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To reduce seismic and environmental vibration noise, ultra-low-frequency vertical vibration isolation systems play an important role in absolute gravimetry. For this purpose, an isolator based on a two-stage beam structure is proposed and demonstrated. The isolator has a simpler and more robust structure than the present ultra-low-frequency vertical active vibration isolators. In the system, two beams are connected to a frame using flexural pivots. The upper beam is suspended from the frame with a normal hex spring and the lower beam is suspended from the upper one using a zero-length spring. The pivot of the upper beam is not vertically above the pivot of the lower beam. With this special design, the attachment points of the zero-length spring to the beams can be moved to adjust the effective stiffness. A photoelectric detector is used to detect the angle between the two beams, and a voice coil actuator attached to the upper beam is controlled by a feedback circuit to keep the angle at a fixed value. The system can achieve a natural period of 100 s by carefully moving the attachment points of the zero-length spring to the beams and tuning the feedback parameters. The system has been used as an inertial reference in the T-1 absolute gravimeter. The experiment results demonstrate that the system has significant vibration isolation performance that holds promise in applications such as absolute gravimeters. [ABSTRACT FROM AUTHOR]

An equivalent model for MgO-based magnetic tunnel junction (MTJ) is used to characterize the frequency behavior of MTJs up to 20 GHz in this paper. From the extracted parasitic factor of MTJ, we found that the parasitic components (beside the junction resistance) significantly yield adverse effect, especially at microwave frequencies. Full parametric studies show that the interfacial capacitance, rather than the geometric capacitance, plays a key role in the drop of efficiency in microwave frequency applications of MTJs. Interfacial resistance engineering is proposed as a solution to improve the parasitic factor, as well as the operation frequency of MTJ. [ABSTRACT FROM AUTHOR]

In the absence of any external applied magnetic field, we have found that a magnetic tunnel junction (MTJ) can produce a significant output direct voltage under microwave radiation at frequencies, which are far from the ferromagnetic resonance condition, and this voltage signal can be increase by at least an order of magnitude by applying a direct current bias. The enhancement of the microwave detection can be explained by the nonlinear resistance/conductance of the MTJs. Our estimation suggests that optimized MTJs should achieve sensitivities for non-resonant broadband microwave detection of about 5000 mV/mW. [ABSTRACT FROM AUTHOR]

The magnetoresistance of a MgO-based magnetic tunnel junction (MTJ) was studied experimentally. The magnetoresistance as a function of current was measured systematically on MTJs for various MgO cross sectional areas and at various temperatures from 7.5 to 290.1 K. The resistance current dependence of the MTJ was also measured for different angles between the two ferromagnetic layers. By considering particle and angular momentum conservation of transport electrons, the current dependence of magnetoresistance can be explained by the changing of spin polarization in the free magnetic layer of the MTJ. The changing of spin polarization is related to the magnetoresistance, its angular dependence and the threshold current where TMR ratio equals zero. A phenomenological model is used which avoid the complicated barrier details and also describes the data. [ABSTRACT FROM AUTHOR]

For the applications in precision measurement such as absolute gravimeter, we have designed and built an ultra-low frequency vertical vibration isolator based on LaCoste spring linkage. In the system, an arm with test mass is suspended by a mechanical extension spring, and one end of the arm is connected to the frame with flexible pivots. The displacement of the arm is detected by an optical reflection method. With the displacement signal, a feedback control force is exerted on the arm to keep it at the balance position. This method can also correct the systematic drift caused by temperature change. In order to study the vibration isolation performance of the system, we analyze the dynamic characteristics of the spring linkage in the general case, and present key methods to adjust the natural oscillating period of the system. With careful adjustment, the system can achieve a steady oscillation with a natural period up to 32 s. This isolator has been tested based on the T-1 absolute gravimeter. A statistical uncertainty of 2 µGal has been achieved within a typical 12 h measurement. The experimental results verify that the isolator has significant vibration isolation performance, and it is very suitable for applications in high precision absolute gravity measurement. [ABSTRACT FROM AUTHOR]

Microwave applicators are widely employed for materials heating in scientific research and industrial applications, such as food processing, wood drying, ceramic sintering, chemical synthesis, waste treatment, and insect control. For the majority of microwave applicators, materials are heated in the standing waves of a resonant cavity, which can be highly efficient in energy consumption, but often lacks the field uniformity and controllability required for a scientific study. Here, we report a microwave applicator for rapid heating of small samples by highly uniform irradiation. It features an anechoic chamber, a 24-GHz microwave source, and a linear-to-circular polarization converter. With a rather low energy efficiency, such an applicator functions mainly as a research tool. This paper discusses the significance of its special features and describes the structure, in situ diagnostic tools, calculated and measured field patterns, and a preliminary heating test of the overall system. [ABSTRACT FROM AUTHOR]

We report the effects of thermal annealing on the characteristics of GeSn epilayers grown on Ge-buffered Si wafers with a high Sn content near a threshold value that affords a direct bandgap. On annealing at temperatures below 400 °C, the characteristics of the epilayer remain unchanged, compared to those of the as-grown samples. On annealing the samples at a temperature in the range of 440-540 °C, strain relaxation in the epilayer is observed, accompanied by generation of misfit dislocations at the GeSn/Ge interface. A further increase in annealing temperature beyond 580 °C causes not only a relaxation in strain but also a change in the microstructure of the epilayer. In addition, Sn forms clusters and segregates to the surface, resulting in a reduction in the Sn content of the epilayer. The present investigation shows changes in the characteristics of the film under thermal treatment, providing an insight into the physical properties of such devices. [ABSTRACT FROM AUTHOR]

We report the observation of mid-infrared room-temperature electroluminescence from a p-i-n Ge/Ge0.922Sn0.078/Ge double heterostructure diode. The device structure is grown using low-temperature molecular beam epitaxy. Emission spectra under various injection current densities in the range of 318 A/cm2-490 A/cm2 show two distinct profiles peaked at 0.545 eV (2.275 μm) and 0.573 eV (2.164 μm), corresponding to indirect and direct bandgaps of the Ge0.922Sn0.078 active layer, respectively. This work represents a step forward towards the goal of an efficient direct-bandgap GeSn light-emitting device on a Si substrate by incorporating higher Sn content of 7.8% in a diode structure that operates at lower current densities. [ABSTRACT FROM AUTHOR]

We report on the optoelectronic properties of thin films of Bi2Se3 grown by molecular beam epitaxy. The films are patterned into circuits with typical extensions of tens of microns. In spatially resolved experiments, we observe submicron photocurrent patterns with positive and negative amplitudes. The patterns are independent of the applied bias voltage, but they depend on the width of the circuits. We interpret the patterns to originate from a local photocurrent generation due to potential fluctuations. [ABSTRACT FROM AUTHOR]

A far-field microwave imaging technique has been developed using a spintronic sensor based on a magnetic tunnel junction (MTJ). Such a sensor can directly rectify a microwave field into a dc voltage signal using the Seebeck effect. Thanks to the high conversion efficiency of the microwave rectification in MTJs, the microwave power sensitivity of the spintronic sensor is on the order of 1-10 mV/mW. This high sensitivity allows the sensor to directly measure the coherent spatial scattered microwave field distribution, which gives it the ability to non-destructively detect hidden objects down to a few wavelengths in size. [ABSTRACT FROM AUTHOR]

Ohmic contact resistivity of a nongold Pd/Ge/Ti/Pt on highly doped molecular beam epitaxy grown n-GaAs and In0.2Ga0.8As/GaAs (∼2 × 1018 cm-3) has been investigated by varying Pd/Ge thicknesses and rapid thermal annealing (RTA) temperature/duration. An optimized Ohmic contact was obtained in the samples with Pd/Ge of 30 nm/30 nm, using RTA at 300 °C for 10 s. Low Ohmic contact resistivity of 5.4 × 10-7 Ω cm2 on n-In0.2Ga0.8As has been achieved. The mechanism of the contact resistivity reduction has been studied using the energy-dispersive x-ray spectroscopy depth profile. [ABSTRACT FROM AUTHOR]

The erratic p-type conductivity in nitrogen-doped ZnO film is still under investigation and has been debated up to now. In this study, the authors have studied the effect of rapid thermal process (RTP) on the properties of N-doped ZnO films grown by metal-organic chemical vapor deposition. Hall-effect measurements show that the sample is of p-type as the RTP temperature is lower than 350 °C while, as the RTP temperature increased up to 550 °C or higher, the conduction-type of the sample changed to be n-type. Correspondingly, obvious D and G peaks, which are related to graphite clusters, are observed to increase their intensity with RTP temperature, indicating that interstitial or substitutional carbon atoms may migrate to form carbon clusters in the grain boundary during RTP. RTP is also found to lead to significant changes on the photoluminescence of the samples, with enhanced visible emissions observed as RTP temperature increased. Similar changes are observed on the intensity ratios of the D over G peaks and the visible emission around 600 nm over the near-band-edge emission. This indicates that besides zinc vacancy (VZn) and oxygen vacancy (VO), which are popularly ascribed as the origins of the visible emissions around 500 and 550 nm, carbon clusters may be a possible origin of the visible emission around 600 nm. Finally, carbon clusters formed in the grain boundary are also supposed to at least partly be responsible for the type transition caused by RTP. [ABSTRACT FROM AUTHOR]

A potential Seebeck microprobe apparatus is described such that a profile of Seebeck coefficients can be detected on a material sample surface for thermopower investigations. Due to its spatially resolved limit on detecting small inhomogeneities of dopants or composition changes, we here propose a constructive combination of numerical modeling and practical measurement to improve the spatial resolution by deconvolution algorithm. The relevant transfer function, obtained from numerical calculations, was successfully applied on real measurement data. Besides, an improvement in detecting the 1.5 μm inhomogeneity has been achieved by applying one transfer function of the known tip setup on a set of Seebeck line scan signals, obtained with 6 μm tip-sample contact length, 3 μs signal capture time after contact, and 0.75 μm scan period. The result showed that a system theoretical approach in terms of deconvolution algorithm could theoretically enhance the spatial resolution anyway, but is practically limited by the systemic preconditions (tip size, signal capture time, and scan period) and the signal to noise ratio of the captured thermovoltages. Therefore, we provide a clear understanding of systemic preconditions and their impacts on the detection of small inhomogeneities and thus the performance in the thermopower analysis. [ABSTRACT FROM AUTHOR]

The drop size distribution was measured from back-lighted spark photographs at the edge of steady sprays in the immediate vicinity of the nozzle exit. The conditions of these liquid-into-gas sprays were such that the outer surface of the liquid jets broke up into small drops at the nozzle exit. The objective was to elucidate the mechanism of breakup. At room temperature, n-hexane and n-tetradecane at pressures from 2.86 to 9.76 MPa were injected into gaseous nitrogen at 1.48 to 2.86 MPa through three straight cylindrical nozzles of different diameters, 127 and 335 μm, and length-to-diameter ratios, 4 and 10. In all cases, the drop sizes could be fitted satisfactorily with a chi-square distribution with degree of freedom equal to 28. The Sauter mean drop diameter and other average diameters were found to decrease with increasing injection velocity and decreasing liquid surface tension, to be insensitive to nozzle diameter and length, and to increase slightly with increasing gas density. The trends and magnitudes are in agreement with those predicted by the supplemented aerodynamic theory of surface breakup if it is assumed that between the jet surface where the drops are formed, that is not visible, and the edge of the spray, where the measurements were made, drops undergo collisions and coalescence. In this region of dense sprays, drop coalescence leads to a rapid increase in drop size, particularly in high gas densities. [ABSTRACT FROM AUTHOR]

A new high-resolution electron energy-loss spectrometer based on the dispersion compensation (DC) concept has been designed, built, and tested. The parallel processing inherent in DC has allowed us to attain signal levels two orders of magnitude in excess of conventional designs while operating at comparable resolution. This is consistent with expectations based on simple model calculations. The resolving power of the spectrometer is not as good as theoretically predicted probably due to uncontrolled beam angle effects. Initial tests on a variety of systems are presented. We analyze the strengths and weaknesses of the design and suggest future improvements. [ABSTRACT FROM AUTHOR]

Direct numerical simulations using high-resolution total variation diminishing (TVD) scheme are performed for studying the shock enhancement of two-dimensional confined (spatially growing) supersonic mixing flows. Several specially designed mixing enhancement schemes are examined with emphasis placed on the study of the fundamental aspects involved in the shock-induced mixing enhancement process. The merits associated with these mixing enhancement schemes are evaluated based on a cost/effectiveness criterion, in which the cost paid for the total pressure loss encountered and the improvement in mixing gained are considered together. The results suggest that mixing enhancement using shock waves can only be effective if the stimulation is spatially persistent, and begins from the very upstream. Being motivated by this observation, an idea of using wavy-wall configuration to generate the desirable periodic shock stimulation is proposed and investigated. The computed results show that, by an appropriate manipulation of the parameters including wall wavelength, relative phase shift between the top and bottom walls, as well as the amplitude of the waviness, considerable improvement in mixing efficiency can be achieved. [ABSTRACT FROM AUTHOR]

Direct numerical simulations using the high-resolution TVD scheme are performed for the study of two-dimensional confined spatially growing supersonic mixing layers. Numerical simulations, being forced by the linear instability wall modes, are justified by comparing the calculated results with both theoretical and experimental data. The numerically visualized downstream compression/expansion wave system explains the so-called ‘‘strange waves’’ observed in experiments and are found to be responsible for the rapid growth of the mixing layer in that region. The existence of shock waves would lead to mixing enhancement by accelerating the process of extracting turbulent kinetic energy from the mean stream. However, the phenomenon of shock-induced mixing improvement is found quite local around the shock impingement area, and often is followed by a saturation in turbulent energy absorption which impedes the further growth of the shear layer in the immediate downstream region. For the present cases considered, the subharmonic instability modes are not dominant, and the baroclinic-torque effect is comparatively less important for the vorticity development. These observations suggest that compressibility effects may behave in a considerably different manner as the underlying instability mechanism changes from the subsonic Kelvin–Helmholtz type to the present supersonic wall instability type. [ABSTRACT FROM AUTHOR]

Presents a study that examined an amorphous silicon/silicon-carbide double barrier structure. Background on double barrier resonant tunneling structures; Schematic diagram of a silicon-carbon-hydrogen double barrier structure.

The dynamics of exciton coupling to photons and LO-phonons in a three-level quantum-dot system is studied using the Wigner–Weisskopf approach. An analytical solution to the system for a rectangular driving pulse is derived, and the wave function of an exciton is found to form a unit vector directed to the surface of an S2 Bloch hemisphere. For long decoherence times, the vector traces out a temperature (T)-dependent Rabi circle with increase pulse area. An increase in T does not deform the Rabi circle but shrinks its radius. Accordingly, a diverse representational scheme is proposed. These properties expand scenarios to T-dependent regimes. [ABSTRACT FROM AUTHOR]

A III-nitride-based photogated transistor using photons to control the channel width of an otherwise gateless field effect transistor (FET) is investigated. This is accomplished by stacking sequential layers of p-GaN/InGaN/n-GaN on a Si substrate in an array of nanorods. The nitride p-i-n diode can be activated by light, whereupon the nanorod device shows phototransistor characteristics in forward bias but behaves like a photoconductor when in reverse bias. An optically pumped FET model, as justified by the low-dimensional nanogeometry, is used in analysis of the device. The resulting photogate efficiency and photocarrier mobilities are estimated to be ∼0.04 V/(W/cm2) and, ∼2000–3000 cm2/V s, respectively. [ABSTRACT FROM AUTHOR]

We demonstrate a direct atomically resolved visualization and quantification of the impact of inhomogeneities in the dopant distribution on the nanoscale potential fluctuations in a two-dimensional semiconducting [formula] Ga overlayer on Si(111) using scanning tunneling microscopy. By a quantitative analysis, two regimes of the potential at nanometer scale are found, which arise from the local distribution of charge carriers in the bands and from electron-electron interactions. [ABSTRACT FROM AUTHOR]

The properties of Cr-doped GaN epilayers grown by rf-plasma-assisted molecular beam epitaxy were studied. The deep acceptor nature of Cr was used to grow semi-insulating GaN epilayers on sapphire substrates for electronic device applications. The room-temperature (RT) sheet resistivity of the epilayers reached 1010 Ω/square. The activation energy of dark conductivity was about 0.48 eV. Step-graded AlxGa1-xN/GaN (x=0.3-0.2) superlattices (SLs) were designed to filter dislocations. Transmission electron microscopy images showed that the SLs can dramatically reduce dislocation density. Al0.35Ga0.65N/GaN heterostructure grown on Cr-doped semi-insulating GaN epilayer exhibited a RT mobility of 960 cm2/V s and sheet carrier density of 2.1×1013 cm-2. [ABSTRACT FROM AUTHOR]

Orthorhombic HoMnO3 films with well-aligned crystallographic orientations were deposited on LaAlO3(110) single crystal substrates by using pulsed laser deposition. The nearly perfect b-axis-oriented films provide the opportunity of investigating the orientation-dependent physical property of this material. The temperature dependent magnetization evidently displays an antiferromagnetic ordering near 42 K, irrespective to the direction of applied field. Furthermore, the theoretically expected lock-in transition was clearly observed at around 30 K when field was applied along the c axis and was undetectable along the a and b axes. The 30 K transition was suppressed to 26 K when the applied field increased from 100 to 500 Oe. [ABSTRACT FROM AUTHOR]

The authors have fabricated NaxCoO2 thin films via lateral diffusion of sodium into Co3O4 (111) epitaxial films (reactive solid-phase epitaxy [Ohta et al., Cryst. Growth Des. 5, 25 (2005)]). The environment of thermal diffusion is key to the control of the sodium content in thin films. From the results of x-ray diffraction and in-plane ρab, the epitaxial growth and the sodium contents of these films were identified. The thermoelectric measurements show a large thermoelectric power similar to that observed in single crystals. The quasiparticle scattering rate is found to approach zero at low temperatures, consistent with the small residual resistivity, indicating high quality of the NaxCoO2 thin films. [ABSTRACT FROM AUTHOR]

The effect of gate voltage on hot-carrier induced degradation in drain extended high-voltage metal-oxide-semiconductor (MOS) transistors with thick gate oxide (100 nm) structure is presented. Different from the conventional low-voltage n-type MOS transistors, under a fixed drain voltage, devices stressed at a higher Vgs result in a greater maximum transconductance and on-resistance degradation. Under higher Vgs, the increase in channel hot-carrier injection is responsible for the greater Gm,max degradation. On the other hand, Kirk effect induced increase in drain avalanche hot carriers near the drain as well as higher electric field in the channel is responsible for the greater Ron degradation. [ABSTRACT FROM AUTHOR]

The viscous torque on a sphere was derived when the sphere is rotating with an arbitrary time varying angular velocity about a given diameter under the low Reynolds number assumption. The torque is composed of a quasisteady torque and an acceleration torque, which is an integral expression depending on the acceleration history. The acceleration torque can be neglected when the acceleration time scale is sufficiently greater than the viscous time scale, which is appropriate for particles with micron size subject to electrokinetic manipulation. [ABSTRACT FROM AUTHOR]

The structures of TaO6 and NbO6 oxygen octahedra in xBa(Mg1/3Ta2/3)O3+(1-x)Ba(Mg1/3Nb2/3)O3 perovskite ceramics with x=0, 0.25, 0.50, 0.75, and 1.0 were investigated by the extended x-ray absorption fine structure method. The decline in the microwave dielectric constant as x increases is caused mainly by the decrease of the mean volume of the oxygen octahedra, regardless of the 1:2 ordered structure and the distortion of the oxygen octahedron. The low Qf values of the TaO6 and NbO6 mixed samples are caused by not only the degrading of the 1:2 ordered structure but also the distortion of oxygen octahedral cages. [ABSTRACT FROM AUTHOR]

We report on atomic layer deposition of an ∼2-nm-thick ZnO layer on the inner surface of ultralow-density (∼0.5% of the full density) nanoporous silica aerogel monoliths with an extremely large effective aspect ratio of ∼105 (defined as the ratio of the monolith thickness to the average pore size). The resultant monoliths are formed by amorphous-SiO2 core/wurtzite-ZnO shell nanoparticles which are randomly oriented and interconnected into an open-cell network with an apparent density of ∼3% and a surface area of ∼100 m2 g-1. Secondary ion mass spectrometry and high-resolution transmission electron microscopy imaging reveal excellent uniformity and crystallinity of ZnO coating. Oxygen K-edge and Zn L3-edge soft x-ray absorption near-edge structure spectroscopy shows broadened O p- as well as Zn s- and d-projected densities of states in the conduction band. [ABSTRACT FROM AUTHOR]

In this study, we present an easy way to create layered-nanoplatelets with well-defined geometry by controlling the cracking process of nanostructured multilayers. The geometrical dimension of layered-nanoplatelets is determined by the multilayer intrinsic size, the total strain, and the elastic mismatch between the substrate and multilayers, which was analyzed by statistical approach. Fracture behaviors characterized by critical strain to nucleate microcrack, fracture toughness, and evolution of fragment width were also studied for nanostructured Cu/Cr multilayers with modulation period (λ) spanning from of 5 to 250 nm and were quantified based on linear elastic theory and shear-lag theory. An optimal modulation period seems to be likely favorable for maximizing the ductility, strength, and fracture toughness of the nanolayered materials. [ABSTRACT FROM AUTHOR]

Phase separation in La[SUB0.67]Ca[SUB0.33] MnO[SUB3] thin films was investigated by scanning tunneling microscopy. The correlation between the grain structure and the spatial distribution of the coexisting metallic and insulating phases was evidently established. At temperatures not far below the metal-insulator transition, the spatial variation of the coexisting metallic and insulating phases is susceptible to magnetic field in an irreversible manner. The irreversibility suggests that the metallic percolation paths can be affected randomly by magnetic field. However, the variation becomes insensitive to magnetic field at lower temperatures. [ABSTRACT FROM AUTHOR]

Structure, magnetization, and dielectric permittivity of (1-x)BiFeO3-xBaTiO3 (BFO-BT) ceramics have been studied as a function of BT content (x = 0.0, 0.1, 0.2, and 0.3). In situ synchrotron x-ray diffraction result of BFO reveals a rhombohedral (R)-orthorhombic (O)-cubic (C) phase transition near 820 and 850 °C upon heating. BFO-10%BT and BFO-20%BT exhibit a R-C transition near 760 and 740 °C, respectively. A C(R)-C transition takes place near 680 °C in BFO-30%BT. C(R) represents that a minor R phase coexists in the C matrix. A local minimum of R distortion angle αR occurs upon heating and implies ionic displacements. This anomaly is likely resulted from the antiferromagnetic (AFM)-paramagnetic (PM) transition and is responsible for the broad frequency-dependent dielectric maximum. BFO and BFO-10-30%BT ceramics exhibit a similar AFM behavior with magnetic susceptibility of about 8.2 × 10-6 emu/g·Oe at room temperature. This work suggests that BTO-substitution can enhance dielectric response and reduce the dielectric loss. [ABSTRACT FROM AUTHOR]

Epitaxial CrO[sub 2] (110)-oriented films were fabricated on Si (100) substrates buffered by rutile TiO[sub 2] derived from oxidation of a pulsed-laser-deposited TiN layer. The epitaxial films of CrO[sub 2] were prepared by chemical vapor deposition in a two-zone furnace with oxygen flow from a CrO[sub 3] precursor. The transport measurements show that the CrO[sub 2] films are metallic with a Curie temperature of about 380 K. The temperature dependence of resistivity was best described by a phenomenological expression ρ(T) = ρ[sub 0] +AT²e[sup (-δ/T)] over the range of 5-350 K with &de;ta= 94 K. The magnetic measurements show the in-plane coercive fields are about 30 and 60 Oe at 300 and 5 K, respectively. The temperature dependent spontaneous magnetization follows Bloch's T[sup 3/2] law and the slope suggests a critical wavelength of λ[sub δ] ∼ 30.6 Å beyond which spin-flip scattering becomes important. [ABSTRACT FROM AUTHOR]

Ga K-edge x-ray absorption measurements were employed to investigate Mg-doping effects in GaN samples. Strong polarization-dependent x-ray absorption near-edge structures become less pronounced with increasing doping concentration, indicating the formation of a mixing-phase structure of cubic and hexagonal phases. Analysis of the extended x-ray absorption region of the spectra revealed doping-related defects such as vacancies, substitutions, and interstitial occupations. They were formed anisotropically in the crystal c axis direction and its perpendiculars. Disorderliness arising from phase mix and defects is believed to have lowered the Debye temperature of the doped GaN films and caused the destructive interference of the absorption fine-structure oscillation functions. These effects were taken into account for the observed large coordination number reductions in our samples. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

An easily fabricated xenon-IV ion laser is described. The various parameters considered are the gas pressures, the excitation voltages, and the repetition rates. Based on our results we believe that there exists an optimal excitation voltage and an optimal repetition rate for achieving largest lasing power and good reliability. [ABSTRACT FROM AUTHOR]

A pulsed laser deposition system with a rotating reflector, which can generate an extended plume with a diameter of about 40 mm, has been proven to be viable in producing large area YBa2Cu3O7 (YBCO) thin films. Films with uniformity of thickness and electrical properties to within a few percents were obtained. By making a further modification on this laser scanning system, YBCO films with various thicknesses can be deposited in situ on several pieces of substrates under essentially identical deposition conditions, and particularly, in a continuous fashion. The atomic force microscopy (AFM) images reveal that films grown on NdGaO3 follow the Stranski–Krastanov (layer then island growth) mode, while films grown on as-polished MgO and on annealed MgO follow the Volmer–Weber (islands without layer growth) mode and step-flow mode, respectively. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Investigates the heterojunctions of solid Carbon 70 and crystalline silicon. Information on the rectifying properties of carbon and silicon compounds; Number of movable-ion charges in the solid C70 compound; Details on how the dielectric constant of the C70 compound can be identified.

Solid C60/n-GaAs heterojunctions have been fabricated by deposition of solid C60 film on the (100)-oriented epitaxial n-type GaAs substrates and their electrical characteristics have been measured. The rectification ratio is greater than 106 at a bias of ±1 V. The current for a fixed forward bias is an exponential function of reciprocal temperature, from which the effective potential barrier height of the heterojunction is determined to be 0.58 eV. A trap with an energy level 0.35 eV below the conduction band of GaAs at the C60/GaAs interface has been observed by the deep level transient spectroscopy technique. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Studies the magnetoresistance of nickel alloy multilayered films. Preparation of the sample; Discussion on the results of the study.

It is shown that a small substitution of praseodymium for bismuth as in Pr0.05Bi1.95Sr2CaCu2.2O8.2+δ produces a significant increase, 13 K, in the onset temperature, 7 K in the temperature at which the whole specimen is superconducting, and 10 K in the transition-midpoint temperature relative to Bi2Sr2CaCu2.2O8.2+δ. Unfortunately the transition is broadened, 20 K versus 14 K. Doubling the amount of praseodymium substitution causes the occurrence of extraneous phase(s), the effect of which is seen only in the resistance versus temperature data, and does not indicate any additional significant increases in the aforementioned temperatures. [ABSTRACT FROM AUTHOR]

YBaCuO superconducting thin films were prepared by multilayer deposition on different kinds of substrates. High Tc with an onset of 94 K and a zero resistance at 84 K has been obtained by deposition on pure ZrO2 substrates. The nature of zero resistance as well as the dR/dT characteristic above Tc and the influence of substrates are discussed. [ABSTRACT FROM AUTHOR]

Relativistic electronics research in recent years has produced powerful millimeter waves on the MW level, while also extending the frequency range into the terahertz (THz) region and beyond. These developments have opened up new horizons in applications. The current study is motivated by the associated need for high-power absorbers not readily available at such frequencies. Our focus is on effective absorber schemes which can handle high power while also possessing a structural simplicity for easy implementation. In and above the THz region, the electrical conductivity can no longer be treated as a real constant. We begin with a derivation of the field penetration depth applicable to all frequencies. Requirements to meet the intended criteria are then determined from the wave penetration and reflection properties. Design examples in the 1-1000 GHz range are illustrated, which consist of a thin lossy conducting layer on the surface of a pyramidal shaped metal base. It is shown in theory that such structures can function effectively in the millimeter and THz regions. [ABSTRACT FROM AUTHOR]

Electron cyclotron maser interactions at terahertz (THz) frequencies require a high-order-mode structure to reduce the wall loss to a tolerable level. To generate THz radiation, it is also essential to employ cyclotron harmonic resonances to reduce the required magnetic field strength to a value within the capability of the superconducting magnets. However, much weaker harmonic interactions in a high-order-mode structure lead to serious mode competition problems. The current paper addresses harmonic mode competition in the gyrotron backward wave oscillator (gyro-BWO). We begin with a comparative study of the mode formation and oscillation thresholds in the gyro-BWO and gyromonotron. Differences in linear features result in far fewer 'windows' for harmonic operation of the gyro-BWO. Nonlinear consequences of these differences are examined in particle simulations of the multimode competition processes in the gyro-BWO, which shed light on the competition criteria between modes of different as well as the same cyclotron harmonic numbers. The viability of a harmonic gyro-BWO is assessed on the basis of the results obtained. [ABSTRACT FROM AUTHOR]

Phonon dynamics in Bi, Bi2Se2, and CuxBi2Se3 (x = 0, 0.1, 0.125) single crystals was investigated using femtosecond pump-probe spectroscopy. Two damped oscillations in measured transient reflectivity changes were observed, respectively, attributed to coherent optical and acoustic phonons. The frequency shift of the phonon modes in Bi-rich Bi2Se2 crystals indicates the intercalation of the additional Bi atoms into the Bi2Se3 matrix and formation of the Bi2 layer between the quintuple layers (QLs). The deformation of QLs in CuxBi2Se3 crystals was also found. These phonon dynamics results suggest that Cu atoms in CuxBi2Se3 crystals are predominantly intercalated between pair of QLs. [ABSTRACT FROM AUTHOR]

We studied ultrafast dynamics and photoinduced mechanical strain of BiFeO3 thin films by dual-color transient reflectivity measurements (ΔR/R). Anisotropic photostriction in BiFeO3 is found to be mainly driven by the optical rectification effect. Results of the photostriction at various thicknesses show that the estimated sound velocity along [110] direction of BiFeO3 is 4.76 km/s. [ABSTRACT FROM AUTHOR]