588 research outputs found
Fine Features in the Primordial Power Spectrum
A possible origin of the anomalous dip and bump in the primordial power
spectrum, which are reconstructed from WMAP data corresponding to the multipole
by using the inversion method, is investigated as a
consequence of modification of scalar field dynamics in the inflation era.
Utilizing an analytic formula to handle higher order corrections to the
slow-roll approximation, we evaluate the relation between a detailed shape of
inflaton potential and a fine structure in the primordial power spectrum. We
conclude that it is unlikely to generate the observed dip and bump in the power
spectrum by adding any features in the inflaton potential. Though we can make a
fine enough shape in the power spectrum by controlling the feature of the
potential, the amplitude of the dip and bump becomes too small in that case.Comment: 15 pages, 11 figures, submitted to JCA
Finite size effects on calorimetric cooperativity of two-state proteins
Finite size effects on the calorimetric cooperatity of the folding-unfolding
transition in two-state proteins are considered using the Go lattice models
with and without side chains. We show that for models without side chains a
dimensionless measure of calorimetric cooperativity kappa2 defined as the ratio
of the van't Hoff to calorimetric enthalpy does not depend on the number of
amino acids N. The average value of kappa2 is about 3/4 which is lower than the
experimental value kappa2=1. For models with side chains kappa2 approaches
unity as kappa2 \sim N^mu, where exponent mu=0.17. Above the critical chain
length Nc =135 these models can mimic the truly all-or-non folding-unfolding
transition.Comment: 3 eps figures. To appear in the special issue of Physica
Cold Nuclear Matter In Holographic QCD
We study the Sakai-Sugimoto model of holographic QCD at zero temperature and
finite chemical potential. We find that as the baryon chemical potential is
increased above a critical value, there is a phase transition to a nuclear
matter phase characterized by a condensate of instantons on the probe D-branes
in the string theory dual. As a result of electrostatic interactions between
the instantons, this condensate expands towards the UV when the chemical
potential is increased, giving a holographic version of the expansion of the
Fermi surface. We argue based on properties of instantons that the nuclear
matter phase is necessarily inhomogeneous to arbitrarily high density. This
suggests an explanation of the "chiral density wave" instability of the quark
Fermi surface in large N_c QCD at asymptotically large chemical potential. We
study properties of the nuclear matter phase as a function of chemical
potential beyond the transition and argue in particular that the model can be
used to make a semi-quantitative prediction of the binding energy per nucleon
for nuclear matter in ordinary QCD.Comment: 31 pages, LaTeX, 1 figure, v2: some formulae corrected, qualitative
results unchange
Mapping the energy landscape of biomolecules using single molecule force correlation spectroscopy (FCS): Theory and applications
In the current AFM experiments the distribution of unfolding times, P(t), is
measured by applying a constant stretching force f_s from which the apparent
unfolding rate is obtained. To describe the complexity of the underlying energy
landscape requires additional probes that can incorporate the dynamics of
tension propagation and relaxation of the polypeptide chain upon force quench.
We introduce a theory of force correlation spectroscopy (FCS) to map the
parameters of the energy landscape of proteins. In the FCS the joint
distribution, P(T,t) of folding and unfolding times is constructed by repeated
application of cycles of stretching at constant fs, separated by release
periods T during which the force is quenched to f_q<f_s. During the release
period, the protein can collapse to a manifold of compact states or refold. We
show that P(T,t) can be used to resolve the kinetics of unfolding as well as
formation of native contacts and to extract the parameters of the energy
landscape using chain extension as the reaction coordinate and P(T,t). We
illustrate the utility of the proposed formalism by analyzing simulations of
unfolding-refolding trajectories of a coarse-grained protein S1 with beta-sheet
architecture for several values of f_s, T and f_q=0. The simulations of
stretch-relax trajectories are used to map many of the parameters that
characterize the energy landscape of S1.Comment: 23 pages, 9 figures; accepted to Biophysical Journa
Mechanism investigation of friction-related effects in single point incremental forming using a developed oblique roller-ball tool
Single point incremental forming (SPIF) is a highly versatile and flexible process for rapid manufacturing of complex sheet metal parts. In the SPIF process, a ball nose tool moves along a predefined tool path to form the sheet to desired shapes. Due to its unique ability in local deformation of sheet metal, the friction condition between the tool and sheet plays a significant role in material deformation. The effects of friction on surface finish, forming load, material deformation and formability are studied using a newly developed oblique roller ball (ORB) tool. Four grades of aluminum sheet including AA1100, AA2024, AA5052 and AA6111 are employed in the experiments. The material deformation under both the ORB tool and conventional rigid tool are studied by drilling a small hole in the sheet. The experimental results suggest that by reducing the friction resistance using the ORB tool, better surface quality, reduced forming load, smaller through-the-thickness-shear and higher formability can be achieved. To obtain a better understanding of the frictional effect, an analytical model is developed based on the analysis of the stress state in the SPIF deformation zone. Using the developed model, an explicit relationship between the stress state and forming parameters is established. The experimental observations are in good agreement with the developed model. The model can also be used to explain two contrary effects of friction and corresponding through-the-thickness-shear: increase of friction would potentially enhance the forming stability and suppress the necking; however, increase of friction would also increase the stress triaxiality and decrease the formability. The final role of the friction effect depends on the significance of each effect in SPIF process
Scaling analysis of electron transport through metal-semiconducting carbon nanotube interfaces: Evolution from the molecular limit to the bulk limit
We present a scaling analysis of electronic and transport properties of
metal-semiconducting carbon nanotube interfaces as a function of the nanotube
length within the coherent transport regime, which takes fully into account
atomic-scale electronic structure and three-dimensional electrostatics of the
metal-nanotube interface using a real-space Green's function based
self-consistent tight-binding theory. As the first example, we examine devices
formed by attaching finite-size single-wall carbon nanotubes (SWNT) to both
high- and low- work function metallic electrodes through the dangling bonds at
the end. We analyze the nature of Schottky barrier formation at the
metal-nanotube interface by examining the electrostatics, the band lineup and
the conductance of the metal-SWNT molecule-metal junction as a function of the
SWNT molecule length and metal-SWNT coupling strength. We show that the
confined cylindrical geometry and the atomistic nature of electronic processes
across the metal-SWNT interface leads to a different physical picture of band
alignment from that of the planar metal-semiconductor interface. We analyze the
temperature and length dependence of the conductance of the SWNT junctions,
which shows a transition from tunneling- to thermal activation-dominated
transport with increasing nanotube length. The temperature dependence of the
conductance is much weaker than that of the planar metal-semiconductor
interface due to the finite number of conduction channels within the SWNT
junctions. We find that the current-voltage characteristics of the metal-SWNT
molecule-metal junctions are sensitive to models of the potential response to
the applied source/drain bias voltages.Comment: Minor revision to appear in Phys. Rev. B. Color figures available in
the online PRB version or upon request to: [email protected]
Liquid 4He: contributions to first principles theory of quantized vortices, thermohydrodynamic properties, and the lambda transition
Liquid 4He has been studied extensively for almost a century, but there are
still a number of outstanding weak or missing links in our comprehension of it.
This paper reviews some of the principal paths taken in previous research and
then proceeds to fill gaps and create an integrated picture with more complete
understanding through first principles treatment of a realistic model that
starts with a microscopic, atomistic description of the liquid. Newly derived
results for vortex cores and thermohydrodynamic properties for a two-fluid
model are used to show that interacting quantized vortices may produce a lambda
anomaly in specific heat near the superfluid transition where flow properties
change. The nature of the order in the superfluid state is explained.
Experimental support for new calculations is exhibited, and a unique specific
heat experiment is proposed to test predictions of the theory. Relevance of the
theory to modern research in cosmology, astrophysics, and Bose-Einstein
condensates is discussed.Comment: 155 pages, 28 figure
Geographical and climatic limits of needle types of one- and two-needled pinyon pines
Aim The geographical extent and climatic tolerances of one- and two-needled pinyon pines (Pinus subsect. Cembroides) are the focus of questions in taxonomy, palaeoclimatology and modelling of future distributions. The identification of these pines, traditionally classified by one- versus two-needled fascicles, is complicated by populations with both one- and two-needled fascicles on the same tree, and the description of two more recently described one-needled varieties: the fallax-type and californiarum-type. Because previous studies have suggested correlations between needle anatomy and climate, including anatomical plasticity reflecting annual precipitation, we approached this study at the level of the anatomy of individual pine needles rather than species.
Location Western North America.
Methods We synthesized available and new data from field and herbarium collections of needles to compile maps of their current distributions across western North America. Annual frequencies of needle types were compared with local precipitation histories for some stands. Historical North American climates were modeled on a c. 1-km grid using monthly temperature and precipitation values. A geospatial model (ClimLim), which analyses the effect of climate modulated physiological and ecosystem processes, was used to rank the importance of seasonal climate variables in limiting the distributions of anatomical needle types.
Results The pinyon needles were classified into four distinct types based upon the number of needles per fascicle, needle thickness and the number of stomatal rows and resin canals. The individual needles fit well into four categories of needle types, whereas some trees exhibit a mixture of two needle types. Trees from central Arizona containing a mixture of Pinus edulis and fallax-type needles increased their percentage of fallax-type needles following dry years. All four needle types occupy broader geographical regions with distinctive precipitation regimes. Pinus monophylla and californiarum-type needles occur in regions with high winter precipitation. Pinus edulis and fallax-type needles are found in regions with high monsoon precipitation. Areas supporting californiarum-type and fallax-type needle distributions are additionally characterized by a more extreme MayâJune drought.
Main conclusions These pinyon needle types seem to reflect the amount and seasonality of precipitation. The single needle fascicle characterizing the fallax type may be an adaptation to early summer or periodic drought, while the single needle of Pinus monophylla may be an adaptation to summerâautumn drought. Although the needles fit into four distinct categories, the parent trees are sometimes less easily classified, especially near their ancestral Pleistocene ranges in the Mojave and northern Sonoran deserts. The abundance of trees with both one- and two-needled fascicles in the zones between P. monophylla, P. edulis and fallax-type populations suggest that needle fascicle number is an unreliable characteristic for species classification. Disregarding needle fascicle number, the fallax-type needles are nearly identical to P. edulis, supporting Littleâs (1968) initial classification of these trees as P. edulis var. fallax, while the californiarum-type needles have a distinctive morphology supporting Baileyâs (1987) classification of this tree as Pinus californiarum
Cyrtograptids from the Telychian (upper Llandovery) of Kinnekulle Mountain, southern Sweden
Investigation of material deformation mechanism in double side incremental sheet forming
Double side incremental forming (DSIF) is an emerging technology in incremental sheet forming (ISF) in recent years. By employing two forming tools at each side of the sheet, the DSIF process can provide additional process flexibility, comparing to the conventional single point incremental forming (SPIF) process, therefore to produce complex geometries without the need of using a backing plate or supporting die. Although this process has been proposed for years, there is only limited research on this process and there are still many unanswered open questions about this process. Using a newly developed ISF machine, the DSIF process is investigated in this work. Focusing on the fundamental aspects of material deformation and fracture mechanism, this paper aims to improve the understanding of the DSIF process. Two key process parameters considered in this study include the supporting force and relative position between master and slave tools. The material deformation, the final thickness distribution as well as the formability under varying conditions of these two process variables are investigated. An analytical model was developed to evaluate the stress state in the deformation zone. Using the developed model, an explicit relationship between the stress state and key process parameters was established and a drop of stress triaxiality was observed in the double contact zone, which explains the enhanced formability in the DSIF process. Based on the analytical and experimental investigation, the advancements and challenges of the DSIF process are discussed with a few conclusions drawn for future research
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