Spin-orbit quenching of positronium during atomic collisions

When positrons are injected into a gas, 75% of the positronium (Ps) is likely to be formed as long-lived ortho-Ps. The main decay mechanisms for the ortho-Ps have been assumed to be natural decay of ortho-Ps and pickoff annihilation of the positron during Ps-atom collisions. A third possibility for annihilation is ortho-Ps-->para-Ps conversion due to the spin-orbit interaction between the atom and colliding Ps. This extra quenching mechanism may explain a number of phenomena observed in the annihilation spectrum of Kr and Xe, including the very small Ps fraction of 3% seen for Xe.     

Anisotropic magnetic quenching of positronium states in oriented crystals

We have carried out a theoretical analysis of the anisotropy of magnetic quenching of positronium states in noncubic crystals oriented relative to the direction of the external magnetic field. We show that an initial polarization of the positrons amplifies the anisotropy of magnetic quenching of positronium and lowers the magnitude of the magnetic field in which the anisotropy is maximum. We have obtained numerical estimates of the magnitude of the experimentally observed effect for quasipositronium in a single crystal of crystalline quartz and for a positronium complex in a single crystal of naphthalene. Fiz. Tverd. Tela (St. Petersburg) 41, 999–1002 (June 1999)     

Bromine and oxygen quenching of positronium in silica gel

Positron lifetime spectra for various concentrations of bromine and oxygen adsorbed on silica gel are investigated. Two-photon angular correlation experiments were also performed on some of these samples to study the quenching process of bromine and oxygen. The cross section for bromine quenching of orthopositronium in silica gel is about two orders of magnitude larger than that for oxygen quenching. The process of bromine quenching of positronium is found to be chemical quenching. Oxygen quenching of positronium in silica gel is found to be conversion quenching. The interaction between positronium and bromine or oxygen in silica gel is found to be similar to that found in the gaseous state. The energy of Ps annihilating in the pores of silica gels is found to be 0.25±0.10 eV.     

What happens in semiconductors during quenching

Turbulent diffusion is anomalously large and considered as universal, i.e. independent of molecular properties. A recent report in Science raised doubts on this, based on measurements of pesticide mixture dispersion in air. This touches the very basis of current understanding of turbulence. We offer an explanation why and how molecular properties affect dispersion despite universal eddy transport.     

Magnetic semiconductors

As far as the electrical conductivity is concerned, solids are usually classified as metals, semiconductors, or insulators. In metals the concentration of the charge carriers responsible for the electrical conductivity is large, whereas in semiconductors and insulators the carrier concentration is much smaller. The distinction between semiconductors and insulators is based on a difference in the nature of the conductivity. For semiconductors the charge carriers (electrons or holes) occupy the states of energy bands; these states are not Iocalized on particular atoms, but spread throughout the entire crystal. In such a situation the mobility of the carriers can be quite high and would, in fact, be infinite in a rigid periodic lattice; in this model the thermal motion of the atoms induces a scattering of the carriers and thus limits the conductivity to finite values. The classical examples of semiconductors are the elements Si and Ge and compounds such as GaAs, InSb, CdTe, ZnS, etc.     

Thermal quenching of luminescence in erbium doped semiconductors

The nature of the temperature dependence of luminescence intensity from Er⁺ ions in GaInAsP, Si, InP, GaAs, AlGaAs, ZnTe, as observed by Favennecet al [1] has been examined in terms of a double exponential model. The smaller activation energy is found to be 58–100 meV, characteristic of a localized energy barrier at the Er⁺ centre while the higher activation energy is approximately 0.8E _g attributed to an Auger non-radiative process of carrier excitation into bands. This model has been found to describe the observed temperature dependences with reasonably good agreement.     

Magnetic excitations in ferromagnetic semiconductors

Magnetic excitations in a series of GaMnAs ferromagnetic semiconductor films were studied by ferromagnetic resonance (FMR). Using the FMR approach, multi-mode spin wave resonance spectra have been observed, whose analysis provides information on magnetic anisotropy (including surface anisotropy), distribution of magnetization precession within the GaMnAs film, dynamic surface spin pinning (derived from surface anisotropy), and the value of exchange stiffness constant D. These studies illustrate a combination of magnetism and semiconductor physics that is unique to magnetic semiconductors.     

Magnetic susceptibility of liquid semiconductors

The temperature dependence of the magnetic susceptibility of semiconducting ZnSb, CdGeAs₂ and CdSnAs₂ is studied in the vicinity of the melting point. It is shown that the susceptibility change at the melting point cannot be quantitatively explained only by the susceptibility of the electron gas present in the melt, but one must take into account other components connected with the susceptibility of the lattice. A quantitative guess of the susceptibility of molten semiconductors that were not yet measured is given.     

Magnetic percolation in diluted magnetic semiconductors

We demonstrate that the magnetic properties of diluted magnetic semiconductors are dominated by short ranged interatomic exchange interactions that have a strong directional dependence. By combining first principles calculations of interatomic exchange interactions with a classical Heisenberg model and Monte Carlo simulations, we reproduce the observed critical temperatures of a broad range of diluted magnetic semiconductors. We also show that agreement between theory and experiment is obtained only when the magnetic atoms are randomly positioned. This suggests that the ordering of diluted magnetic semiconductors is heavily influenced by magnetic percolation, and that the measured critical temperatures should be very sensitive to details in the sample preparation, in agreement with observations.     

Magnetic properties of diluted magnetic semiconductors

A review will be given of the magnetic characteristics of diluted magnetic semiconductors and the relation with the driving exchange mechanisms. II–VI as well as IV–VI compounds will be considered. The relevance of the long-range interaction and the role of the carrier concentration will be emphasized.     

Magnetic susceptibility of multinary diamondlike semiconductors

Summary The magnetic susceptibility of some A^IB ₂ ^II C^IIIX ₄ ^VI (A^I=Cu, Ag; B^II=Zn, Cd; C^III=Ga, In; X^VI=Se, Te) diamondlike chalcogenides has been measured at RT. Coherently with the form of the relative A^IC^IIIX ₂ ^VI -B^IIX^VI phase diagrams, their magnetic susceptibility fulfils the sum rule typical of ideal solid solutions. Separate values of the diamagnetic and paramagnetic contributions to total magnetic susceptibility are reported and discussed according to a chemical-bond approach previously described. Paper presented at the ?V International Conference on Ternary and Multinary Compounds?, held in Cagliari, September 14–16, 1982.     

Magnetic field induced metal-insulator transition in semiconductors

Metal-insulator transition in doped semiconductors is investigated in the presence of intense magnetic fields. Using Thomas-Fermi screening function and a screened coulomb potential in the Hamiltonian, a two-parameter varitional calculation leads to metallization. The correlation effect among the electrons is considered through an effectivemass that depends on the spatial seperation between impurities. Results are provided for a many valley semiconductor (Si) and two single valley semiconductors (GaAs and CdTe). Our results show that in a magnetic field the critical concentration at which metallization occurs increases. The correlation effects bring out one order change in the critical concentration values, in intense magnetic fields.     

Magnetic state around cation vacancies in II-VI semiconductors

The magnetic properties of neutral cation vacancies in II-VI semiconductors are examined using first principles calculations and group theory. A molecular cluster model of a single vacancy in II-VI semiconductors is developed to explain the observed chemical trend. We show that a single Zn vacancy in ZnO yields a total spin ST=1 in agreement with experiments. But for the other less ionic Zn-based II-VI semiconductors ZnA (A=S, Se, Te) this spin triplet state is nearly degenerate with a nonmagnetic spin singlet state.     

Magnetic metastability in tetrahedrally bonded magnetic III-nitride semiconductors

Results of density-functional calculations for isolated transition metal (TM = V, Cr, Mn, Fe, Co, Ni on cation sites) doped GaN demonstrate a novel magnetic metastability in dilute magnetic semiconductors. In addition to the expected high spin ground states (4muB/Mn and 5muB/Fe), there are also metastable low spin states (0muB/Mn and 1muB/Fe)--a phenomenon that can be explained in simple terms on the basis of the ligand field theory. The transition between the high spin and low spin states corresponds to an intraionic transfer of two electrons between the t2 and e orbitals, accompanied by a spin-flip process. The results suggest that TM-doped wideband semiconductors (such as GaN and AlN) may present a new type of light-induced spin-crossover material.     

Magnetic moment of an acceptor center in cubic semiconductors

The magnetic field-induced splitting of the ground-state spin levels of an acceptor center, described by superposition of the Coulomb and the central-cell potentials, has been calculated for diamond-type. An analytic expression for the g factor obtained in the zero-radius potential approximation depends only on the light to heavy hole mass ratio. It is shown that the differences between the values of the g factor for the limiting cases of purely Coulomb and zero-radius potentials do not exceed 5%, thus permitting one to use this analytic expression for estimates. Fiz. Tverd. Tela (St. Petersburg) 39, 58–60 (January 1997)     

Magnetic properties of Mn-doped ZnO diluted magnetic semiconductors

A series of Mn-doped ZnO films have been prepared in different sputtering plasmas by using the inductively coupled plasma enhanced physical vapour deposition. The films show paramagnetic behaviour when they are deposited in an argon plasma. The Hall measurement indicates that ferromagnetism cannot be realized by increasing the electron concentration. However, the room-temperature ferromagnetism is obtained when the films are deposited in a mixed argon-nitrogen plasma. The first-principles calculations reveal that antiferromagnetic ordering is favoured in the case of the substitution of Mn^2＋ for Zn^2＋ without additional acceptor doping. The substitution of N for O （NO^-） is necessary to induce ferromagnetic couplings in the Zn-Mn-O system. The hybridization between N 2p and Mn 3d provides an empty orbit around the Fermi level. The hopping of Mn 3d electrons through the empty orbit can induce the ferromagnetic coupling. The ferromagnetism in the N-doped Zn-Mn-O system possibly originates from the charge transfer between Mn^2＋ and Mn^3＋ via NO^-, The key factor is the empty orbit provided by substituting N for O, rather than the conductivity type or the carrier concentration.     

Magnetic susceptibility of layered 2H-WSe2 semiconductors

Magnetic susceptibility and anisotropy in magnetic susceptibility _L are studied in 2H-WSe₂ layered semiconductor crystals over the temperature interval 77–300 K and magnetic fields 1–10 kOe.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 20–23, November, 1989.The authors are indebted to L. P. Strakhov for his kind evaluation of the results of the study.     

Magnetic field dependent dielectric function for zero-gap semiconductors

Using the random phase approximation and the Yafet wave functions the inter and intraband static dielectric functions of degenerate zero-gap semiconductors placed in a magnetic field are calculated in the long wavelength limit. It is shown that the values of the dielectric function for gapless semiconductors differ considerably from the result obtained previously for the one-band model with parabolic energy dispersion ands-type wave functions. Finally the dielectric function is used to find the static screening of a point impurity charge.     

Magnetic force microscopy of GaAs:Mn ferromagnetic semiconductors

The dependence of magnetic properties of GaAs:Mn and MnAs epitaxial films grown on GaAs (001) by laser ablation of Mn and undoped GaAs in a hydrogen atmosphere under the growth conditions has been studied by magnetic force microscopy (MFM). Magnetic probe calibration for quantitative MFM measurements was performed by scanning across the slit of the magnetic-head of a tape recorder through which controlled direct current was passed. The dipole approximation was used to describe the magnetic properties of the MFM probe. Nonuniformity of the magnetization of GaAs:Mn films related to the formation of MnAs nanoinclusions, which are ferromagnetic at 300 K, has been observed. The typical scales of the spatial nonuniformity of the magnetization of GaAs:Mn films were varied from 270 to 550 nm depending on the film-growth conditions. The MnAs phase was identified by MFM measurements at an elevated temperature (up to 80°N).