A. D. Hillier

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.

Evidence for superconductivity with broken time-reversal symmetry in locally noncentrosymmetric SrPtAs

P. K. Biswas, H. Luetkens, ∗ T. Neupert, 3 T. Stürzer, C. Baines, G. Pascua, A. P. Schnyder, M. H. Fischer, J. Goryo, 3 M. R. Lees, H. Maeter, F. Brückner, H.-H. Klauss, M. Nicklas, P. J. Baker, A. D. Hillier, M. Sigrist, A. Amato, and D. Johrendt Laboratory for Muon Spin Spectroscopy, Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland Condensed Matter Theory Group, Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland Institute for Theoretical Physics, ETH Zurich, 8093 Zurich, Switzerland Department Chemie, Ludwig-Maximilians-Universität München, D-81377 München, Germany Max-Planck-Institut für Festkörperforschung, Heisenbergstrasse 1, D-70569 Stuttgart, Germany Department of Physics, Cornell University, Ithaca, New York 14853, USA Institute of Industrial Science, The University of Tokyo, Meguro, Tokyo 153-0041, Japan Physics Department, University of Warwick, Coventry, CV4 7AL, United Kingdom Institute for Solid State Physics, TU Dresden, D-01069 Dresden, Germany Max Planck Institute for Chemical Physics of Solids, Nöthnitzer Str. 40, 01187 Dresden, Germany ISIS Facility, STFC Rutherford Appleton Laboratory, Didcot OX11 0QX, United Kingdom (Dated: May 5, 2014)

Detection of time-reversal symmetry breaking in the noncentrosymmetric superconductor Re6Zr using muon-spin spectroscopy

We have investigated the superconducting state of the non-centrosymmetric compound Re6Zr using magnetization, heat capacity, and muon-spin relaxation/rotation (muSR) measurements. Re6Zr has a superconducting transition temperature, Tc = 6.75 K. Transverse-field muSR experiments, used to probe the superfluid density, suggest an s-wave character for the superconducting gap. However, zero and longitudinal-field muSR data reveal the presence of spontaneous static magnetic fields below Tc indicating that time-reversal symmetry is broken in the superconducting state and an unconventional pairing mechanism. An analysis of the pairing symmetries identifies the ground states compatible with time-reversal symmetry breaking.

Cascade of Bulk Magnetic Phase Transitions in NaxCoO2 as Studied by Muon Spin Rotation

Using muon spin rotation, well-defined bulk ~ 100% magnetic phases in NaxCoO2 are revealed. A novel magnetic phase is detected for x = 0.85 with the highest transition temperature ever observed for x>= 0.75. This stresses the diversity of x>=0.75 magnetic phases and the link between magnetic and structural degrees of freedom. For the charge-ordered x = 0.50 compound, a cascade of transitions is observed below 85 K. From a detailed analysis of our data, we conclude that the ordered moment varies continuously with temperature and suggest that the two secondary transitions at 48 K and 29 K correspond to a moderate reorientation of antiferromagnetically coupled moments.

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.

Ferromagnetic cluster spin-glass behavior in PrRhSn3

We report the synthesis, structure, and magnetic and transport properties of a new ternary intermetallic compound PrRhSn3 which crystallizes in LaRuSn3-type cubic structure (space group Pm-3n). At low applied fields the dc magnetic susceptibility exhibits a sharp anomaly below 6~K with an irreversible behavior in zero field cooled (ZFC) and field cooled (FC) susceptibility below 5.5 K. The ac susceptibility exhibits a frequency dependent anomaly revealing a spin-glass behavior with a freezing temperature, T_f = 4.3 K. The observation of spin-glass behavior is further supported by a very slow decay of thermo-remnant magnetization (mean relaxation time tau = 2149 s). However, a small jump at very low field in the isothermal magnetization at 2 K and a weak anomaly in the specific heat near 5.5 K reveal the presence of ferromagnetic clusters. The frequency dependence of the transition temperature T_f in the ac susceptibility obeys the Vogel-Fulcher law, nu = nu_0exp[-E_a/k_B(T_f-T_0)] with activation energy E_a/k_B = 19.1 K. This together with an intermediate value of the parameter delta T_f = Delta T_f/T_f Delta(log nu) = 0.086 provide an evidence for the formation of a cluster-glass state in PrRhSn3. The magnetic contribution of the specific heat reveals a broad Schottky-type anomaly centered around 10 K and the analysis based on the crystal electric field model indicates a singlet ground state. Further, below T_f the magnetic part of the specific heat exhibits a T^{3/2} temperature dependence. The strong influence of the crystal electric field and a T^{3/2} temperature dependence are also seen in the electrical resistivity which reveals a metallic character and a high magnetoresistance. We also obtain a surprisingly large value of Sommerfeld-Wilson ratio R_W ~ 247

Relativistic analysis of the pairing symmetry of the noncentrosymmetric superconductor LaNiC2

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Specific heat and mu SR study on the noncentrosymmetric superconductor LaRhSi3

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

Neutron scattering and muon spin relaxation measurements of the noncentrosymmetric antiferromagnet CeCoGe3

We have investigated the superconducting properties of the noncentrosymmetric superconductor LaRhSi

Investigations of the superconducting states of noncentrosymmetric LaPdSi3 and LaPtSi3

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