6,226 research outputs found
Renormalized mean-field analysis of antiferromagnetism and d-wave superconductivity in the two-dimensional Hubbard model
We analyze the competition between antiferromagnetism and superconductivity
in the two-dimensional Hubbard model by combining a functional renormalization
group flow with a mean-field theory for spontaneous symmetry breaking.
Effective interactions are computed by integrating out states above a scale
Lambda_{MF} in one-loop approximation, which captures in particular the
generation of an attraction in the d-wave Cooper channel from fluctuations in
the particle-hole channel. These effective interactions are then used as an
input for a mean-field treatment of the remaining low-energy states, with
antiferromagnetism, singlet superconductivity and triplet pi-pairing as the
possible order parameters. Antiferromagnetism and superconductivity suppress
each other, leaving only a small region in parameter space where both orders
can coexist with a sizable order parameter for each. Triplet pi-pairing appears
generically in the coexistence region, but its feedback on the other order
parameters is very small.Comment: 28 pages, 14 figure
High-performance functional renormalization group calculations for interacting fermions
We derive a novel computational scheme for functional Renormalization Group
(fRG) calculations for interacting fermions on 2D lattices. The scheme is based
on the exchange parametrization fRG for the two-fermion interaction, with
additional insertions of truncated partitions of unity. These insertions
decouple the fermionic propagators from the exchange propagators and lead to a
separation of the underlying equations. We demonstrate that this separation is
numerically advantageous and may pave the way for refined, large-scale
computational investigations even in the case of complex multiband systems.
Furthermore, on the basis of speedup data gained from our implementation, it is
shown that this new variant facilitates efficient calculations on a large
number of multi-core CPUs. We apply the scheme to the , Hubbard model on
a square lattice to analyze the convergence of the results with the bond length
of the truncation of the partition of unity. In most parameter areas, a fast
convergence can be observed. Finally, we compare to previous results in order
to relate our approach to other fRG studies.Comment: 26 pages, 9 figure
Soft Fermi Surfaces and Breakdown of Fermi Liquid Behavior
Electron-electron interactions can induce Fermi surface deformations which
break the point-group symmetry of the lattice structure of the system. In the
vicinity of such a "Pomeranchuk instability" the Fermi surface is easily
deformed by anisotropic perturbations, and exhibits enhanced collective
fluctuations. We show that critical Fermi surface fluctuations near a d-wave
Pomeranchuk instability in two dimensions lead to large anisotropic decay rates
for single-particle excitations, which destroy Fermi liquid behavior over the
whole surface except at the Brillouin zone diagonal.Comment: 12 pages, 2 figures, revised version as publishe
Radiation hardness of CMS pixel barrel modules
Pixel detectors are used in the innermost part of the multi purpose
experiments at LHC and are therefore exposed to the highest fluences of
ionising radiation, which in this part of the detectors consists mainly of
charged pions. The radiation hardness of all detector components has thoroughly
been tested up to the fluences expected at the LHC. In case of an LHC upgrade,
the fluence will be much higher and it is not yet clear how long the present
pixel modules will stay operative in such a harsh environment. The aim of this
study was to establish such a limit as a benchmark for other possible detector
concepts considered for the upgrade.
As the sensors and the readout chip are the parts most sensitive to radiation
damage, samples consisting of a small pixel sensor bump-bonded to a CMS-readout
chip (PSI46V2.1) have been irradiated with positive 200 MeV pions at PSI up to
6E14 Neq and with 21 GeV protons at CERN up to 5E15 Neq.
After irradiation the response of the system to beta particles from a Sr-90
source was measured to characterise the charge collection efficiency of the
sensor. Radiation induced changes in the readout chip were also measured. The
results show that the present pixel modules can be expected to be still
operational after a fluence of 2.8E15 Neq. Samples irradiated up to 5E15 Neq
still see the beta particles. However, further tests are needed to confirm
whether a stable operation with high particle detection efficiency is possible
after such a high fluence.Comment: Contribution to the 11th European Symposium on Semiconductor
Detectors June 7-11, 2009 Wildbad Kreuth, German
A high-precision polarimeter
We have built a polarimeter in order to measure the electron beam
polarization in hall C at JLAB. Using a superconducting solenoid to drive the
pure-iron target foil into saturation, and a symmetrical setup to detect the
Moller electrons in coincidence, we achieve an accuracy of <1%. This sets a new
standard for Moller polarimeters.Comment: 17 pages, 9 figures, submitted to N.I.
SORLA-mediated trafficking of TrkB enhances the response of neurons to BDNF
Stimulation of neurons with brain-derived neurotrophic factor (BDNF) results in robust induction of SORLA, an intracellular sorting receptor of the VPS10P domain receptor gene family. However, the relevance of SORLA for BDNF-induced neuronal responses has not previously been investigated. We now demonstrate that SORLA is a sorting factor for the tropomyosin-related kinase receptor B (TrkB) that facilitates trafficking of this BDNF receptor between synaptic plasma membranes, post-synaptic densities, and cell soma, a step critical for neuronal signal transduction. Loss of SORLA expression results in impaired neuritic transport of TrkB and in blunted response to BDNF in primary neurons; and it aggravates neuromotoric deficits caused by low BDNF activity in a mouse model of Huntington's disease. Thus, our studies revealed a key role for SORLA in mediating BDNF trophic signaling by regulating the intracellular location of TrkB
Generalized modularity matrices
Various modularity matrices appeared in the recent literature on network
analysis and algebraic graph theory. Their purpose is to allow writing as
quadratic forms certain combinatorial functions appearing in the framework of
graph clustering problems. In this paper we put in evidence certain common
traits of various modularity matrices and shed light on their spectral
properties that are at the basis of various theoretical results and practical
spectral-type algorithms for community detection
Performance of prototype BTeV silicon pixel detectors in a high energy pion beam
The silicon pixel vertex detector is a key element of the BTeV spectrometer.
Sensors bump-bonded to prototype front-end devices were tested in a high energy
pion beam at Fermilab. The spatial resolution and occupancies as a function of
the pion incident angle were measured for various sensor-readout combinations.
The data are compared with predictions from our Monte Carlo simulation and very
good agreement is found.Comment: 24 pages, 20 figure
A Spectral Algorithm with Additive Clustering for the Recovery of Overlapping Communities in Networks
This paper presents a novel spectral algorithm with additive clustering
designed to identify overlapping communities in networks. The algorithm is
based on geometric properties of the spectrum of the expected adjacency matrix
in a random graph model that we call stochastic blockmodel with overlap (SBMO).
An adaptive version of the algorithm, that does not require the knowledge of
the number of hidden communities, is proved to be consistent under the SBMO
when the degrees in the graph are (slightly more than) logarithmic. The
algorithm is shown to perform well on simulated data and on real-world graphs
with known overlapping communities.Comment: Journal of Theoretical Computer Science (TCS), Elsevier, A Para\^itr
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