The coincidence problem in f(R)f(R) gravity models

To explore possibilities of avoiding coincidence problem in $f(R)$ gravity we consider models in Einstein conformal frame which are equivalent to Einstein gravity with a minimally coupled scalar field. As the conformal factor determines the coupling term and hence the interaction between matter and dark energy, the function $f(R)$ can in principle be determined by choosing an appropriate function for the deceleration parameter only. Possible behavior of $f(R)$ to avoid coincidence problem are investigated in two such cases.Comment: 19 pages, 9 figures, Accepted for publication in General Relativity and Gravitation. arXiv admin note: substantial text overlap with arXiv:0807.1640 by other author

Spin-orbit interaction induced spin selective transmission through a multi-terminal mesoscopic ring

Spin dependent transport in a multi-terminal mesoscopic ring is investigated in presence of Rashba and Dresselhaus spin-orbit interactions. Within a tight-binding framework we use a general spin density matrix formalism to evaluate all three components ($P_x$, $P_y$ and $P_z$) of the polarization vector associated with the charge current through the outgoing leads. It explores the dynamics of the spin polarization vector of current propagating through the system subjected to the Rashba and/or the Dresselhaus spin-orbit couplings. The sensitivity of the polarization components on the electrode-ring interface geometry is discussed in detail. Our present analysis provides an understanding of the coupled spin and electron transport in mesoscopic bridge systems.Comment: 11 pages, 14 figure