Muonium Centers Research Articles

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 MuBC 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 MuBC 0+ h+\to MuBC + followed by charge exchange reaction (or transition Mu+ BC+ e−→ Mu0 T).

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.

Structure of negatively charged muonium in n-type GaAs.

Muon level-crossing resonance and muon-spin-rotation measurements on heavily doped n-type GaAs:Si and GaAs:Te show that the majority of positive muons implanted at room temperature form an isolated diamagnetic muonium center located at a high-symmetry site with Ga neighbors along the 〈111〉 direction(s). These experiments, together with theoretical considerations, imply that negatively charged muonium is at or near the tetrahedral interstitial site with four Ga nearest-neighbor atoms. Except for zero-point energy differences, these results should model negatively charged isolated hydrogen in GaAs.

Relaxed excited states and anomalous hyperfine structure of muonium centers in KBr

We report on the luminescence induced by positive muons implanted (with 4 MeV) into KBr crystal, which evidences a long-lived (lifetime=13.3 μs excited state produced by muon radiolysis. The temperature dependence of the luminescence yield has a strong correlation with the amplitude of an “anomalous” muonium center: both are observed only below ∼50 K. This correlation strongly suggests that the muonium center is perturbed by the muon-induced excitons to cause the anomalous hyperfine structure. Moreover, the luminescence energy and decay time indicate that the observed luminescence is not associated with the intrinsic or impurity-related self-trapped excitons, but with a relaxed excited state specific to the muon(ium)-KBr system.

Field dependence of the longitudinal muon polarization for anomalous muonium

The constant muon polarization for anomalous muonium exhibits a peculiar field dependence which represents an easily measurable signature of anomalous muonium centers even in polycrystalline materials. Furthermore it can be used to extract information on the dynamical destruction of the state upon temperature variations and it might also be useful to investigate muonium in amorphous materials.

Bond-centered muonium in diamond: Resolved nuclear hyperfine structure.

The nuclear hyperfine structure of bond-centered muonium in $^{13}\mathrm{C}$ enriched diamond has been resolved using time-differential transverse-field muon spin rotation. The measured nearest-neighbor $^{13}\mathrm{C}$ hyperfine parameters are compared to theoretical values from a recent ab initio Hartree-Fock type cluster calculation. The ratios of the measured $^{13}\mathrm{C}$ hyperfine parameters and free atom values indicate that 92% of the unpaired electron spin density resides on the two nearest-neighbor atoms, in marked contrast with the situation for the analogous muonium center in Si.

Recent advances in positive muon spin rotation; muonium as a probe of fullerenes

In non-metallic solids the positive muon often forms paramagnetic muonium centers which are characterized by the hyperfine interactions of the unpaired electron with the positive muon and with the surrounding nuclear spins. The static and fluctuating components of these hyperfine interactions provide information on local molecular dynamics and local electronic structure. Some recent results on C60 and related compounds are presented to illustrate this.

Hyperfine interactions in muonium-impurity complexes in silicon

The properties of paramagnetic complexes formed by muonium located near acceptor and donor impurities in silicon are calculated using the quantum chemical methods. The calculated data are compared to the experimentally observed characteristics of “normal” and “anomalous” muonium centers.

A theoretical investigation of the electronic structure, hyperfine properties and binding energies of muonium centres in cuprous chloride

A series of self-consistent local-spin-density molecular-cluster calculations has been performed to investigate the electronic structure, the hyperfine (hf) properties and binding energies of muonium centres in curprous chloride. Spin-polarization was taken into account on a finite, stoichiometric cluster including Cu 7Cl 7 and an interstitial muonium (MU) atom. Different Mu positions along the direction 〈111〉 of the CuCl zinc blende structure between the two inequivalent tetrahedral interstitial sites (hereafter T Cu and T Cl) were investigated. Our results indicate a localization of Mu close to the T Cu site, shifted about 0.5 Å toward the apical Cu atom of the employed cluster. Such a fact nicely agrees with experimental findings and it has been discussed and rationalized on the basis of the different bonding interaction between MU and the nearest neighbours. In the light of our theoretical outcomes, the single-electron model proposed by Cox and Symons for the reduction of the Fermi contact term A of the hf coupling constant in I–VII semiconductors seems to be rather inadequate. The accord between experimental and theoretical hyperfine parameters is satisfactory. In particular, theoretical data would indicate that the two different muonium centres, experimentally detected in CuCl, have the same chemical environment. These results are encouraging for the study of other compound semiconductors.

Muonium fluorescence: Anomalous muonium center and relaxed excited state in KBr.

A slowly decaying fluorescence [lifetime⇒13.3(5) μs] induced by implanted positive muons has been observed in KBr by using the time-resolved direct-photon-counting method with a total yield of one photon per four incident muons below 30 K. The temperature dependence of the fluorescence yield shows a remarkable coincidence with that of the amplitude of the anomalous muonium center in KBr observed by the muon-spin-rotation method, strongly suggesting a coupled state between muonium centers and radiation-induced relaxed excited states as a model for the anomalous muonium centers

Electronic structure, dynamics, and metastability of muonium in semiconductors

Muonium centers are light hydrogen-like centers formed when positive muons are stopped in crystalline semiconductors. Detailed information on the hyperfine structure, dynamics and metastability of muonium are obtained using a combination of muon spin rotation or relaxation, muon level-crossing resonance and related methods. The expected close similarity to hydrogen, especially with regard to electronic structure, is important since the equivalent information on isolated hydrogen is either less detailed or completely absent. There are also interesting differences between muonium and hydrogen. In particular muonium dynamics are expected to exhibit enhanced quantum mechanical effects since the muon has only 19th the proton mass. In this paper we review the current status of experiments.

Theory of H sites in undoped crystalline semiconductors

The equilibrium sites of atomic hydrogen and muonium in both doped and undoped elemental semiconductors, as investigated by means of several theoretical methods and experimental techniques, are outlined. A particular emphasis is given to the undoped c-Si results. Two controversial points regarding the H and muonium equilibrium sites have been carefully discussed. The bond centered model has been favored with respect to the vacancy associated one. The T site quite reasonably results to be a metastable site and the most favored candidate for the normal muonium center.

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.

Hyperfine interactions of muonium and hydrogen in silicon and diamond: Quantum chemical calculations

The electronic structure of a hydrogen-like atom located at interstitial sites of the silicon and diamond crystals is calculated by the intermediate neglect of differential overlap (INDO) method. Calculations of the electronic g- and hyperfine interaction tensors of the impurity atom are performed. The results obtained are compared with the experimental properties of both “anomalous” muonium and hydrogen centers. It is shown that the most likely model for these centers in silicon and diamond is that in which interstitial neutral hydrogen-like atom locates at the bond-center site.

Nuclear hyperfine structure of muonium in CuCl resolved by means of avoided level crossing.

We report detailed avoided-level-crossing spectra of a muonium center (Mu{sup II}) in single-crystal CuCl in a magnetic field range of 4--5 T and at a temperature of 100 K. The hyperfine parameters of the muon and the closest two shells of nuclei indicate that this center consists of muonium at a tetrahedral interstice with four Cu nearest neighbors and six Cl next-nearest neighbors and that the spin density is appreciable on the muon and on the ten neighboring nuclei but negligible elsewhere.

Structural identification of hydrogen and muonium centers in silicon: First-principles calculations of hyperfine parameters.

First-principles calculations are used to evaluate the hyperfine and superhyperfine parameters for hydrogen and muonium at various sites in the Si lattice. Good agreement is found with muon-spin-rotation experiments, leading to an unambiguous identification of ``anomalous muonium'' with the bond-center site. The results establish the power and reliability of pseudopotential-spin-density-functional theory for calculating signatures of defects.

Theory of structure and hyperfine properties of anomalous muonium in elemental semiconductors: Diamond, silicon, and germanium.

A number of possible models for the anomalous muonium (${\mathrm{Mu}}^{\mathrm{*}}$) center in the elemental semiconductors diamond, silicon, and germanium are investigated in detail, both with respect to their stabilities and abilities to explain the extensive available experimental hyperfine-interaction data, the latter being the major focus of the present work. Using the unrestricted Hartree-Fock cluster procedure, the electronic structures and potential-energy curves associated with muon positions are obtained for the different models. The results are utilized to obtain hyperfine properties associated with the muon and its neighboring nuclei, including vibrational effects associated with the muon. Our results show that stability considerations favor both the vacancy-associated (VA) and bond-centered (BC) models for ${\mathrm{Mu}}^{\mathrm{*}}$. The VA model explains all the experimentally observed features of the muon hyperfine properties and provides reasonably good quantitative agreement with experiment. However, questions remain regarding its formation and ability to explain level-crossing resonance (LCR) data. On the other hand, although the BC model appears to explain the experimental features from LCR measurements, in its present form, it seriously overestimates the strengths of the muon hyperfine interactions as compared to experiment, by more than an order of magnitude in some cases. Additionally, it does not explain the trend from diamond through germanium. On the basis of the results in this paper for the VA and BC models, the direction for future investigations for understanding the nature of the ${\mathrm{Mu}}^{\mathrm{*}}$ center is commented on.

Current status of understanding of anomalous muonium center in semiconductors

Current status of understanding of anomalous muonium center in semiconductors

Structure and Metastabiltty of Muonium Centers in Semiconductors and their Simulation of Isolated Hydrogen Centers

AbstractThe study of isolated hydrogen in semiconductors is difficult because of the tendency for hydrogen to form complexes. An alternative to direct studies of hydrogen is the study of tnuonium in semiconductors. In such experiments the muon is essentially an isotope of hydrogen with th the mass of the proton. Twenty isolated muonium centers have been observed in tetrahedrally-coordinated crystals ranging from Si to the cuprous halides. Muon level-crossing resonance has provided detailed information on neutral interstitial muonium located at a bond center in Si (the analog of the hydrogen center seen by EPR) and very near a bond center in CaP and GaAs. This paper reviews these measurements and discusses the structures and their metastabilities. Comparison is made to EPR for Si and to theoretical studies. Recent results on the metastable and stable muonium centers in CuCl are discussed.

Hyperfine interactions of muonium and hydrogen in semiconductors: Cluster calculations

The electronic structure of muonium (Mu) located at different interstitial sites of the silicon crystal is calculated by the complete neglect of differential overlap (CNDO) and intermediate neglect of differential overlap (INDO) methods. Calculations of the electronicg- and hyperfine interaction tensors of the impurity atom are performed. The results obtained are compared with the experimental properties of both “normal” (Mu′) and “anomalous” (Mu*) muonium centers. It is shown that the most likely dynamic model for Mu′ is that in which neutral Mu diffuses rapidly in the silicon lattice, whereas for Mu* it is the model wherein interstitial Mu is located at the bond-center site. A correlation is made between the characteristics of the hydrogen-bearing Si-AA9 center, recently observed by EPR in a silicon crystal, and those of Mu*. The Si-AA9 center is shown to be a hydrogen-bearing paramagnetic analogue of the Mu* center.