Low-temperature specific heat study of SrCu2(BO3)2 with an exactly solvable ground state

The specific heat of a two-dimensional spin gap system SrCu₂(BO₃)₂ realizing the Shastry-Suther-land model was measured between 1.3 and 25 K under various magnetic fields up to 12 T. The analysis based on an isolated dimer model in a low temperature region revealed that the value of the spin gap at zero field is Δ = 34.4 K. It turned out that Δ decreases in proportion to H due to the Zeeman splitting of the excited triplet levels. This simplest model, however, fails to reproduce the result in a high-temperature region, suggesting rather strong spin-spin correlation of the system.     

Computation of transition to turbulence in the compression stage of a reciprocating engine

The transition to turbulent flow in the compression stage of a reciprocating engine is studied by obtaining the finite-difference numerical solutions to the governing Navier-Stokes equations without using explicit turbulence models. A computational method is developed under the assumption that the flow is in a low-subsonic regime with strong compression. The numerical method is a simple extension of the well known MAC method. Computations were performed for three different chamber geometries at the engine speed of 1400 rpm. The results of the computations clearly demonstrate the transient process in which large tumbling vortices break down into smaller ones near the end of the compression process. The transition process is also caught experimentally by using Mach-Zehnder interferometry.     

Synthesis of carbazole‐based light‐emitting polymers incorporating 2,5‐bis(phenylethenyl)‐4‐decyloxyanisole as collateral fluorophore

The Suzuki coupling polymerization between bis(carbazole) monomer ( CzDB ) and 9,9‐dihexylfluorene‐2,7‐diboronic acid was carried out to obtain PFCz‐PEDA0 having the number‐averaged molecular weight of 7000. The absorption and emission maximum wavelengths were observed at 344 and 408 nm, respectively. The quantum yield (QY) was relatively low (0.12) because of the photo‐induced electron transfer. Subsequently, CzPEDA ‐bearing 2,5‐bis(phenylethenyl)‐4‐decyloxyanisole (PEDA) segment sandwiched with 3‐bromocarbazole units was copolymerized to give PFCz‐PEDAn (n = 05, 10, 20, 35, and 50). The content of PEDA segment in polymer could be controlled by the monomer feed ratio. In CHCl₃ solution, the absorbance at around 400 nm became larger with one isosbestic point at 370 nm, and the emission peak at 448 nm became prominent with increasing the PEDA content. The QY of polymer was increased as the PEDA content, which was a consequence of the fluorescence resonance energy transfer from carbazole‐containing chromophore (energy donor) to PEDA fluorophore (energy acceptor). In spin‐coated film, the maximum QY was obtained in PFCz‐PEDA05 having the most appropriate molar balance of energy donor and acceptor units. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 8141–8148, 2008     

Biologically-inspired stochastic vector matching for noise-robust information processing

“End of Moore’s Law” has recently become a topic. Keeping the signal-to-noise ratio (SNR) at the same level in the future will surely increase the energy density of smaller-sized transistors. Lowering the operating voltage will prevent this, but the SNR would inevitably degrade. Meanwhile, biological systems such as cells and brains possess robustness against noise in their information processing in spite of the strong influence of stochastic thermal noise. Inspired by the information processing of organisms, we propose a stochastic computing model to acquire information from noisy signals. Our model is based on vector matching, in which the similarities between the input vector carrying external noisy signals and the reference vectors prepared in advance as memorized templates are evaluated in a stochastic manner. This model exhibited robustness against the noise strength and its performance was improved by addition of noise with an appropriate strength, which is similar to a phenomenon observed in stochastic resonance. Because the stochastic vector matching we propose here has robustness against noise, it is a candidate for noisy information processing that is driven by stochastically-operating devices with low energy consumption in future. Moreover, the stochastic vector matching may be applied to memory-based information processing like that of the brain.     

Synthesis and optical properties of fluorene‐based polymers incorporating organosilicon unit

The synthesis and optical properties of polymers bearing the repeating unit of terfluorene and various organosilicon groups were investigated. Polymers with high molecular weight and good solubility could be obtained by Suzuki coupling polymerization from silylene‐containing fluorene‐based dibromo monomers and 9,9‐dihexylfluorene‐2,7‐bis(trimethyleneborate). From UV spectra of polymers bearing acyclic silylene bridge, the organosilicon units not only interrupted a π‐conjugation but also contributed to an electronic communication between connected fluorenes. The emission maximum wavelengths (ca. 400 nm) blue‐shifted when compared with that of polyfluorene (418 nm) and the fluorescence quantum yields were considerably high (>0.82) in the CHCl₃ solution. On the other hand, rather broad emission was observed at 480 nm and the fluorescence quantum yield was quite low (0.004) in the solution‐state PL spectrum of tetraphenylsilole‐containing polymer. The polymer emitted visible green light in the spin‐coated film. The fluorescence peak intensity at 486 nm gradually decreased when the film was illuminated with the UV light of 359 nm in air. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 4786–4794, 2007     

Discrete dihedral-angle modulation in porphyrin wheels adsorbed on Cu(1 0 0) observed by scanning tunneling microscopy

Conformations of two dodecameric porphyrin wheels adsorbed on a Cu(1 0 0) were probed by using scanning tunneling microscopy (STM). Whereas a wheel consisting of six meso-meso linked diporphyrins was detected as uniform ring structure, several different images with three discrete molecular heights were detected for a wheel consisting of six meso-meso, β-β,β-β triply-linked planar diporphyrins. These results indicate that the former has a conformation similar to that in a free space, while the latter has various conformations with respect to orientation of planar diporphyrin units toward the metal surface. Several discrete STM images of the latter have been interpreted in terms of possible eight conformations, which vary as to relative orientation of neighboring diporphyrin units.