Electroluminescence in anthracene crystals under time-varying electric fields

Electroluminescence in anthracene crystals has been systematically measured under time varying electric fields at various temperatures using a sodium electrode as the electron-injecting contact and a silver electrode as the hole-injecting contact. The results show that the electroluminescent brightness decreases with increasing frequency over the temperature range from ? 20 to 60°C; and that for a given frequency the brightness increases with increasing temperature, reaches a peak at a critical temperature, and then decreases with increasing temperature. The electroluminescence under a half-wave rectified sinusoidal a.c. field is brighter than that under a sinusoidal a.c. field of the same peak amplitude. There is a delay time between the application of the field and the appearance of electroluminescence, and this delay time decreases with increasing frequency and saturates at a value which may be associated with the transit time of the injected electrons across the specimen. All these results can be explained on the basis of the model that electroluminescence occurs when an electron space charge built up near the anode is sufficient to enhance hole injection from the anode.     

Electroluminescence in lead titanate single crystals

Electroluminescence in PbTiO₃ single crystals is studied with variation in applied electric field, frequency (20 Hz to 5 kHz) and temperature. The EL onset depends on the rate at which the dipole switches. Extremely sharp upward rising nature of the pulses of micro second duration suggest that there is a self maintained discharge in the dielectric due to secondaryγ _p mechanism. Frequency dependence of EL suggests that both the secondary mechanisms, viz. theγ _p andγ _i are active after the application of a high field and the critical field at which this occurs decreases with increase in the frequency of the applied voltage. Similarly the onset voltage decreases with increase in frequency. The temperature dependence of EL at the applied frequency of 50 Hz shows that the onset voltage is intimately connected with the coercive field of the crystal and it is minimum at the Curie point. The study suggests that EL occurs in the bulk and there is a breakdown in the dielectric due to an avalanche formation.     

On the kinetics of slow polarization in the lead magnoniobate ferroelectric relaxor

Polarization and repolarization of a (100)-oriented lead magnoniobate crystal in slowly varying and dc electric fields were measured in a temperature interval including the relaxor and field-induced ferroelectric states. Throughout the interval covered, the polarization was shown to exhibit features characteristic of relaxors, namely, open, nonreproducing polarization trajectories in the first few cycles of quasistatic dielectric hysteresis loops and very long relaxation times. The slow thermally activated relaxation stage follows the universal power law evolution, which permits one to determine possible simple spectra of the relaxation time distribution. Temperature dependences of some relaxation and spectral parameters were derived, and their differences in the relaxor and ferroelectric phases are discussed.     

Biofouling prevention with pulsed electric fields

Temporary immobilization of aquatic nuisance species through application of short electric pulses has been explored as a method to prevent biofouling in cooling water systems where untreated lake, river, or sea water is used. In laboratory experiments, electrical pulses with amplitudes on the order of kilovolts/centimeter and submicrosecond duration were found to be most effective in stunning hydrazoans, a common aquatic nuisance species. Varying pulse amplitude and repetition rate allows us to control the stunning time in a temporal range from minutes to hours. The temporary immobilization is assumed to be caused by reversible membrane breakdown. This assumption is supported by results of measurements of the energy required for stunning. Based on the data obtained in laboratory experiments, field experiments in a tidal mater environment have been performed. The flow velocity was such that the residence time of the aquatic nuisance species in the system was approximately half a minute. The results showed that the pulsed electric field method provides full protection against biofouling when pulses of 0.77 μs width and 6 kV/cm amplitude are applied to the water at the inlet of such a cooling water system. Even at amplitudes of 1 kV/cm, the protection is still in the 90% range, at an energy expenditure of 1 kWh for the treatment of 60 000 gallons of water     

Polarization of strontium-barium niobate crystals in pulsed fields

Polarization (ferroelectric switching) of SBN crystals in pulsed fields differs from the same process in model ferroelectrics. Polarization in most SBN crystals is characterized by slow kinetics, which can be approximated by a power-law dependence, with relaxation times of the order of seconds (in fields

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). The process smoothness results in the absence of a characteristic peak in the switching-current curve. Some crystals are characterized by fast (jumplike) polarization processes, whose characteristics (the kinetics and the field dependence of the switched charge) also differ from model ones.

     

SiN分子外电场情况下的发光特性

为全面分析外电场对分子发光特性的影响, 本文采用密度泛函B3P86方法6-31g(d)基组, 对SiN分子进行了基态结构的优化, 进而使用含时密度泛函方法(time dependent density functional theory, TDDFT), 计算了不同方向及大小的外电场情况下SiN分子的吸收谱、激发能、振子强度、跃迁偶极矩. 通过比较发现外电场对该分子的激发能、吸收谱、跃迁振子强度及跃迁偶极矩影响都比较明显, 说明了电场对SiN分子的激发特性影响比较复杂, 特别是在加场前后分子均有在可见光区波段的吸收谱, 这对研究分子的发光很有意义.同时对该分子所发可见光谱的产生机理进行了分析, 并与已有实验结果进行比较.     

Hysteresis phenomena in the course of polarization evolution in a sinusoidal electric field in lead magnesium niobate crystals

The processes of polarization evolution in single crystals of the PbMg_1/3Nb_2/3O₃ model ferroelectric relaxor in a sinusoidal electric field are investigated at temperatures near and above the temperature T _d 0 of destruction of the induced ferroelectric state upon heating in zero electric field. The polarization switching current loops are measured in the ac electric field applied along the 〈111〉 and 〈110〉 pseudocubic directions. The electroluminescence intensity loops are obtained under the combined action of ac and dc electric fields applied along the 〈100〉 direction. In a certain temperature range above T _d 0 and the freezing temperature T _f in lead magnesium niobate, there are electric current anomalies, that correspond to the dynamic formation and subsequent destruction of the ferroelectric macroregions throughout each half-cycle of the ac electric field. The measurements of electroluminescence hysteresis loops demonstrate that the observed depolarization delay (related to the ac electric field amplitude) increases with an increase in the dc electric field and decreases as the ac field amplitude increases. The nature of the observed phenomena is discussed.     

Quantum-mechanical calculations and analysis of vibrational modes in lead magnoniobate

The vibrational modes in lead magnoniobate have been calculated using the frozen phonon method. It has been shown that lead atoms participate in two radically different vibrational modes, namely, a nonpolar antiferrodistortive mode, which involves the antiphase motion of some lead atoms with respect to other lead atoms, and a polar mode, which includes the polar motion of lead atoms with respect to other atoms of the lattice. The nontrivial properties of lead magnoniobate have been discussed in terms of the unity and conflict of these modes.     

An active antenna for measuring pulsed electric fields

The feasibility of creating an ultra-wideband receiving antenna comprising a dipole loaded on an active element based on field-effect transistors is analyzed. The antenna is intended for registration of waveforms and measurement of instantaneous electric field strengths for electromagnetic radiation pulses of nanosecond and subnanosecond durations. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 5, pp. 82–86, May, 2007.     

Scattering polarization in the presence of magnetic and electric fields

The polarization of radiation by scattering on an atom embedded in combined external quadrupole electric and uniform magnetic fields is studied theoretically. Limiting cases of scattering under Zeeman effect, and Hanle effect in weak magnetic fields are discussed. The theory is general enough to handle scattering in intermediate magnetic fields (Hanle-Zeeman effect) and for arbitrary orientation of magnetic field. The quadrupolar electric field produces asymmetric line shifts, and causes interesting level-crossing phenomena either in the absence of an ambient magnetic field, or in its presence. It is shown that the quadrupolar electric field produces an additional depolarization in the Q/I profiles and rotation of the plane of polarization in the U/I profile over and above that arising from magnetic field itself. This characteristic may have a diagnostic potential to detect steady-state and time-varying electric fields that surround radiating atoms in solar atmospheric layers.     

脉冲强磁场下的电极化测量系统

多铁性材料是当前物质科学研究的热点,具有重要的科学研究意义和应用前景.低温和强磁场实验环境为研究多铁性材料提供了一种有效途径.脉冲强磁场下的电极化测量系统能实现最高磁场强度60 T、最低温度0.5 K的铁电特性测量.该系统采用热释电方法,具有磁场强度高、控温范围广、转角测量等特点,可用于强磁场下的磁电特性研究.本文介绍了该系统的测量装置和实验原理,并展示了其在多铁性材料研究中的一系列应用,揭示了脉冲强磁场电极化测量系统在磁电特性探索中的重要作用.     

Photoinduced decrease in local polarization switching fields in lithium niobate crystals

Variations in local polarization switching fields E _C in lithium niobate crystals have been investigated under laser irradiation of the z surface. It has been established that the simultaneous application of an electric field in the direction opposite to the spontaneous polarization field and laser beam leads to a decrease in the field E _C .     

Out-of-plane spin polarization from in-plane electric and magnetic fields

We show that the joint effect of spin-orbit and magnetic fields leads to a spin polarization perpendicular to the plane of a homogeneous two-dimensional electron system with Rashba spin-orbit coupling and in-plane parallel dc magnetic and electric fields, for angle-dependent impurity scattering or nonparabolic energy spectrum, while only in-plane polarization persists for simplified models. We derive Bloch equations, describing the main features of recent experiments, including the magnetic field dependence of static and dynamic responses.     

A study of the microwave dielectric permittivity of liquid crystals in electric and magnetic fields

A microwave detector based on a self-sustained oscillator circuit is proposed as a means to investigate the real and imaginary components of the dielectric permittivity of liquid crystals in external electric and magnetic fields. Results are given for measurements of a 500 MHz oscillator frequency for two types of nematic crystals, 5CBP and MBBA. Fundamental regularities are identified in the behavior of the microwave dielectric permittivity of samples in electric and magnetic fields. It is shown that the minimum of the high-frequency dielectric loss in liquid crystals correspond to a situation in which the long axes of the molecules are oriented parallel to the direction of the microwave electric field. Zh. Tekh. Fiz. 68, 117–121 (January 1998)     

Electroluminescence and internal screening of domains upon polarization switching of ferroelectric ceramics in a pulsed self-consistent field

This paper reports on the results of investigations into the electroluminescence induced by polarization switching of a ferroelectric ceramic material in an electric field with a rapidly increasing strength. A correlation between the polarization switching and luminescence kinetics is revealed. It is demonstrated that tunneling of electrons to the conduction band and impact ionization in field concentrators are the main mechanisms of generation of free charge carriers responsible for the electroluminescence and internal screening of switching domains.     

Experimental study and computer simulations of the implantation of laser plasma ions in pulsed electric fields

Results are presented from experimental studies of pulsed plasma flows generated by nanosecond laser pulses with an intensity of 7 × 10⁸ W/cm² from a solid-state target in a strong electric field. The current pulses through the laser target and the depth distributions of the iron ions implanted in a silicon substrate to which a negative high-voltage pulse was applied are measured. The physical processes occurring in laser plasma with an initial iron ion density of 6 × 10¹⁰ cm⁻³ are simulated numerically by the particle-in-cell method for different delay times and different shapes of the accelerating high-voltage pulse. The model developed allows one to calculate the ion flows onto the processed substrate, the electron flows onto the target, and the energy spectra of the implanted ions. The results from computer simulations are found to be in good agreement the experimental data.