Thermally stimulated current in polypropylene films charged in strong electric fields

We have observed a unique feature of the thermally stimulated depolarization (TSD) current in polypropylene (PP) films polarized in strong electric fields (up to 1–2 MV/cm). The direction of the TSD current is the same as the direction of the charging (polarizing) current, and the shape of the TSD current curve is much different from the standard curve. We show by computer model that these features can be explained by charge carrier injection in the film by charging from both electrodes. Both the charging curve shape and TSD current direction are explained by a model in which injected carriers form charged regions inside the film at a distance from the electrodes of one-fourth the thickness of the dielectric. Saint Petersburg State Technical University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 4, pp. 12–20, April, 1997.     

Electron-phonon interaction in LaAg

Magnetoacoustic de Haas-van Alphen data on LaAg are compared with a new relativistic band structure calculation including La-4f states. We find convincing agreement of the measured and calculated dHvA frequencies. The deformation of the Fermi surface under strain is determined from the magnetoacoustic dHvA amplitude. The strongest deformation potential coupling is found for three equivalent pockets which become inequivalent under orthorhombic strain. The lifting of their degeneracy leads to the softening of theT ₃ elastic constant. This is different from the useual band Jahn-Teller effect which is based on the lifting of subband degeneracy at a single point ink-space.Dedicated to Professor Harry Thomas on the occasion of his 60th birthday     

Nonlinear interaction of strong laser fields in vacuum

Quantum electrodynamics (QED) predicts that electromagnetic fields interact with each other in vacuum. We study the possibility of revealing this interaction experimentally with intensities on the order of 10²⁴–10²⁶ W/cm², which may be available in the next generation of laser systems. In particular, we investigate high-order harmonic generation in vacuum via the collision of two ultrastrong counterpropagating laser pulses. The experimental feasibility of the related process of stimulated light-by-light scattering is also examined. Finally, the importance of including diffractive effects to describe the nonlinear interaction between an x-ray probe and a strong, focused optical standing wave is pointed out.     

Effects of strong electric fields in a polyacetylene chain

In this work, we study the effects related to the creation of electron/hole pairs via application of an external electric field that acts on a pristine trans-polyacetylene molecular chain at zero-temperature. This phenomenon is termed Schwinger–Landau–Zener (SLZ) effect and arises when a physical system, which can even be the vacuum, is under the action of a strong, static and spatially homogeneous electric field. Initially, we investigate how the electrical conductivity of the polyacetylene changes with the applied field, by considering the carriers production as well as the variation of the interband gap according to certain ab initio models. Next, we analyse the competition between the SLZ effect and another one associated with the incidence of an uniform electric field on one-dimensional crystals – the Bloch oscillations. We evaluate the conditions in which these latter can be destroyed by the particles created through the same field that induces them, and verify the possibility of occurrence of the Bloch oscillations inside the trans-polyacetylene with frequencies equal to or higher than the terahertz scale.     

Electron-phonon interaction in carbon schwarzites

The recent synthesis of random schwarzites has stimulated the present ab initio calculation of the electronic structure and electron-phonon interaction in two different periodic D-type schwarzites, fcc-(C₂₈)₂ (made of 24 seven-membered rings per unit cell) and fcc-(C₆₄)₂ (made of 12 eight membered and 48 six-membered rings per unit cell). Like in fullerenes, also in schwarzites the electron-phonon interaction potential is found to increase with the absolute Gauss curvature, though it remains smaller than for doped fullerenes. Received 19 December 2002 Published online 1st April 2003 RID="a" ID="a"e-mail: marco.bernasconi@unimib.it     

Electron-phonon interaction in impure metals

Making use of the theory for the phonon damping in an impure metal developed recently by Eisenriegler, we study the interaction between electrons and long wave-length longitudinal phonons, and its effects on electronic properties at low temperatures. On account of the inelastic scattering of electrons by impurity ions, a minimum in the electrical resistivity is found when plottedvs temperature, although its value is extremely small.     

Electron-phonon interaction and antiferromagnetic correlations

We study effects of the Coulomb repulsion on the electron-phonon interaction (EPI) in the Holstein-Hubbard model, using the antiferromagnetic (AF) dynamical mean-field approximation. AF correlations strongly enhance EPI effects on the electron Green's function with respect to the paramagnetic correlated system, but the net effect of the Coulomb interaction is a moderate suppression of the EPI. Doping leads to additional suppression. In contrast, the Coulomb interaction strongly suppresses EPI effects on phonons, but the suppression weakens with doping.     

Luminescence of rock salt in very strong electric fields

The luminescence of thin single-crystal layers of NaCl in very strong electric fields was observed. Oscillograms of the brightness waves were obtained. The light flux and luminescence delay were measured as functions of the applied field strength amplitude. The reasons for the delay phenomenon and the drop in the electric strength of samples when electrodes with electron conductivity (metal or graphite) are used are discussed.The authors sincerely thank Prof. G. A. Vorob'ev for his guidance and P. E. Ramazanov for valuable observations.     

Electron-phonon interaction in tetrahedral semiconductors

Considerable progress has been made in recent years in the field of ab initio calculations of electronic band structures of semiconductors and insulators. The one-electron states (and the concomitant two-particle excitations) have been obtained without adjustable parameters, with a high degree of reliability. Also, more recently, the electron-hole excitation frequencies responsible for optical spectra have been calculated. These calculations, however, are performed with the constituent atoms fixed in their crystallographic positions and thus neglect the effects of the lattice vibrations (i.e. electron-phonon interaction) which can be rather large, even larger than the error bars assumed for ab initio calculations.Effects of electron-phonon interactions on the band structure can be experimentally investigated in detail by measuring the temperature dependence of energy gaps or critical points (van Hove singularities) of the optical excitation spectra. These studies have been complemented in recent years by observing the dependence of such spectra on isotopic mass whenever different stable isotopes of a given atom are available at affordable prices. In crystals composed of different atoms, the effect of the vibration of each separate atom can thus be investigated by isotopic substitution. Because of the zero-point vibrations, such effects are present even at zero temperature (T=0).In this paper, we discuss state-of-the-art calculations of the dielectric function spectra and compare them with experimental results, with emphasis on the differences introduced by the electron-phonon interaction. The temperature dependence of various optical parameters will be described by means of one or two (in a few cases three) Einstein oscillators, except at the lowest temperatures where the T⁴ law (contrary to the Varshni T² result) will be shown to apply. Increasing an isotopic mass increases the energy gaps, except in the case of monovalent Cu (e.g. CuCl) and possibly Ag (e.g. AgGaS₂). It will be shown that the gaps of tetrahedral materials containing an element of the first row of the periodic table (C,N,O) are strongly affected by the electron-phonon interaction. It will be conjectured that this effect is related to the superconductivity recently observed in heavily boron-doped carbon.     

Electron-phonon interaction in superconducting Nb

Acoustic phonon damping in Nb due to superconductivity is accurately measured by the inelastic neutron scattering technique. The phonon line widths can be characterized by the reduced parameters $\text{h}\text{?}\text{ω}{}_{\mathrm{p}}\text{/2Δ(0)}$ and T/T_c, and are in good qualitative agreement with the calculation by Bobetic based on the BCS theory.     

Diffusivity of charged carriers in semiconductors in strong electric fields

Expressions for diffusivity parallel (D₆) and perpendicular (D_⊥) to an external electric field in semiconductors are derived by assuming a drifted Maxwellian distribution for the carriers. It is shown that the expressions for diffusivity as obtained by Robson are unchanged for arbitrarily large values of the drift velocity—a result which is not expected in his work.     

Resistive transitions of superconducting thin films in strong electric fields

We report discontinuous current-induced transitions to the normal state of superconducting thin films in the mixed state. The results obtained are in qualitative agreement with the recent theory of Thompson and Hu of dynamic instabilities of vortices.     

Helical modes in carbon nanotubes generated by strong electric fields

Helical modes, conducting opposite spins in opposite directions, are shown to exist in metallic armchair nanotubes in an all-electric setup. This is a consequence of the interplay between spin-orbit interaction and strong electric fields. The helical regime can also be obtained in chiral metallic nanotubes by applying an additional magnetic field. In particular, it is possible to obtain helical modes at one of the two Dirac points only, while the other one remains gapped. Starting from a tight-binding model we derive the effective low-energy Hamiltonian and the resulting spectrum.     

Phase space evolution of pairs created in strong electric fields

We study the process of energy conversion from overcritical electric field into electron–positron–photon plasma. We solve numerically Vlasov–Boltzmann equations for pairs and photons assuming the system to be homogeneous and anisotropic. All the 2-particle QED interactions between pairs and photons are described by collision terms. We evidence several epochs of this energy conversion, each of them associated to a specific physical process. Firstly pair creation occurs, secondly back reaction results in plasma oscillations. Thirdly photons are produced by electron–positron annihilation. Finally particle interactions lead to completely equilibrated thermal electron–positron–photon plasma.     

Dipolar interactions in molecules aligned by strong AC electric fields

We observed magnetization transfer and spectroscopic splittings due to dipolar couplings in the solution NMR spectra of neat nitrobenzene aligned using AC electric fields. Weak dipolar splittings have been previously observed for nitrobenzene in a DC electric field (T. M. Plantenga, et al., Chem. Phys. 66, 1-9, 1982); the use of homogeneous pulsed AC fields has allowed us to establish stable experimental conditions, which were more tolerable to sample impurities and required no sample purification, and to carry out multidimensional experiments. A pulse sequence is discussed in which the electric field is present only for the indirect dimension: this sequence records the dipolar splittings for each proton in the indirect dimension; the direct dimension presents the isotropic chemical shift. Another pulse sequence is discussed that uses the applied electric field only in the mixing period to produce cross peaks between dipolar coupled pairs and correlate their isotropic chemical shifts. The order parameter describing molecular alignment was in good agreement with that previously determined from deuterium quadrupolar measurements of deuterated nitrobenzene in a similar range of electric fields: S(mol) approximately 0.025% for a field strength of 7.0 MV/m (rms). The dipolar splittings for ortho-meta, meta-para, and ortho-para protons were in qualitative agreement with the known geometry. Copyright 2000 Academic Press.