866 research outputs found
Repulsion-Sustained Supercurrent and Flux Quantization in Rings of Symmetric Hubbard Clusters
We test the response to a threading magnetic field of rings of 5-site
-symmetric repulsive Hubbard clusters connected by weak intercell
links; each 5-site unit has the topology of a CuO cluster and a repulsive
interaction is included on every site. In a numerical study of the three-unit
ring with 8 particles, we take advantage of a novel exact-diagonalization
technique which can be generally applied to many-fermion problems. For O-O
hopping we find Superconducting Flux Quantization (SFQ), but for purely Cu-Cu
links bound pair propagation is hindered by symmetry. The results agree with
W=0 pairing theory.Comment: 4 pages, 2 figure
W=0 pairing in Hubbard and related models of low-dimensional superconductors
Lattice Hamiltonians with on-site interaction have W=0 solutions, that
is, many-body {\em singlet} eigenstates without double occupation. In
particular, W=0 pairs give a clue to understand the pairing force in repulsive
Hubbard models. These eigenstates are found in systems with high enough
symmetry, like the square, hexagonal or triangular lattices. By a general
theorem, we propose a systematic way to construct all the W=0 pairs of a given
Hamiltonian. We also introduce a canonical transformation to calculate the
effective interaction between the particles of such pairs. In geometries
appropriate for the CuO planes of cuprate superconductors, armchair
Carbon nanotubes or Cobalt Oxides planes, the dressed pair becomes a bound
state in a physically relevant range of parameters. We also show that W=0 pairs
quantize the magnetic flux like superconducting pairs do. The pairing mechanism
breaks down in the presence of strong distortions. The W=0 pairs are also the
building blocks for the antiferromagnetic ground state of the half-filled
Hubbard model at weak coupling. Our analytical results for the
Hubbard square lattice, compared to available numerical data, demonstrate that
the method, besides providing intuitive grasp on pairing, also has quantitative
predictive power. We also consider including phonon effects in this scenario.
Preliminary calculations with small clusters indicate that vector phonons
hinder pairing while half-breathing modes are synergic with the W=0 pairing
mechanism both at weak coupling and in the polaronic regime.Comment: 42 pages, Topical Review to appear in Journal of Physics C: Condensed
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Rethinking recognition: social context in adult life rather than early experience shapes recognition in a social wasp
Social recognition represents the foundation of social living. To what extent social recognition is hard-wired by early-life experience or flexible and influenced by social context of later life stages is a crucial question in animal behaviour studies. Social insects have represented classic models to investigate the subject, and the acknowledged idea is that relevant information to create the referent template for nest-mate recognition (NMR) is usually acquired during an early sensitive period in adult life. Experimental evidence, however, highlighted that other processes may also be at work in creating the template and that such a template may be updated during adult life according to social requirements. However, currently, we lack an ad hoc experiment testing the alternative hypotheses at the basis of NMR ontogeny in social insects. Thus, to investigate the mechanisms underlying the ontogeny of NMR in Polistes wasps, a model genus in recognition studies, and their different role in determining recognition abilities, we subjected Polistes dominula workers to different olfactory experiences in different phases of their life before inserting them into the social environment of a novel colony and testing them in recognition bioassays. Our results show that workers develop their NMR abilities based on their social context rather than through pre-imaginal and early learning or self-referencing. Our study demonstrates that the social context represents the major component shaping recognition abilities in a social wasp, therefore shedding new light on the ontogeny of recognition in paper wasps and prompting the reader to rethink about the traditional knowledge at the basis of the recognition in social insects. This article is part of the theme issue 'Signal detection theory in recognition systems: from evolving models to experimental tests'
The Fermionic Projector, Entanglement, and the Collapse of the Wave Function
After a brief introduction to the fermionic projector approach, we review how
entanglement and second quantized bosonic and fermionic fields can be described
in this framework. The constructions are discussed with regard to decoherence
phenomena and the measurement problem. We propose a mechanism leading to the
collapse of the wave function in the quantum mechanical measurement process.Comment: 17 pages, LaTeX, 2 figures, minor changes (published version
A network of sex and competition: The promiscuous mating system of an invasive weevil
Abstract
Invasive alien pest insect species represent a major threat for agriculture and biodiversity. Because chemical treatments employed to contrast such pests elicit serious environmental and human health problems, a great effort is currently directed to develop long term and environmentally friendly biological control strategies. However, the successful application of some promising techniques, such as the Sterile Insect Technique (SIT), requires a deep knowledge of the pest basic biology. Here, we argue that understanding pest sexual biology using a social network approach can significantly improve the performance of control strategies. For example, SIT may benefit from understanding how individuals interact and how males accede to reproduction, in order to target the most reproductively active and polygamic males. In this paper we studied the socio-sexual networks of the Asian red palm weevil (RPW) Rhynchophorus ferrugineus, a worldwide invader which is causing heavy economic impacts on several palm species. We found that the RPW has a highly promiscuous mating system, characterized by forced interruptions of pair copulations by additional males. The social network is highly non-random nor regular: few males almost monopolize reproduction, behaving as key-players in the network of matings. Additionally, males have a stable pattern of sexual behaviour over time. We use RPW social network as a case study to direct the development of management techniques such as SIT strategy
Equilibrium and time-dependent Josephson current in one-dimensional superconducting junctions
We investigate the transport properties of a one-dimensional
superconductor-normal metal-superconductor (S-N-S) system described within the
tight-binding approximation. We compute the equilibrium dc Josephson current
and the time-dependent oscillating current generated after the switch-on of a
constant bias. In the first case an exact embedding procedure to calculate the
Nambu-Gorkov Keldysh Green's function is employed and used to derive the
continuum and bound states contributions to the dc current. A general formalism
to obtain the Andreev bound states (ABS) of a normal chain connected to
superconducting leads is also presented. We identify a regime in which all
Josephson current is carried by the ABS and obtain an analytic formula for the
current-phase relation in the limit of long chains. In the latter case the
condition for perfect Andreev reflections is expressed in terms of the
microscopic parameters of the model, showing a limitation of the so called
wide-band-limit (WBL) approximation. When a finite bias is applied to the S-N-S
junction we compute the exact time-evolution of the system by solving
numerically the time-dependent Bogoliubov-deGennes equations. We provide a
microscopic description of the electron dynamics not only inside the normal
region but also in the superconductors, thus gaining more information with
respect to WBL-based approaches. Our scheme allows us to study the ac regime as
well as the transient dynamics whose characteristic time-scale is dictated by
the velocity of multiple Andreev reflections
Boson Dominance in nuclei
We present a new method of bosonization of fermion systems applicable when
the partition function is dominated by composite bosons. Restricting the
partition function to such states we get an euclidean bosonic action from which
we derive the Hamiltonian. Such a procedure respects all the fermion
symmetries, in particular fermion number conservation, and provides a boson
mapping of all fermion operators.Comment: 12 page
Hornets and Honey Bees: A Coevolutionary Arms Race between Ancient Adaptations and New Invasive Threats
Hornets and honey bees have a long history of coevolution resulting in a plethora of captivating adaptations and counteradaptations between predator and prey. From simple physiological mechanisms to complex behavioral strategies, some Vespa hornets have specialized in hunting honey bees, while the latter have put in place effective defenses to counteract their attack. Both hornets and honey bees have evolved the ability to detect the odors and the pheromones emitted by the other to locate the prey or to spot foraging predators. Hornets often rely on their bigger size, heavily armored body and destructive attacks, while honey bees differentiated collective defense responses finely coordinated to deter or kill the hornet menace. However, when new species of hornets and honey bees come into contact, the absence of coevolution can have a heavy impact on the defenseless bees. The evolutionary arms race between hornets and honey bees provides not only compelling examples of adaptations and counteradaptations between predator and prey, but could also represent a starting point for the development of effective and sustainable strategies to protect honey bees and beekeeping activities and to control invasive alien species of hornets
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