Nonlinear dynamics in superlattices driven by high frequency ac-fields

We investigate the dynamical processes taking place in nanodevices driven by high-frequency electromagnetic fields. We want to elucidate the role of different mechanisms that could lead to loss of quantum coherence. Our results show how the dephasing effects of disorder that destroy after some periods coherent oscillations, such as Rabi oscillations, can be overestimated if we do not consider the electron-electron interactions that can reduce dramatically the decoherence effects of the structural imperfections. Experimental conditions for the observation of the predicted effects are discussed.Comment: REVTEX (8 pages) and 4 figures (Postscript

Slip-velocity of large neutrally-buoyant particles in turbulent flows

We discuss possible definitions for a stochastic slip velocity that describes the relative motion between large particles and a turbulent flow. This definition is necessary because the slip velocity used in the standard drag model fails when particle size falls within the inertial subrange of ambient turbulence. We propose two definitions, selected in part due to their simplicity: they do not require filtration of the fluid phase velocity field, nor do they require the construction of conditional averages on particle locations. A key benefit of this simplicity is that the stochastic slip velocity proposed here can be calculated equally well for laboratory, field, and numerical experiments. The stochastic slip velocity allows the definition of a Reynolds number that should indicate whether large particles in turbulent flow behave (a) as passive tracers; (b) as a linear filter of the velocity field; or (c) as a nonlinear filter to the velocity field. We calculate the value of stochastic slip for ellipsoidal and spherical particles (the size of the Taylor microscale) measured in laboratory homogeneous isotropic turbulence. The resulting Reynolds number is significantly higher than 1 for both particle shapes, and velocity statistics show that particle motion is a complex non-linear function of the fluid velocity. We further investigate the nonlinear relationship by comparing the probability distribution of fluctuating velocities for particle and fluid phases

Critical point for the CAF-F phase transition at charge neutrality in bilayer graphene

We report on magneto-transport measurements up to 30 T performed on a bilayer graphene Hall bar, enclosed by two thin hexagonal boron nitride flakes. Our high mobility sample exhibits an insulating state at neutrality point which evolves into a metallic phase when a strong in-plane field is applied, as expected for a transition from a canted antiferromagnetic to a ferromagnetic spin ordered phase. For the first time we individuate a temperature-independent crossing in the four-terminal resistance as a function of the total magnetic field, corresponding to the critical point of the transition. We show that the critical field scales linearly with the perpendicular component of the field, as expected from the underlying competition between the Zeeman energy and interaction-induced anisotropies. A clear scaling of the resistance is also found and an universal behavior is proposed in the vicinity of the transition

Circularly Polarized Resonant Rayleigh Scattering and Skyrmions in the ν\nu = 1 Quantum Hall Ferromagnet

We use the circularly polarized resonant Rayleigh scattering (RRS) to study the quantum Hall ferromagnet at $\nu$ = 1. At this filling factor we observe a right handed copolarized RRS which probes the Skyrmion spin texture of the electrons in the photoexcited grounds state. The resonant scattering is not present in the left handed copolarization, and this can be related to the correlation between Skymionic effects, screening and spin wave excitations. These results evidence that RRS is a valid method for the study of the spin texture of the quantum Hall states

The long shadow of slavery: the persistence of slave owners in Southern law-making

This paper documents the persistence of the Southern slave owning elite in political power after the end of the American Civil War. We draw on a database of Texan state legislators between 1860 and 1900 and link them to their or their ancestors’ slaveholdings in 1860. We then show that former slave owners made up more than half of nearly each legislature’s members until the late 1890s. Legislators with slave owning backgrounds differ systematically from those without, being more likely to represent the Democratic party and more likely to work in an agricultural occupation. Regional characteristics matter for this persistence, as counties with higher soil suitability for growing cotton on average elect more former slave owners

The long shadow of slavery: the persistence of slave owners in southern lawmaking

This paper documents the persistence of Southern slave owners in political power after the American Civil War. Using data from Texas, we show that former slave owners made up more than half of all state legislators until the late 1890s. Legislators with slave-owning backgrounds were more likely to be Democrats and voted more conservatively even conditional on party membership. A county's propensity to elect former slave owners was positively correlated with cotton production, but negatively with Reconstruction-era progress of blacks. Counties that elected more slave owners also displayed worse educational outcomes for blacks in the early twentieth century

Experimental evidence of delocalized states in random dimer superlattices

We study the electronic properties of GaAs-AlGaAs superlattices with intentional correlated disorder by means of photoluminescence and vertical dc resistance. The results are compared to those obtained in ordered and uncorrelated disordered superlattices. We report the first experimental evidence that spatial correlations inhibit localization of states in disordered low-dimensional systems, as our previous theoretical calculations suggested, in contrast to the earlier belief that all eigenstates are localized.Comment: 4 pages, 5 figures. Physical Review Letters (in press

A reinforcement learning framework for optimal operation and maintenance of power grids

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