2,482 research outputs found
Simulation of two-dimensional quantum systems using a tree tensor network that exploits the entropic area law
This work explores the use of a tree tensor network ansatz to simulate the
ground state of a local Hamiltonian on a two-dimensional lattice. By exploiting
the entropic area law, the tree tensor network ansatz seems to produce
quasi-exact results in systems with sizes well beyond the reach of exact
diagonalisation techniques. We describe an algorithm to approximate the ground
state of a local Hamiltonian on a L times L lattice with the topology of a
torus. Accurate results are obtained for L={4,6,8}, whereas approximate results
are obtained for larger lattices. As an application of the approach, we analyse
the scaling of the ground state entanglement entropy at the quantum critical
point of the model. We confirm the presence of a positive additive constant to
the area law for half a torus. We also find a logarithmic additive correction
to the entropic area law for a square block. The single copy entanglement for
half a torus reveals similar corrections to the area law with a further term
proportional to 1/L.Comment: Major rewrite, new version published in Phys. Rev. B with highly
improved numerical results for the scaling of the entropies and several new
sections. The manuscript has now 19 pages and 30 Figure
Beam-size effect and particle losses at Super factory (Italy)
In the colliders, the macroscopically large impact parameters give a
substantial contribution to the standard cross section of the process. These impact parameters may be much larger than the
transverse sizes of the colliding bunches. It means that the standard cross
section of this process has to be substantially modified. In the present paper
such a beam-size effect is calculated for bremsstrahlung at Super factory
developed in Italy. We find out that this effect reduces beam losses due to
bremsstrahlung by about 40%.Comment: 11 pages, 4 figure
Eddy Current measurement of Fiber Volume Fraction in Metal Matrix Composite Extrusions
The objective of this work was to develop an eddy current method for measuring fiber volume fraction in continuous-fiber metal matrix composites. Because an eddy current measurement can be affected by the spatial distribution of fibers as well as the overall fiber density, the measurement method had to be tolerant of possible variations in spatial distribution that might be encountered in practice. For this reason, the work began with the development of models of the effective resistivity tensor for a composite with an arbitrary fiber distribution and the resulting eddy current probe response [1,2]. The intent was to use these models to help design a measurement method and to test the method for ordered and disordered arrangements of fibers
Conformal off-diagonal boundary density profiles on a semi-infinite strip
The off-diagonal profile phi(v) associated with a local operator (order
parameter or energy density) close to the boundary of a semi-infinite strip
with width L is obtained at criticality using conformal methods. It involves
the surface exponent x_phi^s and displays a simple universal behaviour which
crosses over from surface finite-size scaling when v/L is held constant to
corner finite-size scaling when v/L -> 0.Comment: 5 pages, 1 figure, IOP macros and eps
Casimir Forces between Spherical Particles in a Critical Fluid and Conformal Invariance
Mesoscopic particles immersed in a critical fluid experience long-range
Casimir forces due to critical fluctuations. Using field theoretical methods,
we investigate the Casimir interaction between two spherical particles and
between a single particle and a planar boundary of the fluid. We exploit the
conformal symmetry at the critical point to map both cases onto a highly
symmetric geometry where the fluid is bounded by two concentric spheres with
radii R_- and R_+. In this geometry the singular part of the free energy F only
depends upon the ratio R_-/R_+, and the stress tensor, which we use to
calculate F, has a particularly simple form. Different boundary conditions
(surface universality classes) are considered, which either break or preserve
the order-parameter symmetry. We also consider profiles of thermodynamic
densities in the presence of two spheres. Explicit results are presented for an
ordinary critical point to leading order in epsilon=4-d and, in the case of
preserved symmetry, for the Gaussian model in arbitrary spatial dimension d.
Fundamental short-distance properties, such as profile behavior near a surface
or the behavior if a sphere has a `small' radius, are discussed and verified.
The relevance for colloidal solutions is pointed out.Comment: 37 pages, 2 postscript figures, REVTEX 3.0, published in Phys. Rev. B
51, 13717 (1995
Eddy current probe design for second-layer cracks under installed fasteners
The United States Air Force has an operational need to reliably detect second-layer cracks around fastener holes in two-layer airframe structures with the fasteners in place. Because access to the second layer is usually not available, the inspection must be performed by placing a probe on the outer surface of the structure and detecting cracks through the first layer. Eddy current methods have been applied to this inspection problem [1–6], and have met with some success; however, much improvement is still needed to achieve the desired sensitivity to cracks and rejection of signals caused by the geometry of the structure under inspection
Surface Critical Behavior of Binary Alloys and Antiferromagnets: Dependence of the Universality Class on Surface Orientation
The surface critical behavior of semi-infinite
(a) binary alloys with a continuous order-disorder transition and
(b) Ising antiferromagnets in the presence of a magnetic field is considered.
In contrast to ferromagnets, the surface universality class of these systems
depends on the orientation of the surface with respect to the crystal axes.
There is ordinary and extraordinary surface critical behavior for orientations
that preserve and break the two-sublattice symmetry, respectively. This is
confirmed by transfer-matrix calculations for the two-dimensional
antiferromagnet and other evidence.Comment: Final version that appeared in PRL, some minor stylistic changes and
one corrected formula; 4 pp., twocolumn, REVTeX, 3 eps fig
Average trajectory of returning walks
We compute the average shape of trajectories of some one--dimensional
stochastic processes x(t) in the (t,x) plane during an excursion, i.e. between
two successive returns to a reference value, finding that it obeys a scaling
form. For uncorrelated random walks the average shape is semicircular,
independently from the single increments distribution, as long as it is
symmetric. Such universality extends to biased random walks and Levy flights,
with the exception of a particular class of biased Levy flights. Adding a
linear damping term destroys scaling and leads asymptotically to flat
excursions. The introduction of short and long ranged noise correlations
induces non trivial asymmetric shapes, which are studied numerically.Comment: 16 pages, 16 figures; accepted for publication in Phys. Rev.
Eddy Current Detection of Subsurface Cracks in Engine Disk Boltholes
The development of a reliable eddy current inspection system to detect second layer cracks in sleeved engine disk bolt holes poses serious difficulties. This paper discusses some initial results obtained in two separate investigations that are aimed at advancing the state-of-the-art in eddy current detection of subsurface cracks. Both finite element design optimization results of a horseshoe shaped ferrite core probe, and the results of preliminary evaluation of the applicability of electric current perturbation (ECP) technique to the current problem are presented in this paper
Entropic Elasticity of Double-Strand DNA Subject to Simple Spatial Constraints
The aim of the present paper is the study of the entropic elasticity of the
dsDNA molecule, having a cristallographic length L of the order of 10 to 30
persistence lengths A, when it is subject to spatial obstructions. We have not
tried to obtain the single molecule partition function by solving a
Schodringer-like equation. We prefer to stay within a discretized version of
the WLC model with an added one-monomer potential, simulating the spatial
constraints. We derived directly from the discretized Boltzmann formula the
transfer matrix connecting the partition functions relative to adjacent
"effective monomers". We have plugged adequate Dirac delta-functions in the
functional integral to ensure that the monomer coordinate and the tangent
vector are independent variables. The partition function is, then, given by an
iterative process which is both numerically efficient and physically
transparent. As a test of our discretized approach, we have studied two
configurations involving a dsDNA molecule confined between a pair of parallel
plates.Comment: The most formal developments of Section I have been moved into an
appendix and replaced by a direct derivation of the transfer matrix used in
the applications. of Section II. Two paragraphs and two figures have been
added to clarify the physical interpretation of the result
- …