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The penetration depth λ in the filled-skutterudite heavy-fermion superconductor PrOs4Sb12 has been measured using transverse-field muon spin rotation (TF- μ SR ). It is found to be temperature-independent at low temperatures, consistent with a nonzero gap for quasiparticle excitations. In contrast, radiofrequency (RF) measurements yield a stronger temperature dependence of λ ( T ) , indicative of point nodes in the gap. A ∼ 10 % discrepancy is found at low temperatures. This may be due to mechanisms that modify the vortex-state field distribution, or to the surface scattering which breaks pairs in an odd-parity superconductor. Alternatively, it may be a matter of field orientation of nodal gap structure in the μ SR measurements.

The effective superconducting penetration depth measured in the vortex state of PrOs 4 Sb 12 using transverse-field muon spin rotation ( TF - μ SR ) exhibits an activated temperature dependence at low temperatures, consistent with a nonzero gap for quasiparticle excitations. In contrast, Meissner-state radiofrequency (rf) inductive measurements of the penetration depth yield a T 2 temperature dependence, suggestive of point nodes in the gap. A scenario based on the recent discovery of extreme two-band superconductivity in PrOs 4 Sb 12 is proposed to resolve this difference. In this picture a large difference between large- and small-gap coherence lengths renders the field distribution in the vortex state controlled mainly by supercurrents from a fully gapped large-gap band. In zero field all bands contribute, yielding a stronger temperature dependence to the rf inductive measurements.

We have measured the temperature dependence and magnitude of the superfluid density ${\ensuremath{\rho}}_{s}(T)$ via the magnetic field penetration depth $\ensuremath{\lambda}(T)$ in $\mathrm{Pu}\mathrm{Co}{\mathrm{Ga}}_{5}$ (nominal critical temperature ${T}_{c0}=18.5\phantom{\rule{0.3em}{0ex}}\mathrm{K}$) using the muon spin rotation technique in order to investigate the symmetry of the order parameter, and to study the effects of aging on the superconducting properties of a radioactive material. The same single crystals were measured after $25\phantom{\rule{0.3em}{0ex}}\text{days}$ $({T}_{c}=18.25\phantom{\rule{0.3em}{0ex}}\mathrm{K})$ and $400\phantom{\rule{0.3em}{0ex}}\text{days}$ $({T}_{c}=15.0\phantom{\rule{0.3em}{0ex}}\mathrm{K})$ of aging at room temperature. Penetration depths $\ensuremath{\lambda}(0)=265(5)$ and $\ensuremath{\geqslant}498(10)\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ are derived for the fresh and aged samples, respectively. The temperature dependence of the superfluid density ${\ensuremath{\rho}}_{s}(T)∕{\ensuremath{\rho}}_{s}(0)=\ensuremath{\lambda}{(0)}^{2}∕\ensuremath{\lambda}{(T)}^{2}$ is well described in both materials by a model using $d$-wave gap symmetry. Within the context of a strong-coupling dirty $d$-wave model a zero-temperature gap value ${\ensuremath{\Delta}}_{0}=3.0{k}_{B}{T}_{c0}$ is obtained in the fresh sample for a scattering rate $\ensuremath{\Gamma}=0.005\ensuremath{\pi}{k}_{B}{T}_{c0}$, which is consistent with Abrikosov-Gor'kov (AG) pair-breaking theory. This ${\ensuremath{\Delta}}_{0}$ should be compared to the weak-coupling value for a clean $d$-wave gap ${\ensuremath{\Delta}}_{0}=2.14{k}_{B}{T}_{c0}$. In the aged sample the same model yields ${\ensuremath{\Delta}}_{0}=2.4{k}_{B}{T}_{c0}$ for $\ensuremath{\Gamma}=0.010\ensuremath{\pi}{k}_{B}{T}_{c0}$. This value of $\ensuremath{\Gamma}$ is much less than required by the AG pair-breaking formalism. Furthermore, the aged ${\ensuremath{\rho}}_{s}(0)$ is reduced by at least 70% compared to the fresh sample, which is also incompatible with $\ensuremath{\Delta}{T}_{c}∕{T}_{c0}\ensuremath{\sim}20%$, according to AG theory. We conclude that the data in aged $\mathrm{Pu}\mathrm{Co}{\mathrm{Ga}}_{5}$ support the postulate that the scattering from radiation-induced defects is not in the limit of the AG theory of an order parameter which is spatially averaged over impurity sites, but rather in the limit of short-coherence-length superconductivity. We show that a model calculation consistent with this assumption fits ${\ensuremath{\rho}}_{s}(T)$ in the aged sample rather well.

We present $\ensuremath{\mu}\mathrm{SR}$ measurements on Pu metal which set stringent upper limits on the magnitude of the ordered moments ${\ensuremath{\mu}}_{\mathrm{ord}}$ in $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{Pu}$ and $\ensuremath{\delta}$-stabilized Pu (alloyed with $4.3\phantom{\rule{0.3em}{0ex}}\mathrm{at}.%$ Ga). The magnitude of the low-temperature $(4\phantom{\rule{0.3em}{0ex}}\mathrm{K}lTl100--150\phantom{\rule{0.3em}{0ex}}\mathrm{K})$ $\ensuremath{\mu}\mathrm{SR}$ rate in zero applied field is independent of temperature and consistent with nuclear dipolar broadening alone. The Knight shift in $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{Pu}$ in $2\phantom{\rule{0.3em}{0ex}}\mathrm{T}$ applied field is also independent of temperature below about $100\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, but increases abruptly at higher temperatures, an effect likely caused by muon diffusion to paramagnetic impurities. A rough estimate for the muon hopping rate $1∕{\ensuremath{\tau}}_{h}$ at $150\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ in $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{Pu}$ yields $1∕{\ensuremath{\tau}}_{h}\ensuremath{\approx}3\ifmmode\times\else\texttimes\fi{}{10}^{7}\phantom{\rule{0.3em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$; no estimate is yet obtained for $\ensuremath{\delta}\text{\ensuremath{-}}\mathrm{Pu}$. The principal results reported here are limits for ${\ensuremath{\mu}}_{\mathrm{ord}}$ in both $\ensuremath{\alpha}$- and $\ensuremath{\delta}\text{\ensuremath{-}}\mathrm{Pu}\ensuremath{\leqslant}{10}^{\ensuremath{-}3}{\ensuremath{\mu}}_{\mathrm{B}}$ at $\mathrm{T}\ensuremath{\cong}4\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. These limits are discussed in terms of recent models of the electronic properties of $\ensuremath{\delta}\text{\ensuremath{-}}\mathrm{Pu}$, where magnetism has been previously predicted. Our results are consistent with, but cannot unambiguously distinguish between, cases where the moments in $\ensuremath{\delta}\text{\ensuremath{-}}\mathrm{Pu}$ are essentially zero, possess very weak interatomic exchange, are reduced by hybridization, or are washed out by dynamical fluctuations.

Longitudinal-field muon spin relaxation experiments have been carried out in the paramagnetic state of single-crystal Gd_2Ti_2O_7 just above the phase transition at T_m = 1.0 K. At high applied fields the exponential relaxation time T_1 is proportional to field, whereas T_1 saturates below a crossover field B_c that is ~2.5 T at 1.5 K and decreases as T_m is approached. At low fields the relaxation rate increases markedly as the freezing temperature is approached, as expected for critical slowing down of the spin fluctuations, but the increase is suppressed by applied field. This behavior is consistent with the very long autocorrelation function cutoff time implied by the low value of B_c., 4 pages, 2 figures, submitted to 10th International Conference on Muon Spin Rotation, Relaxation, and Resonance, Oxford, UK, August 2005

We report muon spin rotation measurements of the local magnetic susceptibility around a positive muon in the paramagnetic state of the quasi-one-dimensional spin 1/2 antiferromagnet dichlorobis (pyridine) copper (II). Signals from three distinct sites are resolved and have a temperature dependent frequency shift which is significantly different than the magnetic susceptibility. This difference is attributed to a muon induced perturbation of the spin 1/2 chain. The obtained frequency shifts are compared with transfer matrix density-matrix renormalization-group numerical simulations.

Transverse-field muon-spin rotation measurements in the vortex-lattice of the heavy-fermion (HF) superconductor ${\mathrm{P}\mathrm{r}\mathrm{O}\mathrm{s}}_{4}{\mathrm{S}\mathrm{b}}_{12}$ yield a temperature dependence of the magnetic penetration depth $\ensuremath{\lambda}$ indicative of an isotropic or nearly isotropic energy gap. This is not seen to date in any other HF superconductor and is a signature of isotropic pairing symmetry, possibly related to a novel nonmagnetic ``quadrupolar Kondo'' HF mechanism in ${\mathrm{P}\mathrm{r}\mathrm{O}\mathrm{s}}_{4}{\mathrm{S}\mathrm{b}}_{12}$. The $T=0$ relaxation rate ${\ensuremath{\sigma}}_{s}(0)=0.91(1)\text{ }\text{ }\ensuremath{\mu}{\mathrm{s}}^{\ensuremath{-}1}$ yields an estimated magnetic penetration depth $\ensuremath{\lambda}(0)=3440(20)\text{ }\text{ }\AA{}$, which is considerably shorter than in other HF superconductors.

We have performed muon spin rotation ( μ SR ) measurements in the radioactive Pu-based superconductor PuCoGa 5 to study the effects of radiation damage on the superconducting properties. The same single crystals were studied 25 days (fresh) and 400 days (aged) after initial preparation. We find that the μ SR rate σ v , which is proportional to superfluid density ρ s , is steeply reduced in the aged sample compared with its reduction of T c . This result is inconsistent with conventional Abrikosov–Gor’kov pair-breaking theory, but can be explained by the local suppression of the order parameter in a strong-scattering, short-coherence-length superconductor.

Zero-field muon spin relaxation (μSR)(μSR) in non-Fermi liquid Ce0.2La0.8RhIn5 reveals disordered spin freezing with a sharp transition at 0.16 K. The frozen moment at T=0T=0 is estimated as ∼0.13μB/Ce, an order of magnitude smaller than found from thermodynamic data at higher fields and temperatures. Longitudinal-field μSRμSR exhibits no dynamic relaxation at any temperature, showing that the spin freezing is not accompanied by the slow spin fluctuations found in spin glasses above the glass temperature and in other disordered non-Fermi liquid systems.

The local magnetic susceptibility wloc around a positive muon in quasi-one-dimensional spin 1/2 antiferromagnetic chain compounds dichlorobis KCuF3 has been investigated using muon spin rotation/relaxation muSR: We compare chi_loc with the bulk magnetic susceptibility chi measured in a SQUID magnetometer. In all cases there is a dramatic difference between chi_loc and chi; as predicted by theory.