Diffusion, trapping, and relaxation of Mu+ and Mu- in heavily‐doped GaAs

The diamagnetic muonium states in heavily doped GaAs are investigated with a combination of transverse‐field and longitudinal‐field μSR techniques. In metallic n‐type GaAs, formation of Mu^- occurs because of the high Fermi energy. This analog of the hydride ion (H^-) is located in a T_Ga interstice where it is essentially immobile up to about 500 K. At higher temperatures, Mu_T acts as an electron–hole recombination center. In p‐type GaAs, Mu⁺ traps at two different sites, one at low temperatures and a second at higher temperatures after detrapping from the first. 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.     

Mu centers in crystalline Ge under illumination

A slow conversion to a diamagnetic state has been observed for muonium centers at the tetrahedral interstitial site (Mu⁰ _T) in dark Ge at low temperatures. While the conversion process is affected by illumination, no effect of illumination upon the initial (Mu⁰ _T) centers themselves was observed at 10 K. This is in marked contrast with the case of (Mu⁰ _T) centers in Si where strong interaction with photo‐induced carriers is observed, suggesting that the electronic level associated with (Mu⁰ _T) state in Ge is not located in the energy gap. This revised version was published online in August 2006 with corrections to the Cover Date.     

Muon/Muonium in an Isotopically Pure 13C Diamond

A preliminary study of the diamagnetic (μ_d) and the paramagnetic (Mu _T ) states in a synthetic ¹³C diamond has been performed using the Transverse Field Muon Spin Rotation method. This system could be used to verify the quantum diffusion behaviour observed before, however, with a more reliable extraction of the hopping rate. The results were obtained in an applied magnetic field of 7.5 mT and at sample temperatures of 10 K, 100 K and 200 K. The prompt fraction, f, of the μ_d state remains constant at 22(5)% in the range 10–200 K; that of the Mu _T state increases from 53(10)% at 10 K to 78(10)% at 200 K. The fractions of the two states add to 100% at 200 K, suggesting non-population of the bond-centred state, Mu_BC, which is often observed in other diamond samples. The μ_d state has a spin relaxation rate of 0.20(5) μs⁻¹, in contrast to the zero value obtained in type II diamond samples. This indicates appreciable interaction of the μ_d state with the ¹³C atoms. The Mu _T state has a large spin relaxation rate ranging from 3.0(5) μs⁻¹ at 10 K to 7.0(5) μs⁻¹ at 200 K, consistent with values obtained in diamond samples with defects. This work is part of ongoing studies of muon/muonium-defect interactions in diamonds. This revised version was published online in September 2006 with corrections to the Cover Date.     

Interaction of Muons with H2/H3-Centres in Diamond

We report on transverse field muon spin rotation measurements on a nitrogen-rich type Ia diamond, both before and after the conversion of some of the aggregated nitrogen centres to nitrogen-vacancy complexes known as H2/H3-centres. The prompt fractions f and the spin relaxation rates λ were determined for the diamagnetic (μ_d) and the paramagnetic (Mu_T) states in the temperature range 10–300 K. The production of the nitrogen-vacancy complexes had little effect on the parameters of the Mu_T state for which f and λ remained unchanged at approximately 30% and 4 μs⁻¹, respectively. For the μ_d state, on the other hand, the formation of the H2/H3-centres resulted in an increase of the prompt fraction from 10(2)% to 20(3)%, and (for the first time) the spin relaxation rate showed a non-zero value of 0.020(3) μs⁻¹. These results show evidence of strong μ_d interactions with the nitrogen-vacancy complexes in diamond, and suggest a more complex structure for this state than a bare μ⁺. This revised version was published online in September 2006 with corrections to the Cover Date.     

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.     

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.     

Final states in Si and GaAs via RF μSR spectroscopy

The ionization of muonium centers in Si and GaAs have been studied using radio frequency (RF) resonant techniques. In Si all three muonic centers are detectable by RF. No evidence was found for delayed Mu and Mu^* states at any temperature. However, our results on the diamagnetic final state (μ _f ⁺ ) show that it is composed of prompt fractions (as seen by conventional μSR) and delayed fractions arising from the ionization of Mu^* and Mu. We observe a full μ _f ⁺ fraction at 317 K when the Mu relaxation rate is above 10 μs⁻¹. GaAs differs from the situation in Si in that we observed only a partial conversion of Mu^* and Mu to a μ⁺ final state up to 310 K in spite of the fact that the transverse field relaxation rates become very high at 150 and 250 K respectively.     

Electron–muon dynamics in n‐type silicon

We report RF‐μSR results in lightly n‐doped Si samples. Measurement of the diamagnetic amplitude in both the \langle 100\rangle and \langle 111\rangle directions for a sample with N_D\leq5\times 10¹²\ cm^-3 clarifies the charge/spin electron‐exchange dynamics for bond‐centered muonium and yields a 3300 Å ² electron‐capture cross section at Mu⁺ _BC. An increase in the Mu⁰ _BC RF amplitude observed at 30K in a sample of N_D\simeq 2\times 10¹³\ cm^-3 provides direct evidence for enhanced low‐temperature creation of Mu_BC ⁰ at the expense of Mu_T ⁰0 with increased electron concentration.     

Pseudo spin gap in high‐Tc oxides observed by NMR

From the temperature dependence of the ⁶³Cu nuclear spin‐lattice relaxation rate, 1/T₁T, in the planar Cu(2) sites, it is now well established that a highly enhanced and strongly temperature dependent relaxation process due to antiferromagnetic Cu spin fluctuations exists in all of the high‐T_c’s. The data also exhibit the opening of a gap in the low‐lying magnetic excitations with an energy comparable to the superconducting gap, particularly for the so‐called low doping regime. It is also found that, irrespective of the system, the temperature at which the spin‐gap opens, T_sg, determined as the peak of 1/T₁T vs. T, has a linear decrease with increasing of the doping concentration. A spin‐gap phase diagram is proposed. 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.     

Magnetic properties of nickel and platinum quaternary borocarbides

The zero‐field μ⁺SR technique is employed to study the magnetic properties of the non‐superconducting borocarbides Sm_1-xLu_xNi₂B₂C (x=0.0,\ 0.2) and NdPt_1.5Au_0.6B₂C. Coherent ordering of the Sm electronic moments appears below T_N=9.86(3) and 4.15(3) K for the x=0.0 and 0.2 compositions, respectively. No long range order of the Nd₃₊ moments is found in NdPt_1.5Au_0.6B₂C down to 80 mK. Freezing of the electronic moments into a spin‐glass state occurs at T_f\approx 1.2 K. A stretched exponential decay of the μ⁺ spin polarisation is observed above T_f with the exponent \beta smoothly decreasing on approaching T_f from above, reminiscent of the behaviour in other spin glasses. This revised version was published online in July 2006 with corrections to the Cover Date.     

Universal correlations, pseudo‐gaps and condensation mechanisms in high‐Tc superconductors

We combine universal correlations between T_c and n_s/m^* (superconducting carrier density/effective mass) found by μSR with the pseudo gap behavior found in the underdoped region by various techniques to develop a picture for explaining doping dependence of high‐T_c superconductivity in terms of a crossover from Bose–Einstein to BCS condensation. μSR results in overdoped and Zn‐substituted systems suggest that behavior on the high‐density side is more complicated than expected in a simple BCS model. This revised version was published online in August 2006 with corrections to the Cover Date.     

Zero‐field μ+SR study of the colossal magnetoresistance material La0.67Ca0.33MnO3

Zero‐field μ⁺SR and resistivity experiments on La_0.67Ca_0.33MnO₃ powder show that the ferromagnetic transition temperature (T_C=274\ K) and resistivity peak temperature coincide to within 1 K, about 10 K higher than T_C determined from the bulk magnetization. The sublattice magnetization \nu_μ(T) is well described for T ≤ T_C by (1-T/T_C)^β , where β =0.345 ± 0.015. Unusual relaxation dynamics suggest a wide distribution of Mn‐ion correlation times. These results are discussed in terms of the effects of polarons on the spin and charge dynamics. This revised version was published online in July 2006 with corrections to the Cover Date.     

Experimental determination of the chemical changes in the electron density on indium nuclei by the γ‐spectrometric ΔIγ/Iγ method

The γ‐spectrometry method for determination of the relative change Δλ_EC/λ_EC in the partial probability λ_EC for the ^114mIn decay branch by the electron capture (EC), when the nucleus is in different chemical environments, was developed. The method is based on an experimental determination of the relative change ΔI_γ/I_γ in the intensity of the γ‐radiation which is emitted by the excited daughter ¹¹⁴Cd nuclei after EC. The ^114mIn decay by EC in indium metal, In₂O₃ and InAs was compared. The largest ΔI_γ/I_γ=(2.5±0.8)·10⁻⁴ has been found for InAs relatively to In₂O₃. The chemical changes in the electron density on the indium nucleus corresponding to the measured ΔI_γ/I_γ values have been determined. This revised version was published online in July 2006 with corrections to the Cover Date.     

Studies of diluted antiferromagnets MnxMg1-xTiO3 with x=0.55 and 0.70 by muon spin relaxation method

Longitudinal fields μSR measurements have been performed in order to probe the spin dynamics in the diluted antiferromagnets Mn_xMg_1-xTiO₃ with x=0.70 and 0.55. In the x=0.70 sample which forms the antiferromagnetic long‐range order, the static and fluctuating fields coexist at the muon stopping site below T_N. On the other hand, in the x=0.55 sample which shows the spin‐glass behavior, the local fields fluctuate rather fast even below T_SG. We infer that this drastic change occurs when Mn_xMg_1-xTiO₃ transforms from an antiferromagnetic system to a spin‐glass system by dilution. This revised version was published online in July 2006 with corrections to the Cover Date.     

Absolute sign of the Mu* hyperfine parameters in diamond

Standard μSR experiments in diamond have shown that the relative sign of the hyperfine parameters of the anisotropic Mu^* state is negative (A _‖/A _⊥<0). We report an experimental determination of theabsolute sign of the Mu^* hyperfine parameters by studying the transferred muon polarization during the thermally-activated transition from the isotropic Mu state to Mu^*. The results demonstrate that the isotropic part of the Mu^* hyperfine interaction is negative. In a nitrogen-poor diamond, both the Mu disappearance rate and the enhancement of the Mu^* signals are well-described by a single Arrhenius law.     

Electronic properties of HfXY intermetallic compounds (X = Si, Ge; Y = S, Se, Te)

Muonium, with a positive muon as the nucleus is considered a light isotope of hydrogen displaying a close chemical analogy to this atom. It offers a unique opportunity to study the behaviour of hydrogen in diamond at very low concentrations. The mass difference, however, implies that dynamical effects will be distinct. The bond centred muonium (Mu _BC ) state in diamond is easily observed and there is a very good correlation between theoretical and experimental hyperfine parameters (Schneider et al., Phys. Rev. Lett. 71(4):557–560, 1993). Curiously, despite its predicted stability, the bond centred hydrogen state has not yet been observed in diamond. Following the discovery of hydrogen dopant states in certain wide band gap metal oxides, and the possibility of hydrogen related molecular dopants in diamond, the study of hydrogen in diamond is important. Although it is evident from its hyperfine parameters that Mu _BC is not a shallow donor, the question still arises as to where the Mu _BC state in diamond might lie in the band gap. Accordingly, measurements of the high temperature stability of Mu _BC have been performed in a search for its possible ionization. The results are consistent with such an ionization, as the disappearance of Mu _BC polarisation (setting in near 1000 K) is correlated with the slight increase in the population of the diamagnetic μ⁺ species.     

Muonium centers in the alkali halides

Muonium centers (Mu) in single crystals and powdered alkali halides have been studied using the high-timing-resolution transverse field μSR technique. Mu has been observed and its hyperfine parameter (HF) determined inevery alkali halide. For the rocksalt alkali halides, the HF parameter A_μ shows a systematic dependence on the host lattice constant. A comparison of the Mu HF parameter with hydrogen ESR data suggests that the Mu center is the muonic analogue of the interstitial hydrogenH _i ⁰ -center. The rate of Mu diffusion can be deduced from the motional narrowing of the nuclear hyperfine interaction. KBr shows two different Mu states, a low-temperatureMu ^I -state and a high-temperatureMu ^II -state.     

μSR studies of fullerenes and their derivatives

The dynamic properties of pristine C₆₀ and C₇₀ are reviewed, emphasizing the results of the ZF‐ and ALC‐μ⁺SR techniques. In C₆₀, the \mboxfcc\rightarrow\mboxsc transition is accompanied by a change in the dynamics from isotropic reorientational to quasi‐random jump motion between nearly‐degenerate orientations. C₇₀ is frozen on a timescale of 30 ns up to 170 K. At higher temperatures, the motion is found to be complex, consisting of a uniaxial rotation part together with a nutational or jumping motion of the unique axis. Anisotropy on the 30 ns timescale persists to 370 K, well into the fcc phase. The ZF‐μ⁺SR technique has been also employed to study the magnetic properties of fullerides. In the organic salt (TDAE)C₆₀, spontaneous magnetic order is directly observed below a Curie temperature of 16.1 K, higher than any other organic material. In the quasi‐one‐dimensional conductor CsC₆₀, static magnetic order of a random nature is observed to develop in the vicinity of the metal–insulator transition at 30 K with no direct evidence of long range order present. This revised version was published online in August 2006 with corrections to the Cover Date.