External Bias Electric Field Research Articles

History dependence of avalanche dynamics of ferroelectric phase transition in BaTiO3 under external bias field

A ferroelectric phase transition under an external bias electric field after field cooling and zero field cooling in barium titanate single crystals shows scale invariant nucleation and growth of complex domain structures. The avalanche energy exponents vary with the external bias, depending on the cooling history of the sample under applied field or zero field cooling. After field cooling, resulting in a single domain sample, the energy exponent is near the integrated mean field value of 5/3, namely, 1.68 ± 0.022. The sample after field cooling shows the same exponent with an external bias electric field range of up to 4.5 kV/cm. The exponent of a multi-domain sample, after zero field cooling, decreases from ε = 1.85 to the fixed point of 1.66 under high fields. The different behavior is attributed to the greater complexity of domain patterns in the multi-domain sample, which reduces to a single domain state under strong applied fields. Compared with the single-domain state, the multi-domain state has more domain boundaries and the intersections formed by these domain boundaries. These domain boundaries and their intersections will hinder the movement of the phase boundary and act as a pinning effect on the front of the phase interface. The effect is to generate more small energy signals, making the critical exponent high. At the same time, the aftershock time distribution (Omori law) remains the same for all switching conditions with an Omori exponent near −1 and switching time correlations of −1 ± 0.05 for short times (<1 s) and −2 ± 0.10 for long times (>1 s).

Enhanced tunability and temperature-dependent dielectric characteristics at microwaves of K0.5Na0.5NbO3 thin films epitaxially grown on (100)MgO substrates

K0.5Na0.5NbO3 thin films were deposited by pulsed laser deposition on (100)MgO substrates for microwave device applications. A fine epitaxial growth of pure perovskite phase was evidenced by X-ray diffraction. Dielectric characterizations were performed from 1 to 40 GHz using coplanar microwave devices printed on the 500 nm-thick K0.5Na0.5NbO3 thin films. Dielectric permittivity εr = 355 and loss tangent tanδ = 0.35 at 10 GHz were retrieved without biasing. A comparison of the results with those retrieved from the resonant cavity method (to characterize as-deposited films) showed no deleterious influence neither from the device patterning nor the thin film-device interface. A frequency tunability up to 22% was measured under a moderate external DC bias electric field Ebias = 94 kV/cm. Temperature measurements from 20° to 240°C exhibited a permittivity increase up to εr = 975 coupled to a loss decrease tanδ = 0.25 at 10 GHz. According to such measurements, an orthorhombic-tetragonal phase transition was evidenced close to 220 °C with an increase of the frequency tunability up to 34%. Comparison of the properties of such films with those grown on R-plane sapphire substrates demonstrated the benefit brought by the epitaxial growth of K0.5Na0.5NbO3 films on (100) MgO.

Excitation-wavelength-dependent THz wave modulation via external bias electric field**Project supported by the Natural Science Foundation of Beijing, China (Grant No. JQ18015) and the National Natural Science Foundation of China (Grant Nos. 61935001 and 61905271)

A theoretical model was proposed to describe the effects of external bias electric field on terahertz (THz) generated in air plasma. The model predicted that for a plasma in a bias electric field, the amplification effect of the THz wave intensity increases with the increase of the excitation laser wavelength. We experimentally observed the relationship between the THz enhancement effect and the electric field strength at different wavelengths. Experimental results showed a good agreement with the model predictions. These results enhance our understanding of the physical mechanism by which femtosecond lasers excite air to generate THz and extend the practical applications of THz generation and modulation.

Suppression of high-power microwave window breakdown by the sweeping-out-electron effect with an external dc bias electric field

To suppress microwave window breakdown, an external parallel dc bias electric field is applied to sweep out the electrons from the discharge zone. The development of microwave window breakdown under an external dc electric field is investigated using particle-in-cell and Monte Carlo collision simulation. The results show that the number of multipactor electrons decreases with the strength of the external unidirectional dc electric field when the size of the microwave window is finite. In addition, the threshold of breakdown is strongly influenced by the direction of the external bidirectional dc electric field. When the electric field is directed from the center to the edge of the microwave window, breakdown occurs more readily than in the absence of the electric field. However, when the electric field is directed toward the center of the microwave window, the discharge process starts at the edge of the window and develops to the center of the window. This is because a smaller density of electrons at the window edge results in a smaller gas pressure and the low gas pressure above the dielectric surface thus makes formation of the plasma more difficult. As a result, the early stage of breakdown is suppressed and the maximum transmitting power is enhanced.

Design of a Liquid-Crystal-Tunable Terahertz Demultiplexer Based on a Metal-Insulator-Metal Waveguide

A tunable demultiplexer with three output channels infiltrated by liquid crystal (LC) is presented, which is based on a metal-insulator-metal (MIM) waveguide. The operating frequencies of the three output channels can be tuned simultaneously at will by changing the external bias electric field applied to the LC. By analyzing the Fabry-Pérot (FP) resonance modes of the finite-length MIM waveguide both theoretically and numerically, the locations of the three channels are delicately determined to achieve the best demultiplexing effects. Terahertz (THz) signals input from the main channel can be demultiplexed by channels 1, 2 and 3 at 0.7135 THz, 1.068 THz and 1.429 THz, respectively. By applying an external electric field to alter the tilt angle of the infiltrating LC material, the operating frequencies of channels 1, 2 and 3 can be relatively shifted up to 12.3%, 9.6% and 9.7%, respectively. The designed demultiplexer can not only provide a flexible means to demultiplex signals but also tune operating bands of output channels at the same time.

Diffraction of electromagnetic waves on the resonance graphene metasurfaces

By using three independent numerical approaches to solve the 3D diffraction boundary problems the reflection, transmittance coefficients and losses of metasurfaces of graphene strip elements, depending on the frequency and values of the chemical potential were calculated in the terahertz frequency range. The results of numerical simulation of performances of tuned by the external bias electric field THz absorbers and THz polarizers based on the resonance metasurfaces of single square and two connected rectangular graphene strips in the unit cell placed on the dielectric or multilayer graphene-dielectric substrates were obtained.

Propagation dynamics of Airy beams and nonlinear accelerating beams in biased photorefractive media with quadratic electro-optic effect

An investigation of propagation dynamics of Airy beams and nonlinear accelerating beams in biased photorefractive media with quadratic electro-optic effect is presented by means of numerical simulations. For linear Airy beams, it is found that single breathing soliton can be formed by adjusting the initial amplitude and the external bias electric field during the propagation of one Airy beam. The propagation direction of such soliton can be controlled by introducing a launch angle to the initial beam and it can propagate stably against small fluctuations. Interaction of two Airy beams shows that single breathing solitons and soliton pairs can be produced after collision and the external bias electric field has a big impact on their propagation. For nonlinear accelerating beams, because they have infinite energy, we truncate the oscillating tails to study the interaction of two in-phase and out-of-phase accelerating beams. The numerical results show that single solitons and soliton pairs can generate only within a certain range of external bias electric field.

Interactions between self-accelerating beams in photorefractive media.

We investigate interactions between in-phase or out-of-phase Airy beams and nonlinear accelerating beams in photorefractive crystals by means of direct numerical simulations. Since self-accelerating beams possess infinite energy, we evaluated the propagation of two shifted counter beams by truncating the oscillating tails. The numerical results show that the external bias electric field has a profound effect on the propagation dynamics and the solitons shed from the Airy beam. We also found that during interactions between two nonlinear accelerating beams, single solitons, soliton pairs, and bound breathing solitons were produced. When the nonlinear accelerating beam collides with a soliton beam, the main lobe continues accelerating while the soliton intensity fluctuates with the lobes.

Adjustability of resonance frequency by external magnetic field and bias electric field of sandwich magnetoelectric PZT/NFO/PZT composites

Sandwich magnetoelectric composites of PZT/NFO/PZT (PNP) have been prepared by laminating PZT5, NiFe2O4, and PZT5 ceramics in turn with polyvinyl alcohol (PVA) paste. A systematic study of structural, magnetic and ferroelectric properties is undertaken. Structural studies carried out by X-ray diffraction indicate formation of cubic perovskite phase of PZT5 ceramic and cubic spinel phase of NiFe2O4 ceramic. As increasing the content of PZT5 phase, ferroelectric loops and magnetic loops of PNP composites showed increasing remnant electric polarizations and decreasing remnant magnetic moments separately. Both external magnetic fields and bias voltages could regulate the basal radial resonance frequency of the composites, which should be originated with the transformation and coupling of the stress between the piezoelectric phase and magnetostrictive phase. Such magnetoelectric composite provides great opportunities for electrostatically tunable devices.

Magnetic capacitance of the Gd x Bi1–x FeO3 thin films

The capacitance, inductance, and dissipation factor of the GdxBi1–xFeO3 films were measured in the temperature range of 100 K < T < 800 K in magnetic fields of up to 8 kOe at frequencies of 0.1–100 kHz. The magnetic susceptibility maxima in the low-temperature region and dependences of the relaxation time and inductance on prehistory of the films cooled in zero and nonzero magnetic fields are established. The giant increase in magnetic capacitance in the external bias electric field is found. The results obtained are explained by the domain structure transformation in external electric and magnetic fields.

InGaN light emitting diodes with a nanopipe layer formed from the GaN epitaxial layer.

A Si-heavy doped GaN:Si epitaxial layer is transformed into a directional nanopipe GaN layer through a laser-scribing process and a selectively electrochemical (EC) etching process. InGaN light-emitting diodes (LEDs) with an EC-treated nanopipe GaN layer have a high light extraction efficiency. The direction of the nanopipe structure was directed perpendicular to the laser scribing line and was guided by an external bias electric field. An InGaN LED structure with an embedded nanopipe GaN layer can enhance external quantum efficiency through a one-step epitaxial growth process and a selective EC etching process. A birefringence optical property and a low effective refractive index were observed in the directional-nanopipe GaN layer.

Enhancement of the magnetocapacitance effect in an external electric field in La x Bi1-x FeO3 films

The goal of the study is to determine the value of magnetocapacitance under the substitution of lanthanum for bismuth in thin LaxBi1-xFeO3 films and to find out the effect of an external bias electric field on the magnetocapacitance effect. To solve this task, the dielectric permittivity, the magnetic permeability, and the loss tangent are measured in LaxBi1-xFeO3 films in magnetic fields of up to 8 kOe in the range of temperatures 100 K < T < 1000 K. Maxima of the permittivity and permeability at low temperatures and the dependence of the permeability on the prehistory of a sample are found. An increase in the magnetocapacitance due to the substitution of lanthanum for bismuth is observed. A giant enhancement of magnetocapacitance in an external electric bias field is revealed. These phenomena are attributed to the rearrangement of the domain structure.

Effect of a bias electric field on the structure and dielectric response of the ferroelectric smectic-Aliquid crystal in thin planar cells

The phenomenological continuum model is used to study the director structure and dielectric response of the ferroelectric smectic-A phase in thin planar cells. The frequency of the phase mode and the dielectric permittivity are calculated numerically as a function of the bias external DC electric field, cell thickness, the strength of polar surface anchoring, and the ratio between the bend and splay elastic constants. The theoretically obtained dependencies are in agreement with the recently reported experimental measurements, which show that in thin planar cells both the phase mode frequency and the dielectric permittivity decrease with increasing external bias electric field.

Evolution of domain processes during the transition from classical ferroelectric to relaxor ferroelectric

Dependences of the relative permittivity on the external bias electric field of ceramics of the yPZN-mPMN-nPNN-xPT (x = 0.25–0.40) system have been studied. The evolution of domain processes during the concentration transition from classical ferroelectric to relaxor ferroelectric has been shown. Tentative conclusions on the mechanisms of electric field-induced transitions from the relaxor state to the “normal” ferroelectric state have been drawn.

Tunability and Dielectric Properties of Ba1–x Sr x Ti0.94Sn0.06O3 Ceramics

The Ba 1 − x Sr x Ti 0.94 Sn 0.06 O 3 (BSTS) ceramics were prepared by solid-state reaction method. The microstructure was studied by scanning electron microscopy. The dielectric constant and loss as a function of temperature and external bias electric field were investigated. As strontium concentration increasing, the peak permittivity decreases. The Curie temperature decreases at an average rate of 2.86°C/mol% as the concentration of Sr2 + increases in the system of BS x TS 0.06 . The maximum tunability of 63.5% at applied electric field of 6 kV/cm was obtained with the composition of Ba 0.8 Sr 0.2 Ti 0.94 Sn 0.06 O 3 .

Kinetic Theory of the Langevin Saturation in Dilute Solution of Dipolar Symmetric-Top Molecules in Spherical Solvents

Assuming the low molecular reorientation approximation, the formulae for the third-order electric polarization induced in liquids composed of rigid noninteracting dipolar, symmetric-top molecules in spherical solvents were derived. Our medium is acted on by a strong external dc bias electric field superimposed on a weak ac electric field, and the classical Smoluchowski– Debye equation for rotational diffusion of the symmetric-top molecules is applied. In order to highlight the influence of the anisotropy of rotational diffusion tensor components and the orientation of permanent dipole moment of the molecule on the complex linear and nonlinear electric susceptibilities, we present three-dimensional plots of the linear and nonlinear dispersion and absorption spectra, for different values of the frequency of ac electric field.

Structural and dielectric properties of Ba0.80Sr0.20Ti1 − x Sn x O3 ceramics

The Ba0.80Sr0.20Ti1 − xSnxO3 (BSTS) ceramics were prepared by conventional ceramic method. The crystalline structure and morphology were studied by X-ray diffraction and scanning electron microscopy, respectively. The dielectric constant and loss as a function of temperature and external bias electric field were investigated. Experimental results show that with the increasing of sintering temperature, the peak permittivity increases dramatically and tanδ increases at ferroelectric state while does not change apparently at paraelectric state. With the increasing of the Sn4+ concentration in the BSTS structure, the Curie temperature decreases.

Ultrafast photo-conductivity in BAMH-PPV polymer thin-films

The carrier life time in poly(2,5-bis[ N-methyl- N-hexylamino]phenylene vinylene (BAMH-PPV) is measured by photo-conductive sampling. A systematic study of the influence of both external bias electric field and illumination intensity on the photo-conductance of the polymer is carried out. The photo-conductance remained roughly constant with the applied electric field up to 6 × 10 5 V/cm. Electric pulsewidths of 2 ps were measured. The transient carrier mobility is calculated. For 93 nm thick film the lower limit is estimated to be 0.24 (cm 2/V s) when assuming unity quantum efficiency. This transient mobility value could be 480 (cm 2/V s) when taking quantum efficiency to be 0.05% according to Hendry et al. [E. Hendry, M. Koeberg, J. M. Schins, L.D.A. Siebbeles and M. Bonn, Phys. Rev. B. 70 (2004) 033202].

Dielectric nonlinearity of the glassy material Bi1.8Pb0.3Sr2Ca2Cu2.7K0.3Oz

AbstractThe dependence of the dielectric permittivity on the strength of a dc electric field in the glassy dielectric material Bi1.8Pb0.3Sr2Ca2Cu2.7K0.3Oz was studied. It was revealed that the value of the dielectric constant is decreased by a quadratic law as the dc field strength is increased. Such behavior of the dielectric permittivity under external bias electric field speaks in favor of a dielectric nonlinearity, which can be described in frames of a thermodynamic approach. (© 2006 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Effect of defect induced nucleation and growth on switching behavior in ferroelectrics

The switching behavior in ferroelectrics has been simulated using a time-dependent-Ginzburg-Landau approach, incorporating the contributions of dipolar defects. The model also includes the coupling with elastic fields. The dipolar defects are simulated by an inhomogeneous electric field produced by these localized dipoles. Domain pattern evolution was simulated with the sudden application/removal of the external bias electric field. We found that due to defect induced nucleation, a multi-domain twinned state is stabilized when the external electric field is removed from a poled single domain state. On the application of field in the opposite direction, the twinned state can be switched to a single domain with reverse polarization. The associated switching current has also been calculated.