43,498 research outputs found
Boundary effects on the scaling of the superfluid density
We study numerically the influence of the substrate (boundary conditions) on
the finite--size scaling properties of the superfluid density in
superfluid films of thickness within the XY model employing the Monte Carlo
method. Our results suggest that the jump at the
Kosterlitz--Thouless transition temperature depends on the boundary
conditions.Comment: 2 pages, 1 Latex file, 1 postscript figure, 2 style file
Linear response of a grafted semiflexible polymer to a uniform force field
We use the worm-like chain model to analytically calculate the linear
response of a grafted semiflexible polymer to a uniform force field. The result
is a function of the bending stiffness, the temperature, the total contour
length, and the orientation of the field with respect to that of the grafted
end. We also study the linear response of a worm-like chain with a periodic
alternating sequence of positive and negative charges. This can be considered
as a model for a polyampholyte with intrinsic bending siffness and negligible
intramolecular interactions. We show how the finite intrinsic persistence
length affects the linear response to the external field.Comment: 6 pages, 3 figure
Knowledge-based vision and simple visual machines
The vast majority of work in machine vision emphasizes the representation of perceived objects and events: it is these internal representations that incorporate the 'knowledge' in knowledge-based vision or form the 'models' in model-based vision. In this paper, we discuss simple machine vision systems developed by artificial evolution rather than traditional engineering design techniques, and note that the task of identifying internal representations within such systems is made difficult by the lack of an operational definition of representation at the causal mechanistic level. Consequently, we question the nature and indeed the existence of representations posited to be used within natural vision systems (i.e. animals). We conclude that representations argued for on a priori grounds by external observers of a particular vision system may well be illusory, and are at best place-holders for yet-to-be-identified causal mechanistic interactions. That is, applying the knowledge-based vision approach in the understanding of evolved systems (machines or animals) may well lead to theories and models that are internally consistent, computationally plausible, and entirely wrong
Non-Hermitian Delocalization and Eigenfunctions
Recent literature on delocalization in non-Hermitian systems has stressed
criteria based on sensitivity of eigenvalues to boundary conditions and the
existence of a non-zero current. We emphasize here that delocalization also
shows up clearly in eigenfunctions, provided one studies the product of left-
and right-eigenfunctions, as required on physical grounds, and not simply the
squared modulii of the eigenfunctions themselves. We also discuss the right-
and left-eigenfunctions of the ground state in the delocalized regime and
suggest that the behavior of these functions, when considered separately, may
be viewed as ``intermediate'' between localized and delocalized.Comment: 8 pages, 11 figures include
Cellular Ability to Sense Spatial Gradients in the Presence of Multiple Competitive Ligands
Many eukaryotic and prokaryotic cells can exhibit remarkable sensing ability
under small gradient of chemical compound. In this study, we approach this
phenomenon by considering the contribution of multiple ligands to the chemical
kinetics within Michaelis-Menten model. This work was inspired by the recent
theoretical findings from Bo Hu et al. [Phys. Rev. Lett. 105, 048104 (2010)],
our treatment with practical binding energies and chemical potential provides
the results which are consistent with experimental observations.Comment: 5 pages, 4 figure
Interaction effects in non-Hermitian models of vortex physics
Vortex lines in superconductors in an external magnetic field slightly tilted
from randomly-distributed parallel columnar defects can be modeled by a system
of interacting bosons in a non-Hermitian vector potential and a random scalar
potential. We develop a theory of the strongly-disordered non-Hermitian boson
Hubbard model using the Hartree-Bogoliubov approximation and apply it to
calculate the complex energy spectra, the vortex tilt angle and the tilt
modulus of (1+1)-dimensional directed flux line systems. We construct the phase
diagram associated with the flux-liquid to Bose-glass transition and find that,
close to the phase boundary, the tilted flux liquid phase is characterized by a
band of localized excitations, with two mobility edges in its low-energy
spectrum.Comment: 19 pages, 19 figures, To appear in Phys. Rev.
Differential Light Shift Cancellation in a Magnetic-Field-Insensitive Transition of Rb
We demonstrate near-complete cancellation of the differential light shift of
a two-photon magnetic-field-insensitive microwave hyperfine (clock) transition
in Rb atoms trapped in an optical lattice. Up to of the
differential light shift is canceled while maintaining magnetic-field
insensitivity. This technique should have applications in quantum information
and frequency metrology.Comment: 5 pages, 4 figure
Monte Carlo simulations of , a classical Heisenberg antiferromagnet in two-dimensions with dipolar interaction
We study the phase diagram of a quasi-two dimensional magnetic system with Monte Carlo simulations of a classical Heisenberg spin
Hamiltonian which includes the dipolar interactions between
spins. Our simulations reveal an Ising-like antiferromagnetic phase at low
magnetic fields and an XY phase at high magnetic fields. The boundary between
Ising and XY phases is analyzed with a recently proposed finite size scaling
technique and found to be consistent with a bicritical point at T=0. We discuss
the computational techniques used to handle the weak dipolar interaction and
the difference between our phase diagram and the experimental results.Comment: 13 pages 18 figure
Vortices in a cylinder: Localization after depinning
Edge effects in the depinned phase of flux lines in hollow superconducting
cylinder with columnar defects and electric current along the cylinder are
investigated. Far from the ends of the cylinder vortices are distributed almost
uniformly (delocalized). Nevertheless, near the edges these free vortices come
closer together and form well resolved dense bunches. A semiclassical picture
of this localization after depinning is described. For a large number of
vortices their density has square root singularity at the border of
the bunch ( is semicircle in the simplest case). However, by tuning
the strength of current, the various singular regimes for may be
reached. Remarkably, this singular behaviour reproduces the phase transitions
discussed during the past decade within the random matrix regularization of
2d-Gravity.Comment: 4 pages, REVTEX, 2 eps figure
Public Inquiries on Counterterrorism: An Independent Appraisal of New Zealand's Royal Commission of Inquiry into the Christchurch Terrorist Attack
Draft-in-progress; please do not distribute further or citeEven though elected leaders of liberal democracies respond to major terrorist attacks by commissioning public inquiries, scholars have not yet fully explored these inquiries as a practice of the War on Terror. By undertaking an independent appraisal of the Royal Commission of Inquiry into the Terrorist Attack on Christchurch Mosques on 15 March 2019 we seek to introduce empirical evidence of an important case of investigatory oversight to the more critical terrorism studies literature. Specifically, we examine how the Royal Commissioners interpreted their Terms of Reference, explore the ways in which they engaged with New Zealand’s security agencies as well as with various individuals, groups and communities beyond the public sector, and assess the quality of their recommendations. We argue that the Commissioners positioned themselves publicly as undertaking a remedial intervention in New Zealand’s counterterrorism effort, but their inquiry was, in fact, designed to reassure the public that the current security dispensation is largely fit for purpose and does not need major reform. The inquiry did little more than enable two carefully chosen Commissioners, imbued with a veneer of professional detachment but lacking in subject-matter expertise, to call for a strengthening of state security institutions, entrenching the hierarchies that already prevail within the national security system while shielding parliamentarians from accusations of disinterest, negligence or acts of omission. Consequently, New Zealand is no better off when it comes to its ability to understand the nature and scale of the threat posed by terrorism or to respond to an array of routine, as well as novel and surprise, security challenges
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