Universal conductance fluctuations in three dimensional metallic single crystals of Si

In this paper we report the measurement of conductance fluctuations in 3D crystals of Si made metallic by heavy doping. ( L/L(straight phi) approximately 10(3), where L(straight phi) is the phase coherence length.) Temperature and magnetic field dependence of noise strongly indicate the universal conductance fluctuations as a predominant source of the observed magnitude of noise. Conductance fluctuations within a single phase coherent region of L(3)(straight phi) were found to be saturated at <(deltaG(straight phi))(2)> approximately (e(2)/h)(2). An accurate knowledge of the level of disorder enables us to calculate the change in conductance deltaG1 due to movement of a single scatterer as <(deltaG1)(2)> approximately (e(2)/h)(2), which is approximately 2 orders of magnitude higher than its theoretically expected value in 3D systems.     

Epitope Targeting of Tertiary Protein Structure Enables Target‐Guided Synthesis of a Potent In‐Cell Inhibitor of Botulinum Neurotoxin

Botulinum neurotoxin (BoNT) serotype A is the most lethal known toxin and has an occluded structure, which prevents direct inhibition of its active site before it enters the cytosol. Target‐guided synthesis by in situ click chemistry is combined with synthetic epitope targeting to exploit the tertiary structure of the BoNT protein as a landscape for assembling a competitive inhibitor. A substrate‐mimicking peptide macrocycle is used as a direct inhibitor of BoNT. An epitope‐targeting in situ click screen is utilized to identify a second peptide macrocycle ligand that binds to an epitope that, in the folded BoNT structure, is active‐site‐adjacent. A second in situ click screen identifies a molecular bridge between the two macrocycles. The resulting divalent inhibitor exhibits an in vitro inhibition constant of 165 pM against the BoNT/A catalytic chain. The inhibitor is carried into cells by the intact holotoxin, and demonstrates protection and rescue of BoNT intoxication in a human neuron model.     

Pyridine/Oxadiazole‐Based Helical Foldamer Ion Channels with Exceptionally High K+/Na+ Selectivity

Protein channels are characterized by high transport selectivity, which is essential for maintaining cellular function. Efforts to reproduce such high selectivity over the past four decades have not been very successful. We report a novel series of aromatic foldamer‐based polymeric channels where the backbone is stabilized by differential electrostatic repulsions among heteroatoms helically arrayed along the helical backbone. Nanotubes averaging 2.3 and 2.7 nm in length mediate highly efficient transport of K⁺ ions as a consequence of hydrophilic electron‐rich hollow cavities that are 3 Å in diameter. Exceptionally high K⁺ and Na⁺ selectivity values of 16.3 and 12.6, respectively, are achieved.     

A layer framework to investigate student understanding and application of the existence and uniqueness theorems of differential equations

The focus of this paper is on student interpretation and usage of the existence and uniqueness theorems for first-order ordinary differential equations. The inherent structure of the theorems is made explicit by the introduction of a framework of layers concepts–conditions–connectives–conclusions, and we discuss the manners in which students’ understanding of these layers construct their perception of the theorems as a whole.     

Preferential ordering of self-assembled Ge islands on focused ion-beam patterned Si(100)

We report the growth of Ge islands on Si (001) substrates with lithographically defined two-dimensionally periodic pits using focused ion-beam patterning and molecular beam epitaxy. The formation of circularly ordered Ge islands has been achieved by means of nonuniform strain field around the periphery of the holes due to ion bombardment. Lateral ordering of the Ge islands have been controlled by both the pit size and pit separation. Preferential growth at the pit sites has also been achieved by using appropriate pattern shape and size.     

Fabrication and magnetic response probed by RF transverse susceptibility in La0.67Ca0.33MnO3 nanowires

The temperature and magnetic field dependence of the radio-frequency (RF) transverse susceptibility (χ_T) of La_0.67Ca_0.33MnO₃ crystalline nanowires has been studied using a very sensitive self-resonant tunnel-diode oscillator (TDO) technique. The nanowires were synthesized using porous templates of anodized alumina by chemical solution deposition technique, and the crystalline nature of the nanowires with the average diameter of 70 nm was confirmed by TEM, SAED, and HREM. RF transverse susceptibility experiments reveal the presence of a double-peak structure at T≤245 K (the Curie temperature) but a single peak at T>245 K. This distinguishes the low temperature ferromagnetic state from the high temperature paramagnetic state. The effective magnetic anisotropy field (H_K), which corresponds to the peak location of χ_T, has been found to increase with decrease in temperature from the Curie temperature.     

Site-selective imination of an anthracenone sensor: selective fluorescence detection of barium(II)

Site-selective imination of anthraquinone-based macrocyclic crown ethers using titanium tetrachloride as the catalyst yields imines where only the external carbonyl group of the anthraquinone forms Schiff-bases. The following aromatic amines yield monomeric compounds (aniline, 4-nitroaniline, 4-pyrrolaniline, and 1,3-phenylenediamine). Reaction of 2 equiv of the macrocyclic anthraquinone host with 1,2- and 1,4-phenylenediamine yields dimeric imine compounds. The 1,2-diimino host acts as a luminescence sensor, exhibiting enhanced selectivity for Ba(II) ion. Spectroscopic data indicate that two barium ions coordinate to the sensor. Due to E/Z isomerization of the imine, the monomeric complexes are nonluminescent. Restricted rotation about the 1,2 oriented C═N groups or other noncovalent/coordinate-covalent interactions acting between neighboring crown ether rings may inhibit E/Z isomerization in this example, which is different from current examples that employ coordination of a metal cation with a chelating imine nitrogen atom to suppress E/Z isomerization and activate luminescence. The 1,4-diimino adduct, where the crown rings remain widely separated, remains nonluminescent.