2,785 research outputs found
Influence of the contacts on the conductance of interacting quantum wires
We investigate how the conductance G through a clean interacting quantum wire
is affected by the presence of contacts and noninteracting leads. The contacts
are defined by a vanishing two-particle interaction to the left and a finite
repulsive interaction to the right or vice versa. No additional single-particle
scattering terms (impurities) are added. We first use bosonization and the
local Luttinger liquid picture and show that within this approach G is
determined by the properties of the leads regardless of the details of the
spatial variation of the Luttinger liquid parameters. This generalizes earlier
results obtained for step-like variations. In particular, no single-particle
backscattering is generated at the contacts. We then study a microscopic model
applying the functional renormalization group and show that the spatial
variation of the interaction produces single-particle backscattering, which in
turn leads to a reduced conductance. We investigate how the smoothness of the
contacts affects G and show that for decreasing energy scale its deviation from
the unitary limit follows a power law with the same exponent as obtained for a
system with a weak single-particle impurity placed in the contact region of the
interacting wire and the leads.Comment: 10 page, 4 figures included, minor changes in the summary, version
accepted for publication in PR
Boolean decision problems with competing interactions on scale-free networks: Critical thermodynamics
We study the critical behavior of Boolean variables on scale-free networks
with competing interactions (Ising spin glasses). Our analytical results for
the disorder-network-decay-exponent phase diagram are verified using Monte
Carlo simulations. When the probability of positive (ferromagnetic) and
negative (antiferromagnetic) interactions is the same, the system undergoes a
finite-temperature spin-glass transition if the exponent that describes the
decay of the interaction degree in the scale-free graph is strictly larger than
3. However, when the exponent is equal to or less than 3, a spin-glass phase is
stable for all temperatures. The robustness of both the ferromagnetic and
spin-glass phases suggests that Boolean decision problems on scale-free
networks are quite stable to local perturbations. Finally, we show that for a
given decay exponent spin glasses on scale-free networks seem to obey
universality. Furthermore, when the decay exponent of the interaction degree is
larger than 4 in the spin-glass sector, the universality class is the same as
for the mean-field Sherrington-Kirkpatrick Ising spin glass.Comment: 14 pages, lots of figures and 2 table
Thermodynamics of the L\'evy spin glass
We investigate the L\'evy glass, a mean-field spin glass model with power-law
distributed couplings characterized by a divergent second moment. By combining
extensively many small couplings with a spare random backbone of strong bonds
the model is intermediate between the Sherrington-Kirkpatrick and the
Viana-Bray model. A truncated version where couplings smaller than some
threshold \eps are neglected can be studied within the cavity method
developed for spin glasses on locally tree-like random graphs. By performing
the limit \eps\to 0 in a well-defined way we calculate the thermodynamic
functions within replica symmetry and determine the de Almeida-Thouless line in
the presence of an external magnetic field. Contrary to previous findings we
show that there is no replica-symmetric spin glass phase. Moreover we determine
the leading corrections to the ground-state energy within one-step replica
symmetry breaking. The effects due to the breaking of replica symmetry appear
to be small in accordance with the intuitive picture that a few strong bonds
per spin reduce the degree of frustration in the system
Reduction of Dilute Ising Spin Glasses
The recently proposed reduction method for diluted spin glasses is
investigated in depth. In particular, the Edwards-Anderson model with \pm J and
Gaussian bond disorder on hyper-cubic lattices in d=2, 3, and 4 is studied for
a range of bond dilutions. The results demonstrate the effectiveness of using
bond dilution to elucidate low-temperature properties of Ising spin glasses,
and provide a starting point to enhance the methods used in reduction. Based on
that, a greedy heuristic call ``Dominant Bond Reduction'' is introduced and
explored.Comment: 10 pages, revtex, final version, find related material at
http://www.physics.emory.edu/faculty/boettcher
Endo-cannibalism in the making of a recent British ancestor
Following his death in 1975, the ashes of Wally Hope, founder of Stonehenge People's Free Festival, were scattered in the centre of Stonehenge. When a child tasted the ashes the rest of the group followed this lead. In the following decades, as the festival increasingly became the site of contest about British heritage and culture, the story of Wally's ashes was told at significant times. His name continues to be invoked at gatherings today. This paper discusses these events as 'the making of an ancestor', and explores wider contexts in which they might be understood. These include Druidic involvement in the revival of cremation, Amazonian bone-ash endo-cannibalism, and popular means of speaking of and to dead relatives. In addition to considering the role of 'ancestors' in contemporary Britain, the paper contributes to considerations of 'ancestry' as a different way of being dead, of a particular moment in the evolution of an alternative religious neo-tribal movement, of the meanings of 'cannibalism', and of the ways in which human remains might be treated by the bereaved and by various other interested parties
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