Resonant dislocation motion in NaCl crystals in the EPR scheme in the Earth’s magnetic field with pulsed pumping

Resonant dislocation motions in NaCl(Ca) crystals under the simultaneous action of the Earth’s magnetic field B _Earth (～66 μT) and a pulsed pump field $\tilde B$ of sufficient amplitude $\tilde B_m $ and certain duration τ have been detected and studied. The measured dislocation path peaks l(τ) have a maximum at τ = τ _r ≈ 0.53 μs. The resonance criterion has been found to be the ordinary EPR condition in which the g-factor is close to 2 and the optimum inverse pulse duration τ _r ^?1 is used instead of the harmonic pump field frequency ν _r . The largest peak l(τ) height is reached at mutually orthogonal dislocation (L) and magnetic field (B _Earth and $\tilde B$ ) orientations. Pulsed field rotation to the position $\tilde B$ ‖ B _Earth significantly decreases but does not “kill” the effect. For dislocations parallel to the Earth’s field (L ‖ B _Earth), the resonance almost disappears even at $\tilde B$ ⊥ B _Earth. In the optimum geometry of experiments, as the pump field amplitude $\tilde B_m $ decreases from 17.6 to 10 μT, the path peak height l _r = l(τ _r ) decreases only by 7.5%, remaining at the level of l _r ～ 10² μm, and at a $\tilde B_m $ further fall-off to 4 μT, it rapidly decreases to background values. In this case, the relative density of mobile dislocations similarly decreases from ～90 to 40%. Possible physical mechanisms of the observed effect have been discussed.     

Study of excited states in49V with the48Ti (p, γ)49V reaction

In the⁴⁸Ti(p, γ)⁴⁹V reaction gamma decays of thirteen resonances betweenE _p =960 and 1570 keV are investigated. Level energies within ±0.5–±2.0 keV andQ-value 6756.8±1.5 keV are obtained. Branching ratios for the resonance states and strongly populated bound states are given. Gamma-ray angular distribution measurements yield the followingJ(keV) assignments of⁴⁹V bound and resonance states:J(1140)=5/2,J ^π(2235)=5/2^(?),J(2264)=(3/2),J(2308)=3/2,J(3912)=3/2,J(8105,Ep=1374keV)=(1/2) andJ ^π(8289,E _p =1564keV)=3/2^(?). Multipolarity mixing ratios for all measured primary and secondary gamma rays are tabulated. Dopplershift attenuation measurements yield the mean lifetimes τ _m (keV) of the following bound states in⁴⁹V:τ _m (748)=(200± ₁₀₀ ⁴⁰⁰ )fs, τ _m (1140)=(250± ₁₀₀ ⁵⁰⁰ )fs, τ _m (1155)>400 fs, τ _m (1515)=(45± ₂₀ ³⁰ )fs, τ _m (1644)=(55± ₂₀ ³⁰ )fs, τ _m (1661)=(25±5)fs, τ _m (1994)>400 fs, τ _m (2235)=(30± ₁₅ ³⁰ )fs, τ _m (2264)=(45± ₁₅ ³⁰ )fs and τ _m (2308)=(20±10)fs.     

Mass corrections to the tau decay rate

In this note radiative corrections to the total hadronic decay rate of the τ-lepton are studied employing perturbative QCD and the operator product expansion. We calculate quadratic quark mass corrections to the decay rate rationR _τ to the order0(α _s ² m²) and find that they contribute appreciably to the Cabbibo supressed decay modes of the τ-lepton. We also discuss corrections of mass dimensionD=4, where we emphasize the need of a suitable choice of the renormalization scale of the quark and gluon condensates.     

The size effects upon shock plastic compression of nanocrystals

For the first time a theoretical analysis of scale effects upon the shock plastic compression of nanocrystals is implemented in the context of a dislocation kinetic approach based on the equations and relationships of dislocation kinetics. The yield point of crystals τ_y is established as a quantitative function of their cross-section size D and the rate of shock deformation as τ_y ~ ε^2/3 D. This dependence is valid in the case of elastic stress relaxation on account of emission of dislocations from single-pole Frank–Read sources near the crystal surface.     

On the relation between thermally activated and magnetically stimulated processes during dislocation movement in InSb crystals in a magnetic field

It is shown that, in contrast to conventional mobility of dislocations in InSb crystals, which is characterized by a thermally activated temperature dependence of velocity (v ∝ exp(-U/kT), where U is the activation energy), relaxation displacements of dislocation in the same crystals in a magnetic field in the absence of external load are described by a more complex temperature dependence. The V(T) dependence in the temperature range 120–250°C studied here exhibits a tendency to linearization in the ln v vs. 1/T coordinates only in its low-temperature part and rapidly attains saturation upon an increase in temperature. The observed decrease in the thermal sensitivity of relaxation mobility of dislocations in the magnetic field upon heating is interpreted in the framework of the model describing the detachment of a dislocation from a point defect as a sequence of two processes: (i) magnetically stimulated lowering of the barrier, U → U′ (over time τ_sp of the spin evolution in the system) and (ii) expectation of thermal fluctuation (over a time τ_th ∝ exp(U′/kT)). Thus, at low temperatures, we have , and the total time before detachment amounts to τ_th + τ_sp ≈ τ_th. On the contrary, at high temperatures, we have , and τ_th + τ_sp ≈ τ_sp (i.e., the motion becomes athermal). It is shown that this model correctly describes the results of measurements and makes it possible to separate the effects. In particular, it is found that the barrier height decreases from the activation energy U = 0.8 eV under a load of 10 MPa to U′ = 0.25 eV in a magnetic field of B = 0.8 T.     

Angular correlations and the tau-neutrino mass

Effects of a nonzero mass for the tau neutrinov _τ as well as a right-handed charged-current contribution to the τ?v _τ coupling are discussed. Angular correlations of the decay products of τ⁺τ^? ine ⁺ e ^? annihilation are calculated as functions of \(m_{v_\tau } \) and the relative amount of right-handed τ?v _τ coupling.     

Forming abilities of monatomic chains of several fcc and bcc metals in different crystallographic orientations

The forming abilities of monatomic chains (MC) of several fcc and bcc metals stretched in three principal crystallographic orientations of [1 1 1], [1 0 0] and [1 1 0] are analyzed in terms of a ratio between Peierls stress of a bulk crystal with dislocations (τ_p) and theoretical shear stress of a monatomic chain (τ_m). It is found that the structure and orientation dependent τ_m/τ_p values are proportional to the forming abilities of MC while τ_m/τ_p is a function of Possion's ratio ν. The above considerations are in agreement with known experimental and simulation results of Au. In addition, Nb as a candidate for MC formation is suggested.     

Crossing Speeds of Random Walks Among “Sparse” or “Spiky” Bernoulli Potentials on Integers

We consider a random walk among i.i.d. obstacles on $\mathbb {Z}$ under the condition that the walk starts from the origin and reaches a remote location y. The obstacles are represented by a killing potential, which takes value M>0 with probability p and value 0 with probability 1?p, 0<p≤1, independently at each site of $\mathbb {Z}$ . We consider the walk under both quenched and annealed measures. It is known that under either measure the crossing time from 0 to y of such walk, τ _y , grows linearly in y. More precisely, the expectation of τ _y /y converges to a limit as y→∞. The reciprocal of this limit is called the asymptotic speed of the conditioned walk. We study the behavior of the asymptotic speed in two regimes: (1) as p→0 for M fixed (“sparse”), and (2) as M→∞ for p fixed (“spiky”). We observe and quantify a dramatic difference between the quenched and annealed settings.     

Self-similar distribution in “giant” magnetic flux creep

The problem of magnetic field penetration into the half-space is considered in parallel geometry in an external magnetic field increasing with time in accordance with the law B(0, t, τ₀ = B _{c 1} (1 + t/τ₀) ^m , m ≥ 0, t ≥ 0 (τ ₀ is the time of magnetic flux redistribution and B _{c 1} is the lower critical field). It is assumed that the flow of vortices is thermally activated in the “giant” creep mode (i.e., for weak pinning creep and high temperatures). A model equation is derived for describing the magnetic flux evolution. Analytic formulas are obtained for the depth and velocity of magnetic field penetration. It is shown that the giant creep regime is stable for 0 ≤ m ≤ 1/2.     

Strongly positive semigroups and faithful invariant states

Let (?, τ, ω) denote aW*-algebra ?, a semigroupt>0?τ _t of linear maps of ? into ?, and a faithful τ-invariant normal state ω over ?. We assume that τ is strongly positive in the sense that $$\tau _t (A^ * A) \geqq \tau _t (A)^ * \tau _t (A)$$ for allA∈? andt>0. Therefore one can define a contraction semigroupT on ?= \(\overline {\mathcal{M}\Omega } \) by $$T_t A\Omega = \tau _t (A)\Omega ,{\rm A} \in \mathcal{M},$$ where Ω is the cyclic and separating vector associated with ω. We prove 1. the fixed points ?(τ) of τ are given by ?(τ)=?∩T′=?∩E′, whereE is the orthogonal projection onto the subspace ofT-invariant vectors, 2. the state ω has a unique decomposition into τ-ergodic states if, and only if, ?(τ) or {?υE}′ is abelian or, equivalently, if (?, τ, ω) is ?-abelian, 3. the state ω is τ-ergodic if, and only if, ?υE is irreducible or if $$\mathop {\inf }\limits_{\omega '' \in Co\omega 'o\tau } \left\| {\omega '' - \omega '} \right\| = 0$$ for all normal states ω′ where Coω′°τ denotes the convex hull of {ω′°τ _t } _t>0. Subsequently we assume that τ is 2-positive,T is normal, andT* _t ?₊Ω \( \subseteqq \overline {\mathcal{M}_ + \Omega } \) , and then prove 4. there exists a strongly positive semigroup |τ| which commutes with τ and is determined by $$\left| \tau \right|_t \left( A \right)\Omega = \left| {T_t } \right|A\Omega ,$$ 5. results similar to 1 and 2 apply to |τ| but the τ-invariant state ω is |τ|-ergodic if, and only if, $$\mathop {\lim }\limits_{t \to \infty } \left\| {\omega 'o\tau _t - \omega } \right\| = 0$$ for all normal states ω′.     

AnSO(10) model withSU(3)⊗SU(2) L ⊗SU(2) R ⊗U(1) B-L ⊗D intermediate symmetry

We construct aSO(10) model with intermediate symmetry $$SU(3) \otimes SU(2)_L \otimes SU(2)_R \otimes U(1)_{B - L} \otimes D$$ and baryon and lepton number conserved between the two highest scales. The experimental values of sin²θ _w and α _s are consistent with the lower bound on τp→e ⁺+π⁰ and with a mass value forv _τ around 1 eV.     

Electrodynamic Model of Initiation and Extinction of Emitting Sites Interacting Together in Cold Cathode Spots (Type A) in Air

From an electrodynamic and simple quantum-mechanical point of view a model is proposed which explains the phenomena of minimum arc current as well as the formation and extinction of tiny emitting sites interacting together in cold cathode spots (called type A) on the base of a specific coupling between the tunnelling “average” electrons and the metal bulk phonon field. The model seems to be especially applicable to such experimental conditions where typical trumpetlike microcraters with pronounced rims with diameters in the range 0.5—1 μm are left by microspot ensembles on the cathode surface. The model yields emitting-site lifetimes, currents, current densities and radii in the order of τ_p^s ? \documentclass{article}\pagestyle{empty}\begin{document}$ \sqrt {3M/m} $\end{document} τ₀ ? 10^?11 sec, I_min = 4π ? \documentclass{article}\pagestyle{empty}\begin{document}$ \sqrt {n/\mu _0 m} $\end{document}? 0.4 A, j = nev_s ? 4 · 10¹³ A/m² and r_a ? 2c/ω_Pl ? 30 nm (τ_p^s…lifetime of short wave phonons, M … atom mass, m … electron mass, τ₀ … mean free collision time of Fermi electrons at room temperature, n … conduction electron density in the metal bulk, v_s … metal bulk sound velocity, c … light velocity, ω_Pl … metal bulk plasma frequency (values for copper). The lifetime and the interaction diameter of an emitting site (event) ensemble are derived to τ_p^l ?(M/m) τ_p ? 3 nsec and Λ_p^l = ν_sτ_p^l ? 10 μ (τ_pΛ_p^l … lifetime and mean free path of long wave phonons).     

Scattering of electrons due to screw dislocations

The phase dismatching effect on the scattering due to screw dislocations is reformulated to take the discreteness of lattice sites into account. Thet-matrix for an electron scattered from the statep top′ is $$\begin{gathered} t\left( {p,p'} \right) = ip_z T\exp \left\{ {i\left( {p - p'} \right) \cdot m_A } \right\}\exp \left\{ {i\left( {p - p'} \right) \cdot \left( {i + j} \right)/2} \right\} \hfill \\ \cdot \frac{{\left[ {\exp \left( { - ip_y } \right) - \exp \left( {ip'_y } \right)} \right] + \left( {\upsilon _y /\upsilon _x } \right)\left[ {\exp \left( {ip_x } \right) - \exp \left( { - ip'_x } \right)} \right]}}{{1 - \exp \left[ {i\left\{ {\left( {p_x - p'_x } \right) + \left( {\upsilon _y /\upsilon _x } \right)\left( {p_y - p'_y } \right)} \right\}} \right]}} \hfill \\ \end{gathered}$$ for 0≦v _y ≦v _x ≦1 and |p _y |, |p′ _y |?1. Here,v is the group velocity of the incident electron andm _A is the position of the dislocation axis. All vector notations represent vectors in two-dimensional space, the unit vectors of which are represented byi andj. Expressions for |p _y |, |p′ _y |?π and other values ofv are obtained through simple modifications. As an application, the resistivity due to screw dislocations is discussed qualitatively.     

Determination of the positions of impurity centers in a dislocation core in a NaCl crystal from magnetoplasticity spectra

The spectra of the mean free paths l(ν) of edge dislocations have been studied in NaCl crystals exposed in the electron paramagnetic resonance scheme to the crossed magnetic fields: the Earth’s field (50 μT) and the pump field (2.5 μT, 5–440 kHz). The spectra have been measured for a series of angles θ = 0°–5° of rotation of the sample around its edge [100] with respect to the Earth’s field. The fine structure of the spectra contains a series of peaks whose resonance frequencies are described by the empirical expression v _i ^± = Asin(θ ± Δθ _i ) ≈ A(θ ± Δθ _i ). The parameters Δθ _i are independent of the angle θ within the experimental errors. Within the model of “frozen” magnetic moments associated with impurity center Ca⁺-Cl⁰, the angles Δθ _i characterize the deviation of the axis of the center from the 〈100〉 direction in the core of a dislocation. These angles can be expressed in terms of the spectra obtained: Δθ _i = (? _i ⁺ ? v _i ^? )/2A. The computer simulation of the edge dislocation core provides the set of the angles Δθ _i close to the measured values. The spin-lattice relaxation time of the center on dislocation has been estimated from the low-frequency edge of the spectrum l(ν) as τ _{s ? l} ～ 10^?4 s.     

Dislocation behaviour during anelastic strain recovery in zinc single crystals prestrained under basal glide

Anelastic strain recovery experiments of zinc single crystals strained under basal glide were performed as a function of the forest pyramidal dislocation density at room temperature. The instantaneous strain recovery of specimens unloaded after prestraining in compression is proportional to lγ_p^1.8 where l is the average spacing between forest dislocations and γ_p is the prestrain. It is suggested that the instantaneous strain recovery results from the back motion of basal dislocations which multiplied under prestraining. The basal dislocations move to back under the influence of the internal stress unless they are impeded by intersection with forest dislocations. The time dependent strain recovery was measured on all specimens in either the unloaded or partially unloaded states, which enabled the activation volume of the process to be determined. The time dependent strain recovery is explainable in terms of the thermally activated back motion of basal dislocations via cutting the forest dislocations.     

Search for CP violation in Z0 → τ + τ − and an upper limit on the weak dipole moment of the τ lepton

An improved test of invariance in the reaction e⁺e^? τ ⁺ τ ^? on the Z0 peak is performed using the data sample recorded between 1991 and 1995 with the OPAL detector at LEP. Optimal observables, requiring the reconstruction of the τ flight direction and spin, have been used for different final state topologies. From the non-observation of violation we derive 95% confidence level upper limits on the real and imaginary parts of the weak dipole moment of the τ lepton of |Re(d _τ ^w (m _Z ² | < 5.6 × 10^?18 e cm and |Im(d _τ ^w (m _Z ² ))| < 1.5 × 10^?17 e cm, respectively.     

Influence of the shape of the exciting laser pulse on fluorescence saturation in the quantitative analysis of dissolved trace organic substances

Theoretical investigations of the influence of the exciting laser pulse shape on fluorescence saturation in the quantitative analysis of a single-component fluorescing solution, when the Raman signal of the solvent is usedas a reference, are carried out. A quantity α is introduced, responsible for the influence of laser pulse shape on the precision of the analysis. The dependences of α on the exciting radiation photon flux density I_m in the range I_m = 10²² ? 10²⁸ photons/cm²?s and on the duration of the exciting pulses τ_p in the range τ_p = 0.6 ? 60 ns are computed for different pulse shapes in the case of rhodamine 6G dissolved in water. Conditions when α is, in practice, independent of τ_p and of the pulse shape are found. A general analytical expression is found for α, which produces errors not greater than several percent when compared to the computed values of α. An experimental verification of the theoretical results in the case of a rectangular-shaped pulse in the spatial domain and a Gaussian-shaped pulse in the temporal domain is realized.     

Controlling the width of picosecond laser pulses

The relaxation time τ_R of the saturable dye used to mode-lock a Nd: YAG laser has been changed using different dyes or dye solvent mixtures and the laser bandwidth Δω changed by the insertion of an etalon. The pulse duration τ_p was approximately transform-limited for τ_R<2π/Δω but increased to about twice this value when 2π/Δω<τ_p<τ_R. No significant increase in pulse duration was observed for τ_R?2π/Δω but multiple pulses were generated within each round-trip-transit-time.     

Starting stress for the onset of dislocation motion in ruby single crystals

For the purpose of preparing ruby single crystals with a high degree of structural perfection, the evolution of their structural state is investigated using four-point loading at T=1490°C and chemical etching pits. The starting stress τ_st for the onset of the dislocation motion is measured. The regularities of the arrangement of dislocations in single-crystal samples of ruby are established. The starting stress for the onset of the dislocation motion is found to be τ_st=2 MPa. The results obtained make it possible to determine the optimum parameters of thermomechanical treatment of the single crystals and, in the future, to solve the problem of their longterm serviceability.