OH+ in Diffuse Molecular Clouds

Near ultraviolet observations of OH+ and OH in diffuse molecular clouds reveal a preference for different environments. The dominant absorption feature in OH+ arises from a main component seen in CH+ (that with the highest CH+/CH column density ratio), while OH follows CN absorption. This distinction provides new constraints on OH chemistry in these clouds. Since CH+ detections favor low-density gas with small fractions of molecular hydrogen, this must be true for OH+ as well, confirming OH+ and H2O+ observations with the Herschel Space Telescope. Our observed correspondence indicates that the cosmic ray ionization rate derived from these measurements pertains to mainly atomic gas. The association of OH absorption with gas rich in CN is attributed to the need for high enough density and molecular fraction before detectable amounts are seen. Thus, while OH+ leads to OH production, chemical arguments suggest that their abundances are controlled by different sets of conditions and that they coexist with different sets of observed species. Of particular note is that non-thermal chemistry appears to play a limited role in the synthesis of OH in diffuse molecular clouds.Comment: 15 pages, 4 figures, to appear in ApJ Letter

Effects of Non-Uniform Heating on the Location and Magnitude of Critical Heat Flux in a Microchannel Heat Sink

Decreasing form factors and diminishing numbers of thermal interfaces and spreading layers in modern, compact electronic packages result in non-uniform heat generation profiles at the chip level being transmitted directly to the heat sinks. An improved understanding of the effects of non-uniform heating on the heat dissipation limits in microchannel heat sinks has become essential. An experimental investigation is conducted to measure the location and magnitude of critical heat flux (CHF) in a microchannel heat sink exposed to a range of non-uniform heating profiles. A 12.7 mm × 12.7 mm silicon microchannel heat sink with an embedded 5 × 5 array of individually controllable heaters is used in the experiments. The microchannels in the heat sink are 240 mm wide and 370 micrometers deep, and are separated by 110 mm wide fins. The dielectric fluid HFE-7100 is used as the coolant, with an average mass flux in the heat sink of approximately 800 kg/m2s. High-speed visualizations of the flow are recorded to capture the CHF phenomena observed. A central ‘hotspot’ spanning the entire length of the heat sink in the flow direction (formed by heating only the central 20 percent of the base area) produced both the largest wall excess temperature and the lowest CHF of all the heat flux distributions investigated, due to the flow maldistribution induced. A single transverse hotspot spanning the heat sink perpendicular to the flow direction resulted in different CHF values based on its streamwise location; CHF was largest when the hotspot was placed nearest the inlet and smallest when placed nearest the outlet. The visualizations revealed that CHF occurs when there is a sudden and unalleviated upstream expansion of vapor in one or more channels above the hotspot, causing the local wall temperature to rapidly increase. The proximity of the hotspot to the inlet manifold, which communicates between all channels and can relieve downstream vapor expansion, appears to determine the resiliency of the heat sink to conditions leading to CHF

Local Measurement of Flow Boiling Heat Transfer in an Array of Non-Uniformly Heated Microchannels

As electronics packages become increasingly thinner and more compact due to size, weight, and performance demands, the use of large intermediate heat spreaders to mitigate heat generation non-uniformities are no longer a viable option. Instead, non-uniform heat flux profiles produced from chip-scale variations or from multiple discrete devices are experienced directly by the ultimate heat sink. In order to address these thermal packaging trends, a better understanding of the impacts of non-uniform heating on two-phase flow characteristics and thermal performance limits for microchannel heat sinks is needed. An experimental investigation is performed to explore flow boiling phenomena in a microchannel heat sink with hotspots, as well as non-uniform streamwise and transverse peak-heating conditions spanning across the entire heat sink area. The investigation is conducted using a silicon microchannel heat sink with a 5 x 5 array of individually controllable heaters attached to a 12.7 mm x 12.7 mm square base. The channels are 240 lm wide, 370 lm deep, and separated by 110 lm wide fins. The working fluid is the dielectric fluorinert liquid FC-77, flowing at a mass flux of approximately 890 kg/m2 s. High-speed visualizations of the flow are recorded to observe the local flow regimes. Despite the substrate beneath the microchannels being very thin (200 lm), significant lateral conduction occurs and must be accounted for in the calculation of the local heat flux imposed. For non-uniform heat input profiles, with peak heat fluxes along the streamwise and transverse directions, it is found that the local flow regimes, heat transfer coefficients, and wall temperatures deviate significantly from a uniformly heated case. These trends are assessed as a function of an increase in the relative magnitude of the nonuniformity between the peak and background heat fluxes

Power-law spin correlations in pyrochlore antiferromagnets

The ground state ensemble of the highly frustrated pyrochlore-lattice antiferromagnet can be mapped to a coarse-grained ``polarization'' field satisfying a zero-divergence condition From this it follows that the correlations of this field, as well as the actual spin correlations, decay with separation like a dipole-dipole interaction ($1/|R|^3$). Furthermore, a lattice version of the derivation gives an approximate formula for spin correlations, with several features that agree well with simulations and neutron-diffraction measurements of diffuse scattering, in particular the pinch-point (pseudo-dipolar) singularities at reciprocal lattice vectors. This system is compared to others in which constraints also imply diffraction singularities, and other possible applications of the coarse-grained polarization are discussed.Comment: 13 pp, revtex, two figure

Properties of a classical spin liquid: the Heisenberg pyrochlore antiferromagnet

We study the low-temperature behaviour of the classical Heisenberg antiferromagnet with nearest neighbour interactions on the pyrochlore lattice. Because of geometrical frustration, the ground state of this model has an extensive number of degrees of freedom. We show, by analysing the effects of small fluctuations around the ground-state manifold, and from the results of Monte Carlo and molecular dynamics simulations, that the system is disordered at all temperatures, T, and has a finite relaxation time, which varies as 1/T for small T.Comment: 4 pages revtex; 3 figures automatically include

Superconducting Phase with Fractional Vortices in the Frustrated Kagome Wire Network at f=1/2

In classical XY kagome antiferromagnets, there can be a novel low temperature phase where $\psi^3=e^{i3\theta}$ has quasi-long-range order but $\psi$ is disordered, as well as more conventional antiferromagnetic phases where $\psi$ is ordered in various possible patterns ($\theta$ is the angle of orientation of the spin). To investigate when these phases exist in a physical system, we study superconducting kagome wire networks in a transverse magnetic field when the magnetic flux through an elementary triangle is a half of a flux quantum. Within Ginzburg-Landau theory, we calculate the helicity moduli of each phase to estimate the Kosterlitz-Thouless (KT) transition temperatures. Then at the KT temperatures, we estimate the barriers to move vortices and effects that lift the large degeneracy in the possible $\psi$ patterns. The effects we have considered are inductive couplings, non-zero wire width, and the order-by-disorder effect due to thermal fluctuations. The first two effects prefer $q=0$ patterns while the last one selects a $\sqrt{3}\times\sqrt{3}$ pattern of supercurrents. Using the parameters of recent experiments, we conclude that at the KT temperature, the non-zero wire width effect dominates, which stabilizes a conventional superconducting phase with a $q=0$ current pattern. However, by adjusting the experimental parameters, for example by bending the wires a little, it appears that the novel $\psi^3$ superconducting phase can instead be stabilized. The barriers to vortex motion are low enough that the system can equilibrate into this phase.Comment: 30 pages including figure