PubMed Health⌕ Search

SEARCH · PubMed Health

Results for “STRING”

Explore indexed PubMed citations for clinical trials, systematic reviews and public health research. Read source abstracts and follow each citation to its original PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 487 records · Page 27Linked to original sources
PubMed2004

Simple bit-string model for lineage branching.

We introduce a population dynamics model, where individual genomes are represented by bit strings. Selection is described by death probabilities which depend on these genomes, and new individuals continuously replace the ones that die, keeping the population constant. An offspring has the same genome as its (randomly chosen) parent, except for a small amount of (also random) mutations. Chance may thus generate a newborn with a genome that is better than that of its parent, and the newborn will have a smaller death probability. When this happens, this individual is a would-be founder of a new lineage. A new lineage is considered created if the number of its live descendants grows above a certain previously defined threshold. The time evolution of populations evolving under these rules is followed by computer simulations and the probability densities of lineage duration and size, among others, are computed. These densities show a scale-free behavior, in accordance with some conjectures in paleoevolution, and suggesting a simple mechanism as explanation for the ubiquity of these power laws.

Animals↗
PubMed2005

Lagrange mesh, relativistic flux tube, and rotating string.

The Lagrange mesh method is a very accurate and simple procedure to compute eigenvalues and eigenfunctions of nonrelativistic and semirelativistic Hamiltonians. We show here that it can be used successfully to solve the equations of both the relativistic flux tube model and the rotating string model, in the symmetric case. Verifications of the convergence of the method are given.

Journal Article↗
PubMed2005

Critical exponents of the driven elastic string in a disordered medium.

We analyze the harmonic elastic string driven through a continuous random potential above the depinning threshold. The velocity exponent beta = 0.33(2) is calculated. We observe a crossover in the roughness exponent zeta from the critical value 1.26 to the asymptotic (large force) value of 0.5. We calculate directly the velocity correlation function and the corresponding correlation length exponent nu = 1.29(5), which obeys the scaling relation nu = 1/(2 - zeta), and agrees with nu(FS), the finite-size-scaling exponent of fluctuations in the critical force. Surprisingly, the velocity correlation function is nonuniversal at short distances.

Journal Article↗
PubMed2005

Complex networks generated by the Penna bit-string model: emergence of small-world and assortative mixing.

The Penna bit-string model successfully encompasses many phenomena of population evolution, including inheritance, mutation, evolution, and aging. If we consider social interactions among individuals in the Penna model, the population will form a complex network. In this paper, we first modify the Verhulst factor to control only the birth rate, and introduce activity-based preferential reproduction of offspring in the Penna model. The social interactions among individuals are generated by both inheritance and activity-based preferential increase. Then we study the properties of the complex network generated by the modified Penna model. We find that the resulting complex network has a small-world effect and the assortative mixing property.

Journal Article↗
PubMed2006

Field-induced Berezinskii-Kosterlitz-Thouless transition and string-density plateau in the anisotropic triangular antiferromagnetic Ising model.

The field-induced Berezinskii-Kosterlitz-Thouless (BKT) transition in the ground state of the triangular antiferromagnetic Ising model is studied by the level-spectroscopy method. We analyze dimensions of operators around the BKT line, and estimate the BKT point H(c) approximately equal to 0.5229 +/-0.001, which is followed by the level-consistency check to demonstrate the accuracy of our estimate. Further we investigate the anisotropic case to clarify the stability of the field-induced string-density plateau against an incommensurate liquid state by the density-matrix renormalization-group method.

Journal Article↗
PubMed2000

Nonsingular global string compactifications

We consider an exotic "compactification" of spacetime in which there are two infinite extra dimensions, using a global string instead of a domain wall. By having a negative cosmological constant we prove the existence of a nonsingular static solution using a dynamical systems argument. A nonsingular solution also exists in the absence of a cosmological constant with a time-dependent metric. We compare and contrast this solution with the Randall-Sundrum universe and the Cohen-Kaplan spacetime and consider the options of using such a model as a realistic resolution of the hierarchy problem.

Journal Article↗
PubMed2000

Order out of disorder in a gas of elastic quantum strings in 2+1 dimensions.

A limiting case of a dynamical stripe state which is of potential significance to cuprate superconductors is considered: a gas of elastic quantum strings in 2+1 dimensions, interacting merely via a hard-core condition. It is demonstrated that this gas always solidifies, by a mechanism which is the quantum analog of the entropic interactions known from soft condensed matter physics.

Journal Article↗
PubMed2001

TeV strings and the neutrino-nucleon cross section at ultrahigh energies.

In scenarios with the fundamental unification scale at the TeV one expects string excitations of the standard model fields at accessible energies. We study the neutrino-nucleon cross section in these models. We show that duality of the scattering amplitude forces the existence of a tower of massive leptoquarks that mediate the process in the s channel. Using the narrow-width approximation we find a sum rule for the production rate of resonances with different spin at each mass level. We show that these contributions can increase substantially the standard model neutrino-nucleon cross section, although they seem insufficient to explain the cosmic ray events above the Greisen-Zatsepin-Kuz'min cutoff energy.

Journal Article↗
PubMed2001

Numerical analysis of the double scaling limit in the string type IIB matrix model.

The bosonic IIB matrix model is studied using a numerical method. This model contains the bosonic part of the IIB matrix model conjectured to be a nonperturbative definition of the type IIB superstring theory. The large N scaling behavior of the model is shown performing a Monte Carlo simulation. The expectation value of the Wilson loop operator is measured and the string tension is estimated. The numerical results show the prescription of the double scaling limit.

Journal Article↗
PubMed2001

Atomic string holography.

A new diffraction-channeling effect has been discovered, in which Kikuchi or channeling line patterns formed by high energy electrons, neutrons, and positrons are shown to break up into a series of annular disks if the crystal thickness traversed by the beam is small. The disks may be interpreted as Gabor in-line holograms of strings of atoms projected along the beam path. For electrons or positrons the patterns may be detected with little background by detecting characteristic x-ray emission from a thin film as a function of the diffraction conditions of a collimated, ionizing, high energy beam. Uses of the effect for structure determination and atomic-resolution lensless imaging are suggested.

Journal Article↗
PubMed2001

Localized gravity in string theory.

We propose a string realization of the AdS4 brane in AdS5 that is known to localize gravity. Our theory is M D5 branes in the near horizon geometry of N D3 branes, where M and N are appropriately tuned.

Journal Article↗
PubMed2002

Holography and string dynamics in time-dependent backgrounds.

We study the dynamics of D-branes in a smooth time-dependent background. The theory on the branes is a time-dependent noncommutative field theory. We find the metric and fluxes that determine the dual holographic closed string theory. This provides a concrete example of holography in a cosmological setting.

Journal Article↗
PubMed2003

Entangling strings of neutral atoms in 1D atomic pipeline structures.

We study a string of neutral atoms with nearest neighbor interaction in a 1D beam splitter configuration, where the longitudinal motion is controlled by a moving optical lattice potential. The dynamics of the atoms crossing the beam splitter maps to a 1D spin model with controllable time dependent parameters, which allows the creation of maximally entangled states of atoms by crossing a quantum phase transition. Furthermore, we show that this system realizes protected quantum memory, and we discuss the implementation of one- and two-qubit gates in this setup.

Journal Article↗
PubMed2004

1+1 dimensional compactifications of string theory.

We argue that stable, maximally symmetric compactifications of string theory to 1+1 dimensions are in conflict with holography. In particular, the finite horizon entropies of the Rindler wedge in 1+1 dimensional Minkowski and anti-de Sitter space, and of the de Sitter horizon in any dimension, are inconsistent with the symmetries of these spaces. The argument parallels one made recently by the same authors, in which we demonstrated the incompatibility of the finiteness of the entropy and the symmetries of de Sitter space in any dimension. If the horizon entropy is either infinite or zero, the conflict is resolved.

Journal Article↗
PubMed2004

All conjugate-maximal-helicity-violating amplitudes from topological open string theory in twistor space.

It has recently been proposed that the D-instanton expansion of the open topological B model on P(3|4) is equivalent to the perturbative expansion of the maximally supersymmetric Yang-Mills theory in four dimensions. In this letter we show how to construct the gauge theory results for all n-point conjugate-maximal-helicity-violating amplitudes by computing the integral over the moduli space of curves of degree n-3 in P(3|4), providing strong support to the string theory construction.

Journal Article↗
PubMed2004

Nonquantized dirac monopoles and strings in the berry phase of anisotropic spin systems.

The Berry phase of an anisotropic spin system that is adiabatically rotated along a closed circuit C is investigated. It is shown that the Berry phase consists of two contributions: (i) a geometric contribution which can be interpreted as the flux through C of a nonquantized Dirac monopole, and (ii) a topological contribution which can be interpreted as the flux through C of a Dirac string carrying a nonquantized flux, i.e., a spin analogue of the Aharonov-Bohm effect. Various experimental consequences of this novel effect are discussed.

Journal Article↗
PubMed2005

Creep motion of an elastic string in a random potential.

We study the creep motion of an elastic string in a two-dimensional pinning landscape by means of Langevin dynamics simulations. We find that the velocity-force characteristics are well described by the creep formula predicted from phenomenological scaling arguments. We analyze the creep exponent mu and the roughness exponent zeta. Two regimes are identified: when the temperature is larger than the strength of the disorder, we find mu approximately 1/4 and zeta approximately 2/3, in agreement with the quasi-equilibrium-nucleation picture of creep motion; on the contrary, when lowering the temperature enough, the values of mu and zeta increase, showing a strong violation of the latter picture.

Journal Article↗