Three-dimensional MgB2-type superconductivity in hole-doped diamond

We substantiate by numerical and analytical calculations that the recently discovered superconductivity below 4 K in 3% boron-doped diamond is caused by electron-phonon coupling of the same type as in MgB2, albeit in three dimensions. Holes at the top of the zone-centered, degenerate sigma-bonding valence-band couple strongly to the optical bond-stretching modes. The increase from two to three dimensions reduces the mode softening crucial for T(c) reaching 40 K in MgB2. Even if diamond had the same bare coupling constant as MgB2, which could be achieved with 10% doping, T(c) would be only 25 K. Superconductivity above 1 K in Si (Ge) requires hole doping beyond 5% (10%).     

Measurement of Cell Volume Changes by Fluorescence Self-Quenching

At high concentrations, certain fluorophores undergo self-quenching, i.e., fluorescence intensity decreases with increasing fluorophore concentration. Accordingly, the self-quenching properties can be used for measuring water volume changes in lipid vesicles. In cells, quantitative determination of water transport using fluorescence self-quenching has been complicated by the requirement of relatively high (mM) and often toxic loading concentrations. Here we report a simple method that uses low (M) loading concentrations of calcein-acetoxymethyl ester (calcein-AM) to obtain intracellular concentrations of the fluorophore calcein suitable for measurement of changes in cell water volume by self-quenching. The relationship between calcein fluorescence intensity, when excited at 490 nm (its excitation maximum), and calcein concentration was investigated in vitro and in various cultured cell types. The relationship was bell-shaped, with the negative slope in the concentration range where the fluorophore undergoes fluorescence self-quenching. In cultured retinal pigment epithelial cells, calcein fluorescence and extracellular osmolarity were linearly related. A 25-mOsm hypertonic challenge corresponded to a decrease in calcein fluorescence with high signal-to-noise ratio (>15). Similar results were obtained with the fluorophore BCECF when excited at its isosbestic wavelength (436 nm). The present results demonstrate the usefulness of fluorescence self-quenching to measure rapid changes in cell water volume.     

Conditions for the local manipulation of Gaussian states

We present a general necessary and sufficient criterion for the possibility of a state transformation from one mixed Gaussian state to another of a bipartite continuous-variable system with two modes. The class of operations that will be considered is the set of local Gaussian completely positive trace-preserving maps.     

Fourth-order magnetic anisotropy and tunnel splittings in Mn(12) from spin-orbit-vibron interactions

From density-functional-theory based methods, we calculate the vibrational spectrum of the Mn(12)O(12)(COOH)(16)(H(2)O)(4) molecular magnet. Calculated infrared intensities are in accord with experimental studies. There have been no ab initio attempts at determining which interactions account for the fourth-order anisotropy. We show that vibrationally induced distortions of the molecule contribute to the fourth-order anisotropy Hamiltonian and that the magnitude and sign of the effect (-6.2 K) is in good agreement with all experiments. Vibrationally induced tunnel splittings in isotopically pure and natural samples are predicted.     

Enantioselective transport by a steroidal guanidinium receptor

The cationic steroidal receptors 9 and 11 have been synthesized from cholic acid 3. Receptor 9 extracts N-acetyl-alpha-amino acids from aqueous media into chloroform with enantioselectivities (L:D) of 7-10:1. The lipophilic variant 11 has been employed for the enantioselective transport of N-acetylphenylalanine, a) through dichloromethane (DCM) and dichloroethane (DCE) bulk liquid membranes (U-tube apparatus), and b) through 2.5% (v/v) octanol/hexane via hollow fibre membrane contactors. Significant enantioselectivities and multiple turnovers were observed for both types of apparatus.     

A general method for determining the electron self-exchange rates of blue copper proteins by longitudinal NMR relaxation

A general NMR method is presented that allows a precise determination of the second-order rate constant, k(ese), for the electron self-exchange in blue copper proteins, from the longitudinal relaxation rates of the nuclei in the protein. The method relies on the use of partly oxidized (paramagnetic) samples of the protein. In contrast to previous NMR approaches for the determination of electron self-exchange rates, the applicability of the method extends beyond the slow-exchange limit, k(ese)c < R(ip), i = 1, 2, where c is the protein concentration, and R(ip) is the paramagnetic relaxation enhancement of the observed nuclei.     

Organic Nanoparticles in the Aqueous Phase-Theory,Experiment, and Use

Many active organic compounds and organic effect materials are poorly soluble in water, or even insoluble. Aqueous forms of application thus require special formulation techniques to utilize or optimize the physiological (pharmaceuticals, cosmetics, plant protection, nutrition) or technical (varnishes, printing inks, toners) action. The most interesting properties of nanodispersions of active organic compounds and effect materials include the impressive increase in solubility, the improvement in biological resorption, and the modification of optical, electrooptical, and other physical properties which are achievable only with particle sizes in the middle or lower nanometer range (50-500 nm). Hence in addition to economic and ecological constraints there are also technical demands which appear to urgently require the development of new processes for the production of organic nanoparticles as alternatives to the established mechanical milling processes. In this context attention is drawn to the recent increase in research activities which have as their objective the continuous, automatic preparation of nanodispersed systems by precipitation from molecular solution. In this review the current state of knowledge of the fundamentals of particle formation from homogeneous solution and the effect of solvent and polymer additives on the morphology and supramolecular structure of the nanoparticle will be discussed. The practical implementation of this new formulation technology will be explored in detail for the carotenoids, a class of compounds of both physiological and technical interest.     

A comparison of extensional viscosity measurements from various RME rheometers

The transient uniaxial extensional viscosity η _e of linear low density polyethylene (LLDPE) has been measured using the commercial Rheometric Scientific RME and the Münstedt Tensile Rheometer in an effort to compare the performance of available extensional rheometers. The RME indicated a significant strain hardening of the LLDPE, especially at a strain rate of 1 s⁻¹. In contrast, the Münstedt rheometer showed the LLDPE to be only slightly strain hardening. This artificial strain hardening effect in the RME resulted from the strain rate applied to the sample, determined from the sample deformation, being up to 20% less than the set strain rate. These results initiated a round-robin experiment in which the same LLDPE was tested on several RMEs in various locations around the world. All but one of the RMEs indicated a deviation between set and applied strain rates of at least 10%, especially at strain rates above 0.1 s⁻¹. The strain rate deviation was found to depend strongly on the value of the basis length L ₀ , and may result from the upper pair of belts not properly gripping the sample during extension. Thus visual inspection of the sample deformation is necessary to determine the applied strain rate. The most accurate measurements of η _e with respect to the strain rate deviation were obtained when the correct L ₀ value and belt arrangement were used. A list of recommendations for running an RME test is provided. Future work focusing on the fluid mechanics during the test may identify fully the cause of the strain rate deviation, but from a practical point of view the problem can be corrected for in the determination of η _e . Received: 27 September 2000/Accepted: 5 February 2001