5,663 research outputs found
Observation of deconfinement in a cold dense quark medium
In this paper we study the confinement/deconfinement transition in lattice
QCD at finite quark density and zero temperature. The simulations are
performed on an lattice with rooted staggered fermions at a lattice
spacing . This small lattice spacing allowed us to
reach very large baryon density (up to quark chemical potential ) avoiding strong lattice artifacts. In the region we observe for the first time the confinement/deconfinement
transition which manifests itself in rising of the Polyakov loop and vanishing
of the string tension . After the deconfinement is achieved at , we observe a monotonous decrease of the spatial string
tension which ends up with vanishing at . From this observation we draw the conclusion that the
confinement/deconfinement transition at finite density and zero temperature is
quite different from that at finite temperature and zero density. Our results
indicate that in very dense matter the quark-gluon plasma is in essence a
weakly interacting gas of quarks and gluons without a magnetic screening mass
in the system, sharply different from a quark-gluon plasma at large
temperature.Comment: 6 pages, 4 figure
Electron-Electron Interactions in the Vacuum Polarization of Graphene
We discuss the effect of electron-electron interactions on the static
polarization properties of graphene beyond RPA. Divergent self-energy
corrections are naturally absorbed into the renormalized coupling constant
. We find that the lowest order vertex correction, which is the first
non-trivial correlation contribution, is finite, and about 30% of the RPA
result at strong coupling . The vertex correction leads to
further reduction of the effective charge. Finite contributions to dielectric
screening are expected in all orders of perturbation theory.Comment: 5 pages, 2 figures; published versio
Bound states of magnons in the S=1/2 quantum spin ladder
We study the excitation spectrum of the two-leg antiferromagnetic S=1/2
Heisenberg ladder. Our approach is based on the description of the excitations
as triplets above a strong-coupling singlet ground state. The quasiparticle
spectrum is calculated by treating the excitations as a dilute Bose gas with
infinite on-site repulsion. We find singlet (S=0) and triplet (S=1)
two-particle bound states of the elementary triplets. We argue that bound
states generally exist in any dimerized quantum spin model.Comment: 4 REVTeX pages, 4 Postscript figure
Studies and application of bent crystals for beam steering at 70-GeV IHEP accelerator
This report overviews studies accomplished in the U70 proton synchrotron of
IHEP-Protvino during the recent two decades. Major attention is paid to a
routine application of bent crystals for beam extraction from the machine. It
has been confirmed experimentally that efficiency of beam extraction with a
crystal deflector of around 85% is well feasible for a proton beam with
intensity up to 1012 protons per cycle. Another trend is to use bent crystals
for halo collimation in a high energy collider. New promising options emerge
for, say, LHC and ILC based on the "volume reflection" effect, which has been
discovered recently in machine study runs at U70 of IHEP (50 GeV) and SPS of
CERN (400 GeV).Comment: 12 pages, 14 figure
2d Gauge Theories and Generalized Geometry
We show that in the context of two-dimensional sigma models minimal coupling
of an ordinary rigid symmetry Lie algebra leads naturally to the
appearance of the "generalized tangent bundle" by means of composite fields. Gauge transformations of the composite
fields follow the Courant bracket, closing upon the choice of a Dirac structure
(or, more generally, the choide of a "small
Dirac-Rinehart sheaf" ), in which the fields as well as the symmetry
parameters are to take values. In these new variables, the gauge theory takes
the form of a (non-topological) Dirac sigma model, which is applicable in a
more general context and proves to be universal in two space-time dimensions: A
gauging of of a standard sigma model with Wess-Zumino term
exists, \emph{iff} there is a prolongation of the rigid symmetry to a Lie
algebroid morphism from the action Lie algebroid
into (or the algebraic analogue of the morphism in the case of
). The gauged sigma model results from a pullback by this morphism
from the Dirac sigma model, which proves to be universal in two-spacetime
dimensions in this sense.Comment: 22 pages, 2 figures; To appear in Journal of High Energy Physic
Ordering in the pyrochlore antiferromagnet due to Dzyaloshinsky-Moriya interactions
The Heisenberg nearest neighbour antiferromagnet on the pyrochlore (3D)
lattice is highly frustrated and does not order at low temperature where
spin-spin correlations remain short ranged. Dzyaloshinsky-Moriya interactions
(DMI) may be present in pyrochlore compounds as is shown, and the consequences
of such interactions on the magnetic properties are investigated through mean
field approximation and monte carlo simulations. It is found that DMI (if
present) tremendously change the low temperature behaviour of the system. At a
temperature of the order of the DMI a phase transition to a long range ordered
state takes place. The ordered magnetic structures are explicited for the
different possible DMI which are introduced on the basis of symmetry arguments.
The relevance of such a scenario for pyrochlore compounds in which an ordered
magnetic structure is observed experimentally is dicussed
Theoretical Aspects of the Fractional Quantum Hall Effect in Graphene
We review the theoretical basis and understanding of electronic interactions
in graphene Landau levels, in the limit of strong correlations. This limit
occurs when inter-Landau-level excitations may be omitted because they belong
to a high-energy sector, whereas the low-energy excitations only involve the
same level, such that the kinetic energy (of the Landau level) is an
unimportant constant. Two prominent effects emerge in this limit of strong
electronic correlations: generalised quantum Hall ferromagnetic states that
profit from the approximate four-fold spin-valley degeneracy of graphene's
Landau levels and the fractional quantum Hall effect. Here, we discuss these
effects in the framework of an SU(4)-symmetric theory, in comparison with
available experimental observations.Comment: 12 pages, 3 figures; review for the proceedings of the Nobel
Symposium on Graphene and Quantum Matte
Spectrum of elementary and collective excitations in the dimerized S=1/2 Heisenberg chain with frustration
We have studied the low-energy excitation spectrum of a dimerized and
frustrated antiferromagnetic Heisenberg chain. We use an analytic approach,
based on a description of the excitations as triplets above a strong-coupling
singlet ground state. The quasiparticle spectrum is calculated by treating the
excitations as a dilute Bose gas with infinite on-site repulsion. Additional
singlet (S=0) and triplet (S=1) modes are found as two-particle bound states of
the elementary triplets. We have also calculated the contributions of the
elementary and collective excitations into the spin structure factor. Our
results are in excellent agreement with exact diagonalizations and dimer series
expansions data as long as the dimerization parameter is not too small
(), i.e. while the elementary triplets can be treated as localized
objects.Comment: 18 pages, 13 figure
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