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

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

Disorder engineering and conductivity dome in ReS2 with electrolyte gating

Atomically thin rhenium disulphide (ReS2) is a member of the transition metal dichalcogenide (TMDC) family of materials characterized by weak interlayer coupling and a distorted 1T structure. Here, we report on the electrical transport study of mono- and multilayer ReS2 with polymer electrolyte gating. We find that the conductivity of monolayer ReS2 is completely suppressed at high carrier densities, an unusual feature unique to monolayers, making ReS2 the first example of such a material. While thicker flakes of ReS2 also exhibit a conductivity dome and an insulator-metal-insulator sequence, they do not show a complete conductivity suppression at high doping densities. Using dual-gated devices, we can distinguish the gate-induced doping from the electrostatic disorder induced by the polymer electrolyte itself. Theoretical calculations and a transport model indicate that the observed conductivity suppression can be explained by a combination of a narrow conduction band and Anderson localization due to electrolyte-induced disorder.Comment: Submitted versio

Differential behavioral profile induced by the injection of dipotassium chlorazepate within brain areas that project to the nucleus accumbens septi.

The effect of the agonism on g-aminobutyric acid (GABA) receptors was studied within medial prefrontal cortex (mPFC), amygdala (AMY) and ventral hipocampus (VH) in the plus-maze test in male rats bilaterally cannulated. These structures send glutamatergic projections to the nucleus accumbens septi (NAS), in which interaction and integration between these afferent pathways has been described. In a previous study of our group, blockade of glutamatergic transmission within NAS induced an anxiolytic like effect. Methods: Three rat groups received either saline or dipotassium chlorazepate (1 or 2 μg/1 μl solution) 15 min before testing. Time spent in the open arms (TSOA), time per entry (TPE), extreme arrivals (EA), open and closed arms entries (OAE, CAE) and relation- ship between open- and closed-arms quotient (OCAQ) were recorded. Results: In the AMY injected group TSOA, OAE and EA were increased by the higher doses of dipotassium chlorazepate (p < 0.01). In the mPFC, TPE was decreased by both doses (p < 0.05). Injection within ventral hippocampus (VH) decreased TSOA, OAE and OCAQ with lower doses (p < 0.05). When the three studied saline groups were compared, TSOA, OAE, EA and OCAQ were en- hanced in the VH group when compared to mPFC and AMY (p < 0.001). Insertion of inner canula (p < 0.001, p < 0.01, p < 0.01) and saline injection showed an increasing significant difference (p < 0.001 in all cases) with the action of guide cannula alone within VH in TSOA, OAE and EA. Conclusion: We conclude that the injection of dipotassium chlorazepate has a differential effect depending of the brain area, leading to facilitatory and inhibitory effects on anxiety processing.Fil: Llano López, Luis H.. Universidad Nacional de Cuyo; ArgentinaFil: Caif, Fernando. Universidad Nacional de Cuyo; ArgentinaFil: Fraile, Miriam. Universidad Nacional de Cuyo; ArgentinaFil: Tinnirello, Belén. Universidad Nacional de Cuyo; ArgentinaFil: Landa-Gargiulo, Adriana I.. Universidad Nacional de Cuyo; ArgentinaFil: Lafuente, José V.. Universidad del Pais Vasco. Departamento de Neurociencias; España;Fil: Baiardi, Gustavo Carlos. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Córdoba. Instituto de Investigaciones Biológicas y Tecnológicas; ArgentinaFil: Gargiulo, Pascual Angel. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Mendoza. Instituto de Medicina y Biología Experimental de Cuyo; Argentin

The role of disorder in the electronic and transport properties of monolayer and bilayer graphene

This thesis is devoted to the computational study of the electronic and transport properties of monolayer and bilayer graphene in the presence of disorder arising from both topological and point defects. Among the former, we study grain boundaries in monolayer graphene and stacking domain boundaries in bilayer graphene, whereas among the latter we study hydrogen atoms covalently bound on the graphene crystal lattice. The electronic spectrum of disordered graphene has been studied within a tight-binding framework, which has been coupled to the Landauer-Büttiker theory and Green¿s function techniques in order to have access to the properties of coherent transport of graphene charge carriers. We assess the low-energy equilibrium structures of defective graphene by a combination of ab initio density functional theory, classical potentials, and Monte Carlo methods. We study periodic grain boundaries in monolayer graphene and individuate two classes of defects with opposite effects in terms of scattering of low-energy charge carriers. One class, unexpectedly, is highly reflecting in the limit of low defect density, whereas another is highly transparent. Subsequently, we study disordered grain boundaries in order to predict the intrinsic conductance of realistic polycrystalline graphene samples. In two related works, conducted in collaboration with experimentalists, we identify the atomic structure of periodic grain boundaries imaged by scanning tunneling microscopy, and discuss the valley-filtering capabilities of a line defect of graphene that can be grown in a controllable manner. Next, we investigate the electronic transport of graphene with realistic hydrogen adsorbates, whose equilibrium configurations are obtained by means of Monte Carlo simulations. We find that the conductance of graphene dramatically increases upon formation of cluster adatoms, which we predict to happen spontaneously at room temperature. This is due to the non- resonant nature of a large fraction of hydrogen clusters in the room-temperature distribution, which we further elucidate by means of an analytically solvable model. Finally, we study the behavior, in terms of structural and electronic properties, of twisted bilayer graphene in the limit of zero twist angle. We find a critical angle below which the system arranges in a triangular superlattice of Bernal-stacking domains, separated by a hexagonal network of stacking domain boundaries. The presence of stacking domain boundaries is at the base of our interpretation of an experiment reporting oscillations in the electrical conductance of bilayer graphene subjected to mechanical indentation

Grain Boundaries in Graphene on SiC(0001ˉ\bar{1}) Substrate

Grain boundaries in epitaxial graphene on the SiC(000$\bar{1}$) substrate are studied using scanning tunneling microscopy and spectroscopy. All investigated small-angle grain boundaries show pronounced out-of-plane buckling induced by the strain fields of constituent dislocations. The ensemble of observations allows to determine the critical misorientation angle of buckling transition $\theta_c = 19 \pm~2^\circ$. Periodic structures are found among the flat large-angle grain boundaries. In particular, the observed $\theta = 33\pm2^\circ$ highly ordered grain boundary is assigned to the previously proposed lowest formation energy structural motif composed of a continuous chain of edge-sharing alternating pentagons and heptagons. This periodic grain boundary defect is predicted to exhibit strong valley filtering of charge carriers thus promising the practical realization of all-electric valleytronic devices

Commitment, Learning, and Alliance Performance: A Formal Analysis Using an Agent-Based Network Formation Model

Current theoretical arguments highlight a dilemma faced by actors who either adopt a weak or strong commitment strategy for managing their alliances and partnerships. Actors who pursue a weak commitment strategy|i.e. immediately abandon current partners when a more pro table alternative is presented|are more likely to identify the most rewarding alliances. On the other hand, actors who enact a strong commitment approach are more likely to take advantage of whatever opportunities can be found in existing partnerships. Using agent-based modeling, we show that actors who adopt a moderate commitment strategy overcome this dilemma and outperform actors who adopt either weak or strong commitment approaches. We also show that avoiding this dilemma rests on experiencing a related tradeo : moderately-committed actors sacri ce short-term performance for the superior knowledge and information that allows them to eventually do better

Comparative Effect between Antidepressants and D-phenylalanine, a Phenethylamine Precursor, in an Animal Model of Depression

A relevant role has been attributed to phenethylamine in depressive disorders. It has been measured in human urine and rat brain in pathological conditions and after drug administration. Furthermore, a clinical correlation has been proposed between urinary elimination and depressive symptoms. Furthermore, its metabolic predecessor, D-phenylalanine, has been used as an antidepressant drug in the treatment of depressive disorders. The use of this amino acid has been realized alone, or in combination with classical antidepressants. In the present study, we tried to characterize its behavioural profile comparing it with imipramine and fluoxetine. Antidepressant drugs have been studied using diverse animal models. We used here the Porsolt test, or Forced Swimming Test (FST), measuring times of climbing, swimming and resting. When a comparison was performed between groups in climbing behaviour, significant differences were observed between imipramine treated group and saline controls (p < 0.05), and imipramine versus fluoxetine and D-phenylalanine (p < 0.01). When swimming was evaluated, clear differences between D-phenylalanine and the other groups were observed (p < 0.001). Additionally, a significant difference was also observed between imipramine and fluoxetine (p < 0.01). When resting was evaluated, high differences between D-phenylalanine versus all other groups were shown (p < 0.001). Observed behavioural profile was according to serotonergic antidepressant drugs effects. It is supported by the fact that swimming behaviours were increased, and a correlative decrease in resting was also present. We conclude that D-phenylalanine showed higher antidepressant potency than other classical antidepressants, at least at the doses used