50,981 research outputs found
Effective theories and constraints on new phyhsics
Anomalous moments of the top quark arises from one loop corrections to the
vertices and . We study these anomalous couplings
in different frameworks: effective theories, Standard Model and 2HDM. We use
available experimental results in order to get bounds on these anomalous
couplings.Comment: 8 pages, 2 figures, talk presented by R. Martinez at the X Mexican
School of Particles and Fields, Playa del Carmen, Mexico, 200
Quantum spin Hall phase in multilayer graphene
The so called quantum spin Hall phase is a topologically non trivial
insulating phase that is predicted to appear in graphene and graphene-like
systems. In this work we address the question of whether this topological
property persists in multilayered systems. We consider two situations: purely
multilayer graphene and heterostructures where graphene is encapsulated by
trivial insulators with a strong spin-orbit coupling. We use a four orbital
tight-binding model that includes the full atomic spin-orbit coupling and we
calculate the topological invariant of the bulk states as well as the
edge states of semi-infinite crystals with armchair termination. For
homogeneous multilayers we find that even when the spin-orbit interaction opens
a gap for all the possible stackings, only those with odd number of layers host
gapless edge states while those with even number of layers are trivial
insulators. For the heterostructures where graphene is encapsulated by trivial
insulators, it turns out that the interlayer coupling is able to induce a
topological gap whose size is controlled by the spin-orbit coupling of the
encapsulating materials, indicating that the quantum spin Hall phase can be
induced by proximity to trivial insulators.Comment: 7 pages, 6 figure
Quasar-galaxy associations revisited
Gravitational lensing predicts an enhancement of the density of bright,
distant QSOs around foreground galaxies. We measure this QSO-galaxy correlation
w_qg for two complete samples of radio-loud quasars, the southern 1Jy and
Half-Jansky samples. The existence of a positive correlation between z~1
quasars and z~0.15 galaxies is confirmed at a p=99.0% significance level
(>99.9%) if previous measurements on the northern hemisphere are included). A
comparison with the results obtained for incomplete quasar catalogs (e.g. the
Veron-Cetty and Veron compilation) suggests the existence of an `identification
bias', which spuriously increases the estimated amplitude of the quasar-galaxy
correlation for incomplete samples. This effect may explain many of the
surprisingly strong quasar-galaxy associations found in the literature.
Nevertheless, the value of w_qg that we measure in our complete catalogs is
still considerably higher than the predictions from weak lensing. We consider
two effects which could help to explain this discrepancy: galactic dust
extinction and strong lensing.Comment: 9 pages, 6 figures, MNRAS accepte
Real space mapping of topological invariants using artificial neural networks
Topological invariants allow to characterize Hamiltonians, predicting the
existence of topologically protected in-gap modes. Those invariants can be
computed by tracing the evolution of the occupied wavefunctions under twisted
boundary conditions. However, those procedures do not allow to calculate a
topological invariant by evaluating the system locally, and thus require
information about the wavefunctions in the whole system. Here we show that
artificial neural networks can be trained to identify the topological order by
evaluating a local projection of the density matrix. We demonstrate this for
two different models, a 1-D topological superconductor and a 2-D quantum
anomalous Hall state, both with spatially modulated parameters. Our neural
network correctly identifies the different topological domains in real space,
predicting the location of in-gap states. By combining a neural network with a
calculation of the electronic states that uses the Kernel Polynomial Method, we
show that the local evaluation of the invariant can be carried out by
evaluating a local quantity, in particular for systems without translational
symmetry consisting of tens of thousands of atoms. Our results show that
supervised learning is an efficient methodology to characterize the local
topology of a system.Comment: 9 pages, 6 figure
R-Band Imaging of Fields Around 1<z<2 Radiogalaxies
We have taken deep -band images of fields around five radiogalaxies:
0956+47, 1217+36, 3C256, 3C324 and 3C294 with . 0956+47 is found to
show a double nucleus. Our data on 1217+36 suggest the revision of its
classification as a radiogalaxy. We found a statistically significant excess of
bright () galaxies on scales of 2 arcmin around the radiogalaxies
(which have ) in our sample. The excess has been determined
empirically to be at level. It is remarkable that this excess
is not present for galaxies within the same area, suggesting that
the excess is not physically associated to the galaxies but due to intervening
groups and then related to gravitational lensing.Comment: 20 pages, uuencoded compressed PostScript including tables. Figures
available upon request. To appear in the March 1995 issue of The Astronomical
Journa
- …