Quasiparticle excitations and charge transition levels of oxygen vacancies in hafnia

We calculate the quasiparticle defect states and charge transition levels (CTLs) of oxygen vacancies in monoclinic hafnia using density functional theory (DFT) and the GW method. We introduce the criterion that the quality and reliability of CTLs may be evaluated by calculating the same CTL via two physical paths and show that it is necessary to include important electrostatic corrections previously neglected within the supercell DFT + GW approach. Contrary to previous reports, the oxygen vacancies in hafnia are large positive U centers, where U is the defect charging energy.     

Simple approximate physical orbitals for GW quasiparticle calculations

Generating unoccupied orbitals within density functional theory (DFT) for use in GW calculations of quasiparticle energies becomes prohibitive for large systems. We show that, without any loss of accuracy, the unoccupied orbitals may be replaced by a set of simple approximate physical orbitals made from appropriately prepared plane waves and localized basis DFT orbitals that represent the continuum and resonant states of the system, respectively. This approach allows for accurate quasiparticle calculations using only a very small number of unoccupied DFT orbitals, resulting in an order of magnitude gain in speed.     

Using solid-state 31P{19F} REDOR NMR to measure distances between a trifluoromethyl group and a phosphodiester in nucleic acids

REDOR is a solid-state NMR technique frequently applied to biological structure problems. Through incorporation of phosphorothioate groups in the nucleic acid backbone and mono-fluorinated nucleotides, 31P{19F} REDOR has been used to study the binding of DNA to drugs and RNA to proteins through the detection of internuclear distances as large as 13-14 A. In this work, 31P{19F} REDOR is further refined for use in nucleic acids by the combined use of selective placement of phosphorothioate groups and the introduction of nucleotides containing trifluoromethyl (-CF3) groups. To ascertain the REDOR-detectable distance limit between an unique phosphorous spin and a trifluoromethyl group and to assess interference from intermolecular couplings, a series of model compounds and DNA dodecamers were synthesized each containing a unique phosphorous label and trifluoromethyl group or a single 19F nucleus. The dipolar coupling constants of the various 31P and 19F or -CF3 containing compounds were compared using experimental and theoretical dephasing curves involving several models for intermolecular interactions.     

Renormalization of molecular electronic levels at metal-molecule interfaces

The electronic structure of benzene on graphite (0001) is computed using the GW approximation for the electron self-energy. The benzene quasiparticle energy gap is predicted to be 7.2 eV on graphite, substantially reduced from its calculated gas-phase value of 10.5 eV. This decrease is caused by a change in electronic correlation energy, an effect completely absent from the corresponding Kohn-Sham gap. For weakly coupled molecules, this correlation energy change can be described as a surface polarization effect. A classical image potential model illustrates the impact for other conjugated molecules on graphite.     

New generation of massless Dirac fermions in graphene under external periodic potentials

We show that new massless Dirac fermions are generated when a slowly varying periodic potential is applied to graphene. These quasiparticles, generated near the supercell Brillouin zone boundaries with anisotropic group velocity, are different from the original massless Dirac fermions. The quasiparticle wave vector (measured from the new Dirac point), the generalized pseudospin vector, and the group velocity are not collinear. We further show that with an appropriate periodic potential of triangular symmetry, there exists an energy window over which the only available states are these quasiparticles, thus providing a good system to probe experimentally the new massless Dirac fermions. The required parameters of external potentials are within the realm of laboratory conditions.     

Coupling of nonlocal potentials to electromagnetic fields

Nonlocal Hamiltonians are used widely in first-principles quantum calculations; the nonlocality stems from eliminating undesired degrees of freedom, e.g., core electrons. To date, attempts to couple nonlocal systems to external electromagnetic (EM) fields have been heuristic or limited to weak or long wavelength fields. Using Feynman path integrals, we derive an exact, closed-form coupling of arbitrary EM fields to nonlocal systems. Our results justify and clarify the couplings used to date and are essential for systematic computation of linear and especially nonlinear responses.     

Excited-state forces within a first-principles Green's function formalism

We present a new first-principles formalism for calculating forces for optically excited electronic states using the interacting Green's function approach with the GW Bethe-Salpeter-equation method. This advance allows for efficient computation of gradients of the excited-state Born-Oppenheimer energy, allowing for the study of relaxation, molecular dynamics, and photoluminescence of excited states. The approach is tested on photoexcited carbon dioxide and ammonia molecules, and the calculations accurately describe the excitation energies and photoinduced structural deformations.     

Identifying defects in nanoscale materials

We have developed a novel iterative experimental-theoretical technique which can identify the atomic structure of defects in many-atom nanoscale materials from scanning tunneling microscopy and spectroscopy data. A given model for a defect structure is iteratively improved until calculated microscopy and spectroscopy data based on the model converge on the experimental results. We use the technique to identify a defect responsible for the electronic properties of a carbon nanotube intramolecular junction. Our technique can be extended for analysis of defect structures in nanoscale materials in general.     

Excitonic effects and optical spectra of single-walled carbon nanotubes

Many-electron effects often dramatically modify the properties of reduced dimensional systems. We report calculations, based on an ab initio many-electron Green's function approach, of electron-hole interaction effects on the optical spectra of small-diameter single-walled carbon nanotubes. Excitonic effects qualitatively alter the optical spectra of both semiconducting and metallic tubes. Excitons are bound by approximately 1 eV in the semiconducting (8,0) tube and by approximately 100 meV in the metallic (3,3) tube. These large many-electron effects explain the discrepancies between previous theories and experiments.