27,845 research outputs found
Probing protein-protein interactions by dynamic force correlated spectroscopy (FCS)
We develop a formalism for single molecule dynamic force spectroscopy to map
the energy landscape of protein-protein complex (). The joint
distribution of unbinding lifetimes and
measurable in a compression-tension cycle, which accounts for the internal
relaxation dynamics of the proteins under tension, shows that the histogram of
is not Poissonian. The theory is applied to the forced unbinding of
protein , modeled as a wormlike chain, from . We propose a new
class of experiments which can resolve the effect of internal protein dynamics
on the unbinding lifetimes.Comment: 12 pages, 3 figures, accepted to Phys. Rev. Let
Fluctuations, Higher Order Anharmonicities, and Landau Expansion for Barium Titanate
Correct phenomenological description of ferroelectric phase transitions in
barium titanate requires accounting for eighth-order terms in the free energy
expansion, in addition to the conventional sixth-order contributions. Another
unusual feature of BaTiO_3 crystal is that the coefficients B_1 and B_2 of the
terms P_x^4 and P_x^2*P_y^2 in the Landau expansion depend on the temperature.
It is shown that the temperature dependence of B_1 and B_2 may be caused by
thermal fluctuations of the polarization, provided the fourth-order
anharmonicity is anomalously small, i. e. the nonlinearity of P^4 type and
higher-order ones play comparable roles. Non-singular (non-critical)
fluctuation contributions to B_1 and B_2 are calculated in the first
approximation in sixth-order and eighth-order anharmonic constants. Both
contributions increase with the temperature, which is in agreement with
available experimental data. Moreover, the theory makes it possible to
estimate, without any additional assumptions, the ratio of fluctuation
(temperature dependent) contributions to coefficients B_1 and B_2. Theoretical
value of B_1/B_2 appears to be close to that given by experiments.Comment: 5 pages, 1 figur
Disentangling Morphology, Star Formation, Stellar Mass, and Environment in Galaxy Evolution
We present a study of the spectroscopic and photometric properties of
galaxies in six nearby clusters. We perform a partial correlation analysis on
our dataset to investigate whether the correlation between star formation rates
in galaxies and their environment is merely another aspect of correlations of
morphology, stellar mass, or mean stellar age with environment, or whether star
formation rates vary independently of these other correlations. We find a
residual correlation of ongoing star formation with environment, indicating
that even galaxies with similar morphologies, stellar masses, and mean stellar
ages have lower star formation rates in denser environments. Thus, the current
star formation gradient in clusters is not just another aspect of the
morphology-density, stellar mass-density, or mean stellar age-density
relations. Furthermore, the star formation gradient cannot be solely the result
of initial conditions, but must partly be due to subsequent evolution through a
mechanism (or mechanisms) sensitive to environment. Our results constitute a
true ``smoking gun'' pointing to the effect of environment on the later
evolution of galaxies.Comment: 31 pages, including 5 figures; accepted for publication in Ap
Using a Combination of Micro-Computed Tomography, CAD and 3D Printing Techniques to Reconstruct Incomplete 19th-Century Cantonese Chess Pieces
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London Congestion Charging: Successes, Gaps and Future Opportunities Offered by Cooperative ITS
The London Congestion Charging (LCC) scheme was initially introduced on 17 February 2003. Being the largest of its kind and employing advanced technology, it marked a major innovation in the field of urban road user charging and provided inspiration to several other cities worldwide. Nine years on, and following a number of operational changes that have taken place, this study analyzes successes and pitfalls, and identifies potential future opportunities in the light of latest technological developments in the field of cooperative Intelligent Transport Systems (ITS). The analysis concentrates primarily on the LCC scheme itself, but draws broader conclusions about the future of urban road charging in general
Condensation transition in DNA-polyaminoamide dendrimer fibers studied using optical tweezers
When mixed together, DNA and polyaminoamide (PAMAM) dendrimers form fibers
that condense into a compact structure. We use optical tweezers to pull
condensed fibers and investigate the decondensation transition by measuring
force-extension curves (FECs). A characteristic plateau force (around 10 pN)
and hysteresis between the pulling and relaxation cycles are observed for
different dendrimer sizes, indicating the existence of a first-order transition
between two phases (condensed and extended) of the fiber. The fact that we can
reproduce the same FECs in the absence of additional dendrimers in the buffer
medium indicates that dendrimers remain irreversibly bound to the DNA backbone.
Upon salt variation FECs change noticeably confirming that electrostatic forces
drive the condensation transition. Finally, we propose a simple model for the
decondensing transition that qualitatively reproduces the FECs and which is
confirmed by AFM images.Comment: Latex version, 4 pages+3 color figure
Absolute Proper Motion of the Fornax Dwarf Spheroidal Galaxy from Photographic and HST WFPC2 Data
We have measured the absolute proper motion of the Fornax dwarf spheroidal
galaxy from a combination of photographic plate material and HST WFPC2 data
that provide a time baseline of up to 50 years. The extragalactic reference
frame consists of 8 QSO images and 48 galaxies. The absolute proper motion is
mu_alpha cos(delta) = 0.59 +-0.16 mas/yr and mu_delta = -0.15 +- 0.16 mas/yr.
The corresponding orbit of Fornax is polar, with an eccentricity of 0.27, and a
radial period of 4.5 Gyr. Fornax's current location is near pericenter. The
direction of the motion of Fornax supports the notion that Fornax belongs to
the Fornax-LeoI-LeoII-Sculptor-Sextans stream as hypothesized by Lynden-Bell
(1976, 1982) and Majewski (1994).
According to our orbit determination, Fornax crossed the Magellanic plane
\~190 Myr ago, a time that coincides with the termination of the star-formation
process in Fornax. We propose that ram-pressure stripping due to the passage of
Fornax through a gaseous medium denser than the typical intragalactic medium
left behind from the LMC may have caused the end of star formation in Fornax.
The excess, anomalous clouds within the South Galactic pole region of the
Magellanic Stream whose origin has long been debated in the literature as
constituents of either the Magellanic Stream or of the extragalactic Sculptor
group, are found to lie along the orbit of Fornax. We speculate that these
clouds are stripped material from Fornax as the dwarf crossed the Magellanic
Clouds' orbit.Comment: Accepted for publication in Astronomical Journal. The version with
high resolution figures can be found at
ftp://pegasus.astro.yale.edu/pub/dana/paper
Critical Current Oscillations in Strong Ferromagnetic Pi-Junctions
We report magnetic and electrical measurements of Nb Josephson junctions with
strongly ferromagnetic barriers of Co, Ni and Ni80Fe20 (Py). All these
materials show multiple oscillations of critical current with barrier thickness
implying repeated 0-pi phase-transitions in the superconducting order
parameter. We show in particular that the Co barrier devices can be accurately
modelled using existing clean limit theories and so that, despite the high
exchange energy (309 meV), the large IcRN value in the pi-state means Co
barriers are ideally suited to the practical development of superconducting
pi-shift devices.Comment: 4 pages 3 figures 1 table. Revised version as accepted for
publication. To appear in Physical Review Letter
Local Hidden Variable Theories for Quantum States
While all bipartite pure entangled states violate some Bell inequality, the
relationship between entanglement and non-locality for mixed quantum states is
not well understood. We introduce a simple and efficient algorithmic approach
for the problem of constructing local hidden variable theories for quantum
states. The method is based on constructing a so-called symmetric
quasi-extension of the quantum state that gives rise to a local hidden variable
model with a certain number of settings for the observers Alice and Bob.Comment: 8 pages Revtex; v2 contains substantial changes, a strengthened main
theorem and more reference
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