3,076 research outputs found

### A simple topological model with continuous phase transition

In the area of topological and geometric treatment of phase transitions and
symmetry breaking in Hamiltonian systems, in a recent paper some general
sufficient conditions for these phenomena in $\mathbb{Z}_2$-symmetric systems
(i.e. invariant under reflection of coordinates) have been found out. In this
paper we present a simple topological model satisfying the above conditions
hoping to enlighten the mechanism which causes this phenomenon in more general
physical models. The symmetry breaking is testified by a continuous
magnetization with a nonanalytic point in correspondence of a critical
temperature which divides the broken symmetry phase from the unbroken one. A
particularity with respect to the common pictures of a phase transition is that
the nonanalyticity of the magnetization is not accompanied by a nonanalytic
behavior of the free energy.Comment: 17 pages, 7 figure

### Noise-driven Synchronization in Coupled Map Lattices

Synchronization is shown to occur in spatially extended systems under the
effect of additive spatio-temporal noise. In analogy to low dimensional
systems, synchronized states are observable only if the maximum Lyapunov
exponent $\Lambda$ is negative. However, a sufficiently high noise level can
lead, in map with finite domain of definition, to nonlinear propagation of
information, even in non chaotic systems. In this latter case the transition to
synchronization is ruled by a new ingredient : the propagation velocity of
information $V_F$. As a general statement, we can affirm that if $V_F$ is
finite the time needed to achieve a synchronized trajectory grows exponentially
with the system size $L$, while it increases logarithmically with $L$ when, for
sufficiently large noise amplitude, $V_F = 0$ .Comment: 11 pages, Latex - 6 EPS Figs - Proceeding LSD 98 (Marseille

### Topological conditions for discrete symmetry breaking and phase transitions

In the framework of a recently proposed topological approach to phase
transitions, some sufficient conditions ensuring the presence of the
spontaneous breaking of a Z_2 symmetry and of a symmetry-breaking phase
transition are introduced and discussed. A very simple model, which we refer to
as the hypercubic model, is introduced and solved. The main purpose of this
model is that of illustrating the content of the sufficient conditions, but it
is interesting also in itself due to its simplicity. Then some mean-field
models already known in the literature are discussed in the light of the
sufficient conditions introduced here

### Tritium $\beta$-decay in chiral effective field theory

We evaluate the Fermi and Gamow-Teller (GT) matrix elements in tritium
$\beta$-decay by including in the charge-changing weak current the corrections
up to one loop recently derived in nuclear chiral effective field theory
($\chi$ EFT). The trinucleon wave functions are obtained from
hyperspherical-harmonics solutions of the Schrodinger equation with two- and
three-nucleon potentials corresponding to either $\chi$ EFT (the N3LO/N2LO
combination) or meson-exchange phenomenology (the AV18/UIX combination). We
find that contributions due to loop corrections in the axial current are, in
relative terms, as large as (and in some cases, dominate) those from one-pion
exchange, which nominally occur at lower order in the power counting. We also
provide values for the low-energy constants multiplying the contact axial
current and three-nucleon potential, required to reproduce the experimental GT
matrix element and trinucleon binding energies in the N3LO/N2LO and AV18/UIX
calculations.Comment: 19 pages,6 figures, corrections to Text as suggested by Referee
added; Erratum: 4 pages, 3 figures, corrections to Eq.(20), Tables I, II,
III, Figures 4, 5, conclusions unchange

### A chiral effective field theory study of hadronic parity violation in few-nucleon systems

We reconsider the derivation of the nucleon-nucleon parity-violating (PV)
potential within a chiral effective field theory framework. We construct the
potential up to next-to-next-to-leading order by including one-pion-exchange,
two-pion-exchange, contact, and 1/M (M being the nucleon mass) terms, and use
dimensional regularization to renormalize the pion-loop corrections. A detailed
analysis of the number of independent low-energy constants (LEC's) entering the
potential is carried out. We find that it depends on six LEC's: the
pion-nucleon PV coupling constant $h^1_\pi$ and five parameters multiplying
contact interactions. We investigate PV effects induced by this potential on
several few-nucleon observables, including the $\vec{p}$-$p$ longitudinal
asymmetry, the neutron spin rotation in $\vec{n}$-$p$ and $\vec{n}$-$d$
scattering, and the longitudinal asymmetry in the $^3$He$(\vec{n},p)^3$H
charge-exchange reaction. An estimate for the range of values of the various
LEC's is provided by using available experimental data.Comment: 31 pages, 7 figures, submitted to Physical Review

### Accurate quadratic-response approximation for the self-consistent pseudopotential of semiconductor nanostructures

Quadratic-response theory is shown to provide a conceptually simple but
accurate approximation for the self-consistent one-electron potential of
semiconductor nanostructures. Numerical examples are presented for GaAs/AlAs
and InGaAs/InP (001) superlattices using the local-density approximation to
density-functional theory and norm-conserving pseudopotentials without
spin-orbit coupling. When the reference crystal is chosen to be the
virtual-crystal average of the two bulk constituents, the absolute error in the
quadratic-response potential for Gamma(15) valence electrons is about 2 meV for
GaAs/AlAs and 5 meV for InGaAs/InP. Low-order multipole expansions of the
electron density and potential response are shown to be accurate throughout a
small neighborhood of each reciprocal lattice vector, thus providing a further
simplification that is confirmed to be valid for slowly varying envelope
functions. Although the linear response is about an order of magnitude larger
than the quadratic response, the quadratic terms are important both
quantitatively (if an accuracy of better than a few tens of meV is desired) and
qualitatively (due to their different symmetry and long-range dipole effects).Comment: 16 pages, 20 figures; v2: new section on limitations of theor

### GW quasi-particle spectra from occupied states only

We introduce a method that allows for the calculation of quasi-particle
spectra in the GW approximation, yet avoiding any explicit reference to empty
one-electron states. This is achieved by expressing the irreducible
polarizability operator and the self-energy operator through a set of linear
response equations, which are solved using a Lanczos-chain algorithm. We first
validate our approach by calculating the vertical ionization energies of the
benzene molecule and then show its potential by addressing the spectrum of a
large molecule such as free-base tetraphenylporphyrin.Comment: 4 pages, 3 figure

### Electron-vibration coupling constants in positively charged fullerene

Recent experiments have shown that C60 can be positively field-doped. In that
state, fullerene exhibits a higher resistivity and a higher superconducting
temperature than the corresponding negatively doped state. A strong
intramolecular hole-phonon coupling, connected with the Jahn-Teller effect of
the isolated positive ion, is expected to be important for both properties, but
the actual coupling strengths are so far unknown. Based on density functional
calculations, we determine the linear couplings of the two a_g, six g_g, and
eight h_g vibrational modes to the H_u HOMO level of the C60 molecule. The
couplings predict a D_5 distortion, and an H_u vibronic ground state for C60^+.
They are also used to generate the dimensionless coupling constant
which controls the superconductivity and the phonon contribution to the
electrical resistivity in the crystalline phase. We find that is 1.4
times larger in positively-charged C60 than in the negatively-doped case. These
results are discussed in the context of the available transport data and
superconducting temperatures. The role of higher orbital degeneracy in
superconductivity is also addressed.Comment: 22 pages - 3 figures. This revision includes few punctuation
corrections from proofreadin

### Energy saving for air supply in a real WWTP: application of a fuzzy logic controller.

Abstract
An unconventional cascade control system, for the regulation of air supply in activated sludge wastewater treatment plants (WWTPs), was tested. The dissolved oxygen (DO) set point in the aeration tank was dynamically calculated based on effluent ammonia concentration, following a fuzzy logic based approach. First, simulations were conducted, according to the BSM2 protocol, for a general comparison with more conventional control strategies. It turned out that the effluent quality could be improved by 7–8%, based on the EQI parameter. Moreover, the aeration energy requirement could be reduced up to 13%. Subsequently, the system was installed in a full-scale WWTP. While stably complying with the ammonia effluent standard (10 mg/L), excess air supply was prevented, and a reduction of the specific power consumption (kWh/kgCODremoved) of 40–50% was recorded with respect to the previously installed PID controller (fixed DO set point)

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