Counter-rotating Accretion Disks

We consider accretion disks consisting of counter-rotating gaseous components with an intervening shear layer. Configurations of this type may arise from the accretion of newly supplied counter-rotating gas onto an existing co-rotating gas disk. For simplicity we consider the case where the gas well above the disk midplane is rotating with angular rate $+\Omega$ and that well below has the same properties but is rotating with rate $-\Omega$. Using the Shakura-Sunyaev alpha turbulence model, we find self-similar solutions where a thin (relative to the full disk thickness) equatorial layer accretes very rapidly, essentially at free-fall speed. As a result the accretion speed is much larger than it would be for an alpha disk rotating in one direction. Counter-rotating accretion disks may be a transient stage in the formation of counter-rotating galaxies and in the accretion of matter onto compact objects.Comment: 7 pages, 3 figures, aas2pp4.sty, submitted to Ap

Unique gap structure and symmetry of the charge density wave in single-layer VSe2_2

Single layers of transition metal dichalcogenides (TMDCs) are excellent candidates for electronic applications beyond the graphene platform; many of them exhibit novel properties including charge density waves (CDWs) and magnetic ordering. CDWs in these single layers are generally a planar projection of the corresponding bulk CDWs because of the quasi-two-dimensional nature of TMDCs; a different CDW symmetry is unexpected. We report herein the successful creation of pristine single-layer VSe$_2$, which shows a ($\sqrt7 \times \sqrt3$) CDW in contrast to the (4 $\times$ 4) CDW for the layers in bulk VSe$_2$. Angle-resolved photoemission spectroscopy (ARPES) from the single layer shows a sizable ($\sqrt7 \times \sqrt3$) CDW gap of $\sim$100 meV at the zone boundary, a 220 K CDW transition temperature twice the bulk value, and no ferromagnetic exchange splitting as predicted by theory. This robust CDW with an exotic broken symmetry as the ground state is explained via a first-principles analysis. The results illustrate a unique CDW phenomenon in the two-dimensional limit

Bose-Einstein Condensation of Atoms in a Trap

We point out that the local density approximation (LDA) of Oliva is an adaptation of the Thomas-Fermi method, and is a good approximation when $\varepsilon = \hbar\omega/kT 0$, the LDA leads to a quantitative result (14') easily checked by experiments. Critical remarks are made about the physics of the many body problem in terms of the scattering length $a$.Comment: 9 pages, latex. one figure, available from author

Anomalous electronic Raman scattering in Na_xCoO_2 H_2O

Raman scattering experiments on Na_{x}CoO_2 yH_2O single crystals show a broad electronic continuum with a pronounced peak around 100 cm-1 and a cutoff at approximately 560 cm-1over a wide range of doping levels. The electronic Raman spectra in superconducting and non-superconducting samples are similar at room temperature, but evolve in markedly different ways with decreasing temperature. For superconducting samples, the low-energy spectral weight is depleted upon cooling below T* sim 150K, indicating a opening of a pseudogap that is not present in non-superconducting materials. Weak additional phonon modes observed below T* suggest that the pseudogap is associated with charge ordering.Comment: 5 pages, 4 figures, for further information see www.peter-lemmens.d

First and Second Sound Modes of a Bose-Einstein Condensate in a Harmonic Trap

We have calculated the first and second sound modes of a dilute interacting Bose gas in a spherical trap for temperatures ($0.6<T/T_{c}<1.2$) and for systems with $10^4$ to $10^8$ particles. The second sound modes (which exist only below $T_{c}$) generally have a stronger temperature dependence than the first sound modes. The puzzling temperature variations of the sound modes near $T_{c}$ recently observed at JILA in systems with $10^3$ particles match surprisingly well with those of the first and second sound modes of much larger systems.Comment: a shorten version, more discussions are given on the nature of the second sound. A long footnote on the recent work of Zaremba, Griffin, and Nikuni (cond-mat/9705134) is added, the spectrum of the (\ell=1, n_2=0) mode is included in fig.

Competition for inorganic and organic forms of nitrogen and phosphorous between phytoplankton and bacteria during an <i>Emiliania huxleyi</i> spring bloom

Using ¹⁵N and ³³P, we measured the turnover of organic and inorganic nitrogen (N) and phosphorus (P) substrates, and the partitioning of N and P from these sources into two size fractions of marine osmotrophs during the course of a phytoplankton bloom in a nutrient manipulated mesocosm. The larger size fraction (&gt;0.8 μm), mainly consisting of the coccolithophorid <i>Emiliania huxleyi</i>, but also including an increasing amount of large particle-associated bacteria as the bloom proceeded, dominated uptake of the inorganic forms NH₄⁺, NO₃^&minus;, and PO₄^3&minus;. The uptake of N from leucine, and P from ATP and dissolved DNA, was initially dominated by the 0.8&ndash;0.2 μm size fraction, but shifted towards dominance by the &gt;0.8 μm size fraction as the system turned to an increasing degree of N-deficiency. Normalizing uptake to biomass of phytoplankton and heterotrophic bacteria revealed that organisms in the 0.8&ndash;0.2 μm size fraction had higher specific affinity for leucine-N than those in the &gt;0.8 μm size fraction when N was deficient, whereas the opposite was the case for NH₄⁺. There was no such difference regarding the specific affinity for P substrates. Since heterotrophic bacteria seem to acquire N from organic compounds like leucine more efficiently than phytoplankton, our results suggest different structuring of the microbial food chain in N-limited relative to P-limited environments

Drosophila modifier screens to identify novel neuropsychiatric drugs including aminergic agents for the possible treatment of Parkinson's disease and depression.

Small molecules that increase the presynaptic function of aminergic cells may provide neuroprotection in Parkinson's disease (PD) as well as treatments for attention deficit hyperactivity disorder (ADHD) and depression. Model genetic organisms such as Drosophila melanogaster may enhance the detection of new drugs via modifier or 'enhancer/suppressor' screens, but this technique has not been applied to processes relevant to psychiatry. To identify new aminergic drugs in vivo, we used a mutation in the Drosophila vesicular monoamine transporter (dVMAT) as a sensitized genetic background and performed a suppressor screen. We fed dVMAT mutant larvae ∼ 1000 known drugs and quantitated rescue (suppression) of an amine-dependent locomotor deficit in the larva. To determine which drugs might specifically potentiate neurotransmitter release, we performed an additional secondary screen for drugs that require presynaptic amine storage to rescue larval locomotion. Using additional larval locomotion and adult fertility assays, we validated that at least one compound previously used clinically as an antineoplastic agent potentiates the presynaptic function of aminergic circuits. We suggest that structurally similar agents might be used to development treatments for PD, depression and ADHD, and that modifier screens in Drosophila provide a new strategy to screen for neuropsychiatric drugs. More generally, our findings demonstrate the power of physiologically based screens for identifying bioactive agents for select neurotransmitter systems

Trophic ecology of the invasive argentine ant: spatio-temporal variation in resource assimilation and isotopic enrichment

Studies of food webs often employ stable isotopic approaches to infer trophic position and interaction strength without consideration of spatio-temporal variation in resource assimilation by constituent species. Using results from laboratory diet manipulations and monthly sampling of field populations, we illustrate how nitrogen isotopes may be used to quantify spatio-temporal variation in resource assimilation in ants. First, we determined nitrogen enrichment using a controlled laboratory experiment with the invasive Argentine ant (Linepithema humile). After 12 weeks, worker δ15N values from colonies fed an animal-based diet had δ15N values that were 5.51% greater compared to colonies fed a plant-based diet. The shift in δ15N values in response to the experimental diet occurred within 10 weeks. We next reared Argentine ant colonies with or without access to honeydew-producing aphids and found that after 8 weeks workers from colonies without access to aphids had δ15N values that were 6.31% larger compared to colonies with access to honeydew. Second, we sampled field populations over a 1-year period to quantify spatio-temporal variability in isotopic ratios of L. humile and those of a common native ant (Solenopsis xyloni). Samples from free-living colonies revealed that fluctuations in δ15N were 1.6–2.4‰ for L. humile and 1.8–2.9‰ for S. xyloni. Variation was also detected among L. humile castes: time averaged means of δ15N varied from 1.2 to 2.5‰ depending on the site, with δ15N values for queens ≥ workers > brood. The estimated trophic positions of L. humile and S. xyloni were similar within a site; however, trophic position for each species differed significantly at larger spatial scales. While stable isotopes are clearly useful for examining the trophic ecology of arthropod communities, our results suggest that caution is warranted when making ecological interpretations when stable isotope collections come from single time periods or life stages

Spin gap formation in the quantum spin systems TiOX, X=Cl and Br

In the layered quantum spin systems TiOCl and TiOBr the magnetic susceptibility shows a very weak temperature dependence at high temperatures and transition-induced phenomena at low temperatures. There is a clear connection of the observed transition temperatures to the distortion of the octahedra and the layer separation. Band structure calculations point to a relation of the local coordinations and the dimensionality of the magnetic properties. While from magnetic Raman scattering only a small decrease of the magnetic exchange by -5-10% is derived comparing TiOCl with TiOBr, the temperature dependence of the magnetic susceptibility favors a much bigger change.Comment: 5 figures, 15 pages, further information see http://www.peter-lemmens.d

Critical Temperature of a Trapped Interacting Bose Gas in the Local Density Approximation

The Bose gas in an external potential is studied by means of the local density approximation. An analytical result is derived for the dependence of the critical temperature of Bose-Einstein condensation on the mutual interaction in a generic power-law potential.Comment: 6 pages, latex, no figure