We report results of magnetization and 19F NMR measurements in the normal state of as-grown vacuum- annealed LaO0.5F0.5BiS2. The magnetization is dominated by a temperature-independent diamagnetic component and a field- and temperature-dependent paramagnetic contribution M mu (H, T ) from a <<^>> 1000 ppm concentration of local moments, an order of magnitude higher than can be accounted for by measured rare-earth impurity concentrations. M mu (H, T ) can be fit by the Brillouin function B J ( x ) or, perhaps more realistically, a two-level tanh(x) model for magnetic Bi 6p ions in defect crystal fields. Both fits require a phenomenological Curie-Weiss argument x = mu eff H / ( T + T w ), Tw approximate to 1.7 K. There is no evidence for magnetic order down to 2 K, and the origin of Tw is not clear. 19F frequency shifts, linewidths, and spin-lattice relaxation rates are consistent with purely dipolar 19 F / defect-spin interactions. The defect-spin correlation time t c ( T ) obtained from 19F spin-lattice relaxation rates obeys the Korringa relation t c T = const, indicating the relaxation is dominated by conduction- band fluctuations.

The Pr-rich end of the alloy series Pr$_{1-x}$Nd$_x$Os$_4$Sb$_{12}$ has been studied using muon spin rotation and relaxation. The end compound PrOs$_4$Sb$_{12}$ is an unconventional heavy-fermion superconductor, which exhibits a spontaneous magnetic field in the superconducting phase associated with broken time-reversal symmetry. No spontaneous field is observed in the Nd-doped alloys for x $>$ 0.05. The superfluid density is insensitive to Nd concentration, and no Nd$^{3+}$ static magnetism is found down to the lowest temperatures of measurement. Together with the slow suppression of the superconducting transition temperature with Nd doping, these results suggest anomalously weak coupling between Nd spins and conduction-band states.

We report results of a muon spin relaxation study of slow magnetic fluctuations in the pseudogap phase of underdoped single-crystalline YBa$_{2}$Cu$_{3}$O$_{y}$, $y = 6.77$ and 6.83. The dependence of the dynamic muon spin relaxation rate on applied magnetic field yields the rms magnitude~$B\mathrm{_{loc}^{rms}}$ and correlation time~$\tau_c$ of fluctuating local fields at muon sites. The observed relaxation rates do not decrease with decreasing temperature~$T$ below the pseudogap onset at $T^\ast$, as would be expected for a conventional magnetic transition; both $B\mathrm{_{loc}^{rms}}$ and $\tau_c$ are roughly constant in the pseudogap phase down to the superconducting transition. Corresponding NMR relaxation rates are estimated to be too small to be observable. Our results put strong constraints on theories of the anomalous pseudogap magnetism in YBa$_{2}$Cu$_{3}$O$_{y}$.

Hidden magnetic order in the correlated iridate Sr2Ir1-xRhxO4, x = 0.05 and 0.1, has been studied using muon spin relaxation spectroscopy. In zero field (ZF) and weak longitudinal fields (LFs) (less than or similar to 2 mT), the muon spin relaxation data indicate that static and dynamic local fields coexist at each muon site, and can be well described by exponentially damped static Lorentzian Kubo-Toyabe functions. The ZF relaxation rate is dominated by the static-field distribution, and a broad relaxation rate maximum at 175 K for x = 0.1 in ZF is attributed to muon diffusion and trapping. For LF greater than or similar to 2 mT the static rate is completely decoupled, and the exponential decay is due to dynamic spin fluctuations. The temperature dependencies of the relaxation rates exhibit maxima at 215 K (x = 0.05) and 175 K (x = 0.1), in agreement with previous second harmonic generation and polarized neutron diffraction determinations of transition temperatures to a hidden-order state. The maxima are most likely due to critical slowing down of electronic spin fluctuations. The field dependencies of the dynamic spin fluctuation rates can be well described by the Redfield relation, from which the rms width B-loc(rms) and correlation time tau(c) of the fluctuating field are obtained. Values of tau(c) are in the range of 1.5-4 ns for x = 0.1 and shorter than 2 ns for x = 0.05, suggesting an increase with increasing Rh concentration. Values of B-loc(rms) are on the order of 1 mT, consistent with the polarized neutron diffraction cross section.

We report results of a muon spin rotation (mu SR) study of the cuprate-analog nickelate La4Ni3O8, which undergoes a transition at 105 K to a low-temperature phase with charge-stripe and antiferromagnetic (AFM) order on square planar NiO2 layers. Zero-field mu SR shows that the AFM transition is abrupt, commensurate, and has a quasi-two-dimensional character below similar to 25 K. Comparison of observed muon precession frequencies with Ni dipolar field calculations yields Ni moments less than or similar to 0.5 mu(B). Dynamic muon spin relaxation above 105 K suggests critical slowing of Ni spin fluctuations, but is inconsistent with corresponding La-139 NMR results. Critical slowing and an abrupt transition are also observed in the planar cuprate AFM La2CuO4+delta, where they are taken as evidence for weakly interplanar-coupled two-dimensional AFM spin fluctuations, but our mu SR data do not agree quantitatively with theoretical predictions for this scenario when applied to the nickelate.

The superconducting state of the filled skutterudite alloy series Pr(1-x)La(x)Rt(4)Ge(12) has been systematically studied by specific heat and zero-field muon spin relaxation (mu SR), while the critical fields have been determined by magnetization measurements. An additional inhomogeneous local magnetic field, indicative of broken time-reversal symmetry, is observed in the superconducting states of the alloys. For x less than or similar to 0.5, the broken-TRS phase sets in below a temperature T-m, distinctly lower than the superconducting transition temperature T-c. For x greater than or similar to 0.5, T-m approximate to T-c. The local field strength decreases as x -> 1, where LaPt4Ge12 is characterized by conventional pairing. The lower critical field H-c1 (T) of PrPt4Ge12 shows the onset of a second quadratic temperature region below Tq similar to T-m. Upper critical field H-c2 (T ) measurements suggest multiband superconductivity, and point gap nodes are consistent with the specific heat data. In Pr(1-x)La(x)Rt(4)Ge(12), only a single specific-heat discontinuity is observed at T-c, in contrast to the second jump seen in PrOs4Sb12 below T-c. These results suggest that superconductivity in PrPt4Ge12 is characterized by a complex order parameter.

Author(s): Ding, ZF; Zhang, J; Tan, C; Huang, K; Chen, QY; Lum, I; Bernal, OO; Ho, PC; Maclaughlin, DE; Maple, MB; Shu, L | Abstract: © 2019 American Physical Society. We have measured the superconducting penetration depth Λ(T) in the heavy-fermion/intermediate-valent superconducting alloy series Ce1-xYbxCoIn5 using a transverse-field muon spin relaxation to study the effect of intermediate-valent Yb doping on Fermi-liquid renormalization. From Λ(T) we determine the superfluid density ρs(T) and find that it decreases continuously with increasing nominal Yb concentration x, i.e., with increasing intermediate valence. The temperature-dependent renormalization of the "normal" fluid density ρN(T)=ρs(0)-ρs(T) in both the heavy-fermion and intermediate valence limits is proportional to the temperature-dependent renormalization of the specific heat. This indicates that the temperature-dependent Fermi-liquid Landau parameters of the superconducting quasiparticles entering the two different physical quantities are the same. These results represent an important advance in understanding of both intermediate valence and heavy-fermion phenomena in superconductors.

K. Ohishi, R.H. Heffner, 2 G.D. Morris, 3 E.D. Bauer, M.J. Graf, J.-X. Zhu, L.A. Morales, J.L. Sarrao, M.J. Fluss, D.E. MacLaughlin, L. Shu, W. Higemoto, and T.U. Ito 6 Japan Atomic Energy Agency, Tokai, Ibaraki 319-1195, Japan Los Alamos National Laboratory, Los Alamos, New Mexico 87545 USA TRIUMF, 4004 Wesbrook Mall, Vancouver, B.C., Canada V6T 2A3 Lawrence Livermore National Laboratory, P.O. Box 808, Livermore, California 94550 USA Department of Physics, University of California, Riverside, California 92521 USA Department of Physics, Tokyo Institute of Technology, Megro-ku, Tokyo 152-8551, Japan (Dated: May 31, 2018)

We report muon spin relaxation and rotation (mu SR) measurements on hydrothermally grown single crystals of superconducting tetragonal FeS, which help to clarify the controversial magnetic state and superconducting gap symmetry of this compound. mu SR time spectra were obtained from 280 K down to 0.025 K in zero field (ZF) and applied fields up to 75 mT. In ZF, the observed loss of initial asymmetry (signal amplitude) and increase of depolarization rate Lambda(ZF) below 13 K indicate the onset of static magnetism, which coexists with superconductivity below T-c. TF mu SR results indicate a linear temperature dependence of the superfluid density at low temperature, consistent with nodal superconductivity. The s+d-wave model gives the best fit to the observed temperature and field dependencies, and yields an in-plane penetration depth value lambda(ab) (T=0) = 241(3) nm.

We report results of a muon spin relaxation ($\mu$SR) study of YFe$_2$Al$_{10}$, a quasi-2D nearly-ferromagnetic metal in which unconventional quantum critical behavior is observed. No static Fe$^{2+}$ magnetism, with or without long-range order, is found down to 19~mK\@. The dynamic muon spin relaxation rate~$\lambda$ exhibits power-law divergences in temperature and magnetic field, the latter for fields that are too weak to affect the electronic spin dynamics directly. We attribute this to the proportionality of $\lambda(\omega_\mu,T)$ to the dynamic structure factor~$S(\omega_\mu,T)$, where $\omega_\mu \approx 10^5$--$10^7~\mathrm{s}^{-1}$ is the muon Zeeman frequency. These results suggest critical divergences of $S(\omega_\mu,T)$ in both temperature and frequency. Power-law scaling and a 2D dissipative quantum XY (2D-DQXY) model both yield forms for $S(\omega,T)$ that agree with neutron scattering data ($\omega \approx 10^{12}~\mathrm{s}^{-1}$). Extrapolation to $\mu$SR frequencies agrees semi-quantitatively with the observed temperature dependence of $\lambda(\omega_\mu,T)$, but predicts frequency independence for $\omega_\mu \ll T$ in extreme disagreement with experiment. We conclude that the quantum critical spin dynamics of YFe$_2$Al$_{10}$ are not well understood at low frequencies.

K. Huang,1,* C. Tan,1 J. Zhang,1 Z. Ding,1 D. E. MacLaughlin,2 O. O. Bernal,3 P.-C. Ho,4 C. Baines,5 L. S. Wu,6,7 M. C. Aronson,6,7,† and L. Shu1,8,‡ 1State Key Laboratory of Surface Physics, Department of Physics, Fudan University, Shanghai 200433, China 2Department of Physics and Astronomy, University of California, Riverside, California 92521, USA 3Department of Physics and Astronomy, California State University, Los Angeles, California 90032, USA 4Department of Physics, California State University Fresno, Fresno, California 93740, USA 5Laboratory for Muon-Spin Spectroscopy, Paul Sherrer Institute, CH-5232 Villigen, Switzerland 6Department of Physics and Astronomy, Stony Brook University, Stony Brook, New York 11794, USA 7Condensed Matter Physics and Materials Science Department, Brookhaven National Laboratory, Upton, New York 11973, USA 8Collaborative Innovation Center of Advanced Microstructures, Nanjing 210093, China

We reply to the objections raised in a recent Comment (arXiv:1706.03023) regarding our observation of slow magnetic fluctuations and critical slowing down of magnetic fluctuations in the pseudogap phase of YBa$_2$Cu$_3$O$_y$ by zero-field and longitudinal-field muon spin relaxation (arXiv:1703.06799).

We have performed transverse-field muon spin relaxation measurements on ambient-pressure-grown polycrystalline LaO0.5F0.5BiS2. From these measurements, no signature of magnetic order is found down to 25 mK. The value of the magnetic penetration depth extrapolated to 0 K is 0.89 (5) mu m. The temperature dependence of superconducting penetration depth is best described by either a multigap s + s-wave model with Delta(1) = 0.947 (7) meV and Delta(2) = 0.22 (4) meV or the anisotropic s-wave model with Delta(0) = 0.776 meV and anisotropic gap amplitude ratio Delta(min)/Delta(max) = 0.34. Comparisons with other potentially multigap BiS2-based superconductors are discussed. We find that these BiS2-based superconductors, including Bi4O4S3 and the high-pressure synthesized LaO0.5F0.5BiS2, generally conform to the Uemura relation.

The intermediate-valent polymorphs alpha- and beta-YbAlB4 exhibit quantum criticality and other novel properties not usually associated with intermediate valence. Iron doping induces quantum criticality in alpha-YbAlB4 and magnetic order in both compounds. We report results of muon spin relaxation (mu SR) experiments in alpha-YbAl(1-x)Fex B-4, x = 0.014 and 0.25. For x = 0.014 we find no evidence for magnetic order down to 25 mK. The dynamic muon spin relaxation rate.d exhibits a power-law temperature dependence lambda(d) proportional to T-a, a = 0.40(4), in the temperature range 100 mK-2 K, in disagreement with predictions by theories of antiferromagnetic (AFM) or valence quantum critical behavior. For x = 0.25, where AFM order develops in the temperature range 7.5-10 K, we find coexistence of meso-ormacroscopically segregated paramagnetic and AFM phases, with considerable disorder in the latter down to 2 K.