Structural and electronic transformation in low-angle twisted bilayer graphene

Experiments on bilayer graphene unveiled a fascinating realization of stacking disorder where triangular domains with well-defined Bernal stacking are delimited by a hexagonal network of strain solitons. Here we show by means of numerical simulations that this is a consequence of a structural transformation of the moir\'{e} pattern inherent of twisted bilayer graphene taking place at twist angles $\theta$ below a crossover angle $\theta^{\star}=1.2^{\circ}$. The transformation is governed by the interplay between the interlayer van der Waals interaction and the in-plane strain field, and is revealed by a change in the functional form of the twist energy density. This transformation unveils an electronic regime characteristic of vanishing twist angles in which the charge density converges, though not uniformly, to that of ideal bilayer graphene with Bernal stacking. On the other hand, the stacking domain boundaries form a distinct charge density pattern that provides the STM signature of the hexagonal solitonic network.Comment: published version with supplementary materia

Topological Aspects of Charge-Carrier Transmission across Grain Boundaries in Graphene

We systematically investigate the transmission of charge carriers across the grain-boundary defects in polycrystalline graphene by means of the Landauer-B\"uttiker formalism within the tight-binding approximation. Calculations reveal a strong suppression of transmission at low energies upon decreasing the density of dislocations with the smallest Burger's vector ${\mathbf b}=(1,0)$. The observed transport anomaly is explained from the point of view of back-scattering due to localized states of topological origin. These states are related to the gauge field associated with all dislocations characterized by ${\mathbf b}=(n,m)$ with $n-m \neq 3q$ ($q \in \mathbb{Z}$). Our work identifies an important source of charge-carrier scattering caused by topological defects present in large-area graphene samples produced by chemical vapor deposition.Comment: 5 pages, 4 figure

Non-perturbative effects on seven-brane Yukawa couplings

We analyze non-perturbative corrections to the superpotential of seven-brane gauge theories on type IIB and F-theory warped Calabi-Yau compactifications. We show in particular that such corrections modify the holomorphic Yukawa couplings by an exponentially suppressed contribution, generically solving the Yukawa rank-one problem present in F-theory local models. We provide explicit expressions for the non-perturbative correction to the seven-brane superpotential, and check that it is related to a non-commutative deformation to the tree-level superpotential via the Seiberg-Witten map.Comment: 4 pages. v2: minor changes, footnotes and citations added. Version to be published in PR