We confirm the recent prediction that interstitial protium may act as a shallow donor in zinc oxide, by direct spectroscopic observation of its muonium counterpart. On implantation into ZnO, positive muons--chemically analogous to protons in this context--form paramagnetic centers below about 40 K. The muon-electron contact hyperfine interaction, as well as the temperature and activation energy for ionization, imply a shallow level. Similar results for the cadmium chalcogenides suggest that such shallow donor states are generic to the II-VI compounds. The donor level depths should serve as a guide for the electrical activity of interstitial hydrogen.

Using neutron diffraction, 170 Yb Mössbauer and muon spin relaxation spectroscopies, we have examined the pyrochlore Yb 2 Ti 2 O 7 , where the Yb 31 S 0 1͞2 ground state has planar anisotropy. Below ϳ0.24 K, the temperature of the known specific-heat l transition, there is no long range magnetic order. We show that the transition corresponds to a first-order change in the fluctuation rate of the Yb 31 spins. Above the transition temperature, the rate, in the GHz range, follows a thermal excitation law, whereas below, the rate, in the MHz range, is temperature independent, indicative of a quantum fluctuation regime. DOI: 10.1103/PhysRevLett.88.077204 PACS numbers: 75.40. -s, 75.25. +z, 76.75. +i, 76.80. +y Geometrically derived magnetic frustration arises when the spatial arrangement of the spins is such that it prevents the simultaneous minimization of all interaction energies [1 -4]. In the crystallographically ordered pyrochlore structure compounds R 2 Ti 2 O 7 , the rare earth ions ͑R͒ form a sublattice of corner sharing tetrahedra and a number of these compounds have been observed to exhibit frustration related behavior [5][6][7][8][9][10][11][12]. The low temperature magnetic behavior associated with a particular rare earth depends on the properties of the crystal field ground state and on the origin (exchange/dipole), size, and sign of the interionic interactions. For example, the recently identified spin-ice configuration [6,10,11] has been linked with an Ising-like anisotropy and a net ferromagnetic interaction. Most of the published work on the pyrochlores have concerned rare earth ions with Ising-like characteristics [6 -12] and there has also been some interest in the properties of weakly anisotropic Gd 31 [5]. Less attention has been paid to the case, considered here, where the ion has planar anisotropy.To date, in systems where geometrical frustration may be present, two different low temperature magnetic ground states have been considered. First, under the influence of the frustration the system does not experience a magnetic phase transition and remains in a collective paramagnetic state with the spin fluctuations persisting as T ! 0 [3,4,[6][7][8][9][10][11]13]. Second, a long range ordered state is reached through a phase transition which may be first order [5,14,15]. Our results for Yb 2 Ti 2 O 7 , obtained using neutron diffraction, 170 Yb Mössbauer spectroscopy, and muon spin relaxation (mSR), evidence a novel scenario: there is a first-order transition which does not correspond to a transition from a paramagnetic state to a (long or short range) magnetically ordered state. The transition chiefly concerns the time domain, and involves an abrupt slowing down of the dynamics of short range correlated spins; below the transition temperature, these spins continue to fluctuate at a temperature independent rate.We have established the background magnetic characteristics for Yb 2 Ti 2 O 7 in a separate study [16,17]. The Yb 31 ion crystal field ground state is a very well isolated Kramers doubl...

From muon spin relaxation spectroscopy experiments, we show that the sharp peak (lambda-type anomaly) detected by specific heat measurements at 54 mK for the ytterbium gallium garnet compound, Yb3Ga5O12, does not correspond to the onset of a magnetic phase transition, but to a pronounced building up of dynamical magnetic pair correlations. Beside the lambda anomaly, a broad hump is observed at higher temperature in the specific heat of this garnet and other geometrically frustrated compounds. Comparing with other frustrated magnetic systems we infer that a ground state with long-range order is reached only when at least 1/4-1/3 of the magnetic entropy is released at the lambda transition.

We report a positive muon spin relaxation and rotation (µSR) study of the quantum spin ice materials Yb2Ti2O7 and Yb2Sn2O7 focusing on the low field response. In agreement with earlier reports, data recorded in small longitudinal fields evidence anomalously slow spin dynamics in the microsecond range below the temperature Tc at which the specific heat displays an intense peak, namely Tc = 0.24 K and 0.15 K, respectively, for the two systems. We found that slow dynamics extends above Tc up to at least 0.7 K for both compounds. The conventional dynamical Gaussian Kubo-Toyabe model describes the µSR spectra recorded above Tc. At lower temperatures a published analytical extension of the Gaussian Kubo-Toyabe model provides a good description, consistent with the existence of short-range magnetic correlations. While the physical response of the two systems is qualitatively the same, Yb2Ti2O7 exhibits a much larger local magnetic susceptibility than Yb2Sn2O7 below Tc. Considering previously reported ac susceptibility, neutron scattering and µSR results, we suggest the existence of anomalously slow spin dynamics to be a common physical property of pyrochlore magnetic materials. The possibility of molecular spin substructures to be associated to the slow dynamics and therefore the short-range correlations is mentioned. The slow spin dynamics observed under field does not exclude the presence of much faster dynamics detected in extremely low or zero field.

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.