The noncentrosymmetric superconductor Re 24 Ti 5 , a time-reversal symmetry (TRS) breaking candidate with T c = 6 K, was studied by means of muon-spin rotation/relaxation (µSR) and tunnel-diode oscillator (TDO) techniques. At a macroscopic level, its bulk superconductivity was investigated via electrical resistivity, magnetic susceptibility, and heat capacity measurements. The low-temperature penetration depth, superfluid density and electronic heat capacity all evidence an s-wave coupling with an enhanced superconducting gap. The spontaneous magnetic fields revealed by zero-field µSR below T c indicate a time-reversal symmetry breaking and thus the unconventional nature of superconductivity in Re 24 Ti 5 . The concomitant occurrence of TRS breaking also in the isostructural Re 6 (Zr,Hf) compounds, hints at its common origin in this superconducting family and that an enhanced spin-orbital coupling does not affect pairing symmetry.Superconductors with an inversion center can host either even-parity spin-singlet or odd-parity spin-triplet states. These strict symmetry-imposed requirements, however, are relaxed in noncentrosymmetric superconductors (NCSCs), where parity-mixed superconducting states are also allowed. In these materials the lack of an inversion symmetry often induces an antisymmetric spin-orbit coupling (ASOC), which can lift the degeneracy of conduction band electrons. Since the extent of parity-mixing is determined by the strength of SOC, formally similar compounds, but with different spin-orbit couplings, can exhibit different degrees of parity mixing.The recent interest in NCSCs is related to the complex nature of their superconducting properties. 1,2 Because of the mixed pairing, noncentrosymmetric superconductors can display significantly different properties compared to their conventional counterparts. Some NCSCs, such as

The recent discovery of superconductivity in the quasi-one-dimensional compound K2Cr3As3, which consists of double-walled tubes of [(Cr3As3) 2− ] ∞ that run along the c axis, has attracted immediate attention as a potential system for studying superconductors with reduced dimensionality. Here we report clear experimental evidence for the unconventional nature of the superconducting order parameter in K2Cr3As3, by precisely measuring the temperature dependence of the change in the penetration depth ∆λ(T ) using a tunnel diode oscillator. Linear behavior of ∆λ(T ) is observed for T ≪ Tc, instead of the exponential behavior of conventional superconductors, indicating that there are line nodes in the superconducting gap. This is strong evidence for unconventional behavior and may provide key information for identifying the pairing state of this novel superconductor.

The nature of the pairing states of superconducting LaNiC 2 and LaNiGa 2 has to date remained a puzzling question. Broken time reversal symmetry has been observed in both compounds and a group theoretical analysis implies a non-unitary triplet pairing state. However all the allowed non-unitary triplet states have nodal gap functions but most thermodynamic and NMR measurements indicate fully gapped superconductivity in LaNiC 2 . Here we probe the gap symmetry of LaNiGa 2 by measuring the London penetration depth, specific heat and upper critical field. These measurements demonstrate two-gap nodeless superconductivity in LaNiGa 2 , suggesting that this is a common feature of both compounds. These results allow us to propose a novel triplet superconducting state, where the pairing occurs between electrons of the same spin, but on different orbitals. In this case the superconducting wavefunction has a triplet spin component but isotropic even parity gap symmetry, yet the overall wavefunction remains antisymmetric under particle exchange. This model leads to a nodeless twogap superconducting state which breaks time reversal symmetry, and therefore accounts well for the seemingly contradictory experimental results. The breaking of symmetries in addition to gauge symmetry upon entering the superconducting state usually indicates an unconventional order parameter. Several materials have been found to break time reversal symmetry (TRS) in the superconducting state through the detection of spontaneous magnetic fields below T c using zero-field muon-spin relaxation (µSR [14], where it has been argued that as a result of the low symmetry of the orthorhombic crystal structures of both compounds, broken TRS necessarily implies non-unitary triplet superconductivity and all the TRS breaking states have nodes in the gap function [15]. Although evidence for nodal superconductivity was found from some measurements [16,17], recent specific heat [18,19], nuclear quadrapole relaxation [20] and penetration depth [19] measurements indicate fully gapped behavior in LaNiC 2 . In addition, evidence for two-gap superconductivity was found from the specific heat, superfluid density and upper critical field [19]. There have been fewer measurements of superconductivity in LaNiGa 2 [21], which has an orthorhombic centrosymmetric crystal structure in contrast to noncentrosymmetric LaNiC 2 , although fully gapped behavior was inferred from the specific heat [22].In this Letter, we suggest a solution to this apparent contradiction from measurements of the London penetration depth, specific heat and upper critical field, all of which consistently suggest the presence of two-gap superconductivity in LaNiGa 2 . Along with previous results of LaNiC 2 [19], we establish that nodeless, two-gap superconductivity is a common feature of these compounds. We propose that pairing between electrons with the same spins but on different orbitals gives rise to a triplet superconducting state with even parity pairing in both compounds, where the wave function re...

By employing a series of experimental techniques, we provide clear evidence that CaPtAs represents a rare example of a noncentrosymmetric superconductor which simultaneously exhibits nodes in the superconducting gap and broken time-reversal symmetry (TRS) in its superconducting state (below T c ≈ 1.5 K). Unlike in fully gapped superconductors, the magnetic penetration depth λðTÞ does not saturate at low temperatures, but instead it shows a T 2 dependence, characteristic of gap nodes. Both the superfluid density and the electronic specific heat are best described by a two-gap model comprising of a nodeless gap and a gap with nodes, rather than by single-band models. At the same time, zero-field muonspin relaxation spectra exhibit increased relaxation rates below the onset of superconductivity, implying that TRS is broken in the superconducting state of CaPtAs, hence indicating its unconventional nature. Our observations suggest CaPtAs to be a new remarkable material that links two apparently disparate classes, that of TRS-breaking correlated magnetic superconductors with nodal gaps and the weakly correlated noncentrosymmetric superconductors with broken TRS, normally exhibiting only a fully gapped behavior.