Novel substrates for Helium adsorption: Graphane and Graphene-Fluoride

The discovery of fullerenes has stimulated extensive exploration of the resulting behavior of adsorbed films. Our study addresses the planar substrates graphene-fluoride (GF) and graphane (GH) in comparison to graphene. We present initial results concerning the potential energy, energy bands and low density behavior of 4He and 3He films on such different surfaces. For example, while graphene presents an adsorption potential that is qualitatively similar to that on graphite, GF and GH yield potentials with different symmetry, a number of adsorption sites double that on graphene/graphite and a larger corrugation for the adatom. In the case of GF, the lowest energy band width is similar to that on graphite but the He atom has a significantly larger effective mass and the adsorption energy is about three time that on graphite. Implications concerning the monolayer phase diagram of 4He are explored with the exact path integral ground state method. A commensurate ordered state similar to the sqrt{3} x sqrt{3} R30^o state on graphite is found the be unstable both on GF and on GH. The ground states of submonolayer 4He on both GF and GH are superfluids with a Bose Einstein condensate fraction of about 10%.Comment: 6 pages, 3 figures, LT26 proceedings, accepted for publication in Journal of Physics: Conference Serie

The Fine Structure Constant and the CMB Damping Scale

The recent measurements of the Cosmic Microwave Background anisotropies at arcminute angular scales performed by the ACT and SPT experiments are probing the damping regime of CMB fluctuations. The analysis of these datasets unexpectedly suggests that the effective number of relativistic degrees of freedom is larger than the standard value of Neff = 3.04, and inconsistent with it at more than two standard deviations. In this paper we study the role of a mechanism that could affect the shape of the CMB angular fluctuations at those scales, namely a change in the recombination process through variations in the fine structure constant. We show that the new CMB data significantly improve the previous constraints on variations of {\alpha}, with {\alpha}/{\alpha}0 = 0.984 \pm 0.005, i.e. hinting also to a more than two standard deviation from the current, local, value {\alpha}0. A significant degeneracy is present between {\alpha} and Neff, and when variations in the latter are allowed the constraints on {\alpha} are relaxed and again consistent with the standard value. Deviations of either parameter from their standard values would imply the presence of new, currently unknown physics.Comment: 4 pages, 1 figur

Triggering the Formation of Halo Globular Clusters with Galaxy Outflows

We investigate the interactions of high-redshift galaxy outflows with low-mass virialized (Tvir < 10,000K) clouds of primordial composition. While atomic cooling allows star formation in larger primordial objects, such "minihalos" are generally unable to form stars by themselves. However, the large population of high-redshift starburst galaxies may have induced widespread star formation in these objects, via shocks that caused intense cooling both through nonequilibrium H2 formation and metal-line emission. Using a simple analytic model, we show that the resulting star clusters naturally reproduce three key features of the observed population of halo globular clusters (GCs). First, the 10,000 K maximum virial temperature corresponds to the ~ 10^6 solar mass upper limit on the stellar mass of GCs. Secondly, the momentum imparted in such interactions is sufficient to strip the gas from its associated dark matter halo, explaining why GCs do not reside in dark matter potential wells. Finally, the mixing of ejected metals into the primordial gas is able to explain the ~ 0.1 dex homogeneity of stellar metallicities within a given GC, while at the same time allowing for a large spread in metallicity between different clusters. To study this possibility in detail, we use a simple 1D numerical model of turbulence transport to simulate mixing in cloud-outflow interactions. We find that as the shock shears across the side of the cloud, Kelvin-Helmholtz instabilities arise, which cause mixing of enriched material into > 20% of the cloud. Such estimates ignore the likely presence of large-scale vortices, however, which would further enhance turbulence generation. Thus quantitative mixing predictions must await more detailed numerical studies.Comment: 21 pages, 11 figures, Apj in pres

Spin dynamics in hole-doped two-dimensional S=1/2 Heisenberg antiferromagnets: ^{63}Cu NQR relaxation in La_{2-x}Sr_xCuO_4 for x≤0.04x\leq 0.04

The effects on the correlated Cu^{2+} S = 1/2 spin dynamics in the paramagnetic phase of La_{2-x}Sr_xCuO_4 (for $x \lesssim 0.04$) due to the injection of holes are studied by means of ^{63}Cu NQR spin-lattice relaxation time T_1 measurements. The results are discussed in the framework of the connection between T_1 and the in-plane magnetic correlation length $\xi_{2D}(x,T)$. It is found that at high temperatures the system remains in the renormalized classical regime, with a spin stiffness constant $\rho_s(x)$ reduced by small doping to an extent larger than the one due to Zn doping. For $x\gtrsim 0.02$ the effect of doping on $\rho_s(x)$ appears to level off. The values for $\rho_s(x)$ derived from T_1 for $T\gtrsim 500$ K are much larger than the ones estimated from the temperature behavior of sublattice magnetization in the ordered phase ($T\leq T_N$). It is argued that these features are consistent with the hypothesis of formation of stripes of microsegregated holes.Comment: 10 pages, 3 figure

Metal Cooling in Simulations of Cosmic Structure Formation

The addition of metals to any gas can significantly alter its evolution by increasing the rate of radiative cooling. In star-forming environments, enhanced cooling can potentially lead to fragmentation and the formation of low-mass stars, where metal-free gas-clouds have been shown not to fragment. Adding metal cooling to numerical simulations has traditionally required a choice between speed and accuracy. We introduce a method that uses the sophisticated chemical network of the photoionization software, Cloudy, to include radiative cooling from a complete set of metals up to atomic number 30 (Zn) that can be used with large-scale three-dimensional hydrodynamic simulations. Our method is valid over an extremely large temperature range (10 K < T < 10^8 K), up to hydrogen number densities of 10^12 cm^-3. At this density, a sphere of 1 Msun has a radius of roughly 40 AU. We implement our method in the adaptive mesh refinement (AMR) hydrodynamic/N-body code, Enzo. Using cooling rates generated with this method, we study the physical conditions that led to the transition from Population III to Population II star formation. While C, O, Fe, and Si have been previously shown to make the strongest contribution to the cooling in low-metallicity gas, we find that up to 40% of the metal cooling comes from fine-structure emission by S, when solar abundance patterns are present. At metallicities, Z > 10^-4 Zsun, regions of density and temperature exist where gas is both thermally unstable and has a cooling time less than its dynamical time. We identify these doubly unstable regions as the most inducive to fragmentation. At high redshifts, the CMB inhibits efficient cooling at low temperatures and, thus, reduces the size of the doubly unstable regions, making fragmentation more difficult.Comment: 19 pages, 12 figures, significant revision, including new figure

Stability of Magnetized Disks and Implications for Planet Formation

This paper considers gravitational perturbations in geometrically thin disks with rotation curves dominated by a central object, but with substantial contributions from magnetic pressure and tension. The treatment is general, but the application is to the circumstellar disks that arise during the gravitational collapse phase of star formation. We find the dispersion relation for spiral density waves in these generalized disks and derive the stability criterion for axisymmetric $(m=0)$ disturbances (the analog of the Toomre parameter $Q_T$) for any radial distribution of the mass-to-flux ratio $\lambda$. The magnetic effects work in two opposing directions: on one hand, magnetic tension and pressure stabilize the disk against gravitational collapse and fragmentation; on the other hand, they also lower the rotation rate making the disk more unstable. For disks around young stars the first effect generally dominates, so that magnetic fields allow disks to be stable for higher surface densities and larger total masses. These results indicate that magnetic fields act to suppress the formation of giant planets through gravitational instability. Finally, even if gravitational instability can form a secondary body, it must lose an enormous amount of magnetic flux in order to become a planet; this latter requirement represents an additional constraint for planet formation via gravitational instability and places a lower limit on the electrical resistivity.Comment: accepted in Ap

Exciton polaritons in two-dimensional photonic crystals

Experimental evidence of strong coupling between excitons confined in a quantum well and the photonic modes of a two-dimensional dielectric lattice is reported. Both resonant scattering and photoluminescence spectra at low temperature show the anticrossing of the polariton branches, fingerprint of strong coupling regime. The experiments are successfully interpreted in terms of a quantum theory of exciton-photon coupling in the investigated structure. These results show that the polariton dispersion can be tailored by properly varying the photonic crystal lattice parameter, which opens the possibility to obtain the generation of entangled photon pairs through polariton stimulated scattering.Comment: 5 pages, 4 figure