Self-trapping of electron pairs in materials with tunneling states. Electrons in “perfect” covalent glasses

It is shown that in perfect covalent glasses (and some local structures) the self-trapping of negative-U electron (on hole) pairs is realized on selected “flexible” atomic configurations. The basic concepts of the theory of the phenomenon are presented; the criteria and some associated effects are analyzed. The selected configuration contains an appropriate atom (atomic group) in “critical local potential” with unusually small spring constant in respective well. The phenomenon (self-trapping of electron or hole pair) can generate both Anderson negative-U centres in “perfect” glass (i.e. not associated with defects in glasses) and, possibly, a class of deep-level impurity centres in some crystals.     

Radiation-Induced Paramagnetic Centers in the Glasses Of CaO-Ga2O3-GeO2 System

The X-band (v ? 9.4 GHz) ESR spectra of the UV-, X-, and γ-irradiated glasses of CaO-Ga₂O₃-GeO₂ system with compositions similar to Ca₃Ga₂Ge₃O₁₂, Ca₃Ga₂Ge₄O₁₄, and Ca₃Ga₂O₆ crystals have been investigated at 300 and 77 K. It was shown that X- and γ-irradiation of the Ge-contained glasses induce simultaneously electron and hole paramagnetic centers, stable at room temperature. The UV-irradiation of Ge-contained glasses leads to the generation of stable centers of electron type, whereas the X- and γ-irradiation of glasses with Ca₃Ga₂O₆ composition induces stable hole centers, exclusively. The electron centers are assigned to E′ (Ge) centers with different local environments. ESR spectrum of hole centers is ascribed to O^? centers, localized on non-bridging oxygen of the glass network. The E′ (Ge) and O^? centers show high thermal stability in the CaO-Ga₂O₃-GeO₂ glass network. The obtained ESR spectra are described by spin Hamiltonian with g-factor of axial and rhombic symmetry for the E′ (Ge) and O^? centers, respectively. The nature, electron structure and some formation peculiarities of the radiation defects in CaO-Ga₂O₃-GeO₂ glasses are discussed in comparison with other glasses as well as Ca₃Ga₂Ge₃O₁₂ and Ca₃Ga₂Ge₄O₁₄ crystals.     

Radiation induced paramagnetic centres in MgOAl2O3SiO2 glasses containing TiO2

Upon irradiation with ⁶⁰Co γ-rays Cordierite glasses with added TiO₂ display two dominant ESR resonances arising from (a) Ti³⁺ ions (b) holes trapped at non-bridging oxygen ions singly bonded to [SiO₄]^? tetrahedra. The Ti³⁺ ions appear to be in D_4h octahedral sites with the evident distributions between the principal g values arising from an isotropic randomness at the titanium sites. However, the g parameter distributions of the hole resonance, and their changes during the addition of TiO₂ indicate the development of Silicate structures in the glasses which are the precursors of the major low temperature crystalline phases. The invariance of the hole and electron resonance lines with pre-crystallization heat treatments indicates that neither the titanium associated structures or the basic silicate structure of the glass are changed by such treatments.     

Monochromatic neutrinos from the annihilation of fourth-generation massive stable neutrinos in the Sun and in the Earth

The accumulation of relic fourth-generation heavy neutrinos (of mass 50 GeV) in the Earth and the Sun, which is followed by their annihilation, is considered. The most conservative estimates of the fluxes of monochromatic electron, muon, and tau neutrinos and antineutrinos of energy 50 GeV from the annihilation of heavy neutrinos are 4.1×10^?6 cm^?2 s^?1 from Earth’s core and 1.1×10^?7 cm^?2 s^?1 from Sun’s core, whence it follows that an analysis of data from underground neutrino observatories may furnish additional information about the existence of fourth-generation neutrinos. It is shown that, because of kinetic equilibrium between the arrival of cosmic neutrinos and their annihilation, the existence of new U(1) gauge interaction of fourth-generation neutrinos has virtually no effect on the estimates of the annihilation fluxes of electron, muon, and tau neutrinos.     

Elektronenspin-Resonanz und Photochemie des U2-Zentrums in Alkalihalogenid-Kristallen

The photochemical reactions involved in the production and in the bleaching of U₂ centers (interstitial hydrogen atoms) in KCl, KBr and NaCl crystals are investigated by measuring the optical absorption spectra of the crystals at low temperatures. The U₂ centers are prepared by UV irradiation of hydroxide- and hydride-doped crystals. The electron spin resonance of KCl, KBr and NaCl crystals with U₂ centers at low temperatures reveals a well-resolved hyperfine splitting due to interactions of the hydrogen atom with four neighbouring halogen ions. The U₂ center can be described as a tetrahedal [HX₄]^4?-molecule ion, in which one hole, bound mainly at the hydrogen atom and partially at the surrounding halogen ions, gives rise to the spin resonance.     

Pressure-induced delocalization of negative-U centres in semiconducting glasses

Delocalization of the negative-U centres and/or their excitations, strongly localized at ambient pressure, is predicted to occur in semiconducting glasses at the accessible high critical pressure p_c ≈ 10⁵ bar. The phenomenon is shown to exist because of the increase of the centres overlap with growing pressure, which increases the centres concentration. Then, non-localized charge carriers, related to the negative-U centres, become essential at sufficiently high p > p_c, giving rise to dramatic changes in electron (e.g., transport) processes in semiconducting glasses.     

Reentrant phase transition in granular superconductors

The conditions for the appearance of a reentrant superconducting phase in granular materials are studied in mean field approximation applied to periodic models. We assume that the relevant low-lying excitation is the transfer of a Cooper pair from a grain to one of its neighbours, and neglect pair breaking. Both on-grain (U) and nearest neighbour (V) Coulomb interactions are taken into account, and the Coulomb problem is treated in Bethe-Peierls approximation. WhenV/U is not too large, reentrance is predicted ifV/U>(4+3z)^?1/2 wherez is the coordination number. This result is different from a recent criterion suggested by ?imánek, which allows reentrance only in the immediate vicinities of certain discrete values ofV/U. For strong enoughV/U, the models treated here show a transition to an ionic-salt-like charge-ordered state. Reentrant superconductivity is shown to occur also on an ionic background. In actual systems, close-packing effects partially frustrate the ionic ordering and enhance the reentrant feature.     

Sources of tetramuon production by neutrinos

We evaluate four sources of tetramuon production by neutrinos: (a) heavy quark cascade, (b) heavy lepton cascade, (c) charm production with radiative pair, (d) charm production plus an associated charm pair. Rates relative to ν→μ^? of order (a) 10^?6, (b) 10^?5, (c) 10^?7, (d) 10^?9, can be obtained. Invariant mass and azimuthal correlation distributions in 4μ events are presented. Relative multimuon rates differ strikingly in the models considered.     

Proline as a charge stabilizing amino acid in peptide radical cations

Long distance electron transfer in proteins requires relay stations that can be transitorily oxidized or reduced. Although individual prolines cannot assume this function, because of their high ionization energy, it has been shown that polyprolines have the ability to transfer charges. In order to determine the role of the proline in the hole distribution and transport within a PheProPhe tripeptide, the radical cation of a model compound where the phenylalanines carry two or three methoxy groups, respectively, was generated by flash photolysis. Surprisingly, after equilibration, about two thirds of the holes were found to reside on the phen(OMe)₂ instead of the more easily oxidizable phen(OMe)₃ moiety. DFT calculations showed that, in most of the accessible conformations, the phen(OMe)₂^{? +}‐moiety profits more from stabilization by N‐ and/or O‐lone pairs of neighboring amide groups than the phen(OMe)₃^{? +} moiety can, which explains the apparently counterthermodynamic hole distribution. Similar calculations showed that, in several conformers of the natural PheProPhe radical cation, the unpaired electron is delocalized over two amide groups, by residing in a σ MO which links the N‐lone pair of the central proline unit with the O‐lone pair of a proximate amino acid, through hyperconjugation via the intervening C―C_α σ‐bond. The same pattern is found in a model compound, N‐acetylproline dimethylamide. It seems that prolines favor conformers which foster hyperconjugation of two amide groups, which lowers the ionization energy of peptides. One should thus consider such interacting amide groups as potential relay stations in the course of electron transfer in polyprolines. Copyright © 2015 John Wiley & Sons, Ltd.     

Signals of new gauge bosons at futuree + e − colliders

We analyze possible indirect signals of additional neutral gauge bosons at futuree ⁺ e ^? colliders, concentrating onSU(2) _L ×U(1) _y ×U(1) _y , andSU(2) _L ×SU(2) _R ×U(1) effective theories. We develop a simple formalism to describe these effects and make a careful study of radiative corrections, in particular initial state radiation, which will be shown to have important implications. To make realistic estimates of the sensitivity to the new gauge boson effects, we use a model detector fore ⁺ e ^? annihilation at a center of mass energy of 500 GeV. Using a number of selected physical observables we then show that masses considerably higher than the total energy (up to a factor of 6) can be probed and that distinction between various theoretical models is possible.     

Geometry of Real Forms of the Complex Neumann System

In the paper, we study real forms of the complex generic Neumann system. We prove that the real forms are completely integrable Hamiltonian systems. The complex Neumann system is an example of the more general Mumford system. The Mumford system is characterized by the Lax pair (L^?(λ), M^?(λ)) of 2 × 2 matrices, where and U^?(λ), V^?(λ), W^?(λ) are suitable polynomials. The topology of a regular level set of the moment map of a real form is determined by the positions of the roots of the suitable real form of U^?(λ), with respect to the position of the values of suitable parameters of the system. For two families of the real forms of the complex Neumann system, we describe the topology of the regular level set of the moment map. For one of these two families the level sets are noncompact.

In the paper, we also give the formula which provides the relation between two systems of the ?rst integrals in involution of the Neumann system. One of these systems is obtained from the Lax pair of the Mumford type, while the second is obtained from the Lax pair whose matrices are of dimension (n+1) × (n+1).     

Photochemistry and reactions of OH− defects and F centers in Alkali Halides

Additively colored KCl:OH^? crystals show under combined UV and visible irradiation an irreversible destruction of F centers and visible absorption. This process, which is effective over the whole temperature range 4–300 K, was studied systematically in terms of the reaction efficiency and products using UV-visible and IR (local mode) spectroscopy. The UV photo-dissociation of OH^? defects and photo-ionization of F centers produce a combined net effect, in which at higher temperatures all F centers are converted into U centers, thus completely bleaching the colored crystal. By these photo-reactions, high contrast visible images can be produced which are stable under visible light at RT, and are thermally stable up to 650°C. Besides the optical information-storage aspect, these photo-reactions can be used for controlled production of U_A centers if the crystal contains alkali-ion impurities like Na⁺.     

Comparison of electron spin polarization of P 870 + Qa a − observed in Zn2+-containing and Fe2+-depleted bacterial reaction centers

Recently, a general model has been developed to explain electron spin polarized (ESP) electron paramagnetic resonance (EPR) signals found in systems where radical pairs are formed sequentially. The photosynthetic bacterial reaction center (RC) is such a system in which we can experimentally vary parameters (lifetime, structure, and magnetic interactions in the sequentially formed radical pairs) that affect ESP development in order to test this model. In Fe²⁺-depleted transfer step from intermediate radical pair, P ₈₇₀ ⁺ Q _a ^? which is produced in an electron transfer step from intermediate radical pair, P ₈₇₀ ⁺ I^?. (P ₈₇₀ ⁺ is the oxidized primary donor, a special pair of bacteriochlorophyll molecules, I^? is the reduced bacteriopheophytin acceptor, and Q _a ^? is the reduced primary quinone acceptor.) The lifetime of P ₈₇₀ ⁺ I^? can be shortened relative to the lifetime of P ₈₇₀ ⁺ I^? in Fe²⁺-depleted RCs by substitution of Zn²⁺. We report the first observation of X-band and Q-band ESP EPR signals due to P ₈₇₀ ⁺ Q^? from bacterial reaction centers that contain Zn²⁺. Comparison of these signals to those observed from Fe²⁺-depleted bacterial reaction centers shows intensity differences and g-factor shifts. The results are discussed in terms of the general sequential radical pair model.     

Many-body effects in core-level spectroscopy of rare-earth compounds

Many-body effects in core-level photoemission and core-level photoabsorption are discussed for rare-earth systems, especially for Ce and La compounds, both in metallic and insulating forms. Emphasis is put on effects of metallic mixed valency and insulating covalency of 4f electrons on these spectra. For the insulating compound CeO₂, detailed analyses of the 3d core photoemission (3d-XPS) and the 2p core photoabsorption (L₃-XAS) are presented by using the impurity Anderson model with a filled valence band. In order to give a consistent interpretation for 3d-XPS and L₃-XAS, it is shown to be essential to take account of the Coulomb interactions U _fd (between a 4f electron and a photoexcited 5d electron in the L₃-XAS) and -U _dc (between the 5d electron and a core hole), in addition to -U _fc (between the 4f electron and the core hole). Discussions are given on the physical information derived from the analysis, on similarities and differences in spectral features between insulating and metallic systems, and also on some related topics.     

Magnetic field effects on the reaction of the photoexcited triplet of 2-methyl-1,4-naphthoquinone in SDS micellar solution containing the 4-(lauroylamino)-TEMPO radical

A study has ben made of magnetic field effects (MFEs) on the reaction of the photoexcited triplet of 2-methyl-1,4-naphthoquinone (MNQ) in SDS micellar solution containing the 4-lauroylamino-TEMPO radical (L-R^?) under magnetic fields below 1.75 T by a nanosecond laser flash photolysis technique. The triplet MNQ mainly underwent the hydrogen abstraction from an SDS molecule to give a radical pair. The lifetime of the radical pair increased with increasing magnetic field from 0 to 0.62 T. The escaped radical yield also increased from 0 to 1.75 T. The qualitative features of these MFEs were similar to those observed for the photo-reduction of MNQ in SDS micellar solution without L-R^?, and these MFEs can be explained by the relaxation mechanism. However, it was found that L-R^? affected the MFEs for this reaction in two ways: first, L-R^? reacted with the triplet MNQ through H abstraction and/or electron transfer, and second, the spin relaxation of the radical pair was enhanced through the spin-spin interactions of the individual radical with L-R^?.     

Optical properties of Yb3+ ions in fluorophosphate glasses for 1.0 μm solid-state infrared lasers

Yb³⁺-doped fluorophosphate glasses were prepared by melt-quenching technique and characterized their spectroscopic properties to assess the laser performance parameters. The magnitude of absorption (emission) cross-sections at 975 nm for all the studied Yb³⁺-doped glasses is found to be in the range of 0.29–1.50 × 10^?20 (0.59–1.99 × 10^?20 cm²) which is much higher than those of commercial Kigre QX/Yb: 1.06 × 10^?20 (0.5 × 10^?20 cm²) laser glass. The luminescence lifetimes of ²F_5/2 level decrease (1.15–0.45 ms) with increase in Yb₂O₃ concentration (0.1–4.0 mol%). Effect of OH^? content on luminescence properties of Yb³⁺ ions has also been investigated. The effect of radiative trapping has been discussed by using McCumber (McC) and Fuchtbauer–Ladenburge (F–L) methods. The product of experimental lifetimes and emission cross-sections for 0.1 mol% Yb₂O₃-doped glass is found to be 2.28 × 10^?20 cm² ms which indicates that the higher energy storage and extraction capability could be possible. The detailed spectroscopic results suggest that the studied glasses can be considered for high-power and ultrashort pulse laser applications.     

Electron spin polarization in consecutive spin-correlated radical Pairs: Application to short-lived and long-lived precursors in type 1 photosynthetic reaction centres

An analytical treatment of the spin dynamics in sequential photoinduced correlated coupled radical pairs is presented and applied to the spectra of the states P⁺A ₁ ^? and P⁺F _x ^? in type 1 photo-synthetic reaction centres. Expressions for the spin polarized spectra are derived for the specific limiting cases of a very short-lived and very long-lived primary radical pair which correspond to the situation found in heliobacteria and photosystem I (PSI), respectively. The inhomogeneous line-broadening due to the unresolved hyperfine couplings is taken explicitly into account. It is shown that the density matrix of the secondary pair ρ₂ can be written as the sum of two terms corresponding to (i) the part which is independent of the spin dynamics in the precursor, (ii) the additional spin polarization which is generated during the lifetime of the precursor and transferred to the secondary pair. The latter term contains two contributions which arise from the difference of the Zeeman interactions of the radicals in the primary pair and from the inhomogeneous line broadening. The predicted polarization patterns are compared to those established for chemically induced dynamic electron polarization (CIDEP) when uncoupled radicals are generated from a radical pair precursor. The expressions are then used to simulate the experimental spectra of the consecutive pairs P⁺A ₁ ^? and P⁺F _x ^? in PSI using parameters derived entirely from independent experimental data. Excellent agreement with the experimental results is obtained. The spectra of P⁺F _x ^? in heliobacteria at X- and K-band are also simulated and it is shown that the observed polarization patterns can be reproduced assuming direct electron transfer from A₀ to F_x with a time constant ofτ = 600 ps.     

Influence of negative-U centers on the temperature dependence of the carrier concentration in the normal HTSC phase

The temperature dependence of the hole concentration in high-temperature superconductors is considered in terms of the model of negative-U centers. It is shown that this dependence can be nonmonotonic due to the interaction of electron subsystems from the valence band with negative-U centers and the conditions necessary for this have been established. The results obtained are compared with experimental data on the Hall coefficient in cuprates.     

Negative U centers, percolation, and the insulator-metal transition in high-T c superconductors

The mechanism of formation of ?U centers in high-T _c superconductors (HTSCs) is considered. It is shown that the transition from the insulator to the metallic state on doping passes through a certain range of dopant concentrations in which it becomes possible for local transitions of singlet electron pairs to occur from oxygen ions to two neighboring cations (a ?U center), while single-electron transitions are still forbidden. Conduction arises in such systems at a concentration of ?U centers exceeding the percolation threshold for the orbitals of singlet hole pairs. A phase diagram constructed on the basis of the proposed model for the HTSC compounds of the Ln-214 class is in complete agreement with experiment. The mechanisms of formation and relaxation of free hole carriers are considered. It is shown that a distinctive feature of the normal state of HTSCs is the dominant contribution of electron-electron scattering to the charge carrier relaxation processes. It is concluded from the analysis presented that HTSCs comprise a special class of solids in which a nonstandard mechanism of superconductivity, different from the BCS mechanism, is realized.