102 research outputs found
A physical limitation of the Wigner "distribution" function in transport
We present an example revealing that the sign of the "momentum" of the
Wigner "distribution" function is not necessarily associated with the
direction of motion in the real world. This aspect, which is not related to the
well known limitation of the Wigner function that traces back to the
Heisenberg's uncertainty principle, is particularly relevant in transport
studies, wherein it is helpful to distinguish between electrons flowing from
electrodes into devices and vice versa
Reply to Comment on "Critical analysis of a variational method used to describe molecular electron transport"
We show that the failure of the Delaney-Greer (DG) variational ansatz for
transport demonstrated by us in Phys.\ Rev.\ B {\bf 80}, 165301 (2009) (I) is
not related to an unsuitable constraint that prevents a broken time-reversal
symmetry or to real orbitals, as DG incorrectly claim. The complex orbitals
suggested by them as a way-out solution merely represent a particular case of
the general case considered by us in I, which do not in the least affect our
conclusion.Comment: Manuscript as submitted to Phys. Rev. B on 30 November 2010. Sections
VII, VIII, and IX present significant details, which enlarge the analysis of
the published versio
Applying the extended molecule approach to correlated electron transport: important insight from model calculations
Theoretical approaches of electronic transport in correlated molecules
usually consider an extended molecule, which includes, in addition to the
molecule itself, parts of electrodes. In the case where electron correlations
remain confined within the molecule, and the extended molecule is sufficiently
large, the current can be expressed by means of Laudauer-type formulae.
Electron correlations are embodied into the retarded Green function of a
sufficiently large but isolated extended molecule, which represents the key
quantity that can be accurately determined by means of ab initio quantum
chemical calculations. To exemplify these ideas, we present and analyze
numerical results obtained within full CI calculations for an extended molecule
described by the interacting resonant level model. Based on them, we argue that
for organic electrodes the transport properties can be reliably computed,
because the extended molecule can be chosen sufficiently small to be tackled
within accurate ab initio methods. For metallic electrodes, larger extended
molecules have to be considered in general, but a (semi-)quantitative
description of the transport should still be possible particularly in the
typical cases where electron transport proceeds by off-resonant tunneling. Our
numerical results also demonstrate that, contrary to the usual claim, the ratio
between the characteristic Coulomb strength and the level width due to
molecule-electrode coupling is not the only quantity needed to assess whether
electron correlation effects are strong or weak
On a method to calculate conductance by means of the Wigner function: two critical tests
We have implemented the linear response approximation of a method proposed to
compute the electron transport through correlated molecules based on the
time-independent Wigner function [P. Delaney and J. C. Greer, \prl {\bf 93},
36805 (2004)]. The results thus obtained for the zero-bias conductance through
quantum dot both without and with correlations demonstrate that this method is
either quantitatively nor qualitatively able to provide a correct physical
escription of the electric transport through nanosystems. We present an
analysis indicating that the failure is due to the manner of imposing the
boundary conditions, and that it cannot be simply remedied.Comment: 22 pages, 7 figur
Bucharest) ⌠61⌠Nr
The control system, presented in this paper, is dedicated to maintain automatically the process of 15 N separation, by chemical exchange in Nitrox system, in its optimal operation conditions. For this purpose one of the most important task is: the minimization of the transient response and of the disturbances effects over the process. These requirements are fulfilled by applying a neuro-fuzzy feed-forward control configuration. Fuzzy logic control systems have been successfully applied to a wide variety of practical problems. The fuzzy systems have three significant advantages over conventional control techniques. They are cheaper to develop, cover a wider range of operating conditions, and are more flexible in terms of natural language. Unfortunately the parameters of a fuzzy control system are inherently difficult to tune for the purpose of improving behavior By integrating neural networks in fuzzy systems a new class of control systems results: -intelligent control systems. Intelligent control is a technology that replaces the human mind in making decisions, planning control strategies, and learning new functions whenever the environment does not allow or does not justify the presence of a human operator. The use of intelligent control systems has infiltrated the modern world. Specific features of intelligent control include decision making, adaptation to uncertain media, self-organization, planning and scheduling operations. Ver y often, no preferred mathematical model is presumed in the problem formulation, and information is presented in a descriptive manner. Therefore, it may be the most effective way to solve complex control tasks of chemical plants. General Architecture of the Control System The developed control system uses two controllers: feedback controller and feed-forward controller ( (1) The feedback controller, placed in the feedback loop, compares the process output y with the reference input r, and if there is a deviation e = r -y, the controller takes action according to the control strategy. The feedforward controller placed in the feed-forward loop reduces the transient response and compensates all measurable disturbances
Metal-insulator transition in the one-dimensional Holstein model at half filling
We study the one-dimensional Holstein model with spin-1/2 electrons at
half-filling. Ground state properties are calculated for long chains with great
accuracy using the density matrix renormalization group method and extrapolated
to the thermodynamic limit. We show that for small electron-phonon coupling or
large phonon frequency, the insulating Peierls ground state predicted by
mean-field theory is destroyed by quantum lattice fluctuations and that the
system remains in a metallic phase with a non-degenerate ground state and
power-law electronic and phononic correlations. When the electron-phonon
coupling becomes large or the phonon frequency small, the system undergoes a
transition to an insulating Peierls phase with a two-fold degenerate ground
state, long-range charge-density-wave order, a dimerized lattice structure, and
a gap in the electronic excitation spectrum.Comment: 6 pages (LaTex), 10 eps figure
PFO-spectrum disorder: two different cerebrovascular diseases in patients with PFO as detected by AI brain imaging software
BackgroundPatent foramen ovale (PFO) is a prevalent cardiac remnant of fetal anatomy that may pose a risk factor for stroke in some patients, while others can present with asymptomatic white matter (WM) lesions. The current study aimed to test the hypothesis that patients with a PFO who have a history of stroke or transient ischemic attack, compared to those without such a history, have a different burden and distribution of cerebral WM hyperintensities. Additionally, we tested the association between PFO morphological characteristics and severity of shunt, and their impact on the occurrence of ischemic cerebral vascular events and on the burden of cerebral WM lesions.Patients and methodsRetrospective, caseâcontrol study that included patients with PFO confirmed by transesophageal echocardiography. Right-to-left shunt size was assessed using transcranial Doppler ultrasound. Cerebral MRIs were analyzed for all participants using the semi-automated Quantib NDTM software for the objective quantification of WM lesions. WM lesions volume was compared between patients with and without a history of stroke. Additionally, the anatomical characteristics of PFOs were assessed to explore their relation to stroke occurrence and WM lesions volume.ResultsOf the initial 264 patients diagnosed with PFO, 67 met the inclusion criteria and were included in the analysis. Of them, 62% had a history of PFO-related stroke/TIA. Overall burden of WM lesions, including stroke volume, was not significantly different (pâ=â0.103). However, after excluding stroke volume, WM lesions volume was significantly higher in patients without stroke (0.27âcm3, IQR 0.03â0.60) compared to those with stroke/TIA (0.08âcm3, IQR 0.02â0.18), pâ=â0.019. Patients with a history of PFO-related stroke/TIA had a tendency to larger PFO sizes by comparison to those without, in terms of length and height, and exhibited greater right-to-left shunt volumes.DiscussionWe suggest that PFO may be associated with the development of two distinct cerebrovascular conditions (stroke and âsilentâ WM lesions), each characterized by unique imaging patterns. Further studies are needed to identify better the âat-riskâ PFOs and gain deeper insights into their clinical implications
Molecule-Electrode Interface Energetics in Molecular Junction: a Transition Voltage Spectroscopy Study
We assess the performances of the transition voltage spectroscopy (TVS)
method to determine the energies of the molecular orbitals involved in the
electronic transport though molecular junctions. A large number of various
molecular junctions made with alkyl chains but with different chemical
structure of the electrode-molecule interfaces are studied. In the case of
molecular junctions with clean, unoxidized electrode-molecule interfaces, i.e.
alkylthiols and alkenes directly grafted on Au and hydrogenated Si,
respectively, we measure transition voltages in the range 0.9 - 1.4 V. We
conclude that the TVS method allows estimating the onset of the tail of the
LUMO density of states, at energy located 1.0 - 1.2 eV above the electrode
Fermi energy. For oxidized interfaces (e.g. the same monolayer measured with Hg
or eGaIn drops, or monolayers formed on a slightly oxidized silicon substrate),
lower transition voltages (0.1 - 0.6 V) are systematically measured. These
values are explained by the presence of oxide-related density of states at
energies lower than the HOMO-LUMO of the molecules. As such, the TVS method is
a useful technique to assess the quality of the molecule-electrode interfaces
in molecular junctions.Comment: Accepted for publication in J. Phys. Chem C. One pdf file including
manuscript, figures and supporting informatio
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