1,763 research outputs found
Damping mechanism for the strongly renormalized -axis charge transport in high- cuprate superconductors
We analyze the -axis infrared reflectivity of
LaSrCuO single crystals. The plasma edge near 6 meV,
observed below , is due to Cooper-pair tunneling. This low value of the
plasma edge is consistent with the -axis plasma frequency () obtained
from LDA calculations ( eV) if we take into account that the
single-particle charge transport along the axis is strongly incoherent both
above and below . We find no evidence for a reduction of the -axis
scattering rate () below . Our investigation suggests
, which is exactly opposite to the clean limit.
VSGD.94.6.1Comment: 4 pages, figures on request. Revtex, version 2, Materials Science
Center Internal Report Number VSGD.94.6.
Recommended from our members
Flux motion and dissipation in high-temperature superconductors
The effects on flux motion and dissipation of interlayer coupling of the Cu-O planes along the c-axis are considered for the high- temperature superconductors (HTS). It is argued that for the highly-anisotropic HTS, the weak interlayer coupling plays a dominant role that can be described by incoherent Josephson tunneling between superconducting Cu-O bi- or tri-layers. In YBa{sub 2}Cu{sub 3}O{sub 7}, the layers are strongly coupled, presumably because the conducting Cu-O chains short circuit the Josephson tunneling, so that these effects are weak or missing
Deleterious variation shapes the genomic landscape of introgression
While it is appreciated that population size changes can impact patterns of deleterious variation in natural populations, less attention has been paid to how gene flow affects and is affected by the dynamics of deleterious variation. Here we use population genetic simulations to examine how gene flow impacts deleterious variation under a variety of demographic scenarios, mating systems, dominance coefficients, and recombination rates. Our results show that admixture between populations can temporarily reduce the genetic load of smaller populations and cause increases in the frequency of introgressed ancestry, especially if deleterious mutations are recessive. Additionally, when fitness effects of new mutations are recessive, between-population differences in the sites at which deleterious variants exist creates heterosis in hybrid individuals. Together, these factors lead to an increase in introgressed ancestry, particularly when recombination rates are low. Under certain scenarios, introgressed ancestry can increase from an initial frequency of 5% to 30-75% and fix at many loci, even in the absence of beneficial mutations. Further, deleterious variation and admixture can generate correlations between the frequency of introgressed ancestry and recombination rate or exon density, even in the absence of other types of selection. The direction of these correlations is determined by the specific demography and whether mutations are additive or recessive. Therefore, it is essential that null models of admixture include both demography and deleterious variation before invoking other mechanisms to explain unusual patterns of genetic variation.Bernard Y. Kim, Christian D. Huber, Kirk E. Lohmuelle
Inhibition of PI-3K restores nuclear p27(Kip1) expression in a mouse model of Kras-driven lung cancer.
Reduced expression of the CDK inhibitor p27(Kip1) (p27) in human lung cancer correlates with tumor aggressiveness and poor prognosis. However, the regulation of p27 expression and the role of p27 during lung cancer are poorly understood. Urethane-induced lung tumors in mice frequently harbor mutations in the Kras oncogene, and in this study, we use this model to address the regulation of p27 during tumorigenesis. The Ras effector Akt is known to regulate p27 mRNA abundance by phosphorylating and inactivating the FOXO transcription factors. Phosphorylated Akt and FOXO proteins were both increased in lung tumors, correlating with a reduction in p27 mRNA transcript. Akt also directly phosphorylates p27 and regulates its nuclear/cytoplasmic localization. Tumors showed a reduced nuclear/cytoplasmic ratio of p27 protein, together with an increase in phosphorylated Thr197 p27 in the cytoplasmic pool. Treatment of lung tumor-bearing mice with the phosphoinositol-3 kinase inhibitor LY294002 induced a rapid decrease in phosphorylated Akt and phosphorylated p27, concomitant with an increase in nuclear p27. Germline p27 deficiency accelerated both the growth and malignant progression of urethane-induced lung tumors, and did so in a cell autonomous manner, confirming a causal role of p27 in tumor suppression. These results show that p27 is a potent barrier to the growth and malignant progression of Kras-initiated lung tumors. Further, the reduction of nuclear p27 in tumors is mediated by oncogene signaling pathways, which can be reversed by pharmacological agents.Oncogene advance online publication, 3 August 2009; doi:10.1038/onc.2009.226
Orientifolds of Matrix theory and Noncommutative Geometry
We study explicit solutions for orientifolds of Matrix theory compactified on
noncommutative torus. As quotients of torus, cylinder, Klein bottle and
M\"obius strip are applicable as orientifolds. We calculate the solutions using
Connes, Douglas and Schwarz's projective module solution, and investigate
twisted gauge bundle on quotient spaces as well. They are Yang-Mills theory on
noncommutative torus with proper boundary conditions which define the geometry
of the dual space.Comment: 17 pages, LaTeX, minor corrections, two references added, discussions
slightly expanded, to appear in Phys. Rev.
Asymptotic Expansions for Stationary Distributions of Perturbed Semi-Markov Processes
New algorithms for computing of asymptotic expansions for stationary
distributions of nonlinearly perturbed semi-Markov processes are presented. The
algorithms are based on special techniques of sequential phase space reduction,
which can be applied to processes with asymptotically coupled and uncoupled
finite phase spaces.Comment: 83 page
Bose-Einstein condensation of excitons in CuO
We present a parameter-free model which estimates the density of excitons in
CuO, related to experiments that have tried to create an excitonic
Bose-Einstein condensate. Our study demonstrates that the triplet-state
excitons move along adiabats and obey classical statistics, while the
singlet-state excitons are a possible candidate for forming a Bose-Einstein
condensate. Finally we show that the results of this study do not change
qualitatively in a two-dimensional exciton gas, which can be realized in a
quantum well.Comment: 6 pages, RevTex, 1 ps figur
Stabilities of nanohydrated thymine radical cations: insights from multiphoton ionization experiments and ab initio calculations
Multi-photon ionization experiments have been carried out on thymine-water clusters in the gas phase. Metastable H2O loss from T+(H2O)n was observed at n ≥ 3 only. Ab initio quantum-chemical calculations of a large range of optimized T+(H2O)n conformers have been performed up to n = 4, enabling binding energies of water to be derived. These decrease smoothly with n, consistent with the general trend of increasing metastable H2O loss in the experimental data. The lowest-energy conformers of T+(H2O)3 and T+(H2O)4 feature intermolecular bonding via charge-dipole interactions, in contrast with the purely hydrogen-bonded neutrals. We found no evidence for a closed hydration shell at n = 4, also contrasting with studies of neutral clusters
Shot Noise in Nanoscale Conductors From First Principles
We describe a field-theoretic approach to calculate quantum shot noise in
nanoscale conductors from first principles. Our starting point is the
second-quantization field operator to calculate shot noise in terms of single
quasi-particle wavefunctions obtained self-consistently within density
functional theory. The approach is valid in both linear and nonlinear response
and is particularly suitable in studying shot noise in atomic-scale conductors.
As an example we study shot noise in Si atomic wires between metal electrodes.
We find that shot noise is strongly nonlinear as a function of bias and it is
enhanced for one- and two-Si wires due to the large contribution from the metal
electrodes. For longer wires it shows an oscillatory behavior for even and odd
number of atoms with opposite trend with respect to the conductance, indicating
that current fluctuations persist with increasing wire length.Comment: 4 pages, 4 figure
Biomechanical Simulation of Electrode Migration for Deep Brain Stimulation
International audienceDeep Brain Stimulation is a modern surgical technique for treating patients who suffer from affective or motion disorders such as Parkinson's disease. The efficiency of the procedure relies heavily on the accuracy of the placement of a micro-electrode which sends electrical pulses to a specific part of the brain that controls motion and affective symptoms. However, targeting this small anatomical structure is rendered difficult due to a series of brain shifts that take place during and after the procedure. This paper introduces a biomechanical simulation of the intra and postoperative stages of the procedure in order to determine lead deformation and electrode migration due to brain shift. To achieve this goal, we propose a global approach, which accounts for brain deformation but also for the numerous interactions that take place during the procedure (contacts between the brain and the inner part of the skull and falx cerebri, effect of the cerebro-spinal fluid, and biomechanical interactions between the brain and the electrodes and cannula used during the procedure). Preliminary results show a good correlation between our simulations and various results reported in the literature
- …