1,022 research outputs found
Boundary Integral Method for Stationary States of Two-Dimensional Quantum Systems
The boundary integral method for calculating the stationary states of a
quantum particle in nano-devices and quantum billiards is presented in detail
at an elementary level. According to the method, wave functions inside the
domain of the device or billiard are expressed in terms of line integrals of
the wave function and its normal derivative along the domain's boundary; the
respective energy eigenvalues are obtained as the roots of Fredholm
determinants. Numerical implementations of the method are described and applied
to determine the energy level statistics of billiards with circular and stadium
shapes and demonstrate the quantum mechanical characteristics of chaotic
motion. The treatment of other examples as well as the advantages and
limitations of the boundary integral method are discussed.Comment: RevTeX3.0, 24 pages, 9 EPS figures (included); To be published in
Int. J. of Mod. Phys.
3-dimensional Gravity from the Turaev-Viro Invariant
We study the -deformed su(2) spin network as a 3-dimensional quantum
gravity model. We show that in the semiclassical continuum limit the
Turaev-Viro invariant obtained recently defines naturally regularized
path-integral la Ponzano-Regge, In which a contribution from
the cosmological term is effectively included. The regularization dependent
cosmological constant is found to be , where
. We also discuss the relation to the Euclidean Chern-Simons-Witten
gravity in 3-dimension.Comment: 11page
Quantum Zeno Effect Explains Magnetic-Sensitive Radical-Ion-Pair Reactions
Chemical reactions involving radical-ion pairs are ubiquitous in biology,
since not only are they at the basis of the photosynthetic reaction chain, but
are also assumed to underlie the biochemical magnetic compass used by avian
species for navigation. Recent experiments with magnetic-sensitive radical-ion
pair reactions provided strong evidence for the radical-ion-pair
magnetoreception mechanism, verifying the expected magnetic sensitivities and
chemical product yield changes. It is here shown that the theoretical
description of radical-ion-pair reactions used since the 70's cannot explain
the observed data, because it is based on phenomenological equations masking
quantum coherence effects. The fundamental density matrix equation derived here
from basic quantum measurement theory considerations naturally incorporates the
quantum Zeno effect and readily explains recent experimental observations on
low- and high-magnetic-field radical-ion-pair reactions.Comment: 10 pages, 5 figure
Fluctuation-Driven Molecular Transport in an Asymmetric Membrane Channel
Channel proteins, that selectively conduct molecules across cell membranes,
often exhibit an asymmetric structure. By means of a stochastic model, we argue
that channel asymmetry in the presence of non-equilibrium fluctuations, fueled
by the cell's metabolism as observed recently, can dramatically influence the
transport through such channels by a ratchet-like mechanism. For an
aquaglyceroporin that conducts water and glycerol we show that a previously
determined asymmetric glycerol potential leads to enhanced inward transport of
glycerol, but for unfavorably high glycerol concentrations also to enhanced
outward transport that protects a cell against poisoning.Comment: REVTeX4, 4 pages, 3 figures; Accepted for publication in Phys. Rev.
Let
Binding pathway of retinal to bacterio-opsin: a prediction by molecular dynamics simulations
Formation of bacteriorhodopsin (bR) from apoprotein and retinal has been studied experimentally, but the actual pathway, including the point of entry, is little understood. Molecular dynamics simulations provide a surprisingly clear prediction. A window between bR helices E and F in the transmembrane part of the protein can be identified as an entry point for retinal. Steered molecular dynamics, performed by applying a series of external forces in the range of 200â1000 pN over a period of 0.2 ns to retinal, allows one to extract this chromophore from bR once the Schiff base bond to Lys216 is cleaved. Extraction proceeds until the retinal tail forms a hydrogen bond network with Ala144, Met145, and Ser183 side groups lining the exit/entry window. The manipulation induces a distortion with a fitted root mean square deviation of coordinates (ignoring retinal, water, and hydrogen atoms) of less than 1.9 A by the time the retinal carbonyl reaches the protein surface. The forces needed to extract retinal are due to friction and do not indicate significant potential barriers. The simulations therefore suggest a pathway for the binding of retinal. Water molecules are found to play a crucial role in the binding process
Structure and dynamics of the E. coli chemotaxis core signaling complex by cryo-electron tomography and molecular simulations
To enable the processing of chemical gradients, chemotactic bacteria possess large arrays of transmembrane chemoreceptors, the histidine kinase CheA, and the adaptor protein CheW, organized as coupled core-signaling units (CSU). Despite decades of study, important questions surrounding the molecular mechanisms of sensory signal transduction remain unresolved, owing especially to the lack of a high-resolution CSU structure. Here, we use cryo-electron tomography and sub-tomogram averaging to determine a structure of the Escherichia coli CSU at sub-nanometer resolution. Based on our experimental data, we use molecular simulations to construct an atomistic model of the CSU, enabling a detailed characterization of CheA conformational dynamics in its native structural context. We identify multiple, distinct conformations of the critical P4 domain as well as asymmetries in the localization of the P3 bundle, offering several novel insights into the CheA signaling mechanism
Hyperfine interaction and magnetoresistance in organic semiconductors
We explore the possibility that hyperfine interaction causes the recently
discovered organic magnetoresistance (OMAR) effect. Our study employs both
experiment and theoretical modelling. An excitonic pair mechanism model based
on hyperfine interaction, previously suggested by others to explain magnetic
field effects in organics, is examined. Whereas this model can explain a few
key aspects of the experimental data, we, however, uncover several fundamental
contradictions as well. By varying the injection efficiency for minority
carriers in the devices, we show experimentally that OMAR is only weakly
dependent on the ratio between excitons formed and carriers injected, likely
excluding any excitonic effect as the origin of OMAR.Comment: 10 pages, 7 figures, 1 tabl
Dynamics and Efficiency of Brownian Rotors
Brownian rotors play an important role in biological systems and in future
nano-technological applications. However the mechanisms determining their
dynamics, efficiency and performance remain to be characterized. Here the F0
portion of the F-ATP synthase is considered as a paradigm of a Brownian rotor.
In a generic analytical model we analyze the stochastic rotation of F0-like
motors as a function of the driving free energy difference and of the free
energy profile the rotor is subjected to. The latter is composed of the rotor
interaction with its surroundings, of the free energy of chemical transitions,
and of the workload. The dynamics and mechanical efficiency of the rotor
depends on the magnitude of its stochastic motion driven by the free energy
energy difference and its rectification on the reaction-diffusion path. We
analyze which free energy profiles provide maximum flow and how their
arrangement on the underlying reaction-diffusion path affects rectification and
-- by this -- the efficiency.Comment: 22 pages, 11 figures, pdflatex, JCP in pres
The limit of N=(2,2) superconformal minimal models
The limit of families of two-dimensional conformal field theories has
recently attracted attention in the context of AdS/CFT dualities. In our work
we analyse the limit of N=(2,2) superconformal minimal models when the central
charge approaches c=3. The limiting theory is a non-rational N=(2,2)
superconformal theory, in which there is a continuum of chiral primary fields.
We determine the spectrum of the theory, the three-point functions on the
sphere, and the disc one-point functions.Comment: 37 pages, 3 figures; v2: minor corrections in section 5.3, version to
be published in JHE
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