We report on muon spin relaxation measurements of the 4f 2 -based heavy-fermion superconductor filled-skutterudite PrOs4Sb12. The results reveal the spontaneous appearance of static internal magnetic fields below the superconducting transition temperature, providing unambiguous evidence for the breaking of time-reversal symmetry in the superconducting state. A discussion is made on which of the spin or orbital component of Cooper pairs carries a nonzero momentum.PACS numbers: 74.70. Tx, 76.75.+i, 74.70.Dd, 74.25.Ha Many unconventional superconducting (SC) states, as in Ce-and U-based heavy-fermion (HF) compounds or high-T c cuprates, appear in close proximity to magnetic instabilities when a certain parameter (pressure, atomic doping, or oxygen content) is varied [1,2,3]. This fact strongly suggests that the attractive interactions binding electrons into Cooper pairs are mediated by magnetic moment fluctuations. As another possible pairing glue, fluctuations of quadrupole moments-distorted shapes of the electronic clouds of ions, are theoretically considered to be possible [4,5]. An interesting question to be addressed is what is the nature of superconductivity in such a system. For this study, Pr-based compounds with a 4f 2 configuration are likely candidates, since nonmagnetic but quadrupolar active low-energy levels can be realized due to the crystalline-electric-field (CEF) effect; in 5f systems, CEF levels are less clear due to the tendency to be itinerant.One promising candidate material for this study is the recently found superconductor PrOs 4 Sb 12 [6], which is to date the only known Pr-based HF superconductor, with a superconducting transition temperature of 1.82 K (hereafter referred to as T c1 ). The estimated electronic specific heat coefficient γ = 350 − 700 mJ/K 2 mol [6,7] and the enhanced cyclotron-effective masses [8] reflect the existence of strong electron correlations. Specific heat (C), magnetic susceptibility (χ), and inelastic neutron studies provide evidence that PrOs 4 Sb 12 has a nonmagnetic ground state and a magnetic triplet excited state separated by ∆E CEF /k B = 8 K [7,9], which is 5 times larger than T c1 . In the temperature-versus-magneticfield (T -vs-H) phase diagram, a field-induced ordered phase (µ 0 H 4 T) [9] appears close to the superconducting phase (the upper critical field µ 0 H c2 = 2.2 T). It was recently proven to be an antiferro-quadrupolar ordered phase by elastic neutron scattering measurements [10]. This fact strongly indicates that quantum quadrupole fluctuations of the Pr ions play an important role in realizing the HF superconductivity in PrOs 4 Sb 12 , considering that the T -vs-H phase diagram is analogous to those for the HF and cuprate systems, where a magnetically ordered phase exists close to the SC phase in the T -vs-pressure, -atomic-doping, or -oxygen-content phase diagram. This scenario is further supported by the enhanced T c1 compared to T c = 0.74 K for a 4f 0 reference compound LaOs 4 Sb 12 [8,11].The remarkable unconventional SC properties ...

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The magnetism of LixCoO2 (LCO), which has a similar structure to NaxCoO2 (NCO), has been investigated by muon-spin spectroscopy and susceptibility measurements using samples with x=0.1-1 prepared by an electrochemical reaction. In the x range below 0.75, LCO was found to be Pauli paramagnetic down to 1.8 K, suggesting an intermediate- or weak-coupling regime, although disordered local moments, with volume fractions below approximately 20%, appear at low T for LCO with x > or = 0.5. The phase diagram and interactions of LCO are thus strikingly different from NCO, while the differences cannot be explained simply by structural differences between the two systems.

Muon spin rotation experiments are carried out on clinoatacamite, Cu2Cl(OH)3, which is a new geometrically frustrated system featuring a three-dimensional network of corner-sharing tetrahedral 3d Cu2+ spins. A long-range antiferromagnetic order occurs below 18.1 K with a surprisingly small entropy release of about 0.05Rln2/Cu. Below 6.5 K, the static long-range order transforms abruptly into a metastable state with nearly complete depolarization of muon spins which suggests strong fluctuation. The system then enters a state in which partial long-range order and spin fluctuation coexist down to the lowest experimentally attainable temperature of 20 mK. This work presents a novel system for studying geometric frustration.

The electronic structure and the location of muonium centers (Mu) in single-crystalline ZnO were determined for the first time. Two species of Mu centers with extremely small hyperfine parameters have been observed below 40 K. Both Mu centers have an axial-symmetric hyperfine structure along with a <0001> axis, indicating that they are located at the antibonding (AB(O, parallel )) and bond-center (BC( parallel )) sites. It is inferred from their small ionization energy ( approximately 6 and 50 meV) and hyperfine parameters ( approximately 10(-4) times the vacuum value) that these centers behave as shallow donors, strongly suggesting that hydrogen is one of the primary origins of n type conductivity in as-grown ZnO.