The transition from Mu to Mu* in diamond

Evidence is presented for a transition from the isotropic muonium state (Mu) to the [111] axially symmetric anomalous muonium state (Mu^*) in diamond. Amplitude measurements for Mu^* in a powder in zero field and with a single crystal oriented in a magnetic field indicate that such a transition occurs with a temperature-dependent rate(T) and that the electron polarization is conserved during the transition. The possibility of determining the absolute sign of the Mu^* hyperfine parameters is discussed.     

Calculations of the hyperfine parameters of bond-centered muonium in diamond

The electronic structure of hydrogen and muonium at the bond-centered site in diamond is investigated using ab initio cluster calculations. Correlation effects are accounted for by a configuration interaction expansion and by the local density approximation in the density functional approach. The hyperfine and superhyperfine parameters for anomalous muonium are determined by averaging over the spread of the muon wave function. Good agreement with experimental hyperfine parameters is found.This work has partially been supported by the Swiss National Science Foundation.     

High-field ENDOR and the sign of the hyperfine coupling

Two different approaches for assigning electron nuclear double resonance (ENDOR) signals to their respectiveM _s manifolds by a controlled generation of asymmetric ENDOR spectra, are described and applied to a number of systems. This assignment then allows a straightforward determination of the sign of the hyperfine coupling. Both approaches rely on a high thermal polarization that is easily achieved at high fields and low temperatures. For high-spin systems, such asS = 5/2 the assignment is afforded by the selective inversion of the | ?3/2〉 → | ?1/2〉 electron paramagnetic resonance (EPR) transition which is highly populated as compared to its symmetric counterpart, the |1/2〉 → |3/2〉 EPR transition, and therefore is easily identified. ForS = 1/2 the determination of the sign of the hyperfine coupling becomes possible when the cross- and nuclear-spin relaxation rates are much slower than the electron-spin relaxation rate and variable mixing time pulse ENDOR is used to measure the spectrum. Under these conditions the signals of theM _s = 1/2 (α) manifold become negative when the mixing time is on order of the electron-spin relaxation time, whereas those of theM _s =?1/2 (β) manifold remain positive. Under partial saturation of the nuclear transitions and short mixing time the opposite behavior is observed. Pulse W-band¹H ENDOR experiments demonstrating these approaches were applied and the signs of the hyperfine couplings of the water ligands in Mn(H₂O) ₆ ²⁺ , the H_α and H_β histidine protons in the Cu(histidine)₂ complex, the imidazole protons in Cu(imidazole) ₄ ²⁺ and the cysteine β-protons in nitrous oxide reductase were determined.     

Mu to Mu* transition in electron irradiated silicon

A transition from normal muonium (Mu) to anomalous muonium (Mu^*) is observed in electron irradiated silicon. It is suggested that the transformation is induced by the strain field of the defect and takes place some distance away from the defect. The experiment was performed at 15 K.     

Absolute magnitude of non-leptonic hyperon decays and electromagnetic hyperfine splittings

In the quark model with current-current interaction, non-leptonic hyperon decays are controlled by the baryon wave function at small distances. An independent estimate of this quantify is provided by a related phenomenon: the baryon electromagnetic hyperfine splittings, given by the Fermi contact interaction. A good estimate is obtained with the standard Cabibbo-suppressed interaction. Important remaining problems for S-waves are emphasized.     

The electrical hyperfine parameters in synthetic aluminogoethites

The effects of the aluminum substitution and degree of crystallinity upon the lattice-dynamical and electrical properties of synthetic goethites are determined from their Mössbauer spectra. The temperature variations of the centre shift could be calculated using the Debye approximation for the lattice vibrations. The parameters incorporated in this model exhibit a maximum at a certain aluminum content within each of the four series in this study. It has been found that the principal axis of the EFG remains perpendicular to the direction of the magnetic hyperfine field when the aluminum content and/or the crystallinity change.     

Calculation of electric hyperfine interaction parameters in solids

A theoretical approach for the calculation of hyperfine parameters in solids from first principles is presented. These calculations are based on the full potential linearized-augmented-plane-wave (LAPW) band structure method, which is currently one of the most accurate schemes to determine the electronic structure in ordered solids. Exchange and correlation is treated within density functional theory using the generalized gradient approximation. Once the electron density is calculated self-consistently with high accuracy, quantities like electric field gradients (EFG), isomer shifts or hyperfine fields can easily be obtained from this density without further approximations. Using this approach we have studied various systems including metals, insulators, ionic compounds or the highT _c superconductors. In general we find good agreement between theory and experiment, which proves that our method is very accurate. Having these results in mind we are confident that this method is accurate enough to determine the value of the nuclear quadrupole momentQ, provided experimental measurements of the quadrupole coupling constant are available. This procedure is demonstrated forQ of⁷⁷Se and¹⁰⁰Rh, two nuclei recently used in PAC measurements. An extensive study of EFGs at Fe sites in various Fe-compounds has been performed leading to a very reliable quadrupole moment ofQ(⁵⁷Fe)=0.16 b, a value twice as large as that deduced from recent HF calculations but back to older estimates.     

Determination of hyperfine parameters using quantum beats

We have observed the delayed time spectra of the coherently scattered radiation following nuclear Bragg diffraction of incident synchrotron radiation pulses. The measurements were carried out using single crystals of⁵⁷Fe-YIG and⁵⁷FeBO₃ by conventional coincidence techniques. Although the intention of these measurements was only to show effects like speed-up and quantum beats, the spectra also provide the possibility of a sensitive direct measure of the hyperfine interaction parameters. It is demonstrated that even with a counting rate of only 1 Hz it was possible to obtain within 1–2 hours (in the case of⁵⁷FeBO₃) spectra of sufficient statistics to derive precise hyperfine parameters.     

Temperature-dependent hyperfine parameters in CaFeSi2O6

Two samples of natural hedenbergite CaFeSi₂O₆ were studied in the temperature range from 4.2 K to 295 K, and at 78 K and 295 K in magnetic fields from 4.5 T to 7.5 T. The sign of V_ZZ is positive at 4.2 and 78 K and negative at 295 K. The asymmetry parameter depends strongly on the temperature. These results are explained by a simple model assuming a nondegenerated orbital doublet as ground state of Fe²⁺.     

Temperature dependence of the hyperfine parameters of anomalous muonium in germanium

The temperature variation of the anomalous muonium hyperfine interaction in germanium has been measured between 5 and 100 K. The results show that the component perpendicular to the defect axis decreases, while the parallel component increases with increasing temperature. These effects are a result of the interaction of anomalous muonium with the germanium host phonons.Work supported by National Science Foundation Grant DMR-79-09223.     

Effect of Cr on hyperfine parameters in a-Fe-Cr-Si-B alloys

Mössbauer studies on a-Fe_80–xCr_xSi₈B₁₂ (x = 0,2,...12) at 5K have been reported. The average hyperfine field and width of P(H) distribution have been analysed by a model which assumes a random distribution of Cr atoms. On assumption that the hyperfine field distribution of Fe atom with r number of Cr nearest neighbours is given by a gaussian with a mean field H_r =H_o - rC₁ + r²C₂; C and C₂ being constants; broad features of Mössbauer spectra are explained.     

Systematics of Mössbauer hyperfine parameters in Np intermetallics

Data for intermetallic compounds of neptunium obtained with the 60 keV Mössbauer resonance of²³⁷Np are reviewed. Measurements of temperature, pressure and field dependencies are available. The main questions addressed are: (a) the degree of delocalization of 5f-electrons, (b) the formal charge state of Np, and (c) the influence of the ligand on the neptunium electronic structure. For this purpose, we present an evaluation of systematic behavior concerning mainly the hyperfine field and isomer shift in the cubic Laves phase materials NpX₂, the NaCl-type monochalcogenides and monopnictides, and intermetallics with AuCu₃ and ThCr₂Si₂ structures. Analogies to corresponding rare-earth compounds will be pointed out.     

Pressure shifts of F-center hyperfine interaction parameters in alkali chlorides

We report here calculated values of the pressure shift in the shell I hyperfine interaction constants for F-centers in KCl, NaCl and LiCl. The calculation is based on a modified version of the pseudopotential approach of Bartram et al. with electronic polarization included by means of an r-dependent polarization potential. The results agree reasonably well with the available experimental data, and the strong dependence of the pressure shift on ion size ratio is shown to be mainly due to changes in the indirect-overlap contributions to the spin density. These terms result from the requirement of mutual orthogonality of core states on neighbouring ions, and their dependence on the lattice spacing is stronger than that of other contributions to the spin density. Our calculations further indicate that although the predicted value of the pressure shift is sensitive to errors in the pseudopotential and polarization parameters, the relative importance of the indirect overlap contributions is evident in spite of such errors.     

First principles cluster investigation of electronic structure and hyperfine interaction for nitrogen in diamond

The self-consistent field Unrestricted Hartree-Fock cluster procedure has been used to study the location, electronic structure and hyperfine properties of nitrogen impurity in diamond. From the analysis of the potential energy curve for nitrogen along the <111> axis, it was found that nitrogen is located at a position 0.3A away from the substitutional site towards the plane formed by the three nearest neighbour carbon atoms. The calculated values of the magnetic hyperfine constants and nuclear quadrupole coupling constants for¹⁴N agree very well with values obtained from electron paramagnetic resonance and electronnuclear double resonance measurements.     

氮掺杂的金刚石磁性研究

采用第一性原理计算方法研究金刚石中由空位或者N掺杂引起的磁特性. 发现-1价和-2 价的碳空位能分别产生3 μ_B和2μ_B的磁矩; -2价的碳空位能够引发长程有序的铁磁耦合状态, 而-1价的碳空位更倾向于反铁磁耦合.掺杂N元素能有效地控制空位的荷电状态及相应的磁相互作用, 这一结果为在金刚石中实现非过渡族金属掺杂的铁磁性提供了一条新的路径. 关键词： 第一性原理计算 氮掺杂 金刚石 磁性     

The hyperfine fields and isomer shift at Fe-Co metal inclusions in synthetic diamond grains

The thermal changes taking place at the sites of Fe-Co metal inclusions in synthetic diamond grains have been investigated using Mössbauer spectroscopy on as-grown samples and samples annealed at 970–1370 K. The spectra show the presence of Fe-Co and (Fe-Co)-C complexes. The temperature-dependent results suggest that the latter are due to reactions at the metal inclusion-diamond interface, and may be a signature of the processes leading to strength loss in the grains.     

The temperature dependence of the hyperfine parameters of YIG below the Curie point

The temperature dependence of the hyperfine parameters of the ⁵⁷Fe nuclei in yttrium iron garnet Y₃Fe₅O₁₂ (YIG) between room temperature and the Curie point was studied by means of conventional Mößbauer transmission spectroscopy. The critical exponent β, which describes the temperature dependence of the magnetic hyperfine field of ⁵⁷Fe in YIG, was found to be 0.29(1) and 0.33(1) in the [a]- and (d)-sublattice, respectively. In the temperature region just below the Curie point relaxation of the internal hyperfine fields could be observed. The phase transition at the Curie point shows a distinct dependence on the grain size in the sample.