Muon spin rotation and relaxation experiments in the pyrochlore iridate Eu 2 Ir 2 O 7 yield a welldefined muon spin precession frequency below the metal-insulator/antiferromagnetic transition temperature T M = 120 K, indicative of long-range commensurate magnetic order and thus ruling out quantum spin liquid and spin-glass-like ground states. The dynamic muon spin relaxation rate is temperature-independent between 2 K and ∼T M and yields an anomalously long Ir 4+ spin correlation time, suggesting a singular density of low-lying spin excitations. Similar behavior is found in other pyrochlores and geometrically frustrated systems, but also in the unfrustrated iridate BaIrO 3 . Eu 2 Ir 2 O 7 may be only weakly frustrated; if so, the singularity might be associated with the small-gap insulating state rather than frustration.

Magnetic susceptibility and muon spin relaxation (µSR) experiments have been carried out on the quasi-2D triangular-lattice spin S = 2 antiferromagnet FeGa2S4. The µSR data indicate a sharp onset of a frozen or nearly-frozen spin state at T * = 31(2) K, twice the spin-glass-like freezing temperature T f = 16(1) K. The susceptibility becomes field dependent below T * , but no sharp anomaly is observed in any bulk property. A similar transition is observed in µSR data from the spin-1 isomorph NiGa2S4. In both compounds the dynamic muon spin relaxation rate λ d (T ) above T * agrees well with a calculation of spin-lattice relaxation by Chubukov, Sachdev, and Senthil in the renormalized classical regime of a 2D frustrated quantum antiferromagnet. There is no firm evidence for other mechanisms. At low temperatures λ d (T ) becomes temperature independent in both compounds, indicating persistence of spin dynamics. Scaling of λ d (T ) between the two compounds is observed from ∼T f to ∼1.5T* . Although the µSR data by themselves cannot exclude a truly static spin component below T * , together with the susceptibility data they are consistent with a slowly-fluctuating "spin gel" regime between T f and T * . Such a regime and the absence of a divergence in λ d (T ) at T * are features of two unconventional mechanisms: (1) binding/unbinding of Z2 vortex excitations, and (2) impurity spins in a nonmagnetic spin-nematic ground state. The absence of a sharp anomaly or history dependence at T * in the susceptibility of FeGa2S4, and the weakness of such phenomena in NiGa2S4, strongly suggest transitions to low-temperature phases with unconventional dynamics.

Cichorek et al. [1] reported enhancements of the lower critical field H c1 (T ) and critical current I c (T ) in superconducting PrOs 4 Sb 12 below a transition temperature T c3 ≈ 0.6 K, and speculated that this reflects a transition between superconducting phases. Features have been observed near T c3 in other properties, but not in the specific heat. We report muon spin rotation (µSR) measurements of the penetration depth λ in the vortex state of PrOs 4 Sb 12 near H c1 , that to high accuracy exhibit no anomaly T c3 and therefore cast doubt on the putative phase transition.In a Type-II superconductor H c1 = Φ 0 (ln κ + c)/4πλ 2 , c ≈ 0.5, where Φ 0 is the flux quantum and κ is the Ginzburg-Landau parameter [2]. Modification of the superfluid density ρ s by a phase transition should affect both λ = (mc 2 /4πe 2 ρ s ) 1/2 and H c1 . A feature observed near T c3 in rf inductive measurements of λ [3] is too small to account for the observed enhancement ofIn the transverse-field µSR technique the spectrum of muon precession frequencies gives the local-field distribution function, which depends on λ in the vortex state [2]. µSR experiments were carried out at the ISIS µSR facility on a polycrystalline sample of PrOs 4 Sb 12 . Strong de Haas-van Alphen signals obtained from similarlyprepared crystals [4] attest to their high quality. Data were taken in low applied fields H = 25 and 40 Oe, the former corresponding to an internal field H ′ ∼ 32 Oe at H ′ = H c1 (T ) after demagnetization correction. This is close to the estimated unenhanced value of H c1 (0) [1], so that in the absence of enhancement H ′ should be > H c1 . The data are well fit by the Gaussian relaxation function G(t) = exp(− 1 2 σ 2 t 2 ) cos(ω µ t + θ). Figure 1 gives the average muon spin precession frequency ω µ (T ) and relaxation rate σ(T ) at 25 and 40 Oe. There is no discernible anomaly at T c3 , and no evidence that H ′ < H c1 . Similar results are found at higher fields [5].The rms width δB rms of the field distribution in the vortex state is estimated by σ/γ µ , where γ µ is the muon gyromagnetic ratio. In the London model δB other: H c1 /δB rms = 1.31(ln κ + c) ≈ 5.0 in PrOs 4 Sb 12 . Thus a ∼50% enhancement of H c1 (0) [1] implies a similar enhancement of σ(0) contrary to our results. From σ(0) we estimate H c1 (0) ≈ 50 Oe, of the order of the observed value, so that other broadening mechanisms, such as vortex-lattice disorder or a distribution of demagnetizing fields, are unlikely to dominate the muon relaxation.These results and the absence of a specific heat anomaly at T c3 are evidence against a phase transition associated with the enhancement of H c1 at low temperatures in PrOs 4 Sb 12 . The enhanced critical current suggests that flux pinning effects, which generally become stronger at low temperatures, are involved.

Muon spin rotation and relaxation measurements have been carried out on the unconventional antiferromagnet Yb3Pt4. Oscillations are observed below T(N) = 2.22(1) K, consistent with the antiferromagnetic (AFM) Néel temperature observed in bulk experiments. In agreement with neutron diffraction experiments the oscillation frequency ω(μ)(T)/2π follows an S = 1/2 mean-field temperature dependence, yielding a quasistatic local field of 1.71(2) kOe at T = 0. A crude estimate gives an ordered moment of ∼ 0.66 μ(B) at T = 0, comparable to 0.81 μ(B) from neutron diffraction. As T-->T(N) from above the dynamic relaxation rate λ(d) exhibits no critical slowing down, consistent with a mean-field transition. In the AFM phase a T-linear fit to λ(d)(T), appropriate to a Fermi liquid, yields highly enhanced values of λ(d)/T and the Korringa constant K(μ)(2)T/λ(d), with K(μ) the estimated muon Knight shift. A strong suppression of λ(d) by applied field is observed in the AFM phase. These properties are consistent with the observed large Sommerfeld-Wilson and Kadowaki-Woods ratios in Yb3Pt4 (although the data do not discriminate between Fermi-liquid and non-Fermi-liquid states), and suggest strong enhancement of q≈0 spin correlations between large-Fermi-volume band quasiparticles in the AFM phase of Yb3Pt4.