8,931 research outputs found
Spectral Measures of Bipartivity in Complex Networks
We introduce a quantitative measure of network bipartivity as a proportion of
even to total number of closed walks in the network. Spectral graph theory is
used to quantify how close to bipartite a network is and the extent to which
individual nodes and edges contribute to the global network bipartivity. It is
shown that the bipartivity characterizes the network structure and can be
related to the efficiency of semantic or communication networks, trophic
interactions in food webs, construction principles in metabolic networks, or
communities in social networks.Comment: 16 pages, 1 figure, 1 tabl
Paramagnetic Breakdown of Superconductivity in Ultrasmall Metallic Grains
We study the magnetic-field-induced breakdown of superconductivity in
nm-scale metal grains having a mean electron level spacing (bulk gap). Using a generalized variational BCS approach that
yields good qualitative agreement with measured spectra, we argue that Pauli
paramagnetism dominates orbital diamagnetism, as in the case of thin films in a
parallel magnetic field. However, the first-order transition observed for the
latter can be made continuous by finite size effects. The mean-field procedure
of describing the system by a single pairing parameter breaks down for
.Comment: 4 pages of revtex, 3 postscript figures, uses psfrag.sty, epsfig.sty.
Slightly revised and improved version, matching published versio
Numerical Evidence of Luttinger and Fermi Liquid Behaviour in the 2D Hubbard Model
The two dimensional Hubbard model with a single spin-up electron interacting
with a finite density of spin-down electrons is studied using the quantum
Monte Carlotechnique, a new conjugate gradient method for the evaluation of
the Edwards wavefunction ansatz, and the standard second order perturbation
theory. We performed simulations up to 242 sites at reaching the zero
temperature properties with no ``fermion sign problem'' and found a
surprisingly good accuracy of the Edwards wavefunction ansatz at low density or
low doping. The conjugate gradient method was then applied to system up to 1922
sites and infinite for the Edwards state. Fermi liquid theory seems to
remain stable in 2D for all cases studied with the exception of the half
filling case where a ``Luttinger like behavior'' survives in the Hubbard model
, yielding a vanishing quasiparticle weight in the thermodynamic limit.Comment: 10 pages + 4 pictures, RevTex, SISSA 121/93/CM/M
Thermodynamic properties of a small superconducting grain
The reduced BCS Hamiltonian for a metallic grain with a finite number of
electrons is considered. The crossover between the ultrasmall regime, in which
the level spacing, , is larger than the bulk superconducting gap, ,
and the small regime, where , is investigated analytically
and numerically. The condensation energy, spin magnetization and tunneling peak
spectrum are calculated analytically in the ultrasmall regime, using an
approximation controlled by as small parameter, where is the
number of interacting electron pairs. The condensation energy in this regime is
perturbative in the coupling constant , and is proportional to . We find that also in a large regime with
, in which pairing correlations are already rather well developed,
the perturbative part of the condensation energy is larger than the singular,
BCS, part. The condition for the condensation energy to be well approximated by
the BCS result is found to be roughly . We show how
the condensation energy can, in principle, be extracted from a measurement of
the spin magnetization curve, and find a re-entrant susceptibility at zero
temperature as a function of magnetic field, which can serve as a sensitive
probe for the existence of superconducting correlations in ultrasmall grains.
Numerical results are presented which suggest that in the large limit the
1/N correction to the BCS result for the condensation energy is larger than
.Comment: 17 pages, 7 figures, Submitted to Phys. Rev.
Naturally Rehearsing Passwords
We introduce quantitative usability and security models to guide the design
of password management schemes --- systematic strategies to help users create
and remember multiple passwords. In the same way that security proofs in
cryptography are based on complexity-theoretic assumptions (e.g., hardness of
factoring and discrete logarithm), we quantify usability by introducing
usability assumptions. In particular, password management relies on assumptions
about human memory, e.g., that a user who follows a particular rehearsal
schedule will successfully maintain the corresponding memory. These assumptions
are informed by research in cognitive science and validated through empirical
studies. Given rehearsal requirements and a user's visitation schedule for each
account, we use the total number of extra rehearsals that the user would have
to do to remember all of his passwords as a measure of the usability of the
password scheme. Our usability model leads us to a key observation: password
reuse benefits users not only by reducing the number of passwords that the user
has to memorize, but more importantly by increasing the natural rehearsal rate
for each password. We also present a security model which accounts for the
complexity of password management with multiple accounts and associated
threats, including online, offline, and plaintext password leak attacks.
Observing that current password management schemes are either insecure or
unusable, we present Shared Cues--- a new scheme in which the underlying secret
is strategically shared across accounts to ensure that most rehearsal
requirements are satisfied naturally while simultaneously providing strong
security. The construction uses the Chinese Remainder Theorem to achieve these
competing goals
Parity Effect in Ground State Energies of Ultrasmall Superconducting Grains
We study the superconductivity in small grains in the regime when the quantum
level spacing is comparable to the gap . As
is increased, the system crosses over from superconducting
to normal state. This crossover is studied by calculating the dependence of the
ground state energy of a grain on the parity of the number of electrons. The
states with odd numbers of particles carry an additional energy ,
which shows non-monotonic dependence on . Our predictions
can be tested experimentally by studying the parity-induced alternation of
Coulomb blockade peak spacings in grains of different sizes.Comment: 4 pages, revtex, multicol.st
Transport in Double-Crossed Luttinger Liquids
We study transport through two Luttinger liquids (one-dimensional electrons
interacting through a Coulomb repulsion in a metal) coupled together at {\it
two} points. External voltage biases are incorporated through boundary
conditions. We include density-density couplings as well as single-particle
hops at the contacts. For weak repulsive interactions, transport through the
wires remains undisturbed by the inter-wire couplings, which renormalise to
zero. For strong repulsive interactions, the inter-wire couplings become
strong. For symmetric barriers and no external voltage bias, a single gate
voltage is sufficient to tune for resonance transmission in both wires.
However, for asymmetric couplings or for finite external biases, the system is
insulating.Comment: Latex file, 11 pages, one eps figur
Electron Correlation and Jahn-Teller Interaction in Manganese Oxides
The interplay between the electron repulsion and the Jahn-Teller
electron-phonon interation is studied with a large model for the
ferromagnetic state of the manganese oxides. These two interactions collaborate
to induce the local isospin (orbital) moments and reduce the bandwidth .
Especially the retardation effect of the Jahn-Teller phonon with the frequency
is effective to reduce , but the strong -dependence occurs
even when the Coulombic interaction is dominating () as long as
. The phonon spectrum consists of two components, i.e., the
temperature independent sharp peak at and that corresponding to the Kondo peak. These results
compared with the experiments suggest that in the metallic
manganese oxides.Comment: REVTE
Ferromagnetic transition in a double-exchange system
We study ferromagnetic transition in three-dimensional double-exchange model.
The influence of strong spin fluctuations on conduction electrons is described
in coherent potential approximation. In the framework of thermodynamic approach
we construct for the system "electrons (in a disordered spin configuration) +
spins" the Landau functional, from the analysis of which critical temperature
of ferromagnetic transition is calculated.Comment: 4 pages, 1 eps figure, LaTeX2e, RevTeX. References added, text
change
A small superconducting grain in the canonical ensemble
By means of the Lanczos method we analyze superconducting correlations in
ultrasmall grains at fixed particle number. We compute the ground state
properties and the excitation gap of the pairing Hamiltonian as a function of
the level spacing . Both quantities turn out to be parity dependent and
universal functions of the ratio ( is the BCS gap). We
then characterize superconductivity in the canonical ensemble from the scaling
behavior of correlation functions in energy space.Comment: 11 pages Revtex, 5 figures .ep
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