Flow equations for Hamiltonians: Contrasting different approaches by using a numerically solvable model

To contrast different generators for flow equations for Hamiltonians and to discuss the dependence of physical quantities on unitarily equivalent, but effectively different initial Hamiltonians, a numerically solvable model is considered which is structurally similar to impurity models. By this we discuss the question of optimization for the first time. A general truncation scheme is established that produces good results for the Hamiltonian flow as well as for the operator flow. Nevertheless, it is also pointed out that a systematic and feasible scheme for the operator flow on the operator level is missing. For this, an explicit analysis of the operator flow is given for the first time. We observe that truncation of the series of the observable flow after the linear or bilinear terms does not yield satisfactory results for the entire parameter regime as - especially close to resonances - even high orders of the exact series expansion carry considerable weight.Comment: 25 pages, 10 figure

Disorder and interaction effects in two dimensional graphene sheets

The interplay between different types of disorder and electron-electron interactions in graphene planes is studied by means of Renormalization Group techniques. The low temperature properties of the system are determined by fixed points where the strength of the interactions remains finite, as in one dimensional Luttinger liquids. These fixed points can be either stable (attractive), when the disorder is associated to topological defects in the lattice or to a random mass term, or unstable (repulsive) when the disorder is induced by impurities outside the graphene planes. In addition, we analyze mid-gap states which can arise near interfaces or vacancies.Comment: 4 pages, 3 figure

First Order Superfluid to Bose Metal Transition in Systems with Resonant Pairing

Systems showing resonant superfluidity, driven by an exchange coupling of strength $g$ between uncorrelated pairs of itinerant fermions and tightly bound ones, undergo a first order phase transition as $g$ increases beyond some critical value $g_c$. The superfluid phase for $g \leq g_c$ is characterized by a gap in the fermionic single particle spectrum and an acoustic sound-wave like collective mode of the bosonic resonating fermion pairs inside this gap. For $g>g_c$ this state gives way to a phase uncorrelated bosonic liquid with a $q^2$ spectrum.Comment: 5 pages, 3 figure

Entanglement at the boundary of spin chains near a quantum critical point and in systems with boundary critical points

We analyze the entanglement properties of spins (qubits) attached to the boundary of spin chains near quantum critical points, or to dissipative environments, near a boundary critical point, such as Kondo-like systems or the dissipative two level system. In the first case, we show that the properties of the entanglement are significantly different from those for bulk spins. The influence of the proximity to a transition is less marked at the boundary. In the second case, our results indicate that the entanglement changes abruptly at the point where coherent quantum oscillations cease to exist. The phase transition modifies significantly less the entanglement.Comment: 5 pages, 4 figure

Electron-electron interaction and charging effects in graphene quantum dots

We analyze charging effects in graphene quantum dots. Using a simple model, we show that, when the Fermi level is far from the neutrality point, charging effects lead to a shift in the electrostatic potential and the dot shows standard Coulomb blockade features. Near the neutrality point, surface states are partially occupied and the Coulomb interaction leads to a strongly correlated ground state which can be approximated by either a Wigner crystal or a Laughlin like wave function. The existence of strong correlations modify the transport properties which show non equilibrium effects, similar to those predicted for tunneling into other strongly correlated systems.Comment: Extended version accepted for publication at Phys. Rev.

Transboundary Watersheds: Changing Environments and Streams of Thought [abstract]

2 pages

Effect of Holstein phonons on the optical conductivity of gapped graphene

We study the optical conductivity of a doped graphene when a sublattice symmetry breaking is occurred in the presence of the electron-phonon interaction. Our study is based on the Kubo formula that is established upon the retarded self-energy. We report new features of both the real and imaginary parts of the quasiparticle self-energy in the presence of a gap opening. We find an analytical expression for the renormalized Fermi velocity of massive Dirac Fermions over broad ranges of electron densities, gap values and the electron-phonon coupling constants. Finally we conclude that the inclusion of the renormalized Fermi energy and the band gap effects are indeed crucial to get reasonable feature for the optical conductivity.Comment: 12 pages, 4 figures. To appear in Eur. Phys. J.

Au-delà de l'expression « Le privé est politique » : de jeunes mères-activistes chicanas et la lutte pour un intérêt commun

Alors que la maternité a parfois inspiré et guidé les mouvements collectifs de femmes, particulièrement ceux concernant l'éducation et les droits de l'enfant, les pressions sociales et matérielles exercées sur les mères, telles que les contraintes de temps, de budget, et de mobilité, ont plus souvent limité la participation de ces femmes dans des mouvements sociaux de plus grande ampleur, particulièrement les mouvements qui ne visent pas directement les droits de l'enfant. Les grossesses précoces et la maternité chez les jeunes femmes ont été largement étudiées dans la littérature scientifique qui voit ces phénomènes comme limitant les opportunités sociales et la participation citoyenne de ces femmes. Pourtant, dans la ville américaine de Tucson, dans l'Arizona, où les habitants se battent fermement pour les droits des Américains d'origine mexicaine aux États-Unis, un petit contingent de « mères-activistes » est en train de tracer une nouvelle voie d'autodétermination, d'action citoyenne et d'activisme social. Cet article, qui est le résultat d'une étude ethnographique menée sur une période de dix ans, révèle le l’efficacité politique de l'activisme maternel qui se manifeste comme un large mouvement avant-gardiste du vingt-et-unième siècle sur la frontière entre les États-Unis et le Mexique.While motherhood has sometimes inspired and guided women’s collective movements, particularly around children’s education and civil rights, the social and material pressures of mothering – including strains on time, budgets, and mobility – have more often circumscribed women’s participation in broader social movements, particularly those not oriented specifically toward children’s rights. Early childbearing and young motherhood, meanwhile, have been well-documented in the scholarly literature as delimiting young women’s social opportunities and civic participation. Yet in the southwestern U.S. city of Tucson, Arizona – a site quickly moving to the forefront of today’s battles over Mexican American rights and representation in the United States – a small and powerful contingent of “mother-activists” are charting new courses of collective self-determination, civic action, and social activism. This article, which derives from ethnography conducted across ten years, reveals the uniquely powerful form of mother-activist organizing and action that has characterized one vanguard of a broadly visible twenty-first century movement on the U.S.-Mexico border.La maternidad en algunas ocaciones ha inspirado y guiada movimientos colectivos de mujeres, particularlmente en cuanto a la educación infantil y derechos civiles. Sin embargo, las presiones materiales y sociales sobre la maternidad – incluyendo limitaciones de tiempo, presupuestos, y mobilidad social – han circunscrito la participación de las mujeres en movimientos sociales más amplios, particularmente aquellos que no se orientan hacia los derechos de los niños. El tener niños en edad temprana y el ser madre joven, mientras tanto, han sido ampliamente documentados en la literature academica como delimitador de las oportunidades sociales y participación civica en las madres jovenes. A pesar de esto, en la ciudad de Tucson en Arizona, EEUU – un lugar que rapidamente se ha tomado un primer plano sobre las batallas en cuanto a los derechos civiles de Mexico-Americanos y su representación en los EEUU – un contingente pequeño pero poderoso de “madres activistas” están trazando nuevos caminos hacia la autodeterminación colectiva, acción civica, y activismo social. Este artículo, el cual surge de diez años de investigación etnografa, nos revela un ejemplo unico y poderoso de organización y acción de madres que ha caracterizado la vanguardia de un movimiento visible y amplio en la frontera de EEUU con Mexico

Drift-induced modifications to the dynamical polarization of graphene

The response function of graphene is calculated in the presence of a constant current across the sample. For small drift velocities and finite chemical potential, analytic expressions are obtained and consequences on the plasmonic excitations are discussed. For general drift velocities and zero chemical potential, numerical results are presented and a plasmon gain region is identified that is related to interband transitions. © 2015 American Physical Societyopen0

Fluorescence quenching in graphene: a fundamental ruler and evidence for transverse plasmons

Graphene's fluorescence quenching is studied as a function of distance. Transverse decay channels, full retardation and graphene-field coupling to all orders are included, extending previous instantaneous results. For neutral graphene, a virtually exact analytical expression for the fluorescence yield is derived, valid for arbitrary distances and only based on the fine structure constant $\alpha$, the fluorescent wavelength $\lambda$, and distance $z$. Thus graphene's fluorescence quenching measurements provide a fundamental distance ruler. For doped graphene and at appropriate energies, the fluorescence yield at large distances is dominated by transverse plasmons, providing a platform for their detection.Comment: 6 pages, 2 figure