11 research outputs found
A theoretical investigation of the low lying electronic structure of poly(p-phenylene vinylene)
The two-state molecular orbital model of the one-dimensional phenyl-based
semiconductors is applied to poly(p-phenylene vinylene). The energies of the
low-lying excited states are calculated using the density matrix
renormalization group method. Calculations of both the exciton size and the
charge gap show that there are both Bu and Ag excitonic levels below the band
threshold. The energy of the 1Bu exciton extrapolates to 2.60 eV in the limit
of infinite polymers, while the energy of the 2Ag exciton extrapolates to 2.94
eV. The calculated binding energy of the 1Bu exciton is 0.9 eV for a 13
phenylene unit chain and 0.6 eV for an infinite polymer. This is expected to
decrease due to solvation effects. The lowest triplet state is calculated to be
at ca. 1.6 eV, with the triplet-triplet gap being ca. 1.6 eV. A comparison
between theory, and two-photon absorption and electroabsorption is made,
leading to a consistent picture of the essential states responsible for most of
the third-order nonlinear optical properties. An interpretation of the
experimental nonlinear optical spectroscopies suggests an energy difference of
ca. 0.4 eV between the vertical energy and ca. 0.8 eV between the relaxed
energy, of the 1Bu exciton and the band gap, respectively.Comment: LaTeX, 19 pages, 7 eps figures included using epsf. To appear in
Physical Review B, 199
Excited states of linear polyenes
We present density matrix renormalisation group calculations of the Pariser-
Parr-Pople-Peierls model of linear polyenes within the adiabatic approximation.
We calculate the vertical and relaxed transition energies, and relaxed
geometries for various excitations on long chains. The triplet (3Bu+) and even-
parity singlet (2Ag+) states have a 2-soliton and 4-soliton form, respectively,
both with large relaxation energies. The dipole-allowed (1Bu-) state forms an
exciton-polaron and has a very small relaxation energy. The relaxed energy of
the 2Ag+ state lies below that of the 1Bu- state. We observe an attraction
between the soliton-antisoliton pairs in the 2Ag+ state. The calculated
excitation energies agree well with the observed values for polyene oligomers;
the agreement with polyacetylene thin films is less good, and we comment on the
possible sources of the discrepencies. The photoinduced absorption is
interpreted. The spin-spin correlation function shows that the unpaired spins
coincide with the geometrical soliton positions. We study the roles of
electron-electron interactions and electron-lattice coupling in determining the
excitation energies and soliton structures. The electronic interactions play
the key role in determining the ground state dimerisation and the excited state
transition energies.Comment: LaTeX, 15 pages, 9 figure
Density matrix renormalisation group calculations of the low-lying excitations and non-linear optical properties of poly(p-phenylene
The two state molecular orbital (2-MO) model of the phenyl based semiconductors is used to calculate the low-lying spectra of the A + g and B − 1u states of poly(para-phenylene) (PPP). The model parameters are determined by fitting its predictions to exact Pariser-Parr-Pople model calculations of benzene and biphenyl, and it is solved using the density matrix renormalisation group method. It is shown that there exists a band of 1 B − 1u (‘s’-wave) excitons below the band states. In the long chain limit the lowest exciton is situated 3.3 eV above the ground state, consistent with experimental data. The calculated particle-hole separation of these excitons indicates that they are tightly bound, extending over only a few repeat units. The lowest band state is found to be a covalent 2 1 A + g state, whose energy almost coincides with the charge gap EG. Lying just above the 2 1 A + g state is a band 1 B − 1u state (the n1B
Correlation between microstructure and magnetic properties during phase separation in concentrated Fe-Cr alloys
International audienceWe report a theoretical study of microstructure, magnetic properties, and their relationship in relatively concentrated Fe-Cr alloys in both Fe- and Cr-rich regions. Annealing of initially random systems at 500° C for times of the order of 10s substantially changes their microstructure. In both systems, solute atoms form clusters with their sizes increasing with time according to power law, with exponent being close to 0.2. For the Fe-32 at. % Cr alloy, magnetization and the Curie temperature increase with increasing annealing time and cluster size. At large simulation times, the Curie temperature approaches its value for Fe-15 at. % Cr, the concentration of completely phase-separated iron-rich alloy. For the Cr-25 at. % Fe alloy, precipitation also results in an increase of magnetization and the Curie temperature, although characteristic times are about one order of magnitude greater
Modelling phase separation in Fe-Cr system using different atomistic kinetic Monte Carlo techniques
Atomistic kinetic Monte Carlo (AKMC) simulations were performed to study α-α′ phase separation in Fe-Cr alloys. Two different energy models and two approaches to estimate the local vacancy migration barriers were used. The energy models considered are a two-band model Fe-Cr potential and a cluster expansion, both fitted to ab initio data. The classical Kang-Weinberg decomposition, based on the total energy change of the system, and an Artificial Neural Network (ANN), employed as a regression tool were used to predict the local vacancy migration barriers 'on the fly'. The results are compared with experimental thermal annealing data and differences between the applied AKMC approaches are discussed. The ability of the ANN regression method to accurately predict migration barriers not present in the training list is also addressed by performing cross-check calculations using the nudged elastic band method. © 2010 Elsevier B.V. All rights reserved.SCOPUS: cp.jinfo:eu-repo/semantics/publishe
Magnetochemical effects on phase stability and vacancy formation in fcc Fe-Ni alloys
We investigate phase stability and vacancy formation in fcc Fe-Ni alloys over
a broad composition-temperature range, via a density functional theory
parametrized effective interaction model, which includes explicitly spin and
chemical variables. On-lattice Monte Carlo simulations based on this model are
used to predict the temperature evolution of the magnetochemical phase. The
experimental composition-dependent Curie and chemical order-disorder transition
temperatures are successfully predicted. We point out a significant effect of
chemical and magnetic orders on the magnetic and chemical transitions,
respectively. The resulting phase diagram shows a magnetically driven phase
separation around 10-40% Ni and 570-700 K, between ferromagnetic and
paramagnetic solid solutions, in agreement with experimental observations. We
compute vacancy formation magnetic free energy as a function of temperature and
alloy composition. We identify opposite magnetic and chemical disordering
effects on vacancy formation in the alloys with 50% and 75% Ni. We find that
thermal magnetic effects on vacancy formation are much larger in concentrated
Fe-Ni alloys than in fcc Fe and Ni due to a stronger magnetic interaction
An empirical potential for simulating hydrogen isotope retention in highly irradiated tungsten
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