Fluctuating lattice constants of indium under high pressure

Recent high-pressure investigations of elemental In have yielded controversial results. We show that the observed high-pressure face-centered orthorhombic (fco) structure can be explained as an intermediate state between two body-centered tetragonal (bct) structures with different c/a ratios (c/a < square root [2] and c/a > square root [2], respectively). In a pressure range from about 50 to 200 GPa these two bct structures correspond to local minima of the total energy with respect to orthorhombic distortion of the ground-state bct In structure. The fco saddle point represents a tiny barrier and even at low temperatures rapid structural fluctuations should occur. Such a situation has not been identified in any other elemental metal.     

Measuring non-Gaussian fluctuations through incoherent cooper-pair current

We study a Josephson junction (JJ) in the regime of incoherent Cooper-pair tunneling, capacitively coupled to a nonequilibrium noise source. The current-voltage (I-V) characteristics of the JJ are sensitive to the excess voltage fluctuations in the source, and can thus be used for wideband noise detection. Under weak driving, the odd part of the I-V can be related to the second cumulant of noise, whereas the even part is due to the third cumulant. After calibration, one can measure the Fano factors for the noise source, and get information about the frequency dependence of the noise.     

On the vertical excitation energy of cyclopentadiene

The vertical excitation energy for the lowest valence pi-->pi(*) transition of cyclopentadiene is investigated. Using a combination of high-level theoretical methods and spectroscopic simulations, the vertical separation at the ground state geometry is estimated to be 5.43+/-0.05 eV. This value is intermediate between those calculated with coupled-cluster and multireference perturbation theory methods and is about 0.13 eV higher than the observed maximum in the absorption profile.     

Synthesis of linear and tripoidal oligo(phenylene ethynylene)-based building blocks for application in modular DNA-programmed assembly

Rigid linear and tripoidal organic modules based on the oligo(phenylene ethynylene) backbone having salicylaldehyde-derived termini are synthesized. A highly functionalized 5-iodosalicyl aldehyde was prepared and coupled to each ethynyl group of 1,4-diethynylbenzene or 1,3,5-triethynylbenzene in Sonogashira couplings. The two or three termini of the compounds are functionalized for incorporation in linear and branched oligonucleotide strands. For the linear module (LM), the two termini are equipped with amide spacers, and one of these was functionalized with a DMTr (dimethoxytrityl)-protected hydroxy group and the other with a phosphoramidite. One of the tripoidal modules is prepared with DMTr groups in two of its three termini. A tripoidal module is also synthesized with three different groups on its hydroxy termini: a phosphoramidite, a DMTr group, and an Fmoc group. Extended studies have shown that these rigid linear and tripoidal organic modules can be incorporated into short oligonucleotides. Several of these modules can be applied for DNA-directed assembly and covalent coupling into structures of predetermined connectivity. Such structures have potential application for molecular electronics and nanotechnology.     

Variable temperature system using vortex tube cooling and fiber optic temperature measurement for low temperature magic angle spinning NMR

We describe the construction and operation of a variable temperature (VT) system for a high field fast magic angle spinning (MAS) probe. The probe is used in NMR investigations of biological macromolecules, where stable setting and continuous measurement of the temperature over periods of several days are required in order to prevent sample overheating and degradation. The VT system described is used at and below room temperature. A vortex tube is used to provide cooling in the temperature range of -20 to 20 degrees C, while a liquid nitrogen-cooled heat exchanger is used below -20 degrees C. Using this arrangement, the lowest temperature that is practically achievable is -140 degrees C. Measurement of the air temperature near the spinning rotor is accomplished using a fiber optic thermometer that utilizes the temperature dependence of the absorption edge of GaAs. The absorption edge of GaAs also has a magnetic field dependence that we have measured and corrected for. This dependence was calibrated at several field strengths using the well-known temperature dependence of the (1)H chemical shift difference of the protons in methanol.     

Gap and out-gap breathers in a binary modulated discrete nonlinear Schrödinger model

We consider a modulated discrete nonlinear Schrödinger (DNLS) model with alternating on-site potential, having a linear spectrum with two branches separated by a forbidden gap. Nonlinear localized time-periodic solutions with frequencies in the gap and near the gap -- discrete gap and out-gap breathers (DGBs and DOGBs) -- are investigated. Their linear stability is studied varying the system parameters from the continuous to the anti-continuous limit, and different types of oscillatory and real instabilities are revealed. It is shown, that generally DGBs in infinite modulated DNLS chains with hard (soft) nonlinearity do not possess any oscillatory instabilities for breather frequencies in the lower (upper) half of the gap. Regimes of exchange of stability between symmetric and antisymmetric DGBs are observed, where an increased breather mobility is expected. The transformation from DGBs to DOGBs when the breather frequency enters the linear spectrum is studied, and the general bifurcation picture for DOGBs with tails of different wave numbers is described. Close to the anti-continuous limit, the localized linear eigenmodes and their corresponding eigenfrequencies are calculated analytically for several gap/out-gap breather configurations, yielding explicit proof of their linear stability or instability close to this limit.Received: 31 October 2003, Published online: 17 February 2004PACS: 63.20.Pw Localized modes - 63.20.Ry Anharmonic lattice modes - 42.65.Wi Nonlinear waveguides     

Analysis of the phase transitions in alkyl-mica by density and pressure profiles

In a previous work [Heinz, Castelijns, and Suter, J. Am. Chem. Soc. 115, 9500 (2003)], we developed an accurate force field and simulated the phase transitions in C18-mica (octadecyltrimethylammonium-mica) as well as the absence of such transitions in 2C18-mica (dioctadecyldimethylammonium-mica) between room temperature and 100 degrees C. Here we analyze (i) average z coordinates of the carbon atoms and interdigitation of the hydrocarbon bilayers, (ii) density profiles, and (iii) pressure profiles of the structures along all Cartesian axes. In C18-mica, the standard deviation in the z coordinate for the chain atoms is high and more than doubles in the disordered phase. The order-disorder transition is accompanied by a change in the orientation of the ammonium head group, as well as decreasing tensile and shear stress in the disordered phase. In 2C18-mica, the standard deviation in the z coordinate for the chain atoms is low and does not increase remarkably on heating. The backbones display a highly regular structure, which is slightly obscured by rotations in the C18 backbones and minor head group displacements at 100 degrees C. Close contacts between the bulky head groups with sidearms cause significant local pressure which is in part not relieved at 100 degrees C. An increase of the basal-plane spacing at higher temperature is found in both systems due to larger separation between the two hydrocarbon layers and an increased z spacing between adjacent chain atoms (=decreased tilt of the chains relative to the surface normal), and, in C18-mica only, a stronger upward orientation of the C18 chain at the ammonium head group. The likelihood for chain interdigitation between the two hydrocarbon layers is 24%-30% for C18-mica, and 65%-26% for 2C18-mica (for 20-100 degrees C).     

Microfluidic biosensing systems. Part II. Monitoring the dynamic production of glucose and ethanol from microchip-immobilised yeast cells using enzymatic chemiluminescent micro-biosensors

A microfluidic flow injection (microFIA) system was employed for handling and monitoring of cell-released products from living cells immobilised on silicon microchips. The dynamic release of glucose and ethanol produced from sucrose by immobilised Saccharomyces cerevisiae cells was determined using microchip biosensors (micro-biosensors) with either co-immobilised glucose oxidase-horseradish peroxidase (GOX-HRP), or alcohol oxidase-horseradish peroxidase (AOX-HRP), catalysing a series of reactions ending up with chemiluminescence (CL) generated from HRP-catalysed oxidation of luminol in presence of p-iodophenol (PIP). The yeast cells were attached by first treating them with polyethylenimine (PEI) followed by adsorption to the microchip surface. The cell loss during assaying was evaluated qualitatively using scanning electron microscopy (SEM), showing that no cells were lost after 35 min liquid handling of the cell chip at 10 microl min(-1). The enzymes were immobilised on microchips via PEI-treatment followed by glutaraldehyde (GA) activation. The GOX-HRP micro-biosensors could be used during five days without any noticeable decrease in response, while the AOX-HRP micro-biosensors showed continuously decreasing activity, but could still be used employing calibration correction. The glucose and ethanol released from the immobilised yeast chips were quantitatively monitored, by varying the incubation time with sucrose, showing the possibilities and advantages of using a microfluidic system set-up for cell-based assays.