Evidence for muonium emission from an SiO2 layer on n-type Si and the positive muon state in Si

Evidence for the emission of slow muonium atoms from a 3.0-nm-thick SiO₂ layer covered on an n-type Si is reported. Also, upon applying an rf-resonance technique at the muon frequency, a time-differential observation of a delayed state-change from muonium to diamagnetic muon at room temperature was observed. Combining results obtained by use of longitudinal field decoupling and transverse spin rotation methods, the conversion rate was estimated to be 5 to 10 μs⁻¹. Both of the above results, namely the observation of the emission and state-change of muonium, suggest a process in which μ⁺ initially captures an electron from Si, then quickly converts to μ⁺ again during thermal diffusion in the Si towards the SiO₂ layer. Within the oxide layer, muonium is again formed and subsequently is emitted from the SiO₂ surface.     

MuSR/μ+SR studies in KDP and DKDP

The temperature dependence of muon interactions has been studied in ferroelectric KDP ( H₂KPO₄) and DKDP ( D₂KPO₄) using conventional μSR and muon spin resonance spectroscopy. In longitudinal field measurements, a fast relaxing component and a slow relaxing component were observed. The slow relaxing component is attributed to diamagnetic muons. The muon spin resonance measurements indicate that the fast relaxing component results from some muonium like species: either normal or anomalous. In zero field and weak longitudinal field μSR (0–100 G), a remarkable peak in the fast relaxing component is observed around 220 K in both KDP and DKDP. An additional feature is also seen around 300 K. The amplitude of the resonance measurement has a broad minimum around 200 K which corresponds to the maximum in the relaxation rate in longitudinal field (100 G). The temperature dependence of the muonium relaxation rate in KDP is almost identical to that of DKDP. The diamagnetic fraction also shows almost no difference in relaxation rate or asymmetry for DKDP and KDP. This revised version was published online in August 2006 with corrections to the Cover Date.     

Study of condensed nitrogen by μSR method

The temperature dependences of parameters of the muon spin relaxation in liquid and crystalline nitrogen have been studied. It has been established that in condensed nitrogen there takes place a fast depolarization of muons. An anomalous behaviour of the amplitude and phase of muon precession is found in the vicinity of the orientation phase transition in solid nitrogen. It has been shown that muon spin relaxation parameters in nitrogen do not change at reduction of the oxygen impurity content from 0.7·10⁻⁴ to 10⁻⁶. The fast depolarization of muons in condensed nitrogen is apparently due to the formation of muonium atoms. To explain the phenomena observed, a model of the muonium chemical reaction is proposed. The initial phase of the muon precession has been measured as a function of the perpendicular magnetic field to determine the state of short-lived muonium in nitrogen. It has been determined that muonium in nitrogen is in an excited state. Consideration of the nuclear hyperfine interaction of muonium in condensed nitrogen makes it possible to give a qualitative explanation for the temperature dependence of the initial amplitude of the muon precession.     

Theoretical Investigation of Muon and Muonium Trapping and Hyperfine Interactions in the Chemical Ferromagnet p-NPNN (β-phase)

The trapping sites for muon and muonium in β-phase ferromagnetic p-NPNN have been determined by the first-principles Unrestricted Hartree–Fock procedure. Four trapping sites are found for the muon near the two nitrogen and two oxygen atoms of the two NO groups. For the singlet state of trapped muonium, two trapping sites are found near the two oxygens of two NO groups and for the triplet state two trapping sites are found near the two oxygens of the NO₂ group. The observed μSR signal at zero field with frequency 2.1 MHz is assigned to the singlet muonium sites near the two oxygens of the two NO groups and the high frequency signal ascribed to an isotropic hyperfine constant of 400 MHz is assigned to the two trapped muon sites near the two nitrogen atoms of the two NO groups. This revised version was published online in September 2006 with corrections to the Cover Date.     

Muon bonding versus muonium formation: Muon-Spin-Relaxation in α-Al2O3

Results of Muon-Spin-Relaxation (μSR) experiments on well-defined single crystals of corundum (α-Al₂O₃) are reported. Major issue in this study is the controversy of muon bonding or muonium formation in insulators. Transverse, zero-field and longitudinal field measurements were performed as a function of temperature or applied field. The obtained results indicate that muon-oxygen bonding occurs in α-Al₂O₃, having (at least) two (μO) states as found earlier for α-Fe₂O₃. The occurrence of a 10% missing fraction suggests Mu states, although Mu was not observed in our measurements. These results and their implications are discussed in light of other experimental μSR studies on insulating oxides and the above-mentioned controversy.     

Rare muon decays and physics beyond the standard model

Rare muon processes such as muonium to antimuonium conversion, μ→eγ and μ→3e provide sensitive tests of new physics beyond the standard model. Specifically the left-right symmetric models of weak interaction with a low scale for the right-handedW _R boson (in the TeV range) provide a whole range of rare muon processes which are experimentally accessible. In this talk, I discuss the implications of the left-right symmetric models for rare muon processes and also briefly touch on the SUSY models with R-parity violation and their implications for these processes.     

Analysis of the μSR of carbontetrachloride: As a liquid in various transverse magnetic fields with added scavengers,and as crystalline solids

Carbontetrachloride has been widely used in liquid phase μ SR as a calibrant of the asymmetry in view of its supposed 100% diamagnetic muon yield. However, in the present letter, evidences are presented to show that this conventional use of CCl₄ might be wrong and that there exists thermalized muonium in it that partially leads to a lost polarization.     

Quantum diffusion of muonium in GaAs

The diffusion rate of muonium in the III–V compound semiconductor GaAs has been determined from measurements of muon spinT ₁ relaxation induced by motion in the presence of nuclear hyperfine interactions. It is shown for the first time in a semiconductor that (a) there is a crossover of the transport mechanism at about 90 K from stochastic to zero-phonon hopping, as evidenced by a steep rise in the hop rate at lower temperatures, and that (b) the muonium diffuses at the hop rate of 10¹⁰ s⁻¹ (corresponding diffusion constantD≈10⁻⁶ cm²s⁻¹) at lower temperatures as well as at room temperature.     

Studies of the positive muon behavior in solid H2 and D2

Using a pulsed muon beam, we have investigated the microscopic μ⁺ behavior in solid H₂ and D₂ down to 0.6 K by the μ⁺SR method. From the studies of μ⁺ spin relaxation phenomena in solid para‐ H₂ and ortho‐ D₂, we have found that μ⁺ forms three distinct microscopic states; H₂μ⁺( D₂μ⁺)(20\sim25\%), muonium (15\sim20\%) and μ⁺(\sim60\%). In H₂μ⁺, the μ⁺ spin is depolarized in solid para‐ H₂ and a local magnetic field B_loc=1\sim2 G is deduced from LF‐μ⁺SR measurements. The magnitude of B_loc is inconsistent with the magnetic dipolar field (\sim25 G) expected from the magnetic moments of two protons in the H₂μ⁺ molecule and suggests that the H₂μ⁺ molecule might be in the rotationally excited state. From LF‐μ⁺SR measurements, muonium and μ⁺ have been found to diffuse in solid o‐ D_2. The diffusion rate of muonium is two order of magnitude larger than that of μ⁺. This revised version was published online in August 2006 with corrections to the Cover Date.     

Charge exchange of muons in gases: Experimental implications from rate theory

The effects of the charge exchange process on muon spin dynamics have been investigated using a density operator formalism with special interest placed upon the diamagnetic muon and paramagnetic muonium signals observed after thermalization. In the charge exchange region the dynamics of the spin density operator is assumed to be determined by the muonium hyperfine interaction and by electron capture and loss processes for muons. Analytical expressions are obtained for the amplitudes and phases of the diamagnetic muon and paramagnetic muonium signals as a function of the duration of the charge exchange region,t _c, which is inversely proportional to the number density of the moderating gas. The theoretical signals exhibit three features which have, as yet, to be experimentally observed, namely: (i) that the amplitudes associated with the muonium Larmor frequency and with the hyperfine frequency are not, in general, equal, (ii) that all the amplitudes are, in general, damped oscillatory functions oft _c (temperature/pressure) and (iii) that phase jumps occur when an amplitude decreases to zero and then increases with falling pressure. Fits to the experimental argon data are discussed in light of the above points.     

Muonium

Muonium, the bound state of the two structureless leptons μ⁺ and e⁻, is an ideal system to test quantum electrodynamics and tiny contributions of other interactions. This paper reviews the latest state of the measurements and theories for the muonium hyper-fine structure in the ground state and the 1S–2S splitting in muonium. The results of these experiments are indispensable to interpret a more precise measurement of the anomalous magnetic moment of the muon presently underway. Furthermore, the search for muonium-antimuonium conversion is reported and discussed with respect to lepton number conservation.     

Muons and muonium in cryocrystals

We present a collection of measurements of the muon and muonium asymmetries and relaxation parameters in cryocrystals of N₂, CO, Xe,¹³⁶Xe and Ne as functions of temperature. Generally, the fractions of the two species can be attributed to a competition between the formation of muonium or a diamagnetic species, where processes involving transport of spur electrons are important.     

Interaction of muonium with oxygen on silica powder surfaces

Results of the first μSR studies using Merck FO Optipur silica powder, which contains paramagnetic impurities at the ppb level and has a surface area of 610±20 m²/g. are reported. Above 20 K, the transverse field muonium relaxation rate is roughly constant at 0.5 μs⁻¹. Upon the addition of oxygen at ppm levels, the relaxation rate increases linearly with O₂ concentration in the temperature range from 40–100 K yielding two-dimensional depolarization rate constants on the order of 10⁻⁴ cm² molecule⁻¹ s⁻¹. As the temperature is increased further, both oxygen and muonium desorb from the surface yielding a three-dimensional rate constants at 300 K of 3.1(3)×10–10⁻¹⁰ cm³ molecule⁻¹ s⁻¹, in agreement with the gas phase value. Longitudinal field measurements suggest that MuO₂ is formed and is able to spin exchange with other oxygen molecules.     

Radiative recoil corrections to hyperfine splitting: Polarization insertions in the electron factor

We consider three-loop radiative recoil corrections to hyperfine splitting in muonium due to insertions of the one-loop polarization operator in the electron factor. The contribution generated by electron polarization insertions is a cubic polynomial in the large logarithm of the electron—muon mass ratio. The leading logarithm cubed and logarithm squared terms are well known for some time. We calculate all single-logarithmic and nonlogarithmic radiative recoil corrections of the order α³ (m/M)E_F generated by diagrams with the electron and muon polarization insertions.     

Localization of muonium at empty micelles: As evidenced by decreased spin relaxation rates

Muonium apparently localizes preferentially in the hydrocarbon phase of micelles in water. The evidence for this appears in the form of a decrease in the muonium relaxation rate towards a solute (nitrate ions) present in the bulk aqueous phase, when micelles are added. A two-component relaxation function for the muonium signal was used in the ‘fitting’, and showed that the residence time was short compared to the muon or muonium lifetime.     

Muon Physics Possibilities at a Muon-Neutrino Factory

New intense proton accelerators with above GeV energies and MW beam power, such as they are discussed in connection with neutrino factories, appear to be excellently suited for feeding bright muon sources for low-energy muon science. Muon rates with several orders of magnitude increased flux compared to present facilities will become available. This will allow higher precision in experiments which were statistics limited so far such as searches for rare decays, muonium spectroscopy, muon capture, muon catalyzed fusion, muon decay studies and measurements muon moments and parameters. Novel and most important experiments will become possible. For example a permanent electric dipole moment (edm_μ) of a muon could be searched with by far unprecedented accuracy and with a physics potential well beyond the possibilities of present electron, neutron and nuclear edm searches. Investigations of short lived radioactive nuclei using muonic atom spectroscopy would become feasible. This revised version was published online in August 2006 with corrections to the Cover Date.     

Comparative Theoretical Study of Hyperfine Interactions of Muonium in A- and B-Form DNA

Muon Spin Relaxation (μSR) experiments in A- and B-form DNA have shown evidence for an enhanced electron mobility in the more closely-packed A-form. Besides dynamic effects (electronic diffusion) that could cause the observed difference in muon spin relaxation, one should also carefully examine the difference in the strengths of the hyperfine interactions of the muon (μ ⁺) with the moving electron in the two forms of DNA, since this could contribute to the observed difference in the muon spin relaxation rates as well. We have therefore investigated the (static) trapping properties of muon and muonium (μ ⁺ e ⁻) in A-form and B-form DNA from first-principles with the aim to understand how the different structural geometries of A- and B-form DNA can influence the hyperfine interaction of trapped muonium.     

On the change of polarization of positive muons in alkali halides

Positive muons implanted in nonconducting solids form with high probability hydrogenlike muonium atoms (µ ⁺ e ^–) with properties similar to those ofU ₂-centers. The influence of superhyperfine interactions with neighbor nuclei on the evolution of the polarization of the muon is investigated theoretically. The resulting muon polarization in longitudinal magnetic fields is calculated for muonicU ₂-centers in some alkali halides.     

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.     

Hydrogen and muonium in silicon

First principles calculations of the properties of hydrogen and muonium in silicon are presented. H⁺ and H⁻ are shown to have definite preferences for bond-centred and tetrahedral interstitial sites respectively whereas H⁰ (or a muon) is shown to be stable at two sites with almost equal energies, the bond-centred and antibonding sites. The structures of normal and isotropic muonium are discussed. In contrast to common belief the tetrahedral site is shown to be unstable with the muon moving spontaneously towards one of the neighbouring silicon atoms. The barrier to motion between equivalent antibonding sites is low suggesting that the normal muonium signal is isotropic because of motional averaging, not due to the symmetry of a well defined equilibrium site.