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.     

Muonium addition to vinylaromatics and to diacetylenes: formation and stability of arylethyl/vinylcyclohexadienyl and alkynylvinyl radicals

According to TF‐μSR measurements and PM3 M.O.calculations, free radicals formed by muonium addition to styrene, 1‐vinylnaphthalene and 2‐vinylnaphthalene are of the arylethyl type ( ArCHCH₂Mu\mbox$\bullet$) since it is energetically considerably more favourable for Mu\mbox$\bullet$ to add to the vinyl group than to the aromatic rings. In contrast, Mu\mbox$\bullet$ adds to 9‐vinylanthracene at the 10‐position yielding the 4‐vinyldibenzocyclohexadienyl radical, which is more stable by ca 7 kcal/mol than the 9‐anthrylethyl radical. For diacetylenes (i.e. MeC₄Me) the propensity for muonium addition is to the terminal carbon atoms, but the choice is less clear for Me₃SiC₄SiMe₃. For PhC₄Ph, 3 radicals are detected and are assigned to the C1, C2 and ortho ring adducts, with the couplings (and yields): 334 (5.7%), 389 (4.2%), 439 (3.6%). This revised version was published online in August 2006 with corrections to the Cover Date.     

Muonium desorption to vacuum from hot iridium surface

Slow muonium (Mu) emission from the surface of iridium (Ir) foil has been observed in vacuum above \sim1200\ K with a yield of 5(1)% Mu per incident muon stopped in the foil. The relative Mu signal was found to be thermally activated with E_a=1.86(1)\ eV. Analysis of the time‐differential evolution of Mu in vacuum at T_Ir=1580\ K showed that the trapping rate of positive muons during diffusion in the bulk Ir was nearly zero. In situ measurements of the surface of Ir with X‐ray photoelectron spectroscopy verified that the main source of impurities in the 99.9%‐Ir was molybdenum (Mo). This revised version was published online in August 2006 with corrections to the Cover Date.     

Texture effect on vortex‐state TF‐μ+SR in Bi‐2223 high‐TC materials

The influence of texture in Bi‐2223 ingots on the temperature dependence of the μ⁺ spin relaxation rate has been investigated. Texture was induced during cold‐isostatic‐pressing by applying an additional uniaxial load which tends to align single grain c axes along the stress direction, \widehatz. Small plates were cut with faces either \Vert or \perp \widehatz and μSRwas measured in a 15 mT transverse field (TF) during cooling from room temperature to 6 K. While for highly‐textured material, low‐temperature limits are substantially smaller than expected, the large shift (55%) observed when switching from \widehatz\perp TF to \widehatz\,\Vert TF clearly indicates that anisotropy has developed. Relaxation rates for the weakly‐textured sample lie in between. To a lesser extent, sintering also enhanced the low‐temperature relaxation, but did not affect T_C significantly. All samples showed a weak onset of depolarization between 60 K and 90 K, most likely due to the presence of a Bi‐2212 impurity phase. This impurity phase might cause the transition‐temperature smearing in the 100–110 K range in the weakly textured (and less compacted) material. Thus the sharpness of the relaxation drop might be relevant for assessment of material quality and be used as a criterion in the improvement of the production process. This revised version was published online in August 2006 with corrections to the Cover Date.     

Muon implantation in alkali metal hydrides

Zero, longitudinal and transverse field μSR measurements have been made on LiH, LiD and NaH. The primary motivation for the study was to elucidate the behaviour of the muons in the diamagnetic state and analysis of the time‐dependent zero field relaxation data suggests that negatively charged muonium, Mu^-, is formed and takes up a H^- vacancy site in these materials. Evidence is presented for a small (approximately 2%) reduction in the Mu^-–Li distance relative to the unperturbed nearest neighbour anion‐cation distances in the pure crystal lattices. This revised version was published online in August 2006 with corrections to the Cover Date.     

Muon behaviour in diamond grown by chemical vapour deposition

Transverse‐field μSR spectroscopy was used to study the behaviour of positive muons implanted in polycrystalline chemical‐vapour‐deposited (CVD) diamond. Measurements were made at sample temperatures of 10 K, 100 K, and 300 K at a magnetic field of 7.5 mT to study the behaviour of the “normal” (isotropic) muonium state (Mu_T) and the diamagnetic states (μ^d), and at 10 K and 300 K at the so‐called “magic field” of 407.25 mT to study the anomalous (bond‐centred) muonium state (Mu_BC) and μ^d. The absolute fractions of the muonium states in the CVD diamond are observed to be close to those in high‐quality natural type‐IIa single crystal diamond. This revised version was published online in August 2006 with corrections to the Cover Date.     

Diamagnetic muonium states in InP

Transverse and zero‐field muon spin relaxation reveal several diamagnetic muonium states in InP characterized by their static linewidths and diffusion properties. We tentatively associate low‐temperature diamagnetic states with Mu⁺ in the BC and T_P interstitial sites and a missing fraction with Mu⁰ rapidly diffusing through T_In interstices. Trapping peaks above 250 K imply static centers which depend on doping type, consistent with Mu^- at T_In for n‐type samples and Mu coupled with a dopant or other defect for p‐type. This revised version was published online in August 2006 with corrections to the Cover Date.     

Recent progress in NMR studies on organic conductors

Recent ¹³\mathrmC\mbox-NMR studies on a family of quasi‐two‐dimensional organic conductors based on BEDT‐TTF molecules are reviewed in the light of the role of electron–electron correlation in a variety of electronic states and of the symmetry of electron pairing in the superconducting state. Comparison of the nuclear spin‐lattice relaxation rate, conducting property and molecular arrangement indicates a close relationship between the molecular arrangement and manifestation of electron correlation. The metal/nonmetal phases in the isostructural \kappa\mbox- (BEDT\mbox-TTF)₂X systems, which are in the strong dimeric regime, are understood as highly correlated metals and insulators crossing the Mott transition. For the 10‐K superconducting phase situated near the Mott transition, the absence of Hebel–Slichter coherence peak and a low‐temperature T³-dependence of the nuclear spin‐lattice relaxation rate suggest unconventional superconductivity with line nodes in gap parameter or highly anisotropic one. This revised version was published online in July 2006 with corrections to the Cover Date.     

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.     

Muonium centers in heavily n‐doped Si under illumination: Evidence for Mu–hole interaction

A small fraction of implanted muons exists as a paramagnetic state (presumably Mu_BC ⁰, muonium at the Si—Si bond center) in heavily Sb‐doped Si (n-type, [Sb]\ \simeq 10¹⁸\ cm^–3). The paramagnetic state is susceptible to illumination both at 10–20 K and 290 K, providing evidence that holes (minority carriers) play an important role in determining the dynamical properties of muonium centers, where change may occur via a process Mu_BC ⁰+ h⁺\to Mu_BC ⁺ followed by charge exchange reaction (or transition Mu⁺ _BC+ e⁻→ Mu⁰ _T). This revised version was published online in August 2006 with corrections to the Cover Date.     

Electronic structure and dynamics of muonium in semiconductors

In this paper, a selection of recent results on muonium in semiconductors is presented. These are primarily taken from Si and GaAs and encompass the electronic structure of the diamagnetic centers, charge state cycling, spin‐exchange scattering and interconversion between muonium states. These experiments illustrate the power of μSR for investigating the behavior of muonium and, by analogy, the technologically relevant isolated hydrogen centers in semiconductors. This revised version was published online in August 2006 with corrections to the Cover Date.     

Muonium diffusion in “dirty” solid krypton

Muonium spin dynamics in solid krypton in the presence of deliberately introduced impurities has been studied in the temperature range from 2 K to 115 K. At temperatures up to 40 K, muonium transverse relaxation rate turns out to be virtually unaffected by impurities. This implies that the Mu energy level shifts in the neighboring wells induced by impurities are smaller than the characteristic Mu activation energy in the one‐phonon regime of quantum diffusion. This revised version was published online in August 2006 with corrections to the Cover Date.     

Anomalous field dependence of muon Knight shift and line width in the singlet ground state compound PrCu2

We report on ZF and TF‐studies of PrCu₂ above the induced Jahn–Teller transition at 7.3 K. Generally a two‐component signal is found, one showing inhomogeneous the other one homogeneous, temperature dependent line broadening. In ZF the former component is well represented by a Gaussian Kubo–Toyabe function with \varDelta \simeq 6.5\,μs^-1 at 7.5 K, corresponding to a field width of 76 G. This is about 30 times larger than what is calculated to arise from the ¹⁴¹Pr‐nuclear dipole moments alone, pointing to strong hyperfine enhanced features. TF‐field scans at 12 K revealed that the enhancement is suppressed in external fields exceeding 1 kG. In parallel the Knight shift drops from very large values well above 10% at 100 G to shifts of the order of 1% above 1 kG. A scaling of the Knight shifts with the corresponding relaxation rates seems to imply that the strange field dependence below 1 kG is associated with the magnetic susceptibility of the muons’s nearest neighbour Pr³⁺‐ions, a result for which we have no explanation yet to offer. This revised version was published online in July 2006 with corrections to the Cover Date.     

Electric field gradients probed by μ+SR in Sc and \alpha\mbox-ScH_x solid solutions

In Sc and \alpha\mbox-ScH_x below 40 K two μ⁺ states are formed: a tunnelling state extending over two adjacent tetrahedral (T) interstices sharing one face in the ab‐plane of the hcp host lattice and a static μ⁺–Sc–H configuration analog to the T‐pair state formed by two H atoms across a Sc site along the crystalline c‐axis. This explains perfectly the high transverse‐field (TF) μSR measurements at low temperature and allows a qualitative understanding of the temperature dependence. The modelling of the low TF measurements requires in addition the consideration of the electric field gradient (EFG) components acting on the Sc nuclei near the μ⁺, i.e., of the radial EFG due to the μ⁺ and of the axially symmetric crystal field. The magnetic field dependence of the anisotropic μ⁺SR relaxation rate is strongly influenced by the H‐concentration in the solid solution system. This revised version was published online in August 2006 with corrections to the Cover Date.     

Muonium adducts of benzaldehyde: Structural correlation with nitroxides

We have formed adduct radicals (ArCHOMu\bullet) by muonium addition to the carbonyl group of benzaldehyde and its derivatives. The muon coupling is found to be highly sensitive to the nature of substituents in the benzene ring, being increased by electron releasing groups. Similarly, the ¹⁴N coupling in a series of nitroxides [ArN(O\bullet)OSiEt₃], measured using ESR spectroscopy, is increased by electron releasing substituents and a linear correlation is found between the two; their connected nature is discussed. For the radical derived from benzaldehyde itself, a strong dependence is also found on the solvent in which it is solved. We suggest that this may be used in the study of solvent effects, particularly H‐bonding, pertinent to organic and bioorganic media. This revised version was published online in August 2006 with corrections to the Cover Date.     

A μ+SR study of the magnetic properties of ferritin

We present a study of the magnetic properties of the superparamagnetic ferritin system by employing zero‐field (ZF) and longitudinal‐field (LF) μ⁺SR measurements. Two μ⁺ fractions with different depolarisation behaviour are detected, one arising from muons stopped in the organic shell and one from muons stopped in the mineral core. The freezing of the superparamagnetic moments is evident in the temperature evolution of the ZF relaxation rates of both fractions. It occurs gradually below \approx 60\ K and is essentially complete at \approx 11\ K. The results are consistent with a distribution of blocking temperatures which in turn reflects the distribution of core sizes. This revised version was published online in July 2006 with corrections to the Cover Date.     

Local magnetic field and muon site in CeAs

We report on zero field and longitudinal field μSR experiments on a CeAs single crystal between 3.3 and 12 K. Below the antiferromagnetic transition at 7.5 K a spontaneously precessing signal with a saturation frequency of \approx25 MHz representing the full sample amplitude has been found. From an analysis of the field dependence of the relaxation rate of this signal in \langle 100\rangle and \langle 110\rangle crystal orientation parallel to the muon spin and the applied longitudinal field, a \langle 100\rangle orientation of the local field at the muon site is concluded. This supports an AF‐I single‐\veck magnetic ordering. This revised version was published online in July 2006 with corrections to the Cover Date.     

Anomalous frequency shift of negative muon spin precession in n‐type silicon

The dependence of the residual polarization of negative muons in n‐type Si with impurity concentration (1.6\pm 0.2)\times 10¹³\ cm^-3 on temperature in the 10–300 K range has been investigated. Measurements were carried out in external magnetic field of 0.08 T transverse to the muon spin. Muon spin relaxation and frequency shift were observed at temperatures below 30 K. The relaxation rate at 30 K is equal to 0.25\pm 0.08\,μ s^-1. The frequency shift at 20 K is equal to 7\times 10^-3. Both the relaxation rate and the frequency shift grow with decrease of temperature. Below 30 K the relaxation rate is well described by the dependence \varLambda=bT^-q, where q=2.8. An analysis of present and earlier published data on behavior of negative muon polarization in silicon is given. A possible mechanism of relaxation and frequency shift of muon spin precession in silicon is considered. This revised version was published online in August 2006 with corrections to the Cover Date.     

Influence of doping admixture and anisotropic tension on behaviour of muon spin polarization in insulators

Zero‐field μSR measurements have been carried out in the paramagnetic and magnetically ordered states of the heavy fermion alloys: Ce_1-xLa_xRu₂Si₂ (x=0.05,0.13,0.25). In Ce_1-xLa_xRu₂Si₂ system, T_K drastically decreases with x from 24 K for x=0 and becomes very close to the magnetic ordering temperature T_N. As functions of T_K and T_N, the magnetic instabilities were studied in this system. For x=0.05,\ 0.13\ \mboxand\ 0.25 samples, longitudinal muon spin relaxation was measured to study non‐magnetic to magnetic transition and the change from itinerant to localized electron state with increasing La impurities. We discuss the dynamics of muon spins in the paramagnetic and antiferromagnetic state. This revised version was published online in July 2006 with corrections to the Cover Date.     

The low temperature hyperfine structure of quartz under uniaxial pressure

The uniaxial pressure dependence of the muonium hyperfine tensor was measured at 80 K. Uniaxial pressures (0 and 0.2 GPa) were applied along the a‐axis of quartz, and the hyperfine tensor was measured by applying various transverse fields between 0 and 7 G. The structure appears to be the result of the three chemically equivalent sites in quartz. When pressure is applied perpendicular to the crystallographic c‐axis of quartz, the symmetry of these sites is broken such that two chemically inequivalent sites are observed. The resulting hyperfine structure is discussed and qualitative explanations are proposed. This revised version was published online in August 2006 with corrections to the Cover Date.