We have investigated the superconducting state of the noncentrosymmetric compound Re6Zr using magnetization, heat capacity, and muon-spin relaxation or rotation (μSR) measurements. Re6Zr has a superconducting transition temperature, Tc=6.75±0.05  K. Transverse-field μSR experiments, used to probe the superfluid density, suggest an s-wave character for the superconducting gap. However, zero and longitudinal-field μSR 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.

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.

Unconventional superconductors are most frequently identified by the observation of power-law behaviour on low-temperature thermodynamic or transport properties, such as specific heat. Here we show that, in addition to the usual point and line nodes, a much wider class of different nodal types can occur. These new types of nodes typically occur when there are transitions between different types of gap node topology, for example when point or line nodes first appear as a function of some physical parameter. We identify anomalous, non-integer thermodynamic power laws associated with these new nodal types, and give physical examples of superconductors in which they might be observed experimentally, including the noncentrosymmetric superconductor Li2Pd3−xPtxB.A defining feature of many unconventional Fermi superfluids and superconductors is the existence of lines or points on the Fermi surface where the energy gap vanishes and so-called "nodal quasi-particles" can exist at arbitrarily low energies. In Fermi systems with such nodal quasi-particles the low temperature specific heat will show particular power law behaviours as a function of temperature. The expected point and line node power law dependencies were first derived in relation to the proposed low temperature superfluid phases of liquid 3 He [1]. These were subsequently clarified further [2] and are now widely used to identify pairing states in unconventional superconductors [3][4][5]. Here we show that gap nodes in superconductors can occur in a number of more general types than simply the usual line or point zeros and that each of these has a corresponding thermodynamic signature, typically in the form of non-integer power laws in low temperature specific heat. We predict that these anomalous power laws generically occur at points in the phase diagram where there is a topological change in the line or point nodal structure on the Fermi surface, which we illustrate with a specific example: the noncentrosymmetric superconductors Li 2 Pd 3−x Pt x B. In this case the gap node topological changes are also associated with changes of bulk topological quantum numbers for the quasi-particles. The future experimental observation of such non-integer power laws could therefore be used to identify not only superconductors with highly unconventional pairing symmetries, but also topological superconducting and superfluid systems [6]. Similar physics are realised in the high-temperature cobalt doped Pnictide materials Ba(Fe 1−x T x ) 2 As 2 (T=Co,Ni,Pd) [7-9].For the usual types of point and line nodes the quasi-particle energy spectrum is linear in the vicinity of the gap node. A familiar example is the case of * j.quintanilla@kent.ac.uk the 3 He A-phase with triplet pairing order parameter d k ∝ (k x + ik y )ẑ, which has point nodes at the points k = (0, 0, ±k F ) on the Fermi sphere, of radius k F . Near to these points the quasi-particle energy spectrum, E k ,where v F is the Fermi velocity, giving a linear dispersion relation as shown in Fig. 1 a). For this type of ga...