62,020 research outputs found
Thick atomic layers of maximum density as bulk terminations of quasicrystals
The clean surfaces of quasicrystals, orthogonal to the directions of the main
symmetry axes, have a terrace-like appearance. We extend the Bravais' rule for
crystals to quasicrystals, allowing that instead of a single atomic plane a
layer of atomic planes may form a bulk termination.Comment: 4 pages, 4 figure
Searching for hidden mirror symmetries in CMB fluctuations from WMAP 7 year maps
We search for hidden mirror symmetries at large angular scales in the WMAP 7
year Internal Linear Combination map of CMB temperature anisotropies using
global pixel based estimators introduced for this aim. Two different axes are
found for which the CMB intensity pattern is anomalously symmetric (or
anti-symmetric) under reflection with respect to orthogonal planes at the
99.84(99.96)% CL (confidence level), if compared to a result for an arbitrary
axis in simulations without the symmetry. We have verified that our results are
robust to the introduction of the galactic mask. The direction of such axes is
close to the CMB kinematic dipole and nearly orthogonal to the ecliptic plane,
respectively. If instead the real data are compared to those in simulations
taken with respect to planes for which the maximal mirror symmetry is generated
by chance, the confidence level decreases to 92.39 (76.65)%. But when the
effect in question translates into the anomalous alignment between normals to
planes of maximal mirror (anti)-symmetry and these natural axes mentioned. We
also introduce the representation of the above estimators in the harmonic
domain, confirming the results obtained in the pixel one. The symmetry anomaly
is shown to be almost entirely due to low multipoles, so it may have a
cosmological and even primordial origin. Contrary, the anti-symmetry one is
mainly due to intermediate multipoles that probably suggests its
non-fundamental nature. We have demonstrated that these anomalies are not
connected to the known issue of the low variance in WMAP observations and we
have checked that axially symmetric parts of these anomalies are small, so that
the axes are not the symmetry ones.Comment: 18 pages, 10 figures, 2 tables. Consideration and discussion
expanded, 5 figures and 1 table added, main conclusions unchange
Issues on Orientifolds: On the brane construction of gauge theories with SO(2n) global symmetry
We discuss issues related to orientifolds and the brane realization for gauge
theories with orthogonal and symplectic groups. We specifically discuss the
case of theories with (hidden) global SO(2n) symmetry, from three to six
dimensions. We analyze mirror symmetry for three dimensional N=4 gauge
theories, Brane Box Models and six-dimensional gauge theories. We also discuss
the issue of T-duality for D_n space-time singularities. Stuck D branes on ON^0
planes play an interesting role.Comment: 38 pages, 23 figures, uses bibtex and (provided) utphys.bs
Branes and 2d N=(2,2) gauge theories with orthogonal and symplectic groups
We construct two-dimensional N=(2,2) supersymmetric gauge theories with
orthogonal and symplectic groups using branes and orientifold planes in Type
IIA string theory. A number of puzzles regarding the construction, including
the effect of exchanging NS5-branes on an orientifold 2-plane, are resolved by
lifting the configurations to M theory. The low energy properties and
conjectured dualities of these theories are reproduced in the M-brane
description. A similar construction of N=(4,4) theories with orthogonal and
symplectic groups leads to new duality conjectures for these theories.Comment: 22 pages, 27 figures. Type IIA linking number computation corrected
in section 4.5. Clarified and expanded the discussion of N=(4,4) dualities in
section 5. References adde
Observation of a nodal chain with Dirac surface states in TiB2
Topological nodal-line semimetals (TNLSMs) are characterized by
symmetry-protected band crossings extending along one-dimensional lines in
momentum space. The nodal lines exhibit a variety of possible configurations,
such as nodal ring, nodal link, nodal chain, and nodal knot. Here, using
angle-resolved photoemission spectroscopy, we observe nodal rings in the
orthogonal kz = 0 and kx = 0 planes of the Brillouin zone in TiB2. The nodal
rings connect with each other on the intersecting line {\Gamma}-K of the
orthogonal planes, forming a remarkable nodal-chain structure. Furthermore, we
observe surface states (SSs) on the (001) cleaved surface, which are consistent
with the calculated SSs considering the contribution from both Ti and B
terminations. The calculated SSs have novel Dirac-cone-like band structures,
which are distinct from the usual drumhead SSs with a single flat band proposed
in other TNLSMs.Comment: 15 pages, 4 figure
Orientifolds and twisted boundary conditions
It is argued that the T-dual of a crosscap is a combination of an O+ and an
O- orientifold plane. Various theories with crosscaps and D-branes are
interpreted as gauge-theories on tori obeying twisted boundary conditions.
Their duals live on orientifolds where the various orientifold planes are of
different types. We derive how to read off the holonomies from the positions of
D-branes in the orientifold background. As an application we reconstruct some
results from a paper by Borel, Friedman and Morgan for gauge theories with
classical groups, compactified on a 2-- or 3--torus with twisted boundary
conditions.Comment: 23 pages, LaTeX, 2 eps figures; minor corrections, references adde
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