We report the magnetic and superconducting properties of locally noncentrosymmetric SrPtAs obtained by muon-spin-rotation/relaxation (µSR) measurements. Zero-field µSR 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 µSR 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. Transition metal pnictides have attracted considerable scientific interest as they present the second largest family of superconductors after the cuprates [1]. All superconductors of this family share one common structural feature: superconductivity takes place in a square lattice formed by the transition metal elements. Very recently superconductivity with a T c of 2.4 K has been discovered in SrPtAs [2], which has a unique and attractive structural feature: It crystallizes in a hexagonal structure with weakly coupled PtAs layers forming a honeycomb lattice. SrPtAs supports three pairs of split Fermi surfaces, two of which are hole-like and centered around the Γ-point with a cylindrical shape extended along the k z direction and together host only about 30% of the density of states. The remaining 70% of the density of states are hosted by the third pair of split Fermi surfaces that is electron-like, centered around the K and K ′

Superconducting [(Li1−xFex)OH](Fe1−yLiy)Se (x≈0.2, y≈0.08) was synthesized by hydrothermal methods and characterized by single‐crystal and powder X‐ray diffraction. The structure contains alternating layers of anti‐PbO type (Fe1−yLiy)Se and (Li1−xFex)OH. Electrical resistivity and magnetic susceptibility measurements reveal superconductivity at 43 K. An anomaly in the diamagnetic shielding indicates ferromagnetic ordering near 10 K while superconductivity is retained. The ferromagnetism is from the iron atoms in the (Li1−xFex)OH layer. Isothermal magnetization measurements confirm the superposition of ferromagnetic and superconducting hysteresis. The internal ferromagnetic field is larger than the lower, but smaller than the upper critical field of the superconductor. The formation of a spontaneous vortex phase where both orders coexist is supported by 57Fe‐Mössbauer spectra, 7Li‐NMR spectra, and μSR experiments.

or most of its history, the superconductivity of strontium ruthenate (Sr 2 RuO 4) (ref. 1) has been understood in terms of an odd-parity two-component order parameter with equal-spin pairing in the RuO 2 planes: p x ± ip y (refs. 2-5). This order parameter is chiral: the Cooper pairs have angular momentum l = ±1. The evidence for chirality comes from the zero-field muon spin relaxation (ZF-μSR) data 6 , observation of a non-zero Kerr rotation below the critical temperature T c (ref. 7) and signs in the junction experiments of domains in the superconducting state 8,9 , while evidence for equal-spin pairing came from the absence of a change in the Knight shift below T c in nuclear magnetic resonance 10 and polarized neutron scattering 11 measurements. The Knight shift is related to the spin susceptibility; in conventional opposite-spin-pairing superconductors, it is suppressed below T c. However, in new measurements, it has been found that the Knight shift is, in fact, suppressed below T c (refs. 12-14), by a magnitude that is unlikely to be reconcilable with equal-spin pairing. This revision has called into question a number of other results on Sr 2 RuO 4. It raises a particular challenge for experiments that indicate chirality, because opposite-spin pairing implies an even-parity momentum-space gap structure. If the order parameter is constrained to be even parity, chiral, and composed of components that are degenerate on the tetragonal lattice of Sr 2 RuO 4 , the only possibility is d xz ± id yz order 15. Under conventional understanding, this is a highly unlikely order parameter because it

We report muon spin rotation and magnetic susceptibility experiments on in-plane stress effects on the static spin-stripe order and superconductivity in the cuprate system La 2−x Ba x CuO 4 with x ¼ 0.115. An extremely low uniaxial stress of ∼0.1 GPa induces a substantial decrease in the magnetic volume fraction and a dramatic rise in the onset of 3D superconductivity, from ∼10 to 32 K; however, the onset of at-least-2D superconductivity is much less sensitive to stress. These results show not only that largevolume-fraction spin-stripe order is anticorrelated with 3D superconducting coherence but also that these states are energetically very finely balanced. Moreover, the onset temperatures of 3D superconductivity and spin-stripe order are very similar in the large stress regime. These results strongly suggest a similar pairing mechanism for spin-stripe order and the spatially modulated 2D and uniform 3D superconducting orders, imposing an important constraint on theoretical models.

Using a combination of muon-spin relaxation (μSR), inelastic neutron scattering (INS), and nuclear magnetic resonance (NMR), we investigated the novel iron-based superconductor with a triclinic crystal structure (CaFe 1−x Pt x As) 10 Pt 3 As 8 (T c = 13 K), containing platinum-arsenide intermediary layers. The temperature dependence of the superfluid density obtained from the μSR relaxation-rate measurements indicates the presence of two superconducting gaps, 1 2 . According to our INS measurements, commensurate spin fluctuations are centered at the (π, 0) wave vector, like in most other iron arsenides. Their intensity remains unchanged across T c , indicating the absence of a spin resonance typical for many Fe-based superconductors. Instead, we observed a peak in the spin-excitation spectrum around ω 0 = 7 meV at the same wave vector, which persists above T c and is characterized by the ratio ω 0 /k B T c ≈ 6.2, which is significantly higher than typical values for the magnetic resonant modes in iron pnictides (∼ 4.3). The temperature dependence of magnetic intensity at 7 meV revealed an anomaly around T * = 45 K related to the disappearance of this new mode. A suppression of the spin-lattice relaxation rate, 1/T 1 T , observed by NMR immediately below T * without any notable subsequent anomaly at T c , indicates that T * could mark the onset of a pseudogap in (CaFe 1−x Pt x As) 10 Pt 3 As 8 , which is likely associated with the emergence of preformed Cooper pairs.

We report detailed 75 As-NQR investigations of the locally non-centrosymmetric superconductor SrPtAs. The spin-lattice relaxation studies prove weakly coupled multi-gap superconductivity. A retardation of the decay in 1/T1T evidences a nodeless (fully gapped) superconducting state on the complex multi-pocket Fermi surface, which is consistent with an anisotropic s-wave order parameter and with proposed unconventional f -wave and chiral d -wave symmetries. A quantitative analysis of these models favors the unconventional f -wave state.

We present a piezoelectric-driven uniaxial pressure cell that is optimized for muon spin relaxation and neutron scattering experiments and that is operable over a wide temperature range including cryogenic temperatures. To accommodate the large samples required for these measurement techniques, the cell is designed to generate forces up to ∼1000 N. To minimize the background signal, the space around the sample is kept as open as possible. We demonstrate here that by mounting plate-like samples with epoxy, a uniaxial stress exceeding 1 GPa can be achieved in an active volume of at least 5 mm 3 . We show that for practical operation, it is important to monitor both the force and displacement applied to the sample. In addition, because time is critical during facility experiments, samples are mounted in detachable holders that can be rapidly exchanged. The piezoelectric actuators are likewise contained in an exchangeable cartridge.

We present results of 23 Na and 19 F nuclear magnetic resonance (NMR) measurements on NaCaCo2F7, a frustrated pyrochlore magnet with a Curie-Weiss temperature, ΘCW ≈ −140 K, and intrinsic bond disorder. Below 3.6 K both the 23 Na and 19 F spectra broaden substantially in comparison to higher temperatures accompanied by a considerable reduction (80 %) of the NMR signal intensity: This proves a broad quasi-static field distribution. The 19 F spin-lattice relaxation rate 19 (1/T1) exhibits a peak at 2.9 K already starting to develop below 10 K. We attribute the spin freezing to the presence of bond disorder. This is corroborated by large-scale Monte-Carlo simulations of a classical bond-disordered XY model on the pyrochlore lattice. The low freezing temperature, together with the very short magnetic correlation length not captured by the simulations, suggesting that quantum effects play a decisive role in NaCaCo2F7.PACS numbers: 75.10. Nr, 75.40.Gb, The pyrochlore lattice is one of the canonical lattices exhibiting geometric frustration. Pyrochlore oxides with the general formula R 2 T 2 O 7 (R = rare earth ion, T = Ti, Sn, Mo, Ir etc.) are characterized by strong frustration of the rare-earth magnetic moments and have been found to display a variety of fascinating low-temperature phases, including classical and quantum spin-ice regimes, quantum order-by-disorder, and exotic spin liquids [1][2][3].NaCaCo 2 F 7 is a recently synthesized A 2 B 2 X 7 -type pyrochlore [4]. Here, the B site hosts high-spin Co 2+ in CoF 6 octahedra, whereas the A site contains a random distribution of Na and Ca ions, giving rise to exchange (bond) disorder for the magnetic degrees of freedom. Bond disorder can lift degeneracies and lead to new unconventional ground states, both long-range ordered [5,6] and glassy [7][8][9][10].The uniform susceptibility of NaCaCo 2 F 7 displays a Curie-Weiss law with a characteristic temperature Θ CW ≈ −140 K and a moment of 6.1 µ B per Co, the latter suggesting an orbital contribution in addition to the S = 3/2 state of Co 2+ . Despite the Θ CW ≈ −140 K, the material does not display a long-range-ordered magnetic state down to temperatures of 0.6 K [4]. Instead, ac and dc susceptibility data indicate a spin freezing at T f ≈ 2.4 K [4], yielding a frustration index of f = Θ CW /T f ≈ 56. The observed entropy loss at the freezing transition is low, suggesting that magnetic entropy remains present at least down to 0.6 K. Inelastic neutron scattering (INS) results [11] for NaCaCo 2 F 7 show diffuse elastic scattering below 2.4 K. The momentum-space structure of the spin correlations below 2.5 meV has been interpreted in terms of XY-like antiferromagnetic clusters, indicating a local easy-plane anisotropy at low energies, whereas a signal corresponding to collinear correlations was found above this energy. The XY character together with the short detected correlation length of 16Å [11] appears to be at odds with the theoretical suggestion [6] that bond disorder in an XY pyrochlore magnet induces long-range m...