Effect of dimerization on dynamics of spin-charge separation in Pariser-Parr-Pople model: A time-dependent density matrix renormalization group study

We investigate the effect of static electron-phonon coupling, on real-time dynamics of spin and charge transport in $\pi$-conjugated polyene chains. The polyene chain is modeled by the Pariser-Parr-Pople Hamiltonian with dimerized nearest-neighbor parameter $t_{0}(1+\delta)$ for short bonds and $t_{0}(1-\delta)$ for long bonds, and long-range electron-electron interactions. We follow the time evolution of the spin and charge using time-dependent density matrix renormalization group technique, when a hole is injected at one end of the chain in its ground state. We find that spin and charge dynamics followed through spin and charge velocities, depend both on chain length and extent of dimerization, $\delta$. Analysis of the results requires focusing on physical quantities such as average spin and charge polarizations, particularly in the large dimerization limit. In the dimerization range 0.0 $\le$ $\delta$ $\le$ 0.15, spin-charge dynamics is found to have a well defined behavior, with spin-charge separation (measured as the ratio of charge velocity to spin velocity) as well as, the total amount of charge and spin transported in a given time, along the chain, decreasing as dimerization increases. However, in the range 0.3 $\le$ $\delta$ $\le$ 0.5, it is observed that the dynamics of spin and charge transport becomes complicated. It is observed that for large $\delta$ values, spin-charge separation is suppressed and the injected hole fails to travel the entire length of the chain.Comment: Published in Phys. Rev. B; preprint format of published versio

Simulation and modeling of kinetics of silicon oxidation in the thin oxide regime

Thermal oxidation of Silicon in dry O2, in the thin regime(\u3c 500A) is of vital importance to VLSI device engineers, because thin layers of SiO2 are exclusively used as gate dielectric for high performance of MOS devices. There exist a number of models to explain the kinetics of oxidation in this thin regime. However there is considerable variance among them and the reported rate constants, which are supposed to describe the oxidation process. Rather than arriving at an alternative model, the present study aims at an extensive study and simulation of existing models of oxidation in dry oxygen, in the thin regime, with a recent set of experimental data and arrive at the best possible model and provide accurate rate constants for oxidation in dry oxygen. These experimental data have been obtained, earlier, using high-resolution transmission electron microscopy (HRTEM) and ellipsometry techniques to measure thicknesses of silicon oxide, grown at 800°C in dry oxygen, in the thickness range of 2-20 nm

Theory of metal-intercalated phenacenes: Why molecular valence 3 is special

We develop a correlated-electron minimal model for the normal state of charged phenanthrene ions in the solid state, within the reduced space of the two lowest antibonding molecular orbitals of phenanthrene. Our model is general and can be easily extended to study the normal states of other polycyclic aromatic hydrocarbon superconductors. The main difference between our approach and previous correlated-electron theories of phenacenes is that our calculations are exact within the reduced basis space, albeit for finite clusters. The enhanced exchange of electron populations between these molecular orbitals, driven by Coulomb interactions over and above the bandwidth effects, gives a theoretical description of the phenanthrene trianions that is very different from previous predictions. Exact many-body finite cluster calculations show that while the systems with molecular charges of $-$1 and $-$2 are one- and two-band Mott-Hubbard semiconductors, respectively, molecular charge $-$3 gives two nearly $\frac{3}{4}$-filled bands, rather than a completely filled lower band and a $\frac{1}{2}$-filled upper band. The carrier density per active molecular orbital is thus nearly the same in the normal state of the superconducting aromatics and organic charge-transfer solids, and may be the key to understanding unconventional superconductivity in these molecular superconductors.Comment: Published in Phys. Rev. B. Title changed on editorial request. In all, 13 pages, 14 captioned figures, and 2 table

Real-time density matrix renormalization group dynamics of spin and charge transport in push-pull polyenes and related systems

In this paper we investigate the effect of terminal substituents on the dynamics of spin and charge transport in donor-acceptor substituted polyenes ($D-(CH)_{x}-A$) chains, also known as push-pull polyenes. We employ long-range correlated model Hamiltonian for the $D-(CH)_{x}-A$ system, and time-dependent density matrix renormalization group technique for time propagating the wave packet obtained by injecting a hole at a terminal site, in the ground state of the system. Our studies reveal that the end groups do not affect spin and charge velocities in any significant way, but change the amount of charge transported. We have compared these push-pull systems with donor-acceptor substituted polymethine imine (PMI), $D-(CHN)_{x}-A$, systems in which besides electron affinities, the nature of $p_{z}$ orbitals in conjugation also alternate from site to site. We note that spin and charge dynamics in the PMIs are very different from that observed in the case of push-pull polyenes, and within the time scale of our studies, transport of spin and charge leads to the formation of a "quasi-static" state.Comment: 7 pages, 2 tables, 8 figures; published versio

Comparative Evaluation of Taper Preparation Variability of Three Different Niti Files- An In Vitro CBCT Study

The ultimate goal of an endodontic treatment is to achieve the fluid tight seal particularly at the apex as well as in coronal aspect of the root canal system. This is mainly influenced by the cleaning and shaping of root canal system and it has been recognized as an important phase of endodontic therapy. Cleaning and shaping as well as obturation are best accomplished when the file taper claimed by the manufacturer is accurate and the taper of the canal following instrumentation corresponds to the taper of the file. Despite the most advanced technology in manufacturing of dental instruments, variations in endodontic file tapers still exist. Therefore the purpose of this study is to compare the taper variation in root canal preparations among the three different NiTi files using CBCT. Methods: 24 upper 1st molar tooth with mesiobuccal root curvature 15-30 degree were selected and divided into three groups. Mesiobuccal roots (MB1) of three groups were prepared by three different NiTi file system respectively up to size25 taper.06 and CBCT were done separately for each group. The diameter was measured at 1, 3, and orifice in mm and canal taper preparation was calculated using CBCT. Result: Of the 3 file systems, fell within the ±.05 taper variability. All preparations demonstrated variability when compared to the nominal taper .06. Conclusion: Taper preparations of the investigated size 25 taper .06 were favorable but different from the nominal taper

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