26,673 research outputs found
Towards Baxter equation in supersymmetric Yang-Mills theories
We perform an explicit two-loop calculation of the dilatation operator acting
on single trace Wilson operators built from holomorphic scalar fields and an
arbitrary number of covariant derivatives in N=2 and N=4 supersymmetric
Yang-Mills theories. We demonstrate that its eigenspectrum exhibits double
degeneracy of opposite parity eigenstates which suggests that the two-loop
dilatation operator is integrable. Moreover, the two-loop anomalous dimensions
in the two theories differ from each other by an overall normalization factor
indicating that the phenomenon is not sensitive to the presence of the
conformal symmetry. Relying on these findings, we try to uncover integrable
structures behind the two-loop dilatation operator using the method of the
Baxter Q-operator. We propose a deformed Baxter equation which exactly encodes
the spectrum of two-loop anomalous dimensions and argue that it correctly
incorporates a peculiar feature of conformal scalar operators -- the conformal
SL(2) spin of such operators is modified in higher loops by an amount
proportional to their anomalous dimension. From the point of view of spin
chains this property implies that the underlying integrable model is
``self-tuned'' -- the all-loop Hamiltonian of the spin chain depends on the
total SL(2) spin which in its turn is proportional to the Hamiltonian.Comment: Latex, 18 pages, 3 figure
Casimir energy density in closed hyperbolic universes
The original Casimir effect results from the difference in the vacuum
energies of the electromagnetic field, between that in a region of space with
boundary conditions and that in the same region without boundary conditions. In
this paper we develop the theory of a similar situation, involving a scalar
field in spacetimes with compact spatial sections of negative spatial
curvature.Comment: 10 pages. Contribution to the "Fifth Alexander Friedmann
International Seminar on Gravitation and Cosmology," Joao Pessoa, Brazil,
2002. Revised version, with altered Abstract and one new referenc
Anisotropic magnetic behavior of GdBa_2Cu_3O_{6+y} single crystals
Magnetic properties of high-quality Al-free nonsuperconducting
GdBa_2Cu_3O_{6+y} single crystals grown by flux method have been studied. The
magnetic anisotropy below the N\'eel temperature T_N~2.3K corresponds to the
direction of Gd^{3+} magnetic moments along the tetragonal c-axis. At T < T_N
clear indications of spin-flop transitions for H||c have been observed on
magnetization curves at H_{sf}~10kOe. Magnetic phase diagrams have been
obtained for H||c as well as for H||ab. A pronounced anisotropy in the magnetic
susceptibility (unexpected for Gd-based compounds) has been found above T_N.Comment: 2 pages, 3 figures; LT23 (Aug. 2002; Hiroshima), accepted to Physica
Collision of Viscoelastic Spheres: Compact Expressions for the Coefficient of Normal Restitution
The coefficient of restitution of colliding viscoelastic spheres is
analytically known as a complete series expansion in terms of the impact
velocity where all (infinitely many) coefficients are known. While beeing
analytically exact, this result is not suitable for applications in efficient
event-driven Molecular Dynamics (eMD) or Monte Carlo (MC) simulations. Based on
the analytic result, here we derive expressions for the coefficient of
restitution which allow for an application in efficient eMD and MC simulations
of granular Systems.Comment: 4 pages, 4 figure
On the interrelation between monopoles, vortices, topological charge and chiral symmetry breaking: an analysis using overlap fermions for SU(2)
We study the properties of configurations from which P-vortices on one hand
or Abelian monopoles on the other hand have been removed. We find that the zero
modes and the band of non-zero modes close to zero disappear from the spectrum
of the overlap Dirac operator, confirming the absence of topological charge and
quark condensate. The different behavior of the modified ensembles under
smearing compared to the unmodified Monte Carlo ensemble corroborates these
findings. The gluonic topological susceptibility rapidly approaches zero in
accordance with Q_{index}=0. The remaining (ultraviolet) monopoles without
vortices and -- to a less extent -- the remaining vortices without monopoles
are unstable under smearing whereas smearing of the unmodified Monte Carlo
ensemble effects the monopoles and vortices only by smoothing, reducing the
density only slightly.Comment: 13 pages, 5 figures, strongly revised, results added, one figure
added, accepted for publication, title changed
Efficiency of different matrix inversion methods applied to Wilson fermions
We compare different conjugate gradient -- like matrix inversion methods (CG,
BiCGstab1 and BiCGstab2) employing for this purpose the compact lattice quantum
electrodynamics (QED) with Wilson fermions. The main goals of this
investigation are the CPU time efficiency of the methods as well as the
influence of machine precision on the reliability of (physical) results
especially close to the 'critical' line ~\kappa_c(\bt).Comment: 27 pages LaTeX (epsf), all figures include
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