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The recently developed refractory multi-principle element alloy, AlMo0.5NbTa0.5TiZr, shows an interesting microstructure with cuboidal precipitates of a disordered phase (β, bcc) coherently embedded in an ordered phase (β′, B2) matrix, unlike the conventional Ni-based superalloys where the ordered phase (γ′, L12) is the precipitate phase and the disordered phase (γ, fcc) is the matrix phase. It becomes critical to understand the phase transformation pathway (PTP) leading to this microstructure in order to tailor the microstructure for specific engineering applications. In this study, we first propose a possible PTP leading to the microstructure and employ the phase-field method to simulate microstructural evolution along the PTP. We then explore possible PTPs and materials parameters that lead to an inverted microstructure with the ordered phase being the precipitate phase and the disordered phase being the matrix phase, a microstructure similar to those observed in Ni-based superalloys. We find that in order to maintain the precipitates as highly discrete particles along these PTPs, the volume fraction of the precipitate phase needs to be smaller than that of the matrix phase and the elastic stiffness of the precipitate phase should be higher than that of the matrix phase. [ABSTRACT FROM AUTHOR]

We have studied the first phase nucleation behavior of a series of equiatomic Ni-Si multilayers with ultrathin repeating subunits (λ=1.7-5.8 nm) using x-ray reflectivity and x-ray diffraction. The variation in first nucleated phase is understood in terms of the model of Gösele and Tu, who predicted three length-scale dependent reaction routes in thin film diffusion couples: (i) For multilayers with λ>5.0 nm, interfacial nucleation of Ni[SUB2]Si is observed, consistent with studies of bulk diffusion couples. (ii) For multilayers with 3.8< λ<5.0 nm, interfacial nucleation tends towards phases richer in Si due to a waning supply of Ni within the multilayer. (iii) For multilayers with λ<2.0 nm complete mixing of the Ni-Si multilayers prefigures first phase nucleation of NiSi. In this regime the composition of the first nucleation phase is determined by the amorphous precursor, which in turn is dictated by the relative ratios of Ni and Si in the initial multilayer. A fourth scenario, between (ii) and (iii), in which Ni[SUB2]Si is the first nucleated phase is also observed. A possible mechanism for this unexpected result is discussed. [ABSTRACT FROM AUTHOR]

Discusses scanning tunneling microscopy with a photoconductive gate in the tunneling current circuit. Use of a rastercope, a distance measuring equipment; Probe tip design and optimization; Observation of transient signal; Measurement analysis; Spatial resolution.

Topology optimization is a promising method for systematic design of optical devices. As an example, we demonstrate how the method can be used to design a 90° bend in a two-dimensional photonic crystal waveguide with a transmission loss of less than 0.3% in almost the entire frequency range of the guided mode. The method can directly be applied to the design of other optical devices, e.g., multiplexers and wave splitters, with optimized performance. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Investigates the use of spatio-temporal scans through ultrafast scanning tunneling microscope. Influence of transmission line discontinuities on the mode structure of voltage pulses; Information on the generation of common and differential modes of electrical pulses; Details on the capacitive coupling between the tip and the transmission line.

Investigates the voltage transients using an ultrafast scanning tunneling microscope. Dependence of the transient signal amplitude on the tunneling resistance; Geometrical capacitance of the tip-electrode junction; Illumination of the current-gating photoconductive switch.

We obtain femtosecond (200 fs) time resolution using a scanning tunneling microscope on surface plasmon polaritons (SPPs) generated by two 100 fs laser beams in total internal reflection geometry. The tunneling gap dependence of the signal clearly indicates the tunneling origin of the signal and suggests that nanometer spatial resolution can be obtained together with femtosecond temporal resolution. This fast response, in contrast to the picosecond decay time of SPPs revealed by differential reflectivity measurements, can be attributed to a coherent superposition of SPPs rectified at the tunneling junction. © 1998 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Resonant second-harmonic generation is observed at room temperature in reflection from self-assembled InAlGaAs quantum dots grown on a GaAs (001) substrate. The detected second-harmonic signal peaks at a pump wavelength of ∼885 nm corresponding to the quantum-dot photoluminescence maximum. In addition, the second-harmonic spectrum exhibits another smaller but well-pronounced peak at 765 nm not found in the linear experiments. We attribute this peak to the generation of second-harmonic radiation in the AlGaAs spacer layer enhanced by the local symmetry at the quantum-dot interface. We further observe that second-harmonic images of the quantum-dot surface structure show wavelength-dependent spatial variations. Imaging at different wavelength is used to demonstrate second-harmonic generation from the semiconductor quantum dots. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We demonstrate the use of an ultrafast scanning tunneling microscope on a semiconductor surface. Laser-induced transient signals with 1.8 ps rise time are detected. The investigated sample is a low-temperature grown GaAs layer placed on a sapphire substrate with a thin gold layer that serves as a bias contact. For comparison, the measurements are performed with the tip in contact to the sample as well as in tunneling above the surface. In contact and under bias, the transient signals are identified as a transient photocurrent. An additional signal is generated by a transient voltage induced by the nonuniform carrier density created by the absorption of the light (photo Dember effect). The transient depends in sign and in shape on the direction of optical excitation. This signal is the dominating transient in tunneling mode. The signals are explained by a capacitive coupling across the tunneling gap. © 1998 American Institute of Physics. [ABSTRACT FROM AUTHOR]