Robert Cywinski

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.

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.

Relativistic analysis of the pairing symmetry of the noncentrosymmetric superconductor LaNiC2

We present a relativistic symmetry analysis of the allowed pairing states in the noncentroymmetric superconductor LaNiC2. The case of zero spin-orbit coupling (SOC) is discussed first and then the evolution of the symmetry-allowed superconducting instabilities as SOC is adiabatically turned on is described. In addition to mixing singlet with triplet pairing, SOC splits some triplet pairing states with degenerate order-parameter spaces into nondegenerate pairing states with different critical temperatures. We address the breaking of time-reversal symmetry detected in recent muon spin-relaxation experiments and show that it is only compatible with such nonunitary triplet pairing states. In particular, an alternative scenario featuring conventional singlet pairing with a small admixture of triplet pairing is shown to be incompatible with the experimental data

High temperature neutron diffraction studies of 0.9BiFeO3―0.1PbTiO3

Neutron diffraction data were collected from sintered polycrystalline 0.9BiFeO3–0.1PbTiO3 at temperatures between 293 and 793 K and the crystal and magnetic structures refined by the Rietveld method. An antiferromagnetic Neel temperature of 592 K was determined from a Brillouin fit to the refined magnetic moments with a ground state of 4.34μB, showing no loss of moment per Fe site compared to BiFeO3. Some magnetic order was observed to persist above the Neel temperature. A maximum in the spontaneous rhombohedral strain was observed close to the Neel temperature, in a significant departure from typical Landau behavior for a ferroelectric and suggesting a coupling between the magnetic and nuclear structures.

Phase-specific magnetic ordering in BiFeO3−PbTiO3

The multiferroic 0.7u2002BiFeO3–0.3u2002PbTiO3 has been fabricated in both sintered ceramic and powder form using conventional mixed oxide synthesis. Rietveld’s analysis of neutron powder diffraction data has shown that the sintered ceramic and powder are predominantly R3c and P4mm phases, respectively. It is shown explicitly that magnetic ordering does not occur for the P4mm phase at room temperature.

Three Dimensional Mapping of Texture in Dental Enamel

We have used synchrotron x-ray diffraction to study the crystal orientation in human dental enamel as a function of position within intact tooth sections. Keeping tooth sections intact has allowed us to construct 2D and 3D spatial distribution maps of the magnitude and orientation of texture in dental enamel. We have found that the enamel crystallites are most highly aligned at the expected occlusal points for a maxillary first premolar, and that the texture direction varies spatially in a three dimensional curling arrangement. Our results provide a model for texture in enamel which can aid researchers in developing dental composite materials for fillings and crowns with optimal characteristics for longevity, and will guide clinicians to the best method for drilling into enamel, in order to minimize weakening of remaining tooth structure, during dental restoration procedures.

Structural and dynamical study of moment localization in β-Mn_{1−x}In_{x}

We have used neutron-scattering and muon spin relaxation (μSR) to investigate the structural and magnetic properties of the β phase of elemental manganese doped with dilute amounts of indium. β-Mn is an example of a topologically frustrated antiferromagnetically correlated metal—but which remains paramagnetic at all temperatures. The addition of In to β-Mn results in a vast volume expansion of the lattice and would therefore be expected to have a major effect on the stability and localization of the Mn moment as observed in, for example, Ru- and Al-doped β-Mn alloys. We find that In doping in β-Mn results in a short-range ordered spin-glasslike ground state, similar to that of Al-doped β-Mn but with residual low-frequency spin fluctuations. This is in contrast to Ru doping which results in the stabilization of a long-range-ordered Mn moment

A neutron polarization analysis study of moment correlations in (Dy0.4Y0.6)T2 (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.

A μSR study of Er spin dynamics in (Y1-xErx)Ni2B2C superconductors

Zero field μSR has been used to probe rare earth spin dynamics in the magnetic superconductors, Y1−xErxNi2B2C. The muon spin relaxation function is stretched exponential, expu2009(−(λt)β), in form, as usually found for spin glass systems above the glass temperature. However, the Y1−xErx Ni2B2C compounds show no evidence of coexisting superconducting and static spin glass ground states even at concentrations below the critical value (x=0.6) for long range antiferromagnetic order. The temperature dependence of both the muon spin relaxation rate λ and the exponent β suggests that Er spin dynamics change significantly at the superconducting transition temperature.

Geant4 simulations of neutron production in a thorium fuelled Accelerator Driven Subcritical Reactors

Spallation taken from the middle English spalle, is an efficient process for producing an intense neutron flux suitable for exploitation in Accelerator Driven Subcritical Reactors (ADSRs) for energy production and the transmutation. In order to assess spallation driven fission and transmutation we have simulated proton induced neutron production using GEANT4. Benchmarking our simulations against experimental neutron spectra produced from a thick lead target bombarded with 0.5 and 1.5 GeV protons. The Bertini and INCL models available in GEANT4, coupled with the high precision (HP) neutron model, are found to reproduce the published experimental data. Given the confidence in the GEANT4 simulations provided by this benchmarking we then proceed to study neutron production in an ADSR using Thorium in a geometry similar to that of the proposed Belgian MYRRHA project with 0.6 GeV proton beam energy that proposed by SCKCEN.