Field-induced Changes Research Articles

Quantum-confined field-effect wavelength tuning in a three-terminal double quantum well laser

A three-terminal quantum-confined field-effect double quantum well laser device is proposed. The wavelength shift scheme caused by the field-induced change in the energy levels is demonstrated. With the current injected and the electric field applied to different wells, wide range wavelength shift can be achieved by changing the applied field.

Magnetoelastic properties of rare-earth paramagnetic garnets: Magnetostriction and thermal expansion

The differential longitudinal magnetostriction (MS), in fields up to 50 KOe, for the [111], [110] and [001] crystal axes at 4.2 K and the crystal lattice parameter between 5 and 290 K for garnets R 3Ga 5O 12 and R 3Al 5O 12 (R = GdYb) are measured. The MS for all garnets except GdGG is very anisotropic and does not saturate; its value and sign change in an irregular way throughout the series of rare-earth (RE) ions. Within the terms of the crystal-field model the quadrupole approximation for the magnetoelastic energy is discussed. For the garnet structure with inequivalent sites of RE ions, the MS is expressed in terms of the field-induced changes of quadrupole moments which are numerically calculated for the known crystal fields. Semiquantitative analysis of experimental data allows one to account for the variation of the sign and value of MS from one garnet to another. Anomalies of the crystal lattice parameter observed at low temperatures for some RE garnets are described.

Second harmonic generation study of field-induced changes in the phase transition of K1-xLixTaO3

Abstract The phase transition in K1−xLixTaO3 with x=0.036 and x=0.063 has been investigated using optical second harmonic generation (SHG). The application of an electric field during cooling and heating shifts the phase transition to higher temperatures and increases the intensity of the frequency doubled light drastically. This can be interpreted as an increased mean size of ordered regions in the low-temperature domain state by the electric field. The shape of the tensor of the nonlinear susceptibility corresponds to a polar tetragonal low temperature symmetry. The temporal dependence of the SHG intensity upon application of an electric field yields an activation energy similar to that for the Li reorientation.

Directed Growth of Fungal Hyphae in an Electric Field

Abstract The galvanotropic response of the mycelial fungus Neurospora crassa is investigated. The angle distribution function of growing hyphae is described by a generating function which contains two non-trivial terms; one for directional growth and one for bidirectional growth. The following results were obtained, (i) Germ tubes grew towards the anode, (ii) The cellular response was linear for small sized cells and weaker electric fields. The galvanotropic constant, KGROW, which describes the linear response, was small for short germ tubes or hyphae (AGROW -1 = -20 V -cm-1 for I0 = 10 |am) and large for longer cells (AGROW -1 = -1 -7 V ·cm-1 for I0 = 100 μm). The growth coefficient KP of (-18.5 mV) describes the response independent of cell size. The linear response is explained by the field-induced distribution of charged membrane- bound proteins essential for galvanotropism. (iii) For E > E0 = 3.8 V ·cm-1, the linear response is inhibited (inhibition coefficient K1 = 1.11). The inhibition is explained by field-induced changes of the membrane potential, (iv) The galvanotropic response of longer hyphae was bidirectional. The cells grew on average perpendicular to the applied field. The bidirectional response is proportional to Er with a bidirectional growth coefficient K2 of -(0.25 V)-2. The bidirectional growth is explained by the inhibition of the directed growth process. The transition from anode-directed to bidirectional growth was a function of the applied electric field as well as of the tube length (directed growth for E2-l0< 4.4V2·cm-1 and bidirectional growth for E2·l0> 4.4 V2· cm-1).

Electrooptical studies on proton-binding and -release of bacteriorhodopsin

Electric field induced pH changes of purple membrane suspensions were investigated in the pH range from 4.1 to 7.6 by measuring the absorbance change of pH indicators. In connection with the photocycle and proton pump ability, three different states of bacteriorhodopsin were used: (1) the native purple bacteriorhodopsin (magnesium and calcium ions are bound, the M intermediate exists in the photocycle and protons are pumped), (2) the cation-depleted blue bacteriorhodopsin (no M intermediate), and (3) the regenerated purple bacteriorhodopsin which is produced either by raising the pH or by adding magnesium ions (the M intermediate exists). In the native purple bacteriorhodopsin there are, at least, two types of proton binding sites: one releases protons and the other takes up protons in the presence of the electric field. On the other hand, blue bacteriorhodopsin and the regenerated purple bacteriorhodopsin (pH increase) show neither proton release nor proton uptake. When magnesium ions are added to the suspensions, the field-induced pH change is observed again. Thus, the stability of proton binding depends strongly on the state of bacteriorhodopsin and differences in proton binding are likely to be related to differences in proton pump activity. Furthermore, it is suggested that the appearance of the M intermediate and proton pumping are not necessarily related.

Nifedipine Inhibits Magnetic Field-Induced Lysosomal Degranulation

Human peripheral polymorphonuclear leukocytes were exposed to static magnetic fields of variable intensity (0-1.0 T). Following exposure for 30 minutes, the cells were centrifuged and the supernatant was assayed for release of lactate dehydrogenase, a marker of cytotoxicity. Up to 9% of the cells' total content of this lysosomal enzyme was released after exposure to a 1.0-T magnetic field. Significantly less degranulation occurred following pretreatment with 5 μM nifedipine, a calcium channel antagonist. Calcium channel antagonists may serve as magnetoprotective agents against magnetic field-induced changes in Ca2+-dependent cellular processes.

Large displacement transducers based on electric field forced phase transitions in the tetragonal (Pb0.97La0.02) (Ti,Zr,Sn)O3 family of ceramics

Several properties associated with the field-induced phase change between the antiferroelectric and ferroelectric states in tetragonal (Pb0.97 La0.02 )(Sn,Ti,Zr)O3 antiferroelectric ceramic family were investigated for high-strain displacement transducer applications. The longitudinal field-induced strain accompanying the phase change is in the range of 0.2%–0.9%. The single-shot switching time between the two states is on the order of 1–2 μs. Under continuous ac field driving, the hysteretic heating effect introduces a temperature rise, changing the original room-temperature switching behavior. The ceramics degrade under ac field excitation, the average life cycles is in the range of 106 –107 cycles which can be greatly improved by carefully polishing the sample surfaces.

Field-effect-induced metastable conductivity changes in n-type a-Si:H

A lightly doped n-type a-Si:H film, arranged in a field effect configuration, has been subjected to bias-annealing treatments. Depending on the sign and the magnitude of the gate bias, the conductivity of the quenched-in metastable states could be varied throughout the entire range that is accessible by chemical doping. CPM-measurements performed on extreme states suggest that the conductance changes are controlled by field-induced changes in the donor density.

Quantum-confined field-effect light emitters with high-speed switching capability

The field control of luminescent characteristic is observed with the three-terminal quantum-confined field-effect light-emitting device at a high temperature of 100–300 K. The modulation scheme caused by the field-induced change in radiative lifetime without change in carrrier density is demonstrated in the practical device. A fast switching of the spontaneous emission intensity free from lifetime limitation is obtained.

Determination of the third-order piezoelectric constants of quartz using the extentional mode of rods

The dc field-induced change in the frequency of the extentional mode of rods is used to determine the fundamental nonlinear third-order piezoelectric constants of quartz. Five combinations of the total number of eight third-order piezoelectric constants are determined by least-squares fit to data obtained from 50 different rods. The results are notable for their small standard errors of less than 2% on average. They also agree very well with the values obtained independently by the transit-time method.

Measurement of the Hall coefficient using van der Pauw method without magnetic field reversal

The van der Pauw geometry has been widely used for the measurement of resistivities and Hall coefficients. Although the measurement of a Hall coefficient requires a finite magnetic field, it should be noted that van der Pauw’s expression is valid only in the limit of zero field; in addition to the Hall contribution, measurements in a finite magnetic field generally include a term associated with field-induced changes in the longitudinal resistivity. Although a simple solution to this problem entails taking the difference between readings in opposite field directions, there are circumstances where this may be impractical. In this note we present a straightforward extension of the van der Pauw calculation which allows a determination of the Hall coefficient from quantities measured in one field direction only.

The magnetic hyperpolarizability anisotropy of some two-electron systems

The magnetic hyperpolarizability η is calculated at the near-Hartree-Fock level for Li+, He and H2. Connections between η and B hyperpolarizabilities are derived. For H2 values of Δη = 9·76 and 10·31 a.u. (0·26 and 0·28 × 10-42 (4πϵ0) cm3 G-2, respectively) are found in different basis sets. Zero-point averaging adds an estimated 1·7 a.u. to the equilibrium value for the H2 isotopomer, and field-induced changes in vibration a further 0·2 a.u., to give a total ab initio result that is 78 per cent of the value deduced from the most recent experimental measurement of the Cotton-Mouton constant.

On the Lagrangian and Hamiltonian constraint algorithms for the Rarita-Schwinger field coupled to an external electromagnetic field

The complete constraint analysis for the Rarita-Schwinger 3/2 field coupled to an external electromagnetic field is given, showing the parallel between the Lagrangian and Dirac-Bergman algorithms. The analysis confirms the connection between the Velo-Zwanzinger (1969) and Johnson-Sudarshan (1961) pathologies, and illustrates that these do not, in fact, arise because the constraint analysis leading to them is incomplete. On the contrary, a new tier of constraints occurs for the critical field values, reducing the pathology to a field-induced change in degrees of freedom.

Rotational polarization in a Knudsen flow; Various gases on mica between 293 and 666 K

Molecular angular momentum polarization produced in molecule-surface interaction is investigated by observing the magnetic field-induced change in a Knudsen particle flow between parallel plates. Results for H 2, D 2, HD, N 2, CH 4 and SF 6 scattered from a mica surface at temperatures between 293 and 666 K show a strong dependence upon temperature. At 293 K, effects are found to be small in general, with H 2 and D 2 being an order of magnitude smaller than HD. Both odd-in- J polarizations and even-in- J polarizations (orientation and alignment) are observed, their weight being strongly dependent on the type of the molecule. At 666 K, effects are much larger and rather similar for all gases, with the differences between the H 2-isotopes vanishing. Here the orientation (of type J y) becomes dominant for all molecules investigated. Data for HD as a function of temperature give a strong indication of the desorption of adsorbates around 450 K.

Infrared Rotatory Dispersion Studies of Induced Cholesteric Mesophases in Electric Fields I. Texture Change Induced by a DC Electric Field

Abstract The influence of a dc electric field on induced cholesteric mesophases was studied by infrared rotatory dispersion. The reflection Cotton effect was found to vanish above a threshold field of strength about 5.103 V/cm because of a field-induced texture change. The frequency of the reflection Cotton effect is independent of the applied voltage.

クラスター・イオンビーム法によるＣｄ１−ｘＭｎｘＴｅ薄膜のファラデー回転に及ぼす電界効果

Pronounced effects of an electric field on the Faraday rotation of Cd1-xMnxTe films prepared by ionized-cluster beam technique have been investigated in the wavelength region from 800 nm to 480 nm at room temperature. With an increase in an electric field applied to the film plane, a marked shift in dispersion frequency toward a longer wavelength region is observed in Faraday rotation spectra of the Cd1-xMnxTe film, and its peak height decreases. The electric field-induced change of peak height at the dispersion frequency is of the order of 10-4 deg./cmG per one kV/cm, and enlarges with increase in composition x. This can be explained by the ionization of excitons due to the Franz-Keldysh effect.

Measurements of the polarization dependence of field-induced refractive index changes in GaAs/AlAs multiple quantum well structures

The measurements of the polarization dependence of field-induced refractive index changes in GaAs/AlAs multiple quantum wells, which are directly measured by using the modulation spectroscopy method for the TE and TM modes, are described. Clear polarization dependences, such as differences in the spectra shape and the peak wavelength, were observed. The polarization dependences originate from the different excitonic transitions: the n=1 heavy-hole and light-hole exciton for the TE mode and the n=1 light-hole exciton for the TM mode. The experimental results show qualitatively good agreement with the theoretical calculation.

Thermodynamics of electron transport in Photosystem I studied by electric field-stimulated charge recombination

The kinetics of chlorophyll luminescence induced by externally applied electric-field pulses in osmotically swollen thylakoid systems are known to consist of a fast and a slow phase, attributed to charge recombinations in Photosystem I and II, respectively (Symons, M., Korenstein, R. and Malkin, S. (1986) Biochim. Biophys. Acta 806, 305–310). We confirm this assignment and show that conditions can be created under which the phases can be studied separately. A previously unresolved 80 μs decay of the precursor of Photosystem II electroluminescence is attributed to manganese oxidation. After illumination the precursor of Photosystem I electroluminescence decayed biexponentially with a major component with a half-time of 70 ms and a minor component with a half-time of about 15 ms at room temperature. The same kinetics were observed in the absorbance difference at 700 nm and in ‘normal’ Photosystem I luminescence. On the basis of these findings the Photosystem I electroluminescence is attributed to charge recombination between the oxidized primary donor P +-700 and the reduced electron acceptor F A − (70 ms component) and possibly F B −. The enthalpy of activation of the first reaction was determined at 460 ± 30 meV. Field-induced absorbance changes showed that charge recombination occurred in up to 20% of the reaction centers. It is further shown that in Photosystem I, in contrast to Photosystem II, the luminescence yield is enhanced by an external electric field with at least a factor of 150 at higher field strengths. The results are discussed in terms of a thermodynamical model. It is concluded that in PS I the primary charge separation, P-700A 0 → P +-700A − 0, as well as subsequent electron transport to F A and F B are electrogenic.

Stark spectroscopy of Langmuir-Blodgett monolayers and multilayers

The electrical (capacity and conductivity) and electro-optical (Stark spectra) properties were investigated for monolayers of an anthraquinone dye. The polar structure of the monolayers results in the appearance of the linear Stark effect with a field-induced change δ T in the optical transmission proportional to the difference δ μ of the dipole moments for an excited and the ground molecular states. Multilayers were shown to be non-polar owing to the effects of molecular association. In the latter case only the quadratic Stark effect is observed with δ T proportional to the difference δα of the polarizabilities of an associated excited state and the ground state. It was also shown that the electric field distribution across an ultrathin Langmuir-Blodgett film is extremely non-uniform.

Polarized electroabsorption spectra of amorphous semiconductors

Abstract Transverse electroabsorption spectra of amorphous Se and hydrogenated amorphous Si (a-Si: H) are presented. These spectra are anisotropic, with fields F parallel to the polarization E of light, causing much larger signals than F perpendicular to E. At temperatures below 350 K the signal depends little on temperature. At higher temperatures the field-induced change δα in the absorption increases in a-Si: H. In Se the anisotropy of δαdepends on photon energy. At low energies the spectrum becomes isotropic and may arise from a quadratic Stark effect of tail states with rather large polarizabilities. The electroabsorption spectra cannot be explained by the Franz-Keldysh effect or by an exciton model. It is proposed that they arise from field mixing of tail states which increases their dynamic polarizability. The mixing is most effective for states near the mobility edges.