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Expressions for the stabilization energies of tetrahedral complexes in doped II–VI:3d crystals with a wurtzite structure have been obtained by the example of a CdSe:Cr2+ crystal within the linear problem of the Jahn–Teller effect. It has been shown that the crystallographic anisotropy of a hexagonal lattice leads to an increase in one of the global minima of the lower sheet of the adiabatic potential, which significantly differs from the adiabatic potential of the same complexes in cubic II–VI:3d crystals and affects isothermal elastic moduli and the manifestation of the Jahn–Teller effect in an ultrasonic experiment. [ABSTRACT FROM AUTHOR]

A novel type of sub-lattice of the Jahn-Teller (JT) centers was arranged in Ti-doped barium hexaferrite BaFe12O19. In the un-doped crystal all iron ions, sitting in five different crystallographic positions, are Fe3+ in the high-spin configuration (S = 5/2) and have a non-degenerate ground state. We show that the electron-donor Ti substitution converts the ions to Fe2+ predominantly in tetrahedral coordination, resulting in doubly-degenerate states subject to the E ⊗ e problem of the JT effect. The arranged JT complexes, Fe2+O4, their adiabatic potential energy, non-linear and quantum dynamics, have been studied by means of ultrasound and terahertz-infrared spectroscopies. The JT complexes are sensitive to external stress and applied magnetic field. For that reason, the properties of the doped crystal can be controlled by the amount and state of the JT complexes. [ABSTRACT FROM AUTHOR]

The temperature dependence of the speed of sound in crystalline mercury selenide with low concentrations of iron impurities is studied. Experiments are conducted in the ranges of concentration and temperature where hybridized electronic states in iron impurities have been observed previously. It is found that at temperatures below 10 K the speed of slow transverse ultrasonic waves has an anomalous nonmonotonic segment of its temperature variation that is related to the influence of the impurities and reflects the existence of hybridized states. The observed anomalies in the sound speed are described in terms of a theory for the electron contribution to the elastic moduli that includes hybridization of impurity states and electron-electron interactions. Fits of the theoretical dependences to the experimental data yield quantitative information on the parameters of the hybridized states and of the Fermi-liquid interaction. [ABSTRACT FROM AUTHOR]

Abstract: Generalized mutual entropy is defined for networks and applied in the analysis of complex network structures. The method is tested for the case of computer simulated scale-free networks, random networks, and their mixtures. The possible applications for real network analysis are discussed. [Copyright &y& Elsevier]

We have studied the propagation of transverse ultrasonic waves with a frequency of 52 MHz in an iron-doped mercury selenide (HgSe:Fe) crystal with an impurity concentration of 1019 cm−3, for which the effects of resonant electron scattering on the impurity are most pronounced in the conduction behavior. The temperature dependences of the absorption coefficient α and the phase velocity v of the transverse acoustic waves, which have been studied in the temperature interval T = 1.48-80 K, exhibit anomalies at a temperature of about 5 K: a peak in the α( T) curve for a slow transverse wave and a minimum in the v( T) curve. These anomalies are explained using the theory of interaction between ultrasound and electrons in hybridized states on iron impurity atoms. [ABSTRACT FROM AUTHOR]

A family of matrix solutions of nonlinear wave equations is extended and its application to modelling is given. It is shown that a similarity transformation, induced by the matrix solution, is equivalent to the rotation. Matrix solutions are used for modelling helical motions and vortex rings, simulations rotations and particles collision, mapping contraction and pulsating spheres. Geometrical interpretation of the doubling of rotation angle in each step of sequential mapping contraction is given. [ABSTRACT FROM AUTHOR]

The temperature dependence of the phase velocity and absorption of ultrasound propagating along the [110] axis in a ZnSe crystal doped with Cr2+ ions is investigated in the frequency interval 32–158 MHz. It is found that the absorption peak and anomalies of the phase velocity observed in the temperature interval 10–20 K are of a relaxation nature. The temperature dependence of the relaxation time and the relaxed (isothermal) and unrelaxed (adiabatic) elastic moduli Cl=12(C11+C12+2C44) are recovered from the data. Oscillations of the amplitude and phase of the signal are observed at 32 MHz. An interpretation of this phenomenon is proposed. [ABSTRACT FROM AUTHOR]

Ultrasonic experiments on ZnSe and ZnSe:Ni crystals with impurity concentrations of 5.5×1019 cm-3 have revealed precursor phenomena at temperatures much higher than the temperature of the structural transition (Tc=14.5 K). It is found that the elastic constant C44 softens while C11 and C12 become stiffer as Tc is approached from above; the strains associated with the order parameter are transverse deformations of the [variant_greek_epsilon]4 type. A brief symmetry analysis of the possibilities of formation of a tetragonal phase is given. The Jahn–Teller effect and displacement of the charge density on the chemical bonds are discussed as probable causes of the low-temperature phase transition. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A method for producing quasi-regular ensembles of Co nanocylinders with controllable height in anodic alumina pores on the surface of GaAs structures is improved. Alumina pores are filled by galvanic deposition of cobalt from CoCl 2 solution, which ensures uniform growth of Co in pores. It is established that, even at a small ratio of length of Co nanocylinders to their diameter l/d ≈ 2, the preferred direction of magnetic anisotropy coincides with the axis of nanocylinders. [ABSTRACT FROM AUTHOR]

The article presents a study that evaluates the mileage of motor vehicle tyres affected by mountainous conditions. The study is undertaken through the use of the existing procedures in evaluating the service life of tyres. It also considers the wear intensity on the tires through the tread pattern design. The results of the study indicate that tyre wear depends on the complexity of the service condition particularly in mountainous routes.

The temperature dependence of the absorption and velocity of transverse ultrasonic waves propagating in the [110] direction in a ZnSe:Fe2+ crystal with the iron impurity of n=2.2×1019 cm-3 is investigated in the temperature interval 1.4–100 K at a frequency of 53 MHz. For the slow transverse mode an absorption peak at 7.3 K and a decrease of the phase velocity below 40 K are found, while no such features are observed for the fast mode. The anomalies are interpreted as manifestations of the Jahn–Teller effect. The softening of the elastic modulus (C11–C12)/2 indicates that the local distortions in ZnSe:Fe2+ are tetragonal. Under the assumption that the absorption maximum is of a relaxational nature, the temperature dependence of the relaxation time is recovered from the data, and the values of the potential barrier height and vibronic frequency are obtained. [ABSTRACT FROM AUTHOR]

Coherent forward neutron propagation in gas is discussed as a new approach to search for neutron-antineutron oscillations (n − n ¯), which violate both B and B − L conservation. We show that one can increase the probability of neutron - antineutron transitions in the presence of a nonzero external magnetic field to essentially free neutron oscillation probability by tuning the density of an appropriate mixture of gases so that the neutron optical potential of the gas cancels that from the magnetic field. [ABSTRACT FROM AUTHOR]

A methodology is worked out to retrieve the numerical values of all the main parameters of the six-dimensional adiabatic potential energy surface (APES) of a polyatomic system with a quadratic T-term Jahn-Teller effect (JTE) from the ultrasound experiments. The method is based on a verified assumption that ultrasound attenuation and speed encounter anomalies when the direction of propagation and polarization of its wave of strain coincides with the characteristic directions of symmetry breaking in the JTE. For the SrF2:Cr crystal, employed as a basic example, we observed anomaly peaks in the temperature dependence of attenuation of ultrasound at frequencies of 50-160 MHz in the temperature interval of 40-60K for the wave propagating along the [110] direction, for both the longitudinal and the shear modes, the latter with two polarizations along the [001] and [1̄10] axes, respectively. We show that these anomalies are due to the ultrasound relaxation by the system of non-interacting Cr2+ JT centers with orthorhombic local distortions. The interpretation of the experimental findings is based on the T2g ⊗ (eg + t2g) JTE problem including the linear and the quadratic terms of vibronic interactions in the Hamiltonian and the same-symmetry modes reduced to one interaction mode. Combining the experimental results with a theoretical analysis, we show that on the complicated six-dimensional APES of this system with three tetragonal, four trigonal, and six orthorhombic extrema points, the latter are global minima, while the former are saddle points, and we estimate numerically all the main parameters of this surface, including the linear and quadratic vibronic coupling constants, the primary force constants, the coordinates of all the extrema points and their energies, the energy barrier between the orthorhombic minima, and the tunneling splitting of the ground vibrational states. To our knowledge, such a based-on-experimental-data numerical reconstruction of the APES of a JTE problem in the five-dimensional space of all active tetragonal and trigonal displacements has not been reported before. [ABSTRACT FROM AUTHOR]

An observation of neutron-antineutron oscillations (n-n), which violate both B and B-L conservation, would constitute a scientific discovery of fundamental importance to physics and cosmology. A stringent upper bound on its transition rate would make an important contribution to our understanding of the baryon asymmetry of the Universe by eliminating the postsphaleron baryogenesis scenario in the light quark sector. We show that one can design an experiment using slow neutrons that in principle can reach the required sensitivity of τn-n∼1010 s in the oscillation time, an improvement of ~104 in the oscillation probability relative to the existing limit for free neutrons. The improved statistical accuracy needed to reach this sensitivity can be achieved by allowing both the neutron and antineutron components of the developing superposition state to coherently reflect from mirrors. We present a quantitative analysis of this scenario and show that, for sufficiently small transverse momenta of n/n and for certain choices of nuclei for the n/n guide material, the relative phase shift of the n and n components upon reflection and the n annihilation rate can be small enough to maintain sufficient coherence to benefit from the greater phase space acceptance the mirror provides. [ABSTRACT FROM AUTHOR]

Transition-metal-doped semiconductor GaAs crystals are used as model systems for spintronic research, as well as in other applications. To explore the structure and properties of such impurities, we extended the methodology of ultrasonic investigation of the Jahn-Teller effect in dielectric impurity-centers to the study of significantly different semiconductor impurities using the GaAs:Cu crystal as an example. Phase velocity and attenuation of ultrasound in this system were measured in the temperature interval of 1.9-80 K at 52 MHz and 156 MHz. The anomaly in the velocity and a peak of attenuation found for the longitudinal and slow shear waves indicate the presence of the Jahn-Teller effect with the e-type local distortions of the CuGa4As impurity complex. The temperature dependence of the elastic modulus and relaxation time shows that below 5 K, the thermal activation mechanism of relaxation is possibly replaced by resonance type transitions. The main parameters of the Jahn-Teller effect, stabilization energy, minima positions and the barrier between them, frequency of pseudorotation of the distortions, and the tunneling splitting of the ground state energy, as well as the constant of exchange interaction between the two holes in Cu2+ centers and the concentration of the centers were estimated. [ABSTRACT FROM AUTHOR]

A method is developed to determine the symmetry properties of strains and the type of Jahn–Teller effect in crystals with impurity ions in a triply degenerate electronic T state. This method is based on a calculation of the isothermal contribution of the impurity subsystem to the elastic moduli of a crystal and the absorption and velocity of normal modes for all three possible problems, namely, T ⊗ e, T ⊗ t2, and T ⊗ (e + t2). The calculation results are compared with experimental data. The efficiency of the method is demonstrated for a CdSe:Cr2+ crystal. The CrSe4 center is found to be described in terms of the problem T ⊗ e. The parameters of the ground-state adiabatic potential are determined. [ABSTRACT FROM AUTHOR]

The influence of an external magnetic field on the complex elastic moduli of crystals with a sphalerite or zinc blende (ZnSe) and wurtzite (CdSe) structure lightly doped by Cr2+ ions has been investigated. Measurements have been performed in the frequency interval 26–32 MHz at 1.4 K. In II–VI crystals, bivalent chromium cations are triply orbital-degenerate in the ground state and, being in a tetrahedral environment, produce Jahn–Teller complexes. These complexes are described in terms of the T (e + t2)-problem and exhibit the adiabatic potential energy surface with the global minima of tetragonal symmetry. It has been found that in ZnSe:Cr2+ crystals, a magnetic field directed along the [001] and [110] axes influences modulus (c11 – c12)/2 and does not influence modulus c44. In CdSe:Cr2+ crystals, however, moduli c55 and c66, which are the analogs of (c11 – c12)/2 and c44, depend on the magnetic field directed along the [ ] and [ ] axes, respectively. The discovered anomalous behavior of the elastic moduli with magnetic field has been treated in terms of a model that takes into account the crystal field, vibronic and spin-orbital interactions, and the contribution of the Jahn–Teller subsystem to isothermal moduli determined at a constant magnetic induction. Good agreement with experimental dependences of the elastic moduli in strong magnetic fields has been obtained, and it has been shown that the nonmonotonic variation in weak magnetic fields (below 2 T) should be associated with a magnetic field dependence of the relaxation time. [ABSTRACT FROM AUTHOR]

We present the results of our ultrasonic studies in a fluorite (CaF2) single crystal with isovalent substitution of Ca2+ ions by copper ions performed at frequencies 23–163 MHz in the temperature range 4–240 K. Anomalies typical for the relaxational contributions of systems of Jahn–Teller complexes have been found in the temperature dependences of the attenuation and velocity of the longitudinal and transverse normal modes associated with the elastic moduli cL = c44 + (c11 + c12)/2 and cE = (c11 – c12)/2 at temperatures below 80 K. The Cu2+ ions in a cubic environment in the ground state possess threefold orbital degeneracy, are described by the T (e + t2) Jahn–Teller effect problem, and have an adiabatic potential energy surface specified in the five-dimensional space of trigonal and tetragonal symmetrized coordinates. An analysis of the experimental data has allowed us to determine the symmetry properties of the extrema (global minima and saddle points) of the lowest adiabatic potential energy sheet and to find the linear (tetragonal and trigonal) and quadratic vibronic coupling constants of the Jahn–Teller Cu2+ complexes in the CaF2 matrix appearing in the vibronic Hamiltonian, the Jahn–Teller stabilization energies, and the coordinates of these extrema in the five-dimensional space of normal coordinates. [ABSTRACT FROM AUTHOR]

We show that the structure, properties, and concentration of vacancies in crystals can be studied by ultrasonic experiments previously employed for impurity centres only. Measurements of the temperature dependence of attenuation and phase velocities of ultrasonic shear waves of 52 MHz propagating along the crystallographic axis [110] of nominally pure ZnSe single crystals (grown by the seeded physical vapour transport method) show strong anomalies which are typical for relaxation processes in system with isolated Jahn-Teller (JT) centres. The observed JT distortion mode is trigonal, subject to a threefold orbitally degenerate T-term interaction with trigonal and tetragonal nuclear displacements. In the absence of sufficiently high concentrations of impurity atoms with such properties we attributed the observed JT centres to zinc vacancies. The temperature dependence of the isothermal and adiabatic forms of the appropriate elastic modulus and the relaxation time show that the relaxation mechanism changes from thermal activation at higher temperatures to tunnelling through a potential energy barrier below 18 K. We provide an estimate of the magnitude of the potential barrier, as well as the pseudorotation frequency and concentration of vacancies. Also we determine the extremum points of the adiabatic potential energy surface of the vacancy centre. [ABSTRACT FROM AUTHOR]

New research results on phenomena due to the existence of electronic resonance energy levels and hybridized states at impurities of transition elements in semiconductors are presented. The data show that the thermal conductivity and ultrasonic parameters of mercury selenide containing iron impurities have resonance anomalies due to the influence of these impurities. A consistent and detailed interpretation is offered for the set of observed effects of hybridized states in mercury selenide with iron impurities. The proposed interpretation of the data obtained on other systems is discussed. [ABSTRACT FROM AUTHOR]

Radiative corrections to the lifetime and angular correlation coefficients of neutron beta-decay are evaluated in effective field theory. We also evaluate the lowest order nucleon recoil corrections, including weak-magnetism. Our results agree with those of the long-range and model-independent part of previous calculations. In an effective theory the model-dependent radiative corrections are replaced by well-defined low-energy constants. The effective field theory allows a systematic evaluation of higher order corrections to our results to the extent that the relevant low-energy constants are known. [Copyright &y& Elsevier]

The self-consistent vibrating potential model (VPM) is extended for the description of Eλ surface collective excitations in alkali metal clusters with practically any kind of static deformation. The case of spherical clusters is also covered. Any single-particle potentials and valence electron densities for which the coefficients of the multipole expansion are known can be used within the model. The strength function method incorporated into the model allows one to avoid solving the equations for every state and, as a result, simplifies the calculations drastically. The model is of a quite general character and can also be used for description of giant resonances in atomic nuclei if the Coulomb terms are neglected. The VPM is applied to calculate the E1, E2 and E3 excitations in spherical (Na, Na and Na) and deformed (Na, Na and Na) clusters. [ABSTRACT FROM AUTHOR]

The results of the study of the temperature dependence of attenuation and velocity of ultrasonic waves are presented for CaF2:Cr and CaF2:Ni crystals in which Cr2+ and Ni2+ ions substitute calcium ions, forming Jahn–Teller (JT) complexes [CrF8]6– and [NiF8]6–. The Cr2+ and Ni2+ ions in the fluorite structure have threefold orbital degeneracy in the ground state, are described by the T (e + t2) Jahn–Teller effect (JTE) problem, and have an adiabatic potential energy surface (APES) defined in the five-dimensional space of trigonal and tetragonal symmetrized coordinates whose lower sheet is a simply connected surface with orthorhombic minima separated by potential energy barriers. The temperature dependence of the ultrasonic attenuation and velocity exhibits anomalies typical of the relaxation contributions of systems of JT complexes in the low-temperature region. The effect of tunneling relaxation mechanisms (direct and two-phonon transitions) on the JT contribution to the complex elastic moduli and to the error in determining the relaxation time is analyzed. Based on the approach that takes into account both tunneling and activation mechanisms of relaxation, the activation energies and the constants characterizing these relaxation mechanisms are determined in CaF2:Cr2+ and CaF2:Ni2+ crystals. [ABSTRACT FROM AUTHOR]

The temperature dependences of attenuation and the velocity of ultrasonic waves at frequencies of 26–158 MHz in a CaF2 fluorite crystal at the substitution of Jahn–Teller Cr2+ centers for calcium ions have been studied. An abnormally high relaxation rate, which is two orders of magnitude higher than the relaxation rate in other previously studied CaF2:Ni2+ and SrF2:Cr2+ fluorites, has been found in the system of Jahn–Teller complexes in the low-temperature region. It has been shown that the global minima of the adiabatic potential energy surface of the Cr2+ complexes in the CaF2:Cr2+ crystal also have orthorhombic symmetry but are separated by significantly lower potential energy barriers than in CaF2:Ni2+ and SrF2:Cr2+ crystals. It has been found that tunneling relaxation mechanisms (direct and two-phonon transitions) are dominant, rather than thermal activation, in CaF2:Cr2+ in the temperature range where the Jahn–Teller effect is manifested in an ultrasonic experiment. The parameters characterizing these relaxation mechanisms have been determined. [ABSTRACT FROM AUTHOR]

For silicon carbide, the problem of obtaining low-resistance ohmic contacts with improved service characteristics remains topical despite a significant body of experimental data. The influence exerted by the composition of Ni and TiAl metallization, alloying parameters, additional doping of the contact layer of silicon carbide (SiC) with N+ nitrogen ions, and the crystallographic Si- or C-face on the resistance of ohmic contacts to n-6H-SiC is examined. It is found that the greatest influence on how ohmic contacts to n-6H-SiC are formed is exerted by the alloying process resulting in a decrease in the contact resistance by approximately six times. The process of additional doping with N+ also reduces the contact resistance by nearly a factor of four. It is found that low-resistance contacts can be obtained on both faces with approximately the same low resistance. TiAl metallization is optimal for the C-face, and Ni metallization, for the Si-face. This choice of metallization makes it possible to obtain ohmic contacts on both polar faces with approximately the same resistances on the order of 2.5 × 10–4 Ω cm2. [ABSTRACT FROM AUTHOR]

We report the effect of magnetic field induced quantum tunneling and relaxation transitions between orthogonal configurations in polyatomic systems where no tunneling is expected. Typical situations of this kind occur in molecular systems and local centers in crystals in ground and excited electronic T states, subject to the T ⊗ e problem of the Jahn-Teller effect, where the wave functions of the three tetragonally distorted configurations are orthogonal. A detailed microscopic theory of this effect shows how the magnetic field violates the orthogonality between the latter allowing for tunneling and relaxations, which decrease in strong fields due to the induced decoherence. The novel effect is demonstrated experimentally as a big, sharp peak in ultrasound attenuation by Cr2+ centers in ZnSe: Cr2+ in the magnetic field B = 0.15 T at the temperature below 8 K. It may influence a variety of magnetic, electronic, and photonic properties of any system in an electronic T state. [ABSTRACT FROM AUTHOR]

The interaction of ultrasound with Cu4As in a GaAs:Cu crystal has been experimentally studied. The temperature dependences of the attenuation of all normal ultrasonic modes propagating in the 〈110〉 direction both in doped copper and in nominally pure gallium arsenide crystals have been measured. In the GaAs:Cu crystal, the attenuation peak has been revealed for a transverse wave polarized along the 〈110〉 axis whose elastic shifts correspond to the symmetry of the tetragonal mode of the Jahn-Teller effect. The temperature dependence of the attenuation of this wave indicates that two types of attenuation-relaxation and resonance-occur. The constructed temperature dependence of the relaxation time indicates that tunneling through the potential barrier between the minima of the adiabatic potential energy is the main relaxation mechanism at temperatures below 10 K. Tunneling splitting estimated from experimental data is in good agreement with the theoretical estimate. [ABSTRACT FROM AUTHOR]

Abstract: The Nab collaboration will perform a precise measurement of a, the electron–neutrino correlation parameter, and b, the Fierz interference term in neutron beta decay, in the Fundamental Neutron Physics Beamline at the SNS, using a novel electric/magnetic field spectrometer and detector design. The experiment is aiming at the accuracy level in , and will provide an independent measurement of , the ratio of axial-vector to vector coupling constants of the nucleon. Nab also plans to perform the first ever measurement of b in neutron decay, which will provide an independent limit on the tensor weak coupling. [Copyright &y& Elsevier]

Results are presented from experimental and theoretical studies of the heating of a hydrogen plasma with a lithium admixture at the fundamental ion-cyclotron frequency of hydrogen in the T-11M tokamak. It is found experimentally that the action of RF radiation on a hydrogen plasma containing a small amount (less than 4%) of lithium increases the duration of the discharge current pulse. The effect of the increase in the discharge current pulse under the action of RF radiation is simulated numerically. [ABSTRACT FROM AUTHOR]

Results are presented from investigations of the possibility of heating a hydrogen plasma at the fundamental harmonic of the ion cyclotron frequency in the T-11M tokamak. The fluxes of charge-exchange atoms that escape from the plasma in the radial direction and across the toroidal magnetic field (transverse neutrals) were recorded by a Lakmus neutral particle analyzer. Measurements by the analyzer show that, during an RF pulse, the ion temperature increases by approximately 50–100 eV. Such plasma parameters as the ion temperature, rotation velocity, and isotopic composition were measured by a high-resolution spectrometer. According to the data from high-resolution spectroscopy, the ion temperature increases by approximately 150 eV. Results from numerical simulations of the ion cyclotron resonance heating of a hydrogen plasma in the T-11M tokamak are also given. [ABSTRACT FROM AUTHOR]

In interpreting the SNO experiments, accurate estimates of the νd reaction cross sections are of great importance. We improve the previous estimates of our group by updating some of its inputs and by taking into account the results of a recent effective-field-theoretical calculation. The new cross sections are slightly (∼1%) larger than the previously reported values. It is shown to be reasonable to assign 1% uncertainty to the νd cross sections reported here; this error estimate does not include radiative corrections, for which we refer to the literature. [Copyright &y& Elsevier]