Field-induced Changes Research Articles

External Electric Field Effects on Absorption and Fluorescence Spectra of Monomer and Dimer of Oxacarbocyanine in Langmuir−Blodgett Films

The dimer of oxacarbocyanine (OCC) formed in mixed Langmuir−Blodgett (LB) films shows very different electric field effects from the monomer both in the absorption spectrum and in the fluorescence spectrum. The magnitude of the change in electric dipole moment between the S1 excited state and the ground state of the dimer along the normal to the surface is much larger than that of the monomer. The fluorescence quantum yield of the OCC dimer also shows a remarkable electric field effect. The sign of the field-induced change in the quantum yield of the dimer fluorescence becomes opposite when the direction of the applied electric field is inverted, suggesting that the excitation dynamics of the dimer that is affected by an external electric field is a vectorial process along the normal to the surface. The process affected by an electric field is attributed to the interlayer electron transfer between the excited state of the OCC dimer and the cadmium salt of the fatty acid deposited in contact with each other.

Polarization-independent large field-induced refractive index change in a strained five-step GaAs–InAlGaAs asymmetric coupled quantum well

A tensile-strained five-step asymmetric coupled quantum well (FACQW) structure is proposed for large field-induced refractive index change without polarization dependence and redshift of the absorption edge. A strong exciton absorption peak is caused by electron-hole transitions between symmetric wave functions and antisymmetric wave functions with a small applied electric field. The field-induced refractive index change of strained the FACQW is larger by one order of magnitude compared to that of a rectangular quantum well and the difference in the refractive index change of TE and TM modes is under 2% when the operation wavelength is not at the absorption edge.

Calculation of the Static Permittivity of Suspensions from the Stored Energy

It is shown that the static permittivity of suspensions of charged particles in electrolyte solution can be deduced from the field-induced change of the Gibbs free energy stored outside the double layer. This energy is related to the changes of the electrolyte concentration around the particle and is obtained solving a purely static problem. The method is first verified deducing well-known results corresponding to suspensions of spherical particles. It is then used to calculate the static permittivity of suspensions of spheroidal particles, leading to a new result that cannot be analytically obtained using the classical approach.

Field-induced relaxation of crystal structure in dipole-glass phase of PMN — PT single crystal: X-ray study

X-ray studies of the electric field-induced structural changes in 0.8PMN — 0.2PT single crystal were made in the temperature range 300–400 K including the points of the supposed dipole-glass (T DG ≈ 352 K) and ferroelectric (T F ≈ 324) transitions. The changes in the form and intensities of the 005 and 224 reflexes were studied after the application the different electric fields (E < 5 × 105 V/m) along [001] cubic direction. At T > T DG and T < T F the instantaneous changes in the form and the intensity of the 224 reflex are in accordance with the cubic and rhombohedral symmetries of the parent and ferroelectric phases correspondingly, while at T F < T < T DG the field above some (temperature dependent) threshold value induces the smearing of this Bragg peak followed by the slow (during 30–40 min.) formation of the broad multiplet structure. Also the influence of the slow ac field (frequency 0.01 Hz) on the averaged over the period Bragg peak form was studied at 300 K < T < 400 K.

Effect of an external electric field on photoinduced interlayer electron transfer in Langmuir–Blodgett films as revealed by the effect of the field on fluorescence: a D–A system of thiacyanine and viologen

Electron donor and acceptor layers separated by a spacer layer were prepared with thiacyanine and viologen using the Langmuir–Blodgett technique, and the effect of an external electric field on the fluorescence of thiacyanine was examined. Interlayer electron transfer from the photoexcited molecules of thiacyanine to viologen was affected by the external electric field. The magnitude and direction of the field-induced change in the electron transfer rate were evaluated on the basis of measurements of the field-induced change in the fluorescence intensity combined with fluorescence decay measurements. © 1997 Elsevier Science S.A.

Large field-induced refractive index change without red shift of absorption edge in five-step asymmetric coupled quantum wells with modified potential

A structure composed of five-step asymmetric coupled quantum wells (FACQWs) is first proposed for large field-induced refractive index change without red shift of absorption edge. A strong exciton absorption peak is caused by e1hh2 and e2hh1 transitions with a small applied electric field. The field-induced refractive index change of FACQWs is larger by one order compared to that of rectangular quantum well when the operation wavelength is apart from absorption edge.

Thermodynamic theory of magnetic field effects on chemical equilibria and applications to metal–hydrogen systems

A general thermodynamic theory was constructed on magnetic field effects on chemical equilibria. Introducing the magnetochemical potential or the electromagnetic chemical potential leads to the magnetic field-induced changes in the equilibrium constant in an ideal gas reaction and the electromotive force in an electrochemical system. Applying these general results to metal–hydrogen systems gives the relationships between the equilibrium hydrogen pressure or the equilibrium electrode potential and magnetic fields, which correspond to experimental data.

Magnetoresistance and Hall effect of the magnetic semiconductor HgCr2Se4 in strong magnetic fields

The giant decrease of the electrical resistance of HgCr2Se4 (more than by a factor of 200) caused by magnetic field-induced changes in the carrier mobility and concentration, the quadratic dependences of magnetoresistance and normal Hall constant on magnetic induction in the paramagnetic region, as well as the deviations from these dependences observed to occur as one approaches the Curie temperature, are discussed within a model involving carriers of several types (holes in the valence band, electrons localized at ferron-type impurity centers, and electrons hybridized in the impurity and conduction bands).

Electric field effects in heterogeneous catalysis

Coadsorbed ions on metal surfaces generate electrostatic fields of the order of V A ̊ which induce substantial changes in the rate of catalytic reactions. The mechanism of the interaction of the electric field with adsorbed reactants or products on the catalyst surface is analyzed by means of ab initio cluster model wavefunctions. Two examples of catalytic processes modified by coadsorbed ions are considered: (a) The trimerization of acetylene to benzene on the Cu(110) surface and (b) the oxidation of ethylene and other organic molecules on supported Pt catalysts under the effect of an external applied potential. In both cases the trend observed experimentally is reproduced by the calculations and attributed to a field-induced change in the strength of the bond of the reactants or products. The way the field affects the bond, however, is different in the two cases. This can be understood in terms of ionic-covalent character of the adsorbates. For covalent adsorbates, like benzene on Cu(110), the field changes the metal work function and consequently the extent of the adsorbate-surface dative bonding. The effect is entirely chemical. For ionic or partially ionic adsorbates, like the oxygen atoms involved in the oxidation reactions, the effect is largely electrostatic and arises primarily from the interaction of the non-uniform electric field with the polar metal-oxygen bond.

Field-induced conductivity changes in CMR manganites

A very large increase in electrical conductivity by several orders of magnitude is obtained when an external magnetic field is applied to colossal magnetoresistance materials such as La−Ca−Mn−O. The magnetoresistance is strongly temperature dependent and exhibits a sharp peak below room temperature that can be shifted by adjusting the composition or processing parameters. The control of lattice geometry or strain (e.g., by chemical substitution, epitaxial growth, or post-deposition anneal of thin films) appears to be crucial in obtaining colossal magnetoresistance properties. The orders-of-magnitude change in electrical resistivity could be useful for various magnetic and electric device applications.

Field-induced refractive index variations in GaAs/AlGaAs multiple quantum well waveguide modulator

A quantum well electroabsorption waveguide modulator utilizing a field-induced refractive index change was designed and fabricated. The on/off ratios of the device were investigated as a function of wavelength over the spectral range of 850 nm to 910 nm for the various reverse biases. The field-induced refractive index variations associated with quantum-confined Stark effect was theoretically obtained based on the measured on/off ratios. The resulting maximum refractive index change( ${\DELTA}n) of ~7.5 {\times} 10^{-4}$ at -8 V was estimated.

Electric-field-induced luminescence quenching in an electroluminescent organic semiconductor

We report on the effect of a static electric field on the photoluminescence (PL) of laddertype poly(paraphenylene) (m-LPPP) in devices of polymer-light-emitting diodes. External electric fields of 4.5 MV/cm reduce the integral PL intensity down to 24% of the zero-field value and alter the shape of the PL spectra. The field-induced PL quenching in the m-LPPP polymer is ascribed to field dissociation of the emitting species. Time-resolved spectroscopy showed that the emission is composed of contributions from the radiative recombination of free singlet excitons and of a second emitting species, which we describe as self-trapped excitons. The observed field-induced changes of the shape of the PL emission can be attributed to the different field response of these two emitting species. Thus the PL emission color can be changed with the electric field in the blue-green spectral range. The magnitude of the binding energy of free singulet excitons is discussed.

Large field-induced refractive-index change on TM-polarized light in an InGaAlAs-InAlAs MQW waveguide structure

For optical modulation devices with low-loss and high efficiency, we have investigated field-induced refractive index change (/spl Delta/n) and absorption change (/spl Delta//spl alpha/) in the large detuning wavelength region where absorption is almost negligible using an InGaAlAs-InAlAs MQW waveguide structure. The result shows larger /spl Delta/n induced on TM-polarized light than that on TE-polarized light. Moreover, in induced on TM-polarized tight can become smaller than that on TE-polarized light. This suggests that TM-polarized light is more useful for low-loss and high-efficiency phase-modulation devices, especially large-scale matrix switches.

Electronic properties of H2, H2, and LiH in high magnetic fields

Advances in magnet construction technology have made magnets available with continuous fields of nearly 50 T and with bores of sufficient diameter for experiments. In addition to these magnets, already in use at the National High Magnetic Field Laboratory (NHMFL), semicontinuous pulsed sources of 100 T are anticipated in the near future. At its Los Alamos campus, the NHMFL has detonated pulsed magnets of over 1000 T. It thus becomes possible to investigate the behavior of molecules in strong fields with an eye to field-induced changes in such quantities as geometrical and electronic properties, spectroscopic properties, and reactivities. Theory is a useful probe for these quantities and serves to screen among possible candidates for experiments. In this contribution, we report preliminary results on the calculation of electronic properties of H2+, H2, and LiH, the simplest of molecules. Initial indications are that for increasing applied field strength, molecular bond lengths decrease and binding energies increase, with a concomitant increase in vibrational frequencies. Field-induced changes in these quantities, as well as in ground-state molecular potential energy surfaces are discussed, and suggestions are made for further investigations, both theoretical and experimental. © 1997 John Wiley & Sons, Inc. Int J Quant Chem 64: 561–570, 1997

Optical characterization of InGaAs–GaAs multiple quantum wells using variable angle spectroscopic ellipsometry for designing tunable modulators

Variable angle spectroscopic ellipsometry (VASE) technique is used to measure the optical constants of InGaAs/GaAs multiple quantum well (MQW) structures for the purpose of designing tunable optical modulators. The VASE measurements also include field-induced changes in index of refraction and absorption. These measured changes in MQWs are in agreement with theoretical computations. In addition, the design and simulated performance of an InGaAs/GaAs MQWs Fabry–Perot modulator using the VASE data is presented.

Model for magnetic field effects on radical pair recombination in enzyme kinetics

A prototypical model for describing magnetic field effects on the reaction kinetics of enzymes that exhibit radical pair recombination steps in their reaction cycle is presented. The model is an extended Michaelis-Menten reaction scheme including an intermediate enzyme-substrate complex where a spin-correlated radical pair state exists. The simple structure of the scheme makes it possible to calculate the enzyme reaction rate explicitly by combining chemical kinetics with magnetic field-dependent spin kinetics (radical pair mechanism). Recombination probability is determined by using the exponential model. Simulations show that the size of the magnetic field effect depends on relations between different rate constants, such as 1) the ratio between radical pair-lifetime and the magnetic field-sensitive intersystem crossing induced by the hyperfine interaction and the delta g mechanisms and 2) the chemical rate constants of the enzyme reaction cycle. An amplification factor that is derived from the specific relations between the rate constants is defined. It accounts for the fact that although the magnetic field-induced change in radical pair recombination probability is very small, the effect on the enzyme reaction rate is considerably larger, for example, by a factor of 1 to 100. Model simulations enable a qualitative comparison with recent experimental studies reporting magnetic field effects on coenzyme B12-dependent ethanolamine ammonia lyase in vitro activity that revealed a reduction in Vmax/KM at low flux densities and a return to the zero-field rate or an increase at high flux densities.

Phase-separation mechanism for giant magnetoresistance of lanthanum manganites

Based on the fact that the nonsaturated magnetic state of La 1− x Ca x MnO 3 and related materials is a result of their mixed ferro-antiferromagnetic state, the possibility is investigated that such a state is caused by a thermodynamical equilibrium nonuniform oxygen distribution. Its magnetic field-induced change should lead to a giant magnetoresistance of these materials.

Normal state resistivity of Bi 2Sr 2CaCu 2O 8 + x

We have found that for electric fields larger than about half of the break down field ( E > 1 2 E BD ) the measured resistivity varies as the square root of the electric field, ϱ ∝ E 1 2 . The resistivity calculated for the field-driven movement of the mobile charge carries in the normal state of a superconductor is found to vary as the square of the electric field, ϱ ∝ E 2. Thus, the mechanism of the field-driven movement of charge carriers is not in agreement with the experimental measurements of the normal state resistivity of Bi 2Sr 2CaCu 2O 8 + x . We predict that the resistivity of a system with conduction electrons localised along a one-dimensional chain varies as the square root of the applied electric field. The mechanism of the field-induced changes in the density of states of the mobile charge carriers along a one-dimensional chain is in agreement with the experimental measurements.

Field-induced thickness change of ferroelectric liquid crystal films.

We observe an increase of the thickness of a chiral ferroelectric liquid crystal film when subjected to ac fields. At 1 kHz at voltages slightly above the threshold for helix unwinding, the sample thickness increases by 0.3 \ensuremath{\mu}m in a few minutes. Under this field the cover plate vibrates vertically with a dominating component at twice the field frequency of an amplitude of almost 1 nm. This vibration is expected by the continuum theory of Leslie, Stuart, and Nakagawa [Mol. Cryst. Liq. Cryst. 198, 443 (1991)]; but a theoretically unexplained small asymmetry in the vibration is needed to explain the pumping. The reason for the asymmetry is not yet understood.

Coherent Raman response and spectral characteristics of ultrashort solitons in fibers.

The Raman self-frequency shift of a soliton is studied for pulse durations shorter than the optical phonon oscillation period and with respect to the influence of the field-induced change in the vibrational level populations. It is shown that under certain conditions the self-frequency shift can be suppressed by the coherent saturation of the Raman response. \textcopyright{} 1996 The American Physical Society.