394 research outputs found
Influence of dimensionality on superconductivity in carbon nanotubes
We investigate the electronic instabilities in carbon nanotubes (CNs),
looking for the break-down of the one dimensional Luttinger liquid regime due
to the strong screening of the long-range part of the Coulomb repulsion. We
show that such a breakdown is realized both in ultra-small single wall CNs and
multi wall CNs, while a purely electronic mechanism could explain the
superconductivity (SC) observed recently in ultra-small (diameter ) single wall CNs () and entirely end-bonded multi-walled
ones (). We show that both the doping and the screening of
long-range part of the electron-electron repulsion, needed to allow the SC
phase, are related to the intrinsically 3D nature of the environment where the
CNs operate.Comment: 5 pages, 3 figures, PACS: 71.10.Pm,74.50.+r,71.20.Tx, to appear in J.
Phys. Cond. Ma
Surface ozone measurements at Taliarte, Gran Canaria (Canary Islands, Spain)
From December 1990 tropospheric ozone concentrations have been measured at Taliarte station (Gran Canaria, Canary Islands). Taliarte’s measurements are of great interest because of its proximity to Izaña BAPMoN observatory
(Tenerife, Canary Islands) and its relative distance to high-pollution regions. A comparative study with other North-Atlantic stations has been carried out. In order to
compare possible analogies and differences, a comparative study has been carried out. Studies involve seasonal cycle, diurnal oscillation and long-range transport. Ozone concentrations at Taliarte show high sensitivity to transport from higher latitudes. For diurnal variations, two different patterns have been found: “trade winds” and “marine breeze” situations
Comparative analysis of human and mouse expression data illuminates tissue-specific evolutionary patterns of miRNAs.
MicroRNAs (miRNAs) constitute an important class of gene regulators. While models have been proposed to explain their appearance and expansion, the validation of these models has been difficult due to the lack of comparative studies. Here, we analyze miRNA evolutionary patterns in two mammals, human and mouse, in relation to the age of miRNA families. In this comparative framework, we confirm some predictions of previously advanced models of miRNA evolution, e.g. that miRNAs arise more frequently de novo than by duplication, or that the number of protein-coding gene targeted by miRNAs decreases with evolutionary time. We also corroborate that miRNAs display an increase in expression level with evolutionary time, however we show that this relation is largely tissue-dependent, and especially low in embryonic or nervous tissues. We identify a bias of tag-sequencing techniques regarding the assessment of breadth of expression, leading us, contrary to predictions, to find more tissue-specific expression of older miRNAs. Together, our results refine the models used so far to depict the evolution of miRNA genes. They underline the role of tissue-specific selective forces on the evolution of miRNAs, as well as the potential co-evolution patterns between miRNAs and the protein-coding genes they target
Suppression of electron-electron repulsion and superconductivity in Ultra Small Carbon Nanotubes
Recently, ultra-small-diameter Single Wall Nano Tubes with diameter of have been produced and many unusual properties were observed, such as
superconductivity, leading to a transition temperature , much
larger than that observed in the bundles of larger diameter tubes.
By a comparison between two different approaches, we discuss the issue
whether a superconducting behavior in these carbon nanotubes can arise by a
purely electronic mechanism. The first approach is based on the Luttinger Model
while the second one, which emphasizes the role of the lattice and short range
interaction, is developed starting from the Hubbard Hamiltonian. By using the
latter model we predict a transition temperature of the same order of magnitude
as the measured one.Comment: 7 pages, 3 figures, to appear in J. Phys.-Cond. Ma
An Instruction to Accelerate Software Caches
In this paper we propose an instruction to accelerate software caches. While DMAs are very efficient for predictable data sets that can be fetched before they are needed, they introduce a large latency overhead for computations with unpredictable access behavior. Software caches are advantageous when the data set is not predictable but exhibits locality. However, software caches also incur a large overhead. Because the main overhead is in the access function, we propose an instruction that replaces the look-up function of the software cache. This instruction is evaluated using the Multidimensional Software Cache and two multimedia kernels, GLCM and H.264 Motion Compensation. The results show that the proposed instruction accelerates the software cache access time by a factor of 2.6. This improvement translates to a 2.1 speedup for GLCM and 1.28 for MC, when compared with the IBM software cache
Dust-Bounded ULIRGs? Model Predictions for Infrared Spectroscopic Surveys
The observed faintness of infrared fine-structure line emission along with
the warm far-infrared (FIR) colors of ultraluminous infrared galaxies (ULIRGs)
is a long-standing problem. In this work, we calculate the line and continuum
properties of a cloud exposed to an Active Galactic Nucleus (AGN) and starburst
spectral energy distribution (SED). We use an integrated modeling approach,
predicting the spectrum of ionized, atomic, and molecular environments in
pressure equilibrium. We find that the effects of high ratios of impinging
ionizing radiation density to particle density (i.e. high ionization
parameters, or U) can reproduce many ULIRG observational characteristics.
Physically, as U increases, the fraction of UV photons absorbed by dust
increases, corresponding to fewer photons available to photoionize and heat the
gas, producing what is known as a "dust-bounded" nebula. We show that high U
effects can explain the "[C II] deficit", the ~1 dex drop in the [C II] 158
micron /FIR ratio seen in ULIRGs when compared to starburst or normal galaxies.
Additionally, by increasing U through increasing the ionizing photon flux,
warmer dust and thus higher IRAS F(60)/F(100) ratios result. High U effects
also predict an increase in [O I]63 micron /[C II] 158 micron and a gradual
decline in [O III] 88 micron /FIR, similar to the magnitude of the trends
observed, and yield a reasonable fit to [Ne V]14 micron /FIR ratio AGN
observations.Comment: 34 pages, 13 figures, accepted for publication in the Astrophysical
Journa
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