5,472 research outputs found
A toy model of polymer stretching
We present an extremely simplified model of multiple-domains polymer
stretching in an atomic force microscopy experiment. We portray each module as
a binary set of contacts and decompose the system energy into a harmonic term
(the cantilever) and long-range interactions terms inside each domain. Exact
equilibrium computations and Monte Carlo simulations qualitatively reproduce
the experimental saw-tooth pattern of force-extension profiles, corresponding
(in our model) to first-order phase transitions. We study the influence of the
coupling induced by the cantilever and the pulling speed on the relative
heights of the force peaks. The results suggest that the increasing height of
the critical force for subsequent unfolding events is an out-of-equilibrium
effect due to a finite pulling speed. The dependence of the average unfolding
force on the pulling speed is shown to reproduce the experimental logarithmic
law.Comment: New revised versio
Gauge Invariance and Symmetry Breaking by Topology and Energy Gap
For the description of observables and states of a quantum system, it may be
convenient to use a canonical Weyl algebra of which only a subalgebra , with a non-trivial center , describes observables, the other
Weyl operators playing the role of intertwiners between inequivalent
representations of . In particular, this gives rise to a gauge
symmetry described by the action of . A distinguished case is when
the center of the observables arises from the fundamental group of the manifold
of the positions of the quantum system. Symmetries which do not commute with
the topological invariants represented by elements of are then
spontaneously broken in each irreducible representation of the observable
algebra, compatibly with an energy gap; such a breaking exhibits a mechanism
radically different from Goldstone and Higgs mechanisms. This is clearly
displayed by the quantum particle on a circle, the Bloch electron and the two
body problem.Comment: 23 page
Two-walker discrete-time quantum walks on the line with percolation
One goal in the quantum-walk research is the exploitation of the intrinsic
quantum nature of multiple walkers, in order to achieve the full computational
power of the model. Here we study the behaviour of two non-interacting
particles performing a quantum walk on the line when the possibility of lattice
imperfections, in the form of missing links, is considered. We investigate two
regimes, statical and dynamical percolation, that correspond to different time
scales for the imperfections evolution with respect to the quantum-walk one. By
studying the qualitative behaviour of three two-particle quantities for
different probabilities of having missing bonds, we argue that the chosen
symmetry under particle-exchange of the input state strongly affects the output
of the walk, even in noisy and highly non-ideal regimes. We provide evidence
against the possibility of gathering information about the walkers
indistinguishability from the observation of bunching phenomena in the output
distribution, in all those situations that require a comparison between
averaged quantities. Although the spread of the walk is not substantially
changed by the addition of a second particle, we show that the presence of
multiple walkers can be beneficial for a procedure to estimate the probability
of having a broken link.Comment: 16 pages, 9 figure
Localization-like effect in two-dimensional alternate quantum walks with periodic coin operations
Exploiting multi-dimensional quantum walks as feasible platforms for quantum
computation and quantum simulation is attracting constantly growing attention
from a broad experimental physics community. Here, we propose a two-dimensional
quantum walk scheme with a single-qubit coin that presents, in the considered
regimes, a strong localization-like effect on the walker. The result could
provide new possible directions for the implementation of quantum algorithms or
from the point of view of quantum simulation. We characterize the
localization-like effect in terms of the parameters of a step-dependent qubit
operation that acts on the coin space after any standard coin operation,
showing that a proper choice can guarantee a non-negligible probability of
finding the walker in the origin even for large times. We finally discuss the
robustness to imperfections, a qualitative relation with coherences behavior,
and possible experimental realizations of this model with the current
state-of-the-art settings.Comment: 5 pages, 4 figures, RevTeX
Stochastic gravitational background from inflationary phase transitions
We consider true vacuum bubbles generated in a first order phase transition
occurring during the slow rolling era of a two field inflation: it is known
that gravitational waves are produced by the collision of such bubbles. We find
that the epoch of the phase transition strongly affects the characteristic peak
frequency of the gravitational waves, causing an observationally interesting
redshift in addition to the post-inflationary expansion. In particular it is
found that a phase transition occurring typically 1020 foldings
before the reheating at GeV may be detected by the next Ligo
gravity waves interferometers. Moreover, for recently proposed models capable
of generating the observed large scale voids as remnants of the primordial
bubbles (for which the characteristic wave lengths are several tens of Mpc), it
is found that the level of anisotropy of the cosmic microwave background
provides a deep insight upon the physical parameters of the effective
Lagrangian.Comment: 12 pages, 3 figures. Phys.Rev.D in pres
Synthesis And Characterization Of Polyynes End-Capped By Biphenyl Groups ({\Alpha},{\Omega}-Biphenylpolyynes)
Stable polyyne chains terminated with biphenyl end groups
(a,u-biphenylpolyynes) were synthesized in a single step through a simple
procedure by using the Cadiot-Chodkiewicz reaction conditions. The
a,ubiphenylpolyynes were separated through HPLC analysis and identified by
means of their electronic absorption spectra. The a,u-biphenylpolyynes were
studied by FT-IR and Raman spectroscopy and the spectral interpretation was
supported with DFT calculations. A peculiarly low reactivity of
a,u-biphenylpolyynes with ozone was observed.Comment: The research leading to these results has received funding from the
European Research Council Consolidator Grant EspLORE (ERC-2016-CoG Grant
No.724610
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