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.
We report a comprehensive study of the noncentrosymmetric superconductor Mo 3 P. Its bulk superconductivity, with T c = 5.5 K, was characterized via electrical resistivity, magnetization, and heat-capacity measurements, while its microscopic electronic properties were investigated by means of muon-spin rotation/relaxation (µSR) and nuclear magnetic resonance (NMR) techniques. In the normal state, NMR relaxation data indicate an almost ideal metallic behavior, confirmed by band-structure calculations, which suggest a relatively high electron density of states, dominated by the Mo 4d-orbitals. The low-temperature superfluid density, determined via transverse-field µSR and electronic specific heat, suggest a fully-gapped superconducting state in Mo 3 P, with ∆ 0 = 0.83 meV, the same as the BCS gap value in the weak-coupling case, and a zero-temperature magnetic penetration depth λ 0 = 126 nm. The absence of spontaneous magnetic fields below the onset of superconductivity, as determined from zero-field µSR measurements, indicates a preserved time-reversal symmetry in the superconducting state of Mo 3 P and, hence, spin-singlet pairing. Superconductivity and spin-orbit coupling in non-centrosymmetric materials:A review, Rep. Prog. Phys. 80, 036501 (2017).
Results of NMR studies of 23 Na in NaNO 2 confined within molecular sieves MCM-41 with pore size 37 and 20Å and SBA-15 with pore size 52Å are presented. 23 Na spin-lattice relaxation and line shape were measured in a large temperature range up to 535 K covering the bulk ferroelectric phase transition point. It is shown that confined NaNO 2 below the bulk sodium nitrite melting point consists of two parts with relaxation times which differ by two orders in magnitude. A portion of NaNO 2 exhibits bulk-like properties with the ferroelectric phase transition in the vicinity of the bulk transition temperature. The bulk-like NaNO 2 prevails below and near the ferroelectric phase transition and its amount decreases strongly when temperature approaches the bulk melting point. Fast nuclear relaxation in another portion of confined NaNO 2 revealed very high molecular mobility. This portion increases with increasing temperature and dominates above 510 K. It was suggested that fast relaxation corresponds to the melted or premelted state of confined NaNO 2 caused by confinement. Temperature evolution of the 23 Na NMR line confirms such a suggestion. The amount of NaNO 2 which possesses high molecular mobility depends on pore size and is maximal for the MCM-41 porous matrix with 20Å pore size. The correlation time of electric field gradient fluctuations was found for this part to be similar to those in viscous liquids with the activation energy of about 0.42 eV.
Magnetic properties of a superconducting lead-porous glass composite were studied. The glass pore size was 7 nm. The onset of superconductivity was observed at 7.22 K with complete diamagnetic screening at lower temperatures. Strong magnetic instabilities were found on the magnetization-versus-field loops in the temperature range from 1.8 to 5.5 K at a sweep rate of 20 Oe/s. The shape of the hysteresis loops above 2.5 K was typical for other types of hard type-II superconductors in the adiabatic limit, the field of the first jump on the virgin magnetization being maximal at 3.5 K. Below 2.5 K the hysteresis loops become complex, showing different behavior at lower and higher fields. The nature of such a loop was discussed. The smooth hysteresis loops just below the superconducting transition had fishtails that completely disappeared down to 6 K. The evolution of magnetization instabilities until complete smoothing away the hysteresis loops with increasing or decreasing the sweep rate was observed at 1.8 or 5 K, respectively.
Results of NMR studies of nuclear spin-lattice relaxation in liquid metallic gallium confined within random pore networks of two different porous glasses with 16 and 2 nm pore sizes are presented. The measurements were run in the temperature range from 330 K to confined gallium freezing. Relaxation for both gallium isotopes 71 Ga and 69 Ga was found to accelerate remarkably compared to the bulk melt, the dominant mechanism of relaxation changing from magnetic to quadrupolar. The correlation time of electric field gradient fluctuations caused by atomic motion was estimated at various temperatures using data for quadrupolar relaxation contribution and was found to increase drastically compared to bulk, which corresponded to a pronounced slowdown of atomic mobility in confined liquid gallium. The influence of confinement was more effective for smaller pore sizes. The temperature dependence of the correlation time for confined gallium was found to be noticeably stronger than in bulk, an additional slowdown of atomic mobility being observed at low temperatures.
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