Ryosuke Kadono

Electronic structure of the muonium center as a shallow donor in ZnO.

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.

Quantum diffusion of muonium in insulators

In this paper we review our recent experiments conducted at TRIUMF on muonium diffusion in alkali halides. First, the technique of longitudinal-field muonium spin relaxation (T1) due to nuclear hyperfine interaction, an indispensabletour de force for the present work. is described. It is demonstrated in KCl that the technique provides spectacular sensitivity for muonium diffusion as well as determining the average nuclear hyperfine coupling constant. The muonium hop rate shows a minimum (T*≃80 K) and steep increase with decreasing temperature. The result is compared with the current theory of quantum diffusion in non-metallic crystals. A few more sets of new data may be presented for other alkali halides. In addition, we show that muonium forms a delocalized state in NaCl as evidenced by a large change of the average nuclear hyperfine parameter. Related topics of local tunneling system may be briefly reviewed.

Muonium as a shallow center in GaN

A paramagnetic muonium (Mu) state with an extremely small hyperfine parameter was observed for the first time in single-crystalline GaN below 25 K. It has a highly anisotropic hyperfine structure with axial symmetry along the <0001> direction, suggesting that it is located either at a nitrogen-antibonding or a bond-centered site oriented parallel to the c axis. Its small ionization energy (<or=14 meV) and small hyperfine parameter (approximately 10(-4) times the vacuum value) indicate that muonium in one of its possible sites produces a shallow state, raising the possibility that the analogous hydrogen center could be a source of n-type conductivity in as-grown GaN.

Multigap superconductivity in sesquicarbides La2C3 and Y2C3.

A complex structure of the superconducting order parameter in Ln2C3 (Ln=La,Y) is demonstrated by muon spin relaxation measurements in their mixed state. The muon depolarization rate sigma v(T)] exhibits a characteristic temperature dependence that can be perfectly described by a phenomenological double-gap model for nodeless superconductivity. While the magnitude of two gaps is similar between La2C3 and Y2C3, a significant difference in the interband coupling between those two cases is clearly observed in the behavior of sigma v(T).

µSR Studies on Heavy Fermion Superconductors CeIrIn5 and CeCoIn5

We report that the muon spin relaxation rate under zero-external field in the heavy-fermion compound Ce Tr In 5 ( Tr =Ir, Co) is independent of temperature down to 20 mK, demonstrating the absence of magnetism due to long-range magnetic order and/or to the breakdown of time-reversal symmetry in the superconducting state. The magnetic penetration depth deduced by the muon spin relaxation rate under a transverse field is λ∼6700 A for CeIrIn 5 at 402 G and ∼5500 A for CeCoIn 5 at 3 kG.

Full Gap Superconductivity in Ba

Superfluid density (

_{0.6}

n_s

K

) in the mixed state of an iron pnictide superconductor Ba

_{0.4}

_{0.6}

Fe

K