R.L. Lichti

Shallow donor state of hydrogen in indium nitride

The nature of the electron states associated with hydrogen in InN has been inferred by studying the behavior of positive muons, which mimic protons when implanted into semiconductors. The muons capture electrons below 60 K, forming paramagnetic centers with a binding energy of about 12 meV. Together with an exceedingly small muon-electron hyperfine constant indicative of a highly delocalized electron wave function, the results confirm the recently predicted shallow-donor properties of hydrogen in InN.

Shallow versus deep hydrogen states in ZnO and HgO

The muonium states mimicking interstitial hydrogen in ZnO and HgO are compared. Whereas in ZnO a theoretically predicted shallow donor state is confirmed, in HgO we find a considerably deeper state. The respective ionization temperatures are around 40 K and 150 K and the donor ionization energies are 19±1 and 136±3 meV, deduced from the temperature dependence of the µSR (muon spin-rotation) signal amplitudes. The µSR spectra provide a comprehensive characterization of the undissociated paramagnetic states: the hyperfine parameters, which measure the electron spin density on and near the muon, differ by a factor of ~30. These define a hydrogenic radius of 1.1 nm in ZnO but indicate a much more compact electronic wavefunction in HgO, more akin to those of Mu* and the AA9 centre in Si. These data should largely carry over to hydrogen as a guide to its electrical activity in these materials.

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.

Coupling strengths and flux pinning in oxide superconductors

Muon spin relaxation measurements have been performed to obtain penetration depth versus temperature for several high‐temperature oxide superconductors. Comparison with theoretical results based on the Eliashberg equations indicates that at 1 kOe the (RE)Ba2Cu3O7 materials (data for RE=Gd, Er, and Eu) are in the very strong coupling, extremely dirty limit, while data at 5 kOe for Bi2CaSr2Cu2O8 and Tl2Ca2Ba2Cu3O10 follow the strong coupling, clean limit curve.

Disruption of antiferromagnetic order in Zn-doped La2CuO4

Abstract The magnetic phase diagram of La 2 (Cu 1− x Zn x )O 4 has been determined from zero-field muon-spin-rotation (ZF-μSR) data taken at LAMPF for 0 ≤ x ≤ 0.10. Antiferromagnetic onset temperatures follow T N ( x ) from susceptibility measurements on the same samples. However, the order becomes long range, as evidenced by a well-defined internal magnetic field, only at temperatures well below T N . Extrapolation of our results yields T N → 0 K at x = 0.11 for a single (Cu 1− x Zn x )O 2 plane, and comparison with YBa 2 (Cu 1− x Zn x ) 3 O 6 implies identical disruption of magnetism by Zn doping in the single- and double-plane systems.

Zero-field μSR in crystalline C60

The Co60Mu radical in polycrystalline C60 has been studied in zero magnetic field between 9 K and 200 K, and at room temperature. At low temperatures, we observe three low-frequency oscillations which correspond to the intra-triplet transitions of a completely anisotropic muon-electron hyperfine interaction. These signals exhibit a strongly temperature-dependent T1, attributed to thermally-activated jump rotational diffusion of the C60Mu radical. A fit to an Arhenius law yields an activation energy of 200(20) meV for temperatures below the fcc-sc structural phase transition. At room temperature only the motionally-narrowed 325 MHz singlet-triplet transition is observed.

Observation of the anisotropic spin-glass transition and transverse spin ordering in pseudo-brookite through muon spin relaxation

Zero-field longitudinal muon-spin-relaxation (μSR) experiments have been performed on single crystals of pseudo-brookite (Fe2−xTil+xO5; x=0.25), an anisotropic spin-glass system. The spinglass temperature (Tg) is determined to be 44.0±0.5K. Above Tg, a distinct exponential muon-spin-relaxation rate (λ) is observed, while below Tg a square-root exponential decay is seen, indicating fast spin fluctuations in the ‘frozen’ state. Near 8K, a maximum in λ is observed, which is due to transverse spin ordering at these low temperatures. Even near Tg, λ is very low (<1 μs−1), likely due to a well-defined muon-oxygen state in the single crystals. The sharp λ-increase (with decreasing temperatures) above Tg allows a comparison between spinfreezing models like the Vogel-Fulcher law and a power law. The results of these initial measurements indicate that dynamic (and static) magnetism in oxide spin glasses can be directly monitored through μSR.

Magnetic ordering in (Y1−xPrx)Ba2Cu3O7 as evidenced by muon spin relaxation

Using the zero‐field‐muon‐spin‐relaxation (μSR) technique clear evidence has been found for antiferromagnetic ordering of Cu moments within the CuO planes of (Y1−xPrx)Ba2Cu3O7. The Neel temperatures are approximately 285, 220, 35, 30, and 20 K for x=1, 0.8, 0.6, 0.58, and 0.54, respectively. For x=0.50 we observe a fast‐relaxing component of the muon polarization in addition to a long‐time tail, reminiscent of spin‐glass behavior. This region of the phase diagram (0.5≤x≤0.54) corresponds to the existence of both superconductivity and magnetism. The fully developed local magnetic field for x>0.54 is found to be ∼16 mT, but decreases to ∼12 mT at T=17 K for the x=1 sample, presumably due to the onset of Pr‐ion ordering. Magnetic ordering also occurs in PrBa2Cu3O6; the Neel temperature is ∼325 K.

MUONIUM IN SEMICONDUCTORS AND SOME IMPLICATIONS FOR HYDROGEN IMPURITIES

A description of the structures, charge states, and the transitions among them for isolated interstitial muonium in covalent semiconductors is presented. This description appears to be general, explaining a wealth of experimental observations on group IV and III–V semiconductors. Observations which support various features of this general description will be presented. A brief mention of some implications for hydrogen in semiconductors will follow.

Magnetism and candidate muon-probe sites in RBa2Cu3O y

Key results of zero-field (ZF) and transverse-field (TF) muon-spin-relaxation (μSR) experiments on superconducting and insulating RBa2Cu3Oy (R123y, with R=Eu, Gd, Pr and Pr/Y:y=6, 7) are examined. The chemical behavior of the positive muon probe is addressed, and muon-oxygen bonding is shown to occur in all these cuprates. To explain magnetic fields at muon-probe sites in PrxY1−xBa2Cu3Oy (0<=x<0.5,y=7 andx=0,y=6) samples, improvements on the reported magnetic structures from neutron diffraction are necessary. Cu magnetism in Pr123y (y=6,7) is observed belowTN1, which is near RT. The magnetism seen belowTN2 can be interpreted assuming an additional ordering in the Cutt-O chain layers. Alternatively, Pr ordering is also considered as the cause of the second phase transition. Considering the specific muon-probe location, a more detailed interpretation can be provided for the μSR parameters, measured in the normal and mixed states of these unconventional superconductors.