998 research outputs found
Coherence lengths and anisotropy in MgB2 superconductor
Field and temperature microwave measurements have been carried out on MgB2
thin film grown on Al2O3 substrate. The analysis reveals the mean field
coherence length xi_{MF} in the mixed state and a temperature independent
anisotropy ratio gamma_{MF} = xi_{MF}^{ab} / xi_{MF}^c approximately 2. At the
superconducting transition, the scaling of the fluctuation conductivity yields
the Ginzburg-Landau coherence length with a different anisotropy ratio
gamma_{GL} = 2.8, also temperature independent.Comment: submitted to PR
D-Theory: Field Theory via Dimensional Reduction of Discrete Variables
A new non-perturbative approach to quantum field theory --- D-theory --- is
proposed, in which continuous classical fields are replaced by discrete
quantized variables which undergo dimensional reduction. The 2-d classical O(3)
model emerges from the (2+1)-d quantum Heisenberg model formulated in terms of
quantum spins. Dimensional reduction is demonstrated explicitly by simulating
correlation lengths up to 350,000 lattice spacings using a loop cluster
algorithm. In the framework of D-theory, gauge theories are formulated in terms
of quantum links --- the gauge analogs of quantum spins. Quantum links are
parallel transporter matrices whose elements are non-commuting operators. They
can be expressed as bilinears of anticommuting fermion constituents. In quantum
link models dimensional reduction to four dimensions occurs, due to the
presence of a 5-d Coulomb phase, whose existence is confirmed by detailed
simulations using standard lattice gauge theory. Using Shamir's variant of
Kaplan's fermion proposal, in quantum link QCD quarks appear as edge states of
a 5-d slab. This naturally protects their chiral symmetries without
fine-tuning. The first efficient cluster algorithm for a gauge theory with a
continuous gauge group is formulated for the U(1) quantum link model. Improved
estimators for Wilson loops are constructed, and dimensional reduction to
ordinary lattice QED is verified numerically.Comment: 15 pages, LaTeX, including 9 encapsulated postscript figures.
Contribution to Lattice 97 by 5 authors, to appear in Nuclear Physics B
(Proceeding Supplements). Requires psfig.tex and espcrc2.st
Self-adapting method for the localization of quantum critical points using Quantum Monte Carlo techniques
A generalization to the quantum case of a recently introduced algorithm (Y.
Tomita and Y. Okabe, Phys. Rev. Lett. {\bf 86}, 572 (2001)) for the
determination of the critical temperature of classical spin models is proposed.
We describe a simple method to automatically locate critical points in
(Quantum) Monte Carlo simulations. The algorithm assumes the existence of a
finite correlation length in at least one of the two phases surrounding the
quantum critical point. We illustrate these ideas on the example of the
critical inter-chain coupling for which coupled antiferromagnetic S=1 spin
chains order at T=0. Finite-size scaling relations are used to determine the
exponents, and in agreement with previous
estimates.Comment: 5 pages, 3 figures, published versio
A macroscopic multifractal analysis of parabolic stochastic PDEs
It is generally argued that the solution to a stochastic PDE with
multiplicative noise---such as , where denotes
space-time white noise---routinely produces exceptionally-large peaks that are
"macroscopically multifractal." See, for example, Gibbon and Doering (2005),
Gibbon and Titi (2005), and Zimmermann et al (2000). A few years ago, we proved
that the spatial peaks of the solution to the mentioned stochastic PDE indeed
form a random multifractal in the macroscopic sense of Barlow and Taylor (1989;
1992). The main result of the present paper is a proof of a rigorous
formulation of the assertion that the spatio-temporal peaks of the solution
form infinitely-many different multifractals on infinitely-many different
scales, which we sometimes refer to as "stretch factors." A simpler, though
still complex, such structure is shown to also exist for the
constant-coefficient version of the said stochastic PDE.Comment: 41 page
Field-induced XY behavior in the S=1/2 antiferromagnet on the square lattice
Making use of the quantum Monte Carlo method based on the worm algorithm, we
study the thermodynamic behavior of the S=1/2 isotropic Heisenberg
antiferromagnet on the square lattice in a uniform magnetic field varying from
very small values up to the saturation value. The field is found to induce a
Berezinskii-Kosterlitz-Thouless transition at a finite temperature, above which
a genuine XY behavior in an extended temperature range is observed. The phase
diagram of the system is drawn, and the thermodynamic behavior of the specific
heat and of the uniform and staggered magnetization is discussed in sight of an
experimental investigation of the field-induced XY behavior.Comment: 4 pages, 4 figure
Relating the Cosmological Constant and Supersymmetry Breaking in Warped Compactifications of IIB String Theory
It has been suggested that the observed value of the cosmological constant is
related to the supersymmetry breaking scale M_{susy} through the formula Lambda
\sim M_p^4 (M_{susy}/M_p)^8. We point out that a similar relation naturally
arises in the codimension two solutions of warped space-time varying
compactifications of string theory in which non-isotropic stringy moduli induce
a small but positive cosmological constant.Comment: 7 pages, LaTeX, references added and minor changes made, (v3) map
between deSitter and global cosmic brane solutions clarified, supersymmetry
breaking discussion improved and references adde
The phase diagram of quantum systems: Heisenberg antiferromagnets
A novel approach for studying phase transitions in systems with quantum
degrees of freedom is discussed. Starting from the microscopic hamiltonian of a
quantum model, we first derive a set of exact differential equations for the
free energy and the correlation functions describing the effects of
fluctuations on the thermodynamics of the system. These equations reproduce the
full renormalization group structure in the neighborhood of a critical point
keeping, at the same time, full information on the non universal properties of
the model. As a concrete application we investigate the phase diagram of a
Heisenberg antiferromagnet in a staggered external magnetic field. At long
wavelengths the known relationship to the Quantum Non Linear Sigma Model
naturally emerges from our approach. By representing the two point function in
an approximate analytical form, we obtain a closed partial differential
equation which is then solved numerically. The results in three dimensions are
in good agreement with available Quantum Monte Carlo simulations and series
expansions. More refined approximations to the general framework presented here
and few applications to other models are briefly discussed.Comment: 17 pages, 7 figure
Multiplicity Studies and Effective Energy in ALICE at the LHC
In this work we explore the possibility to perform ``effective energy''
studies in very high energy collisions at the CERN Large Hadron Collider (LHC).
In particular, we focus on the possibility to measure in collisions the
average charged multiplicity as a function of the effective energy with the
ALICE experiment, using its capability to measure the energy of the leading
baryons with the Zero Degree Calorimeters. Analyses of this kind have been done
at lower centre--of--mass energies and have shown that, once the appropriate
kinematic variables are chosen, particle production is characterized by
universal properties: no matter the nature of the interacting particles, the
final states have identical features. Assuming that this universality picture
can be extended to {\it ion--ion} collisions, as suggested by recent results
from RHIC experiments, a novel approach based on the scaling hypothesis for
limiting fragmentation has been used to derive the expected charged event
multiplicity in interactions at LHC. This leads to scenarios where the
multiplicity is significantly lower compared to most of the predictions from
the models currently used to describe high energy collisions. A mean
charged multiplicity of about 1000-2000 per rapidity unit (at ) is
expected for the most central collisions at .Comment: 12 pages, 19 figures. In memory of A. Smirnitski
The COSINE-100 liquid scintillator veto system
This paper describes the liquid scintillator veto system for the COSINE-100 dark matter experiment and its performance. The COSINE-100 detector consists of eight NaI(Tl) crystals immersed in 2200 L of linear alkylbenzene-based liquid scintillator. The liquid scintillator tags between 65 and 75% of the internal 40K background in the 2–6 keV energy region. We also describe the background model for the liquid scintillator, which is primarily used to assess its energy calibration and threshold
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