177 research outputs found
Spontaneous symmetry breaking and Lifshitz transition in bilayer graphene
We derive the renormalization group equations describing all the short-range
interactions in bilayer graphene allowed by symmetry and the long range Coulomb
interaction. For certain range of parameters, we predict the first order phase
transition to the uniaxially deformed gapless state accompanied by the change
of the topology of the electron spectrum.Comment: 4 pages, 3 figure
Magnetism and domain formation in SU(3)-symmetric multi-species Fermi mixtures
We study the phase diagram of an SU(3)-symmetric mixture of three-component
ultracold fermions with attractive interactions in an optical lattice,
including the additional effect on the mixture of an effective three-body
constraint induced by three-body losses. We address the properties of the
system in by using dynamical mean-field theory and variational Monte
Carlo techniques. The phase diagram of the model shows a strong interplay
between magnetism and superfluidity. In the absence of the three-body
constraint (no losses), the system undergoes a phase transition from a color
superfluid phase to a trionic phase, which shows additional particle density
modulations at half-filling. Away from the particle-hole symmetric point the
color superfluid phase is always spontaneously magnetized, leading to the
formation of different color superfluid domains in systems where the total
number of particles of each species is conserved. This can be seen as the SU(3)
symmetric realization of a more general tendency to phase-separation in
three-component Fermi mixtures. The three-body constraint strongly disfavors
the trionic phase, stabilizing a (fully magnetized) color superfluid also at
strong coupling. With increasing temperature we observe a transition to a
non-magnetized SU(3) Fermi liquid phase.Comment: 36 pages, 17 figures; Corrected typo
Evidence for a Novel Reaction Mechanism of a Prompt Shock-Induced Fission Following the Fusion of 78Kr and 40Ca Nuclei at E/A =10 MeV
An analysis of experimental data from the inverse-kinematics ISODEC
experiment on 78Kr+40Ca reaction at a bombarding energy of 10 AMeV has revealed
signatures of a hitherto unknown reaction mechanism, intermediate between the
classical damped binary collisions and fusion-fission, but also substantially
different from what is being termed in the literature as fast fission or quasi
fission. These signatures point to a scenario where the system fuses
transiently while virtually equilibrating mass asymmetry and energy and, yet,
keeping part of the energy stored in a collective shock-imparted and, possibly,
angular momentum bearing form of excitation. Subsequently the system fissions
dynamically along the collision or shock axis with the emerging fragments
featuring a broad mass spectrum centered around symmetric fission, relative
velocities somewhat higher along the fission axis than in transverse direction,
and virtually no intrinsic spin. The class of massasymmetric fission events
shows a distinct preference for the more massive fragments to proceed along the
beam direction, a characteristic reminiscent of that reported earlier for
dynamic fragmentation of projectile-like fragments alone and pointing to the
memory of the initial mass and velocity distribution.Comment: 5 PAGES, 6 FIGURE
A statistical interpretation of the correlation between intermediate mass fragment multiplicity and transverse energy
Multifragment emission following Xe+Au collisions at 30, 40, 50 and 60 AMeV
has been studied with multidetector systems covering nearly 4-pi in solid
angle. The correlations of both the intermediate mass fragment and light
charged particle multiplicities with the transverse energy are explored. A
comparison is made with results from a similar system, Xe+Bi at 28 AMeV. The
experimental trends are compared to statistical model predictions.Comment: 7 pages, submitted to Phys. Rev.
Isolation and Characterization of the Saccharomyces cerevisiae DPP1 Gene Encoding Diacylglycerol Pyrophosphate Phosphatase
Diacylglycerol pyrophosphate (DGPP) is involved in a putative novel lipid signaling pathway. DGPP phosphatase (DGPP phosphohydrolase) is a membrane-associated 34-kDa enzyme fromSaccharomyces cerevisiae which catalyzes the dephosphorylation of DGPP to yield phosphatidate (PA) and then catalyzes the dephosphorylation of PA to yield diacylglycerol. Amino acid sequence information derived from DGPP phosphatase was used to identify and isolate the DPP1(diacylglycerol pyrophosphatephosphatase) gene encoding the enzyme. Multicopy plasmids containing the DPP1 gene directed a 10-fold overexpression of DGPP phosphatase activity in S. cerevisiae. The heterologous expression of the S. cerevisiae DPP1 gene in Sf-9 insect cells resulted in a 500-fold overexpression of DGPP phosphatase activity over that expressed in wild-type S. cerevisiae. DGPP phosphatase possesses a Mg2+-independent PA phosphatase activity, and its expression correlated with the overexpression of DGPP phosphatase activity in S. cerevisiae and in insect cells. DGPP phosphatase was predicted to be an integral membrane protein with six transmembrane-spanning domains. The enzyme contains a novel phosphatase sequence motif found in a superfamily of phosphatases. Adpp1Δ mutant was constructed by deletion of the chromosomal copy of the DPP1 gene. The dpp1Δ mutant was viable and did not exhibit any obvious growth defects. The mutant was devoid of DGPP phosphatase activity and accumulated (4-fold) DGPP. Analysis of the mutant showed that the DPP1 gene was not responsible for all of the Mg2+-independent PA phosphatase activity in S. cerevisiae
Contemporary presence of dynamical and statistical production of intermediate mass fragments in midperipheral Ni+Ni collisions at 30 MeV/nucleon
The reaction at 30 MeV/nucleon has been experimentally
investigated at the Superconducting Cyclotron of the INFN Laboratori Nazionali
del Sud. In midperipheral collisions the production of massive fragments
(4Z12), consistent with the statistical fragmentation of the
projectile-like residue and the dynamical formation of a neck, joining
projectile-like and target-like residues, has been observed. The fragments
coming from these different processes differ both in charge distribution and
isotopic composition. In particular it is shown that these mechanisms leading
to fragment production act contemporarily inside the same event.Comment: 9 pages, minor correction
Temperature Dependence of Backbone Dynamics in Human Ileal Bile Acid-Binding Protein: Implications for the Mechanism of Ligand Binding
Human ileal bile acid-binding protein (I-BABP), a member of the family of intracellular lipid binding proteins plays a key role in the cellular trafficking and metabolic regulation of bile salts. The protein has two internal and, according to a recent study, an additional superficial binding site and binds di- and trihydroxy bile salts with positive cooperativity and a high degree of site-selectivity. Previously, in the apo form, we have identified an extensive network of conformational fluctuations on the millisecond time scale, which cease upon ligation. Additionally, ligand binding at room temperature was found to be accompanied by a slight rigidification of picosecond-nanosecond (ps-ns) backbone flexibility. In the current study, temperature-dependent N-15 NMR spin relaxation measurements were used to gain more insight into the role of dynamics in human I-BABP-bile salt recognition. According to our analysis, residues sensing a conformational exchange in the apo state can be grouped into two clusters with slightly different exchange rates. The entropy-enthalpy compensation observed for both clusters suggests a disorder-order transition between a ground and a sparsely populated higher energy state in the absence of ligands. Analysis of the faster, ps-ns motion of N-15-H-1 bond vectors indicates an unusual nonlinear temperature-dependence for both ligation states. Intriguingly, while bile salt binding results in a more uniform response to temperature change throughout the protein, the temperature derivative of the generalized order parameter shows different responses to temperature increase for the two forms of the protein in the investigated temperature range. Analysis of both slow and fast motions in human I-BABP indicates largely different energy landscapes for the apo and halo states suggesting that optimization of binding interactions might be achieved by altering the dynamic behavior of specific segments in the protein
Isospin Physics in Heavy-Ion Collisions at Intermediate Energies
In nuclear collisions induced by stable or radioactive neutron-rich nuclei a
transient state of nuclear matter with an appreciable isospin asymmetry as well
as thermal and compressional excitation can be created. This offers the
possibility to study the properties of nuclear matter in the region between
symmetric nuclear matter and pure neutron matter. In this review, we discuss
recent theoretical studies of the equation of state of isospin-asymmetric
nuclear matter and its relations to the properties of neutron stars and
radioactive nuclei. Chemical and mechanical instabilities as well as the
liquid-gas phase transition in asymmetric nuclear matter are investigated. The
in-medium nucleon-nucleon cross sections at different isospin states are
reviewed as they affect significantly the dynamics of heavy ion collisions
induced by radioactive beams. We then discuss an isospin-dependent transport
model, which includes different mean-field potentials and cross sections for
the proton and neutron, and its application to these reactions. Furthermore, we
review the comparisons between theoretical predictions and available
experimental data. In particular, we discuss the study of nuclear stopping in
terms of isospin equilibration, the dependence of nuclear collective flow and
balance energy on the isospin-dependent nuclear equation of state and cross
sections, the isospin dependence of total nuclear reaction cross sections, and
the role of isospin in preequilibrium nucleon emissions and subthreshold pion
production.Comment: 101 pages with embedded epsf figures, review article for
"International Journal of Modern Physics E: Nuclear Physics". Send request
for a hard copy to 1/author
Comparing nuclear power trajectories in Germany and the UK: from ‘regimes' to ‘democracies’ in sociotechnical transitions and Discontinuities
This paper focuses on arguably the single most striking contrast in contemporary major energy politics in Europe (and even the developed world as a whole): the starkly differing civil nuclear policies of Germany and the UK. Germany is seeking entirely to phase out nuclear power by 2022. Yet the UK advocates a ‘nuclear renaissance’, promoting the most ambitious new nuclear construction programme in Western Europe.Here,this paper poses a simple yet quite fundamental question: what are the particular divergent conditions most strongly implicated in the contrasting developments in these two countries. With nuclear playing such an iconic role in historical discussions over technological continuity and transformation, answering this may assist in wider understandings of sociotechnical incumbency and discontinuity in the burgeoning field of‘sustainability transitions’. To this end, an ‘abductive’ approach is taken: deploying nine potentially relevant criteria for understanding the different directions pursued in Germany and the UK. Together constituted by 30 parameters spanning literatures related to socio-technical regimes in general as well as nuclear technology in particular, the criteria are divided into those that are ‘internal’ and ‘external’ to the ‘focal regime configuration’ of nuclear power and associated ‘challenger technologies’ like renewables.
It is ‘internal’ criteria that are emphasised in conventional sociotechnical regime theory, with ‘external’ criteria relatively less well explored. Asking under each criterion whether attempted discontinuation of nuclear power would be more likely in Germany or the UK, a clear picture emerges. ‘Internal’ criteria suggest attempted nuclear discontinuation should be more likely in the UK than in Germany– the reverse of what is occurring.
‘External’ criteria are more aligned with observed dynamics –especially those relating to military nuclear commitments and broader ‘qualities of democracy’. Despite many differences of framing concerning exactly what constitutes ‘democracy’, a rich political science literature on this point is unanimous in characterising Germany more positively than the UK. Although based only on a single case,a potentially important question is nonetheless raised as to whether sociotechnical regime theory might usefully give greater attention to the general importance of various aspects of democracy in constituting conditions for significant technological discontinuities and transformations. If so, the policy implications are significant. A number of important areas are identified for future research, including the roles of diverse understandings and specific aspects of democracy and the particular relevance of military nuclear commitments– whose under-discussion in civil nuclear policy literatures raises its own questions of democratic accountability
Spallation reactions. A successful interplay between modeling and applications
The spallation reactions are a type of nuclear reaction which occur in space
by interaction of the cosmic rays with interstellar bodies. The first
spallation reactions induced with an accelerator took place in 1947 at the
Berkeley cyclotron (University of California) with 200 MeV deuterons and 400
MeV alpha beams. They highlighted the multiple emission of neutrons and charged
particles and the production of a large number of residual nuclei far different
from the target nuclei. The same year R. Serber describes the reaction in two
steps: a first and fast one with high-energy particle emission leading to an
excited remnant nucleus, and a second one, much slower, the de-excitation of
the remnant. In 2010 IAEA organized a worskhop to present the results of the
most widely used spallation codes within a benchmark of spallation models. If
one of the goals was to understand the deficiencies, if any, in each code, one
remarkable outcome points out the overall high-quality level of some models and
so the great improvements achieved since Serber. Particle transport codes can
then rely on such spallation models to treat the reactions between a light
particle and an atomic nucleus with energies spanning from few tens of MeV up
to some GeV. An overview of the spallation reactions modeling is presented in
order to point out the incomparable contribution of models based on basic
physics to numerous applications where such reactions occur. Validations or
benchmarks, which are necessary steps in the improvement process, are also
addressed, as well as the potential future domains of development. Spallation
reactions modeling is a representative case of continuous studies aiming at
understanding a reaction mechanism and which end up in a powerful tool.Comment: 59 pages, 54 figures, Revie
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