symmetry of the crystal, which is possible only in a noncentrosymmetric superconductor.

Generalized spin-glass relaxation

Spin relaxation close to the glass temperature of CuMn and AuFe spin glasses is shown, by neutron spin echo, to follow a generalized exponential function which explicitly introduces hierarchically constrained dynamics and macroscopic interactions. The interaction parameter is directly related to the normalized Tsallis nonextensive entropy parameter q and exhibits universal scaling with reduced temperature. At the glass temperature q=5/3 corresponding, within Tsallis' q statistics, to a mathematically defined critical value for the onset of strong disorder and nonlinear dynamics.

Evidence for a spin emulsion

Magnetic small-angle neutron scattering from the itinerant electron magnet, Y(Mn1-xFex)2, in which ferromagnetic and antiferromagnetic spin correlations compete, is found to follow an anomalous Q(-6) dependence (Q = 4pisintheta/lambda). It is suggested that this scattering is the magnetic analogue of that predicted for a structural microemulsion by Teubner's extension of conventional Kirste-Porod scattering to well-defined interfaces with extreme differences between mean and Gaussian curvatures. The "spin-emulsion-like" morphology of magnetic interfaces in Y(Mn1-xFex)2 is confirmed both qualitatively and quantitatively by a simple model based upon reported near neighbor Mn and Fe spin correlations.

Non-fermi-liquid behavior of electron-spin fluctuations in an elemental paramagnet

We report for the first time, the observation of non-Fermi-liquid scaling behavior in an elemental paramagnetic metal. Both the dynamical susceptibility and the resistivity of beta-Mn are shown to display non-Fermi-liquid scaling over a relatively large temperature range at ambient pressure. The temperature dependence of the resistivity in beta-Mn is consistent with the existence of an antiferromagnetic zero-temperature phase transition or "quantum critical point." Since there is no site disorder in this pure element, we show that non-Fermi-liquid behavior observed in beta-Mn is not a consequence of summing over different local atomic environments, but a much more fundamental phenomenon.