Search

We have developed an efficient real-time time-dependent density functional theory (TDDFT) method that can increase the effective time step from $l1$ as in traditional methods to $0.1--0.5\text{ }\text{ }\mathrm{fs}$. With this algorithm, the TDDFT simulation can have comparable speed to the Born-Oppenheimer (BO) ab initio molecular dynamics (MD). As an application, we simulated the process of an energetic Cl particle colliding onto a monolayer of ${\mathrm{MoSe}}_{2}$. Our simulations show a significant energy transfer from the kinetic energy of the Cl particle to the electronic energy of ${\mathrm{MoSe}}_{2}$, and the result of TDDFT is very different from that of BO-MD simulations.

Despite the lack of translational symmetry in random substitutional alloys, their description in terms of single Bloch states has been used in most phenomenological models and spectroscopic practices. We present a new way of analyzing the alloy electronic structures based on a {open_quotes}majority representation{close_quotes} phenomenon of the reciprocal space spectrum P({bold k}) of the wave function. This analysis provides a quantitative answer to the questions: When can an alloy state be classified according to the crystal Bloch state symmetry, and under what circumstances are the conventional theoretical alloy models applicable. {copyright} {ital 1998} {ital The American Physical Society}

We have developed a ``linear combination of bulk bands'' method that permits atomistic, pseudopotential electronic structure calculations for $\ensuremath{\sim}{10}^{6}$ atom nanostructures. Application to $(\mathrm{GaAs}{)}_{n}/(\mathrm{AlAs}{)}_{n}$ (001) superlattices (SL's) reveals even-odd oscillations in the $\ensuremath{\gamma}\ensuremath{-}X$ coupling magnitude ${V}_{\ensuremath{\gamma}X}(n)$, which vanishes for $n\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}\mathrm{odd}$, even for abrupt and segregated SL's, respectively. Surprisingly, in contrast with recent expectations, 0D quantum dots are found here to have a smaller $\ensuremath{\gamma}\ensuremath{-}X$ coupling than equivalent 2D SL's. Our analysis shows that for large quantum dots this is largely due to the existence of level repulsion from many $X$ states.

An ab initio method is presented to calculate shallow impurity levels in bulk semiconductors. This method combines the GW calculation for the treatment of the central-cell potential with a potential patching method for large systems (with 64,000 atoms) to describe the impurity state wave functions. The calculated acceptor levels in Si, GaAs, and an isovalent bound state of GaP are in excellent agreement with experiments with a root-mean-square error of 8.4 meV.

Orbital-free (OF) density functional theory (DFT) directly solves for the electron density rather than the wave function of many electron systems, greatly simplifying and enabling large scale first principles simulations. However, the required approximate noninteracting kinetic energy density functionals and local electron-ion pseudopotentials severely restrict the general applicability of conventional OFDFT. Here, we present a new generation of OFDFT called angular-momentum-dependent (AMD)-OFDFT to harness the accuracy of Kohn-Sham DFT and the simplicity of OFDFT. The angular momenta of electrons are explicitly introduced within atom-centered spheres so that the important ionic core region can be accurately described. In addition to conventional OF total energy functionals, we introduce a crucial nonlocal energy term with a set of AMD energies to correct errors due to the kinetic energy density functional and the local pseudopotential. We find that our AMD-OFDFT formalism offers substantial improvements over conventional OFDFT, as we show for various properties of the transition metal titanium.

Nonradiative carrier recombination is of both applied and fundamental interest. Here a novel algorithm is introduced to calculate such a deep level nonradiative recombination rate using the ab initio density functional theory. This algorithm can calculate the electron-phonon coupling constants all at once. An approximation is presented to calculate the phonon modes for one impurity in a large supercell. The neutral Zn impurity site together with a N vacancy is considered as the carrier-capturing deep impurity level in bulk GaN. Its capture coefficient is calculated as 5.57 × 10(-10)cm(3)/s at 300 K. We found that there is no apparent onset of such a nonradiative process as a function of temperature.

Quantum mechanical pseudopotential calculations of the absorption spectra and static dielectric constant ${\ensuremath{\epsilon}}_{s}$ of Si quantum dots with \ensuremath{\sim}100-1300 atoms are presented. The predicted ${\ensuremath{\epsilon}}_{s}$ is found to be significantly reduced relative to the bulk value, but is considerably larger than the value deduced from currently available model calculations. A convenient parametrization of ${\ensuremath{\epsilon}}_{s}$ vs size $R$ is provided. We find that for quantum dots with $Rl20$ \AA{} the electron-hole pair is confined by the physical dimension of the dot, not by the Coulomb attraction.

There are materials that exist in unusual solid-liquid hybrid phases, for example, the superionics at high temperatures of 700 °C. Using ab initio molecular dynamics, we show that the intensely studied Cu(2)S high chalcocite phase is actually a solid-liquid hybrid phase which exists in relatively low temperature (>105 °C). Its formation mechanism is different from the superionics. We also show that the previously proposed atomic structure for high chalcocite is incorrect, and the low chalcocite to high chalcocite transition should be described as a sublattice solid to liquid transition.

A new model Hamiltonian is developed to describe the ab initio energy differences of the nonisovalent alloy configurations based on the semiconductor electron counting rule. Monte Carlo simulations using this Hamiltonian show strong short range order of the GaN/ZnO alloy, which has significant effects on its electronic structure. We also predict further reduction of the band gap by increasing the synthesis temperature.

We discuss theoretically a novel approach to tailoring the properties of a new family of organic-inorganic hybrid superlattices, using two isostructural materials, $\mathrm{ZnSe}(\mathrm{en}{)}_{0.5}$ and $\mathrm{ZnTe}(\mathrm{en}{)}_{0.5}$, as examples. Replacing Se with Te leads to a number of nontrivial changes: the conduction band parity, singularity type, conductivity in the superlattice direction, and the $p$-type dopability. Experimentally, we report the first unambiguous observation of exciton-polariton emission in a hybrid semiconductor, i.e., $\mathrm{ZnTe}(\mathrm{en}{)}_{0.5}$. The band-edge excitonic transitions in both emission and absorption are explained by the calculated electronic structures.

A motif based charge patching method is presented for large system electronic structure calculations. It produces ab initio quality charge densities for large systems without actually doing self-consistent calculations for them. It represents a general faster alternative to the conventional O(N) methods. This method is applied here to unconventional semiconductor binary systems, and the resulting eigenenergies are found to be almost the same as the original ab initio eigenenergies (with 20-50 meV errors).