5,945 research outputs found
Rigid unit modes in tetrahedral crystals
The 'rigid unit mode' (RUM) model requires unit blocks, in our case
tetrahedra of SiO_4 groups, to be rigid within first order of the displacements
of the O-ions. The wave-vectors of the lattice vibrations, which obey this
rigidity, are determined analytically. Lattices with inversion symmetry yield
generically surfaces of RUMs in reciprocal space, whereas lattices without this
symmetry yield generically lines of RUMs. Only in exceptional cases as in
beta-quartz a surface of RUMs appears, if inversion symmetry is lacking. The
occurence of planes and bending surfaces, straight and bent lines is discussed.
Explicit calculations are performed for five modifications of SiO_2 crystals.Comment: 18 pages, 6 figures, improved notatio
Running coupling and mass anomalous dimension of SU(3) gauge theory with two flavors of symmetric-representation fermions
We have measured the running coupling constant of SU(3) gauge theory coupled
to Nf=2 flavors of symmetric representation fermions, using the Schrodinger
functional scheme. Our lattice action is defined with hypercubic smeared links
which, along with the larger lattice sizes, bring us closer to the continuum
limit than in our previous study. We observe that the coupling runs more slowly
than predicted by asymptotic freedom, but we are unable to observe fixed point
behavior before encountering a first order transition to a strong coupling
phase. This indicates that the infrared fixed point found with the thin-link
action is a lattice artifact. The slow running of the gauge coupling permits an
accurate determination of the mass anomalous dimension for this theory, which
we observe to be small, gamma_m < 0.6, over the range of couplings we can
reach. We also study the bulk and finite-temperature phase transitions in the
strong coupling region.Comment: 17 pages, 16 figures. Substantial modifications to explain why the
fat-link result for the beta function supersedes our thin-link result; also
updated the phase diagram to reflect additional numerical work. Added
references. Final versio
An Introduction to Finite Temperature Quantum Chromodynamics on the Lattice
In these lectures, we introduce finite temperature QCD on the lattice to
non-experts of the subject. We first formulate lattice QCD both at zero and
finite temperatures. Then a section is devoted to the topic of improved lattice
actions which are becoming an essential ingredient of precision studies of QCD
on the lattice. We then discuss about finite temperature SU(3) gauge theory,
i.e. QCD without dynamical quarks (quenched QCD). Finally, we report recent
status of studies in full QCD taking into account the effects of dynamical
quarks.Comment: Lectures presented at the 1997 Yukawa International Seminar (YKIS'97)
on ``Non-Perturbative QCD --- Structure of the QCD Vacuum ---'', YITP, Kyoto,
Japan, 2--12 Dec. 1997. To be published in the proceedings [Prog. Theor.
Phys. Suppl.
Massless Spectra and Gauge Couplings at One-Loop on Non-Factorisable Toroidal Orientifolds
So-called `non-factorisable' toroidal orbifolds can be rewritten in a
factorised form as a product of three two-tori by imposing an additional shift
symmetry. This finding of Blaszczyk et al., arXiv:1111.5852, provides a new
avenue to Conformal Field Theory methods, by which the vector-like massless
matter spectrum - and thereby the type of gauge group enhancement on
orientifold invariant fractional D6-branes - and the one-loop corrections to
the gauge couplings in Type IIA orientifold theories can be computed in
addition to the well-established chiral matter spectrum derived from
topological intersection numbers among three-cycles. We demonstrate this
framework for the orientifolds on the
-type torus. As observed before for factorisable
backgrounds, also here the one-loop correction can drive the gauge groups to
stronger coupling as demonstrated by means of a four-generation Pati-Salam
example.Comment: 65 pages, 10 figures; v3: matches published version (ref +
explanatory remarks added
- …