Although the interstitial hydrogen atom would seem to be one of the simplest defect centres in any lattice, its solid state chemistry is in fact unknown in many materials, not least amongst the elements. In semiconductors, the realization that hydrogen can profoundly influence electronic properties even as a trace impurity has prompted its study by all available means-but still only in the functionally important or potentially important materials-for the elements, Si, Ge and diamond. Even here, it was not studies of hydrogen itself but of its pseudo-isotope, muonium, that first provided the much needed microscopic pictures of crystallographic site and local electronic structure-now comprehensively confirmed by ab initio computation and such data as exists for monatomic, interstitial hydrogen centres in Si. Muonium can be formed in a variety of neutral paramagnetic states when positive muons are implanted into non-metals. The simple trapped atom is commonly only metastable. It coexists with or reacts to give defect centres with the unpaired electron in somewhat more extended orbitals. Indications of complete delocalization into effective mass states are discussed for B, α-Sn, Bi and even Ge, but otherwise all the muonium centres seen in the elemental semiconductors are deep and relatively compact. These are revealed, distinguished and characterized by μSR spectroscopy-muon spin rotation and resonance informing on sites and spin-density distributions, muon spin relaxation on motional dynamics and charge-state transitions. This Report documents the progress of μSR studies for all the semiconductors and semimetals of the p-block elements, Groups III-VI of the Periodic Table. The striking spectra and originally unanticipated results for Group IV are for the most part well known but deserve summarizing and updating; the sheer diversity of muonium states found is still remarkable, especially in carbon allotropes. The interplay of crystallographic site and charge state in Si and Ge at high temperatures, or under illumination, reflects the capture and loss of charge carriers that should model the electrical activity of monatomic hydrogen but still challenges theoretical descriptions. Spin-flip scattering of conduction electrons by the paramagnetic centres is revealed in heavily doped n-type material, as well as some modification of the local electronic structures. The corresponding spectroscopy for the solid elements of Groups III, V and VI is rather less well known and is reviewed here for the first time; a good deal of previously unpublished data is also included. Theoretical expectations and computational modelling are sparse, here. Recent results for B suggest a relatively shallow centre with molecular character; P and As show deeper quasi-atomic states, but still with substantial overlap of spin density onto surrounding host atoms. Particular attention is paid to the chalcogens. Muonium centres in Te show charge-state transitions already around room temperature; the identification of those in S and Se has been complicated by unusual spin dynamics of a different character, here attributed to spin-orbit coupling and interstitial reorientation. In the metals and semimetals, muonium is not formed as a paramagnetic centre. Here the implanted muons mimic interstital protons and interest shifts to a variety of other topics, including aspects of charge screening (α-Sn, Sb, Bi), site preference and quantum mobility (Al, β-Sn, Pb). The post-transition metals receive only a brief mention, by way of contrast with the nonmetals. Systematic studies of local susceptibility via measurements of muon Knight shifts extends in favourable cases to revealing the elusive high-field Condon domains (Al, Sn, Pb, Bi). Some new information is available on the superconducting phases. Appendices include a derivation of the spin Hamiltonian for paramagnetic muonium centres or molecular radicals having varying admixtures of orbital angular momentum, including the extreme case of orbital degeneracy, and examine the consequences of significant spin-orbit coupling for μSR spectroscopy and muon spin relaxation. This is the framework for the tentative assignments made here for the muonium defect centres formed in sulphur and selenium, namely diatomic species resembling the chalcogen monohydrides. Equally, it provides guidelines for eventual solid-state detection of OMu-the elusive muoniated hydroxyl radical.

https://iopscience.iop.org/article/10.1088/0034-4885/72/11/116501/pdf

Muonium as a model for interstitial hydrogen in the semiconducting and semimetallic elements

Publisher Summary The muon spin rotation/relaxation/resonance (μSR) spectroscopy has become an important tool for probing local static and dynamic magnetic properties. In over two decades a quite substantial body of μSR data has been collected from rare-earth and actinide materials, the latter, however, mostly restricted to samples containing uranium. μSR is the measurement of the temporal development of the spatial orientation of the spins of muons, which have been implanted in the material of interest with all spins initially fixed in one direction (complete muon spin polarization). μSR can be performed either with positive or negative muons. The latter behave like heavy electrons and will be captured into Bohr orbits by the atoms in the sample material. They will quickly reach the ls ground state, whose orbital radius, however, is comparable to nuclear radii. Negative muons thus sense predominantly the effects of the protonic charge distribution and the result is primarily of interest to nuclear physics. The magnetic field at the site of the muon in a magnetic compound is, at least in part, created by the dipole moments on neighboring paramagnetic atoms or ions.

Chapter 206 μSR studies of rare-earth and actinide magnetic materials

Publisher Summary This chapter discusses the compounds of transition elements with nonmetals The ferromagnetism of transition metal intermetallic compounds are covered by Booth The compounds of transition metals with nonmetallic and semimetallic elements are described The scope of the combined physics and chemistry conference series International Conference of Solid Compounds of Transition Elements is reviewed However, sulfides are only treated when there is a direct connection to related selenides and tellurides The chapter discusses ferromagnetism, ferri-, antiferro-, and helimagnetism The compounds are listed according to the stoichiometric composition in the chapter The large groups TX and T 2 X, TT’X, where T is a transition element and X a nonmetal element is emphasized Various T m X n compounds are also described The compounds are arranged according to the nonmetallic elements of the third, fourth, fifth, and sixth group of the periodic table In the chemical formulas, the elements are arranged according to increasing atomic number The magnetic phase diagrams are presented according to the same rule The chapter also presents a survey of the relevant crystallographic structures

Chapter 3 Compounds of transition elements with nonmetals

Neutron elastic and inelastic scattering experiments have been performed on Fe 2 P single crystals in the temperature range from 8 K to 800 K. Fe 2 P is of the hexagonal C-22 type structure and exhibits c -axis ferromagnetism below 130 K. In the intermediate range between 130 K and 217 K, the canted ferromagnetism with a uniform component along the c -axis M 0 and a staggered component in the c -plane M Q appears, and the first order transition takes place at T c =217 K from the canted ferromagnetism to paramagnetism. The most interesting behaviour is the existence of giant short range order up to T =3 T c . It originates in the small stiffness constants D and leads to the strong deviation of susceptibility from the Curie-Weiss law. The amplitude of the spin fluctuation observed In Fe 2 P In the pararnagnetic phase is understood by the simple model with parameters deduced from the spin wave dispersion at low temperature, the magnetization and uniform susceptibility.

Neutron Scattering Investigation of Itinerant Electron System Fe2P

The spatial and temporal correlations of magnetic moments in the paramagnetic regime of ${(\mathrm{Mn},\mathrm{Fe})}_{2}(\mathrm{P},\mathrm{Si})$ have been investigated by means of polarized neutron diffraction and muon-spin relaxation techniques. Short-range magnetic correlations are present at temperatures far above the ferromagnetic transition temperature $({T}_{C})$. This leads to deviations of paramagnetic susceptibility from Curie-Weiss behavior. These short-range magnetic correlations extend in space, slow down with decreasing temperature, and finally develop into long-range magnetic order at ${T}_{C}$.

/pdf/short-range-magnetic-correlations-and-spin-dynamics-in-the-1qkkmsd79t.pdf

Short-range magnetic correlations and spin dynamics in the paramagnetic regime of (Mn,Fe)2(P,Si)

μSR study of Fe2P in the paramagnetic state

The local fields at140Ce in the cubic intermetallic compounds GdAl2 and DyAl2 have been measured with the DPAC method. At our lowest temperatures we obtainBeff (30K)=54(2) T for GdAl2 andBeff(12.5K)=27(1) T for DyAl2 which are considerably lower than the hyperfine field of the free Ce3+ ion (183 T). The (Ce)GdAl2 field is quantitatively explained by a cubic crystal field splitting of 4f states but for DyAl2 additional effects are discussed.

Hyperfine fields at Ce in GdAl2 and DyAl2

The magnetic field dependence of the transverseμSR linewidth has been studied in anAlAg single crystal. The muon site is found to be tetrahedral at 22 K and octahedral at 0.05 K. Both the site change and the magnitudes of the electric field gradients are similar to previous observations onAlMn, although these two impurities create different strain fields in Al.

On muon localization in doped aluminium samples

The muon spin relaxation in the paramagnetic states of GdNi2 and DyNi2 compounds has been measured, as a complement to earlier studies of the rare earth dialuminides (REAl2). In addition, improved data have been taken on a GdAl2-sample, up to a temperature of 4Tc. At high temperatures the Moriya molecular field model predicts a rare earth spin fluctuation rate proportional to the Curie temperature of the compund, which would lead to a Tc−1 dependence of the muon relaxation rate. This relation is fulfilled for the pair GdNi2/GdAl2, but not for the DyNi2/DyAl2 ratios. Possible reasons for this discrepancy are discussed.

E. Wäckelgård

Papers

μSR study of Fe2P in the paramagnetic state

Hyperfine fields at Ce in GdAl2 and DyAl2

On muon localization in doped aluminium samples

Comparison of the paramagnetic fluctuations in GdM2 and DyM2 (M=Al, Ni)

Muon spin rotation in gadolinium above the Curie temperature