713 research outputs found
First-principles nonequilibrium Green's function approach to transient photoabsorption: Application to atoms
We put forward a first-principle NonEquilibrium Green's Function (NEGF)
approach to calculate the transient photoabsorption spectrum of optically thin
samples. The method can deal with pump fields of arbitrary strength, frequency
and duration as well as for overlapping and nonoverlapping pump and probe
pulses. The electron-electron repulsion is accounted for by the correlation
self-energy, and the resulting numerical scheme deals with matrices that scale
quadratically with the system size. Two recent experiments, the first on helium
and the second on krypton, are addressed. For the first experiment we explain
the bending of the Autler-Townes absorption peaks with increasing the
pump-probe delay \t, and relate the bending to the thickness and density of
the gas. For the second experiment we find that sizable spectral structures of
the pump-generated admixture of Kr ions are fingerprints of {\em dynamical
correlation} effects, and hence they cannot be reproduced by time-local
self-energy approximations. Remarkably, the NEGF approach also captures the
retardation of the absorption onset of Kr with respect to Kr as a
function of \t.Comment: 13 pages, 8 captioned figure
Charge dynamics in molecular junctions: Nonequilibrium Green's Function approach made fast
Real-time Green's function simulations of molecular junctions (open quantum
systems) are typically performed by solving the Kadanoff-Baym equations (KBE).
The KBE, however, impose a serious limitation on the maximum propagation time
due to the large memory storage needed. In this work we propose a simplified
Green's function approach based on the Generalized Kadanoff-Baym Ansatz (GKBA)
to overcome the KBE limitation on time, significantly speed up the
calculations, and yet stay close to the KBE results. This is achieved through a
twofold advance: first we show how to make the GKBA work in open systems and
then construct a suitable quasi-particle propagator that includes correlation
effects in a diagrammatic fashion. We also provide evidence that our GKBA
scheme, although already in good agreement with the KBE approach, can be
further improved without increasing the computational cost.Comment: 13 pages, 13 figure
Correlated Nanoscopic Josephson Junctions
We discuss correlated lattice models with a time-dependent potential across a
barrier and show how to implement a Josephson-junction-like behavior. The
pairing occurs by a correlation effect enhanced by the symmetry of the system.
In order to produce the effect we need a mild distortion which causes avoided
crossings in the many-body spectrum. The Josephson-like response involves a
quasi-adiabatic evolution in the time-dependent field. Besides, we observe an
inverse-Josephson (Shapiro) current by applying an AC bias; a supercurrent in
the absence of electromotive force can also be excited. The qualitative
arguments are supported by explicit exact solutions in prototype 5-atom
clusters with on-site repulsion. These basic units are then combined in
ring-shaped systems, where one of the units sits at a higher potential and
works as a barrier. In this case the solution is found by mapping the
low-energy Hamiltonian into an effective anisotropic Heisenberg chain. Once
again, we present evidence for a superconducting flux quantization, i.e. a
Josephson-junction-like behavior suggesting the build-up of an effective order
parameter already in few-electron systems. Some general implications for the
quantum theory of transport are also briefly discussed, stressing the
nontrivial occurrence of asymptotic current oscillations for long times in the
presence of bound states.Comment: 12 pages, 2 figures, to appear in J. Phys. - Cond. Ma
Cheers: a linear-scaling KBE+GKBA code
The interaction of electrons with quantized phonons and photons underlies the
ultrafast dynamics of systems ranging from molecules to solids, giving rise to
a plethora of physical phenomena experimentally accessible using time-resolved
techniques. Green's function methods offer an invaluable interpretation tool
since scattering mechanisms of growing complexity can be selectively
incorporated in the theory. cheers is a general-purpose nonequilibrium Green's
function code that implements virtually all known many-body approximations and
is designed for first principles studies of ultrafast processes in molecular
and model solid state systems. The aims of generality, extensibility,
efficiency, and user friendliness of the code are achieved through the
underlying theory development and the use of modern software design practices.
Here, we motivate the necessity for the creation of such a code and overview
its design and capabilities.Comment: 15 pages, 4 figure
Time-linear quantum transport simulations with correlated nonequilibrium Green's functions
We present a time-linear scaling method to simulate open and correlated
quantum systems. The method inherits from many-body perturbation theory the
possibility to choose selectively the most relevant scattering processes in the
dynamics, thereby paving the way to the real-time characterization of
correlated ultrafast phenomena in quantum transport. The open system dynamics
is described in terms of an embedding correlator from which the time-dependent
current can be calculated using the Meir-Wingreen formula. We show how to
efficently implement the method through a simple grafting into recently
proposed time-linear Green's function schemes for closed systems.
Electron-electron and electron-phonon interactions can be treated on equal
footing while preserving all fundametal conservation laws.Comment: 6 pages, 3 figure
Equilibrium and time-dependent Josephson current in one-dimensional superconducting junctions
We investigate the transport properties of a one-dimensional
superconductor-normal metal-superconductor (S-N-S) system described within the
tight-binding approximation. We compute the equilibrium dc Josephson current
and the time-dependent oscillating current generated after the switch-on of a
constant bias. In the first case an exact embedding procedure to calculate the
Nambu-Gorkov Keldysh Green's function is employed and used to derive the
continuum and bound states contributions to the dc current. A general formalism
to obtain the Andreev bound states (ABS) of a normal chain connected to
superconducting leads is also presented. We identify a regime in which all
Josephson current is carried by the ABS and obtain an analytic formula for the
current-phase relation in the limit of long chains. In the latter case the
condition for perfect Andreev reflections is expressed in terms of the
microscopic parameters of the model, showing a limitation of the so called
wide-band-limit (WBL) approximation. When a finite bias is applied to the S-N-S
junction we compute the exact time-evolution of the system by solving
numerically the time-dependent Bogoliubov-deGennes equations. We provide a
microscopic description of the electron dynamics not only inside the normal
region but also in the superconductors, thus gaining more information with
respect to WBL-based approaches. Our scheme allows us to study the ac regime as
well as the transient dynamics whose characteristic time-scale is dictated by
the velocity of multiple Andreev reflections
VUV-Vis optical characterization of Tetraphenyl-butadiene films on glass and specular reflector substrates from room to liquid Argon temperature
The use of efficient wavelength-shifters from the vacuum-ultraviolet to the
photosensor's range of sensitivity is a key feature in detectors for Dark
Matter search and neutrino physics based on liquid argon scintillation
detection. Thin film of Tetraphenyl-butadiene (TPB) deposited onto the surface
delimiting the active volume of the detector and/or onto the photosensor
optical window is the most common solution in current and planned experiments.
Detector design and response can be evaluated and correctly simulated only when
the properties of the optical system in use (TPB film + substrate) are fully
understood. Characterization of the optical system requires specific, sometimes
sophisticated optical methodologies. In this paper the main features of TPB
coatings on different, commonly used substrates is reported, as a result of two
independent campaigns of measurements at the specialized optical metrology labs
of ENEA and University of Tor Vergata. Measured features include TPB emission
spectra with lineshape and relative intensity variation recorded as a function
of the film thickness and for the first time down to LAr temperature, as well
as optical reflectance and transmittance spectra of the TPB coated substrates
in the wavelength range of the TPB emission
Antimicrobial peptide human ÎČ-defensin-2 improves in vitro cellular viability and reduces pro-inflammatory effects induced by enteroinvasive Escherichia coli in Caco-2 cells by inhibiting invasion and virulence factorsâ expression
Escherichia coli is one of the commensal species most represented in the intestinal microbiota. However, there are some strains that can acquire new virulence factors that enable them to adapt to new intestinal niches. These include enteroinvasive E. coli (EIEC) that is responsible for the bacillary dysentery that causes severe diarrheal symptoms in both children and adults. Due to the increasing onset of antibiotic resistance phenomena, scientific research is focused on the study of other therapeutic approaches for the treatment of bacterial infections. A promising alternative could be represented by antimicrobial peptides (AMPs), that have received widespread attention due to their broad antimicrobial spectrum and low incidence of bacterial resistance. AMPs modulate the immune defenses of the host and regulate the composition of microbiota and the renewal of the intestinal epithelium. With the aim to investigate an alternative therapeutic approach, especially in the case of antibiotic resistance, in this work we created a line of intestinal epithelial cells able to express high concentrations of AMP human ÎČ-defensin-2 (HBD-2) in order to test its ability to interfere with the pathogenicity mechanisms of EIEC. The results showed that HBD-2 is able to significantly reduce the expression of the proinflammatory cytokines by intestinal epithelial cells, the invasiveness ability of EIEC and the expression of invasion-associated genes
Time-dependent quantum transport with superconducting leads: a discrete basis Kohn-Sham formulation and propagation scheme
In this work we put forward an exact one-particle framework to study
nano-scale Josephson junctions out of equilibrium and propose a propagation
scheme to calculate the time-dependent current in response to an external
applied bias. Using a discrete basis set and Peierls phases for the
electromagnetic field we prove that the current and pairing densities in a
superconducting system of interacting electrons can be reproduced in a
non-interacting Kohn-Sham (KS) system under the influence of different Peierls
phases {\em and} of a pairing field. An extended Keldysh formalism for the
non-equilibrium Nambu-Green's function (NEGF) is then introduced to calculate
the short- and long-time response of the KS system. The equivalence between the
NEGF approach and a combination of the static and time-dependent
Bogoliubov-deGennes (BdG) equations is shown. For systems consisting of a
finite region coupled to superconducting semi-infinite leads we
numerically solve the static BdG equations with a generalized wave-guide
approach and their time-dependent version with an embedded Crank-Nicholson
scheme. To demonstrate the feasibility of the propagation scheme we study two
paradigmatic models, the single-level quantum dot and a tight-binding chain,
under dc, ac and pulse biases. We provide a time-dependent picture of single
and multiple Andreev reflections, show that Andreev bound states can be
exploited to generate a zero-bias ac current of tunable frequency, and find a
long-living resonant effect induced by microwave irradiation of appropriate
frequency.Comment: 20 pages, 9 figures, published versio
Real-world versus trial patients with transthyretin amyloid cardiomyopathy
Transthyretin (TTR) amyloid cardiomyopathy (ATTRâAC) is caused either by singleâpoint mutations in the TTR gene (ATTRvâAC) or by deposition of the wildâtype protein (ATTRwtâAC).1 Long been considered a rare disease, ATTRâAC has been increasingly recognized in recent years, particularly among the elderly,1 mostly due to the possibility of a nonâinvasive diagnosis through bone scintigraph
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