Studies Of Vibrational Surface Modes In Ionic Crystals. I. Detailed Shell-model Studies For The Unrelaxed (001) Face Of Seven Crystals Having The Rocksalt Structure

We present a comprehensive, unified account of the lattice dynamics of unrelaxed (001) faces of seven crystals having the rock salt structure, three of which have an absolute gap between the acoustical and optical bulk bands (RbF, RbCl, and NaI) and four of which have overlap between these bulk bands (NaF, NaCl, LiF, and MgO). The results are obtained from calculations of the normal modes of a thin crystal film in which the ionic interactions are described by the rigid-shell model. An outline of the formulation of the shell model for the film is given, and the rapid convergence of surface-phonon frequencies with increasing film thickness is demonstrated. Surface- and pseudo surface-phonon dispersion curves and bulk bands are displayed in the form of the dispersion curves of a 15-layer film over the two-dimensional surface Brillouin zone. The surface-excess phonon density of states fs(ω) is given, and its peaks and valleys are correlated with features in the film dispersion curves. The domains of existence and the vibrational character of the more prominent surface-phonon bands are discussed. We establish the relation between the surface modes of the lattice models and the macroscopic surface waves of the dielectric and elastic continuum theories (Fuchs-Kliewer and Rayleigh waves, respectively). Comparison with experiments is made; the agreement with low-energy atom scattering from single-crystal surfaces of LiF and with inelastic neutron scattering from micro crystallite samples of MgO is particularly notable, although the latter comparison also displays some significant discrepancies. © 1977 The American Physical Society

On the representation of the electric charge distribution in ethane for calculations of the molecular quadrupole moment and intermolecular electrostatic energy

doi:10.1063/1.449776It is shown that a simple atom‐site monopole model is inferior to one which includes higher‐order local multipoles to represent the intramolecular charge distribution of ethane. Unlike the latter model, the local monopole representation predicts the wrong sign for the molecular quadrupole moment and gives a repulsive rather than an attractive electrostatic interaction at typical intermolecular distances. In the local multipole model, the atom‐site dipoles give the largest contribution to both the molecular quadrupole moment and the intermolecular interaction.This work was partially supported by the Danish Natural Science Foundation, the Otto Mensteds Fond, and National Science Foundation Grants No. DMR-8214518 and No. DMR-8304366

Spectroscopic scanning tunneling microscopy insights into Fe-based superconductors

In the first three years since the discovery of Fe-based high Tc superconductors, scanning tunneling microscopy (STM) and spectroscopy have shed light on three important questions. First, STM has demonstrated the complexity of the pairing symmetry in Fe-based materials. Phase-sensitive quasiparticle interference (QPI) imaging and low temperature spectroscopy have shown that the pairing order parameter varies from nodal to nodeless s\pm within a single family, FeTe1-xSex. Second, STM has imaged C4 -> C2 symmetry breaking in the electronic states of both parent and superconducting materials. As a local probe, STM is in a strong position to understand the interactions between these broken symmetry states and superconductivity. Finally, STM has been used to image the vortex state, giving insights into the technical problem of vortex pinning, and the fundamental problem of the competing states introduced when superconductivity is locally quenched by a magnetic field. Here we give a pedagogical introduction to STM and QPI imaging, discuss the specific challenges associated with extracting bulk properties from the study of surfaces, and report on progress made in understanding Fe-based superconductors using STM techniques.Comment: 36 pages, 23 figures, 229 reference

Effect of type and concentration of ballasting particles on sinking rate of marine snow produced by the Appendicularian Oikopleura dioica

Ballast material (organic, opal, calcite, lithogenic) is suggested to affect sinking speed of aggregates in the ocean. Here, we tested this hypothesis by incubating appendicularians in suspensions of different algae or Saharan dust, and observing the sinking speed of the marine snow formed by their discarded houses. We show that calcite increases the sinking speeds of aggregates by ~100% and lithogenic material by ~150% while opal only has a minor effect. Furthermore the effect of ballast particle concentration was causing a 33 m d(-1) increase in sinking speed for a 5×10(5) µm(3) ml(-1) increase in particle concentration, near independent on ballast type. We finally compare our observations to the literature and stress the need to generate aggregates similar to those in nature in order to get realistic estimates of the impact of ballast particles on sinking speeds

A vertically resolved model for phytoplankton aggregation

This work presents models of the vertical distribution and flux of phytoplankton aggregates, including changes with time in the distribution of aggregate sizes and sinking speeds. The distribution of sizes is described by two parameters, the mass and number of aggregates, which greatly reduces the computational cost of the models. Simple experiments demonstrate the effects of aggregation on the timing and depth distribution of primary production and export. A more detailed ecological model is applied to sites in the Arabian Sea; it demonstrates that aggregation can be important for deep sedimentation even when its effect on surface concentrations is small, and it presents the difference in timing between settlement of aggregates and fecal pellets

How Cooper pairs vanish approaching the Mott insulator in Bi2Sr2CaCu2O8+d

The antiferromagnetic ground state of copper oxide Mott insulators is achieved by localizing an electron at each copper atom in real space (r-space). Removing a small fraction of these electrons (hole doping) transforms this system into a superconducting fluid of delocalized Cooper pairs in momentum space (k-space). During this transformation, two distinctive classes of electronic excitations appear. At high energies, the enigmatic 'pseudogap' excitations are found, whereas, at lower energies, Bogoliubov quasi-particles -- the excitations resulting from the breaking of Cooper pairs -- should exist. To explore this transformation, and to identify the two excitation types, we have imaged the electronic structure of Bi2Sr2CaCu2O8+d in r-space and k-space simultaneously. We find that although the low energy excitations are indeed Bogoliubov quasi-particles, they occupy only a restricted region of k-space that shrinks rapidly with diminishing hole density. Concomitantly, spectral weight is transferred to higher energy r-space states that lack the characteristics of excitations from delocalized Cooper pairs. Instead, these states break translational and rotational symmetries locally at the atomic scale in an energy independent fashion. We demonstrate that these unusual r-space excitations are, in fact, the pseudogap states. Thus, as the Mott insulating state is approached by decreasing the hole density, the delocalized Cooper pairs vanish from k-space, to be replaced by locally translational- and rotational-symmetry-breaking pseudogap states in r-space.Comment: This is author's version. See the Nature website for the published versio