13,430 research outputs found
Hopping polaronic motion in high-Tc superconductors
No full textThe motion of small polarons is studied as a function of temperature in the context of a molecular crystal model in which the discreteness of the lattice is accounted for. The polaron bandwidth and the site jump hopping probability have been calculated vs. temperature and dimensionality. The crossover temperature Td* between band-like and hopping motion is reduced in low-dimensional systems due to the enhanced importance of the off-diagonal scattering processes. We discuss the relevance of the model to high-Tc superconductors in which polaronic features in the transport properties have been pointed out [M. Zoli, Phys. Rev. B 53 (1996) 9074; M. Zoli, Phys. Rev. B 56 (1997) 111]
Presentazione
No full textIl saggio introduttivo fa da prefazione al volume che costituisce il risultato della ricerca interuniversitaria Working Poor NEEDS: NEw Equity, Decent work and Skills. In particolare, spiega i vari aspetti e punti di vista che vengono in rilievo nell’affrontare il tema della povertà nonostante l’occupazione: il lavoro dignitoso, come obiettivo e antagonista dell’in-work poverty, la dimensione economica del fenomeno, l'importanza dell'investimento per le politiche attive del lavoro, l’attenzione alle politiche per l’occupazione, le politiche per assicurare a tutte le persone che entrano nel mercato del lavoro di ottenere un livello di reddito non solo minimo, ma anche adeguato, e il divario di genere nel mercato del lavoro
Dal lavoro povero al lavoro dignitoso. Politiche, strumenti, proposte
No full textLa pubblicazione costituisce uno dei principali risultati di una ricerca interuniversitaria – ampia e multidisciplinare – finanziata dal Ministero nell’ambito del PRIN 2017 sul tema Working Poor NEEDS: NEw Equity, Decent work and Skills che ha visto coinvolte quattro Università (Dipartimento di Scienze giuridiche dell’Università degli Studi di Udine, Dipartimento di Scienze giuridiche dell’Università degli Studi di Bologna, Dipartimento di Ingegneria industriale e dell’Informazione e dell’Economia dell’Università degli Studi di L’Aquila, Dipartimento di Scienze giuridiche “Cesare Beccaria” dell’Università degli Studi di Milano) con i rispettivi gruppi di lavoro coor- dinati da responsabili locali, rispettivamente in persona dei professori Marina Brollo (Principal Investigator del PRIN), Carlo Zoli, Pietro Lambertucci e Marco Biasi
Estimate of the Polaron Mass in High Tc Superconductors
No full textI study the motion of polarons as a function of temperature in the context of a molecular crystal model in which the discreteness of the lattice is accounted for. The model is based on a non linear Schrödinger equation which can be solved perturbatively if the conditions for the existence of small polarons are assumed. The polaron bandwidth and the site jump hopping probability have been calculated versus temperature and dimensionality. The crossover temperature View the MathML source between band-like and hopping motion is reduced in low-dimensional systems due to the enhanced importance of the off-diagonal scattering processes. An Einstein phonon spectrum leads to wrong estimates of the polaron bandwidths. The first and second neighbors intermolecular force constants which parametrize the pair interactions strongly affect the values of the ground state polaron bandwidth and of the hopping probability. We discuss the relevance of the model to high-Tc superconductors in which polaronic features in the transport properties have been pointed out [1]. The estimated effective polaron masses are consistent with Tc values of order ≃100 K
Thermodynamics of Twisted DNA with Solvent Interaction
No full textThe imaginary time path integral formalism is applied to a nonlinear Hamiltonian for a short fragment
of heterogeneous DNA with a stabilizing solvent interaction term. Torsional effects are modeled by
a twist angle between neighboring base pairs stacked along the molecule backbone. The base pair
displacements are described by an ensemble of temperature dependent paths thus incorporating those
fluctuational effects which shape the multisteps thermal denaturation. By summing over ∼107−108
base pair paths, a large number of double helix configurations is taken into account consistently with
the physical requirements of the model potential. The partition function is computed as a function
of the twist. It is found that the equilibrium twist angle, peculiar of B-DNA at room temperature,
yields the stablest helicoidal geometry against thermal disruption of the base pair hydrogen bonds.
This result is corroborated by the computation of thermodynamical properties such as fractions of
open base pairs and specific heat. © 2011 American Institute of Physic
Twist-stretch profiles of DNA chains
No full textHelical molecules change their twist number under the effect of a mechanical load. We study the twist-stretch relation for a set of short DNA molecules modeled by a mesoscopic Hamiltonian. Finite temperature path integral techniques are applied to generate a large ensemble of possible configurations for the base pairs of the sequence. The model also accounts for the bending and twisting fluctuations between adjacent base pairs along the molecules stack. Simulating a broad range of twisting conformation, we compute the helix structural parameters by averaging over the ensemble of base pairs configurations. The method selects, for any applied force, the average twist angle which minimizes the molecule's free energy. It is found that the chains generally over-twist under an applied stretching and the over-twisting is physically associated to the contraction of the average helix diameter, i.e. to the damping of the base pair fluctuations. Instead, assuming that the maximum amplitude of the bending fluctuations may decrease against the external load, the DNA molecule first over-twists for weak applied forces and then untwists above a characteristic force value. Our results are discussed in relation to available experimental information albeit for kilo-base long molecules
Short DNA persistence length in a mesoscopic helical model
No full textThe flexibility of short DNA chains is investigated via computation of the average
correlation function between dimers which defines the persistence length. Path integration tech-
niques have been applied to confine the phase space available to base pair fluctuations and derive
the partition function. The apparent persistence lengths of a set of short chains have been com-
puted as a function of the twist conformation both in the over-twisted and the untwisted regimes,
whereby the equilibrium twist is selected by free energy minimization. The obtained values are
significantly lower than those generally attributed to kilo-base long DNA. This points to an in-
trinsic helix flexibility at short length scales, arising from large fluctuational effects and local
bending, in line with recent experimental indications. The interplay between helical untwisting
and persistence length has been discussed for a heterogeneous fragment by weighing the effects of
the sequence specificities through the non-linear stacking potential
Twisting short dsDNA with applied tension
No full textThe twisting deformation of mechanically stretched DNA molecules is studied by a coarse grained Hamiltonian model incorporating the fundamental interactions that stabilize the double helix and accounting for the radial and angular base pair fluctuations. The latter are all the more important at short length scales in which DNA fragments maintain an intrinsic flexibility. The presented computational method simulates a broad ensemble of possible molecule conformations characterized by a specific average twist and determines the energetically most convenient helical twist by free energy minimization. As this is done for any external load, the method yields the characteristic twist-stretch profile of the molecule and also computes the changes in the macroscopic helix parameters i.e. average diameter and rise distance. It is predicted that short molecules under stretching should first over-twist and then untwist by increasing the external load. Moreover, applying a constant load and simulating a torsional strain which over-twists the helix, it is found that the average helix diameter shrinks while the molecule elongates, in agreement with the experimental trend observed in kilo-base long sequences. The quantitative relation between percent relative elongation and superhelical density at fixed load is derived. The proposed theoretical model and computational method offer a general approach to characterize specific DNA fragments and predict their macroscopic elastic response as a function of the effective potential parameters of the mesoscopic Hamiltonian
Entropic Penalties in Circular DNA Assembly
No full textThe thermodynamic properties of DNA circular molecules are investigated by a new path integral computational method which treats in the real space the fundamental forces stabilizing the molecule. The base pair and stacking contributions to the classical action are evaluated separately by simulating a broad ensemble of twisted conformations. We obtain, for two short sequences, a free energy landscape with multiple wells corresponding to the most convenient values of helical repeat. Our results point to a intrinsic flexibility of the circular structures in which the base pair fluctuations move the system from one well to the next thus causing the local unwinding of the helix.
The latter is more pronounced in the shorter sequence whose cyclization causes a higher bending stress. The entropic reductions associated to the formation of the ordered helicoidal structure are estimated
Path Integral Description of a Semiclassical Su-Schrieffer-Heeger Model
No full textThe electron motion along a chain is described by a continuum version of the Su-Schrieffer-Heeger Hamiltonian,
in which phonon fields and electronic coordinates are mapped onto the time scale. The path-integral
formalism allows us to derive the nonlocal source action for the particle interacting with the oscillators bath.
The method can be applied for any value of the e-ph coupling. The path-integral dependence on the model
parameters has been analyzed by computing the partition function and some thermodynamical properties from
T51 K up to room temperature. A peculiar upturn in the low-temperature heat capacity over temperature ratio
~pointing to a glassy like behavior! has been ascribed to the time-dependent electronic hopping along the chain
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