919 research outputs found
Enzyme activities in liver and muscle biopsy specimens from thyrotoxic and hypothyroid patiens
Streaming instability of slime mold amoebae: An analytical model
During the aggregation of amoebae of the cellular slime mould Dictyostelium, the interaction of chemical waves of the signaling molecule cAMP with cAMP-directed cell movement causes the breakup of a uniform cell layer into branching patterns of cell streams. Recent numerical and experimental investigations emphasize the pivotal role of the cell-density dependence of the chemical wave speed for the occurrence of the streaming instability. A simple, analytically tractable, model of Dictyostelium aggregation is developed to test this idea. The interaction of cAMP waves with cAMP-directed cell movement is studied in the form of coupled dynamics of wave front geometries and cell density. Comparing the resulting explicit instability criterion and dispersion relation for cell streaming with the previous findings of model simulations and numerical stability analyses, a unifying interpretation of the streaming instability as a cAMP wave-driven chemotactic instability is proposed
Observation of an orbital interaction-induced Feshbach resonance in 173-Yb
We report on the experimental observation of a novel inter-orbital Feshbach
resonance in ultracold 173-Yb atoms, which opens the possibility of tuning the
interactions between the 1S0 and 3P0 metastable state, both possessing
vanishing total electronic angular momentum. The resonance is observed at
experimentally accessible magnetic field strengths and occurs universally for
all hyperfine state combinations. We characterize the resonance in the bulk via
inter-orbital cross-thermalization as well as in a three-dimensional lattice
using high-resolution clock-line spectroscopy.Comment: 5 pages, 4 figure
Spectroscopy and dynamics of unoccupied electronic states of the topological insulators Sb2Te3 and Sb2Te2S
Time- and angle-resolved two-photon photoemission (2PPE) was used to study the electronic structure and ultrafast electron dynamics of the p-doped topological insulator Sb2Te3 and its derivative Sb2Te2S. Our 2PPE experiments directly reveal that the massless Dirac-cone like energy dispersion of topological surface states is realized above the Fermi energy in both materials. The observed bulk conduction bands of Sb2Te2S are found to be shifted to higher energies as compared to Sb2Te3. This shift has, however, surprisingly almost no influence on the electron dynamics in the topological surface state, which proceed on a picosecond time scale.We acknowledge funding by the Deutsche Forschungsgemeinschaft through SPP1666.Peer Reviewe
Quantum Monte Carlo calculations of H dissociation on Si(001)
We present quantum Monte Carlo calculations for various reaction pathways of
H with Si(001), using large model clusters of the surface. We obtain
reaction energies and energy barriers noticeably higher than those from
approximate exchange-correlation functionals. In improvement over previous
studies, our adsorption barriers closely agree with experimental data. For
desorption, the calculations give barriers for conventional pathways in excess
of the presently accepted experimental value, and pinpoint the role of coverage
effects and desorption from steps.Comment: 4 pages, 1 figur
Highly site-specific H2 adsorption on vicinal Si(001) surfaces
Experimental and theoretical results for the dissociative adsorption of H_2
on vicinal Si(001) surfaces are presented. Using optical second-harmonic
generation, sticking probabilities at the step sites are found to exceed those
on the terraces by up to six orders of magnitude. Density functional theory
calculations indicate the presence of direct adsorption pathways for
monohydride formation but with a dramatically lowered barrier for step
adsorption due to an efficient rehybridization of dangling orbitals.Comment: 5 pages, 4 figures, submitted to Phys. Rev. Lett. (1998). Other
related publications can be found at
http://www.fhi-berlin.mpg.de/th/paper.htm
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