Imaging Pauli repulsion in scanning tunneling microscopy.

A scanning tunneling microscope (STM) has been equipped with a nanoscale force sensor and signal transducer composed of a single D2 molecule that is confined in the STM junction. The uncalibrated sensor is used to obtain ultrahigh geometric image resolution of a complex organic molecule adsorbed on a noble metal surface. By means of conductance-distance spectroscopy and corresponding density functional calculations the mechanism of the sensor and transducer is identified. It probes the short-range Pauli repulsion and converts this signal into variations of the junction conductance.

First-principles calculations of the atomic and electronic structure of SrZrO3 and PbZrO3 (001) and (011) surfaces.

We present the results of calculations of surface relaxations, rumplings, energetics, optical band gaps, and charge distribution for the SrZrO(3) and PbZrO(3) (001) and (011) surfaces using the ab initio code CRYSTAL and a hybrid description of exchange and correlation. We consider both SrO(PbO) and ZrO(2) terminations of the (001) surface and Sr(Pb), ZrO, and O terminations of the polar SrZrO(3) and PbZrO(3) (011) surfaces. On the (001) surfaces, we find that all upper and third layer atoms relax inward, while outward relaxations of all atoms in the second layer are found with the sole exception of the SrO-terminated SrZrO(3) (001) surface second layer O atom. Between all (001) and (011) surfaces the largest relaxations, more than 15% of the bulk lattice constant, are for the Sr- and Pb-terminated SrZrO(3) and PbZrO(3) (011) surface upper layer Sr and Pb atoms. Our calculated surface rumpling for the SrO- and PbO-terminated SrZrO(3) and PbZrO(3) (001) surfaces (6.77 and 3.32% of a(0)) are by a factor of ten larger than the surface rumpling for the ZrO(2)-terminated (001) surfaces (-0.72 and 0.38% of a(0), respectively). In contrast to the surface rumpling, the (001) surface energies are comparable and in the energy range from 0.93 eV/cell for the ZrO(2)-terminated PbZrO(3) surface to 1.24 eV/cell for the ZrO(2)-terminated SrZrO(3) surface. In contrast to the (001) surface, different terminations of the SrZrO(3) and PbZrO(3) (011) surfaces lead to very different surface energies ranging from 1.74 eV/cell for the Pb-terminated PbZrO(3) (011) surface to 3.61 eV/cell for the ZrO-terminated SrZrO(3) (011) surface. All our calculated (011) surface energies are considerably larger than the (001) surface energies. Our calculated optical band gap for the SrZrO(3) bulk, 5.31 eV, is in fair agreement with the experimental value of 5.6 eV. All our calculated optical band gaps for the SrZrO(3) and PbZrO(3) (001) and (011) surfaces are reduced with respect to the bulk. We predict a considerable increase in the Zr-O chemical bond covalency near the SrZrO(3) and PbZrO(3) (011) surfaces as compared both to the bulk and to the (001) surface.

Structure and bonding of the multifunctional amino acid L-DOPA on Au(110).

In investigations of the proteins which are responsible for the surface adhesion of the blue mussel Mytilus edulis, an unusually frequent appearance of the otherwise rare amino acid 3-(3,4-dihydroxyphenyl)-L-alanine (L-DOPA) has been observed. This amino acid is thought to play a major role in the mechanism of mussel adhesion. Here we report a detailed structural and spectroscopic investigation of the interface between L-DOPA and a single-crystalline Au(110) model surface, with the aim of understanding fundamentals about the surface bonding of this amino acid and its role in mussel adhesion. Molecular layers are deposited by organic molecular beam deposition (OMBD) in an ultrahigh-vacuum environment. The following experimental techniques have been applied: ex situ Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), low-energy electron diffraction (LEED), high-resolution electron energy loss spectroscopy (HREELS), and scanning tunneling microscopy (STM). Vibrational spectra of isolated L-DOPA molecules and the zwitterionic bulk have been calculated using density functional theory (DFT). The predicted modes are assigned to observed spectra, allowing conclusions regarding the molecule-substrate and molecule-molecule interactions at the L-DOPA/Au(110) interface. We find that zwitterionic L-DOPA forms a monochiral, one-domain commensurate monolayer on Au(110), with the catechol rings on top of [110] gold rows, oriented parallel to the surface. The (2 x 1)-Au(110) surface reconstruction is not lifted. The carboxylate group is found in a bidentate or bridging configuration, the amino group is tilted out of the surface plane, and the hydroxyl groups do not dehydrogenate on Au(110). Similar to the case for the bulk, molecules form dimers on Au(110). However, the number of hydrogen bridge bonds between L-DOPA molecules is reduced as compared to the bulk. Thicker layers which are deposited onto the commensurate interface do not order in the bulk structure. In conclusion, our study shows that the aromatic ring system of L-DOPA functions as a surface anchor. Since it is also known that the hydroxyl groups support cross-link reactions between L-DOPA residues in the mussel glue protein, we can conclude that the catechol ring supports surface adhesion of mussel proteins via two independent functions.

Molecular distortions and chemical bonding of a large pi-conjugated molecule on a metal surface.

Normal incidence x-ray standing wave experiments and density functional theory reveal that 3,4,9,10-perylene-tetracarboxylic-dianhydride chemisorbs on Ag(111) in a nonplanar but vertically distorted configuration. The carboxylic O atoms are 0.18 +/- 0.03 angstroms closer to the surface than the perylene core. The distortion is related to weak, local bonds between carboxylic O atoms and the Ag surface which are coupled--through charge transfer into the former lowest unoccupied molecular orbital--to the primary, extended chemisorption bond via the perylene skeleton.

Structural relaxations in electronically excited poly(para-phenylene).

Structural relaxations in electronically excited poly(para-phenylene) are studied using many-body perturbation theory and density-functional-theory methods. A sophisticated description of the electron-hole interaction is required to describe the excitonic energies, but the associated structural relaxations can be obtained quite accurately within a constrained density-functional-theory approach. We find that the structural relaxations in the low-energy excitonic states extend over about eight monomers, leading to an energy reduction of 0.22 eV and a Stokes shift of 0.40 eV.

High accuracy many-body calculational approaches for excitations in molecules.

Two state-of-the-art computational approaches: quantum Monte Carlo and GW with exciton effects [GW-BSE (Bethe-Salpeter equation)] are employed to calculate ionization potentials, electron affinities, and first excited singlet and triplet energies for the silane and methane molecules. Results are in excellent agreement between these dramatically different approaches and with available experiment. The optically forbidden triplet excitation in silane is predicted to lie roughly 1 eV higher than previously reported. In the GW-BSE method, we demonstrate that inclusion of off-diagonal matrix elements in the self-energy operator is crucial for an accurate picture.

U parameter of the mott-hubbard insulator 6H-SiC(0001)-(sqrt

The 6H-SiC(0001)-(sqrt[3]xsqrt[3])R30 degrees surface exhibits one half-filled localized dangling-bond orbital per surface unit cell. Its electronic structure can accurately be described as a Mott-Hubbard insulator. We investigate its spectrum by a spin-polarized ab initio quasiparticle calculation. The resulting band structure shows one occupied and one empty surface band, separated by a direct band gap of 1.95 eV. Since the band gap in the spectrum of the Hubbard model is directly given by the on-site Coulomb-interaction parameter U of the dangling-bond orbital, our results allow for a reliable determination of U = 1.95 eV.

Excitons and optical properties of alpha-quartz

We present an ab initio study of the optical properties of alpha-quartz. The absorption spectrum is calculated by solving the Bethe-Salpeter equation for the interacting electron-hole system and found to be in excellent agreement with the measured spectrum up to 10 eV above the absorption threshold. We find that excitonic effects are crucial in understanding the sharp features in the absorption spectrum in this energy range. They are also crucial in the ab initio computation of the static dielectric constant, significantly enhancing its value.

Structural and optical properties of the Ge(111)-(2 x 1) surface.

We study the two lowest-energy isomers of the Ge(111)-(2 x 1) surface, by a state-of-the-art first-principles calculation of their optical spectra, including the electron-hole interaction effects. A comparison of our results with the available experimental data suggests that, at difference with the silicon case, the stablest isomer differs from the standard "buckled Pandey chains" reconstruction. This conclusion is supported by accurate total-energy results.

Contamination of cerebrospinal fluid specimens with hematogenous blasts in patients with leukemia.

Hematogenous blasts in cytologic preparations of cerebrospinal fluid (CSF) have generally been thought to indicate leukemic involvement of the central nervous system (CNS). However, two leukemic patients who came to autopsy had no CNS involvement despite positive CSF preparations shortly before death. Both patients had over 10,000 blasts/cu mm in their peripheral blood. The CSF specimen from one patient was grossly bloody, but the specimen from the second patient was clear. An experiment was designed to test the hypothesis that membrane filter techniques are sufficiently sensitive to detect blasts in CSF contaminated by amounts of leukemic blood too small to be detected visually. Blood from a leukemic patient with 130,000 cells/cu mm was serially diluted in CSF from 1:20 to 1:1,600,000 and membrane filters from each dilution were examined. No blood could be detected visually at the 1:8,000 and further dilutions. Blasts forms were seen on membrane filter preparations down to the 1:1,600,000 dilution, which contained 0.000000125 ml of blood, equivalent to about 1/40,000th the volume of an 18-gauge spinal needle. These findings suggest that contamination of CSF specimens with minute amounts of blood, undetectable by visual inspection, may lead to erroneous interpretation of leukemic involvement of the CNS.

Primary amyloidosis, pure red cell aplasia, and Kaposi's sarcoma in a single patient.

A patient who developed primary amyloidosis, pure red cell aplasia, and Kaposi's sarcoma is described. This is the second reported coincidence of Kaposi's sarcoma and pure red cell aplasia and the first coincidence of Kaposi's sarcoma and primary amyloid, thus enlarging the spectrum of plasma cell and immunoglobulin abnormalities seen in Kaposi's sarcoma. Because immunologic abnormalities have been described in all these diseases, it is felt that some primary immune dysfunction is the underlying cause of the three diseases in this patient.