Location of pattern-disturbing structures in the vicinity of an antenna Final report, Jun. 5, 1963 - Sep. 15, 1966

Location of radiation pattern disturbing structures in vicinity of satellite antenn

Continuous Transition between Antiferromagnetic Insulator and Paramagnetic Metal in the Pyrochlore Iridate Eu2Ir2O7

Our single crystal study of the magneto-thermal and transport properties of the pyrochlore iridate Eu2Ir2O7 reveals a continuous phase transition from a paramagnetic metal to an antiferromagnetic insulator for a sample with stoichiometry within ~1% resolution. The insulating phase has strong proximity to an antiferromagnetic semimetal, which is stabilized by several % level of the off-stoichiometry. Our observations suggest that in addition to electronic correlation and spin-orbit coupling the magnetic order is essential for opening the charge gap.Comment: 6 pages, 6 figure

Wave functions in the neighborhood of a toroidal surface; hard vs. soft constraint

The curvature potential arising from confining a particle initially in three-dimensional space onto a curved surface is normally derived in the hard constraint $q \to 0$ limit, with $q$ the degree of freedom normal to the surface. In this work the hard constraint is relaxed, and eigenvalues and wave functions are numerically determined for a particle confined to a thin layer in the neighborhood of a toroidal surface. The hard constraint and finite layer (or soft constraint) quantities are comparable, but both differ markedly from those of the corresponding two dimensional system, indicating that the curvature potential continues to influence the dynamics when the particle is confined to a finite layer. This effect is potentially of consequence to the modelling of curved nanostructures.Comment: 4 pages, no fig

The Vector Analyzing Power in Elastic Electron-Nucleus Scattering

The vector analyzing power A_n is calculated for elastic electron scattering from a variety of spin zero nuclei at energies from 14 MeV to 3 GeV. Time reversal symmetry insures that A_n vanish in first Born approximation. Therefore A_n depends on Coulomb distortions and can be large for scattering from heavy nuclei. The vector analyzing power is a potential source of systematic error for parity violation experiments. We find that A_n=-0.361 ppm for the kinematics of the Parity Radius Experiment (PREX) involving 850 MeV electrons scattering at six degrees from 208Pb. This is comparable to the parity violating asymmetry. However for HAPPEX He involving 3 GeV electrons scattering on 4He we find that A_n is very small.Comment: 6 pages, 6 figures, submitted to Phys. Rev.

Metal-Insulator transitions in the periodic Anderson model

We solve the Periodic Anderson model in the Mott-Hubbard regime, using Dynamical Mean Field Theory. Upon electron doping of the Mott insulator, a metal-insulator transition occurs which is qualitatively similar to that of the single band Hubbard model, namely with a divergent effective mass and a first order character at finite temperatures. Surprisingly, upon hole doping, the metal-insulator transition is not first order and does not show a divergent mass. Thus, the transition scenario of the single band Hubbard model is not generic for the Periodic Anderson model, even in the Mott-Hubbard regime.Comment: 5 pages, 4 figure

Strongly correlated metal interfaces in the Gutzwiller approximation

We study the effect of spatial inhomogeneity on the physics of a strongly correlated electron system exhibiting a metallic phase and a Mott insulating phase, represented by the simple Hubbard model. In three dimensions, we consider various geometries, including vacuum-metal-vacuum, a junction between a weakly and a strongly correlated metal, and finally the double junctions metal-Mott insulator-metal and metal-strongly correlated metal- metal. We applied to these problems the self-consistent Gutzwiller technique recently developed in our group, whose approximate nature is compensated by an extreme flexibility,ability to treat very large systems, and physical transparency. The main general result is a clear characterization of the position dependent metallic quasiparticle spectral weight. Its behavior at interfaces reveals the ubiquitous presence of exponential decays and crossovers, with decay lengths of clear physical significance. The decay length of metallic strength in a weakly-strongly correlated metal interface is due to poor screening in the strongly correlated side. The decay length of metallic strength from a metal into a Mott insulator (or into vacuum) is due to tunneling. In both cases, the decay length is a bulk property, and diverges with a critical exponent ($\sim 1/2$ in the present approximation, mean field in character) as the (continuous, paramagnetic) Mott transition is approached.Comment: 19 pages, 19 figure

Transport in gapped bilayer graphene: the role of potential fluctuations

We employ a dual-gated geometry to control the band gap \Delta in bilayer graphene and study the temperature dependence of the resistance at the charge neutrality point, RNP(T), from 220 to 1.5 K. Above 5 K, RNP(T) is dominated by two thermally activated processes in different temperature regimes and exhibits exp(T3/T)^{1/3} below 5 K. We develop a simple model to account for the experimental observations, which highlights the crucial role of localized states produced by potential fluctuations. The high temperature conduction is attributed to thermal activation to the mobility edge. The activation energy approaches \Delta /2 at large band gap. At intermediate and low temperatures, the dominant conduction mechanisms are nearest neighbor hopping and variable-range hopping through localized states. Our systematic study provides a coherent understanding of transport in gapped bilayer graphene.Comment: to appear in Physical Review B: Rapid Com

Doping Dependence of Polaron Hopping Energies in La(1-x)Ca(x)MnO(3) (0<= x<= 0.15)

Measurements of the low-frequency (f<= 100 kHz) permittivity at T<= 160 K and dc resistivity (T<= 430 K) are reported for La(1-x)Ca(x)MnO(3) (0<= x<= 0.15). Static dielectric constants are determined from the low-T limiting behavior of the permittivity. The estimated polarizability for bound holes ~ 10^{-22} cm^{-3} implies a radius comparable to the interatomic spacing, consistent with the small polaron picture established from prior transport studies near room temperature and above on nearby compositions. Relaxation peaks in the dielectric loss associated with charge-carrier hopping yield activation energies in good agreement with low-T hopping energies determined from variable-range hopping fits of the dc resistivity. The doping dependence of these energies suggests that the orthorhombic, canted antiferromagnetic ground state tends toward an insulator-metal transition that is not realized due to the formation of the ferromagnetic insulating state near Mn(4+) concentration ~ 0.13.Comment: PRB in press, 5 pages, 6 figure

Sangamonian Forest History and Climate in Atlantic Canada

Seven of the more than twenty five buried organic deposits in Atlantic Canada assigned to pre-Wisconsinan non-glacial intervals possibly relate to the climatic optimum of the Sangamon Interglaciation, that is substage 5e of the deep-sea oxygen isotope record. These sites are East Bay and Green Point on Cape Breton Island. Addington Forks and East Milford in mainland Nova Scotia. Le Bassin and Portage-du-Cap on the Iles de la Madeleine, Québec, and Woody Cove, Newfoundland. Except for Woody Cove, none of the sites records a complete climatic cycle, and the sequence of events must be pieced together from their disparate records. The spectra, characterized by significant amounts of thermophilous taxa that are not as abundant or present in the region today, are similar in general to Holocene spectra at sites immediately south of the lower Great Lakes. Comparison of the fossil spectra from five sites with modern surface spectra from eastern North America yields modern analogs which, if valid, indicate that the climate in Atlantic Canada during the climatic optimum of the last interglacial interval was more continental in character and considerably warmer than present.Des quelque vingt-cinq dépôts organiques enfouis datant d'intervalles non glaciaires pré-wisconsiniens, sept d'entre eux pourraient être attribués à l'optimum climatique de l'Interglaciaire du Sangamonien. c'est-à-dire le stade isotopique 5e. Ces sites sont les suivants: East Bay et Green Point, à l'île du Cap-Breton, Addington Forks et East Milford. en Nouvelle-Ecosse. Le Bassin et Portage-du-Cap, aux îles de la Madeleine, et Woody Cove, à Terre-Neuve. Aucun des sites, sauf celui de Woody Cove, n'a enregistré un cycle climatique complet, et la séquence des événements doit être reconstituée à partir d'inventaires disparates. Les spectres, caractérisés par des quantités importantes de taxons thermophiles qui ne sont plus aussi abondants ou même présents dans la région aujourd'hui, sont semblables de façon générale aux spectres holocènes de sites localisés immédiatement au sud des Grands Lacs inférieurs. La comparaison entre les spectres fossiles de cinq sites et des spectres de surface actuels de l'est de l'Amérique du Nord fait ressortir des sites analogues actuels qui démontreraient que le climat des Maritimes au cours de l'optimum climatique du dernier interglaciaire était plus continental et passablement plus chaud que maintenant.Von den mehr als 25 vergrabenen organischen Ablagerungen im atlantischen Kanada, die prâ-wisconsinischen nichtglazialen Intervallen zugeschrieben werden, gehôren sieben môglicherweise dem klimatischen Optimum des sangamonischen Interglazial an. das heisst Unterphase 5e des Tiefsee-Sauerstoff-lsotop-Belegs. Die sieben Vorkommen sind East Bay und Green Point auf Cape Breton Island, Addington Forks und East Milford auf dem Festland von Nova Scotia. Le Bassin und Portage-du-Cap auf den îles de la Madeleine, Québec, und Woody Cove, Newfoundland. Ausser Woody Cove belegt keiner dieser Plàtze einen ganzen klimatischen Zyklus und die Abfolge der Ereignisse muss aus ihren disparaten Belegen zusammengesetzt werden. Die Spectra, die durch signifikante Mengen von thermophilen Taxa gekennzeichnet sind, welche heute in der Gegend nicht so reichlich vorhanden oder nicht so zugegen sind, àhneln im allgemeinen den Holozàn-Spectra an Plàtzen unmitellbar sùdlich der unteren grossen Seen. Ein Vergleich der Fossil-Sprectra von fùnf Plâtzen mit modernen Oberflâchen-Spectra von Ost Nordamerika liefert moderne Analoge. welche, wenn sie gùltig sind, darauf hinweisen. dass das Klima im atlantischen Kanada wàhrend des klimatischen Optimums des letzten Interglazial einen kontinentalen Charakter hatte und bedeutend warmer war als gegenwàrtig