Transport reversal in a delayed feedback ratchet

Feedback flashing ratchets are thermal rectifiers that use information on the state of the system to operate the switching on and off of a periodic potential. They can induce directed transport even with symmetric potentials thanks to the asymmetry of the feedback protocol. We investigate here the dynamics of a feedback flashing ratchet when the asymmetry of the ratchet potential and of the feedback protocol favor transport in opposite directions. The introduction of a time delay in the control strategy allows one to nontrivially tune the relative relevance of the competing asymmetries leading to an interesting dynamics. We show that the competition between the asymmetries leads to a current reversal for large delays. For small ensembles of particles current reversal appears as the consequence of the emergence of an open-loop like dynamical regime, while for large ensembles of particles it can be understood as a consequence of the stabilization of quasiperiodic solutions. We also comment on the experimental feasibility of these feedback ratchets and their potential applications.Comment: LaTeX, 7 pages, 6 figure

How occasional backstepping can speed up a processive motor protein

Fueled by the hydrolysis of ATP, the motor protein kinesin literally walks on two legs along the biopolymer microtubule. The number of accidental backsteps that kinesin takes appears to be much larger than what one would expect given the amount of free energy that ATP hydrolysis makes available. This is puzzling as more than a billion years of natural selection should have optimized the motor protein for its speed and efficiency. But more backstepping allows for the production of more entropy. Such entropy production will make free energy available. With this additional free energy, the catalytic cycle of the kinesin can be speeded up. We show how measured backstep percentages represent an optimum at which maximal net forward speed is achieved.Comment: LaTeX, 5 pages, 3 figure

Baryonic Signatures in Large-Scale Structure

We investigate the consequences of a non-negligible baryon fraction for models of structure formation in Cold Dark Matter dominated cosmologies, emphasizing in particular the existence of oscillations in the present-day matter power spectrum. These oscillations are the remnants of acoustic oscillations in the photon-baryon fluid before last scattering. For acceptable values of the cosmological and baryon densities, the oscillations modulate the power by up to 10%, with a `period' in spatial wavenumber which is close to Delta k approximately 0.05/ Mpc. We study the effects of nonlinear evolution on these features, and show that they are erased for k > 0.2 h/ Mpc. At larger scales, the features evolve as expected from second-order perturbation theory: the visibility of the oscillations is affected only weakly by nonlinear evolution. No realistic CDM parameter combination is able to account for the claimed feature near k = 0.1 h/ Mpc in the APM power spectrum, or the excess power at 100 Mpc/h wavelengths quoted by several recent surveys. Thus baryonic oscillations are not predicted to dominate existing measurements of clustering. We examine several effects which may mask the features which are predicted, and conclude that future galaxy surveys may be able to detect the oscillatory features in the power spectrum provided baryons comprise more than 15% of the total density, but that it will be a technically challenging achievement.Comment: 16 pages, 13 Figures, to be published in MNRA

Submm-bright QSOs at z~2: signposts of co-evolution at high z

We have assembled a sample of 5 X-ray and submm-luminous z~2 QSOs which are therefore both growing their central black holes through accretion and forming stars copiously at a critical epoch. Hence, they are good laboratories to investigate the co-evolution of star formation and AGN. We have performed a preliminary analysis of the AGN and SF contributions to their UV-to-FIR SEDs, fitting them with simple direct (disk), reprocessed (torus) and star formation components. All three are required by the data and hence we confirm that these objects are undergoing strong star formation in their host galaxies at rates 500-2000 Msun/y. Estimates of their covering factors are between about 30 and 90%. In the future, we will assess the dependence of these results on the particular models used for the components and relate their observed properties to the intrinsice of the central engine and the SF material, as well as their relevance for AGN-galaxy coevolution.Comment: 6 pages, 2 figures, contributed talk to "Nuclei of Seyfert galaxies and QSOs - Central engine & conditions of star formation" November 6-8, 2012. MPIfR, Bonn, Germany. Po

Threshold feedback control for a collective flashing ratchet: threshold dependence

We study the threshold control protocol for a collective flashing ratchet. In particular, we analyze the dependence of the current on the values of the thresholds. We have found analytical expressions for the small threshold dependence both for the few and for the many particle case. For few particles the current is a decreasing function of the thresholds, thus, the maximum current is reached for zero thresholds. In contrast, for many particles the optimal thresholds have a nonzero finite value. We have numerically checked the relation that allows to obtain the optimal thresholds for an infinite number of particles from the optimal period of the periodic protocol. These optimal thresholds for an infinite number of particles give good results for many particles. In addition, they also give good results for few particles due to the smooth dependence of the current up to these threshold values.Comment: LaTeX, 10 pages, 7 figures, improved version to appear in Phys. Rev.

Pinned Low Energy Electronic Excitation in Metal Exchanged Vanadium Oxide Nanoscrolls

We measured the optical properties of mixed valent vanadium oxide nanoscrolls and their metal exchanged derivatives in order to investigate the charge dynamics in these compounds. In contrast to the prediction of a metallic state for the metal exchanged derivatives within a rigid band model, we find that the injected charges in Mn$^{2+}$ exchanged vanadium oxide nanoscrolls are pinned. A low-energy electronic excitation associated with the pinned carriers appears in the far infrared and persists at low temperature, suggesting that the nanoscrolls are weak metals in their bulk form, dominated by inhomogeneous charge disproportionation and Madelung energy effects.Comment: 4 figure

Submm-bright X-ray absorbed QSOs at z~2: insights into the co-evolution of AGN and star-formation

We have assembled a sample of 5 X-ray-absorbed and submm-luminous type 1 QSOs at $z \sim 2$ which are simultaneously growing their central black holes through accretion and forming stars copiously. We present here the analysis of their rest-frame UV to submm Spectral Energy Distributions (SEDs), including new Herschel data. Both AGN (direct and reprocessed) and Star Formation (SF) emission are needed to model their SEDs. From the SEDs and their UV-optical spectra we have estimated the masses of their black holes $M_{BH}\sim 10^{9}-10^{10}\,M_{\odot}$, their intrinsic AGN bolometric luminosities $L_{BOL}\sim(0.8 - 20)\times 10^{13} L_{\odot}$, Eddington ratios $L_{BOL}/L_{Edd}\sim 0.1 - 1.1$ and bolometric corrections $L_{BOL}/L_{X,2-10}\sim 30 - 500$. These values are common among optically and X-ray-selected type 1 QSOs (except for RX~J1249), except for the bolometric corrections, which are higher. These objects show very high far-infrared luminosities $L_{FIR}\sim$ (2 - 8)$\times10^{12}\,M_{\odot}$ and Star Formation Rates SFR$\sim 1000 M_{\odot}/$y. From their $L_{FIR}$ and the shape of their FIR-submm emission we have estimated star-forming dust masses of $M_{DUST}\sim 10^9\,M_\odot$. We have found evidence of a tentative correlation between the gas column densities of the ionized absorbers detected in X-ray (N$_{H_{ion}}$) and $SFR$. Our computed black hole masses are amongst the most massive known.Comment: Accepted for publication in MNRAS, December 22, 2014, 17 pages, 5 figure

Closed-loop control strategy with improved current for a flashing ratchet

We show how to switch on and off the ratchet potential of a collective Brownian motor, depending only on the position of the particles, in order to attain a current higher than or at least equal to that induced by any periodic flashing. Maximization of instant velocity turns out to be the optimal protocol for one particle but is nevertheless defeated by a periodic switching when a sufficiently large ensemble of particles is considered. The protocol presented in this article, although not the optimal one, yields approximately the same current as the optimal protocol for one particle and as the optimal periodic switching for an infinite number of them.Comment: 4 pages, 4 figure

Information and maximum power in a feedback controlled Brownian ratchet

Closed-loop or feedback controlled ratchets are Brownian motors that operate using information about the state of the system. For these ratchets, we compute the power output and we investigate its relation with the information used in the feedback control. We get analytical expressions for one-particle and few-particle flashing ratchets, and we find that the maximum power output has an upper bound proportional to the information. In addition, we show that the increase of the power output that results from changing the optimal open-loop ratchet to a closed-loop ratchet also has an upper bound that is linear in the information.Comment: LaTeX, 6 pages, 4 figures, improved version to appear in Eur. Phys. J.