2,650 research outputs found
Phase transition in the assignment problem for random matrices
We report an analytic and numerical study of a phase transition in a P
problem (the assignment problem) that separates two phases whose
representatives are the simple matching problem (an easy P problem) and the
traveling salesman problem (a NP-complete problem). Like other phase
transitions found in combinatoric problems (K-satisfiability, number
partitioning) this can help to understand the nature of the difficulties in
solving NP problems an to find more accurate algorithms for them.Comment: 7 pages, 5 figures; accepted for publication in Europhys. Lett.
http://www.edpsciences.org/journal/index.cfm?edpsname=ep
Excited state entanglement in homogeneous fermionic chains
We study the Renyi entanglement entropy of an interval in a periodic
fermionic chain for a general eigenstate of a free, translational invariant
Hamiltonian. In order to analytically compute the entropy we use two technical
tools. The first one is used to reduce logarithmically the complexity of the
problem and the second one to compute the R\'enyi entropy of the chosen
subsystem. We introduce new strategies to perform the computations, derive new
expressions for the entropy of these general states and show the perfect
agreement of the analytical computations and the numerical outcome. Finally we
discuss the physical interpretation of our results and generalise them to
compute the entanglement entropy for a fragment of a fermionic ladder.Comment: 31 pages, 1 table, 8 figures. Final version published in J. Phys. A.
References and section added. Typos correcte
Entanglement in fermionic chains with finite range coupling and broken symmetries
We obtain a formula for the determinant of a block Toeplitz matrix associated
with a quadratic fermionic chain with complex coupling. Such couplings break
reflection symmetry and/or charge conjugation symmetry. We then apply this
formula to compute the Renyi entropy of a partial observation to a subsystem
consisting of contiguous sites in the limit of large . The present work
generalizes similar results due to Its, Jin, Korepin and Its, Mezzadri, Mo. A
striking new feature of our formula for the entanglement entropy is the
appearance of a term scaling with the logarithm of the size of . This
logarithmic behaviour originates from certain discontinuities in the symbol of
the block Toeplitz matrix. Equipped with this formula we analyse the
entanglement entropy of a Dzyaloshinski-Moriya spin chain and a Kitaev
fermionic chain with long range pairing.Comment: 27 pages, 5 figure
Magnetic-field-dependent quasiparticle energy relaxation in mesoscopic wires
In order to find out if magnetic impurities can mediate interactions between
quasiparticles in metals, we have measured the effect of a magnetic field B on
the energy distribution function f(E) of quasiparticles in two silver wires
driven out-of-equilibrium by a bias voltage U. In a sample showing sharp
distributions at B=0, no magnetic field effect is found, whereas in the other
sample, rounded distributions at low magnetic field get sharper as B is
increased, with a characteristic field proportional to U. Comparison is made
with recent calculations of the effect of magnetic-impurities-mediated
interactions taking into account Kondo physics.Comment: 4 pages, 3 figures, to be published in Physical Review Letter
On the M\"obius transformation in the entanglement entropy of fermionic chains
There is an intimate relation between entanglement entropy and Riemann
surfaces. This fact is explicitly noticed for the case of quadratic fermionic
Hamiltonians with finite range couplings. After recollecting this fact, we make
a comprehensive analysis of the action of the M\"obius transformations on the
Riemann surface. We are then able to uncover the origin of some symmetries and
dualities of the entanglement entropy already noticed recently in the
literature. These results give further support for the use of entanglement
entropy to analyse phase transition.Comment: 29 pages, 5 figures. Final version published in JSTAT. Two new
figures. Some comments and references added. Typos correcte
Influence of Magnetic Field on Effective Electron-Electron Interactions in a Copper Wire
We have measured in a copper wire the energy exchange rate between
quasiparticles as a function of the applied magnetic field. We find that the
effective electron-electron interaction is strongly modified by the magnetic
field, suggesting that magnetic impurities play a role on the interaction
processes.Comment: latex anthore.tex, 8 files, 6 figures, 7 pages in: Proceedings of the
XXXVIth Rencontres de Moriond `Electronic Correlations: From Meso- to
Nano-physics' Les Arcs, France January 20-27, 2001 [SPEC-S01/027
Ballistic effects in a proximity induced superconducting diffusive metal
Using a Scanning Tunneling Microscope (STM), we investigate the Local Density
of States (LDOS) of artificially fabricated normal metal nano-structures in
contact with a superconductor. Very low temperature local spectroscopic
measurements (100 mK) reveal the presence of well defined subgap peaks at
energy |E|<Delta in the LDOS at various positions of the STM tip. Although no
clear correlations between the LDOS and the shape of the samples have emerged,
some of the peak features suggest they originate from quasi-particle bound
states within the normal metal structures (De Gennes St James states).
Refocusing of electronic trajectories induced by the granular srtucture of the
samples can explain the observation of spatially uncorrelated interference
effects in a non-ballistic medium.Comment: 4 pages, 4 figure
Agaricus guachari, sp. nov. (Agaricales), encontrada en Venezuela.
[EN] Agaricus guachari is proposed and described as a new species. It was found growing on dung of a nocturnal bird called "guácharo" (Steatornis caripensis Humboldt), in a cave, showing white basidiomata, which reddenned quickly when cut or robbed. Tbe habitat, presence of a well-developed universal veil and negative Schaeffer's reaction distinguished this species from the rest of taxa included in this genus. Data, including iconography, micro-and ultramicroscopic features, are also added.[ES] Se propone y describe Agaricus guachari como especie nueva para la ciencia. Este hongo se encontró viviendo sobre excrementos de "guácharo" (Steatomis caripensis Humboldt), que es un ave nocturna que habita en cuevas y se alimenta de frotos de plantas tropicales en Suramérica. El hongo presenta basidiomas blancos en fresco, que al rozarlos o cortarlos pasan rápidamente a tomar un color rojizo. Este hábitat tan característico, unido al hecho de presentar velo universal bien desarrollado y reacción negativa frente al reactivo de Schaeffer, confiere a esta especie una identidad propia que la separa y diferencia de los otros táxones de Agaricus conocidos hasta ahora. Además de la descripción completa, se aporta una detallada iconografía con los datos más relevantes sobre sus características micro- y ultramicroscópicas.Peer reviewe
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