@article{ranjan_yu_nakhmanson_bernholc_nardelli_2010, title={Polarization effects and phase equilibria in high-energy-density polyvinylidene-fluoride-based polymers}, volume={66}, ISSN={["2053-2733"]}, DOI={10.1107/s0108767310026358}, abstractNote={Using first-principles calculations, the phase diagrams of polyvinylidene fluoride (PVDF) and its copolymers under an applied electric field are studied and phase transitions between their nonpolar α and polar β phases are discussed. The results show that the degree of copolymerization is a crucial parameter controlling the structural phase transition. In particular, for tetrafluoroethylene (TeFE) concentration above 12%, PVDF–TeFE is stabilized in the β phase, whereas the α phase is stable for lower concentrations. As larger electric fields are applied, domains with smaller concentrations (\leq 12%) undergo a transition from the α to the β phase until a breakdown field of ~600 MV m−1 is reached. These structural phase transitions can be exploited for efficient storage of electrical energy.}, journal={ACTA CRYSTALLOGRAPHICA A-FOUNDATION AND ADVANCES}, author={Ranjan, V. and Yu, L. and Nakhmanson, Serge and Bernholc, Jerry and Nardelli, M. Buongiorno}, year={2010}, month={Sep}, pages={553–557} }
@article{saha_lu_bernholc_meunier_2009, title={First-principles methodology for quantum transport in multiterminal junctions}, volume={131}, ISSN={0021-9606 1089-7690}, url={http://dx.doi.org/10.1063/1.3247880}, DOI={10.1063/1.3247880}, abstractNote={We present a generalized approach for computing electron conductance and I-V characteristics in multiterminal junctions from first-principles. Within the framework of Keldysh theory, electron transmission is evaluated employing an O(N) method for electronic-structure calculations. The nonequilibrium Green function for the nonequilibrium electron density of the multiterminal junction is computed self-consistently by solving Poisson equation after applying a realistic bias. We illustrate the suitability of the method on two examples of four-terminal systems, a radialene molecule connected to carbon chains and two crossed-carbon chains brought together closer and closer. We describe charge density, potential profile, and transmission of electrons between any two terminals. Finally, we discuss the applicability of this technique to study complex electronic devices.}, number={16}, journal={The Journal of Chemical Physics}, publisher={AIP Publishing}, author={Saha, Kamal K. and Lu, Wenchang and Bernholc, J. and Meunier, Vincent}, year={2009}, month={Oct}, pages={164105} }
@article{yu_ranjan_lu_bernholc_nardelli_2008, title={Equivalence of dipole correction and Coulomb cutoff techniques in supercell calculations}, volume={77}, ISSN={["1098-0121"]}, DOI={10.1103/physrevb.77.245102}, abstractNote={In ab initio calculations of surfaces or nonperiodic systems, one frequently relies on the supercell approximation, where the periodic replicas of the system are separated by enough empty space to avoid spurious interactions between the successive images. However, a vacuum separation is not sufficient to screen the dipolar interaction that appears in asymmetrically charged or polar systems. The dipole correction and Coulomb cutoff methods are often used to eliminate such interactions between the periodic replicas. In this work, these methods are compared under the same conditions in the framework of plane-wave based density-functional theory. The dipole correction method is shown to be equivalent to the rigorous Coulomb cutoff formalism in the calculations of total energy, force, charge density, and self-consistent potential. We demonstrate that the band structures obtained by these methods coincide for the localized bound states and that the corrections have essentially no influence on the occupied energy bands, only substantially affecting the unoccupied bands. By comparing the results of the two methods, the localized bound states of interest can be easily distinguished from the highly delocalized unoccupied states using a relatively small supercell. This comparison offers substantial savings in the computational time when ascertaining convergence with supercell size. The accuracy of the dipole correction method is also confirmed by comparing the results for a model ferroelectric ${\text{BaTiO}}_{3}$ slab with a Berry-phase calculation of polarization for the bulk system.}, number={24}, journal={PHYSICAL REVIEW B}, author={Yu, Liping and Ranjan, V. and Lu, W. and Bernholc, J. and Nardelli, M. Buongiorno}, year={2008}, month={Jun} }