Automatic tuned MRI RF coil for multinuclear imaging of small animals at 3T

We have developed an MRI RF coil whose tuning can be adjusted automatically between 120 and 128 MHz for sequential spectroscopic imaging of hydrogen and fluorine nuclei at field strength 3 T. Variable capacitance (varactor) diodes were placed on each rung of an eight-leg low-pass birdcage coil to change the tuning frequency of the coil. The diode junction capacitance can be controlled by the amount of applied reverse bias voltage. Impedance matching was also done automatically by another pair of varactor diodes to obtain the maximum SNR at each frequency. The same bias voltage was applied to the tuning varactors on all rungs to avoid perturbations in the coil. A network analyzer was used to monitor matching and tuning of the coil. A Pentium PC controlled the analyzer through the GPIB bus. A code written in LABVIEW was used to communicate with the network analyzer and adjust the bias voltages of the varactors via D/A converters. Serially programmed D/A converter devices were used to apply the bias voltages to the varactors. Isolation amplifiers were used together with RF choke inductors to provide isolation between the RF coil and the DC bias lines. We acquired proton and fluorine images sequentially from a multicompartment phantom using the designed coil. Good matching and tuning were obtained at both resonance frequencies. The tuning and matching of the coil were changed from one resonance frequency to the other within 60 s.     

Mode-hop-free fine tuning of an external-cavity diode laser with an intracavity liquid crystal cell

A planar nematic liquid crystal (NLC) cell is incorporated into a Littman-type external cavity as the wavelength-tuning device for a semiconductor laser diode. By varying the driving voltage of the NLC cell and laser diode bias current simultaneously, we demonstrate single-mode oscillation and mode-hop-free tuning over 19.2 GHz at 775 nm. The result is in good agreement with the theoretical predictions.     

A Gunn-Diode Active Leaky-Wave Frequency Scanning Antenna

A frequency tunable active leaky-wave scanning antenna using Gunn-diode voltage control oscillator (VCO) as source is developed. The frequency tuning controlled by changing either the varactor diode dc bias or the Gunn diode dc bias is demonstrated. The measured scanning angle of active antenna is close to 15 degree as the Gunn VCO frequency tuned from 12.58GHz to 12.98GHz. To excite the first higher order mode of the microstrip leaky-wave antenna is fed asymmetrically. The dominant mode excitation has been successfully suppressed by adding a sequence of covered wire in the middle line of the microstrip leaky wave antenna. This is a prototype of frequency scanning antenna using two terminal device, which can be easily scaled up to millimeter wave frequency region.     

CrAs(0 0 1)/AlAs(0 0 1) heterogeneous junction as a spin current diode predicted by first-principles calculations

We report on first-principles calculations of spin-dependent quantum transport in a CrAs(0 0 1)/AlAs(0 0 1) heterogeneous junction and predict a strong diode effect of charge and spin current. The minority spin current is absolutely inhibited when the bias voltage is applied to the terminals of both CrAs and AlAs. The majority spin current is inhibited when the bias voltage is applied to the terminal of CrAs and “relaxed” when the bias voltage is applied to the terminal of AlAs. The charge and spin current diode are promising for reprogrammable logic applications in the field of spintronics.     

Fabrication and electrical characterization of thermally deposited amorphous Se0.82In0.18 on n-Si (100)

Amorphous (a-) Se_0.82In_0.18 thin films have been deposited onto n-type silicon (n-Si) single crystal, using the three-temperature technique, in the fabricated configuration of Au/a-Se_0.82In_0.18/n-Si/Al. The current density-voltage (J–V) characteristics have been measured at different isotherms in the range of 198–313 K, thus inspecting the conduction mechanisms comparing with Au/a-Se/n-Si/Al heterojunctions. The analysis proved that the forward bias is characterized by two parts: current increasing exponentially with the applied voltage (low voltage bias region, V<0.2 V), and non-exponentially in the higher voltage region (V>0.2 V). At the low bias region, the current was dominated by a multi-tunneling capture-emission process with a rather temperature-independent effect in the temperature range investigated. However, at the high voltage region, the effect of temperature becomes more pronounced with an ohmic character in the range of 198 to 273 K. For temperatures higher than 273 K, and below the glass transition temperature of a-Se_0.82In_0.18 (T _g～330 K), the high voltage region could be subdivided into two parts: an ohmic conduction range that limited at bias voltage of 0.20 V<V<0.46 V, and a space charge limited current region for bias voltage of V>0.46 V. The reverse J–V characteristics showed a deviation from that of the ideal diode behavior, analogous to that of pure a-Se/n-Si heterojunctions.     

快上升沿电磁脉冲作用下PIN二极管中的电流过冲现象

 为了研究瞬态电磁脉冲对PIN二极管的干扰，利用基于扩散漂移模型的基本半导体方程，采用半导体器件一维瞬态数值仿真的方法,对快上升沿阶跃电磁脉冲作用下PIN二极管中的电流密度和电荷密度分布的变化进行了研究，分别观察了正反偏电压脉冲作用下过冲电流的产生过程并进行了分析。分析表明，过冲电流是和PIN二极管在高频下的容性表现相关的，无论是在正电压还是负电压情况下，脉冲上升沿时间越短、初始正偏压越高，则过冲电流密度的峰值越高。     

磁台阶势垒结构中二维电子气的自旋极化输运

运用散射矩阵方法,研究了台阶磁势垒量子结构中二维电子气的隧穿输运性质.结果表明：在零偏压下,电子传输概率的自旋极化曲线随入射能量的增加而振荡衰减;随着磁台阶数的增加,电子传输概率的自旋极化度最大值减小,同时电子传输概率的自旋极化度振荡衰减也越来越慢;随着磁台阶的总宽度增加,电子传输概率的自旋极化曲线出现更明显的振荡,电子隧穿磁台阶势垒表现出明显的量子尺寸效应;在偏置电压的作用下,电子传输概率的自旋极化度在宽广的入射能量区出现明显的振荡增大,电子隧穿磁台阶势垒表现出更明显的自旋过滤效应. 关键词： 磁台阶势垒 自旋极化 自旋过滤     

Reversal of spin polarization in Fe/GaAs (001) driven by resonant surface states: first-principles calculations

A minority-spin resonant state at the Fe/GaAs(001) interface is predicted to reverse the spin polarization with the voltage bias of electrons transmitted across this interface. Using a Green's function approach within the local spin-density approximation, we calculate the spin-dependent current in a Fe/GaAs/Cu tunnel junction as a function of the applied bias voltage. We find a change in sign of the spin polarization of tunneling electrons with bias voltage due to the interface minority-spin resonance. This result explains recent experimental data on spin injection in Fe/GaAs contacts and on tunneling magnetoresistance in Fe/GaAs/Fe magnetic tunnel junctions.     

Pyroelectric properties of an antiferroelectric liquid crystal

The effects of temperature and applied voltage on the pyroelectric properties of an antiferroelectric liquid crystal are given. It has been found that the pyroelectric signal depends strongly on the bias voltage across the sample. The pyroelectric signal behaviour is interpreted with the aid of spontaneous polarization data and good agreement is found between the results from the pyroelectric and polarization techniques. The spontaneous polarization of the sample exhibits the temperature- and field-induced `Devil's staircase' behaviour, as predicted by the Ising model.     

Simulation Investigation of Millimeter Wave Varactor-tuned Transmission Cavity-stabilized Oscillator

A new circuit configuration for millimeter wave varactor-tuned transmission cavity-stabilized oscillator has been proposed in this paper. Compared to conventional varactor-tuned reflection cavity-stabilized oscillator, in this configuration, a high quality factor transmission cavity directly coupled to varactor diode is employed to improve the performances of the oscillator. The operation frequency of this oscillator can be tuned by varying the resonant frequency of the transmission cavity through changing bias voltage of the varactor diode. An equivalent circuit model for the oscillator has been presented in order to theoretically investigate the performance characteristics of the oscillator. On the basis of this model, electrical tuning characteristics have been studied. Mode jumping phenomena during electrical tuning process have been analyzed for obtaining stable operations of the oscillator. The analytical formulae of quality factor and efficiency have been derived in terms of relevant circuit parameters. Particular emphasis has been paid on several circuit parameters which have a substantial impact on circuit performance. Some design considerations have been pointed out according to the simulation results, which are useful to the design and fabrication of this type of oscillators.     

Ⅱ-Ⅵ族稀磁半导体多层结构中的自旋极化隧穿

采用转移矩阵法和Airy函数,研究了ZnSe/ZnMnSe/ZnSe/ZnBeSe/ZnSe/ZnBeSe/ZnSe异质结构的自旋极化输运。在外加偏压和磁场对电子透射系数和自旋极化率的影响方面,所得到的结论显现出复杂而有趣的特性。磁场对自旋向上和向下电子隧穿的影响是不同的:对于自旋向上情况,出现双共振向单共振转换现象。     

Generation and detection of spin polarization in parallel coupled double quantum dots connected to four terminals

A four-terminal parallel double quantum dots (QDs) device is proposed to generate and detect the spin polarization in QDs. It is found that the spin accumulation in QDs and the spin-polarized currents in the upper and down leads can be generated when a bias voltage is applied between the left and right leads. It is more interesting that the spin polarization in the QDs can be detected using the upper and down leads. Moreover, the direction and magnitude of the spin polarization in the QDs, and in the upper and down leads can be tuned by the energy levels of QDs and the bias.     

Influence of contact shape on AlGaN/GaN Schottky diode prepared on Si with thick buffer layer

A report on the fabrication and characterization of high performance conventional and ring-shaped AlGaN/GaN Schottky barrier diode on Si is presented. The resulting device exhibited low leakage current, which led to a detectivity performance of 3.48×10¹³ and 1.76×10¹³ cm?Hz^1/2 W^?1, respectively, for both conventional and ring-shaped Schottky diode. The differential resistances of both devices were obtained at approximately 1.37×10¹² and 1.41×10¹³ Ω, respectively. The zero bias peak responsivities of conventional and ring-shaped Schottky diodes were estimated to be 3.18 and 2.08 A?cm^?2/W, respectively. The typical UV to visible rejection ratio was observed over three orders of magnitude at zero bias. The C–V measurements was used to calculate and analyze the polarization sheet charge density of the AlGaN barrier layer by using self-consistently solving Schrodinger’s and Poisson’s equations. It is demonstrated that the ring shape of the Schottky barrier has higher polarization sheet charge density, which has the consequence that the Schottky shape has influence on the strain of the AlGaN barrier layer.     

Separating spins by dwell time of electrons across parallel double δ-magnetic-barrier nanostructure applied by bias

The dwell time and spin polarization(SP)of electrons tunneling through a parallel doubleδ-magnetic-barrier nanostructure in the presence of a bias voltage is studied theoretically in this work.This nanostructure can be constructed by patterning two asymmetric ferromagnetic stripes on the top and bottom of InAs/AlxIn1-xAs heterostructure,respectively.An evident SP effect remains after a bias voltage is applied to the nanostructure.Moreover,both magnitude and sign of spin-polarized dwell time can be manipulated by properly changing the bias voltage,which may result in an electrically-tunable temporal spin splitter for spintronics device applications.     

含有源频率选择表面可调复合吸波体

基于传输线等效理论,设计了含有源频率选择表面(active frequency selective surface,AFSS)的三层可调复合吸波体,第一层是表面层,为AFSS衬底;中间层是AFSS层,由频率选择表面(frequency selective surface,FSS)和PIN二极管阵列构成;第三层是介质层.反射率测量结果表明,通过调节PIN二极管阵列偏置电压可以动态调节吸波体反射特性,在偏置电压为5 V时,可获得最佳吸波性能,在5—15 GHz和5.3—13 GHz频段分别可获得-8 dB和- 关键词： 频率选择表面 复合吸波材料 反射率 PIN二极管     

Mode-hopping-free scanning over 2 THz by means of quasi-synchronous tuning

We report mode-hopping-free tuning (MHF) over 2 THz (6 nm) of a Littman-Metcalf-type external-cavity diode laser by means of quasi-synchronous tuning, wherein the pivot point is displaced 65 mm away from the conventional rigorous synchronous rotation center. An experimental method for judging the relative position between actual pivot location and the quasi-synchronous tuning line is developed. An adjustment structure with only one freedom of movement is enough to accurately locate the pivot point, and the precise requirement is relaxed to more than 300 μm for achieving a MHF tuning range of over 1THz in our configuration.     

Generation-mode operation of the tunnel diode stimulated by external microwave radiation

We show that oscillations in the generating circuit are stimulated under the action of microwave radiation when bias voltage, which is smaller than peak voltage, is applied to the diode. In this case, the frequency and amplitude of the generation stimulated by microwave radiation depend on the radiation power level. Saratov State University, Saratov, Russia. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 42, No. 10, pp. 1009–1012, October 1999.     

Hardening measures for bipolar transistors against microwave-induced damage

In the present paper we study the influences of the bias voltage and the external components on the damage progress of a bipolar transistor induced by high-power microwaves. The mechanism is presented by analyzing the variation in the internal distribution of the temperature in the device. The findings show that the device becomes less vulnerable to damage with an increase in bias voltage. Both the series diode at the base and the relatively low series resistance at the emitter, Re, can obviously prolong the burnout time of the device. However, Re will aid damage to the device when the value is sufficiently high due to the fact that the highest hot spot shifts from the base-emitter junction to the base region. Moreover, the series resistance at the base Rb will weaken the capability of the device to withstand microwave damage.     

Voltage-controlled negative refractive index in vertically coupled quantum dot systems

The authors demonstrate that negative index of refraction can be achieved by tuning the tunneling rate between InGaAs quantum dots layers via simply applying a bias voltage across the layers. As the bias voltage is changed, the index of refraction is tunable from negative through zero to positive. Moreover, the large negative refractive index and little loss can be achieved at the same time.     

Investigation on the electric-field-induced Pockels effect and optical rectification in near-intrinsic silicon samples

Based on the classical polarization theory, we studied and specified the physical mechanism of the electric-field-induced (EFI) Pockels effect and optical rectification in the space charge region of a near-intrinsic silicon sample with the planar capacitor structure. Especially, the effect of the applied DC bias on these EFI effects was investigated. The results show that the electro-optic signal from Pockels effect in silicon linearly increases with the applied DC voltage and the modulating voltage, and the signal of optical rectification is linearly enhanced by the DC bias as well, but the polarization characteristic of optical rectification does not vary. The enhancement of these EFI effects is mainly owed to the strengthening of the built-in field and the extension of the space charge region in the silicon sample. The Kerr effect of silicon was also detected and contrasted against the EFI Pockels effect, and it is verified that the EFI Pockels effect is much stronger than the Kerr effect in the silicon sample. These EFI effects are significant for the development of silicon photonics or silicon optoelectronics.