Surface and friction characterization by thermoelectric measurements during ultrasonic friction processes

Even though friction is one of the oldest problems in physics many aspects of friction processes are not clear today. We present an experimental setup, which permits the study of tribological systems by measuring the dissipated heat at the interface of two surfaces during a friction process with a time resolution of 1 ms. The apparatus is based on a standard ultrasonic wire-bond machine used in semiconductor industries to connect the internal semiconductor die to the external leads, but the standard bond wire is replaced by a thermocouple. To demonstrate the ability of the apparatus it will be shown that bond substrates used in semiconductor industries can be unequivocally characterized.     

Magnetic phase in the near-surface region of an FeBO3 single crystal

An FeBO3 single crystal was studied from 291 K up to the Neel temperature T(N) = 348.35 K by depth selective conversion electron Mossbauer spectroscopy in ultrahigh vacuum (10(-9) mbar). A new magnetic near-surface phase was found. Its thickness D diverges on approaching T(N) and gives a critical exponent for the correlation length of nu = 0.59(4). The phase boundary between the bulk and near-surface phase could be identified.     

Collective modes in an open microwave billiard

Numerical calculations for a microwave Sinai billiard coupled strongly to a lead are performed as a function of the coupling strength between billiard and lead. They prove the formation of different time scales in an open quantum system at large coupling strength. The short-lived collective states are formed together with many long-lived trapped states.     

Adsorption of Diblock Copolymers of Poly(ethylene oxide) and Poly(lactide) at Hydrophilic Silica from Aqueous Solution

The inverse osmotic compressibility of the poly(propylene imine) dendrimers in deuterated methanol has been measured as a function of concentration with small-angle neutron scattering. The experimental results reveal three different regimes. First, there is a dilute regime going up to a maximum in the inverse osmotic compressibility. This region can be subdivided into a very dilute region, where the behavior is hard-sphere-like, and a denser region, where the solvation layers overlap. The maximum, occurring around volume fraction 0.30 for each generation, is found to be the concentration where the distance between the centers of two dendrimers is twice their radius of gyration. It designates the crossover to the second regime of a semidilute phase with shrinking dendrimers. Interpenetration of the dendrimers does not seem to take place. Finally, for the higher generations, at high concentrations, the dendrimers are collapsed and the inverse osmotic compressibility starts to increase again. As dendrimers are regularly and very highly branched molecules, they can be considered as ultimately hyperbranched polymers. For both types, the experimental inverse osmotic compressibility shows similar features. The dendrimers seem to be more compact, however. Copyright 2000 Academic Press.     

Measurement of cdk4 kinase activity using an affinity peptide-tagging technology

Cyclin-dependent kinases such as Cdk4 are involved in the control of cell cycle progression, and misregulation of Cdk4 has been implicated in many types of cancers. In the present study, we report the development of a novel homogeneous assay using an affinity peptide-tagging technology for rapidly discovering Cdk4 inhibitors. The DNA sequence encoding a streptavidin recognition motif, or StrepTag (AWRHPQFGG), was cloned and expressed at the C-terminus of a fusion protein of a 152-amino acid hyperphosphorylation domain (Rb152) of the retinoblastoma protein (Rb) linked to GST at the N-terminus. This affinity peptide-tagged protein (GST-Rb152-StrepTag), which contains the two known phosphorylation sites of Rb, specifically phosphorylated by Cdk4 in vivo, was used as a substrate in the current in vitro kinase assay. After phosphorylation, scintillation proximity assay (SPA) scintillant beads coated with streptavidin were added. Radiolabeled GST-Rb152-StrepTag was brought in close proximity to the SPA scintillant beads through the interaction between StrepTag and streptavidin, resulting in the emission of light from beads. By applying the affinity peptide-tagging technology, we have eliminated the separation and wash steps which are normally required in a radioactive filtration assay. Therefore, this homogeneous method is simple, robust, and highly amenable to high-throughput screening of Cdk4-specific inhibitors. Furthermore, the affinity peptide tagging technique reported here is a simple, generic method that can be applied to many recombinant proteins for the development of kinase and protein-protein interaction assays.     

A novel 1,5-benzoheteroazepine synthesis via a one-Pot coupling-isomerization-cyclocondensation sequence

2,4-Di(hetero)aryl substituted 2,3-dihydro 1,5-benzoheteroazepines (hetero = NH, O, S) can be readily sythesized in a one-pot process initiated by a coupling-isomerization sequence of an electron poor (hetero)aryl halide and a terminal propargyl alcohol subsequently followed by a cyclocondensation with 2-amino, 2-hydroxy, or 2-mercapto anilines. In addition, the structures were established unambiguously by an X-ray structure analysis of 7a.     

Comparative analysis of the 1H NMR relaxation enhancement produced by iron oxide and core-shell iron-iron oxide nanoparticles

Physicochemical and magnetorelaxometric characterization of the colloidal suspensions consisting of Fe-based nanoparticles coated with dextran have been carried out. Iron oxide and iron core/iron oxide shell nanoparticles were obtained by laser-induced pyrolysis of Fe(CO)₅ vapours. Under different magnetic field strengths, the colloidal suspension formed by iron oxide nanoparticles showed longitudinal (R₁) and transverse (R₂) nuclear magnetic relaxation suspension (NMRD) profiles, similar to those previously reported for other commercial magnetic resonance imaging (MRI) contrast agents. However, colloidal suspension formed by ferromagnetic iron-core nanoparticles showed a strong increase of the R₁ values at low applied magnetic fields and a strong increase of the R₂ measured at high applied magnetic field. This behaviour was explained considering the larger magnetic aggregate size and saturation magnetization values measured for this sample, 92 nm and 31 emu/g Fe, respectively, with respect to those measured for the colloidal suspensions of iron oxide nanoparticles (61 nm and 23 emu/g Fe). This suspension can be used both as T₁ and T₂ contrast agent.     

Valence changes and structural distortions in "charge ordered" manganites quantified by atomic-scale scanning transmission electron microscopy

We investigate the microscopic nature of the "charge ordering" modulation in mixed-valent manganites in real space using scanning transmission electron microscopy. The modulation in Bi0.5Sr0.4Ca0.1MnO3 has a uniform periodicity appearing as stripes in high angle annular dark field images. Geometric phase analysis shows the modulation to be a displacement wave with transverse amplitude (0.008+/-0.001)a and longitudinal amplitude (0.003+/-0.001)a. Series of energy loss spectra taken across the stripes show no periodic changes and place an upper bound of +/-0.04 on any valence changes of the Mn ions.     

Resonance fluorescence from a coherently driven semiconductor quantum dot in a cavity

We show that resonance fluorescence, i.e., the resonant emission of a coherently driven two-level system, can be realized with a semiconductor quantum dot. The dot is embedded in a planar optical microcavity and excited in a waveguide mode so as to discriminate its emission from residual laser scattering. The transition from the weak to the strong excitation regime is characterized by the emergence of oscillations in the first-order correlation function of the fluorescence, g(tau), as measured by interferometry. The measurements correspond to a Mollow triplet with a Rabi splitting of up to 13.3 microeV. Second-order correlation measurements further confirm nonclassical light emission.