MW Field Research Articles

Microwave-induced parametric instability of 2D magnetoplasmons

Recent experiments on microwave-irradiated ultraclean 2DES revealed a colossal narrow photoresistivity peak in the vicinity of the second cyclotron resonance harmonic 2ωc. Here we propose an explanation of this puzzle phenomenon in terms of the parametric instability of 2D magnetoplasmons. We develop the hydrodynamic theory of 2D magnetoplasmons under an inhomogeneous electric field of the MW pumping with frequency Ω. Such a strong inhomogeneous MW field is always induced by lateral metal contacts, which are used to measure photoresistivity. We show that the parametric instability of the 2D magnetoplasmons arises near Ω = 2ωc when the gradient of the electric field of the pumping exceeds a threshold value. The instability leads to heating of 2DES, which in turn leads to the sharp photoresistivity peak.

Wireless microfluidic control with integrated shape-memory-alloy actuators operated by field frequency modulation

This paper reports wireless microfluidic control enabled by the selective operation of multiple bulk-micromachined shape-memory-alloy actuators using external radiofrequency magnetic fields. Each shape-memory-alloy actuator is driven by a wireless resonant heater which generates heat only when the field frequency is tuned to the resonant frequency of the heater. Multiple actuators coupled with the heater circuits that are designed to have different resonant frequencies in the range of 135–295 MHz are selectively and simultaneously controlled by modulating the field frequency to the resonant frequencies of the corresponding heaters. A wireless microsyringe device that has three actuator–heater components and a flexible parylene reservoir is developed. The 5 µl reservoir is squeezed by the 5 mm long cantilever-type actuators to eject controlled amount of liquid from the reservoir. Using the device with an acidic solution loaded in the reservoir, sequential modifications of the pH level in the liquid are experimentally demonstrated through the selective control of the three actuators. The thermal characterization of the actuator using infrared imaging shows a temperature increase of 50 °C in 4 s and the full activation of the actuator in 8 s with 300 mW field output power.

A low-cost implementation of EPR detection in a dissolution DNP setup

The implementation of electron paramagnetic resonance (EPR) detection in a low-temperature dissolution dynamic nuclear polarization (DNP) setup is presented. Using a coil oriented parallel to the static magnetic field, the change of the longitudinal magnetization of free radicals is measured upon resonant irradiation of an amplitude or frequency modulated microwave (mw) field. The absorption EPR spectrum is measured if the amplitude of the mw field is modulated, whilst the first derivative of the spectrum is obtained with frequency modulation. Using a burst of pulses, it is also possible to perform pump–probe experiments such as saturation-recovery or electron–electron double resonance experiments. Furthermore, the magnetization could be monitored in a time-resolved manner during amplitude modulation, thus making it possible to record its transient as it is approaching an equilibrium value. Experimental examples are shown with frozen solutions of trityl radical and TEMPO, two commonly used radicals for dissolution DNP experiments.

Nonlinear transportgoverned enhancement of electro-optic nonlinearity by a strong-microwave field

Nonlinear hot carrier transport features and relatedoptical nonlinearities are investigated under nonuniformillumination and carrier heating in microwave (mw)-biased high-resistivity GaAs crystals. The dynamics of light-interferencepattern-induced nonequilibrium carriers and evolution of internal electric field are studied in mw fieldsextending above a region of negative differential conductivity.We analysed the peculiarities of nonlinear transport in a spatiallymodulated structure at various photoexcited carrierconcentrations, which was kept below its threshold for mw-induced high-field Gunn-domain grating formation. Atlow-illumination levels, we found a three-fold increase in thespace-charge (SC) field due to electron gas heating. At higher lightintensities, enhancement of the internal field is determined bya fast screening of the external mw field and the transportof nonuniformly heated carriers. Hot carrier transport effectsin a region of negative differential conductivity, even atnearly homogeneous but high illumination, lead to SC-wave formation with amplitudes, which may exceed the diffusiveSC-field amplitude by 100 times. The latter effectwas confirmed experimentally using a four-wave mixingtechnique in mw-biased GaAs crystals and short laserpulses.

Solid effect in the electron spin dressed state: A new approach for dynamic nuclear polarization

We describe a new type of solid effect for dynamic nuclear polarization (DNP) that is based on simultaneous, near resonant microwave (mw) and radio frequency (rf) irradiation of a coupled electron nuclear spin system. The interaction of the electron spin with the mw field is treated as an electron spin dressed state. In contrast to the customary laboratory frame solid effect, it is possible to obtain nuclear polarization with the dressed state solid effect (DSSE) even in the absence of nonsecular hyperfine coupling. Efficient, selective excitation of dressed state transitions generates nuclear polarization in the nuclear laboratory frame on a time scale of tens of μs, depending on the strength of the electron–nuclear coupling, the mw and rf offset and field strength. The experiment employs both pulsed mw and rf irradiation at a repetition rate comparable to T1e−1, where T1e is the electronic spin lattice relaxation time. The DSSE is demonstrated on a perdeuterated BDPA radical in a protonated matrix of polystyrene.

The spectral forms of transient ESR spectra of radical pairs and the origin of the antiphase structure

We show that transient electron spin resonance (ESR) spectra of short lived radical pairs can be written formally as a superposition of Lorentzian-like lines and the corresponding dispersive lines. The coefficients to these spectral components depend strongly on the frequency and amplitude of the microwave (mw) field and consequently a variety of spectral forms may result. The coefficients, and thus the spectral form, are determined by the interradical interaction induced rate of change of the longitudinal and transversal electron spin polarization. The longitudinal polarization is responsible for the well known chemically induced dynamic electron polarization (CIDEP) which changes the intensity of the lines without affecting the line shape. The transversal polarization is shown to be responsible for the antiphase structure (APS) in accordance with our previous proposition [J. Chem. Phys. 109, 8743 (1998)]. A simple one re-encounter model calculation illustrates and explains qualitatively most of the observed APS characteristics. Numerical calculations are used to discuss anomalously large APS splitting.

Theory of non-Markovian relaxation of single triplet electron spins using time- and frequency-domain magnetic resonance spectroscopy measured via optical fluorescence: Application to single pentacene molecules in crystallinep-terphenyl

Fluorescence-detected magnetic resonance (FDMR) allows one to monitor magnetic resonance phenomena via fluorescence. Experimental FDMR data obtained using single triplet-state chromophore guest molecules in a low-temperature organic host matrix are analyzed using a stochastic approach to describe triplet electron spin dephasing resulting from frequency fluctuations ${U}^{t}$ induced by host-matrix proton spin dynamics. Modeling the fluctuations ${U}^{t}$ by a sum of N independent random telegraph processes with the same jump rate $\ensuremath{\nu}$ but different variances ${\ensuremath{\sigma}}_{k}$ we construct an exact set of equations for the density matrix of a five-level molecule averaged over fluctuation histories ${U}^{t}.$ These equations provide a basis to study non-Markovian effects of microwave- (MW-) field-dependent dephasing in the FDMR response of a molecule undergoing slow fluctuations ${U}^{t} ({\ensuremath{\sigma}}^{2}/{\ensuremath{\nu}}^{2}>~1, {\ensuremath{\sigma}}^{2}=\ensuremath{\sum}{\ensuremath{\sigma}}_{k}^{2})$ to a MW field that is resonant with a transition between triplet spin substates. Both frequency- and time-domain FDMR phenomena such as (i) power-broadened FDMR line shapes, (ii) FDMR Hahn echo signals, and (iii) FDMR free induction decay are studied. Analytical expressions for the FDMR response are obtained in the case $\ensuremath{\nu}\ensuremath{\gg}{k}_{j}^{i}$ where ${k}_{j}^{i}$ is an intersystem crossing rate. Experimental data on power-broadened line shapes for a pentacene+p-terphenyl pair which demonstrate a pronounced effect of MW-field-suppressed dephasing are explained in the context of the theory.

Numerical calculation and statistical analysis of MW field intensity on synchronized emitters

Usually, there is one MF emitter for an AM station working at one frequency in the metropolis. The authors consider three same-frequency low power AM emitters working on synchronized broadcasting. Previous field measurements in Shanghai have shown that the reception quality of three low power emitters working synchronously was rather better than a single high power emitter. This article proposes the specifications and the statistical rules of a synchronized emitter by means of computer visualization, and presents field intensity maps through calculation, and makes comparisons and analyses the results.

The manifestation of degenerate electron exchange in stimulated nuclear polarization at high magnetic fields

A detailed theoretical and experimental investigation of the degenerate electron exchange (DEE) effect on the stimulated nuclear polarization (SNP) intensity and spectra at high magnetic fields has been made. The theoretical description of DEE processes is performed in two different ways: (i) The numerical solution of equations of noncorrelated frequency migration theory and (ii) the analytical solution for simple two-site model for the dependence of the SNP intensity on the rate of DEE at low microwave (mw) field amplitudes ( B 1 ⪡ a, a is a constant of hyperfine interaction (HFI) of radical-ions) and low DEE rates ( aτ e ⪢ 1, τ e is the characteristic time of DEE). It is shown that the increase of DEE rate leads to the decrease of splitting in SNP spectra, and also to a substantial decrease of the SNP signal intensity due to an overlapping of hyper-fine structure lines with opposite signs. A numerical evaluation of DEE rate constants can be done by comparison of experimental and calculated spectrum shape in the case of fast DEE rates and from the dependence of SNP intensity in resonance on the concentration of diamagnetic molecules.

Stability analysis of MHD disk generators and application to power systems with CO 2 recovery

Preliminary design and stability analyses of magnetohydrodynamic (MHD) disk generators have been carried out. These are used in electric power-generation systems with CO 2 recovery. Inflow disk channels show better performance than outflow disk channels. An inflow channel with supersonic flow shows an enthalpy extraction ratio of 20.2% when the thermal input is 1100 MW and the magnetic field is 8 T. The calculated enthalpy extraction is 23.5% with 2000 MW and 10 T. A stability analysis has shown that it is possible to stabilize the open-cycle disk channels, although the channels tend to become unstable at the exit region when they are operated with 10 T. Cycle analyses have been carried out for an electric power system in which coal is burnt with 95% oxygen and CO 2 is recovered as liquid. A high cycle efficiency of 43% (higher heating value, HHV) can be obtained for a thermal input of about 1000 MW and magnetic field of 8 T while 46% HHV results from the system with about 2000 MW and 10 T, even when the power required for CO 2 liquefaction is considered.

Effects of weak microwave fields amplitude modulated at ELF on EEG of symmetric brain areas in rats

Averaged electroencephalogram (EEG) frequency spectra were studied in eight unanesthetized and unmyorelaxed adult male rats with chronically implanted carbon electrodes in symmetrical somesthetic areas when a weak (0.1-0.2 mW/cm) microwave (MW, 945 MHz) field, amplitude-modulated at extremely low frequency (ELF) (4 Hz), was applied. Intermittent (1 min "On," 1 min "Off") field exposure (10-min duration) was used. Hemispheric asymmetry in frequency spectra (averaged data for 10 or 1 min) of an ongoing EEG was characterized by a power decrease in the 1.5-3 Hz range on the left hemisphere and by a power decrease in the 10-14 and 20-30 Hz ranges on the right hemisphere. No differences between control and exposure experiments were shown under these routines of data averaging. Significant elevations of EEG asymmetry in 10-14 Hz range were observed during the first 20 s after four from five onsets of the MW field, when averaged spectra were obtained for every 10 s. Under neither control nor pre- and postexposure conditions was this effect observed. These results are discussed with respect to interaction of MW fields with the EEG generators.

Metallorganic and Microwave Processing of Cermets

ABSTRACTPreliminary results are decribed on synthesis of ceramic-metal composites with a functional gradient material character utilizing infiltration of a porous metallic matrix with ceramic precursors and thermal processing by MW-heating. Infiltration of the metal matrix with a ceramic precursor facilitates bonding between metal and ceramic. By repeated infiltration and decomposition, complete filling of the pores with a ceramic phase is achieved.Using SiC as MW absorbing material in a sandwich-like configuration, heating of the metal-ceramic composite in a strong MW field is possible. Temperatures of apr. 1000°C were achieved with this method.