Imaging coherent electron flow from a quantum point contact

Scanning a charged tip above the two-dimensional electron gas inside a gallium arsenide/aluminum gallium arsenide nanostructure allows the coherent electron flow from the lowest quantized modes of a quantum point contact at liquid helium temperatures to be imaged. As the width of the quantum point contact is increased, its electrical conductance increases in quantized steps of 2 e(2)/h, where e is the electron charge and h is Planck's constant. The angular dependence of the electron flow on each step agrees with theory, and fringes separated by half the electron wavelength are observed. Placing the tip so that it interrupts the flow from particular modes of the quantum point contact causes a reduction in the conductance of those particular conduction channels below 2 e(2)/h without affecting other channels.     

Measuring the spin polarization of a metal with a superconducting point contact

A superconducting point contact is used to determine the spin polarization at the Fermi energy of several metals. Because the process of supercurrent conversion at a superconductor-metal interface (Andreev reflection) is limited by the minority spin population near the Fermi surface, the differential conductance of the point contact can reveal the spin polarization of the metal. This technique has been applied to a variety of metals where the spin polarization ranges from 35 to 90 percent: Ni0.8Fe0.2, Ni, Co, Fe, NiMnSb, La0.7Sr0.3MnO3, and CrO2.     

基于品牌接触点视角的景区情感体验营销策略研究

新媒体时代背景下,旅游产品消费者的消费习惯和审美情趣已逐渐由物质实用性转向了精神享受性。景区经营者应从旅游产品的虚拟景象阶段、实质接触阶段和连续消费阶段着手,分别针对消费者的接触点进行情感体验设计,进而影响旅游产品消费者的购买行为。     

Designing small-molecule switches for protein-protein interactions

Mutations introduced into human growth hormone (hGH) (Thr175 --> Gly-hGH) and the extracellular domain of the hGH receptor (Trp104 --> Gly-hGHbp) created a cavity at the protein-protein interface that resulted in binding affinity being reduced by a factor of 10(6). A small library of indole analogs was screened for small molecules that bind the cavity created by the mutations and restore binding affinity. The ligand 5-chloro-2-trichloromethylimidazole was found to increase the affinity of the mutant hormone for its receptor more than 1000-fold. Cell proliferation and JAK2 phosphorylation assays showed that the mutant hGH activates growth hormone signaling in the presence of added ligand. This approach may allow other protein-protein and protein-nucleic acid interactions to be switched on or off by the addition or depletion of exogenous small molecules.     

Juvenile hormone regulates butterfly larval pattern switches

Insect color patterns can be very diverse. This variation is also seen among many larval instar stages, which can take on vastly different phenotypes. Young caterpillars of the swallowtail butterfly, Papilio xuthus, are mimics of bird droppings, whereas the fifth larval instar is camouflaged among the leaves of host plants (cryptic pattern). We find that juvenile hormone (JH) titers decrease during the fourth larval instar. Furthermore, treatment with JH analog at the beginning of the fourth instar stage resulted in reproducing the mimetic pattern instead of the usual cryptic one and likewise altered gene expression patterns to that associated with the mimetic pattern. These findings suggest that JH regulates the progressive larval pattern switch of this insect.     

Hydrogen sensors and switches from electrodeposited palladium mesowire arrays

Hydrogen sensors and hydrogen-activated switches were fabricated from arrays of mesoscopic palladium wires. These palladium "mesowire" arrays were prepared by electrodeposition onto graphite surfaces and were transferred onto a cyanoacrylate film. Exposure to hydrogen gas caused a rapid (less than 75 milliseconds) reversible decrease in the resistance of the array that correlated with the hydrogen concentration over a range from 2 to 10%. The sensor response appears to involve the closing of nanoscopic gaps or "break junctions" in wires caused by the dilation of palladium grains undergoing hydrogen absorption. Wire arrays in which all wires possessed nanoscopic gaps reverted to open circuits in the absence of hydrogen gas.     

Widespread genetic switches and toxicity resistance proteins for fluoride

Most riboswitches are metabolite-binding RNA structures located in bacterial messenger RNAs where they control gene expression. We have discovered a riboswitch class in many bacterial and archaeal species whose members are selectively triggered by fluoride but reject other small anions, including chloride. These fluoride riboswitches activate expression of genes that encode putative fluoride transporters, enzymes that are known to be inhibited by fluoride, and additional proteins of unknown function. Our findings indicate that most organisms are naturally exposed to toxic levels of fluoride and that many species use fluoride-sensing RNAs to control the expression of proteins that alleviate the deleterious effects of this anion.     

Quantum gases

Ultracold quantum gases are proving to be a powerful model system for strongly interacting electronic many-body systems. This Perspective explores how such atomic ensembles can help to unravel some of the outstanding open questions in the field.     

Conformational switches modulate protein interactions in peptide antibiotic synthetases

Protein dynamics plays an important role in protein function. Many functionally important motions occur on the microsecond and low millisecond time scale and can be characterized by nuclear magnetic resonance relaxation experiments. We describe the different states of a peptidyl carrier protein (PCP) that play a crucial role in its function as a peptide shuttle in the nonribosomal peptide synthetases of the tyrocidine A system. Both apo-PCP (without the bound 4'-phosphopantetheine cofactor) and holo-PCP exist in two different stable conformations. We show that one of the apo conformations and one of the holo conformations are identical, whereas the two remaining conformations are only detectable by nuclear magnetic resonance spectroscopy in either the apo or holo form. We further demonstrate that this conformational diversity is an essential prerequisite for the directed movement of the 4'-PP cofactor and its interaction with externally acting proteins such as thioesterases and 4'-PP transferase.     

Quantum liquids

Quantum liquids are systems in which not only the effects of quantum mechanics but also those of the characteristic indistinguishability of elementary particles are important. The most spectacular of these are the systems of bosons (liquid 4He, the Bose alkali gases), which undergo the phenomenon of Bose condensation, and the fermion systems (liquid 3He, the electrons in some metals), which display the related phenomenon of Cooper pairing. I discuss these phenomena and the relation between them.