Z-pinch dynamics under the combined breakdown in bismuth-antimony alloys

Time scans of the electric field in n-Bi_0.9Sb_0.1 samples are calculated under the impact ionization for various field orientations with respect to the crystal axes (the conditions of current given). The strong anisotropy of the pinch characteristics in the electron-hole plasma of such alloys is shown to be caused by the joint effect of the hole mobility anisotropy in the T-valley and the combined breakdown, the impact ionization being produced both by the external electric field and the Hall field. The magnitude of the latter is governed by the proper magnetic field of the current. The results explain the experimental data of Brandt et al. [1] who have observed the dynamical characteristics of the pinch in the above mentioned alloys to be strongly anisotropic.     

The effect of an external electric field on the optical properties of a quantum-dot molecule with a resonant state of the D 2 − center

The effect of an external electric field on the probability of the electron radiative transition from a resonant u-state to the localized g-state of the D ₂ ^? center in the presence of dissipative tunneling has been investigated in the model of zero-radius potential. It is shown that the probability of radiative transition increases by about two orders of magnitude in the case of the external electric field intensity for which the initially asymmetric double-well oscillator potential simulating a quantum-dot molecule becomes symmetrical.     

The nature of anomalies in the magnetoresistance of antiferromagnetic YBa2Cu3O6+x

The characteristic d-type angular dependence of the in-plane magnetoresistance on the orientation of the external magnetic field in the (ab) plane for the antiferromagnetic tetragonal crystal YBa₂Cu₃O_6+x (x ～ 0.3) is considered theoretically. This dependence is interpreted in terms of the efficient hole transfer through the low-lying purely oxygen O 2pe _u doublet, which is not hybridized with the $b_{1g} (d_{{}_x2_ - {}_y2} )$ ground state. The external magnetic field determines the orientation of the strong exchange field acting on the b _1g e _u : ³ E _u triplet state of the hole-type CuO₄ center. The spin-orbit interaction results in orbital polarization of the E _u doublet, which is responsible for the d-type spatial anisotropy of the hole transport. The available experimental data make it possible to evaluate the parameter of the effective spin Hamiltonian. The influence of spin-vibronic effects on the hole transfer is analyzed.     

Electron spin resonance in the photo-excited triplet state of free base porphin in a single crystal of n-octane

E.S.R. experiments have been performed on the lowest triplet state of free base porphin (H₂P) in a n-octane single crystal at 1·3 K. The results demonstrate that the large majority of guest molecules occur in two orientations. While the molecules in these two orientations are coplanar, they have their N-H H-N axes at right angles. The fine structure results show that the two molecular orientations have zero-field splittings that differ by a few per cent in magnitude. Further, the 65 cm^-1 doublet separation which appears in the fluorescence spectrum of H₂P is related to the occurrence of these two orientations.

Resolved hyperfine structure is obtained for the two in-plane canonical orientations of the magnetic field and also when the field bisects the angle between these two directions. From an analysis of the fine structure and hyperfine structure results it is established that the zero-field splitting pattern is described by the parameters (average over the two orientations) the x axis is taken along the N-H H-N direction and z is the out-of-plane axis. From a computer simulation of the hyperfine structure it further follows that this structure is dominated by a high spin density at the methine carbons; with the coupling constants of the C-H fragment proposed by Hirota et al. the methine density is found to be ρ_m = 0·163.

In order to interpret the experimental results the zero-field splitting parameters and spin density distribution have been calculated for the lower triplet states of H₂P on the basis of a set of PPP-SCF-MO-CI calculations. From these calculations it follows that the lowest triplet state must correspond to the excitation e_g ←a _2u in Gouterman's four-orbital model. In terms of the D _2h symmetry of the H₂P molecule the assignment is ³ B _2u (b _3g ←b _1u ). For this assignment the calculations yield .     

Magnetic circular dichroism of transition metal ions in solids-I. MgO:Cr3+

Absorption and magnetic circular dichroism (MCD) spectra of MgO: Cr³⁺ have been studied over the temperature range 5–300 °K and with magnetic fields up to 77 kG. A theory for intensity calculation using a weak field coupling scheme is discussed. Spin-forbidden transitions to ²E and ²T₁, are observed in MCD and are interpreted as magnetic dipole zero-phonon lines. The spin allowed bands ⁴T₂ and ⁴T₁ are found to be vibration-induced electric dipole transitions in which the t_1u lattice modes are the dominant contributors to the intensity.     

Electric-magnetic duality in non-abelian gauge theories

The duality transformation of the vacuum expectation value of the operator which creates magnetic vortices (the 't Hooft loop operator in the Higgs phase), is performed in the radial gauge (x_uA_u^a(x) = 0). It is found that in the weak coupling region (small g) of a pure Yang-Mills theory the dual operator creates electric vortices whose strength is $\text{1}\text{g}$. The theory is self-dual in this region, and the effective coupling of the dual Lagrangian is $\text{1}\text{g}$. (It is self-dual also in the extreme strong coupling region.) Thus the above duality transformation reduces to electric-magnetic duality where the electric field in the 't Hooft loop operators transforms into a magnetic field in the dual operator. In a spontaneously broken gauge theory these results are valid only within the region where the vortices (or the monopoles) are concentrated, or in directions of the algebra space of unbroken symmetry, as self-duality holds only for this subset of fields. Noting that the 't Hooft loop operator project into the subspace of these field configurations we find that it is an electric-magnetic duality for the spontaneously broken theory as well. In the strong coupling region a strong coupling expansion in powers $\text{1}\text{g}$ is suggested.     

Linear electric shift in the paramagnetic resonance of Mn2+ in ZnTe

The influence of an electric field on the EPR transition has been observed in ZnTe:Mn²⁺ at 29 K by applying an ac electric field. The value of the component of the third rank tensor describing the effect is R₁₄ = 164 HzV^-1 cm. A strong correlation between the magnitude of the component R₁₄ and the amount of covalency was found.     

Stability of bipolarons in the presence of a magnetic field

The stability of large Fröhlich bipolarons in the presence of a static magnetic field is investigated with the path integral formalism. We find that the application of a magnetic field (characterized by the cyclotron frequence ω _c) favors bipolaron formation: (i) the critical electronphonon coupling parameter α _c (above which the bipolaron is stable) decreases with increasing ω _c and (ii) the critical Coulomb repulsion strength U _c (below which the bipolaron is stable) increases with increasing ω _c. The binding energy and the corresponding variational parameters are calculated as a function of α, U and ω _c. Analytical results are obtained in various limiting cases. In the limit of strong electron-phonon coupling (α ? 1) we obtain for ω _c ? 1 that E _estim ? E _estim(ω _c = 0) + c(u)ω _c/α ⁴ with c(u) an explicitly calculated constant, dependent on the ratio u = U/α where U is the strength of the Coulomb repulsion. This relation applies both in 2D and in 3D, but with a different expression for c(u). For ω _c ? α ²? 1 we find in 3D E _estim ? ω _c - α ² A(u) ln²(ω _c/α ²), (also with an explicit analytical expression for A(u)) whereas in 2D E _estim ^2D ? ω _c - α√ω _cπ(u-2-√2)/2. The validity region of the Feynman-Jensen inequality for the present problem, bipolarons in a magnetic field, remains to be examined.     

Nonlinear optical properties of nanospherical layer in the presence of an electric field

The linear and nonlinear optical properties in a spherical nanolayer quantum system subjected to an uniform applied electric field directed with respect to the z-axis have been theoretically investigated within the compact-density matrix formalism and the iterative method. The dependence of the optical absorption coefficients (ACs) and refractive index (RI) changes on the core radius R₁, on the inner radius of the clad R₂, and on the applied electric field F has been investigated detailedly. The results show that the optical ACs and RI changes of the nanospherical layer have been strongly affected by these factors. Moreover, the outcome of the calculation also suggests that all the factors mentioned above can give rise to blue-shift or red-shift significantly.     

Magnetoelectricity in the PbFe1/2Nb1/2O3 Ceramics

The magnetodielectric effect (the influence of a magnetic field H on the dielectric constant ?) and the magnetoelectric effect (the influence of an electric field E on the magnetoelectric constant ??) of the PbFe_1/2Nb_1/2O₃ ceramics have been investigated at temperatures T in the range from 50 to 200°C, including the Curie point T _C ? 98°C. It has been demonstrated that there is a correlation of these effects with the shift of the ferroelectric-paraelectric phase transition temperature in a magnetic field.     

Hysteresis phenomena in the course of polarization evolution in a sinusoidal electric field in lead magnesium niobate crystals

The processes of polarization evolution in single crystals of the PbMg_1/3Nb_2/3O₃ model ferroelectric relaxor in a sinusoidal electric field are investigated at temperatures near and above the temperature T _d 0 of destruction of the induced ferroelectric state upon heating in zero electric field. The polarization switching current loops are measured in the ac electric field applied along the 〈111〉 and 〈110〉 pseudocubic directions. The electroluminescence intensity loops are obtained under the combined action of ac and dc electric fields applied along the 〈100〉 direction. In a certain temperature range above T _d 0 and the freezing temperature T _f in lead magnesium niobate, there are electric current anomalies, that correspond to the dynamic formation and subsequent destruction of the ferroelectric macroregions throughout each half-cycle of the ac electric field. The measurements of electroluminescence hysteresis loops demonstrate that the observed depolarization delay (related to the ac electric field amplitude) increases with an increase in the dc electric field and decreases as the ac field amplitude increases. The nature of the observed phenomena is discussed.     

Electroabsorption near the 1S line of the green series of Cu2O

The effect of an electric field on the n = 1 line of the green series of Cu₂O has been studies at 4.2°K, using single crystals, polarized light and a modulation technique. Under the action of an electric field, an ignition of this line (λ^g_1S = 5 830 Å) is observed. This ignition effect depends on the geometry of the experiments, and gives informations on the nature of this transition as well as on the bands involved in the formation of this exciton.     

Beweglichkeitsanisotropie warmer Elektronen in n-Typ Germanium bei 35 GHz

The field dependent electric conductivity has been investigated withn-type germanium crystals of various orientations using microwaves of a frequency of 35 Gc/s. The orientation effect yields values ofSchmidt-Tiedemann's constantsβ ₀ andγ ₀. The ratioγ ₀/(?β ₀) is nearly independent of the lattice temperature with an average value of 0·65 in the temperature range between 100 and 273° K.     

F-center induced Raman scattering in RbCl

Experimental Raman spectra of RbCl containing F-centers are reported. Lines of A_1u, E_u and F_2u symmetry were observed. Calculations of the first-order Raman spectra are presented; they have been based on the assumption that the electron-phonon interaction is linear with displacements of F-center nearest neighbours. A shell-model fitted to neutron data was used to evaluate the Green-function matrix elements and the projected densities of states. The best agreement between theory and experiment was found to occur when using a zero-force-constant change for the F_2u spectrum and a 0·35 fractional decrease in the longitudinal force-constant A₁₂ for the A_3u and E_u spectra.     

Characteristic numbers of U1-valued lattice gauge fields

If a U₁-valued latice gauge field u defined on a periodic, 2-dimensional lattice satisfies the generic continuity condition uuuu ≠ − 1, it can be used to construct a principal U₁-bundle over the torus and in that bundle a connection such that parallel transport along bonds is given by u. In higher dimensions this construction can only be carried out if u is monopole-free (otherwise no such bundle can exist). The characteristic classes and numbers of this bundle can then be calculated from u in a straightforward way. Examples are given of u's with maximum possible characteristic numbers, along with a discussion of the behavior of u and of its topology under an action-decreasing deformation.     

Direct observation of the torsional fundamental of gaseous C2F6 in the far infrared

The forbidden a_1u torsional vibration of C₂F₆ has been observed in far-infrared absorption at 67.5 cm^?1 with an integrated intensity of 0.20 ± 0.08 × 10^?2 cm^?2. The observed intensity is compared with that expected theoretically from Coriolis coupling of the torsion to the infarred-active a_2u and e_u vibrations. For certain force fields and choices of sign for the dipole moment derivatives the agreement is satisfactory. The potential barrier to internal rotation has been recomputed from the observed torsional frequency to be 1367 cm^?1. (3.91 kcal mole^?1). This value is compared with those of ethane and partially fluorinated ethanes.     

Quantum Monte Carlo simulation of the ferroelectric or ferrielectric nanowire with core shell morphology

By using the Quantum Monte Carlo simulation; the electric properties of a nanowire, consisting of a ferroelectric core of spin-1/2 surrounded by a ferroelectric shell of spin-1/2 with ferro- or anti-ferroelectric interfacial coupling have been studied within the framework of the Transverse Ising Model (TIM). We have examined the effects of the shell coupling J_s, the interfacial coupling J_Int, the transverse field Ω, and the temperature T on the hysteresis behavior and on the electric properties of the system. The remanent polarization and the coercive field as a function of the transverse field and the temperature are examined. A number of characteristic behavior have been found such as the appearance of triple hysteresis loops for appropriate values of the system parameters.     

Influence of an external electric field on self-focusing of light beams in cubic photorefractive crystals

Self-focusing of an optical paraxial beam in a photorefractive cubic crystal exposed to a strong external electric field E ₀ of arbitrary orientation has been considered with regard to the Pockels effect. The best localization of radiation is shown to be attained when the vector E ₀ is oriented along diagonals of the cubic cell. Numerical modeling revealed that the beam width increased significantly for non-optimal electric field orientations.