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

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

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

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

Anemia and Iron Deficiency Diagnosis, Management and Treatment in Chronic Kidney Disease - Consensus and Agreement through a Delphi Panel

Background: Anemia is a common complication of Chronic Kidney Disease (CKD), in which iron deficiency’s (ID) role is frequently underrated. In CKD, anemia has been associated with higher morbidity and lower quality of life. Nonetheless, reported treatment rates of anemia in CKD are low and guidelines’ variability and/or absence for its management and treatment may be preventing patients from receiving optimal treatment. Within this context, we aimed to assess the agreement level on anemia and iron deficiency diagnosis, management, and treatment in CKD patients, by Portuguese physicians in Nephrology, through a Delphi Panel. Methods: A group of seven medical experts in Nephrology and Transfusion Medicine was assembled, and a focus group was conducted, in which 28 statements were agreed upon. Then, a two-round Delphi Panel using a Likert scale was conducted online, inviting Portuguese Society of Nephrology associates to participate. Results: Answers were collected from 76 participants in Round 1 and consensus level was obtained for 1 statement, 57 (75%) respondents fully disagreeing on transfusing all patients with hemoglobin below 9 g/dl, regardless of the clinical situation. The remaining 27 statements were used in Round 2, none obtaining consensus level and 14 statements being categorized as qualified majority: 4 on diagnosis, 3 on disease management, and 7 on treatment. Discussion: Our study showed a lack of consensus on diagnosis, management, and treatment of anemia in CKD patients among the nephrology community in Portugal. Overall, our results illustrated the heterogeneity of national clinical practices in: laboratory parameters’ choice; cutoff values defining anemia and/or ID; parameter-based therapeutic decisions. Nonetheless, it was shown clear that patient’s individual characteristics, clinical settings, and the physician’s “clinical sense” seem to be considered to a further extent than the available guidelines. Future studies should be considered to develop recommendations that can be widely accepted.info:eu-repo/semantics/publishedVersio

Electron waves in chemically substituted graphene

We present exact analytical and numerical results for the electronic spectra and the Friedel oscillations around a substitutional impurity atom in a graphene lattice. A chemical dopant in graphene introduces changes in the on-site potential as well as in the hopping amplitude. We employ a T-matrix formalism and find that disorder in the hopping introduces additional interference terms around the impurity that can be understood in terms of bound, semi-bound, and unbound processes for the Dirac electrons. These interference effects can be detected by scanning tunneling microscopy.Comment: 4 pages, 7 figure