1,287 research outputs found
Transition to superfluid turbulence governed by an intrinsic parameter
Hydrodynamic flow in both classical and quantum fluids can be either laminar
or turbulent. To describe the latter, vortices in turbulent flow are modelled
with stable vortex filaments. While this is an idealization in classical
fluids, vortices are real topologically stable quantized objects in
superfluids. Thus superfluid turbulence is thought to hold the key to new
understanding on turbulence in general. The fermion superfluid 3He offers
further possibilities owing to a large variation in its hydrodynamic
characteristics over the experimentally accessible temperatures. While studying
the hydrodynamics of the B phase of superfluid 3He, we discovered a sharp
transition at 0.60Tc between two regimes, with regular behaviour at
high-temperatures and turbulence at low-temperatures. Unlike in classical
fluids, this transition is insensitive to velocity and occurs at a temperature
where the dissipative vortex damping drops below a critical limit. This
discovery resolves the conflict between existing high- and low-temperature
measurements in 3He-B: At high temperatures in rotating flow a vortex loop
injected into superflow has been observed to expand monotonically to a single
rectilinear vortex line, while at very low temperatures a tangled network of
quantized vortex lines can be generated in a quiescent bath with a vibrating
wire. The solution of this conflict reveals a new intrinsic criterion for the
existence of superfluid turbulence.Comment: Revtex file; 5 pages, 2 figure
The effect of intervertebral cartilage on neutral posture and range of motion in the necks of sauropod dinosaurs
The necks of sauropod dinosaurs were a key factor in their evolution. The habitual posture and range of motion of these necks has been controversial, and computer-aided studies have argued for an obligatory sub-horizontal pose. However, such studies are compromised by their failure to take into account the important role of intervertebral cartilage. This cartilage takes very different forms in different animals. Mammals and crocodilians have intervertebral discs, while birds have synovial joints in their necks. The form and thickness of cartilage varies significantly even among closely related taxa. We cannot yet tell whether the neck joints of sauropods more closely resembled those of birds or mammals. Inspection of CT scans showed cartilage:bone ratios of 4.5% for Sauroposeidon and about 20% and 15% for two juvenile Apatosaurus individuals. In extant animals, this ratio varied from 2.59% for the rhea to 24% for a juvenile giraffe. It is not yet possible to disentangle ontogenetic and taxonomic signals, but mammal cartilage is generally three times as thick as that of birds. Our most detailed work, on a turkey, yielded a cartilage:bone ratio of 4.56%. Articular cartilage also added 11% to the length of the turkey's zygapophyseal facets. Simple image manipulation suggests that incorporating 4.56% of neck cartilage into an intervertebral joint of a turkey raises neutral posture by 15°. If this were also true of sauropods, the true neutral pose of the neck would be much higher than has been depicted. An additional 11% of zygapophyseal facet length translates to 11% more range of motion at each joint. More precise quantitative results must await detailed modelling. In summary, including cartilage in our models of sauropod necks shows that they were longer, more elevated and more flexible than previously recognised
Enhanced Supersymmetry of Nonrelativistic ABJM Theory
We study the supersymmetry enhancement of nonrelativistic limits of the ABJM
theory for Chern-Simons level . The special attention is paid to the
nonrelativistic limit (known as `PAAP' case) containing both particles and
antiparticles. Using supersymmetry transformations generated by the monopole
operators, we find additional 2 kinematical, 2 dynamical, and 2 conformal
supercharges for this case. Combining with the original 8 kinematical
supercharges, the total number of supercharges becomes maximal: 14
supercharges, like in the well-known PPPP limit. We obtain the corresponding
super Schr\"odinger algebra which appears to be isomorphic to the one of the
PPPP case. We also discuss the role of monopole operators in supersymmetry
enhancement and partial breaking of supersymmetry in nonrelativistic limit of
the ABJM theory.Comment: 22 pages, references added, version to appear in JHE
The time-profile of cell growth in fission yeast: model selection criteria favoring bilinear models over exponential ones
BACKGROUND: There is considerable controversy concerning the exact growth profile of size parameters during the cell cycle. Linear, exponential and bilinear models are commonly considered, and the same model may not apply for all species. Selection of the most adequate model to describe a given data-set requires the use of quantitative model selection criteria, such as the partial (sequential) F-test, the Akaike information criterion and the Schwarz Bayesian information criterion, which are suitable for comparing differently parameterized models in terms of the quality and robustness of the fit but have not yet been used in cell growth-profile studies. RESULTS: Length increase data from representative individual fission yeast (Schizosaccharomyces pombe) cells measured on time-lapse films have been reanalyzed using these model selection criteria. To fit the data, an extended version of a recently introduced linearized biexponential (LinBiExp) model was developed, which makes possible a smooth, continuously differentiable transition between two linear segments and, hence, allows fully parametrized bilinear fittings. Despite relatively small differences, essentially all the quantitative selection criteria considered here indicated that the bilinear model was somewhat more adequate than the exponential model for fitting these fission yeast data. CONCLUSION: A general quantitative framework was introduced to judge the adequacy of bilinear versus exponential models in the description of growth time-profiles. For single cell growth, because of the relatively limited data-range, the statistical evidence is not strong enough to favor one model clearly over the other and to settle the bilinear versus exponential dispute. Nevertheless, for the present individual cell growth data for fission yeast, the bilinear model seems more adequate according to all metrics, especially in the case of wee1Δ cells
Virtual endocranial and inner ear endocasts of the Paleocene ‘condylarth’ Chriacus: New insight into the neurosensory system and evolution of early placental mammals
Liquid-gas phase transition in nuclear multifragmentation
The equation of state of nuclear matter suggests that at suitable beam
energies the disassembling hot system formed in heavy ion collisions will pass
through a liquid-gas coexistence region. Searching for the signatures of the
phase transition has been a very important focal point of experimental
endeavours in heavy ion collisions, in the last fifteen years. Simultaneously
theoretical models have been developed to provide information about the
equation of state and reaction mechanisms consistent with the experimental
observables. This article is a review of this endeavour.Comment: 63 pages, 27 figures, submitted to Adv. Nucl. Phys. Some typos
corrected, minor text change
Mutually beneficial host exploitation and ultra-biased sex ratios in quasisocial parasitoids
Selfish interests usually preclude resource sharing, but under some conditions collective actions enhance per capita gains. Such Allee effects underlay early explanations of social evolution but current understanding focusses on kin selection (inclusive fitness). We find an Allee effect that explains unusual quasisociality (cooperative brood care) among parasitoid wasps without invoking or precluding kin selection effects. In Sclerodermus harmandi, individual females produce most offspring when exploiting small hosts alone. However, larger hosts are more successfully exploited by larger groups of females, with the per-female benefits outweighing the costs of host sharing. Further, the extremely biased sex ratios (97% female) are better explained by mutually beneficial female–female interactions that increase the reproductive value of daughters (local resource enhancement), rather than by the usually invoked local mate competition between males. Thus, atypical quasisocial behaviour in a parasitoid wasp directly enhances reproductive success and selects for very extremely female-biased sex ratios
Habitat use and population biology of the Danube Clouded Yellow butterfly Colias myrmidone (Lepidoptera: Pieridae) in Romania
Detection of human papillomavirus DNA and p53 codon 72 polymorphism in prostate carcinomas of patients from Argentina
BACKGROUND: Infections with high-risk human papillomaviruses (HPVs), causatively linked to cervical cancer, might also play a role in the development of prostate cancer. Furthermore, the polymorphism at codon 72 (encoding either arginine or proline) of the p53 tumor-suppressor gene is discussed as a possible determinant for cancer risk. The HPV E6 oncoprotein induces degradation of the p53 protein. The aim of this study was to analyse prostate carcinomas and hyperplasias of patients from Argentina for the presence of HPV DNA and the p53 codon 72 polymorphism genotype. METHODS: HPV DNA detection and typing were done by consensus L1 and type-specific PCR assays, respectively, and Southern blot hybridizations. Genotyping of p53 codon 72 polymorphism was performed both by allele specific primer PCRs and PCR-RFLP (Bsh1236I). Fischer's test with Woolf's approximation was used for statistical analysis. RESULTS: HPV DNA was detected in 17 out of 41 (41.5 %) carcinoma samples, whereas all 30 hyperplasia samples were HPV-negative. Differences in p53 codon 72 allelic frequencies were not observed, neither between carcinomas and hyperplasias nor between HPV-positive and HPV-negative carcinomas. CONCLUSION: These results indicate that the p53 genotype is probably not a risk factor for prostate cancer, and that HPV infections could be associated with at least a subset of prostate carcinomas
Hindered rolling and friction anisotropy in supported carbon nanotubes
Carbon nanotubes (CNTs) are well known for their exceptional thermal,
mechanical and electrical properties. For many CNT applications it is of the
foremost importance to know their frictional properties. However, very little
is known about the frictional forces between an individual nanotube and a
substrate or tip. Here, we present a combined theoretical and experimental
study of the frictional forces encountered by a nanosize tip sliding on top of
a supported multiwall CNT along a direction parallel or transverse to the CNT
axis. Surprisingly, we find a higher friction coefficient in the transverse
direction compared with the parallel direction. This behaviour is explained by
a simulation showing that transverse friction elicits a soft 'hindered rolling'
of the tube and a frictional dissipation that is absent, or partially absent
for chiral CNTs, when the tip slides parallel to the CNT axis. Our findings can
help in developing better strategies for large-scale CNT assembling and sorting
on a surface.Comment: 8 pages, 5 figure
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