10,291 research outputs found
Crossover from Reptation to Rouse dynamics in the Extended Rubinstein-Duke Model
The competition between reptation and Rouse Dynamics is incorporated in the
Rubinstein-Duke model for polymer motion by extending it with sideways motions,
which cross barriers and create or annihilate hernias. Using the Density-Matrix
Renormalization-Group Method as solver of the Master Equation, the renewal time
and the diffusion coefficient are calculated as function of the length of the
chain and the strength of the sideways motion. These new types of moves have a
strong and delicate influence on the asymptotic behavior of long polymers. The
effects are analyzed as function of the chain length in terms of effective
exponents and crossover scaling functions.Comment: 16 Pages RevTeX and 13 PostScript figures included, accepted for
publication in Phys. Rev.
Kadanoff-Baym approach to time-dependent quantum transport in AC and DC fields
We have developed a method based on the embedded Kadanoff-Baym equations to
study the time evolution of open and inhomogeneous systems. The equation of
motion for the Green's function on the Keldysh contour is solved using
different conserving many-body approximations for the self-energy. Our
formulation incorporates basic conservation laws, such as particle
conservation, and includes both initial correlations and initial embedding
effects, without restrictions on the time-dependence of the external driving
field. We present results for the time-dependent density, current and dipole
moment for a correlated tight binding chain connected to one-dimensional
non-interacting leads exposed to DC and AC biases of various forms. We find
that the self-consistent 2B and GW approximations are in extremely good
agreement with each other at all times, for the long-range interactions that we
consider. In the DC case we show that the oscillations in the transients can be
understood from interchain and lead-chain transitions in the system and find
that the dominant frequency corresponds to the HOMO-LUMO transition of the
central wire. For AC biases with odd inversion symmetry odd harmonics to high
harmonic order in the driving frequency are observed in the dipole moment,
whereas for asymmetric applied bias also even harmonics have considerable
intensity. In both cases we find that the HOMO-LUMO transition strongly mixes
with the harmonics leading to harmonic peaks with enhanced intensity at the
HOMO-LUMO transition energy.Comment: 16 pages, 9 figures. Submitted at "Progress in Nonequilibrium Green's
Functions IV" conferenc
Total energies from variational functionals of the Green function and the renormalized four-point vertex
We derive variational expressions for the grand potential or action in terms
of the many-body Green function which describes the propagation of
particles and the renormalized four-point vertex which describes the
scattering of two particles in many-body systems. The main ingredient of the
variational functionals is a term we denote as the -functional which plays
a role analogously to the usual -functional studied by Baym (G.Baym,
Phys.Rev. 127, 1391 (1962)) in connection with the conservation laws in
many-body systems. We show that any -derivable theory is also
-derivable and therefore respects the conservation laws. We further set
up a computational scheme to obtain accurate total energies from our
variational functionals without having to solve computationally expensive sets
of self-consistent equations. The input of the functional is an approximate
Green function and an approximate four-point vertex
obtained at a relatively low computational cost. The
variational property of the functional guarantees that the error in the total
energy is only of second order in deviations of the input Green function and
vertex from the self-consistent ones that make the functional stationary. The
functionals that we will consider for practical applications correspond to
infinite order summations of ladder and exchange diagrams and are therefore
particularly suited for applications to highly correlated systems. Their
practical evaluation is discussed in detail.Comment: 21 pages, 10 figures. Physical Review B (accepted
Crossover from reptation to Rouse dynamics in a one-dimensional model
A simple one-dimensional model is constructed for polymer motion. It exhibits
the crossover from reptation to Rouse dynamics through gradually allowing
hernia creation and annihilation. The model is treated by the density matrix
technique which permits an accurate finite-size-scaling analysis of the
behavior of long polymers.Comment: 5 Pages RevTeX and 5 PostScript figures included (to appear in
Physical Review E
Comparison of Viscosities from the Chapman-Enskog and Relaxation Time Methods
A quantitative comparison between the results of shear viscosities from the
Chapman-Enskog and relaxation time methods is performed for selected test cases
with specified elastic differential cross sections: (i) the non-relativistic,
relativistic and ultra-relativistic hard sphere gas with angle and energy
independent differential cross section, (ii) the Maxwell gas, (iii) chiral
pions and (iv) massive pions. Our quantitative results reveal that the extent
of agreement (or disagreement) depends very sensitively on the energy
dependence of the differential cross sections employed.Comment: Submitted to Cent. Eur. J.Phy
Usporedna procjena oksidacije amalgama kronopotenciometrijom i voltametrijom
The characteristic features of scanned deposition potential curves constructed from stripping chronopotentiometry (SSCP) and various modes of stripping voltammetry (SSV) are critically evaluated. The strengths and weaknesses of each method for identification of metal ion speciation features and susceptibility to typical interferences are described for conventional (HMDE) and microelectrodes, i.e. irreversibility in the electron transfer reaction, multi-metal resolution, intermetallic compound formation, homogeneous kinetics, induced metal adsorption, and requirement for excess ligand to avoid saturation at the electrode surface during reoxidation. The most advantageous stripping modes are those in which practically complete depletion of the accumulated metal is achieved during the reoxidation step, i.e. SCP with low stripping current and DC-SV with slow potential scan rate. Under these conditions there is a straightforward quantitative relationship between the amount of metal accumulated and the analytical signal. The slow rate of oxidation with these modes renders them practically immune to induced metal adsorption; they have a lower requirement for excess ligand in the sample solution and greater resistance to both irreversibility in the electrochemical oxidation and to interference from intermetallic compounds. Even in the case of nonreversible electrode processes, or for systems limited by complex formation/dissociation kinetics, depletive scanned deposition potential stripping curves allow complexation parameters to be determined from the shift in half-wave deposition potential, analogous to the DeFord-Hume approach for conventional voltammetry. SSCP has greater sensitivity, and provides greater resolution in multi-metal systems than does depletive DC-SSV, while SSV provides useful complementary information in some cases.Kritički su procijenjena karakteristična svojstva pseudopolarograma konstruiranih korištenjem dvaju metoda oksidacije amalgama: kronopotenciometrije i voltametrije. Pseudopolarogram je prikaz ovisnosti najvećeg intenziteta odziva oksidacije amalgama o potencijalu redukcije metalnih iona. Opisane su prednosti i mane obaju metoda pri određivanju raspodjele metalnih kompleksa i njihova podložnost tipičnim smetnjama. Analizirani su odzivi na visećoj živinoj kapi i na mikroelektrodama. Razmatrane su sljedeće pojave: reverzibilnost i brzina prijenosa elektrona, razdvajanje odziva većeg broja metala u živi, stvaranje intermetalnih spojeva u živi, utjecaji kemijskih reakcija koje prethode redukciji metalnih iona ili slijede oksidaciju amalgama, inducirana adsorpcija metalnih iona na površinu živine elektrode i minimalni višak slobodnog liganda potreban da se tijekom oksidacije amalgama svi metalni ioni uz površinu elektrode odmah kompleksiraju. Najbolja metoda oksidacije jest ona kojom se postiže potpuna oksidacija svih metalnih atoma akumuliranih u živi. Taj uvjet zadovoljavaju kronopotenciometrija sa slabom strujom i voltametrija sa sporom promjenom potencijala. Pod tim uvjetima postoji kvantitativna funkcionalna veza intenziteta odziva i količine akumuliranih metalnih atoma. Metode spore oksidacije su imune na induciranu adsorpciju, trebaju manji višak slobodnog liganda, manje ovise o brzini izmjene elektrona i manje im smetaju intermetalni spojevi. Ovim metodama mogu se odrediti konstante stabilnosti metalnih kompleksa i u slučajevima kinetički kontroliranih elektrodnih reakcija i sporih reakcija kompleksiranja ili disocijacije kompleksa. Pokazano je da je kronopotenciometrija osjetljivija od voltametrije i da se njenim korištenjem postiže bolja rezolucija odziva većeg broja metala, ali se voltametrijom mogu dobiti dodatne informacije o istraživanom sustavu
Intraparticulate speciation analysis of soft nanoparticulate metal complexes:The impact of electric condensation on the binding of Cd2+/Pb2+/Cu2+ by humic acids
In aqueous dispersions of soft, charged nanoparticles, the physicochemical conditions prevailing within the particle body generally differ substantially from those in the bulk medium. Accordingly it is necessary to define intrinsic descriptors that appropriately reflect the chemical speciation inside the particle's microenvironment. Herein the speciation of divalent metal ions within the body of negatively charged soft nanoparticulate complexants is elaborated for the example case of humic acid association with Cd(ii), Pb(ii) and Cu(ii). The electrostatic effects are described by a two-state model that accounts for counterion condensation in the intraparticulate double layer shell at the particle/medium interface and Donnan partitioning within the bulk of the particle body. Inner-sphere complex formation is defined by an intrinsic binding constant expressed in terms of local reactant concentrations as controlled by the pertinent electrostatic conditions. For the high particle charge density case (Debye length smaller than charged site separation), three distinct intraparticulate metal species are identified, namely free hydrated ions, electrostatically condensed ions, and inner-sphere metal-humic complexes. For all metal ions studied, the electrostatic contribution to the association of the metal ion with the oppositely charged particle is found to account for a substantial fraction of the total metal bound.</p
Atom lithography without laser cooling
Using direct-write atom lithography, Fe nanolines are deposited with a pitch
of 186 nm, a full width at half maximum (FWHM) of 50 nm, and a height of up to
6 nm. These values are achieved by relying on geometrical collimation of the
atomic beam, thus without using laser collimation techniques. This opens the
way for applying direct-write atom lithography to a wide variety of elements.Comment: 7 pages, 11 figure
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