138,045 research outputs found
Continuous symmetry of C60 fullerene and its derivatives
Conventionally, the Ih symmetry of fullerene C60 is accepted which is
supported by numerous calculations. However, this conclusion results from the
consideration of the molecule electron system, of its odd electrons in
particular, in a close-shell approximation without taking the electron spin
into account. Passing to the open-shell approximation has lead to both the
energy and the symmetry lowering up to Ci. Seemingly contradicting to a
high-symmetry pattern of experimental recording, particularly concerning the
molecule electronic spectra, the finding is considered in the current paper
from the continuous symmetry viewpoint. Exploiting both continuous symmetry
measure and continuous symmetry content, was shown that formal Ci symmetry of
the molecule is by 99.99% Ih. A similar continuous symmetry analysis of the
fullerene monoderivatives gives a reasonable explanation of a large variety of
their optical spectra patterns within the framework of the same C1 formal
symmetry exhibiting a strong stability of the C60 skeleton.Comment: 11 pages. 5 figures. 6 table
Average Symmetry and Complexity of Binary Sequences
The concept of complexity as average symmetry is here formalised by
introducing a general expression dependent on the relevant symmetry and a
related discrete set of transformations. This complexity has hybrid features of
both statistical complexities and of those related to algorithmic complexity.
Like the former, random objects are not the most complex while they still are
more complex than the more symmetric ones (as in the latter). By applying this
definition to the particular case of rotations of binary sequences, we are able
to find a precise expression for it. In particular, we then analyse the
behaviour of this measure in different well-known automatic sequences, where we
find interesting new properties. A generalisation of the measure to statistical
ensembles is also presented and applied to the case of i.i.d. random sequences
and to the equilibrium configurations of the one-dimensional Ising model. In
both cases, we find that the complexity is continuous and differentiable as a
function of the relevant parameters and agrees with the intuitive requirements
we were looking for.Comment: 9 pages, 5 figure
Discovering the manifold facets of a square integrable representation: from coherent states to open systems
Group representations play a central role in theoretical physics. In
particular, in quantum mechanics unitary --- or, in general, projective unitary
--- representations implement the action of an abstract symmetry group on
physical states and observables. More specifically, a major role is played by
the so-called square integrable representations. Indeed, the properties of
these representations are fundamental in the definition of certain families of
generalized coherent states, in the phase-space formulation of quantum
mechanics and the associated star product formalism, in the definition of an
interesting notion of function of quantum positive type, and in some recent
applications to the theory of open quantum systems and to quantum information.Comment: 13 page
Isotropy of Angular Frequencies and Weak Chimeras With Broken Symmetry
The notion of a weak chimeras provides a tractable definition for chimera
states in networks of finitely many phase oscillators. Here we generalize the
definition of a weak chimera to a more general class of equivariant dynamical
systems by characterizing solutions in terms of the isotropy of their angular
frequency vector - for coupled phase oscillators the angular frequency vector
is given by the average of the vector field along a trajectory. Symmetries of
solutions automatically imply angular frequency synchronization. We show that
the presence of such symmetries is not necessary by giving a result for the
existence of weak chimeras without instantaneous or setwise symmetries for
coupled phase oscillators. Moreover, we construct a coupling function that
gives rise to chaotic weak chimeras without symmetry in weakly coupled
populations of phase oscillators with generalized coupling
Operators versus functions: from quantum dynamical semigroups to tomographic semigroups
Quantum mechanics can be formulated in terms of phase-space functions,
according to Wigner's approach. A generalization of this approach consists in
replacing the density operators of the standard formulation with suitable
functions, the so-called generalized Wigner functions or (group-covariant)
tomograms, obtained by means of group-theoretical methods. A typical problem
arising in this context is to express the evolution of a quantum system in
terms of tomograms. In the case of a (suitable) open quantum system, the
dynamics can be described by means of a quantum dynamical semigroup 'in
disguise', namely, by a semigroup of operators acting on tomograms rather than
on density operators. We focus on a special class of quantum dynamical
semigroups, the twirling semigroups, that have interesting applications, e.g.,
in quantum information science. The 'disguised counterparts' of the twirling
semigroups, i.e., the corresponding semigroups acting on tomograms, form a
class of semigroups of operators that we call tomographic semigroups. We show
that the twirling semigroups and the tomographic semigroups can be encompassed
in a unique theoretical framework, a class of semigroups of operators including
also the probability semigroups of classical probability theory, so achieving a
deeper insight into both the mathematical and the physical aspects of the
problem.Comment: 12 page
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