We report the magnetic and superconducting properties of locally noncentrosymmetric SrPtAs obtained by muon-spin-rotation/relaxation ($\mu$SR) measurements. Zero-field $\mu$SR reveals the occurrence of small spontaneous static magnetic fields with the onset of superconductivity. This finding suggests that the superconducting state of SrPtAs breaks time-reversal symmetry. The superfluid density as determined by transverse field $\mu$SR is nearly flat approaching $T=0$ K proving the absence of extended nodes in the gap function. By symmetry, several superconducting states supporting time-reversal-symmetry breaking in SrPtAs are allowed. Out of these, a dominantly $d+id$ (chiral $d$-wave) order parameter is most consistent with our experimental data.

In a class of frustrated magnets known as spin ice, magnetic monopoles emerge as classical defects and interact via the magnetic Coulomb law. With quantum-mechanical interactions, these magnetic charges are carried by fractionalized bosonic quasi-particles, spinons, which can undergo Bose–Einstein condensation through a first-order transition via the Higgs mechanism. Here, we report evidence of a Higgs transition from a magnetic Coulomb liquid to a ferromagnet in single-crystal Yb₂Ti₂O₇. Polarized neutron scattering experiments show that the diffuse [111]-rod scattering and pinch-point features, which develop on cooling are suddenly suppressed below T_C~0.21 K, where magnetic Bragg peaks and a full depolarization of the neutron spins are observed with thermal hysteresis, indicating a first-order ferromagnetic transition. Our results are explained on the basis of a quantum spin-ice model, whose high-temperature phase is effectively described as a magnetic Coulomb liquid, whereas the ground state shows a nearly collinear ferromagnetism with gapped spin excitations.

We have investigated the superconducting state of the noncentrosymmetric compound ${\mathrm{Re}}_{6}\mathrm{Zr}$ using magnetization, heat capacity, and muon-spin relaxation or rotation ($$\mu${}\mathrm{SR}$) measurements. ${\mathrm{Re}}_{6}\mathrm{Zr}$ has a superconducting transition temperature, ${T}_{c}=6.75\ifmmode\pm\else\textpm\fi{}0.05\text{ }\text{ }\mathrm{K}$. Transverse-field $$\mu${}\mathrm{SR}$ experiments, used to probe the superfluid density, suggest an $s$-wave character for the superconducting gap. However, zero and longitudinal-field $$\mu${}\mathrm{SR}$ data reveal the presence of spontaneous static magnetic fields below ${T}_{c}$ 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.

For the spin-chain compound ${\mathrm{Ca}}_3{\mathrm{Co}}_2{\mathrm{O}}_6$, the magnetization curves as a function of the magnetic field are strongly out-of-equilibrium at low temperature, and they exhibit several steps whose origins are still a matter for debate. In the present paper we report on a detailed investigation of the temperature and time dependence of these features. First, it is found that some of the magnetization steps can disappear as the characteristic time of the measurement is increased. A comparison of the influence of temperature and time points to the existence of a thermally activated process that plays an important role in determining the form of the magnetization curves. Second, direct investigations of the magnetic response as a function of time show that this thermally activated process competes with a second relaxation mechanism of a very different nature, which becomes dominant at the lowest temperatures. These results shed new light on the peculiar magnetization process of this geometrically frustrated, Ising-like spin-chain compound.