Measurement of17O quadrupole interactions and identification of the diamagnetic fraction in ice by muon level crossing resonance

By means of Level Crossing Resonance in a sample of ice which is enriched in H₂ ¹⁷O, the final diamagnetic state of implanted positive muons is determined to be the muonium-substituted molecule HMuO, accommodated in the regular and fully relaxed Ih structure. The¹⁷O quadrupole coupling constant is measured to be 6.1 MHz at 200 K assuming an asymmetry parameter close to unity, a decrease of about 5% relative to that in normal ice Ih at 77 K. The isotope effect is attributed to a greater polarization in the vicinity of a muonium (as opposed to a normal hydrogen) bond. At 50 K, an additional resonance is observed which could correspond to a precursor state, so far not definitely identified. One possibility is a muon trapped at a Bjerrum L-defect, giving a {H₂O−Mu−OH₂}⁺ species with an,¹⁷O quadrupole coupling constant of 8.2 MHz and asymmetry parameter of 0.55. Above this temperature, the fall in the (Gaussian) line-width parameter is attributed to the increasing rate of proton or muon migration, the correlation time dropping from 4 μs at 80 K to 1 μs near the melting-point. The increase in the diamagnetic fraction with rise in temperature is attributed to the increasing proportion of trapping sites available for muon capture.     

Observation of “decoupled” diamagnetic muon states in Alkali halides by muon spin resonance

The states of positive muons in KCl, NaCl and KI were studied with the muon spin resonance method under a 3 kG decoupling longitudinal field, revealing a considerably larger fraction of diamagnetic muon state than observed by the conventional spin rotation method. The origin of this fraction, which increases with temperature, is attributed to a muonium to muon transition in solids.     

A study of diamagnetic muon bonding in α-quartz using muon spin resonance

Diamagnetic muon spin resonance experiments were carried out in a-quartz at two temperatures: 300 K and 130 K. A strong diamagnetic resonance peak was observed at both temperatures. The spectrum exhibits an anisotropy in the chemical shift of 15 ppm (300K) and 25 ppm (130K).     

Muon level crossing resonance in diamagnetic systems-general considerations

This paper is a general discussion of the nature of the level crossing phenomena in diamagnetic systems, with particular emphasis on the quadrupolar LCR as evidenced in Cu. The system exhibits some surprises at higher temperatures which indicate that the level crossing technique contains hitherto unsuspected capabilities for μSR.     

Muonium to diamagnetic muon conversion in KCl and NaCl revealed by time-differential muon spin resonance

The conversion of muonium into a diamagnetic muon state in KCl and NaCl was directly observed by detecting a time-delayed appearance of diamagnetic muon states with muon spin resonance. The conversion rate, determined from the time-differential resonance signals, shows a thermal activation characteristic in temperature variation.     

Level crossing resonance in muonium-like systems

This paper describes the phenomenon of level crossing resonance (or avoided level crossing) encountered in paramagnetic systems involving a muon. Recent experiments on muonated radicals and muonium defect centers in semiconductors are reviewed which demonstrate how the technique provides detailed information on nuclear hyperfine structure, which is unresolvable with more standard techniques. The science implications are discussed.     

Level crossing mechanism in new coherence resonance

The mechanism of level crossing in the new coherence resonance for the case that a spin-$\text{1}\text{2}$ atom interacts with an oscillating rf field in optical pumping is theoretically analyzed considering the higher-order effects of a strong rf field.     

Electron scavenger effects on diamagnetic muon polarization in cyclohexane

The effect of electron scavengers on diamagnetic polarization P_D in cyclohexane were examined and compared with the experimental yield of Ps formation (inhibition and anti-inhibition effects) for the same solutions. The effect of C₆F₆ on P_D has been shown to sharply contrast, as we call non anti-inhibition, to the anti-inhibition effect in Ps formation. These results suggest that Muonium formation is different from Ps formation, and does not agree with the simple spur reaction model.     

Level crossing resonance detection of a muonic radical in sulphur

Detection of a level crossing resonance establishes that a molecular radical is created in elemental sulphur following the implantation of positive muons. Formally this is the product of the reaction of atomic muonium with the S₈ rings and must serve as a model for the fate of interstitial atomic hydrogen in this element. The isotropic component of the muon-electron hyperfine interaction is determined to be 233 ± 5 MHz. Preliminary data for the temperature dependence of the resonance linewidth indicates a substantial motional narrowing of the anisotropic components.     

Level crossing resonance due to chlorine nuclei in a free radical

Muonium adds to allyl chloride, CH₂=CHCH₂Cl, to form two radicals: MuCH₂CHCH₂Cl (main product) and CH₂CHMuCH₂Cl (minor product). Both radicals were fully characterized bySR andLCR. In the main product, the LCR lines due to the³⁵Cl and³⁷Cl nuclei were observed. Also, the temperature dependence of various hyperfine coupling constants (hfc) indicates that both Mu and Cl eclipse the unpaired electronp ₂-orbital in the minimum energy conformation. For the fragment-CH₂Cl, the presence of Mu in the-position is found to affect significantly the hfc of Cl in the-position; an internal rotational barrier of 12 kJ mol^–1 was estimated using a simpleV ₂ torsional potential.     

Competition between benzene and styrene in forming radicals under different solvent conditions observed by muon level crossing resonance

The muon level-crossing-resonance technique has been used to resolve major discrepancies that exist in muon-spin-resonance studies (both free-radical formation and muonium decay rates) in the competition between benzene and styrene. The results, obtained for 30 mM solutions in ethanol and for 2.5 mM aqueous micelles solutions, show that muonium atoms (Mu) react 8 (±2) times faster with styrene than with benzene. In the above cases thermalized Mu is unquestionably the reactive species, which is known to show nucleophilic intra-molecular selectivity in the case of styrene. But a similar value, 9 (±2), was also obtained for undiluted mixtures of liquid benzene and styrene (neat mixture) — where the precursor might have been hot Mu (which should display weaker selectivity than Mu) or cations derived from ⁺ (which should show higher selectivity). These results support the view that thermalized Mu is the predominant reactive species in liquid benzene and styrene.     

Squeezed states in resonance fluorescence of two interacting atoms

The possible existence of so-called “squeezed” states in two-atom resonance fluorescence is discussed in Lehmberg's master equation approach. It is shown that squeezing strongly depends on interatomic separations r₁₂. For large r₁₂ one of the quadrature components is squeezed, and as r₁₂ decreases its squeezing decreases in order to appear in the other quadrature component for certain value of r₁₂. For very small r₁₂ fluctuations in both components tend to zero.     

Muon level crossing resonance in niobium

A useful guide to detecting quadrupole level crossing resonance (QLCR) spectra is to search in the vicinity of B_E\cdot(\gamma_n/\gamma_μ), where B_E is the onset magnetic field for the decoupling of the quadrupole interaction, as seen in muon spin rotation linewidths. More detailed predictions of the positions and intensities of the resonances require numerical simulations taking account of the local geometry of the muon site. We present such simulations for muons adjacent to nuclei of spin 9/2 and demonstrate a pronounced dependence on the anisotropy of the quadrupole coupling tensor. Simulations for the specific cases of muons located at the octahedral and tetrahedral interstices in niobium metal are compared with the experimentally detected spectrum in a polycrystalline sample. This revised version was published online in August 2006 with corrections to the Cover Date.     

Spin ice: magnetic excitations without monopole signatures using muon spin rotation

Theory predicts the low temperature magnetic excitations in spin ices consist of deconfined magnetic charges, or monopoles. A recent transverse-field (TF) muon spin rotation (μSR) experiment [S.?T. Bramwell et al., Nature (London) 461, 956 (2009)] reports results claiming to be consistent with the temperature and magnetic field dependence anticipated for monopole nucleation-the so-called second Wien effect. We demonstrate via a new series of μSR experiments in Dy(2)Ti(2)O(7) that such an effect is not observable in a TF μSR experiment. Rather, as found in many highly frustrated magnetic materials, we observe spin fluctuations which become temperature independent at low temperatures, behavior which dominates over any possible signature of thermally nucleated monopole excitations.     

Muon level crossing resonance in aluminium

Cross relaxation between implanted positive muons and²⁷Al nuclei in Al metal, lightly doped with Cu to impede the muon diffusion, shows weak resonances at 2.3 and 4.0 mT. Assignment of these to polarization transfervia the 1/2 3/2 and 3/2 5/2 transitions of the (J=5/2) spins leads to a quadrupole coupling constante ²qQ/h=1.8(1) MHz,i.e. an electric field gradient on Al nuclei immediately adjacent to the muon siteq=0.048(3) a.u., with a small departure from axial symmetry.     

Band crossing and surface states in solids

A symmetry criterion for the existence of surface states in model calculations in solids is derived. The criterion applies to surfaces that coincide with symmetry planes of the ideal crystal. A detailed analysis is carried out for one-dimensional model calculations and it is shown how band crossing can be interpreted in the framework of the crystal symmetry.