Dc Bias Electric Field Research Articles

Phase shift of terahertz Bloch oscillations induced by interminiband mixing in a biased semiconductor superlattice

We investigate terahertz waveforms emitted from a GaAs-based superlattice with interminiband mixing under various dc bias electric fields, injecting electrons into the conduction first miniband with femtosecond optical pulses. Bloch oscillations are observed under relatively low bias fields; surprisingly, the oscillation phase is found to shift by π/2 from those reported previously for Wannier–Stark ladder states in nearly isolated minibands. The oscillatory feature of terahertz emission is gradually destroyed under higher bias fields. We show that the observed π/2 phase shift can be ascribed to significant delocalization of electron wavefunctions driven by interminiband mixing.

Modeling ferroelectric domain wall interaction with point defects and losses: the damping function method

The displacement of the ferroelectric domain wall l is modeled by the equation of a particle in a force field. The forces that act in the domain walls are due to the external electric field and the internal electric and elastic interactions, which are represented by the effective potential . The potential is chosen to describe a nonlinear dielectric response in a dc bias electric field. This model also correctly reproduces a real component of dielectric permittivity in a subswitching alternating electric field, but the predicted losses are lower than the experimental values. Subswitching dielectric losses are produced mostly by the interaction of domain walls with defects. Point defects can be modeled as a perturbation of the potential. Another way to model point defects is introducing a generalized damping function so that the average displacement of domain walls is determined by a single equation. The equivalence of both theoretical approaches is demonstrated. An analytical expression of the damping function is derived from the Rayleigh law. Simulations show that the method of the damping function correctly reproduces a nonlinear dielectric response and losses. The damping function method could be a useful theoretical framework to elucidate the friction force generated by the interaction of ferroelectric domain walls with defects, and it can be extended to study similar phenomena such as ferroelasticity and ferromagnetism.

Dielectric properties of four room temperature ferroelectric and antiferroelectric multi-component liquid crystalline mixtures

ABSTRACTDielectric properties of four recently formulated room temperature multi-component liquid crystalline mixtures with paraelectric (SmA*), ferroelectric (SmC*) and antiferroelectric (SmC*A) phases have been studied as a function of temperature and frequency. Under planer anchoring condition, dielectric spectroscopy revealed all the characteristic modes: low frequency PL and high frequency PH mode in SmC*A phase, Goldstone mode (GM) in SmC* phase and soft mode (SM) in SmA* phase. Dielectric behaviour has also been studied under the application of DC bias electric field. With bias electric field, we have been able to study the soft mode dielectric behaviour in the SmC* phase. An unknown high frequency mode (X-mode) with and without bias is also observed in SmC* phase. Dielectric results are explained in the light of generalised Landau theory. The mixtures show very high soft mode electroclinic coefficient in the SmA* phase in addition to fast switching in SmC*A and SmC* phases [30].

Electric-field-dependent mechanical and electrical properties of barium strontium titanate films for tunable device applications

Developing the novel electric field tunable, reconfigurable, adaptive and frequency-agile microwave devices with specific properties involves technological and scientific activities in the studying of material properties. Measurements of its properties could be a difficult and expensive issue, especially when it concerns thin-film ferroelectrics. Their electromechanical properties change significantly under applied DC bias field. The material coefficients of barium strontium titanate thin films are still unknown; therefore, it is expedient to carry out their calculations. In the present paper, the linear equations of piezoelectric effect have been written for thin barium strontium titanate films under DC bias electric field in paraelectric phase (induced piezoelectric effect) based on the grand potential of Landau theory of phase transitions. The dependence of material constants for thin films of Ba0.65Sr0.35TiO3 (BST65) on the applied planar field at different lattice misfit strains has been studied for this phase. All induced piezoelectric moduli show extreme behavior during field alteration. Elastic moduli show similar abnormal behavior. The alterations increase as misfit strain is approaching the critical value.

A Twofold Approach in Loss Reduction of KTa0.5Nb0.5O3 Ferroelectric Layers for Low-Loss Tunable Devices at Microwaves.

Ferroelectric oxide films are attractive to design and fabricate reconfigurable and miniaturized planar devices operating at microwaves due to the large electric field dependence of their dielectric permittivity. In particular, KTa1-xNbxO3 (KTN) ferroelectric material presents a high tunability under moderate dc bias electric field. However, its intrinsic dielectric loss strongly contributes to the global loss of the related devices and limits their application areas at microwaves. In this paper, a twofold approach is investigated to reduce the device loss. The intrinsic loss of KTN is first reduced by doping the ferroelectric material with a low-loss dielectric material, namely, MgO. Second, the doped ferroelectric films are confined using an original laser microetching process. Both routes have been implemented here to provide a synergic effect on the total insertion loss of the microwave test device, namely, a coplanar waveguide stub resonator. The experimental data demonstrate a decrease of the intrinsic loss by a factor of ~2 and a decrease of the global loss by a factor of ~4 with a frequency tunability close to 10% at ~10 GHz under a moderate biasing (80 kV/cm).

Non-trivial behavior of the low temperature maximum of dielectric constant and location of the end critical point in Na0.5Bi0.5TiO3-0.06BaTiO3 lead free relaxor ferroelectrics crystals detected by acoustic emission

[001] lead free relaxor ferroelectrics crystals of Na0.5Bi0.5TiO3–0.06BaTiO3 were studied by means of dielectric and acoustic emission methods in the temperature range of 25–240 °C and under a dc bias electric field up to 0.4 kV/cm. A temperature maximum of the dielectric constant was found near 170 °C, as well as the acoustic emission bursts pointed out to both the depolarization temperature near 120 °C and the temperature, corresponding to the maximum of dielectric constant, near 170 °C. While the depolarization temperature increased linearly, the temperature of the dielectric constant maximum was shown to exhibit a V-shape behavior under an electric field: it initially decreases, reaches a sharp minimum at some small threshold electric field of 0.15 kV/cm, and then starts to increase similar to the Curie temperature of the normal ferroelectrics, as the field enhances. Acoustic emission bursts, accompanying the depolarization temperature, weakened with the enhancing field, whereas the ones accompanying the temperature of the dielectric constant maximum exhibited two maxima: near 0.1 kV/cm and near 0.3 kV/cm. The meaning of these two acoustic emission maxima is discussed.

Reducing the sensitivity of Rydberg atoms to dc electric fields using two-frequency ac field dressing

We propose a method for reducing the sensitivity of atomic ground to Rydberg transitions to stray dc electric fields, using microwave-induced dressing of Rydberg states. Calculations are presented for the Cs $\state{90S}{1/2}$ and $\state{90P}{3/2}$ states. With zero dc bias electric field, a two-frequency ac field is used to simultaneously reduce the sensitivity of both states to dc field variations. The sensitivity reduction is a factor of 95 for the $\state{90S}{1/2}$ state and a factor of 1600 for the $\state{90P}{3/2}, m_J=3/2$ state. We also show how the two-frequency ac field can be used to cancel both second- and fourth-order terms in the polarizability of a single Rydberg state. These results are relevant to improving the stability of experiments that seek to excite Rydberg atoms in the proximity of charged surfaces.

Debye-like relaxation behavior and electric field induced dipole re-orientation of the 0.6BaTiO3-0.4Bi(Mg1/2Ti1/2)O3 ceramic

Mechanisms of the dielectric relaxation and electric field induced dipole re-orientation process in the 0.6BaTiO3-0.4Bi(Mg1/2Ti1/2)O3 ceramic have been investigated by a combination of experiments and simulations. Using the modified Debye relaxation theory, the observed strong relaxation is related to the evolution of the distribution of relaxation times with increasing temperatures. The increased freezing temperature under dc bias electric fields indicates that the nonlinear dielectric behavior has positive contributions to the total polarization. In relaxor state, the dielectric response of the 0.6BaTiO3-0.4Bi(Mg1/2Ti1/2)O3 ceramic shows a maximum value with the increasing external fields, which is attributed to two main polarization mechanisms, lattice phonon polarization and re-orientation of polar clusters. The contribution of these two polarization mechanisms is separated by the multi-polarization model, indicating that the re-orientation of polar clusters is responsible for the increase of the dielectric constant. At last, Monte Carlo method is performed to simulate the dipole re-orientation process, and the simulated results are consistent with experimental findings. This study provides a different perspective to investigate the relaxor materials with isolated polar clusters.

High Efficient THz Emission From Unbiased and Biased Semiconductor Nanowires Fabricated Using Electron Beam Lithography

Besides having perfect control on structural features, such as vertical alignment and uniform distribution by fabricating the wires via e-beam lithography and etching process, we also investigated the THz emission from these fabricated nanowires when they are applied DC bias voltage. To be able to apply a voltage bias, an interdigitated gold (Au) electrode was patterned on the high-quality InGaAs epilayer grown on InP substrate by molecular beam epitaxy. Afterwards, perfect vertically aligned and uniformly distributed nanowires were fabricated in between the electrodes of this interdigitated pattern so that we could apply voltage bias to improve the THz emission. As a result, we achieved enhancement in the emitted THz radiation by ∼four times, about 12 dB increase in power ratio at 0.25 THz with a DC biased electric field compared with unbiased NWs.

Dielectric Responses of Polyurethane Films Related to Temperature and Frequency

The dielectric properties of polyurethane were investigated at the temperature range of −170 to 100°C and at the frequency range of 0.01 Hz to 3 kHz. Two dielectric relaxations were observed related to the glass transition of the soft segments (α‐relaxation) close to −15°C and related to the mobility of the extender connecting the hard segments (I‐relaxation) near 50–85°C. For α‐relaxation process, the relaxation time, τ = 0.012 and 5.3 × 10−3 s for a film of 2.2 mm and 42 µm in thickness at −15°C were estimated. As temperature increased, the newly appeared I‐relaxation at the temperatures above 25°C resulted in a different relaxation process. The two relaxations followed the temperature–frequency superposition principle with different shifting factors. The polyurethane films with two different thicknesses exhibited equivalent types of relaxations but different relaxation times and shifting rates depending on their intermolecular arrangements. They showed stable dielectric responses and losses under varied DC bias electric fields.

Dielectric relaxation, impedance spectra, piezoelectric properties of (Ba, Ca)(Ti, Sn)O 3 ceramics and their multilayer piezoelectric actuators

Abstract In this study, an improved method was developed for the preparation of lead-free (Ba1−xCax)(Ti0.91Sn0.09)O3 (BCTS) (0.02 ≤ x ≤ 0.07) ceramics by combining the conventional solid-state reaction with a gel precursor mixing method. The effects of Ca content on the microstructural, dielectric, impedance, piezoelectric and ferroelectric properties of the ceramics were systematically investigated. Dense ceramics with a uniform distribution of grain size and sharp grain boundaries were developed. Extremely high electrical properties of d33 ∼620 pC/N, kp ∼46% and e ∼14500 were obtained at x = 0.03. A dielectric loss peak was observed, correlating to the oxygen vacancy relaxation at elevated temperature. Moreover, the dielectric properties vs. DC bias electric field and the temperature dependences of electrical properties were studied as well. Lastly, BCTS-based monolithic multilayer piezoelectric actuators were successfully fabricated showing large displacements under low driven voltages. These results would be valuable for understanding the electric properties of BCTS ceramics that would in turn extend their applications in multilayer piezoelectric devices.

Degraded grain size effect of barium strontium titanate ceramics under a direct current bias electric field

The influence of a DC bias electric field (E) on the grain size effect of Ba0.70Sr0.30TiO3 (BST) ceramics was systematically investigated. As E increased from 0 to 800 V mm−1, the difference of phase-transition diffuseness (ΔTd), dielectric constant maximum (εp) and corresponding tanδp of BST samples with different grain sizes (0.8–8 μm) decreased from 44.6% to 13.2%, 64.9% to 17.5% and 84.2% to 22.3%, respectively. The greatly degraded size-effect could be attributed to the stabilizing effect of E on the ferroelectric phase. A critical field (Ecrit) denoting the grain size effect vanishes and is evaluated to be about 1500 V mm−1. The relation and physical mechanisms are useful for ferroelectric applications.

Field induced changes in the ring/chain equilibrium of hydrogen bonded structures: 5-methyl-3-heptanol.

Using non-linear dielectric techniques, we have measured the dynamics of 5-methyl-3-heptanol at a temperature at which the Kirkwood correlation factor gK indicates the coexistence of ring- and chain-like hydrogen-bonded structures. Steady state permittivity spectra recorded in the presence of a high dc bias electric field (17 MV/m) reveal that both the amplitude and the time constant are increased by about 10% relative to the low field limit. This change is attributed to the field driven conversion from ring-like to the more polar chain-like structures, and a direct observation of its time dependence shows that the ring/chain structural transition occurs on a time scale that closely matches that of the dielectric Debye peak. This lends strong support to the picture that places fluctuations of the end-to-end vector of hydrogen bonded structures at the origin of the Debye process, equivalent to fluctuations of the net dipole moment or gK. Recognizing that changes in the ring/chain equilibrium constant also impact the spectral separation between Debye and α-process may explain the difference in their temperature dependence whenever gK is sensitive to temperature, i.e., when the structural motifs of hydrogen bonding change considerably.

Negative Capacitance in BaTiO3/BiFeO3 Bilayer Capacitors.

Negative capacitances provide an approach to reduce heat generations in field-effect transistors during the switch processes, which contributes to further miniaturization of the conventional integrated circuits. Although there are many studies about negative capacitances using ferroelectric materials, the direct observation of stable ferroelectric negative capacitances has rarely been reported. Here, we put forward a dc bias assistant model in bilayer capacitors, where one ferroelectric layer with large dielectric constant and the other ferroelectric layer with small dielectric constant are needed. Negative capacitances can be obtained when external dc bias electric fields are larger than a critical value. Based on the model, BaTiO3/BiFeO3 bilayer capacitors are chosen as study objects, and negative capacitances are observed directly. Additionally, the upward self-polarization effect in the ferroelectric layer reduces the critical electric field, which may provide a method for realizing zero and/or small dc bias assistant negative capacitances.

Electrostrictive effect for active control of surface plasmon signals.

Real-time surface plasmon modulation was achieved by electrically varying the pitch of a nanoscale surface relief diffraction grating inscribed on an azobenzene thin film covered with a layer of silver. The azobenzene film was spin coated on an electrostrictive Lead Lanthanum Zirconate Titanate (PLZT) ceramic substrate and a combination of DC bias and AC electric fields were applied longitudinally on the PLZT ceramic causing a change in the grating's pitch as well as the surface plasmon's resonance wavelength. This method permits extremely accurate control of the surface plasmon wavelength for tunable optics applications.

Effect of electric field on the dielectric properties of the Barium Strontium Titanate film

We investigated the tunability of the dielectric characterization of the Ferroelectric Barium Strontium Titanate (Ba0.5Sr0.5TiO3) thin film on the Strontium Titanate substrate by a dc bias electric field through the terahertz time-domain spectroscopy. The tunability of the real part of the permittivity reached up to 74.8% and the imaginary part of the permittivity reached up to 33.6% when the bias electric field up to 30 V. And the results are attributed to the soft mode hardening caused by the electric field.

Mechanical energy harvesting via a plasticizer-modified electrostrictive polymer

A plasticizer-modified electrostrictive terpolymer with improved electromechanical properties was investigated for mechanical energy harvesting via a pseudo-piezoelectric cycle. Experimental results indicate that the modified terpolymer exhibited improved energy harvesting performances in contrast with the pure terpolymer. A maximum generated short-circuit current of 3.635 μA (much higher than the leakage current) and a power density of 607 μW/cm3 were observed for modified terpolymer. Based on the quadric relationship between the energy conversion efficiency and the DC bias electric field, an energy conversion efficiency as high as 34% and a corresponding electrical power density of 4.31 mW/cm3 could be achieved for the modified terpolymer working at a DC bias electric field of 30 MV/m.

Reduction of the dc-electric-field sensitivity of circular Rydberg states using nonresonant dressing fields

Non-resonant dressing fields can make the transition frequency between two circular Rydberg states insensitive to second order variations in the dc electric field. Perturbation theory can be used to establish the required dressing field amplitude and frequency. The same perturbative approach may be used to understand removal of the first order dependence of the transition frequency on electric field about a bias dc electric field [Hyafil et al. Phys. Rev. Lett., v. 93, 103001 (2004)]. The directional alignment of the dressing and dc fields is critical in determining the electric field sensitivity of the dressed transition frequencies. This sensitivity is significantly larger for circular Rydberg states compared to low-angular momentum Rydberg states of Rb.

Discharging and energy-releasing properties of Pb0.90La0.04Ba0.04[(Zr0.6Sn0.4)0.85Ti0.15]O3 antiferroelectric ceramics under different electric fields

Pb0.90La0.04Ba0.04[(Zr0.6Sn0.4)0.85Ti0.15]O3 (PLBZST) antiferroelectric (AFE) ceramics were fabricated via the traditional solid-state ceramics processing method and the pulsed discharging current waveforms were measured under different electric fields. The discharging current of the AFE ceramic with an area of only 15 mm2 could be as high as 50 A under 2.8 kV/mm. The analysis of hysteresis loops, dielectric constant under DC bias electric field and the discharging current waveforms indicate that the AFE dielectrics are almost linear under low electric field but show remarkable nonlinearity under high electric field. Under 2.8 kV/mm, 0.33 J/cm3 energy could be released from the stored 0.47 J/cm3 energy. The discharging efficiency is 70 %.

Dielectric Tunability and Relaxor Properties of the 0.96Pb(Ni1/3Nb2/3)O3-0.04PbZrO3 Ceramics

0.96Pb(Ni1/3Nb2/3)O3-0.04PbZrO3 (PNN-PZ) ceramics prepared via columbite method exhibit high crystallinity and high density, typical relaxor behavior is demonstrated by the dielectric data. The high dielectric tunability (˜87%) and low dielectric loss (˜0.007) were obtained for PNN-PZ at room temperature around 30kV/cm and 1 kHz, at fields below 13kV/cm, a double-wells model work well between tunability and dc bias electric field, in contrast, at all fields, the thermodynamic model can do better in fitting the dielectric nonlinear tunability behavior of the PNN-PZ ceramics than the double-wells model. These properties indicate that PNN-PZ ceramics might be a promising candidate for tunable materials.