Quantum instability in a dc-SQUID with strongly asymmetric dynamical parameters

A classical system cannot escape out of a metastable state at zero temperature. However, a composite system made from both classical and quantum degrees of freedom may drag itself out of the metastable state by a sequential process. The sequence starts with the tunneling of the quantum component which then triggers a distortion of the trapping potential holding the classical part. Provided this distortion is large enough to turn the metastable state into an unstable one, the classical component can escape. This process reminds of the famous baron Muenchhausen who told the story of rescuing himself from sinking in a swamp by pulling himself up by his own hair--we thus term this decay the `Muenchhausen effect'. We show that such a composite system can be conveniently studied and implemented in a dc-SQUID featuring asymmetric dynamical parameters. We determine the dynamical phase diagram of this system for various choices of junction parameters and system preparations.Comment: 12 pages, 12 figure

Time-Resolved Coherent Photoelectron Spectroscopy of Quantized Electronic States on Metal Surfaces

Time-resolved two-photon photoemission in combination with the coherent excitation of several quantum states was used to study the ultrafast electron dynamics of imagepotential states on metal surfaces. For a (100) surface of copper, the spectroscopy of quantum beats made previously unresolved high-order states (quantum number n Ն 4) experimentally accessible. By exciting electrons close to the vacuum level, electron wave packets could be created and detected that described the quasi-classical periodic motion of weakly bound electrons. They traveled more than 200 Å away from the surface and oscillated back and forth with a period of 800 femtoseconds. Photoelectron spectroscopy has developed into one of the most versatile and successful tools for surface studies. Particularly attractive features of this technique are the high surface sensitivity associated with the low escape depth of the photoelectrons and the capability of angle-resolved photoemission to completely characterize electronic states in energy and momentum space (1). Recently, these features have been combined with ultrafast laser excitation for direct time-domain investigations of electron dynamics at surfaces (2). Here, we demonstrate another facet of this powerful technique, the investigation of coherence phenomena in real time. In contrast to experimental methods that rely merely on intensities, coherent spectroscopies offer the unique capability of accessing not only the amplitudes but also the phases of the wave functions of interest (3). This technique dramatically increases the amount of information that one is able to obtain about the temporal evolution of fast processes. In this report, we discuss the dynamics of image-potential states, that is, the quantized excited states of electrons that exist in front of many metal surfaces (4, 5). Using femtosecond time-resolved two-photon photoemission (2PPE), we observed the interference between the wave functions of neighboring eigenstates and the quasi-classical motion of electron wave packets created by the coherent superposition of several quantum states. Recently, the imaging of the static charge density of related surface electronic (ground) states in real space with the scanning tunneling microscope has attracted considerable interest (6); the present results reveal the dynamical evolution of excited electrons in real time. Image-potential states are conceptually rather simple. An electron at a distance z in front of a conducting metal surface experiences an attractive force F(z) ϭ Ϫe 2 /(2z) 2 identical to that produced by a positive (mirror image) charge at a distance z inside the metal converging toward the vacuum energy, where the influence of the surface potential on the binding energy E B ϭ ϪE n is approximated by a quantum defect 0 Յ a Յ 0.5. Experimentally, image-potential states have been studied with 2PPE on many metal surfaces including surfaces covered with adsorbates and metallic overlayers (5, 7-11). One photon with energy ប a (ប is Planck's constant h divided by 2 and is the photon frequency times 2) excites an electron out of an occupied state below the Fermi energy E F into the image-potential state n. A second photon with energy ប b excites the electron to an energy above E vac The experimental setup consisted of a 80-MHz Ti:sapphire laser system that generated infrared (IR) pulses of 70-fs duration. Frequency-tripled 95-fs ultraviolet (UV) pulses from this laser were used for the excitation step (ប a ϭ 4.7 eV). The photoelectrons were emitted by the fundamental IR pulses (ប b ϭ 1.57 eV) and were detected in a hemispherical analyzer with an energy resolution of 30 meV and an angular acceptance of Ϯ0.6°about the surface normal. The preparation of the Cu(111) and Cu(100) samples and details of the ultrahigh-vacuum chamber have been described elsewhere (5). The samples were kept at room temperature. Typical energy-resolved 2PPE spectra of C

Condensation of zinc vapor in a supersonic carrier gas

A study of the condensation of a metal vapor in an inert carrier gas is made. Superheated zinc vapor is generated in a hot shot wind tunnel in a helium carrier gas and expanded in a converging-diverging nozzle. Static pressure measurements along the length of the nozzle are made to determine the location of the onset of condensation. A conical nominal Mach 5 (helium) nozzle is employed. The amount of supercooling before the onset of condensation is found to be approximately 430 K, measured along the isentrope over a range of initial zinc mass fractions of .35 to .70 for saturation partial pressures of zinc between 10 psia and 70 psia. The measurements are compared with results of an analysis based on the classical liquid drop model of nucleation. The computed results agree reasonably well with the measurements.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/41794/1/10494_2004_Article_BF00382286.pd

VLPs and particle strategies for cancer vaccines

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Transfer RNA-dependent cognate amino acid recognition by an aminoacyl-tRNA synthetase.

An investigation of the role of tRNA in the catalysis of aminoacylation of Escherichia coli glutaminyl-tRNA synthetase (GlnRS) has revealed that the accuracy of specific interactions between GlnRS and tRNAGln determines amino acid affinity. Mutations in GlnRS at D235, which makes contacts with nucleotides in the acceptor stem of tRNAGln, and at R260 in the enzyme's active site were found to be independent during tRNA binding but interactive for aminoacylation. Characterization of mutants of GlnRS at position 235, showed amino acid recognition to be tRNA mediated. Aminoacylation of tRNA(CUA)Tyr [tyrT (UAG)] by GlnRS-D235H resulted in a 4-fold increase in the Km for the Gln, which was reduced to a 2-fold increase when A73 was replaced with G73. These and previous results suggest that specific interactions between GlnRS and tRNAGln ensure the accurate positioning of the 3' terminus. Disruption of these interactions can change the Km for Gln over a 30-fold range, indicating that the accuracy of aminoacylation is regulated by tRNA at the level of both substrate recognition and catalysis. The observed role of RNA as a cofactor in optimizing amino acid activation suggests that the tRNAGln-GlnRS complex may be partly analogous to ribonucleoprotein enzymes where protein-RNA interactions facilitate catalysis