Structural and electrical properties of granular metal films

Granular metal films (50–200,000 Å thick) were prepared by co-sputtering metals (Ni, Pt, Au) and insulators (SiO₂, Al₂O₃), where the volume fraction of metal, x, was varied from x = 1 to x = 0.05. The materials were characterized by electron micrography, electron and X-ray diffraction, and measurements of composition, density and electrical resistivity at electric fields ε up to 10⁶ V/cm and temperatures T in the range of 1.3 to 291 K. In the metallic regime (isolated insulator particles in a metal continuum) and in the transition regime (metal and insulator particles in a metal continuum) and in the transition regime (metal and insulator labyrinth structure) the conduction is due to percolation with a percolation threshold at x?0.5. Tunnelling measurements on superconductor-insulator-granular metal junctions reveals that the transition from the metallic regime to the dielectric regime (10–50 Å size isolated metal particles in an insulator continuum) is associated with the breaking up of a metal continuum into isolated metal particles. In the dielectric regime the temperature dependence of the low-field resistivity is given by ρL = ρo exp [2√(C/kT)], and the field dependence of the high-field, low-temperature resistivity is given by ρH = ρ∞ exp (εo/ε), where ρo, ρ∞, C, and εo are material constants. A simple theory based on the assumption that the ratio s/d (d-metal particle size and s-separation between particles) is a function only of composition yields expressions for ρ(ε, T) in excellent agreement with experiment. Furthermore, the theory predicts the experimental finding that the resistivity can be expressed in terms of a universal function of the reduced variables kT/C and ε/εo. The inter-relationship between all the important physical properties of granular metals and their structure is also discussed.     

Nonmonotonicity in the quantum-classical transition: chaos induced by quantum effects

The classical-quantum transition for chaotic systems is understood to be accompanied by the suppression of chaotic effects as the relative variant Planck's over 2pi is increased. We show evidence to the contrary in the behavior of the quantum trajectory dynamics of a dissipative quantum chaotic system, the double-well Duffing oscillator. The classical limit in the case considered has regular behavior, but as the effective variant Planck's over 2pi is increased we see chaotic behavior. This chaos then disappears deeper into the quantum regime, which means that the quantum-classical transition in this case is nonmonotonic in variant Planck's over 2pi.     

Temperature and wavelength dependent refractive index equations for MgO-doped congruent and stoichiometric LiNbO<Subscript>3</Subscript>

We present wavelength- and temperature-dependent refractive index equations for 5% MgO-doped congruent PPLN and for 1% MgO-doped stoichiometric PPLN crystals valid for a wide spectral and temperature range. The dispersion equations were derived from quasi-phase-matched nonlinear interactions with these two crystal compositions in the near and mid-infrared. The results show a good agreement with previously published frequency conversion experiments. PACS 42.65.Ky; 42.65.Yj; 42.70.Mp     

A Voltage-Responsive Synthetic Cl−-Channel Regulated by pH

Transmembrane protein channels are an important inspiration for the design of artificial ion channels. Their dipolar structure helps overcome the high energy barrier to selectively translocate water and ions sharing one pathway, across the cell membrane. Herein, we report that the amino-imidazole (Imu) amphiphiles self-assemble via multiple H-bonding to form stable artificial Cl⁻-channels within lipid bilayers. The alignment of water/Cl⁻ wires influences the conduction of ions, envisioned to diffuse along the hydrophilic pathways; at acidic pH, Cl⁻/H⁺ symport conducts along a partly protonated channel, while at basic pH, higher Cl⁻/OH⁻ antiport translocate through a neutral channel configuration, which can be greatly activated by applying strong electric field. This voltage/pH regulated channel system represents an unexplored alternative for ion-pumping along artificial ion-channels, parallel to that of biology.     

Structure‐Driven Selection of Adaptive Transmembrane Na+ Carriers or K+ Channels

Self‐assembled alkyl‐ureido‐benzo‐15‐crown‐5‐ethers are selective ionophores for K⁺ cations, which are preferred to Na⁺ cations. The transport mechanism is determined by the optimal coordination rather than classical dimensional compatibility between the crown ether hole and the cation diameter. Herein, we demonstrate that systematic changes of the structure lead to unexpected modifications in the cation‐transport activity and suffice to produce adaptive selection. We show that the main contribution to performance arises from optimal constraints on the conformational freedom, which are determined by the binding macrocycles, the nature of the hydrogen‐bonding groups, and the hydrophobic tails. Simple changes to the flexible 15‐crown‐5‐ether lead to selective carriers for Na⁺. Hydrophobic stabilization of the channels through mutual interactions between lipids and variable hydrophobic tails appears to be an important cause of increased activity. Oppositely, restricted translocation is achieved when constrained hydrogen‐bonded macrocyclic relays are less dynamic in a pore superstructure.     

Highly sensitive dynamic strain measurements by locking lasers to fiber Bragg gratings

A novel, sensitive, simple, and robust strain interrogation technique is analyzed and experimentally tested. By locking a laser wavelength to the midreflection wavelength of a standard fiber Bragg grating and measuring the error signal, we achieve high dynamic strain sensitivity of 45 picostrain/ radicalHz rms at 3 kHz, where the dominant noise in the experiment is the laser frequency noise.     

Internal structure of a thin film of mixed polymeric micelles on a solid/liquid interface

The adsorption of mixed micelles of poly(4-(2-amino hydrochloride-ethylthio)-butylene)- block-poly(ethylene oxide), PAETB 49- b-PEO 212 and poly(4-(2-sodium carboxylate-ethylthio)-butylene)- block-poly(ethylene oxide), PCETB 47- b-PEO 212 on solid/liquid interfaces has been studied with light, X-ray, and neutron reflectometry. The structure of the adsorbed layer can be described with a two-layer model consisting of an inner layer formed by the coacervate of the polyelectrolyte blocks PAETB 49 and PCETB 47 ( approximately 1 nm) and an outer layer of PEO 212 blocks ( approximately 6 nm). The micelles unfold upon adsorption forming a rather homogeneous flat layer that exposes its polyethylene oxide chains into the solution, thus rendering the surface antifouling after modification with the micelles.