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Ultra-fast coherent magnetization reversal, which is indispensable for high-speed spintronic devices, is expected to be realized by irradiation with a nearly monocyclic terahertz pulse. In this letter, using MnAs ferromagnetic nanoparticles embedded in a GaAs host matrix, we observe a large modulation up to 20% of the magnetization of these nanoparticles with terahertz pump pulse irradiation. This modulation ratio is the largest value ever reported for terahertz magnetization modulation. We reveal that the modulation is induced by the electric-field component of the terahertz pulse via spin-carrier interactions. Ferromagnetic nanoparticle systems are promising, and material designing is the key for ultrafast magnetization reversal using terahertz pulses. [ABSTRACT FROM AUTHOR]

Reports on transmission electron microscope observation on cross sections of molecular beam epitaxy-grown Al[subx]Ga[sub1-x]As-Al[suby]Ga[sub1-y] superlattice samples. Determination of layer thicknesses; Differences in the shape of the lattice points between the samples; Computer simulation of the lattice image of superlattice for the electron beam incident.

Our numerical experiments aim to study the applicability of the renormalization group (RG) idea to multichain lattice systems. The systems are composed of multiple chains on a simple cubic lattice. The nearest-neighbor interaction is assumed between nonbonded chain elements. The RG parameter space is spanned by the inverse of the chain length 1/n, the solvent condition η, and the chain number concentration ρ. We first tentatively determine parameter flows by using two RG invariants (weak conditions). Then we examine whether the flows satisfy the RG requirements for the correlations (strong conditions). Simultaneous satisfaction will guarantee us the applicability of the RG idea. We found that the RG idea is applicable to the systems in the very good solvent region but becomes inapplicable to those in the poorer solvent regions and to the shorter spatial distances. In the range where the linearized RG defined by a fixed point in the good solvent region works successfully, we arrive at scaling forms in several fashions. One of them corresponds to Freed’s crossover form. The RG appears to work even in the nonlinear range, i.e., concentrated solutions outside the semidilute region. The RG failure in the poor solvent region is briefly discussed. [ABSTRACT FROM AUTHOR]

According to the recent polymer theories, the thermodynamic properties of polymer solutions in a good solvent, at the scaling limit, are governed by a scaling variable composed of the polymerization degree n, the concentration [lowercase_phi_synonym], and the polymer dimension exponent ν. Our computer experiments on multichain lattice systems with the nearest-neighbor interaction afford us the configurational free energy or the osmotic pressure. The nearest-neighbor interaction or the solvent condition where the free energy best obeys the scaling is reinterpreted as the fixed point solvent condition in a lattice space RG context. We approach the scaling limit by extrapolating the data obtained at that solvent condition to the limit n → ∞. The osmotic pressure obtained by the procedure shows an excellent agreement with Ohta and Oono’s theory. The nonextrapolated chain dimensions at that solvent condition are also in a good agreement with the theory. It is found that a weak nearest-neighbor attractive interaction works between the chain elements at that solvent condition. We, therefore, should take care for saying about the scaling from experiments on athermal solutions. Many of our data appear to be successfully interpreted in terms of the RG context by the help of some optimistic expectation for the convergence. [ABSTRACT FROM AUTHOR]

Examines the ratio dependence of molecular-beam epitaxy grown gallium-aluminum-antimony material system. Description of X-ray diffraction; Discussion on growth mechanisms; Details of the Raman scattering measurement.

Sub-picosecond modulation of the optical reflectivity (R) using terahertz electric-field (ETHz) pulses was achieved in a typical one-dimensional Mott insulator, the bromine-bridged nickel compound, [Ni(chxn)2Br]Br2 (chxn: cyclohexanediamine). The reflectivity change (ΔR/R) at around the Mott-gap transition peak (∼1.3 eV) was ∼1% for ETHz ∼45 kV/cm, and proportional to the square of ETHz. The relaxation time of ΔR/R was under 0.1 ps, enabling optical switching with a high repetition rate in the near-infrared region. The electric-field and probe-energy dependences of ΔR/R demonstrate that the modulation is due to large third-order optical nonlinearity of one-dimensional Mott insulators. [ABSTRACT FROM AUTHOR]

The design and performance of a cryogenic low energy electron point source microscope are described. The microscope can be immersed in liquid helium, thus allowing for investigations at cryogenic temperatures. The cryogenic environment is associated with a low fluctuation in the field emission electron beam current and, combined with a compact and rigid mechanical design, low mechanical drift characteristics. [ABSTRACT FROM AUTHOR]

The design and performance of an ultrahigh vacuum compatible cryogenic dual-tip scanning tunneling microscope is described. The microscope is attached at the bottom of a low-loss liquid helium Dewar and can be operated down to 4.2 K. The coarse positioning system consists of five linear steppers driven by piezo-tubes. The displacement of each stepper can be monitored by its own embedded capacitive position sensor with a submicron resolution, thus allowing accurate control of the tip navigation process. An alignment procedure, using a specimen made of three mutually nonparallel planes, is introduced to bring the two tips into overlapped scan ranges without the help of an additional guiding device such as an electron microscope. The overall system exhibits good mechanical rigidity and atomic resolution has been achieved with either tip. This instrument is well suited for investigating low temperature quantum properties of atomically clean nanostructures in a three-terminal configuration. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

An ultrahigh vacuum compatible compact helium cryostat with a 48 h hold time for a 2.5 l of liquid helium capacity has been realized. The design exploits the large enthalpy of the gas phase helium, by using two layers of vapor-cooled radiation shield in place of the conventional liquid nitrogen jacket. The mechanical rigidity, the absence of liquid nitrogen bubbling, and a low helium boil-off rate make this cryostat particularly suitable for applications that require a low vibration environment such as scanning probe microscopy. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Proposes a method for estimating the amplitude of measurement noise present in chaotic time series. Developing the initial error using the nearest-neighbor model for prediction of a time series; Use of initial error to estimate the amplitude of noise in a time series; Point density term in the initial error.

Picosecond light pulses synchronized with successive pulses of synchrotron radiation have been generated using a cw mode-locked Nd:YAG laser. By improvement of the time characteristics of the synchronized pulses, transient absorption measurements have been made in a short time domain of 0.05-11 ns using a delay-time modulation technique. [ABSTRACT FROM AUTHOR]

Discusses the abrupt collapse transition of a linear flexible polymer with van der Waals intramolecular attraction. Information on the transition of a lattice polymer under stretching force; Details of computer experiments on flexible polymers in poor solvents; Definition of solvent condition.

We have investigated the effects of the formation temperature and postmetallization annealing (PMA) on the interface properties of GeNx/p-Ge fabricated by the plasma nitridation of Ge substrates using an electron-cyclotron-resonance-generated nitrogen plasma. The nitridation temperature is found to be a critical parameter in improving the finally obtained GeNx/Ge interface properties. The GeNx/Ge formed at room temperature and treated by PMA at 400 °C exhibits the best interface properties with an interface trap density of 1 × 1011 cm-2 eV-1. The GeNx/Ge interface is unpinned and the Fermi level at the Ge surface can move from the valence band edge to the conduction band edge. [ABSTRACT FROM AUTHOR]

Most of the countercharge in the conventional electret-based electromechanical transducer, where the electret is formed on a conducting or semiconducting substrate, is induced in the substrate. Here we introduce a type of electret transducer that contains a freestanding electret without a conducting substrate, thereby enhancing the electric field between the electret and the electrodes where the output current is generated. A measurement of the power output from an electromechanical transducer yielded approximately five times larger power when a freestanding electret film was used than when the same electret material was deposited on a conducting substrate. [ABSTRACT FROM AUTHOR]

We demonstrate 1240 nm single-photon emissions from InAs quantum dots (QDs) embedded in a hybrid pillar microcavity consisting of dielectric and semiconductor distributed Bragg reflectors. The QDs are grown by metal organic chemical vapor deposition. Discrete emission lines corresponding to isolated QDs in the hybrid pillar are observed with a spontaneous emission rate enhanced by a factor of 2. Single-photon emissions are confirmed by antibunching in a second-order photon correlation function. [ABSTRACT FROM AUTHOR]

Low-dose electron microscopy of radiation sensitive biological specimens achieves only limited resolution because of the inherently weak signal buried in noise. Here, we propose to reduce the noise by using a deformable mirror similar to the Castaing–Henry device to manipulate phase of the electron waves. The method takes advantage of quantum nature of the measurement and nonrandomness of any specimen structure of interest. The simulation revealed 9.7% reduction of noise for the representative case of the ribosome. Increasingly better figures are expected as knowledge about a specimen improves. [ABSTRACT FROM AUTHOR]

Third-order optical nonlinearity was investigated in two charge-transfer (CT)-type conjugated polymers, poly(aryleneethynylene) and poly(thiophene-alt-thiazole), using the third-harmonic generation (THG) method. Comparison of the results with previously obtained THG data for polythiophene and the strong CT polymer poly(thiophene-alt-quinoxaline) revealed that the ratio of the maximum value of third-order nonlinear susceptibility and the absorption coefficient max|χ(3)|/αmax increased as the degree of CT character increased for the four polymers. This variation in max|χ(3)|/αmax is discussed based on the magnitude of the transition dipole moment between the one-photon-allowed and the one-photon-forbidden excited states. [ABSTRACT FROM AUTHOR]

We show that radiation damage to unstained biological specimens is not an intractable problem in electron microscopy. When a structural hypothesis of a specimen is available, quantum mechanical principles allow us to verify the hypothesis with a very low electron dose. Realization of such a concept requires precise control of the electron wave front. Based on a diffractive electron optical implementation, we demonstrate the feasibility of this new method by both experimental and numerical investigations. [ABSTRACT FROM AUTHOR]

The magnetic-field effect on electroluminescence (EL) has been investigated for the tris-(8-hydroxyquinolino) aluminum (Alq3)-based organic light emitting diode. The EL intensity sharply increases up to 8% with increasing magnetic field to 500 Oe at room temperature. The magnetic field effect on EL depends on the interface structure between a hole transporting and a light emitting layers, indicating the importance of the spin-state dynamics of the electron-hole pairs at the interface. [ABSTRACT FROM AUTHOR]

We have investigated the third-order optical nonlinearity of poly(3-hexylthiophenes) with various head-to-tail coupling ratios (r), using the third-harmonic generation method. An increase in r leads to a reduction in optical gap energy (Eg) and an increase in the third-order nonlinear susceptibility (χ(3)). For r=0 to 0.80, χ(3) is scaled by Eg as χ(3)∝Eg-6.7, while for r∼1, χ(3) is considerably enhanced beyond this scaling law. We discuss how the behavior of χ(3) is based upon the conjugation-length dependence of the transition dipole moments. [ABSTRACT FROM AUTHOR]

Wurtzite InN films were grown on a thick GaN layer by metalorganic vapor phase epitaxy. Growth of a (0001)-oriented single crystalline layer was confirmed by Raman scattering, x-ray diffraction, and reflection high energy electron diffraction. We observed at room temperature strong photoluminescence (PL) at 0.76 eV as well as a clear absorption edge at 0.7-1.0 eV. In contrast, no PL was observed, even by high power excitation, at ∼1.9 eV, which had been reported as the band gap in absorption experiments on polycrystalline films. Careful inspection strongly suggests that a wurtzite InN single crystal has a true bandgap of 0.7-1.0 eV, and the discrepancy could be attributed to the difference in crystallinity. [ABSTRACT FROM AUTHOR]

A homogeneous, amorphous peroxopolytungstic acid film can be formed easily with the spin-coating method. The film is found to provide a homogeneous tungsten metal film after reduction with hydrogen at ∼400 °C. Although the electric resistivity (∼450 μΩ cm) is higher than that for the bulk (5.65 μΩ cm), this method has an advantage in addition to spin coatability: tungsten metal films with fine patterns can be easily obtained because the starting material is photosensitive. [ABSTRACT FROM AUTHOR]

Voltage-controlled optical bistable laser operation is demonstrated for the first time using nonlinear optical absorption of two-dimensional excitons in a GaAs-AlGaAs multiple quantum well (MQW) laser. The laser used here is the conventional MQW laser diode but with tandem electrode configuration. The L-I curve of light output as a function of current injected into one segment of electrodes can be changed by a bias voltage applied to the other segment of electrodes. At the onset of laser oscillation, a hysteresis loop appears, the shape of which depends strongly on the applied bias voltage. This bistability is ascribed to the nonlinear absorption of two-dimensional excitons. [ABSTRACT FROM AUTHOR]

Recently reported amorphous polytungstic acids containing peroxo groups are found to provide a new inorganic resist. A homogeneous resist film can be formed easily with the spin coating method. This film is made insoluble in water by deep ultraviolet, x ray, and electron beam irradiation. The oxygen reactive ion etching (O2 RIE) durability is found to be high enough that a bilayer resist scheme employing this resist as a top imaging layer gives a high resolution pattern (line and space: 0.38 μm). The mechanism for the radiation induced reaction is also discussed. [ABSTRACT FROM AUTHOR]

This letter describes the monolithic integration of a laser diode and an optical waveguide modulator that have the same GaAs/AlGaAs quantum well double heterostructure grown on the same substrate. The intensity of the light emitted from the laser diode is modulated by the modulator, utilizing the effect of an electric field on exciton absorption. The absorption loss coefficient of the modulator waveguide with no applied voltage was estimated to be 60 cm-1. This value is much smaller than the previously reported corresponding value of a veguide having a conventional GaAs double heterostructure. The modulation depth achieved was 7 dB for a driving voltage of 2.3 V; the cut-off modulation frequency was 0.88 GHz. A prospect for improving these characteristics is also presented. [ABSTRACT FROM AUTHOR]

Optical pulses of less than 4 ps full width at half maximum have been generated by electrical gain switching of long-wavelength multiple quantum well lasers, and were observed directly on a streak camera. This is the first ever generation of optical pulses of such short duration using direct electrical gain switching. We have identified the higher differential gain due to the second quantized state transition in the multiple quantum well laser to be responsible for the generation of these ultrashort optical pulses. We also discuss the possible resolution limits of the streak camera measurement system. [ABSTRACT FROM AUTHOR]

The application of microwave-generated atomic oxygen as an oxidant is found to change the manner of atomic layer deposition (ALD) of an Al2O3 layer on a Ge substrate, leading to the spontaneous formation of aluminum germanate with a deposition rate higher than that of conventional ALD with water oxidant. Electrical characterization of the Al/aluminum germanate (11 nm)/p-Ge(100) structure indicates that both the bulk and the interface properties of the aluminum germanate are promising with small capacitance-voltage hysteresis of less than 20 mV and interface trap densities ranging from 2×1011 to 6×1011 cm-2 eV-1 in the upper half of the Ge band gap. [ABSTRACT FROM AUTHOR]

We have investigated optical nonlinearity in beryllium-doped low-temperature (LT) molecular-beam-epitaxy-grown GaAs/AlAs multiple quantum wells (MQWs). The response time of the nonlinearity is reduced by Be doping in the MQW. While the undoped LT MQW shows a 0.7–0.9 ps response, the response time of the Be-doped LT MQW is as short as 0.25 ps. The saturation density of the Be-doped MQW is almost the same as that of the undoped MQW, and is smaller than that of bulk GaAs. These results demonstrate that the Be-doped LT MQW exhibits a faster response than the undoped LT MQW, and a faster response as well as larger nonlinearity than LT bulk GaAs. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]