16,194 research outputs found
Lepton masses and mixings in orbifold models with three Higgs families
We analyse the phenomenological viability of heterotic Z(3) orbifolds with
two Wilson lines, which naturally predict three supersymmetric families of
matter and Higgs fields. Given that these models can accommodate realistic
scenarios for the quark sector avoiding potentially dangerous flavour-changing
neutral currents, we now address the leptonic sector, finding that viable
orbifold configurations can in principle be obtained. In particular,it is
possible to accomodate present data on charged lepton masses, while avoiding
conflict with lepton flavour-violating decays. Concerning the generation of
neutrino masses and mixings, we find that Z(3) orbifolds offer several
interesting possibilities.Comment: 28 pages, 11 figures. References adde
The non-linear q-voter model
We introduce a non-linear variant of the voter model, the q-voter model, in
which q neighbors (with possible repetition) are consulted for a voter to
change opinion. If the q neighbors agree, the voter takes their opinion; if
they do not have an unanimous opinion, still a voter can flip its state with
probability . We solve the model on a fully connected network (i.e.
in mean-field) and compute the exit probability as well as the average time to
reach consensus. We analyze the results in the perspective of a recently
proposed Langevin equation aimed at describing generic phase transitions in
systems with two ( symmetric) absorbing states. We find that in mean-field
the q-voter model exhibits a disordered phase for high and an
ordered one for low with three possible ways to go from one to the
other: (i) a unique (generalized voter-like) transition, (ii) a series of two
consecutive Ising-like and directed percolation transition, and (iii) a series
of two transitions, including an intermediate regime in which the final state
depends on initial conditions. This third (so far unexplored) scenario, in
which a new type of ordering dynamics emerges, is rationalized and found to be
specific of mean-field, i.e. fluctuations are explicitly shown to wash it out
in spatially extended systems.Comment: 9 pages, 7 figure
Dynamical phase coexistence: A simple solution to the "savanna problem"
We introduce the concept of 'dynamical phase coexistence' to provide a simple
solution for a long-standing problem in theoretical ecology, the so-called
"savanna problem". The challenge is to understand why in savanna ecosystems
trees and grasses coexist in a robust way with large spatio-temporal
variability. We propose a simple model, a variant of the Contact Process (CP),
which includes two key extra features: varying external
(environmental/rainfall) conditions and tree age. The system fluctuates locally
between a woodland and a grassland phase, corresponding to the active and
absorbing phases of the underlying pure contact process. This leads to a highly
variable stable phase characterized by patches of the woodland and grassland
phases coexisting dynamically. We show that the mean time to tree extinction
under this model increases as a power-law of system size and can be of the
order of 10,000,000 years in even moderately sized savannas. Finally, we
demonstrate that while local interactions among trees may influence tree
spatial distribution and the order of the transition between woodland and
grassland phases, they do not affect dynamical coexistence. We expect dynamical
coexistence to be relevant in other contexts in physics, biology or the social
sciences.Comment: 8 pages, 7 figures. Accepted for publication in Journal of
Theoretical Biolog
Effect of spin-orbit interaction on a magnetic impurity in the vicinity of a surface
We propose a new mechanism for surface-induced magnetic anisotropy to explain
the thickness-dependence of the Kondo resistivity of thin films of dilute
magnetic alloys. The surface anisotropy energy, generated by spin-orbit
coupling on the magnetic impurity itself, is an oscillating function of the
distance d from the surface and decays as 1/d^2. Numerical estimates based on
simple models suggest that this mechanism, unlike its alternatives, gives rise
to an effect of the desired order of magnitude.Comment: 4 pages, 4 figure
Unfolding Rates for the Diffusion-Collision Model
In the diffusion-collision model, the unfolding rates are given by the
likelihood of secondary structural cluster dissociation. In this work, we
introduce an unfolding rate calculation for proteins whose secondary structural
elements are -helices, modeled from thermal escape over a barrier which
arises from the free energy in buried hydrophobic residues. Our results are in
good agreement with currently accepted values for the attempt rate.Comment: Shorter version of cond-mat/0011024 accepted for publication in PR
- …