2,760 research outputs found
Min-oscillations in Escherichia coli induced by interactions of membrane-bound proteins
During division it is of primary importance for a cell to correctly determine
the site of cleavage. The bacterium Escherichia coli divides in the center,
producing two daughter cells of equal size. Selection of the center as the
correct division site is in part achieved by the Min-proteins. They oscillate
between the two cell poles and thereby prevent division at these locations.
Here, a phenomenological description for these oscillations is presented, where
lateral interactions between proteins on the cell membrane play a key role.
Solutions to the dynamic equations are compared to experimental findings. In
particular, the temporal period of the oscillations is measured as a function
of the cell length and found to be compatible with the theoretical prediction.Comment: 17 pages, 5 figures. Submitted to Physical Biolog
Signatures of Majorana Kramers pairs in superconductor-Luttinger liquid and superconductor-quantum dot-normal lead junctions
Time-reversal invariant topological superconductors are characterized by the
presence of Majorana Kramers pairs localized at defects. One of the transport
signatures of Majorana Kramers pairs is the quantized differential conductance
of when such a one-dimensional superconductor is coupled to a
normal-metal lead. The resonant Andreev reflection, responsible for this
phenomenon, can be understood as the boundary condition change for lead
electrons at low energies. In this paper, we study the stability of the Andreev
reflection fixed point with respect to electron-electron interactions in the
Luttinger liquid. We first calculate the phase diagram for the Luttinger
liquid-Majorana Kramers pair junction and show that its low-energy properties
are determined by Andreev reflection scattering processes in the spin-triplet
channel, i.e. the corresponding Andreev boundary conditions are similar to that
in a spin-triplet superconductor - normal lead junction. We also study here a
quantum dot coupled to a normal lead and a Majorana Kramers pair and
investigate the effect of local repulsive interactions leading to an interplay
between Kondo and Majorana correlations. Using a combination of renormalization
group analysis and slave-boson mean-field theory, we show that the system flows
to a new fixed point which is controlled by the Majorana interaction rather
than the Kondo coupling. This Majorana fixed point is characterized by
correlations between the localized spin and the fermion parity of each spin
sector of the topological superconductor. We investigate the stability of the
Majorana phase with respect to Gaussian fluctuations.Comment: 26 pages, 8 figure
Constraints on the Existence of Chiral Fermions in Interacting Lattice Theories
It is shown that an interacting theory, defined on a regular lattice, must
have a vector-like spectrum if the following conditions are satisfied:
(a)~locality, (b)~relativistic continuum limit without massless bosons, and
(c)~pole-free effective vertex functions for conserved currents.
The proof exploits the zero frequency inverse retarded propagator of an
appropriate set of interpolating fields as an effective quadratic hamiltonian,
to which the Nielsen-Ninomiya theorem is applied.Comment: LaTeX, 9 pages, WIS--93/56--JUNE--P
Non-adiabatic Effects in the Dissociation of Oxygen Molecules at the Al(111) Surface
The measured low initial sticking probability of oxygen molecules at the
Al(111) surface that had puzzled the field for many years was recently
explained in a non-adiabatic picture invoking spin-selection rules [J. Behler
et al., Phys. Rev. Lett. 94, 036104 (2005)]. These selection rules tend to
conserve the initial spin-triplet character of the free O2 molecule during the
molecule's approach to the surface. A new locally-constrained
density-functional theory approach gave access to the corresponding
potential-energy surface (PES) seen by such an impinging spin-triplet molecule
and indicated barriers to dissociation which reduce the sticking probability.
Here, we further substantiate this non-adiabatic picture by providing a
detailed account of the employed approach. Building on the previous work, we
focus in particular on inaccuracies in present-day exchange-correlation
functionals. Our analysis shows that small quantitative differences in the
spin-triplet constrained PES obtained with different gradient-corrected
functionals have a noticeable effect on the lowest kinetic energy part of the
resulting sticking curve.Comment: 17 pages including 11 figures; related publications can be found at
http://www.fhi-berlin.mpg.de/th/th.htm
Big Bang Nucleosynthesis Constraints on Hadronically and Electromagnetically Decaying Relic Neutral Particles
Big Bang nucleosynthesis in the presence of decaying relic neutral particles
is examined in detail. All non-thermal processes important for the
determination of light-element abundance yields of 2H, 3H, 3He, 4He, 6Li, and
7Li are coupled to the thermonuclear fusion reactions to obtain comparatively
accurate results. Predicted light-element yields are compared to
observationally inferred limits on primordial light-element abundances to infer
constraints on the abundances and properties of relic decaying particles with
decay times in the interval 0.01 sec < tau < 10^(12) sec. Decaying particles
are typically constrained at early times by 4He or 2H, at intermediate times by
6Li, and at large times by the 3He/2H ratio. Constraints are shown for a large
number of hadronic branching ratios and decaying particle masses and may be
applied to constrain the evolution of the early Universe.Comment: 24 pages (revtex), 11 figures, title changed, matches published
versio
Constitutive modeling of indentation cracking in fused silica
Fused silica shows three distinct regimes during nanoindentation, that is, plastic deformation, inelastic densification, and cracking. Cohesive zone FEM is used to study these regimes for different indenter geometries. In a three-dimensional model, the median/radial cracking is considered by introducing cohesive element planes that are aligned along the indenter edges perpendicular to the indented surface. In addition to comparing indentation cracking data with experimental data, the role of densification on indentation crack growth is critically examined using a pressure independent von Mises and a pressure dependent Drucker-Prager Cap constitutive model. The results show that the Drucker-Prager Cap model delivers an accurate description of the elastic-plastic deformation conditions for all examined indenter geometries. Material densification leads to shorter crack lengths and thus the approach by Lawn, Evans and Marshall (LEM) [1] results in larger indentation-based fracture toughness values (Equation 1).
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Electron-hole pairs during the adsorption dynamics of O2 on Pd(100) - Exciting or not?
During the exothermic adsorption of molecules at solid surfaces dissipation
of the released energy occurs via the excitation of electronic and phononic
degrees of freedom. For metallic substrates the role of the nonadiabatic
electronic excitation channel has been controversially discussed, as the
absence of a band gap could favour an easy coupling to a manifold of
electronhole pairs of arbitrarily low energies. We analyse this situation for
the highly exothermic showcase system of molecular oxygen dissociating at
Pd(100), using time-dependent perturbation theory applied to first-principles
electronic-structure calculations. For a range of different trajectories of
impinging O2 molecules we compute largely varying electron-hole pair spectra,
which underlines the necessity to consider the high-dimensionality of the
surface dynamical process when assessing the total energy loss into this
dissipation channel. Despite the high Pd density of states at the Fermi level,
the concomitant non-adiabatic energy losses nevertheless never exceed about 5%
of the available chemisorption energy. While this supports an electronically
adiabatic description of the predominant heat dissipation into the phononic
system, we critically discuss the non-adiabatic excitations in the context of
the O2 spin transition during the dissociation process.Comment: 20 pages including 7 figures; related publications can be found at
http://www.fhi-berlin.mpg.de/th/th.html [added two references, changed
V_{fsa} to V_{6D}, modified a few formulations in interpretation of spin
asymmetry of eh-spectra, added missing equals sign in Eg.(2.10)
TransCom N2O model inter-comparison - Part 2:Atmospheric inversion estimates of N2O emissions
This study examines N2O emission estimates from five different atmospheric inversion frameworks based on chemistry transport models (CTMs). The five frameworks differ in the choice of CTM, meteorological data, prior uncertainties and inversion method but use the same prior emissions and observation data set. The posterior modelled atmospheric N2O mole fractions are compared to observations to assess the performance of the inversions and to help diagnose problems in the modelled transport. Additionally, the mean emissions for 2006 to 2008 are compared in terms of the spatial distribution and seasonality. Overall, there is a good agreement among the inversions for the mean global total emission, which ranges from 16.1 to 18.7 TgN yr(-1) and is consistent with previous estimates. Ocean emissions represent between 31 and 38% of the global total compared to widely varying previous estimates of 24 to 38%. Emissions from the northern mid- to high latitudes are likely to be more important, with a consistent shift in emissions from the tropics and subtropics to the mid- to high latitudes in the Northern Hemisphere; the emission ratio for 0-30A degrees N to 30-90A degrees N ranges from 1.5 to 1.9 compared with 2.9 to 3.0 in previous estimates. The largest discrepancies across inversions are seen for the regions of South and East Asia and for tropical and South America owing to the poor observational constraint for these areas and to considerable differences in the modelled transport, especially inter-hemispheric exchange rates and tropical convective mixing. Estimates of the seasonal cycle in N2O emissions are also sensitive to errors in modelled stratosphere-to-troposphere transport in the tropics and southern extratropics. Overall, the results show a convergence in the global and regional emissions compared to previous independent studies
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