347 research outputs found
Universal insulating-to-metallic crossover in tight-binding random geometric graphs
Within the scattering matrix approach to electronic transport, the scattering
and transport properties of tight-binding random graphs are analyzed. In
particular, we compute the scattering matrix elements, the transmission, the
channel-to-channel transmission distributions (including the total transmission
distribution), the shot noise power, and the elastic enhancement factor. Two
graph models are considered: random geometric graphs and bipartite random
geometric graphs. The results show an insulating to a metallic crossover in the
scattering and transport properties by increasing the average degree of the
graphs from small to large values. Also, the scattering and transport
properties are shown to be invariant under a scaling parameter depending on the
average degree and the graph size. Furthermore, for large connectivity and in
the perfect coupling regime, the scattering and transport properties of both
graph models are well described by the random matrix theory predictions of
electronic transport, except for bipartite graphs in particular scattering
setups.Comment: 13 pages, 16 figure
Assessing the Hierarchical Hamiltonian Splitting Integrator for Collisionless N-body Simulations
The N-body problem has become one of the hottest topics in the fields of
computational dynamics and cosmology. The large dynamical range in some
astrophysical problems led to the use of adaptive time steps to integrate
particle trajectories, however, the search of optimal strategies is still
challenging. We quantify the performance of the hierarchical time step
integrator Hamiltonian Splitting (HamSp) for collisionless multistep
simulations. We compare with the constant step Leap-Frog (LeapF) integrator and
the adaptive one (AKDK). Additionally, we explore the impact of different time
step assigning functions. There is a computational overhead in HamSp however
there are two interesting advantages: choosing a convenient time-step function
may compensate and even turn around the efficiency compared with AKDK. We test
both reversibility and time symmetry. The symmetrized nature of the HamSp
integration is able to provide time-reversible integration for medium time
scales and overall deliver better energy conservation for long integration
times, and the linear and angular momentum are preserved at machine precision.
We address the impact of using different integrators in astrophysical systems.
We found that in most situations both AKDK and HamSp are able to correctly
simulate the problems. We conclude that HamSp is an attractive and competitive
alternative to AKDK, with, in some cases, faster and with better energy and
momentum conservation. The use of recently discussed Bridge splitting
techniques with HamSp may allow to reach considerably high efficiency.Comment: 13 pages, 16 figure
Analog quantum chemistry simulation
Quantum computers hold the promise to provide outstanding computational speed
ups in chemical problems, like the determination of the electronic ground state
energy of molecules. Here, we demonstrate that the same goal can be achieved
with an analog quantum simulator which combines two well-established
technologies, namely, ultra-cold atoms in optical lattices and cavity QED. In
the proposed simulator, fermionic atoms hopping in an optical potential play
the role of electrons, additional optical potentials provide the nuclear
attraction, while a single spin excitation over a Mott insulator mediates the
electronic Coulomb repulsion. We analyze the impact of discretization and
finite size effects of the lattice, and provide the working conditions required
for the precise determination of the electronic energy of simple molecules.Comment: 12 pages, 5 figure
Transport studies in three-terminal microwave graphs with orthogonal, unitary, and symplectic symmetry
The Landauer-B\"uttiker formalism establishes an equivalence between the
electrical conduction through a device, e.g., a quantum dot, and the
transmission. Guided by this analogy we perform transmission measurements
through three-port microwave graphs with orthogonal, unitary, and symplectic
symmetry thus mimicking three-terminal voltage drop devices. One of the ports
is placed as input and a second one as output, while a third port is used as a
probe. Analytical predictions show good agreement with the measurements in the
presence of orthogonal and unitary symmetries, provided that the absorption and
the influence of the coupling port are taken into account. The symplectic
symmetry is realized in specifically designed graphs mimicking spin 1/2
systems. Again a good agreement between experiment and theory is found. For the
symplectic case the results are marginally sensitive to absorption and coupling
strength of the port, in contrast to the orthogonal and unitary case.Comment: 6 pages, 6 figure
Nonlinear software sensor for monitoring genetic regulation processes with noise and modeling errors
Nonlinear control techniques by means of a software sensor that are commonly
used in chemical engineering could be also applied to genetic regulation
processes. We provide here a realistic formulation of this procedure by
introducing an additive white Gaussian noise, which is usually found in
experimental data. Besides, we include model errors, meaning that we assume we
do not know the nonlinear regulation function of the process. In order to
illustrate this procedure, we employ the Goodwin dynamics of the concentrations
[B.C. Goodwin, Temporal Oscillations in Cells, (Academic Press, New York,
1963)] in the simple form recently applied to single gene systems and some
operon cases [H. De Jong, J. Comp. Biol. 9, 67 (2002)], which involves the
dynamics of the mRNA, given protein, and metabolite concentrations. Further, we
present results for a three gene case in co-regulated sets of transcription
units as they occur in prokaryotes. However, instead of considering their full
dynamics, we use only the data of the metabolites and a designed software
sensor. We also show, more generally, that it is possible to rebuild the
complete set of nonmeasured concentrations despite the uncertainties in the
regulation function or, even more, in the case of not knowing the mRNA
dynamics. In addition, the rebuilding of concentrations is not affected by the
perturbation due to the additive white Gaussian noise and also we managed to
filter the noisy output of the biological systemComment: 21 pages, 7 figures; also selected in vjbio of August 2005; this
version corrects a misorder in the last three references of the published
versio
Options of partners improve carbon for phosphorus trade in the arbuscular mycorrhizal mutualism
The mutualism between plants and arbuscular mycorrhizal fungi (AMF) is widespread and has persisted for over 400 million years. Although this mutualism depends on fair resource exchange between plants and fungi, inequality exists among partners despite mechanisms that regulate trade. Here, we use (33) P and (14) C isotopes and a split-root system to test for preferential allocation and reciprocal rewards in the plant-AMF symbiosis by presenting a plant with two AMF that differ in cooperativeness. We found that plants received more (33) P from less cooperative AMF in the presence of another AMF species. This increase in (33) P resulted in a reduced (14) C cost per unit of (33) P from less cooperative AMF when alternative options were available. Our results indicate that AMF diversity promotes cooperation between plants and AMF, which may be an important mechanism maintaining the evolutionary persistence of and diversity within the plant-AMF mutualism
Parámetros de calidad de la canal y la carne de cabritos lechales de raza payoya en sistema de explotación convencional
En el trabajo se han elegido 24 cabritos pertenecientes a la raza Payoya,
12 machos y 12 hembras con el objeto de comparar los parámetros de la calidad de la canal y de la carne. Los resultados nos muestran que aunque existen algunas diferencias en algunas medidas de calidad de la carne (pH, CRA, L*, a*, b*, C y Hº), el resto de los parámetros estudiados no presentaron diferencias significativas entre los dos sexos
Negatively charged amino acids at the foot-and-mouth disease virus capsid reduce the virion-destabilizing effect of viral RNA at acidic pH
Elucidation of the molecular basis of the stability of foot-and-mouth disease virus (FMDV) particles is relevant to understand key aspects of the virus cycle. Residue N17D in VP1, located at the capsid inner surface, modulates the resistance of FMDV virion to dissociation and inactivation at acidic pH. Here we have studied whether the virion-stabilizing effect of amino acid substitution VP1 N17D may be mediated by the alteration of electrostatic charge at this position and/or the presence of the viral RNA. Substitutions that either introduced a positive charge (R,K) or preserved neutrality (A) at position VP1 17 led to increased sensitivity of virions to inactivation at acidic pH, while replacement by negatively charged residues (D,E) increased the resistance of virions to acidic pH. The role in virion stability of viral RNA was addressed using FMDV empty capsids that have a virtually unchanged structure compared to the capsid in the RNA-filled virion, but that are considerably more resistant to acidic pH than WT virions, supporting a virion-destabilizing effect of the RNA. Remarkably, no differences were observed in the resistance to dissociation at acidic pH between the WT empty capsids and those harboring replacement N17D. Thus, the virion-destabilizing effect of viral RNA at acidic pH can be partially restored by introducing negatively charged residues at position VP1 N17Work in F.S´s laboratory was funded by grants from MINECO-FEDER EU (AGL2017–84097-C2–1-R), Comunidad de Madrid co-fnanced with ECFEDER funds (P2018/BAA-4370). Work in M.G.M.´s laboratory was funded by grants from MINECO-FEDER EU (BIO2015–69928-R and RTI2018–096635-B-I00). Work by both groups was also funded by an institutional grant from Fundación Ramón Areces M.G.M. is an associate member of the Institute for Biocomputation and Physics of Complex Systems, Zaragoza, Spai
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