Klein tunneling through an oblique barrier in graphene ribbons

We study a transmission coefficient of graphene nanoribbons with a top gate which acts as an oblique barrier. Using a Green function method based on the Dirac-like equation, scattering among transverse modes due to the oblique barrier is taken into account numerically. In contrast to the 2-dimensional graphene sheet, we find that the pattern of transmission in graphene ribbons depends strongly on the electronic structure in the region of the barrier. Consequently, irregular structures in the transmission coefficient are predicted while perfect transmission is still calculated in the case of metallic graphene independently of angle and length of the oblique barrier

M Theory Fivebrane Interpretation for Strong Coupling Dynamics of SO(N_c) Gauge Theories

We study M theory fivebranes to understand the moduli space of vacua of N=1 supersymmetric SO(N_c) gauge theory with N_f flavors in four dimensions. We discuss how the various branches of this theory arise in the string/M theory brane configurations and compare our results with the ones obtained earlier by Intriligator and Seiberg in the context of field theory. In the M theory approach, we explain the various branches from the asymptotic position of semi-infinite D4 branes in the $w=x^8+i x^9$ direction, which is closely related to the eigenvalues of the meson matrix $M^{ij} = Q^{i}_a Q^{j}_a$ where $Q^{i}_a$ is a squark multiplet ( $i=1, \cdots, 2N_f$ and $a=1, \cdots, N_c$).In M theory, these branches are explained by observing a new phenomena which did not occur for the gauge groups SU(N_c) or Sp(N_c).Comment: 15 pages, Latex, English corrected, Version to Appear in Physics Letters

Branes, Geometry and N=1 Duality with Product Gauge Groups of SO and Sp

We study N=1 dualities in four dimensional supersymmetric gauge theories as the worldvolume theory of D4 branes with one compact direction in type IIA string theory. We generalize the previous work for SO(N_{c1}) x Sp(N_{c2}) with the superpotential W=Tr X^4 to the case of W= Tr X^4(k+1) in terms of brane configuration. We conjecture that the new dualities for the product gauge groups of SO(N_{c1}) x Sp(N_{c2}) x SO(N_{c3}), SO(N_{c1}) x Sp(N_{c2}) x SO(N_{c3}) x Sp(N_{c4}) and higher multiple product gauge groups can be obtained by reversing the ordering of NS5 branes and D6 branes while preserving the linking numbers. We also describe the above dualities in terms of wrapping D6 branes around 3 cycles of Calabi-Yau threefolds in type IIA string theory. The theory with adjoint matter can be regarded as taking multiple copies of NS5 brane in the configuration of brane or geometric approaches.Comment: 26 pages, latex, 6 figures, 4 tables, 1 appendix, v2: changed the sign of A^5 in page 4, corrected geometric configuration of Calabi-Yau 3-fold in the text, added O6 plane charge in page 7, and inserted the appendi