1,667 research outputs found
Correlation-induced corrections to the band structure of boron nitride: a wave-function-based approach
We present a systematic study of the correlation-induced corrections to the
electronic band structure of zinc-blende BN. Our investigation employs an ab
initio wave-function-based local Hamiltonian formalism which offers a rigorous
approach to the calculation of the polarization and local charge redistribution
effects around an extra electron or hole placed into the conduction or valence
bands of semiconducting and insulating materials. Moreover, electron
correlations beyond relaxation and polarization can be readily incorporated.
The electron correlation treatment is performed on finite clusters. In
conducting our study, we make use of localized Wannier functions and embedding
potentials derived explicitly from prior periodic Hartree-Fock calculations.
The on-site and nearest-neighbor charge relaxation bring corrections of several
eV to the Hartree-Fock band gap. Additional corrections are caused by
long-range polarization effects. In contrast, the dispersion of the
Hartree-Fock bands is marginally affected by electron correlations. Our final
result for the fundamental gap of zinc-blende BN compares well with that
derived from soft x-ray experiments at the B and N K-edges.Comment: 18 pages, 8 figures; the following article has been submitted to J.
Chem. Phy
Electronic, Optical, Structural, and Elastic Properties of MAX Phases and (Cr2Hf)2Al3C3
Title from PDF of title pages, viewed on July 13, 2015Dissertation advisor: Wai-Yim ChingVitaIncludes bibliographic references (pages 124-140)Thesis (Ph.D.)--Department of Physics and Astronomy and Department of Chemistry. University of Missouri--Kansas City, 2015The term “MAX phase” refers to a very interesting and important class of
layered ternary transition-metal carbides and nitrides with a novel combination of both
metal- and ceramic-like properties that have made these materials highly regarded
candidates for numerous technological and engineering applications. In the present
dissertation work, the electronic structure and optical conductivities of 20 MAX phases
Ti3AC2 (A = Al, Si, Ge), Ti2AC (A = Al, Ga, In, Si, Ge, Sn, P, As, S), Ti2AlN, M2AlC
(M = V, Nb, Cr), and Tan+1AlCn (n = 1 to 4) are studied using the first-principles
orthogonalized linear combination of atomic orbitals (OLCAO) method. It is
confirmed that the N(Ef) (total density of states at the Fermi level Ef) increases as the
number of valence electrons of the composing elements increases. The local feature of
total density of states (TDOS) near Ef is used to predict structural stability. The
calculated effective charge on each atom shows that the M (transition-metal) atoms
always lose charge to the X (C or N) atoms, whereas the A-group atoms mostly gain
charge but some lose charge. Bond order values are obtained and critically analyzed
for all types of interatomic bonds in the 20 MAX phases. Also included in this work is
the exploration [using (Cr2Hf)2Al3C3 as an example] of the possibility of incorporating more types of elements into a MAX phase while maintaining the crystallinity, instead
of creating solid solution phases. The crystal structure and elastic properties of
(Cr2Hf)2Al3C3 are studied using the Vienna ab initio Simulation Package. Unlike
MAX phases with a hexagonal symmetry (P63/mmc, #194), (Cr2Hf)2Al3C3 crystallizes
in the monoclinic space group of P21/m (#11). Its structure is found to be energetically
much more favorable against the allotropic segregation and solid solution phases.
Calculations using a stress versus strain approach and the VRH approximation for
polycrystals also show that (Cr2Hf)2Al3C3 has outstanding elastic moduliIntroduction of max phases -- Scope and motivation of research -- Theory and methodology -- Results and discussion on the twenty max phases -- Results and discussion on the derivative (Cr2Hf)2Al3C3 -- Summary -- Appendix A. Full basis of titanium atomic orbitals -- Appendix B. The relaxed unit cell of (Cr2Hf)2Al3C3 -- Appendix C. The relaxed 1 x 1 x 3 supercell of (Cr2Hf)2Al3C3 -- Appendix D. The relaxed segregation model -- Appendix E. The relaxed 3 x 3 x1 supercell of (Cr2Hf)2Al3C3 -- Appendix F. The relaxed solid solution mode
Wavefunction-based method for excited-state electron correlations in periodic systems - application to polymers
A systematic method for determining correlated wavefunctions of extended
systems in the ground and excited states is presented. It allows to fully
exploit the power of quantum-chemical programs designed for correlation
calculations of finite molecules. Using localized Hartree-Fock (HF) orbitals
(both occupied and virtual ones), an effective Hamiltonian which can easily be
transferred from finite to infinite systems is built up. Correlation
corrections to the matrix elements of the effective Hamiltonian are derived
from clusters. To treat correlation effects, multireference configuration
interaction (MRCI) calculations with singly and doubly excited configurations
(SD) are performed. This way one is able to generate both valence and
conduction bands where all correlation effects in the excited states as well as
in the ground state of the system are taken into account. An appropriate
size-extensivity correction to the MRCI(SD) correlation energies is developed
which takes into account the open-shell character of the excited states. This
approach is applicable to a wide range of polymers and crystals. In the present
work trans-polyacetylene is chosen as a test system. The corresponding band
structure is obtained with the correlation of all electrons in the system being
included on a high level of sophistication. The account of correlation effects
leads to substantial shifts of the "center-of-mass" positions of the bands
(valence bands are shifted upwards and conduction bands downwards) and a
flattening of all bands compared to the corresponding HF band structure.
Further an extention of the above approach to excitons (optical excitations) in
crystals is developed which allows to use standard quantum-chemical methods to
describe the electron-hole pairs and to finally obtain excitonic bands.Comment: 111 pages, 23 figures, Ph.D. Thesi
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