Robust ultrafast currents in molecular wires through Stark shifts

A novel way to induce ultrafast currents in molecular wires using two incident laser frequencies, $\omega$ and $2\omega$, is demonstrated. The mechanism relies on Stark shifts, instead of photon absorption, to transfer population to the excited states and exploits the temporal profile of the field to generate phase controllable transport. Calculations in a \emph{trans}-polyacetylene oligomer coupled to metallic leads indicate that the mechanism is highly efficient and robust to ultrafast electronic dephasing processes induced by vibronic couplings.Comment: 4 pages, 2 figures, accepted to Physical Review Letter

Explosions and Outflows during Galaxy Formation

We consider an explosion at the center of a halo which forms at the intersection of filaments inside a cosmological pancake, a convenient test-bed model for galaxy formation. ASPH/P3M simulations reveal that such explosions are anisotropic. The energy and metals are channeled into the low density regions, away from the pancake. The pancake remains essentially undisturbed, even if the explosion is strong enough to blow away all the gas located inside the halo and reheat the IGM surrounding the pancake. Infall quickly replenishes this ejected gas and gradually restores the gas fraction as the halo continues to grow. Estimates of the collapse epoch and SN energy-release for galaxies of different mass in the CDM model can relate these results to scale-dependent questions of blow-out and blow-away and their implication for early IGM heating and metal enrichment and the creation of gas-poor dwarf galaxies.Comment: To appear in "The 20th Texas Symposium on Relativistic Astrophysics", eds. H. Martel and J.C. Wheeler, AIP, in press (2001) (3 pages, 2 figures

Formation and Evolution of Self-Interacting Dark Matter Halos

We study the formation and evolution of self-interacting dark matter (SIDM) halos. We find analytical, fully cosmological similarity solutions taking account of the collisional interaction of SIDM particles. This interaction results in a thermal conductivity that heats the halo core and flattens its density profile. These similarity solutions are relevant to galactic and cluster halo formation in the CDM model. We assume an initial mass profile dM/M M^{-eps}, as in the familiar secondary infall model. If eps=1/6, SIDM halos will evolve self-similarly, with a cold, supersonic infall terminated by a strong accretion shock. Different solutions arise for different values of the collisionality parameter, Q= sigma rho_b r_s, where sigma is the scattering cross section, rho_b is the cosmic mean density, and r_s is the shock radius. For all these solutions, a flat-density, isothermal core is present which grows in size as a fixed fraction of r_s. We find two different regimes for these solutions: 1) for Q \leq Q_{th}, the core density decreases and core size increases as Q increases; 2) for Q \geq Q_{th}, the core density increases and core size decreases as Q increases. Our similarity solutions are in agreement with previous N-body simulations of SIDM halos, which correspond to the low-Q regime, if Q=[8.4e-4 - 4.9e-2]Q_{th} (low-Q), or sigma=[0.56-5.6]cm^2/g. As Q=\infty, our similarity solution aquires a central density cusp, in agreement with some simulation results which used an ordinary collisional fluid to approximate the effects of SIDM collisionality. When Q=[18.6-231]Q_{th} or sigma=[1.2e4 - 2.71e4]cm^2/g, for which we find flat-density cores comparable to those of the observationally acceptable low-Q solutions, has not previously been identified. Further study of this regime is warranted.Comment: 7 pages, 5 figures, talk presented at the Second Korean Astrophysics Workshop (APCTP Workshop) on Formation and Interaction of Galaxies, published in a special issue of Journal of Korean Astronomical Society, ed. H. M Le

A Convenient Set of Comoving Cosmological Variables and Their Application

We present a set of cosmological variables, called "supercomoving variables," which are particularly useful for describing the gas dynamics of cosmic structure formation. For ideal gas with gamma=5/3, the supercomoving position, velocity, density, temperature, and pressure are constant in time in a uniform, isotropic, adiabatically expanding universe. Expressed in terms of these supercomoving variables, the cosmological fluid conservation equations and the Poisson equation closely resemble their noncosmological counterparts. This makes it possible to generalize noncosmological results and techniques to cosmological problems, for a wide range of cosmological models. These variables were initially introduced by Shandarin for matter-dominated models only. We generalize supercomoving variables to models with a uniform component corresponding to a nonzero cosmological constant, domain walls, cosmic strings, a nonclumping form of nonrelativistic matter (e.g. massive nettrinos), or radiation. Each model is characterized by the value of the density parameter Omega0 of the nonrelativistic matter component in which density fluctuation is possible, and the density parameter OmegaX of the additional, nonclumping component. For each type of nonclumping background, we identify FAMILIES within which different values of Omega0 and OmegaX lead to fluid equations and solutions in supercomoving variables which are independent of Omega0 and OmegaX. We also include the effects of heating, radiative cooling, thermal conduction, viscosity, and magnetic fields. As an illustration, we describe 3 familiar cosmological problems in supercomoving variables: the growth of linear density fluctuations, the nonlinear collapse of a 1D plane-wave density fluctuation leading to pancake formation, and the Zel'dovich approximation.Comment: 38 pages (AAS latex) + 2 figures (postscript) combined in one gzip-ed tar file. Identical to original posted version, except for addition of 2 references. Monthly Notices of the R.A.S., in pres