bluuebunny/crossref_metadata_2025 huggingface dataset:
Clinical Trials in Dementia
Authors: Raya-Ampil Encarnita , Cummings Jeffrey L.
Published: June 2006

ABSTRACT

Dementia, particularly Alzheimer's disease (AD), is increasing by patient population included in clinical trials. The methodology for trials of AD patients have been defined in terms of outcomes, standard measures and analytic techniques. Trial methodology is evolving with experience as new potential therapies become available. Screening criteria, instrumentation choices, duration of trials and analytic strategies may have a profound impact on the conclusions that can be derived from trials. The components of AD trials are reviewed in detail in this chapter.
Signal strength measurements at frequencies of around 300 MHz over two sea paths in the British Channel Islands
Authors: Sim C. Y. D. , Warrington E. M.
Published: June 2006

In order to gather information about the occurrence of ducting/superrefraction and signal‐fading effects at frequencies around 300 MHz with antenna heights appropriate to intership communications, an experimental investigation has been undertaken with a transmitter and two receivers deployed in the British Channel Islands. Signal strength measurements made over a period of 17 months for a path from Jersey to Guernsey from April 2001 to September 2002 and 8 months of data for a path from Jersey to Alderney from November 2001 to September 2002 have been analyzed. Comparisons have been made between the received signal characteristics and several meteorological parameters such as sea state, weather conditions, and season, and the statistics of the occurrence of enhancements in signal strength due to superrefraction and ducting are presented.
Three‐dimensional visualization of a near‐vertical slab tear beneath the southern Mariana arc
Authors: Miller Meghan S. , Gorbatov Alexei , Kennett Brian L. N.
Published: June 2006

The use of a three‐dimensional ray‐tracing inversion algorithm has greatly enhanced the resolution of gradients and strong variations in wave speeds to create improved P wave tomographic images of the Mariana arc region. The images obtained from the Mariana arc region show relatively low amplitudes of heterogeneity due to the limited number of seismic stations in the area. Despite these limitations, detailed interpretations of the three‐dimensional geometry and morphology of the Pacific Plate subducting beneath the Philippine Sea Plate have provided a three‐dimensional model of the steep dip of the Pacific plate and the curvature of the slab beneath the Mariana arc in unprecedented detail. The new P wave tomography and seismicity depict a previously unidentified E‐W trending near‐vertical tear in the subducting plate at the southern end of the Mariana arc that divides the arc into two distinct segments: a steeply dipping curved slab penetrating the lower mantle and a short (∼250 km depth) slab along the Challenger Deep segment of the arc. The slab tear is likely to be the result of the need to accommodate the reduced volume the slab must occupy as it is subducted beneath the Philippine Sea plate along a curved arc.
Time series of the partial pressure of carbon dioxide (2001–2004) and preliminary inorganic carbon budget in the Scheldt plume (Belgian coastal waters)
Authors: Schiettecatte Laure‐Sophie , Gazeau Frédéric , van der Zee Claar , Brion Natacha , Borges Alberto V.
Published: June 2006

A 4‐year time series (2001–2004) of the partial pressure of CO2 (pCO2) and air‐sea CO2 fluxes is reported in the Scheldt estuarine plume. This system is oversaturated in CO2 with respect to the atmosphere, except during the spring phytoplanktonic bloom, and acts as a net source of CO2 to the atmosphere of 0.7 mol C m−2 yr−1 that represents 7 to 27% of the inner Scheldt estuary CO2 emission. Results also highlight that a high spatial and temporal coverage of the surface pCO2 in coastal ecosystems is crucial for reliable estimations of air‐sea CO2 fluxes. The seasonal variations of pCO2 seem to be more dominated by biological activities (photosynthesis/respiration) than by temperature change. A stochiometrically linked C/P mass balance budget failed to provide net ecosystem production estimates consistent with the pCO2 dynamics in this area. It is hypothesized that this discrepancy is related to physiological characteristics of the dominant phytoplanktonic species (Phaeocystis sp.) within the studied area. On the basis of a preliminary dissolved inorganic carbon input/output budget, the annual emission of CO2 toward the atmosphere seems to be largely due to the outgassing of the inputs of CO2 from the inner Scheldt estuary, rather than due to organic carbon degradation.
Substorms during the 10–11 August 2000 sawtooth event
Authors: Henderson M. G. , Skoug R. , Donovan E. , Thomsen M. F. , Reeves G. D. , Denton M. H. , Singer H. J. , McPherron R. L. , Mende S. B. , Immel T. J. , Sigwarth J. B. , Frank L. A.
Published: June 2006

Sawtooth events have been identified at geosynchronous orbit as large‐amplitude quasiperiodic (2–4 hour period) modulations of the energetic electron and ion fluxes. They are called sawtooth events because the shape of the flux versus time profiles are composed of rapid increases followed by gradual decreases that resemble the teeth on a saw blade. Although much of the phenomenology associated with sawtooth events is substorm‐like, there is still debate as to whether the individual teeth are substorms or not. Here we examine each of the teeth associated with the 10–11 August 2000 sawtooth event in detail. We find that all but one of the teeth were associated with injections at geosynchronous orbit and that most of the teeth were consistent with the hypothesis that they are predominantly caused by unusually large and longitudinally extended substorms. A few were unclear or complex, and the final flux enhancement at 1845:36 UT was not a substorm but a solar wind shock‐associated disturbance. In addition, the presence of numerous dispersionless flux perturbations in the LANL SOPA data provides support for the hypothesis that solar wind pressure variations can modulate the flux profiles to some extent. For the substorm events we find that the geosynchronous particle injections were neither globally simultaneous nor globally dispersionless but were instead consistent with a nightside/duskside source in most cases. Similarly, we show that the field dipolarizations were also not global and simultaneous. Each of the substorms was also associated with high‐latitude negative H bays, middle‐ and low‐latitude positive H bays, a partial recovery in Sym‐H, and the onset of Pi2 ULF pulsations. In addition, we show that the auroral distribution develops in a systematic way during each cycle of a sawtooth substorm event. Specifically, a localized auroral onset develops on the lower branch of a thinned double‐oval distribution. The location of onset is typically premidnight and often occurs to the west of intense omega band forms. This is followed by westward, eastward, and poleward expansion and the copious production of auroral streamers which can develop in complex patterns including a “spoke‐like” morphology. The double‐oval configuration thins again during the stretching phase until the next onset occurs and the cycle repeats. A schematic representation of the auroral dynamics associated with sawtooth substorms is also presented.
Divergence‐free magnetic field interpolation and charged particle trajectory integration
Authors: Mackay F. , Marchand R. , Kabin K.
Published: June 2006

An interpolation method is presented for calculating a divergence‐free magnetic field at arbitrary locations in space from a representation of that field on a discrete grid. This interpolation method is used along with symplectic integration to perform particle trajectory integrations with good conservation properties. These integrations are better at conserving constants of motion and adiabatic invariants than standard, nonsymplectic Runge‐Kutta integration schemes. In particular, we verify that carrying out particle integrations with interpolated magnetic fields that satisfy ∇ · B = 0 yields a better conservation of the first adiabatic invariant μ. Comparisons are made between the different integration methods for proton trajectories in an ideal dipole magnetic field and in the cusp region of the magnetosphere as determined numerically from a global magnetohydrodynamic (MHD) model for realistic solar wind conditions. In the case considered, we find that particle trapping can occur in the cusp only if the self‐consistent electric field obtained from the MHD code is not taken into account. When that electric field is included in the calculation of particle trajectories, we fail to find any particle trapping in the cusp region.
Estimation of planetary surface roughness using radio sounder A‐scope data
Authors: Kobayashi T. , Ono T.
Published: June 2006

We propose a simple technique to estimate a roughness parameter, RMS gradient, of a planetary surface. The technique is practiced on A‐scope data of radio sounding observation of planet with a simple formula. The formula was derived analytically for the case of Gaussian random rough surface using a geometrical optics model. The technique was examined by simulations of planetary sounding observations which is based on physical optics, and demonstrated to estimate the RMS gradient of the planetary surface in the range from some fraction of degrees to about ten degrees. Finally, the upper limit of valid range of RMS gradient was formulized as a function of observation altitude and the maximum detection range.
Spatial organization of soil depths using a landform evolution model
Authors: Saco Patricia M. , Willgoose Garry R. , Hancock Greg R.
Published: June 2006

The evolution of soil depths is investigated by modeling the interaction between soil production and surface erosion within a landform evolution model. An enhanced version of the landform evolution model SIBERIA that incorporates a soil evolution module is used to simulate evolving landforms and soils depths over geologic timescales. The spatial and temporal evolution of soil depths are examined at the hillslope scale. Though it is widely accepted among the geomorphology community that soil water enhances chemical, physical and biological weathering processes, its effect has not been explicitly included in published models of soil production. The main scientific questions that we address are (1) what are the implications of incorporating soil moisture dependency in the soil production function and (2) what type of soil production dynamics is needed to generate a bedrock topography that has a different spatial pattern from that of the ground surface. A range of physics for the soil production model is explored. The effect of soil moisture is included using the wetness index obtained from drainage analysis of either surface elevations or the bedrock topography. The results show that the various soil production functions that incorporate either a wetness index or subsurface flow depth based on the bedrock topography give rise to soils that self‐organize with well‐defined spatial patterns and bedrock elevations with spatial organization significantly different from that of the surface. The model that incorporates the influence of subsurface water on soil production is able to naturally generate a soil production rate with a maximum value for a nonzero soil depth and overcomes an inconsistency of previously published “humped” soil production models.
Water column dynamics during the last interglacial anoxic event in the Mediterranean (sapropel S5)
Authors: Rohling E. J. , Hopmans E. C. , Sinninghe Damsté J. S.
Published: June 2006

Marine organic‐rich sediments are important for long‐term carbon sequestration, and as a source of fossil hydrocarbons, but the processes underlying their formation remain elusive. We present a multiproxy analysis of a relatively recent, well‐preserved Mediterranean organic‐rich deposit known as sapropel S5, which formed 124–119 thousand years ago (ka B.P., where B.P. references year 1950). Combining planktonic foraminiferal abundance records with stable isotope and organic biomarker data, we reconcile the apparently contradicting coexistence of deep‐dwelling foraminiferal faunas with green sulphur bacteria that indicate photic zone euxinia. S5 started with a rapid freshwater‐induced increase of density stratification that inhibited vertical mixing and deepwater ventilation. Through the first 900 years, anoxic to euxinic deepwater conditions developed and extended up to roughly 200 m water depth. Then followed a 4100‐year period of variability in the depth/intensity of winter mixing with a roughly 1400‐year periodicity, coincident with fluctuations in water column stability. S5 terminated with a marked decrease in density stratification and consequently renewed convective overturn.
Allometric relationships between traveltime channel networks, convex hulls, and convexity measures
Authors: Tay Lea Tien , Sagar B. S. Daya , Chuah Hean Teik
Published: June 2006

The channel network (S) is a nonconvex set, while its basin [C(S)] is convex. We remove open‐end points of the channel connectivity network iteratively to generate a traveltime sequence of networks (Sn). The convex hulls of these traveltime networks provide an interesting topological quantity, which has not been noted thus far. We compute lengths of shrinking traveltime networks L(Sn) and areas of corresponding convex hulls C(Sn), the ratios of which provide convexity measures CM(Sn) of traveltime networks. A statistically significant scaling relationship is found for a model network in the form L(Sn) ∼ A[C(Sn)]0.57. From the plots of the lengths of these traveltime networks and the areas of their corresponding convex hulls as functions of convexity measures, new power law relations are derived. Such relations for a model network are CM(Sn) ∼ and CM(Sn) ∼ . In addition to the model study, these relations for networks derived from seven subbasins of Cameron Highlands region of Peninsular Malaysia are provided. Further studies are needed on a large number of channel networks of distinct sizes and topologies to understand the relationships of these new exponents with other scaling exponents that define the scaling structure of river networks.
Data assimilation and adaptive forecasting of water levels in the river Severn catchment, United Kingdom
Authors: Romanowicz Renata J. , Young Peter C. , Beven Keith J.
Published: June 2006

This paper describes data assimilation (DA) and adaptive forecasting techniques for flood forecasting and their application to forecasting water levels at various locations along a 120 km reach of the river Severn, United Kingdom. The methodology exploits the top‐down, data‐based mechanistic (DBM) approach to the modeling of environmental processes, concentrating on the identification and estimation of those “dominant modes” of dynamic behavior that are most important for flood prediction. In particular, hydrological processes active in the catchment are modeled using the state‐dependent parameter (SDP) method of estimating a nonlinear, effective rainfall transformation together with a linear stochastic transfer function (STF) method for characterizing both the effective rainfall–river level behavior and the river level routing processes. The complete model consists of these lumped parameter, linear and nonlinear stochastic, dynamic elements connected in a quasi‐distributed manner that represents the physical structure of the catchment. The adaptive forecasting system then utilizes a state‐space form of the complete catchment model, including allowance for heteroscedasticity in the errors, as the basis for data assimilation and forecasting using a Kalman filter forecasting engine. Here the predicted model states (water levels) and adaptive parameters are updated recursively in response to input data received in real time from sensors in the catchment. Direct water level forecasting is considered, rather than flow, because this removes the need to transform the level measurement through the rating curve and tends to decrease the forecasting errors.
Comparison of methods to calculate relative permeability from capillary pressure in consolidated water‐wet porous media
Authors: Li Kewen , Horne Roland N.
Published: June 2006

The Brooks and Corey relative permeability model has been accepted widely as a way to calculate relative permeability using capillary pressure data. However, the Purcell model was found to be the best fit to the experimental data of the wetting‐phase relative permeability in the cases studied here, as long as the measured capillary pressure curve had the same residual saturation as the relative permeability curve. The differences between the experimental data of relative permeability and the data calculated using the Purcell relative permeability model for the wetting phase were almost negligible. A physical model was developed to explain the insignificance of the effect of tortuosity on the calculation of the wetting‐phase relative permeability. For the nonwetting‐phase, the relative permeabilities calculated using the models were very close to the experimental values in drainage except for the Purcell model. However, in the case of imbibition, the relative permeabilities calculated using the models were different from the experimental data. This study showed that relative permeability could be calculated satisfactorily by choosing a suitable model, especially in drainage processes. In the reverse procedure, capillary pressure could also be computed once relative permeability data are available.
Geophysical implications of reduction in thermal conductivity due to hydration
Authors: Hofmeister Anne M. , Pertermann Maik , Branlund Joy M. , Whittington Alan G.
Published: June 2006

Laser‐flash measurements show that hydration lowers the lattice component of thermal diffusivity (D) for calcic and Fe‐Mg garnets, and glasses. Hydration depresses D above the glass transition, suggesting that melts respond likewise. For garnets, which have a wide range of total water and OH− contents (X), ∂(lnD)/∂X = −0.003%/ppm H2O. For olivine and quartz, D is constant, but thermal conductivity (k = ρCPD) decreases because hydration lowers density (ρ) but negligibly changes heat capacity (CP). This finding is geophysically significant: specifically, it suggests that hydration leads to heat retention, promoting convective instabilities and melting.
Seasonal evolution of the upper‐tropospheric westerly jet core over East Asia
Authors: Zhang Yaocun , Kuang Xueyuan , Guo Weidong , Zhou Tianjun
Published: June 2006

The location change of the westerly jet core at upper troposphere in June and July is investigated by using the NCEP/NCAR reanalysis data. The results show that the location of the westerly jet core changes rapidly from 140°E to 90°E during 35th–39th pentads, which corresponds to the plum rain period over East Asia. The location change of the jet core is actually the relative intensity change of the different westerly jet centers. The meridional temperature contrast in the troposphere is associated with the rapid location change of the jet core. The diabatic heating changes are the primary factor determining the seasonal evolution of the westerly jet core over East Asia.
Vinylidene-acetylene cation isomerization investigated by large scale ab initio calculations
Authors: Boyé-Péronne Séverine , Gauyacq Dolores , Liévin Jacques
Published: June 2006

The planar isomerization routes of the vinylidene/acetylene cation in the lowest electronic states are accurately examined for the first time, by using large scale MRCI and CCSD(T) calculations in a complementary way. They are compared with the similar calculations performed for the neutral ground state isomerization. An accurate value of the adiabatic ionization potential of vinylidene (11.26eV) is predicted. The vinylidene cation lowest state, 1A12, follows an almost flat pathway with a shallow secondary minimum on the 1A′2 potential energy surface, before suddenly dropping to the stable acetylene cation ground state, X̃Πu2. It is therefore confirmed to be completely unstable with respect to isomerization. The first excited state of the vinylidene cation, 1B12, which also correlates with the Πu2 ground state of acetylene cation along a A′2 isomerization route, has been studied at the same level of calculation. This 1B12 state is lying only 0.15eV above the 1A12 state, and exhibits a potential energy barrier of 0.55eV which explains the earlier assignment of this symmetry to the ground state of vinylidene cation. In addition to large scale calculations, a comprehensive description of the important steps of isomerization drawn from a very simple model involving monoconfigurational states is presented. In particular, the behavior of one unique orbital, namely, the 5a1 outer molecular orbital, is shown to completely govern the molecular geometry and energy evolution along the isomerization route of the ground state cation C2H2+.
Multipole-multimode Floquet theory of rotational resonance width experiments: C13–C13 distance measurements in uniformly labeled solids
Authors: Ramachandran Ramesh , Lewandowski Józef R. , van der Wel Patrick C. A. , Griffin Robert G.
Published: June 2006

A formal description of zero-quantum (ZQ) NMR processes using multipole-multimode Floquet theory is proposed for studying polarization transfer in magic angle spinning experiments. Specifically, we investigate the factors affecting the accuracy and precision of C13–C13 distance measurements that are based on ZQ-magnetization exchange processes in rotational resonance width experiments. With suitable examples drawn from measurements in N-acetyl-[U-C13,N15]-L-valine-L-leucine, we substantiate our approach and propose methods for improving the accuracy and reliability of such C13–C13 distance measurements in uniformly C13, N15-labeled solids. In addition, the theoretical model presented in this article provides a more general framework for describing relaxation phenomena involving multiple decay rate constants in zero-quantum processes.
Aging of the Johari-Goldstein relaxation in the glass-forming liquids sorbitol and xylitol
Authors: Yardimci Hasan , Leheny Robert L.
Published: June 2006

Employing frequency-dependent dielectric susceptibility we characterize the aging in two supercooled liquids, sorbitol and xylitol, below their calorimetric glass transition temperatures. In addition to the alpha relaxation that tracks the structural dynamics, the susceptibility of both liquids possesses a secondary Johari-Goldstein relaxation at higher frequencies. Following a quench through the glass transition, the susceptibility slowly approaches the equilibrium behavior. For both liquids, the magnitude of the Johari-Goldstein relaxation displays a dependence on the time since the quench, or aging time, that is quantitatively very similar to the age dependence of the alpha peak frequency. The Johari-Goldstein relaxation time remains constant during aging for sorbitol while it decreases slightly with age for xylitol. Hence, one cannot sensibly assign a fictive temperature to the Johari-Goldstein relaxation. This behavior contrasts with that of liquids lacking distinct Johari-Goldstein peaks for which the excess wing of the alpha peak tracks the main part of the peak during aging, enabling the assignment of a single fictive temperature to the entire spectrum. The aging behavior of the Johari-Goldstein relaxation time further calls into question the possibility that the relaxation time possesses stronger temperature dependence in equilibrium than is observed in the out-of-equilibrium state below the glass transition.
A macroion electrokinetics algorithm
Authors: Fixman Marshall
Published: June 2006

A numerical algorithm is presented for the standard model of macroion electrokinetics and certain generalizations of it. The macroion consists of a cylindrical section with identical, hemispheroidal endcaps, each piece having arbitrary length. The system of one macroion and adjoining salt solution is subjected to an arbitrary sequence of pulsed electrical fields and pulsed translational and rotational velocities. Numerical solutions are obtained for the time dependent electrostatic and mobile ion concentration fields and the solvent velocity. From these fields the dielectric response, force, and torque are calculated. Generalizations of the standard model include the diffusive motion of macroion surface charges, partial slip of solvent motion at the macroion surface, and a simple model for the reactive exchange of surface charge with solution ions. The primary illustrative application is to recent measurements of electric birefringence versus applied field frequency for poly-(tetrafluorothylene) colloidal particles, but a few results are presented for the dielectric response of DNA fragments and of spherical colloidal particles. The source code and additional details are provided as supplementary documentation.
Multiple photon excitation and ionization of NO in and on helium droplets
Authors: Polyakova E. , Stolyarov D. , Wittig C.
Published: June 2006

The photoexcitation of NO embedded in superfluid Hen nanodroplets having ⟨n⟩∼104 has been examined. Two-photon excitation prepares electronically excited states (NO*), most notably, the embedded analog of the AΣ2 state of gas phase NO. Vertical excitation to this low Rydberg state is blueshifted and broadened relative to its gas phase counterpart because of the repulsive electron-helium interaction. Transport to the droplet surface is believed to be facile in the superfluid. For example, NO* prefers (energetically) to reside at the droplet surface rather than at the droplet center, in contrast to NO. Photoionization of surface-bound NO* occurs over a significant photon energy range. This yields small cluster ions NO+Hek, with ∼90% of these clusters having k⩽10. The variation of ion yield with photon energy displays a precipitous change in the region of 24300–24400cm−1 for all values of k. Possible photoionization mechanisms are discussed and it is suggested that intermediate levels with high-n Rydberg character play a role. This work underscores the important role played by transport in the photophysics of species embedded in the superfluid host.
Present status of the theory of the high-Tc cuprates
Authors: Anderson P. W.
Published: June 2006

The Gutzwiller-projected mean-field theory, also called plain vanilla or renormalized mean-field theory, is explained, and its successes and possible extensions in describing the phenomenology of the cuprate superconductors are discussed. Throughout, we emphasize that while this is a Hartree–Fock-based BCS theory, it embodies fundamental differences from conventional perturbative many-body theory which may be characterized by calling it a theory of the doped Mott insulator.
Complexity in high-temperature superconductors
Authors: Alvarez G. , Moreo A. , Dagotto E.
Published: June 2006

A spin-fermion and Landau–Ginzburg model for high-temperature superconductors are formulated and analyzed with unbiased numerical techniques. A study of the competition between d-wave superconductivity and antiferromagnetism in the clean limit suggests that the phase diagram is not universal. When chemical disorder is considered, a “clustered state” with short-range order but without long-range order appears, leading to the possibility of a giant proximity effect. The influence of disorder on the spectral function is also addressed, and results are compared to available-angle resolved photoemission data for cuprates.
ARPES on high-temperature superconductors: Simplicity vs. complexity (Review Article)
Authors: Kordyuk A. A. , Borisenko S. V.
Published: June 2006

A notable role in understanding of the microscopic electronic properties of high-temperature superconductors (HTSC) belongs to angle-resolved photoemission spectroscopy (ARPES). This technique supplies a direct window into the reciprocal space of solids: the momentum-energy space where quasiparticles (electrons dressed in clouds of interactions) dwell. Any interaction in the electronic system, e.g., superconducting pairing, leads to modification of the quasiparticle spectrum—to redistribution of the spectral weight over the momentum-energy space probed by ARPES. Continued development of the technique had the effect that the picture seen through the ARPES window became clearer and sharper until the complexity of the electronic band structure of the cuprates had been resolved. Now, in the doping range optimal for superconductivity, the cuprates much resemble a normal metal with well-predicted electronic structure, though with rather strong electron-electron interaction. This principal disentanglement of the complex physics from complex structure reduced the mystery of HTSC to the tangible problem of the interaction responsible for quasiparticle formation. Here we present a short overview of recent ARPES results, which, we believe, suggest a way to resolve the HTSC puzzle.
Stripe phases—possible ground state of the high-Tc superconductors
Authors: Raczkowski M. , Oleś A. M. , Frésard R.
Published: June 2006

Based on the mean-field method applied either to the extended single-band Hubbard model or to the single-band Peierls–Hubbard Hamiltonian we study the stability of both site-centered and bond-centered charge domain walls. The difference in energy between these phases is found to be small. Therefore, moderate perturbations to the pure Hubbard model, such as next-nearest-neighbor hopping, lattice anisotropy, or coupling to the lattice, induce phase transitions, shown in the corresponding phase diagrams. In addition, we determine the charge for stable phases and magnetization densities, double occupancy, and kinetic and magnetic energies and investigate the role of a finite electron-lattice coupling. We also review experimental signatures of stripes in the superconducting copper oxides.
Magnetic field induced finite-size effect in type-II superconductors
Authors: Schneider T.
Published: June 2006

We explore the occurrence of a magnetic-field-induced finite-size effect on the specific heat and correlation lengths of anisotropic type-II superconductors near the zero-field transition temperature Tc. Since near the zero-field transition thermal fluctuations are expected to dominate and with increasing field strength these fluctuations become one-dimensional, whereupon the effect of fluctuations increases, it appears unavoidable to account for thermal fluctuations. Invoking the scaling theory of critical phenomena it is shown that the specific heat data of nearly optimally doped YBa2Cu3O7−δ are inconsistent with the traditional mean-field and lowest-Landau-level predictions of a continuous superconductor to normal state transition along an upper critical field Hc2(T). On the contrary, we observe agreement with a magnetic-field-induced finite-size effect, whereupon even the correlation length longitudinal to the applied field H cannot grow beyond the limiting magnetic length LH∝Φ0∕H. It arises because with increasing magnetic field the density of vortex lines becomes greater, but this cannot continue indefinitely. LH is then roughly set on the proximity of vortex lines by the overlapping of their cores. Thus the shift and the rounding of the specific heat peak in an applied field is traced back to a magnetic-field-induced finite-size effect in the correlation length longitudinal to the applied field.
Applying BCS–BEC crossover theory to high-temperature superconductors and ultracold atomic Fermi gases (Review Article)
Authors: Chen Q. , Levin K. , Stajic J.
Published: June 2006

This review is written at the time of the twentieth anniversary of the discovery of high-temperature superconductors, which nearly coincides with the important discovery of the superfluid phases of ultracold trapped fermionic atoms. We show how these two subjects have much in common. Both have been addressed from the perspective of the BCS—Bose–Einstein condensation (BEC) crossover scenario, which is designed to treat short coherence length superfluids with transition temperatures which are “high” with respect to the Fermi energy. A generalized mean field treatment of BCS–BEC crossover at general temperatures T, based on the BCS–Leggett ground state, has met with remarkable success in the fermionic atomic systems. Here we summarize this success in the context of four different cold atom experiments, all of which provide indications, direct or indirect, for the existence of a pseudogap. This scenario also provides a physical picture of the pseudogap phase in the underdoped cuprates which is a central focus of high Tc research. We summarize successful applications of BCS–BEC crossover to key experiments in high Tc systems, including the phase diagram, specific heat, and vortex core STM data, along with the Nernst effect, and exciting recent data on the superfluid density in very underdoped samples.
Pseudogap and high-temperature superconductivity from weak to strong coupling. Towards a quantitative theory (Review Article)
Authors: Tremblay A.-M. S. , Kyung B. , Sénéchal D.
Published: June 2006

This is a short review of the theoretical work on the two-dimensional Hubbard model performed in Sherbrooke in the last few years. It is written on the occasion of the twentieth anniversary of the discovery of high-temperature superconductivity. We discuss several approaches, how they were benchmarked and how they agree sufficiently with each other that we can trust that the results are accurate solutions of the Hubbard model. Then comparisons are made with experiment. We show that the Hubbard model does exhibit d-wave superconductivity and antiferromagnetism essentially where they are observed for both hole- and electron-doped cuprates. We also show that the pseudogap phenomenon comes out of these calculations. In the case of electron-doped high temperature superconductors, comparisons with angle-resolved photoemission experiments are nearly quantitative. The value of the pseudogap temperature observed for these compounds in recent photoemission experiments had been predicted by theory before it was observed experimentally. Additional experimental confirmation would be useful. The theoretical methods that are surveyed include mostly the two-particle self-consistent approach, variational cluster perturbation theory (or variational cluster approximation), and cellular dynamical mean-field theory.
Correlated band structure of electron-doped cuprate materials
Authors: Dahnken C. , Potthoff M. , Arrigoni E. , Hanke W.
Published: June 2006

We present a numerical study of the doping dependence of the spectral function of the n-type cuprates. Using a variational cluster-perturbation theory approach based upon the self-energy-functional theory, the spectral function of the electron-doped two-dimensional Hubbard model is calculated. The model includes the next-nearest-neighbor electronic hopping amplitude t′ and a fixed on-site interaction U=8t at half-filling and doping levels ranging from x=0.077 to x=0.20. Our results support the fact that a comprehensive description of the single-particle spectrum of electron-doped cuprates requires a proper treatment of strong electronic correlations. In contrast to previous weak-coupling approaches, we obtain a consistent description of the ARPES experiments without the need to (artificially) introduce a doping-dependent on-site interaction U.
Interaction of strongly correlated electrons and acoustical phonons
Authors: Moskalenko V. A. , Entel P. , Digor D. F.
Published: June 2006

We investigate the interaction of correlated electrons with acoustical phonons using the extended Hubbard–Holstein model. The Lang–Firsov canonical transformation allows one to obtain mobile polarons for which a new diagram technique and generalized Wick’s theorem is used. The physics of emission and absorption of the collective phonon-field mode by the polarons is discussed in detail. In the strong-coupling limit of the electron–phonon interaction, and in the normal as well as in the superconducting phase, chronological thermodynamic averages of products of acoustical phonon-cloud operators can be expressed by the products of one-cloud operator averages. While the normal one-cloud propagator has the form of a Lorentzian, the anomalous one is of Gaussian form and considerably smaller. We have established the Mott–Hubbard and superconducting phase transitions in this model.
Investigation of the superconducting energy gap in the compound LuNi2B2C by the method of point contact spectroscopy: two-gap approximation
Authors: Bobrov N. L. , Beloborod’ko S. I. , Tyutrina L. V. , Chernobay V. N. , Yanson I. K. , Naugle D. G. , Rathnayaka K. D. D.
Published: June 2006

It is shown that the two-gap approximation is applicable for describing the dV∕dI(V) spectra of LuNi2B2C–Ag point contacts in a wide interval of temperatures. The values and the temperature dependences of the large and the small gaps in the ab plane and in the c direction were estimated using the generalized BTK model [A. Plecenik, M. Grajacar, S. Benacka, P. Seidel, and A. Pfuch, Phys. Rev. B 49, 10016 (1994)] and the equations of S. I. Beloborodko, Fiz. Nizk. Temp. 29, 868 (2003) [Low Temp. Phys. 29, 650 (2003)]. In the BCS extrapolation the critical temperature of the small gap is 10K in the ab plane and 14.5K in the c direction. The absolute values of the gaps are Δ0ab=2.16meV and Δ0c=1.94meV. For the large gaps the critical temperature Tc coincides with the bulk, Tcbulk=16.8K, and their absolute values are very close, being about 3meV in both orientations. In the c direction the contributions to the conductivity from the small and the large gaps remain practically identical up to 10–11K. In the ab plane the contribution from the small gap is much smaller and decreases rapidly as a temperature rises.
Microwave response of single crystal YBa2Cu3O7−δ films as a probe for pairing symmetry
Authors: Pan V. M. , Kalenyuk O. A. , Kasatkin O. L. , Komashko V. A. , Ivanyuta O. M. , Melkov G. A.
Published: June 2006

Temperature dependences of the microwave surface impedance, Zs(T), are measured in the c-axis oriented single-crystal high-Tc superconducting cuprate YBa2Cu3O7−δ (YBCO) thin films deposited by the off-axis dc magnetron sputtering onto CeO2-buffered single-crystal sapphire substrates (film thickness is d≈150, 300, 480nm). Measurements are performed by a use of the coplanar resonator as well as the end-plate cylindrical cavity resonator techniques at several discrete frequencies within the range 5–134GHz. The measurements have revealed unexpected peculiarities on the Zs(T)-dependences for the most perfect films under study. The peculiarities appear to be most strongly pronounced on the temperature dependences of the film surface resistance Rs(T)=Re{Zs(T)}. The most important features of the unusual surface resistance behavior are: (i) the temperature dependence Rs(T) of the YBCO films under study at low temperatures obeys the exponential law: Rs(T)=Rres+R0exp[−Δs∕T] with a small energy gap Δs(Δs≈0.5Tc at f=5GHz); (ii) the most perfect films reveal a distinct two-peak structure of the Rs(T) dependence with peaks positioned at 27–30K and 48–51K, while such peaks are not observed in less perfect films. The peaks are mostly pronounced at moderate (e.g., 34GHz) frequencies and gradually disappear both at higher and lower frequencies, while their temperature positions remain unchanged. These features of perfect single-crystalline YBCO films are believed to reveal their intrinsic electron properties. Taking into account the possibility of a mixed (s+id)-type pairing symmetry as well as a significant role of extended out-of-plane crystal defects (such as dislocation lines or twin planes) in Bogolyubov’s quasiparticle scattering within the most perfect YBCO films, one can suggest a consistent explanation for the anomalies observed in the Zs(T) behavior.
Modern superconductive materials for electrical machines and devices working on the principle of levitation
Authors: Prikhna T. A.
Published: June 2006

The peculiarities of high-pressure synthesis of highly dense nanostructural MgB2-based superconductive materials, of thermobaric treatment of MT–YBCO (melt-textured YBa2Cu3O7−δ-based superconductor), high-pressure sintering of YBa2Cu3O7−δ, oxygenation of MT–YBCO under high isostatic pressure of oxygen and processes of formation of superconductive junctions between MT–YBCO blocks are considered. The attained level of superconductive and mechanical properties of such materials and junctions make them promising for application in cryogenic devices working on the principle of levitation: electric motors, generators, pumps for liquid–gas transfer, magnetic bearings, flywheels, fault-current limiters, maglev transport, etc. High-pressure synthesized MgB2 (with Ti additions) blocks were for the first time tried in the superconductive electric motor at 20K and demonstrated an efficiency similar to MT–YBCO (at the same working temperature).
Legendre-transform functionals for spin-density-functional theory
Authors: Ayers Paul W. , Yang Weitao
Published: June 2006

We provide a rigorous proof that the Hohenberg-Kohn theorem holds for spin densities by extending Lieb’s Legendre-transform formulation to spin densities. The resulting spin-density-functional theory resolves several troublesome issues. Most importantly, the present paper provides an explicit construction for the spin potentials at any point along the adiabatic connection curve, thus providing a formal basis for the use of exchange-correlation functionals of the spin density in the Kohn-Sham density-functional theory (DFT). The practical implications of this result for unrestricted Kohn-Sham DFT calculations is considered, and the existence of holes below the Fermi level is discussed. We argue that an orbital’s energy tends to increase as its occupation number increases, which provides the basis for a computational algorithm for determining the occupation numbers in Kohn-Sham DFT and helps explain the origin of Hund’s rules and holes below the Fermi level.
Interaction of diatomic germanium with lithium atoms: Electronic structure and stability
Authors: Gopakumar G. , Lievens Peter , Nguyen Minh Tho
Published: June 2006

Quantum chemical calculations were applied to investigate the electronic structure of mono-, di-, and trilithiated digermanium (Ge2Lin) and their cations (n=0–3). Computations using a multiconfigurational quasidegenerate perturbation approach based on complete active space self-consistent-field wave functions, and density functional theory reveal that Ge2Li has a B12 ground state with a doublet-quartet energy gap of 33kcal∕mol. Ge2Li2 has a singlet ground state with a Au3-A11 gap of 29kcal∕mol, and Ge2Li3 a doublet ground state with a B24-A22 separation of 22kcal∕mol. The cation Ge2Li+ has a B13 ground state, being 13kcal∕mol below the open-shell B11 state. The computed electron affinities for diatomic germanium are EA(1)=1.9eV, EA(2)=−2.5eV, and EA(3)=−6.0eV, for Ge2−, Ge22−, and Ge23−, respectively, indicating that only the monoanion is stable with respect to electron detachment, in such a way that Ge2Li is composed by Ge2−∙Li+ ions. An “atoms-in-molecules” analysis shows the absence of a ring critical point in Ge2Li. An electron localization function analysis on Ge2Li supports the view that the Ge–Li bond is predominantly ionic; however, a small covalent character could be anticipated from the analysis of the Laplacian at the Ge–Li bond critical point. The ionic picture of the Ge–Li bond is further supported by a natural-bond-order analysis and the Laplacian of the electron density. The calculated Li affinity value for Ge2 is 2.08eV, while the Li+ cation affinity value for Ge2− is 5.7eV. The larger Li+ cation affinity value of Ge2− suggests a Ge2−Li+ interaction and thus supports the ionic nature of Ge–Li bond. In GeLi4 and Ge2Li, the presence of trisynaptic basins indicates a three-center bond connecting the germanium and lithium atoms.
Effect of additional hydrogen peroxide to H2O2⋯(H2O)n, n=1 and 2 complexes: Quantum chemical study
Authors: Kulkarni Anant D. , Pathak Rajeev K. , Bartolotti Libero J.
Published: June 2006

Hydrogen peroxide, H2O2, acts as a particularly strong reactant in aqueous environment. It has been demonstrated earlier that agglomerates with a single peroxide interacting with one and two water molecules manifest in several stable conformers within a narrow energy range. In the present study we seek structural changes brought out by adding an extra H2O2 to these systems at molecular level employing ab initio quantum chemical methods, viz., restricted Hartree-Fock and the second order Møller-Plesset perturbation theory. These clusters exhibit consistent trends in energy hierarchy at both the levels. Further, a many body interaction energy analysis quantifies the strength and cooperativity of hydrogen bonding in the (H2O2)2⋯(H2O)n, (n=1 and 2) clusters, bringing out structuring/destructuring effects attributed to attachment of water and hydrogen peroxide molecules.
Energy gradients with respect to atomic positions and cell parameters for the Kohn-Sham density-functional theory at the Γ point
Authors: Weber Valéry , Tymczak Christopher J. , Challacombe Matt
Published: June 2006

The application of theoretical methods based on density-functional theory is known to provide atomic and cell parameters in very good agreement with experimental values. Recently, construction of the exact Hartree-Fock exchange gradients with respect to atomic positions and cell parameters within the Γ-point approximation has been introduced [V. Weber et al., J. Chem. Phys. 124, 214105 (2006)]. In this article, the formalism is extended to the evaluation of analytical Γ-point density-functional atomic and cell gradients. The infinite Coulomb summation is solved with an effective periodic summation of multipole tensors [M. Challacombe et al., J. Chem. Phys. 107, 9708 (1997)]. While the evaluation of Coulomb and exchange-correlation gradients with respect to atomic positions are similar to those in the gas phase limit, the gradients with respect to cell parameters needs to be treated with some care. The derivative of the periodic multipole interaction tensor needs to be carefully handled in both direct and reciprocal space and the exchange-correlation energy derivative leads to a surface term that has its origin in derivatives of the integration limits that depend on the cell. As an illustration, the analytical gradients have been used in conjunction with the QUICCA algorithm [K. Németh and M. Challacombe, J. Chem. Phys. 121, 2877 (2004)] to optimize one-dimensional and three-dimensional periodic systems at the density-functional theory and hybrid Hartree-Fock/density-functional theory levels. We also report the full relaxation of forsterite supercells at the B3LYP level of theory.
Photoionization-induced dynamics of ammonia: Ab initio potential energy surfaces and time-dependent wave packet calculations for the ammonia cation
Authors: Viel Alexandra , Eisfeld Wolfgang , Neumann Stefanie , Domcke Wolfgang , Manthe Uwe
Published: June 2006

An analytical anharmonic six-dimensional three-sheeted potential energy surface for the ground and first excited states of the ammonia cation has been developed which is tailored to model the ultrafast photoinduced dynamics. Selected ab initio cuts, obtained by multireference configuration interaction calculations, have been used to determine the parameters of a diabatic representation for this Jahn-Teller and pseudo-Jahn-Teller system. The model includes higher-order coupling terms both for the Jahn-Teller and for the pseudo-Jahn-Teller matrix elements. The relaxation to the ground state is possible via dynamical pseudo-Jahn-Teller couplings involving the asymmetric bending and stretching coordinates. The photoelectron spectrum of NH3 and the internal conversion dynamics of NH3+ have been determined by wave packet propagation calculations employing the multiconfigurational time-dependent Hartree method. Three different time scales are found in the dynamics calculations for the second absorption band. The ultrafast Jahn-Teller dynamics of the two excited states occurs on a 5fs time scale. The major part of the internal conversion to the ground state takes place within a short time scale of 20fs. This fast internal conversion is, however, incomplete and the remaining excited state population does not decay completely even within 100fs.