3,453 research outputs found
On the evolutionary behaviour of BL Lac objects
We present a new well defined sample of BL Lac objects selected from the
ROSAT All-Sky Survey (RASS). The sample consists of 39 objects with 35 forming
a flux limited sample down to f_X = 8 x 10^{-13} cgs, redshifts are known for
33 objects (and 31 of the complete sample). X-ray spectral properties were
determined for each object individually with the RASS data. The luminosity
function of RASS selected BL Lac objects is compatible with results provided by
objects selected with the Einstein observatory, but the RASS selected sample
contains objects with luminosities at least tenfold higher. Our analysis
confirms the negative evolution for X-ray selected BL Lac objects found in a
sample by the Einstein observatory, the parameterization provides similar
results. A subdivision of the sample into halves according to the X-ray to
optical flux ratio yielded unexpected results. The extremely X-ray dominated
objects have higher redshifts and X-ray luminosities and only this subgroup
shows clear signs of strong negative evolution. The evolutionary behaviour of
objects with an intermediate spectral energy distribution between X-ray and
radio dominated is compatible with no evolution at all. Consequences for
unified schemes of X-ray and radio selected BL Lac objects are discussed.We
suggest that the intermediate BL Lac objects are the basic BL Lac population.
The distinction between the two subgroups can be explained if extreme X-ray
dominated BL Lac objects are observed in a state of enhanced X-ray activity.Comment: 14 pages incl. 8 figures, accepted by A&
Critical behaviour and scaling functions of the three-dimensional O(6) model
We numerically investigate the three-dimensional O(6) model on 12^3 to 120^3
lattices within the critical region at zero magnetic field, as well as at
finite magnetic field on the critical isotherm and for several fixed couplings
in the broken and the symmetric phase. We obtain from the Binder cumulant at
vanishing magnetic field the critical coupling J_c=1.42865(3). The universal
value of the Binder cumulant at this point is g_r(J_c)=-1.94456(10). At the
critical coupling, the critical exponents \gamma=1.604(6), \beta=0.425(2) and
\nu=0.818(5) are determined from a finite-size-scaling analysis. Furthermore,
we verify predicted effects induced by massless Goldstone modes in the broken
phase. The results are well described by the perturbative form of the model's
equation of state. Our O(6)-result is compared to the corresponding Ising, O(2)
and O(4) scaling functions. Finally, we study the finite-size-scaling behaviour
of the magnetisation on the pseudocritical line.Comment: 13 pages, 20 figures, REVTEX, fixed an error in the determination of
R_\chi and changed the corresponding line in figure 13
Towards a Realistic Equation of State of Strongly Interacting Matter
We consider a relativistic strongly interacting Bose gas. The interaction is
manifested in the off-shellness of the equilibrium distribution. The equation
of state that we obtain for such a gas has the properties of a realistic
equation of state of strongly interacting matter, i.e., at low temperature it
agrees with the one suggested by Shuryak for hadronic matter, while at high
temperature it represents the equation of state of an ideal ultrarelativistic
Stefan-Boltzmann gas, implying a phase transition to an effectively weakly
interacting phase.Comment: LaTeX, figures not include
Dual variables for the SU(2) lattice gauge theory at finite temperature
We study the three-dimensional SU(2) lattice gauge theory at finite
temperature using an observable which is dual to the Wilson line. This
observable displays a behaviour which is the reverse of that seen for the
Wilson line. It is non-zero in the confined phase and becomes zero in the
deconfined phase. At large distances, it's correlation function falls off
exponentially in the deconfined phase and remains non-zero in the confined
phase. The dual variable is non-local and has a string attached to it which
creates a Z(2) interface in the system. It's correlation function measures the
string tension between oppositely oriented Z(2) domains. The construction of
this variable can also be made in the four-dimensional theory where it measures
the surface tension between oppositely oriented Z(2) domains.Comment: 13 pages, LaTeX, 4 figures are included in the latex fil
Block Spin Effective Action for 4d SU(2) Finite Temperature Lattice Gauge Theory
The Svetitsky-Yaffe conjecture for finite temperature 4d SU(2) lattice gauge
theory is confirmed by observing matching of block spin effective actions of
the gauge model with those of the 3d Ising model. The effective action for the
gauge model is defined by blocking the signs of the Polyakov loops with the
majority rule. To compute it numerically, we apply a variant of the IMCRG
method of Gupta and Cordery.Comment: LaTeX2e, 22 pages, 8 Figure
Bose-Einstein Condensation from a Rotating Thermal Cloud: Vortex Nucleation and Lattice Formation
We develop a stochastic Gross-Pitaveskii theory suitable for the study of
Bose-Einstein condensation in a {\em rotating} dilute Bose gas. The theory is
used to model the dynamical and equilibrium properties of a rapidly rotating
Bose gas quenched through the critical point for condensation, as in the
experiment of Haljan et al. [Phys. Rev. Lett., 87, 21043 (2001)]. In contrast
to stirring a vortex-free condensate, where topological constraints require
that vortices enter from the edge of the condensate, we find that phase defects
in the initial non-condensed cloud are trapped en masse in the emerging
condensate. Bose-stimulated condensate growth proceeds into a disordered vortex
configuration. At sufficiently low temperature the vortices then order into a
regular Abrikosov lattice in thermal equilibrium with the rotating cloud. We
calculate the effect of thermal fluctuations on vortex ordering in the final
gas at different temperatures, and find that the BEC transition is accompanied
by lattice melting associated with diminishing long range correlations between
vortices across the system.Comment: 15 pages, 12 figure
Zero-Field Satellites of a Zero-Bias Anomaly
Spin-orbit (SO) splitting, , of the electron Fermi surface
in two-dimensional systems manifests itself in the interaction-induced
corrections to the tunneling density of states, . Namely, in
the case of a smooth disorder, it gives rise to the satellites of a zero-bias
anomaly at energies . Zeeman splitting, , in a weak parallel magnetic field causes a narrow {\em plateau} of
a width at the top of each sharp satellite peak.
As exceeds , the SO satellites cross over to the
conventional narrow maxima at with SO-induced
plateaus at the tops.Comment: 7 pages including 2 figure
Spin Injection in a Ballistic Two-Dimensional Electron Gas
We explore electrically injected, spin polarized transport in a ballistic
two-dimensional electron gas. We augment the Buettiker-Landauer picture with a
simple, but realistic model for spin-selective contacts to describe multimode
reservoir-to-reservoir transport of ballistic spin 1/2 particles. Clear and
unambiguous signatures of spin transport are established in this regime, for
the simplest measurement configuration that demonstrates them directly. These
new effects originate from spin precession of ballistic carriers; they exhibit
strong dependence upon device geometry and vanish in the diffusive limit. Our
results have important implications for prospective ``spin transistor''
devices.Comment: Submitted to Phys. Rev. Let
Z2 Monopoles, Vortices, and the Deconfinement Transition in Mixed Action SU(2) Gauge Theory
Adding separate chemical potentials lambda and gamma for Z2 monopoles and
vortices respectively in the Villain form of the mixed fundamental-adjoint
action for the SU(2) lattice gauge theory, we investigate their role in the
interplay between the deconfinement and bulk phase transitions using Monte
Carlo techniques. Setting lambda to be nonzero, we find that the line of
deconfinement transitions is shifted in the coupling plane but it behaves
curiously also like the bulk transition line for large enough adjoint coupling,
as for lambda=0. In a narrow range of couplings, however, we find separate
deconfinement and bulk phase transitions on the same lattice for nonzero and
large lambda, suggesting the two to be indeed coincident in the region where a
first order deconfinement phase transition is seen. In the limit of large
lambda and gamma, we obtain only lines of second order deconfinement phase
transitions, as expected from universality.Comment: 18 pages, 10 figures include
Lattice QCD equation of state : improving the differential method
We propose an improvement of the differential method for the computation of
the equation of state of QCD from lattice simulations. In contrast to the
earlier differential method our technique yields positive pressure for all
temperatures including in the transition region. Employing it on temporal
lattices of 8, 10 and 12 sites and by extrapolating to zero lattice spacing we
obtained the pressure, energy density, entropy density, specific heat and speed
of sound in quenched QCD for 0.9 < T/Tc < 3. A comparison of our results is
made with those from the dimensional reduction approach and a conformal
symmetric theory at high-temperature.Comment: Version to be published in Pramana. Title modified. Explanatory
material added. Figure 6b redrawn with corrected normalization for the
AdS/CFT lin
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