532 research outputs found
Double Time Window Targeting Technique: Real time DMRG dynamics in the PPP model
We present a generalized adaptive time-dependent density matrix
renormalization group (DMRG) scheme, called the {\it double time window
targeting} (DTWT) technique, which gives accurate results with nominal
computational resources, within reasonable computational time. This procedure
originates from the amalgamation of the features of pace keeping DMRG
algorithm, first proposed by Luo {\it et. al}, [Phys.Rev. Lett. {\bf 91},
049701 (2003)], and the time-step targeting (TST) algorithm by Feiguin and
White [Phys. Rev. B {\bf 72}, 020404 (2005)]. Using the DTWT technique, we
study the phenomena of spin-charge separation in conjugated polymers (materials
for molecular electronics and spintronics), which have long-range
electron-electron interactions and belong to the class of strongly correlated
low-dimensional many-body systems. The issue of real time dynamics within the
Pariser-Parr-Pople (PPP) model which includes long-range electron correlations
has not been addressed in the literature so far. The present study on PPP
chains has revealed that, (i) long-range electron correlations enable both the
charge and spin degree of freedom of the electron, to propagate faster in the
PPP model compared to Hubbard model, (ii) for standard parameters of the PPP
model as applied to conjugated polymers, the charge velocity is almost twice
that of the spin velocity and, (iii) the simplistic interpretation of
long-range correlations by merely renormalizing the {\it U} value of the
Hubbard model fails to explain the dynamics of doped holes/electrons in the PPP
model.Comment: Final (published) version; 39 pages, 13 figures, 1 table; 2 new
references adde
Electronic Control of Spin Alignment in pi-Conjugated Molecular Magnets
Intramolecular spin alignment in pi-conjugated molecules is studied
theoretically in a model of a Peierls-Hubbard chain coupled with two localized
spins. By means of the exact diagonalization technique, we demonstrate that a
spin singlet (S=0) to quartet (S=3/2) transition can be induced by electronic
doping, depending on the chain length, the positions of the localized spins,
and the sign of the electron-spin coupling. The calculated results provides a
theoretical basis for understanding the mechanism of spin alignment recently
observed in a diradical donor molecule.Comment: 4 pages, 4 figures, Physical Review Letters (in press
Unscreened Coulomb repulsion in the one dimensional electron gas
A tight binding model of electrons interacting via bare Coulomb repulsion is
numerically investigated by use of the Density Matrix Renormalization Group
method which we prove applicable also to very long range potentials. From the
analysis of the elementary excitations, of the spin and charge correlation
functions and of the momentum distribution, a picture consistent with the
formation of a one dimensional "Wigner crystal" emerges, in quantitative
agreement with a previous bosonization study. At finite doping, Umklapp
scattering is shown to be ineffective in the presence of long range forces.Comment: RevTex, 5 pages with 8 eps figures. To be published on Phys. Rev.
Dynamic ductile to brittle transition in a one-dimensional model of viscoplasticity
We study two closely related, nonlinear models of a viscoplastic solid. These
models capture essential features of plasticity over a wide range of strain
rates and applied stresses. They exhibit inelastic strain relaxation and steady
flow above a well defined yield stress. In this paper, we describe a first step
in exploring the implications of these models for theories of fracture and
related phenomena. We consider a one dimensional problem of decohesion from a
substrate of a membrane that obeys the viscoplastic constitutive equations that
we have constructed. We find that, quite generally, when the yield stress
becomes smaller than some threshold value, the energy required for steady
decohesion becomes a non-monotonic function of the decohesion speed. As a
consequence, steady state decohesion at certain speeds becomes unstable. We
believe that these results are relevant to understanding the ductile to brittle
transition as well as fracture stability.Comment: 10 pages, REVTeX, 12 postscript figure
Second-order electronic correlation effects in a one-dimensional metal
The Pariser-Parr-Pople (PPP) model of a single-band one-dimensional (1D)
metal is studied at the Hartree-Fock level, and by using the second-order
perturbation theory of the electronic correlation. The PPP model provides an
extension of the Hubbard model by properly accounting for the long-range
character of the electron-electron repulsion. Both finite and infinite version
of the 1D-metal model are considered within the PPP and Hubbard approximations.
Calculated are the second-order electronic-correlation corrections to the total
energy, and to the electronic-energy bands. Our results for the PPP model of 1D
metal show qualitative similarity to the coupled-cluster results for the 3D
electron-gas model. The picture of the 1D-metal model that emerges from the
present study provides a support for the hypothesis that the normal metallic
state of the 1D metal is different from the ground state.Comment: 21 pages, 16 figures; v2: small correction in title, added 3
references, extended and reformulated a few paragraphs (detailed information
at the end of .tex file); added color to figure
Hydrogen Bonding Controls Excited-State Decay of the Photoactive Yellow Protein Chromophore
International audienceWe have performed excited-state dynamics simulations of a Photoactive Yellow Protein chromophore analogue in water. The results of the simulations demonstrate that in water the chromophore predominantly undergoes single-bond photoisomerization, rather than double-bond photoisomerization. Despite opposite charge distributions in the chromophore, excited-state decay takes place very efficiently from both single- and double-bond twisted minima in water. Radiationless decay is facilitated by ultrafast solvent reorganization, which stabilizes both minima by specific hydrogen bond interactions. Changing the solvent to the slightly more viscous D(2)O leads to an increase of the excited-state lifetime. Together with previous simulations, the present results provide a complete picture of the effect of the protein on the photoisomerization of the chromophore in PYP: the positive guanidinium group of Arg52 favors double-bond isomerization over single-bond isomerization by lowering the barrier for double-bond isomerization, while the hydrogen bonds with Tyr42 and Glu46 enhance deactivation from the double-bond twisted minimum
Flat-Band Ferromagnetism in Organic Polymers Designed by a Computer Simulation
By coupling a first-principles, spin-density functional calculation with an
exact diagonalization study of the Hubbard model, we have searched over various
functional groups for the best case for the flat-band ferromagnetism proposed
by R. Arita et al. [Phys. Rev. Lett. {\bf 88}, 127202 (2002)] in organic
polymers of five-membered rings. The original proposal (poly-aminotriazole) has
turned out to be the best case among the materials examined, where the reason
why this is so is identified here. We have also found that the ferromagnetism,
originally proposed for the half-filled flat band, is stable even when the band
filling is varied away from the half-filling. All these make the ferromagnetism
proposed here more experimentally inviting.Comment: 11 pages, 13figure
Curve Crossing Problem with Arbitrary Coupling: Analytically Solvable Model
We give a general method for finding an exact analytical solution for the two
state curve crossing problem. The solution requires the knowledge of the
Green's function for the motion on the uncoupled potential. We use the method
to find the solution of the problem in the case of parabolic potentials coupled
by Gaussian interaction. Our method is applied to this model system to
calculate the effect of curve crossing on electronic absorption spectrum and
resonance Raman excitation profile
miR-132, an experience-dependent microRNA, is essential for visual cortex plasticity
Using quantitative analyses, we identified microRNAs (miRNAs) that were abundantly expressed in visual cortex and that responded to dark rearing and/or monocular deprivation. The most substantially altered miRNA, miR-132, was rapidly upregulated after eye opening and was delayed by dark rearing. In vivo inhibition of miR-132 in mice prevented ocular dominance plasticity in identified neurons following monocular deprivation and affected the maturation of dendritic spines, demonstrating its critical role in the plasticity of visual cortex circuits.National Eye Institute (Ruth L. Kirschstein Postdoctoral Fellowship 1F32EY020066-01)Simons Foundation (Postdoctoral Fellowship)National Institutes of Health (U.S.) (EY017098)National Institutes of Health (U.S.) (EY007023
Fluorescence studies on new potential antitumoral benzothienopyran-1-ones in solution and in liposomes
Fluorescence properties of four new potential
antitumoral compounds, 3-arylbenzothieno[2,3-c]pyran-1-ones, were studied in solution and in lipid membranes of dipalmitoyl phosphatidylcholine (DPPC), egg yolk phosphatidylcholine (Egg-PC) and dioctadecyldimethylammonium bromide (DODAB). The 3-(4-methoxyphenyl)benzothieno[2,3-c]pyran-1-one (1c) exhibits the higher fluorescence quantum yields in all solvents studied. All compounds present a solvent sensitive emission, with significant red shifts in polar solvents for the methoxylated compounds. The results point to an ICT character of the excited state, more pronounced for compound 1c. Fluorescence (steady-state) anisotropy measurements of the compounds incorporated in liposomes of DPPC, DODAB and Egg-PC indicate that all compounds have two different locations, one due to a deep penetration in the lipid membrane and another corresponding to a more hydrated environment. In general, the methoxylated compounds prefer hydrated environments inside the liposomes. The 3-(4-
fluorophenyl)benzothieno[2,3-c]pyran-1-one (1a) clearly prefers a hydrated environment, with some molecules located at the outer part of the liposome interface. On the contrary, the preferential location of 3-(2-fluorophenyl)benzothieno[2,3-c]pyran-1-one (1b) is in the region of lipid hydrophobic tails. Compounds with a planar geometry (1a and 1c) have higher mobility in the lipid membranes when phase transition occurs.Portugal and FEDER (Fundo Europeu de Desenvolvimento Regional), for financial support through Centro de FÃsica (CFUM) and Centro de QuÃmica (CQ-UM) of University of Minho and through the Project PTDC/QUI/81238/2006. M.S.D. Carvalho and R.C. Calhelha acknowledge FCT for their PhD grants SFRH/BD/47052/2008 and SFRH/BD/29274/2006, respectively.Fundação para a Ciência e a Tecnologia (FCT
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