2,220 research outputs found
Disorder promotes ferromagnetism: Rounding of the quantum phase transition in Sr_{1-x}Ca_xRuO_3
The subtle interplay of randomness and quantum fluctuations at low
temperatures gives rise to a plethora of unconventional phenomena in systems
ranging from quantum magnets and correlated electron materials to ultracold
atomic gases. Particularly strong disorder effects have been predicted to occur
at zero-temperature quantum phase transitions. Here, we demonstrate that the
composition-driven ferromagnetic-to-paramagnetic quantum phase transition in
Sr1-xCaxRuO3 is completely destroyed by the disorder introduced via the
different ionic radii of the randomly distributed Sr and Ca ions. Using a
magneto-optical technique, we map the magnetic phase diagram in the
composition-temperature space. We find that the ferromagnetic phase is
significantly extended by the disorder and develops a pronounced tail over a
broad range of the composition x. These findings are explained by a microscopic
model of smeared quantum phase transitions in itinerant magnets. Moreover, our
theoretical study implies that correlated disorder is even more powerful in
promoting ferromagnetism than random disorder.Comment: 15 pages, 4 figures, submitted to Phys. Rev. Let
Fluctuating-friction molecular motors
We show that the correlated stochastic fluctuation of the friction
coefficient can give rise to long-range directional motion of a particle
undergoing Brownian random walk in a constant periodic energy potential
landscape. The occurrence of this motion requires the presence of two
additional independent bodies interacting with the particle via friction and
via the energy potential, respectively, which can move relative to each other.
Such three-body system generalizes the classical Brownian ratchet mechanism,
which requires only two interacting bodies. In particular, we describe a simple
two-level model of fluctuating-friction molecular motor that can be solved
analytically. In our previous work [M.K., L.M and D.P. 2000 J. Nonlinear Opt.
Phys. Mater. vol. 9, 157] this model has been first applied to understanding
the fundamental mechanism of the photoinduced reorientation of dye-doped liquid
crystals. Applications of the same idea to other fields such as molecular
biology and nanotechnology can however be envisioned. As an example, in this
paper we work out a model of the actomyosin system based on the
fluctuating-friction mechanism.Comment: to be published in J. Physics Condensed Matter
(http://www.iop.org/Journals/JPhysCM
Dark matter annihilation at the galactic center
If cold dark matter is present at the galactic center, as in current models
of the dark halo, it is accreted by the central black hole into a dense spike.
Particle dark matter then annihilates strongly inside the spike, making it a
compact source of photons, electrons, positrons, protons, antiprotons, and
neutrinos. The spike luminosity depends on the density profile of the inner
halo: halos with finite cores have unnoticeable spikes, while halos with inner
cusps may have spikes so bright that the absence of a detected neutrino signal
from the galactic center already places interesting upper limits on the density
slope of the inner halo. Future neutrino telescopes observing the galactic
center could probe the inner structure of the dark halo, or indirectly find the
nature of dark matter.Comment: 4 pages, 5 figure
Stereotyped odor-evoked activity in the mushroom body of Drosophila revealed by green fluorescent protein-based Ca2+ imaging
To study the representation of olfactory information in higher brain centers, we expressed a green fluorescent protein-based Ca2+ sensor, G-CaMP, in the Drosophila mushroom body (MB). Using two-photon microscopy, we imaged odor-evoked G-CaMP fluorescence transients in MB neurons [Kenyon cells (KCs)] with single-cell resolution. Odors produced large fluorescence transients in a subset of KC somata and in restricted regions of the calyx, the neuropil of the MB. In different KCs, odor-evoked fluorescence transients showed diverse changes with odor concentration: in some KCs, fluorescence transients were evoked by an odor at concentrations spanning several orders of magnitude, whereas in others only at a narrow concentration range. Different odors produced fluorescence transients in different subsets of KCs. The spatial distributions of KCs showing fluorescence transients evoked by a given odor were similar across individuals. For some odors, individual KCs with fluorescence transients evoked by a particular odor could be found in similar locations in different flies with spatial precisions on the order of the size of KC somata. These results indicate that odor-evoked activity can have remarkable spatial specificity in the MB
Experimentação em rede de genótipos de trigo para duplo propósito no Rio Grande do Sul em 2000.
bitstream/item/84111/1/CNPT-BOL.-PESQ.-5-01.pd
Perturbation Theory for Path Integrals of Stiff Polymers
The wormlike chain model of stiff polymers is a nonlinear -model in
one spacetime dimension in which the ends are fluctuating freely. This causes
important differences with respect to the presently available theory which
exists only for periodic and Dirichlet boundary conditions. We modify this
theory appropriately and show how to perform a systematic large-stiffness
expansions for all physically interesting quantities in powers of ,
where is the length and the persistence length of the polymer. This
requires special procedures for regularizing highly divergent Feynman integrals
which we have developed in previous work. We show that by adding to the
unperturbed action a correction term , we can calculate
all Feynman diagrams with Green functions satisfying Neumann boundary
conditions. Our expansions yield, order by order, properly normalized
end-to-end distribution function in arbitrary dimensions , its even and odd
moments, and the two-point correlation function
Experimentação de trigo para duplo propósito no Rio Grande do Sul, em 1999.
bitstream/item/84110/1/CNPT-BOL.-PESQ.-5-00.pd
The Classic: Integration of Deoxyribonucleic Acid Specific for Rous Sarcoma Virus after Infection of Permissive and Nonpermissive Hosts: (RNA tumor viruses/reassociation kinetics/duck cells)
A relatively simple but stringent technique was developed to detect the integration of virus-specific DNA into the genomes of higher organisms. In both permissive (duck) and nonpermissive (mammalian) cells which normally contain no nucleotide sequences specific for Rous sarcoma virus, transformation by the virus results in the appearance of DNA specific for Rous sarcoma virus covalently integrated into strands of host-cell DNA containing reiterated sequences. Early after infection of mouse or duck cells by Rous sarcoma virus, unintegrated DNA specific for the virus can be demonstrated
Expression of Interest: The Atmospheric Neutrino Neutron Interaction Experiment (ANNIE)
Submitted for the January 2014 Fermilab Physics Advisory Committee meetingSubmitted for the January 2014 Fermilab Physics Advisory Committee meetingSubmitted for the January 2014 Fermilab Physics Advisory Committee meetingSubmitted for the January 2014 Fermilab Physics Advisory Committee meetingNeutron tagging in Gadolinium-doped water may play a significant role in reducing backgrounds from atmospheric neutrinos in next generation proton-decay searches using megaton-scale Water Cherenkov detectors. Similar techniques might also be useful in the detection of supernova neutrinos. Accurate determination of neutron tagging efficiencies will require a detailed understanding of the number of neutrons produced by neutrino interactions in water as a function of momentum transferred. We propose the Atmospheric Neutrino Neutron Interaction Experiment (ANNIE), designed to measure the neutron yield of atmospheric neutrino interactions in gadolinium-doped water. An innovative aspect of the ANNIE design is the use of precision timing to localize interaction vertices in the small fiducial volume of the detector. We propose to achieve this by using early production of LAPPDs (Large Area Picosecond Photodetectors). This experiment will be a first application of these devices demonstrating their feasibility for Water Cherenkov neutrino detectors
Spin effects in the magneto-drag between double quantum wells
We report on the selectivity to spin in a drag measurement. This selectivity
to spin causes deep minima in the magneto-drag at odd fillingfactors for
matched electron densities at magnetic fields and temperatures at which the
bare spin energy is only one tenth of the temperature. For mismatched densities
the selectivity causes a novel 1/B-periodic oscillation, such that negative
minima in the drag are observed whenever the majority spins at the Fermi
energies of the two-dimensional electron gasses (2DEGs) are anti-parallel, and
positive maxima whenever the majority spins at the Fermi energies are parallel.Comment: 4 pages, 3 figure
- …