879 research outputs found
Flutter and forced response of mistuned rotors using standing wave analysis
A standing wave approach is applied to the analysis of the flutter and forced response of tuned and mistuned rotors. The traditional traveling wave cascade airforces are recast into standing wave arbitrary motion form using Pade approximants, and the resulting equations of motion are written in the matrix form. Applications for vibration modes, flutter, and forced response are discussed. It is noted that the standing wave methods may prove to be more versatile for dealing with certain applications, such as coupling flutter with forced response and dynamic shaft problems, transient impulses on the rotor, low-order engine excitation, bearing motion, and mistuning effects in rotors
Instabilities at [110] Surfaces of d_{x^2-y^2} Superconductors
We compare different scenarios for the low temperature splitting of the
zero-energy peak in the local density of states at (110) surfaces of
d_{x^2-y^2}-wave superconductors, observed by Covington et al.
(Phys.Rev.Lett.79 (1997), 277). Using a tight binding model in the
Bogolyubov-de Gennes treatment we find a surface phase transition towards a
time-reversal symmetry breaking surface state carrying spontaneous currents and
an s+id-wave state. Alternatively, we show that electron correlation leads to a
surface phase transition towards a magnetic state corresponding to a local spin
density wave state.Comment: 4 pages, 5 figure
Representations of the discrete inhomogeneous Lorentz group and Dirac wave equation on the lattice
We propose the fundamental and two dimensional representation of the Lorentz
groups on a (3+1)-dimensional hypercubic lattice, from which representations of
higher dimensions can be constructed. For the unitary representation of the
discrete translation group we use the kernel of the Fourier transform. From the
Dirac representation of the Lorentz group (including reflections) we derive in
a natural way the wave equation on the lattice for spin 1/2 particles. Finally
the induced representation of the discrete inhomogeneous Lorentz group is
constructed by standard methods and its connection with the continuous case is
discussed.Comment: LaTeX, 20 pages, 1 eps figure, uses iopconf.sty (late submission
Causality violation and singularities
We show that singularities necessarily occur when a boundary of causality
violating set exists in a space-time under the physically suitable assumptions
except the global causality condition in the Hawking-Penrose singularity
theorems. Instead of the global causality condition, we impose some
restrictions on the causality violating sets to show the occurrence of
singularities.Comment: 11 pages, latex, 2 eps figure
One-loop Beta Functions for the Orientable Non-commutative Gross-Neveu Model
We compute at the one-loop order the beta-functions for a renormalisable
non-commutative analog of the Gross Neveu model defined on the Moyal plane. The
calculation is performed within the so called x-space formalism. We find that
this non-commutative field theory exhibits asymptotic freedom for any number of
colors. The beta-function for the non-commutative counterpart of the Thirring
model is found to be non vanishing.Comment: 16 pages, 9 figure
Network synchronization: Optimal and Pessimal Scale-Free Topologies
By employing a recently introduced optimization algorithm we explicitely
design optimally synchronizable (unweighted) networks for any given scale-free
degree distribution. We explore how the optimization process affects
degree-degree correlations and observe a generic tendency towards
disassortativity. Still, we show that there is not a one-to-one correspondence
between synchronizability and disassortativity. On the other hand, we study the
nature of optimally un-synchronizable networks, that is, networks whose
topology minimizes the range of stability of the synchronous state. The
resulting ``pessimal networks'' turn out to have a highly assortative
string-like structure. We also derive a rigorous lower bound for the Laplacian
eigenvalue ratio controlling synchronizability, which helps understanding the
impact of degree correlations on network synchronizability.Comment: 11 pages, 4 figs, submitted to J. Phys. A (proceedings of Complex
Networks 2007
Dynamical generalization of a solvable family of two-electron model atoms with general interparticle repulsion
Holas, Howard and March [Phys. Lett. A {\bf 310}, 451 (2003)] have obtained
analytic solutions for ground-state properties of a whole family of
two-electron spin-compensated harmonically confined model atoms whose different
members are characterized by a specific interparticle potential energy
u(). Here, we make a start on the dynamic generalization of the
harmonic external potential, the motivation being the serious criticism
levelled recently against the foundations of time-dependent density-functional
theory (e.g. [J. Schirmer and A. Dreuw, Phys. Rev. A {\bf 75}, 022513 (2007)]).
In this context, we derive a simplified expression for the time-dependent
electron density for arbitrary interparticle interaction, which is fully
determined by an one-dimensional non-interacting Hamiltonian. Moreover, a
closed solution for the momentum space density in the Moshinsky model is
obtained.Comment: 5 pages, submitted to J. Phys.
Constraints on Beta Functions from Duality
We analyze the way in which duality constrains the exact beta function and
correlation length in single-coupling spin systems. A consistency condition we
propose shows very concisely the relation between self-dual points and phase
transitions, and implies that the correlation length must be duality invariant.
These ideas are then tested on the 2-d Ising model, and used towards finding
the exact beta function of the -state Potts model. Finally, a generic
procedure is given for identifying a duality symmetry in other single-coupling
models with a continuous phase transition.Comment: LaTeX, 6 page
M5-brane geometries, T-duality and fluxes
We describe a duality relation between configurations of M5-branes in
M-theory and type IIB theory on Taub-NUT geometries with NSNS and RR 3-form
field strength fluxes. The flux parameters are controlled by the angles between
the M5-brane and the (T)duality directions. For one M5-brane, the duality leads
to a family of supersymmetric flux configurations which interpolates between
imaginary self-dual fluxes and fluxes similar to the Polchinski-Strassler kind.
For multiple M5-branes, the IIB configurations are related to fluxes for
twisted sector fields in orbifolds. The dual M5-brane picture also provides a
geometric interpretation for several properties of flux configurations (like
the supersymmetry conditions, their contribution to tadpoles, etc), and for
many non-trivial effects in the IIB side. Among the latter, the dielectric
effect for probe D3-branes is dual to the recombination of probe M5-branes with
background ones; also, a picture of a decay channel for non-supersymmetric
fluxes is suggested.Comment: 30 pages, 3 figure
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