3,650 research outputs found
Hybrid Monte-Carlo simulation of interacting tight-binding model of graphene
In this work, results are presented of Hybrid-Monte-Carlo simulations of the
tight-binding Hamiltonian of graphene, coupled to an instantaneous long-range
two-body potential which is modeled by a Hubbard-Stratonovich auxiliary field.
We present an investigation of the spontaneous breaking of the sublattice
symmetry, which corresponds to a phase transition from a conducting to an
insulating phase and which occurs when the effective fine-structure constant
of the system crosses above a certain threshold .
Qualitative comparisons to earlier works on the subject (which used larger
system sizes and higher statistics) are made and it is established that
is of a plausible magnitude in our simulations. Also, we discuss
differences between simulations using compact and non-compact variants of the
Hubbard field and present a quantitative comparison of distinct discretization
schemes of the Euclidean time-like dimension in the Fermion operator.Comment: 7 pages, 1 figure, presented at the 31st International Symposium on
Lattice Field Theory (Lattice 2013), 29 July - 3 August 2013, Mainz, German
Spectrum of QCD at Finite Isospin Density
We study the phase diagram of QCD at finite isospin density using two flavors
of staggered quarks. We investigate the low temperature region of the phase
diagram where we find a pion condensation phase at high chemical potential. We
started a basic analysis of the spectrum at finite isospin density. In
particular, we measured pion, rho and nucleon masses inside and outside of the
pion condensation phase. In agreement with previous studies in two-color QCD at
finite baryon density we find that the Polyakov loop does not depend on the
density in the staggered formulation.Comment: 8 pages, 7 figures, proceedings of Lattice2017, Granada, Spai
Effective potential for SU(2) Polyakov loops and Wilson loop eigenvalues
We simulate SU(2) gauge theory at temperatures ranging from slightly below
to roughly for two different values of the gauge coupling. Using a
histogram method, we extract the effective potential for the Polyakov loop and
for the phases of the eigenvalues of the thermal Wilson loop, in both the
fundamental and adjoint representations. We show that the classical potential
of the fundamental loop can be parametrized within a simple model which
includes a Vandermonde potential and terms linear and quadratic in the Polyakov
loop. We discuss how parametrizations for the other cases can be obtained from
this model.Comment: 16 pages, 39 figure
On the universal critical behavior in 3-flavor QCD
We analyze the universal critical behavior at the chiral critical point in
QCD with three degenerate quark masses. We confirm that this critical point
lies in the universality class of the three dimensional Ising model. The
symmetry of the Ising model, which is Z(2), is not directly realized in the QCD
Hamiltonian. After making an ansatz for the magnetization- and energy-like
operators as linear admixtures of the chiral condensate and the gluonic action,
we determine several non-universal mixing and normalization constants. These
parameters determine an unambiguous mapping of the critical behavior in QCD to
that of the 3d-Ising model. We verify its validity by showing that the thus
obtained orderparameter scales in accordance with the magnetic equation of
state of the 3d-Ising model.Comment: 7 pages, contribution to Lattice 2011 proceeding
Lattice simulation of a center symmetric three-dimensional effective theory for SU(2) Yang-Mills
We perform simulations of an effective theory of SU(2) Wilson lines in three
dimensions. Our action includes a kinetic term, the one-loop perturbative
potential for the Wilson line, a non-perturbative "fuzzy-bag" contribution and
spatial gauge fields. We determine the phase diagram of the theory and confirm
that, at moderately weak coupling, the non-perturbative term leads to
eigenvalue repulsion in a finite region above the deconfining phase transition.Comment: To appear in the proceedings of "Strong and Electroweak Matter",
Amsterdam, the Netherlands, August 26-29, Nucl. Phys. A, in prin
Low-metallicity star formation: Relative impact of metals and magnetic fields
Low-metallicity star formation poses a central problem of cosmology, as it
determines the characteristic mass scale and distribution for the first and
second generations of stars forming in our Universe. Here, we present a
comprehensive investigation assessing the relative impact of metals and
magnetic fields, which may both be present during low-metallicity star
formation. We show that the presence of magnetic fields generated via the
small-scale dynamo stabilises the protostellar disc and provides some degree of
support against fragmentation. In the absence of magnetic fields, the
fragmentation timescale in our model decreases by a factor of ~10 at the
transition from Z=0 to Z>0, with subsequently only a weak dependence on
metallicity. Similarly, the accretion timescale of the cluster is set by the
large-scale dynamics rather than the local thermodynamics. In the presence of
magnetic fields, the primordial disc can become completely stable, therefore
forming only one central fragment. At Z>0, the number of fragments is somewhat
reduced in the presence of magnetic fields, though the shape of the mass
spectrum is not strongly affected in the limits of the statistical
uncertainties. The fragmentation timescale, however, increases by roughly a
factor of 3 in the presence of magnetic fields. Indeed, our results indicate
comparable fragmentation timescales in primordial runs without magnetic fields
and Z>0 runs with magnetic fields.Comment: MNRAS in pres
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