Bilayer graphene: gap tunability and edge properties

Bilayer graphene -- two coupled single graphene layers stacked as in graphite -- provides the only known semiconductor with a gap that can be tuned externally through electric field effect. Here we use a tight binding approach to study how the gap changes with the applied electric field. Within a parallel plate capacitor model and taking into account screening of the external field, we describe real back gated and/or chemically doped bilayer devices. We show that a gap between zero and midinfrared energies can be induced and externally tuned in these devices, making bilayer graphene very appealing from the point of view of applications. However, applications to nanotechnology require careful treatment of the effect of sample boundaries. This being particularly true in graphene, where the presence of edge states at zero energy -- the Fermi level of the undoped system -- has been extensively reported. Here we show that also bilayer graphene supports surface states localized at zigzag edges. The presence of two layers, however, allows for a new type of edge state which shows an enhanced penetration into the bulk and gives rise to band crossing phenomenon inside the gap of the biased bilayer system.Comment: 8 pages, 3 fugures, Proceedings of the International Conference on Theoretical Physics: Dubna-Nano200

Localized states at zigzag edges of bilayer graphene

We report the existence of zero energy surface states localized at zigzag edges of bilayer graphene. Working within the tight-binding approximation we derive the analytic solution for the wavefunctions of these peculiar surface states. It is shown that zero energy edge states in bilayer graphene can be divided into two families: (i) states living only on a single plane, equivalent to surface states in monolayer graphene; (ii) states with finite amplitude over the two layers, with an enhanced penetration into the bulk. The bulk and surface (edge) electronic structure of bilayer graphene nanoribbons is also studied, both in the absence and in the presence of a bias voltage between planes.Comment: 4 pages, 5 figure

Comportamiento ético de las empresas y corrupción en el sector público – un análisis de clústers de países a nivel mundial

In countries where corruption is high, there is less incentive for corporate organizations to behave ethically. Ethics, either in the private and public sectors, can be related to the level of economic development, economic freedom, and the quality of governance. The objective of this paper is to analyze the relationship between corporate ethics and corruption in the public sector, to group countries according to these indicators, and investigate the characteristics of these clusters. A hierarchical followed by a K-means cluster analysis was performed to group 127 countries. Non-parametric and post-hoc tests were used to examine the significant differences among clusters in some features. The correlation between the ethical behavior of firms and corruption in the public sector was strong and significant. The cluster analysis led to three significantly different clusters, one formed by countries with high levels of corporate ethics and low corruption and it is composed only by countries with high-income, another with medium levels and finally a group of countries with low levels of corporate ethics and high levels of corruption. The results also suggest significant differences among the clusters in several economic, institutional, and development indicators. Improving these indicators potentially lead to high standards of ethics in the private and public sectors.En los países donde la corrupción es elevada, hay menos incentivos para que las organizaciones empresariales se comporten éticamente. La ética, tanto en el sector privado como en el público, puede estar relacionada con el nivel de desarrollo económico, la libertad económica y la calidad de la gobernanza. El objetivo de este trabajo es analizar la relación entre la ética empresarial y la corrupción en el sector público, agrupar a los países según estos indicadores e investigar las características de estas clústers. Se realizó un análisis jerárquico seguido de un análisis de clústers de K-means para agrupar 127 países. Se utilizaron pruebas no paramétricas y post hoc para examinar las diferencias significativas entre clústers en algunas características. La correlación entre el comportamiento ético de las empresas y la corrupción en el sector público fue fuerte y significativa. El análisis de clústers dio lugar a tres clústers significativamente diferentes, uno formado por países con altos niveles de ética empresarial y baja corrupción y compuesto únicamente por países con altos rendimientos, otro con niveles medios y, por último, un grupo de países con bajos niveles de ética empresarial y altos niveles de corrupción. Los resultados también sugieren diferencias significativas entre los clústers en varios indicadores económicos, institucionales y de desarrollo. La mejora de estos indicadores puede conducir potencialmente a altos niveles de ética en los sectores público y privado.info:eu-repo/semantics/publishedVersio

Disorder Induced Localized States in Graphene

We consider the electronic structure near vacancies in the half-filled honeycomb lattice. It is shown that vacancies induce the formation of localized states. When particle-hole symmetry is broken, localized states become resonances close to the Fermi level. We also study the problem of a finite density of vacancies, obtaining the electronic density of states, and discussing the issue of electronic localization in these systems. Our results also have relevance for the problem of disorder in d-wave superconductors.Comment: Replaced with published version. 4 pages, 4 figures. Fig. 1 was revise

Phenomenological study of the electronic transport coefficients of graphene

Using a semi-classical approach and input from experiments on the conductivity of graphene, we determine the electronic density dependence of the electronic transport coefficients -- conductivity, thermal conductivity and thermopower -- of doped graphene. Also the electronic density dependence of the optical conductivity is obtained. Finally we show that the classical Hall effect (low field) in graphene has the same form as for the independent electron case, characterized by a parabolic dispersion, as long as the relaxation time is proportional to the momentum.Comment: 4 pages, 1 figur

Modeling disorder in graphene

We present a study of different models of local disorder in graphene. Our focus is on the main effects that vacancies -- random, compensated and uncompensated --, local impurities and substitutional impurities bring into the electronic structure of graphene. By exploring these types of disorder and their connections, we show that they introduce dramatic changes in the low energy spectrum of graphene, viz. localized zero modes, strong resonances, gap and pseudogap behavior, and non-dispersive midgap zero modes.Comment: 16 pages, lower resolution figure

String Theory Effects on Five-Dimensional Black Hole Physics

We review recent developments in understanding quantum/string corrections to BPS black holes and strings in five-dimensional supergravity. These objects are solutions to the effective action obtained from M-theory compactified on a Calabi-Yau threefold, including the one-loop corrections determined by anomaly cancellation and supersymmetry. We introduce the off-shell formulation of this theory obtained through the conformal supergravity method and review the methods for investigating supersymmetric solutions. This leads to quantum/string corrected attractor geometries, as well as asymptotically flat black strings and spinning black holes. With these solutions in hand, we compare our results with analogous studies in four-dimensional string-corrected supergravity, emphasizing the distinctions between the four and five dimensional theories.Comment: 85 pages; uses ws-ijmpa-mod.cls article class; Invited review for IJMP

5D Black Holes and Strings with Higher Derivatives

We find asymptotically flat black hole and string solutions to 5D supergravity in the presence of higher derivative terms. In some cases, including the fundamental heterotic string solution, the higher derivative terms smooth out naked singularities into regular geometries carrying zero entropy. We also compute corrections to the entropy of 5D Calabi-Yau black holes, and discuss the relation to previous results.Comment: 33 pages, 2 figs., harvmac; v2: typos corrected, references added v3: refs correcte

Extremal black holes in D=5: SUSY vs. Gauss-Bonnet corrections

We analyse near-horizon solutions and compare the results for the black hole entropy of five-dimensional spherically symmetric extremal black holes when the N=2 SUGRA actions are supplied with two different types of higher-order corrections: (1) supersymmetric completion of gravitational Chern-Simons term, and (2) Gauss-Bonnet term. We show that for large BPS black holes lowest order \alpha' corrections to the entropy are the same, but for non-BPS are generally different. We pay special attention to the class of prepotentials connected with K3\times T^2 and T^6 compactifications. For supersymmetric correction we find beside BPS also a set of non-BPS solutions. In the particular case of T^6 compactification (equivalent to the heterotic string on $T^4\times S^1$) we find the (almost) complete set of solutions (with exception of some non-BPS small black holes), and show that entropy of small black holes is different from statistical entropy obtained by counting of microstates of heterotic string theory. We also find complete set of solutions for K3\times T^2 and T^6 case when correction is given by Gauss-Bonnet term. Contrary to four-dimensional case, obtained entropy is different from the one with supersymmetric correction. We show that in Gauss-Bonnet case entropy of small ``BPS'' black holes agrees with microscopic entropy in the known cases.Comment: 28 pages; minor changes, version to appear in JHE