Symmetry classification of energy bands in graphene and silicene

We present the results of the symmetry classification of the electron energy bands in graphene and silicene using group theory algebra and the tight--binding approximation. The analysis is performed both in the absence and in the presence of the spin-orbit coupling. We also discuss the bands merging in the Brillouin zone symmetry points and the conditions for the latter to become Dirac points.Comment: LaTeX, 6 pages, 2 eps Figures. A Figure and a citation were added. Accepted for publication in Graphen

No-go for Partially Massless Spin-2 Yang-Mills

There are various no-go results forbidding self-interactions for a single partially massless spin-2 field. Given the photon-like structure of the linear partially massless field, it is natural to ask whether a multiplet of such fields can interact under an internal Yang-Mills like extension of the partially massless symmetry. We give two arguments that such a partially massless Yang-Mills theory does not exist. The first is that there is no Yang-Mills like non-abelian deformation of the partially massless symmetry, and the second is that cubic vertices with the appropriate structure constants do not exist.Comment: 18 pages. v2 small corrections and ref

Up Quark Masses from Down Quark Masses

The quark and charged lepton masses and the angles and phase of the CKM mixing matrix are nicely reproduced in a model which assumes SU(3)xSU(3) flavour symmetry broken by the v.e.v.'s of fields in its bi-fundamental representation. The relations among the quark mass eigenvalues, m_u/m_c \approx m_c/m_t \approx m^2_d/m^2_s \approx m^2_s/m^2_b \approx \Lambda^2_{GUT}/M^2_{Pl}, follow from the broken flavour symmetry. Large tan(beta) is required which also provides the best fits to data for the obtained textures. Lepton-quark grandunification with a field that breaks both SU(5) and the flavour group correctly extends the predictions to the charged lepton masses. The seesaw extension of the model to the neutrino sector predicts a Majorana mass matrix quadratically hierarchical as compared to the neutrino Dirac mass matrix, naturally yielding large mixings and low mass hierarchy for neutrinos.Comment: 11 pages, 1 figure. Published version: model improved, references adde