Linear and nonlinear photoexcitation dynamics in pi-conjugated polymers

Linear and nonlinear recombination kinetics with various lifetime distributions were identified for long-lived photoexcitations in a series of pi-conjugated polymer films using modulation frequency and excitation intensity dependencies of the photoinduced absorption. This includes monomolecular, bimolecular, and defect-limited recombination processes that lead to saturation. Using generalized kinetics parameters, we found characteristic plots for all recombination processes. Specifically, the bimolecular recombination process shows superlinear intensity dependence away from the steady state; on the contrary, dispersive bimolecular recombination leads to sublinear dependence.     

Discovery of temperature-dependent phenomena of muon-catalyzed fusion in solid deuterium and tritium mixtures

A systematic experimental study on muon-catalyzed fusion was conducted using a series of solid deuterium and tritium mixtures. A variety of conditions were investigated, i.e., tritium concentrations from 20% to 70%, and temperatures from 5 to 16 K. With decreasing temperature, we observed an unexpected decrease in the muon cycling rate (lambda(c)) and an increase in the muon loss probability (W). The origins of these observed changes were interpreted by the temperature-dependence in the dt mu formation process for lambda(c) and that in the muon reactivation process after muon-to-alpha sticking for W.     

III-Nitride Quantum Devices—Microphotonics

Photonic devices based on III-nitrides offer benefits such as UV/blue emission, large band offsets of InN/GaN/AlN heterostructures allowing novel quantum well (QW) device design, and inherently high-emission efficiencies. Furthermore, due to their mechanical hardness and larger band gaps (when compared with conventional semiconductor devices), III-nitride-based devices may operate at much higher temperatures and voltages/power levels for any dimensional configuration and in harsher environments than other semiconductor devices and are expected to provide much lower temperature sensitivities. These are crucial advantages for many applications. Over the last decade, the physics of microsize photonic devices has been investigated. New physical phenomena and properties are expected to dominate as the device size scales down. The microsize light emitters offer benefits over edge emitters such as the ability to create arrays of individually controllable pixels on a single chip, enhanced quantum efficiency, and greatly reduced lasing threshold. Rapid progress in the area of III-nitride microphotonics has been made. The growth and fabrication of micron and submicron size photonic structures based on III-nitride wide bandgap semiconductors has been achieved, and the technology has made it possible to integrate arrays of optical elements to form active photonic-integrated devices. One example is an interconnected µ-LED with enhanced emission efficiency over the conventional LEDs for the same device area. Another example is a µ-LED array with independently addressed pixels or III-nitride microdisplay. III-nitride microdisplay may offer performance that is superior to microdisplays fabricated from liquid crystals and organic LEDs. The third example presented is III-nitride UV Focal Plane Arrays (UV-FPA) of detectors. So far, the operation of AlGaN UV-FPA with size up to 256×256 pixels with 30×30?μm² unit cells has been demonstrated. Together with the nature of their two-dimensional array, these active micro-photonic devices show promise in many important applications, such as optical communications, signal and image processing, optical interconnects, computing, enhanced energy conversion and storage, chemical, biohazard substances, and disease detection, missile and shellfire, atmospheric ozone-level, and flame sensing. III-nitride microlens arrays have been fabricated successfully for blue and UV wavelength applications on GaN and AlN. The successful fabrication of microlens arrays based on III-nitride materials opens the possibility for monolithically integrating nitride-based micro-size photonic devices, as well as coupling light into, out of, and between arrays of III-nitride emitters and detectors, especially for short wavelengths covering the green-blue to deep UV (200?nm) region. Nanofabrication and characterization of photonic crystals with diameter/periodicity as small as 100/180?nm on InGaN/GaN MQW has been achieved. An unprecedented maximum enhancement factor of 20 was obtained under optical pumping. Single-mode ridged optical waveguide devices using GaN/AlGaN heterostructures have been designed, fabricated, and characterized for operation in 1550?nm wavelength window. The feasibility of developing novel photonic integrated circuits based on III-nitride wide bandgap semiconductors for fiber-optical communications has been investigated.     

Analytical method for steady state vibration of system with localized non-linearities using convolution integral and Galerkin method

The analytical method using transfer function or impulse response is very effective for analyzing non-linear systems with localized non-linearities. This is because the number of non-linear equations can be reduced to that of the equations with respect to points connected with the non-linear element. In the present paper, analytical method for the steady state vibration of non-linear system including subharmonic vibration is proposed by utilizing convolution integral and the impulse response. The Galerkin method is introduced to solve the non-linear equations formulated by the convolution integral, and then the steady state vibration is obtained. An advantage of the present method is that stability or instability of the steady state vibration can be discriminated by the transient analysis from convolution integral. The three-degree-of-freedom mass-spring system is shown as a numerical example and the proposed method is verified by comparing with the result by Runge-Kutta-Gill method.     

Growth and optical properties of In:Nd:LiNbO3 crystals

In, Nd double-doped LiNbO₃ (LN) crystals have been grown for the first time. Their infrared (IR) transmission spectra were measured and discussed to investigate their structure and defects. The optical damage resistance of Nd:In:LiNbO₃ crystals were characterized by straightly observing transmission facula distortion method. The optical damage resistance of In (4.0 mol%):Nd:LiNbO₃ was much higher than that of Nd:LiNbO₃. The defects were discussed in this paper to explain the optical damage resistance in the In:Nd:LiNbO₃ crystals.     

Superconductivity in CoSr2(Y1−xCax)Cu2O7+δ

The roles of aliovalent Ca^II-for-Y^III substitution and high-pressure-oxygen annealing in the process of ‘superconducterizing’ the Co-based layered copper oxide, CoSr₂(Y_1−xCa_x)Cu₂O_7+δ (Co-1212), were investigated. The as-air-synthesized samples up to x=0.4 were found essentially oxygen stoichiometric (−0.03≤δ≤0.00). These samples, however, were not superconducting, suggesting that the holes created by the divalent-for-trivalent cation substitution are trapped on Co in the charge reservoir. Ultra-high-pressure heat treatment carried out at 5 GPa and 500 °C for 30 min in the presence of Ag₂O₂ as an excess oxygen source induced bulk superconductivity in these samples. The highest T_c was obtained for the high-oxygen-pressure treated x=0.3 sample at ∼40 K.     

Extremely large binding energy of biexcitons in an organic-inorganic quantum-well material (C4H9NH3)2PbBr4

We have confirmed biexciton formation in an organic-inorganic hybrid quantum-well material (C₄H₉NH₃)₂PbBr₄ by photoluminescence and two-photon absorption measurements. The biexciton has extremely large binding energy, 60 meV, which to our knowledge is the largest value ever reported for a semiconductor. By analyzing the spectrum of biexciton luminescence, the biexciton gas temperature was found to be much higher than the bath temperature due to a higher local temperature arising from the large biexciton binding energy.     

Conductance in normal metal/insulator/ferromagnetic superconductor tunnel junctions

In this paper, the superconducting order parameter and the energy spectrum of the Bogoliubov excitations are obtained from the Bogoliubov-de Gennes equation for a ferromagnetic superconductor (FS). In the framework of the Blonder-Tinkham-Klapwijk model, we present the differential conductance of the normal metal/insulator/FS junctions. It is shown that the exchange energy h in the FS can lead to the Zeeman splitting of the conductance peaks and the energy difference between the two splitting peaks is equal to 2h. The observation of such Zeeman splitting in the conductance spectrum can be taken as evidence for the coexistence between superconductivity and ferromagnetism.     

Atomic structure of Cs layer grown on Si(0 0 1)(2 × 1) surface at room temperature

The atomic structure of Cs atoms adsorbed on the Si(0 0 1)(2 × 1) surface has been investigated by coaxial impact collision ion scattering spectroscopy. When 0.5 ML of Cs atoms are adsorbed on Si(0 0 1) at room temperature, it is found that Cs atoms occupy a single absorption site on T3 with a height of 3.18 ± 0.05 Å from the second layer of Si(0 0 1)(2 × 1) surface, and the bond length between Cs and the nearest Si atoms is 3.71 ± 0.05 Å.     

Dynamical significance in alkali metasilicate glasses

In this work, we elucidate the effect of the less mobile ions on the dynamics of the more mobile ions by molecular dynamics simulations of lithium ions motion in lithium metasilicate glass by freezing some randomly chosen lithium ions (5%, 10% and 25%) at their initial locations at 700 K. A remarkable slowing down of the dynamics of the majority mobile Li ions was observed both in the self-part of the density–density correlation function, F_s(k,t), and in the diffusion coefficients. On the other hand, there is no significant change in the structure. These results show many similarities to the mixed alkali effect (MAE) with mixing of the small content of foreign alkali (10% and 25% of K₂O), where large reduction of the dynamics was also observed in both experiments and MD simulations. This immobilization of faster ions causes the large MAE as already discussed in relation to the mechanism of the cooperative ion jump motions. Although of lesser importance, the ion dynamics are influenced by the matrix of oxygen atoms, because the jump motions of Li ions are assisted by the localized motions of oxygen atoms.