bluuebunny/crossref_metadata_2025 huggingface dataset:
S bursts and the Jupiter ionospheric Alfvén resonator
Authors: Ergun R. E. , Su Y.‐J. , Andersson L. , Bagenal F. , Delemere P. A. , Lysak R. L. , Strangeway R. J.
Published: June 2006

This article explores a possible relationship between S bursts and trapped Alfvén waves in Jupiter's upper ionosphere. Eigenmodes of inertial Alfvén waves in Jupiter's ionosphere are predicted to have frequencies (∼20 Hz) that match the repetition frequency of S bursts and the two phenomena are colocated, suggesting such an association is possible. Electron acceleration or modulation may provide the physical mechanism that transfers energy from the Alfvén wave to the S burst. Inertial Alfvén waves are known to accelerate electrons with fluxes that are modulated at Alfvén wave eigenmode frequencies. The modulated electron fluxes, in turn, may generate or modulate the generation of the S burst emissions. The exact growth mechanism has not been identified, but we put forth and discuss two possibilities, an anti‐Jovian electron beam or a ring distribution created from impulsive acceleration and mirroring. Since the Alfvén wave eigenmode phenomena and electron acceleration are seen on Earth, we rely heavily on analogy with Earth‐based observations.
Determination of the quiet time current sheet thickness using Geotail CPI data and nonlinear dynamics modeling
Authors: Holland D. L. , Richards B. H. , Ronquist I. H. M. , Ansher J. A. , Paterson W. R. , Frank L. A.
Published: June 2006

In this paper we present a survey of the quiet (Kp ≤ 1+) current sheet thickness for XGSE between −20 RE and −80 RE as determined from an ion distribution function signature of nonlinear particle dynamics in current sheet‐like magnetic fields. The signature manifests itself as a series of peaks in the ion distribution function whose separation depend on the fourth root of the energy and parameters that describe the current sheet structure. We have found clear evidence of the distribution function signature throughout the entire region of interest. Analysis of the data shows that the current sheet thickness is remarkably uniform in the region under study with an average thickness of 0.76 ± 0.56 RE. This result is consistent with measurements of the quiet time current sheet thickness made using other techniques.
Detecting slope deformation using two‐pass differential interferometry: Implications for landslide studies on Earth and other planetary bodies
Authors: Bulmer M. H. , Petley D. N. , Murphy W. , Mantovani F.
Published: June 2006

Landslide features have been identified on Earth and the Moon, Mars, Venus, as well on the Jovian moons. By focusing on a terrestrial landslide complex we test the operational parameters of RADARSAT‐1 and the use of two‐pass differential interferometry to detect change, to map its extent, and to measure the amount of movement over a given time period. RADARSAT‐1 was chosen because of its variable imaging modes and geometry. For investigations of landslide motions using remote sensing techniques, repeat‐pass data are required. Synthetic aperture radar (SAR) interferometry (InSAR) can ideally monitor movements across the whole surface of a landslide to a millimeteric precision, yielding a coverage significantly better than that obtained by ground instrumentation. Obtaining optimal data for InSAR analysis requires controlled orbital characteristics and imaging geometries, an understanding of the landslide characteristics and behavior, a cooperative surface, and mitigation of the factors that can affect phase. Using two‐pass differential interferometry, a slope deformation map has been generated from RADARSAT‐1 data for part of the Black Ven landslide (2°52′W, 50°40′N), on the south coast of England. Four months separate the InSAR pair during which time 0.03 m of subsidence was measured. From this a movement rate of 0.09 m/yr can be calculated. This agrees well with ground observations and an in situ record of movement, thus demonstrating that the technique can be used to investigate landslides. With further refinement it can provide more direct measurements of landslide deformation on Earth and other planetary bodies than are currently available.
A multiple wetting front gravitational infiltration and redistribution model for water balance applications
Authors: Struthers Iain , Hinz Christoph , Sivapalan Murugesu
Published: June 2006

A unit gradient multiple wetting front model is described, in which the infiltration and redistribution behavior of individual “square wave” fronts is simulated for unponded rainfall infiltration. Drainage recession behavior of this model is demonstrated to be equivalent to that of a capacitance (bucket) model for certain scenarios, suggesting a possible physical basis for capacitance model parameters with respect to drainage prediction. The ability of the wetting front model to account for the variable time lag between surface infiltration and drainage generation at depth, which cannot be achieved using a capacitance model, is demonstrated to improve predictions of the 25‐year average drainage regime of a free‐draining lysimeter in Colbitz, Germany, when compared to capacitance model predictions. Improvement in predictions compared to the capacitance model is most apparent in deeper and drier soils, where the infiltration‐drainage time lag is significant. Comparison with a numerical Richards equation solution suggests that drainage behavior predicted by the model is reasonable for the conditions considered, despite neglecting capillary impacts. The ability of the model to differentiate between drainage response time and the age of draining water gives the approach general applicability for problems requiring simulation of the age of water involved in hydrological response to storms.
Cluster observations of energetic ionospheric ion beams in the auroral region: Acceleration and associated energy‐dispersed precipitation
Authors: Bosqued J. M. , Ashour‐Abdalla M. , Marchaudon A. , Laakso H. , Umeda T. , El Alaoui M. , Peroomian V. , Rème H. , Paschmann G. , Dunlop M. , Fazakerley A.
Published: June 2006

This paper presents a detailed study of the Feb. 14, 2001 Cluster northern auroral pass at mid‐altitudes (4–5 RE), characterized by observations of a series of energy‐dispersed ion structures in a region of poleward convection. In contradiction with one current view, that ions populating these energy‐dispersed signatures originate sporadically in the magnetotail, Cluster directly observed energetic (0.2–15 keV), field‐aligned H+ ions of ionospheric origin. The ions were ejected at the top of a steady auroral acceleration region near 72.5° ILAT, then bounced on closed field‐lines, and were finally dispersed poleward in latitude by the E × B drift effect. Simple but realistic latitudinal drift computations demonstrate that the anticipated location of successive bouncing echoes coincides rather well with the Cluster observations. Best agreement is reached when the particles are further accelerated (presumably nonadiabatically) by 1–2 keV, as they periodically cross the tail neutral sheet.
Current subsidence rates due to compaction of Holocene sediments in southern Louisiana
Authors: Meckel T. A. , ten Brink U. S. , Williams S. Jeffress
Published: June 2006

Relative contributions of geologic and anthropogenic processes to subsidence of southern Louisiana are vigorously debated. Of these, shallow sediment compaction is often considered dominant, although this has never been directly observed or effectively demonstrated. Quantitative understanding of subsidence is important for predicting relative sea level rise, storm surge flooding due to hurricanes, and for successful wetland restoration. Despite many shallow borings, few appropriate stratigraphic and geotechnical data are available for site‐specific calculations. We overcome this by determining present compaction rates from Monte Carlo simulations of the incremental sedimentation and compaction of stratigraphies typical of the Holocene of southern Louisiana. This approach generates distributions of present compaction rates that are not expected to exceed 5 mm/yr, but may locally. Locations with present subsidence rates greater than the predicted maximum probable shallow compaction rates are likely influenced by additional processes.
INSM1 functions as a transcriptional repressor of the neuroD/β2 gene through the recruitment of cyclin D1 and histone deacetylases
Authors: Liu Wei-Dong , Wang Hong-Wei , Muguira Michelle , Breslin Mary B. , Lan Michael S.
Published: June 2006

INSM1/IA-1 (insulinoma-associated 1) is a developmentally regulated zinc-finger transcription factor, exclusively expressed in the foetal pancreas and nervous system, and in tumours of neuroendocrine origin. We have identified an INSM1 binding site in the neuroD/β2 promoter and demonstrated transcriptional repressor activity of INSM1 by transient transfection assay. A chromatin immunoprecipitation assay confirmed that in vivo INSM1 is situated on the promoter region of the neuroD/β2 gene. In an attempt to elucidate the molecular mechanism of transcriptional repression by the INSM1 gene, cyclin D1 was identified as an interacting protein by using a 45-day-old human foetal brain cDNA library and a yeast two-hybrid screen. The physical association between INSM1 and cyclin D1 was confirmed by in vitro and in vivo pull-down assay. Cyclin D1 co-operates with INSM1 and suppresses neuroD/β2 promoter activity. Co-immunoprecipitation of INSM1, cyclin D1 and HDACs (histone deacetylases) in mammalian cells revealed that INSM1 interacts with HDAC-1 and -3 and that this interaction is mediated through cyclin D1. Overexpression of cyclin D1 and HDAC-3 significantly enhanced the transcriptional repression activity of INSM1 on the neuroD/β2 promoter. A further chromatin immunoprecipitation assay confirmed that HDAC-3 occupies this same region of the neuroD/β2 promoter, by forming a transcription complex with INSM1. Thus we conclude that INSM1 recruits cyclin D1 and HDACs, which confer transcriptional repressor activity.
Identification and characterization of Thermoplasma acidophilum glyceraldehyde dehydrogenase: a new class of NADP+-specific aldehyde dehydrogenase
Authors: Jung Jin Hwa , Lee Sun Bok
Published: June 2006

Thermoacidophilic archaea such as Thermoplasma acidophilum and Sulfolobus solfataricus are known to metabolize D-glucose via the nED (non-phosphorylated Entner–Doudoroff) pathway. In the present study, we identified and characterized a glyceraldehyde dehydrogenase involved in the downstream portion of the nED pathway. This glyceraldehyde dehydrogenase was purified from T. acidophilum cell extracts by sequential chromatography on DEAE-Sepharose, Q-Sepharose, Phenyl-Sepharose and Affi-Gel Blue columns. SDS/PAGE of the purified enzyme showed a molecular mass of approx. 53 kDa, whereas the molecular mass of the native protein was 215 kDa, indicating that glyceraldehyde dehydrogenase is a tetrameric protein. By MALDI–TOF-MS (matrix-assisted laser-desorption ionization–time-of-flight MS) peptide fingerprinting of the purified protein, it was found that the gene product of Ta0809 in the T. acidophilum genome database corresponds to the purified glyceraldehyde dehydrogenase. The native enzyme showed the highest activity towards glyceraldehyde, but no activity towards aliphatic or aromatic aldehydes, and no activity when NAD+ was substituted for NADP+. Analysis of the amino acid sequence and enzyme inhibition studies indicated that this glyceraldehyde dehydrogenase belongs to the ALDH (aldehyde dehydrogenase) superfamily. BLAST searches showed that homologues of the Ta0809 protein are not present in the Sulfolobus genome. Possible differences between T. acidophilum (Euryarchaeota) and S. solfataricus (Crenarchaeaota) in terms of the glycolytic pathway are thus expected.
Signalling cross-talk between hepatocyte nuclear factor 4α and growth-hormone-activated STAT5b
Authors: Park Soo-Hee , Wiwi Christopher A. , Waxman David J.
Published: June 2006

In the present study, we have characterized signalling cross-talk between STAT5b (signal transducer and activator of transcription 5b) and HNF4α (hepatocyte nuclear factor 4α), two major regulators of sex-dependent gene expression in the liver. In a HepG2 liver cell model, HNF4α strongly inhibited β-casein and ntcp (Na+/taurocholate cotransporting polypeptide) promoter activity stimulated by GH (growth hormone)-activated STAT5b, but had no effect on interferon-γ-stimulated STAT1 transcriptional activity. By contrast, STAT5b synergistically enhanced the transcriptional activity of HNF4α towards the ApoCIII (apolipoprotein CIII) promoter. The inhibitory effect of HNF4α on STAT5b transcription was associated with the inhibition of GH-stimulated STAT5b tyrosine phosphorylation and nuclear translocation. The short-chain fatty acid, butyrate, reversed STAT5b transcriptional inhibition by HNF4α, but did not reverse the inhibition of STAT5b tyrosine phosphorylation. HNF4α inhibition of STAT5b tyrosine phosphorylation was not reversed by pervanadate or by dominant-negative phosphotyrosine phosphatase 1B, suggesting that it does not result from an increase in STAT5b dephosphorylation. Rather, HNF4α blocked GH-stimulated tyrosine phosphorylation of JAK2 (Janus kinase 2), a STAT5b tyrosine kinase. Thus STAT5b and HNF4α exhibit bi-directional cross-talk that may augment HNF4α-dependent gene transcription while inhibiting STAT5b transcriptional activity via the inhibitory effects of HNF4α on JAK2 phosphorylation, which leads to inhibition of STAT5b signalling initiated by the GH receptor at the cell surface.
Nitric oxide modulation of ophthalmic artery blood flow velocity waveform morphology in healthy volunteers
Authors: Lockhart Christopher J. , Gamble Andrew J. , Rea Derrick , Hughes Sinead , McGivern R. Canice , Wolsley Clive , Stevenson Michael , Harbinson Mark T. , Plumb Richard D. , McVeigh Gary E.
Published: June 2006

Quantitative analysis of the arterial pressure pulse waveform recorded by applanation tonometry of the radial artery can track NO (nitric oxide)-mediated modulation of arterial smooth muscle tone. The changes in pressure pulse waveform morphology result from pulse wave reflection arising predominantly from smaller arteries and arterioles. Employing Doppler ultrasound to record the spectral flow velocity waveform in the ophthalmic artery, we studied the effects of NO modulation on waveforms recorded in the proximity of the terminal ocular microcirculatory bed. In healthy young men (n=10; age 18–26 years), recordings were made at baseline, following 300 μg of sublingual GTN (glyceryl trinitrate) and during the intravenous infusion of 0.25 and 0.5 mg/kg of L-NAME (NG-nitro-L-arginine methyl ester). Peaks (P1, P2 and P3) and nodes (N1, N2 and N3) on the arterial flow velocity waveform were identified during the cardiac cycle and employed to quantify waveshape change in response to the haemodynamic actions of the pharmacological interventions. The administration of GTN resulted in a significant (P<0.05) increase in heart rate without significant alteration in blood pressure. At the doses employed, L-NAME did not significantly alter systemic haemodynamics. With the exception of peak Doppler systolic velocity, all other peaks and nodes decreased significantly in response to GTN (P<0.05 for all points compared with baseline). In response to the administration of L-NAME, all peaks and nodes decreased significantly (P<0.05 for all points compared with baseline). The resistive index, a ratio calculated from the peak and trough flow velocities employed to assess change in flow resistance, increased significantly in response to GTN (0.77 at baseline compared with 0.85; P<0.05). Quantification of changes in the flow velocity spectral waveform during the cardiac cycle sensitively identified NO modulation of smooth muscle tone prior to alteration in systemic haemodynamics. Focusing on the resistive index, which identifies isolated points on the waveform describing the excursions of flow, may provide misleading information in relation to the haemodynamic effects of drug interventions.
The expanding role of NADPH oxidases in health and disease: no longer just agents of death and destruction
Authors: Quinn Mark T. , Ammons Mary Cloud B. , DeLeo Frank R.
Published: June 2006

The NADPH oxidase was originally identified as a key component of human innate host defence. In phagocytes, this enzyme complex is activated to produce superoxide anion and other secondarily derived ROS (reactive oxygen species), which promote killing of invading micro-organisms. However, it is now well-established that NADPH oxidase and related enzymes also participate in important cellular processes not directly related to host defence, including signal transduction, cell proliferation and apoptosis. These enzymes are present in essentially every organ system in the body and contribute to a multitude of physiological events. Although essential for human health, excess NADPH-oxidase-generated ROS can promote numerous pathological conditions. Herein, we summarize our current understanding of NADPH oxidases and provide an overview of how they contribute to specific human diseases.
Are structures with Al–H bonds represented in the photoelectron spectrum of Al3O4H2−?
Authors: Guevara-García Alfredo , Martínez Ana , Ortiz J. V.
Published: June 2006

Photoelectron spectra of Al3O4H2− clusters formed by reactions of Al3O3− with water molecules have been interpreted recently in terms of dissociative absorption products with hydroxide and oxide anions that are coordinated to aluminum cations. Alternative isomers with Al–H bonds have lower energies, but barriers to hydrogen migrations that break O–H bonds and create Al–H bonds are high. Ab initio electron propagator calculations of the vertical electron detachment energies of the anions indicate that the species with hydrides cannot be assigned to the chief features in the photoelectron spectrum. Therefore, the previously studied dissociative absorption products are the structures that are most likely to be probed in the photoelectron spectra.
The effect of broken conjugation on the excited state: Ether linkage in the cyano-substituted poly(p-phenylene vinylene) conjugated polymer poly(2,5,2′,5′-tetrahexyloxy-8,7′-dicyano-di-p-phenylene vinylene)
Authors: Chasteen S. V. , Carter S. A. , Rumbles G.
Published: June 2006

We investigate the effect of broken conjugation on the excited state dynamics of excimers in cyano-substituted phenylene-vinylene polymers. We compare previous studies on the well-characterized poly(2,5,2′,5′-tetrahexyloxy-8,7′-dicyano-di-p-phenylene vinylene) (CN-PPV) with poly[oxa-1,4-phenylene-1,2-(1-cyano)-ethenylene-2,5-dioctyloxy-1,4-phenylene-1,2-(2-cyano)-ethenylene-1,4-phenylene] (CN-ether-PPV), in which the conjugation is disrupted by the insertion of an oxygen atom within the polymer backbone. Despite the broken conjugation, the spectroscopic behavior of the two materials is similar, indicating that the cyano group dominates the photophysics in these materials. The emission in CN-ether-PPV is due to a single-chain exciton in solution and due to an interchain excimer in thin film, as previously reported for CN-PPV; however, the excimer absorption and emission in thin film are blueshifted by ∼0.2eV relative to CN-PPV, implying that the excimer in CN-ether-PPV is less stable. Furthermore, substitution of an ether group along the chain results in decay times in both solution and film that are twice as long than in CN-PPV due to the broken conjugation which restricts the exciton within a conjugation segment and reduces its access to internal quenching sites. These properties result in a decay time of 14ns for CN-ether-PPV film, one of the longest decay times observed in a conjugated polymer film. The long lifetime indicates a large exciton diffusion length, making these species particularly vulnerable to quenching by other materials. This work has implications for the design of conjugated polymers for efficient optoelectronic devices, such as photovoltaics.
Scale invariance in the spectral action
Authors: Chamseddine Ali H. , Connes Alain
Published: June 2006

The arbitrary mass scale in the spectral action for the Dirac operator is made dynamical by introducing a dilaton field. We evaluate all the low-energy terms in the spectral action and determine the dilaton couplings. These results are applied to the spectral action of the noncommutative space defined by the standard model. We show that the effective action for all matter couplings is scale invariant, except for the dilaton kinetic term and Einstein-Hilbert term. The resulting action is almost identical to the one proposed for making the standard model scale invariant as well as the model for extended inflation and has the same low-energy limit as the Randall-Sundrum model. Remarkably, all desirable features with correct signs for the relevant terms are obtained uniquely and without any fine tuning.
Charge transfer interaction in the effective fragment potential method
Authors: Li Hui , Gordon Mark S. , Jensen Jan H.
Published: June 2006

An approximate formula is derived and implemented in the general effective fragment potential (EFP2) method to model the intermolecular charge transfer interaction. This formula is based on second order intermolecular perturbation theory and utilizes canonical molecular orbitals and Fock matrices obtained with preparative self-consistent field calculations. It predicts charge transfer energies that are in reasonable agreement with the reduced variational space energy decomposition analysis. The formulas for the charge transfer gradients with respect to EFP translational and rotational displacements are also derived and implemented.
Dynamic properties of inhomogeneous states in cuprates (Review Article)
Authors: Lorenzana J. , Seibold G.
Published: June 2006

We review the static and dynamical properties of stripe textures with regard to their relevance in high-Tc superconductors. Our investigations are based on the time-dependent Gutzwiller approximation which allows for the computation of random-phase approximation-like (RPA) fluctuations on top of the Gutzwiller approximation (GA). No restrictions are imposed on the charge and spin configurations, which makes the method suitable for the calculation of linear excitations around symmetry-broken solutions. Within this approach we calculate the optical conductivity of stripes and compare our results with reflectivity experiments on cuprates. Further on, we use the spin-rotational invariant extension of the GA+RPA method in order to investigate the characteristics of magnetic excitations in the stripe phase of cuprates. Our results are in very good agreement with neutron scattering and optical experiments on lanthanum cuprate superconductors.
Nonadiabatic breakdown and pairing in high-Tc compounds
Authors: Pietronero L. , Cappelluti E.
Published: June 2006

The electron-phonon interaction plays a fundamental role in the superconducting and normal-state properties of all the high-Tc materials, from cuprates to fullerenes. Another common element of these compounds is in addition the extremely small Fermi energy EF, which is comparable with the range ωph of the phonon frequencies. In such a situation the adiabatic principle ωph∕EF⪡1, on which the standard theory of the electron-phonon interaction and of the superconductivity relies, breaks down. In this contribution we discuss the physical consequences of the breakdown of the adiabatic assumption, with special interest on the superconducting properties. We review the microscopic derivation of the nonadiabatic theory of the electron-phonon coupling which explicitly takes into account higher-order electron-phonon scattering not included in the conventional picture. Within this context we discuss also the role of the repulsive electron-electron correlation and the specific phenomenology of cuprates and fullerides.
Gauge theory of pairing and spin fluctuations near the quantum critical point and superhigh-temperature superconductivity
Authors: Schrieffer J. R.
Published: June 2006

We develop a new theory of pairing and magnetic effect near the quantum critical point. Several novel properties are predicted: based on a spin fermion model, we derive two new interactions: i) a spin deformational potential Hsdp proportional to the bandwidth W (as opposed to the considerably smaller exchange coupling J of the nearly antiferromagnetic Fermi-liquid theory), and ii) a diamagnetic potential Hdia, quadratic in a gauge potential A. A dramatic increase of Tc is predicted for 0.01W⩽J⩽10W. This should have immense technological impact in electric energy production, storage, and transmission, as well as for medical electronics, microwave electronics, computer memory and information storage, separations technology, and maglev, among others. The striking prediction to be confirmed by experiment is that the pairing order parameter Δ(k) is predicted to be p-wave, i.e., l=1, S=1, as compared to l=2 and S=1 for conventional HTS materials. In addition, a novel collective model is predicted whose frequency ωL is in the optical range and is determined by Hsdp.
Theory of antiferromagnetic pairing in cuprate superconductors (Review article)
Authors: Plakida N. M.
Published: June 2006

A review of the antiferromagnetic exchange and spin-fluctuation pairing theory in the cuprate superconductors is given. We briefly discuss a phenomenological approach and a theory in the limit of weak Coulomb correlations. A microscopic theory in the strong correlation limit is presented in more detail. In particular, results of our recently developed theory for the effective p-d Hubbard model and the reduced t-J model are given. We have proved that retardation effects for the antiferromagnetic exchange interaction are unimportant, which results in pairing of all the charge carriers in the conduction band and a high Tc proportional to the Fermi energy. The spin-fluctuation interaction caused by kinematic interaction gives an additional contribution to the d-wave pairing. The dependence of Tc on the hole concentration and lattice constant (or pressure) and an oxygen isotope shift are discussed.
Spin dynamics in cuprate perovskites
Authors: Sherman A. , Schreiber M.
Published: June 2006

Results obtained with the use of the t-J model of Cu–O planes and Mori’s projection operator formalism are compared with data of neutron scattering experiments in lanthanum and yttrium cuprates. This comparison allows us to interpret the intensive peak at the antiferromagnetic wave vector observed in yttrium cuprates as a manifestation of excitations of localized Cu spins. The high-frequency incommensurability detected both in lanthanum and yttrium cuprates is connected with the dispersion of these excitations, while the low-frequency incommensurability arises due to a dip in the spin-excitation damping at the antiferromagnetic wave vector. For moderate doping the dip stems from the weakness of the interaction between the spin excitations and holes near hot spots. It is conjectured that the dissimilarity of the susceptibility frequency dependencies in yttrium and lanthanum cuprates may be connected with different values of the hole bandwidth and damping in these crystals.
Superconductivity and antiferromagnetism in quasi-one-dimensional organic conductors (Review Article)
Authors: Dupuis N. , Bourbonnais C. , Nickel J. C.
Published: June 2006

We review the current understanding of superconductivity in the quasi-one-dimensional organic conductors of the Bechgaard and Fabre salt families. We discuss the interplay between superconductivity, antiferromagnetism, and charge-density-wave fluctuations. The connection to recent experimental observations supporting unconventional pairing and the possibility of a triplet spin order parameter for the superconducting phase is also presented.
Pseudogaps: introducing the length scale into dynamical mean-field theory
Authors: Kuchinskii E. Z. , Nekrasov I. A. , Sadovskii M. V.
Published: June 2006

Pseudogap physics in strongly correlated systems is essentially scale dependent. We generalize the dynamical mean-field theory (DMFT) by introducing into the DMFT equations dependence on the correlation length of pseudogap fluctuations via an additional (momentum-dependent) self-energy Σk. This self-energy describes nonlocal dynamical correlations induced by short-ranged collective SDW-like antiferromagnetic spin (or CDW-like charge) fluctuations. At high enough temperatures these fluctuations can be viewed as a quenched Gaussian random field with finite correlation length. This generalized DMFT +Σk approach is used for the numerical solution of the weakly doped one-band Hubbard model with repulsive Coulomb interaction on a square lattice with nearest- and next-nearest-neighbor hopping. The effective single impurity problem is solved by the numerical renormalization group (NRG). Both types of strongly correlated metals, namely (i) the doped Mott insulator and (ii) the case of bandwidth W≲U (U is the value of local Coulomb interaction) are considered. Densities of states, spectral functions and ARPES spectra calculated within DMFT +Σk show a pseudogap formation near the Fermi level of the quasiparticle band. We also briefly discuss effects of random impurity scattering. Finally we demonstrate a qualitative picture of Fermi surface “destruction” due to pseudogap fluctuations and formation of “Fermi arcs” which agrees well with ARPES observations.
The role of the Coulomb interaction in the formation of superconducting and pseudogap states in cuprate metal-oxides
Authors: Pashitskii E. A. , Pentegov V. I.
Published: June 2006

It is shown that the key role in the mechanism of high-Tc superconductivity in the layered cuprate metal-oxides with anisotropic quasi-two-dimensional electronic spectrum and d-wave symmetry of the superconducting order parameter is played by the retarded screened Coulomb interaction and many-body correlations. We argue that the pseudogap observed in these materials is the anisotropic dielectric gap, which appears due to the electron-hole pairing on the flat portions of the Fermi surface in the vicinity of the extended saddle points in the quasiparticle spectrum. This gap coexists with the superconducting gap and is partially suppressed by scattering of current carriers on the charged point defects. The suppression of dielectric gap is analogous to the suppression of superconducting gap by magnetic impurities in “gapless” superconductors. The complete destruction of the pseudogap by charged impurities is assumed to increase Tc significantly.
Electron structure and electron–phonon interaction in the strongly correlated electron system of cuprates
Authors: Ovchinnikov S. G. , Gavrichkov V. A. , Korshunov M. M. , Shneyder E. I.
Published: June 2006

The generalized tight-binding method presents a practical realization of the scheme that describes quasiparticles in a strongly correlated electron system and consists of exact intra-cell diagonalization of the model Hamiltonian and perturbative treatment of the inter-cell hoppings. In the present paper this method and its ab initio modification are applied to undoped and weakly doped HTSC cuprates. Results are in very good agreement with the experimental ARPES data on various compounds. Starting with a multiband p–d model the realistic effective low-energy Hamiltonian of strongly correlated electrons interacting with spin fluctuations and phonons is derived both for hole- and electron-doped systems. Without electron–phonon interaction the pure magnetic mechanism of pairing does not provide the correct value of Tc even for single-layer La2−xSrxCuO4 and Nd2−xCexCuO4.
Comparison of three effective Hamiltonian models of increasing complexity: Triazene in water as a test case
Authors: Fdez. Galván I. , Martín M. E. , Aguilar M. A. , Ruiz-López M. F.
Published: June 2006

A critical comparison of widely used solvation models is reported. It is illustrated by a study of the triazene molecule in liquid water. We consider the following approaches: (1) a continuum model based on multicentric multipole expansions of the charge distribution, (2) the averaged solvent electrostatic potential from molecular dynamics (ASEP/MD) method, and (3) molecular dynamics simulations using a combined quantum mechanics/molecular mechanics potential (QM/MM/MD). We find that the solvation induces appreciable changes in the geometry and charge distribution of triazene. These changes are only qualitatively reproduced by the dielectric continuum model, which clearly underestimates induced dipole moments and solute-solvent interaction energy. We also show that the use of effective point charges placed on solute nuclei during the classical simulations may cause significant errors in the description of the solvent structure. The addition of charges representing nitrogen atom lone pairs is compulsory to reproduce the QM/MM/MD simulation results. Moreover, our results validate the use of the mean field approximation in the study of solvent effects. A major conclusion of this study is that the ASEP/MD method constitutes a reliable alternative to the much more computationally demanding QM/MM/MD methods.
The discrete representation correspondence between quantum and classical spatial distributions of angular momentum vectors
Authors: Anderson Roger W. , Aquilanti Vincenzo
Published: June 2006

This work demonstrates that the quantum mechanical moments of a state described by the density matrix correspond to discrete spherical harmonic moments of the classical multipole expansion of the spatial distribution of the angular momentum vectors. For the diagonal density matrix elements, this work exploits the fact that the quantum mechanical vector coupling (Clebsch-Gordan) coefficients become increasingly accurate discrete representations of spherical harmonics as j increases. A Schwinger-type basis accounts for nonaxially symmetric angular distributions, which result in nonzero off-diagonal elements of the density matrix. The resulting discrete minimum uncertainty picture of the classical moments has a stringent equivalence with the quantum mechanical one for all j and provides an unambiguous connection for the classical and quantum moments in the large j limit. The equivalence is numerically tested for simple models, and there is a satisfying equivalence even for small j. Applications, implications, and extensions are indicated, and the relevance of this work for the interpretation of classical mechanical simulations of inelastic and reactive molecular collisions will be documented elsewhere.
Glassy correlations and microstructures in randomly cross-linked homopolymer blends
Authors: Wald Christian , Goldbart Paul M. , Zippelius Annette
Published: June 2006

We consider a microscopic model of a polymer blend that is prone to phase separation. Permanent cross-links are introduced between randomly chosen pairs of monomers, drawn from the Deam-Edwards distribution. Thereby, not only density but also concentration fluctuations of the melt are quenched-in in the gel state, which emerge upon sufficient cross-linking. We derive a Landau expansion in terms of the order parameters for gelation and phase separation, and analyze it on the mean-field level, including Gaussian fluctuations. The mixed gel is characterized by thermal as well as time-persistent (glassy) concentration fluctuations. Whereas the former are independent of the preparation state, the latter reflect the concentration fluctuations at the instant of cross-linking, provided the mesh size is smaller than the correlation length of phase separation. The mixed gel becomes unstable to microphase separation upon lowering the temperature in the gel phase. Whereas the length scale of microphase separation is given by the mesh size, at least close to the transition, the emergent microstructure depends on the composition and compressibility of the melt. Hexagonal structures, as well as lamellas or random structures with a unique wavelength, can be energetically favorable.
The OH radical-H2O molecular interaction potential
Authors: Du Shiyu , Francisco Joseph S. , Schenter Gregory K. , Iordanov Tzvetelin D. , Garrett Bruce C. , Dupuis Michel , Li Jun
Published: June 2006

The OH radical is one of the most important oxidants in the atmosphere due to its high reactivity. The study of hydrogen-bonded complexes of OH with the water molecules is a topic of significant current interest. In this work, we present the development of a new analytical functional form for the interaction potential between the rigid OH radical and H2O molecules. To do this we fit a selected functional form to a set of high level ab initio data. Since there is a low-lying excited state for the H2O∙OH complex, the impact of the excited state on the chemical behavior of the OH radical can be very important. We perform a potential energy surface scan using the CCSD(T)/aug-cc-pVTZ level of electronic structure theory for both excited and ground states. To model the physics of the unpaired electron in the OH radical, we develop a tensor polarizability generalization of the Thole-type all-atom polarizable rigid potential for the OH radical, which effectively describes the interaction of OH with H2O for both ground and excited states. The stationary points of (H2O)n∙OH clusters were identified as a benchmark of the potential.
Aspects of quantum coherence in the optical Bloch equations
Authors: Sanz A. S. , Han H. , Brumer P.
Published: June 2006

Aspects of coherence and decoherence are analyzed within the optical Bloch equations. By rewriting the analytic solution in an alternate form, we are able to emphasize a number of unusual features: (a) despite the Markovian nature of the bath, coherence at long times can be retained; (b) the long-time asymptotic degree of coherence in the system is intertwined with the asymptotic difference in level populations; (c) the traditional population-relaxation and decoherence times, T1 and T2, lose their meaning when the system is in the presence of an external field, and are replaced by more general overall time scales; (d) increasing the field strength, quantified by the Rabi frequency Ω, increases the rate of decoherence rather than reducing it, as one might expect; and (e) maximum asymptotic coherence is reached when the system parameters satisfy Ω2=1∕(T1T2).
Multidimensional reactive scattering with quantum trajectories: Dynamics with 50–200 vibrational modes
Authors: Babyuk Dmytro , Wyatt Robert E.
Published: June 2006

The dynamics of ensembles containing thousands of quantum trajectories are studied for multidimensional systems undergoing reactive scattering. The Hamiltonian and equations of motion are formulated in curvilinear reaction path coordinates, for the case of a planar (zero-torsion) reaction path. In order to enhance the computational efficiency, an improved least squares fitting procedure is introduced. This scheme involves contracted basis sets and the use of inner and outer stencils around points where fitting is performed. This method is applied to reactive systems with 50–200 harmonic vibrational modes which are coupled to motion along the reaction coordinate. Dynamical results, including trajectory evolution and time-dependent reaction probabilities, are presented and power law scaling of computation time with the number of vibrational modes is described.
Dynamics and fragmentation of van der Waals clusters: (H2O)n, (CH3OH)n, and (NH3)n upon ionization by a 26.5eV soft x-ray laser
Authors: Dong F. , Heinbuch S. , Rocca J. J. , Bernstein E. R.
Published: June 2006

A tabletop soft x-ray laser is applied for the first time as a high energy photon source for chemical dynamics experiments in the study of water, methanol, and ammonia clusters through time of flight mass spectroscopy. The 26.5eV/photon laser (pulse time duration of ∼1ns) is employed as a single photon ionization source for the detection of these clusters. Only a small fraction of the photon energy is deposited in the cluster for metastable dissociation of cluster ions, and most of it is removed by the ejected electron. Protonated water, methanol, and ammonia clusters dominate the cluster mass spectra. Unprotonated ammonia clusters are observed in the protonated cluster ion size range 2⩽n⩽22. The unimolecular dissociation rate constants for reactions involving loss of one neutral molecule are calculated to be (0.6–2.7)×104, (3.6–6.0)×103, and (0.8–2.0)×104s−1 for the protonated water (9⩽n⩽24), methanol (5⩽n⩽10), and ammonia (5⩽n⩽18) clusters, respectively. The temperatures of the neutral clusters are estimated to be between 40 and 200K for water clusters (10⩽n⩽21), and 50–100K for methanol clusters (6⩽n⩽10). Products with losses of up to five H atoms are observed in the mass spectrum of the neutral ammonia dimer. Large ammonia clusters (NH3)n (n>3) do not lose more than three H atoms in the photoionization/photodissociation process. For all three cluster systems studied, single photon ionization with a 26.5eV photon yields near threshold ionization. The temperature of these three cluster systems increases with increasing cluster size over the above-indicated ranges.
Aging in a free-energy landscape model for glassy relaxation. II. Fluctuation-dissipation relations
Authors: Diezemann Gregor , Böhmer Roland
Published: June 2006

Several fluctuation-dissipation relations are investigated for a simple free-energy landscape model designed to describe the primary relaxation in supercooled liquids. The calculations of the response and of the correlation functions are performed for a quench from a high temperature to a low temperature. In the model, all dynamical quantities reach equilibrium after long times, but for times shorter than the re-equilibration time they do not exhibit time-translational invariance and the fluctuation-dissipation theorem is violated. Two measures for these violations are considered. One such measure is given by the slope in a plot of the integrated response versus the correlation function and another one by the so-called fluctuation-dissipation ratio. It is found that these measures do not coincide and furthermore are not independent of the dynamical variable considered in the calculation. We propose to determine the fluctuation-dissipation ratio experimentally via measurements of the deuteron spin-lattice relaxation rate and the dielectric loss.
Evaluation of pressure tensor in constant-volume simulations of hard and soft convex bodies
Authors: Allen Michael P.
Published: June 2006

A method for calculating the pressure tensor in constant-volume Monte Carlo simulations of convex bodies is presented. In contrast to other approaches, the method requires only an isotropic scaling of the simulation box and the counting of simple geometric quantities characterizing overlapping pairs. Nonsphericity presents no special difficulties. The result is expressed as a sum of pairwise contributions and can therefore be used to compute pressure tensor profiles in a conventional way.
Comment on “Comment on ‘Simple reversible molecular dynamics algorithms for Nosé-Hoover chain dynamics’ ” [J. Chem. Phys. 110, 3623 (1999)]
Authors: Yamamoto Takeshi
Published: June 2006

It is shown that some of the reversible integrators for Nosé-Hoover chain dynamics developed by Jang and Voth [J. Chem. Phys.107, 9514 (1997)] can be derived from a factorization of the classical propagator and that they are measure preserving as opposed to the Comment by Tuckerman and Martyna [J. Chem. Phys.110, 3623 (1999)]. In light of this we will reexamine the different numerical behavior of the Jang-Voth and Martyna-Tuckerman integrators.
Reliable molecular simulations of solute-solvent systems with a minimum number of solvent shells
Authors: Brancato Giuseppe , Rega Nadia , Barone Vincenzo
Published: June 2006

In this work, the mean field (MF) method, a continuum-based model designed for treating complex molecular systems, such as liquids and solutions, recently presented by Brancato et al. [J. Chem. Phys. 122, 154109 (2005)], has been further developed and improved especially in the treatment of the electrostatics. The revised model has been used to investigate the size effects on several physical properties of various solute-solvent systems by increasing the number of explicitly included solvent molecules from few tens up to thousands. Results on simple ions, such as sodium and chloride ions, and on a small peptide, such as alanine dipeptide analog (AcAlaNHMe), have shown that solvation structures and dynamics, as well as solvent-induced changes in the solute conformation, can be correctly reproduced by the MF model, providing that only two or three solvent layers are treated explicitly.
Polyelectrolyte stars in planar confinement
Authors: Konieczny Martin , Likos Christos N.
Published: June 2006

We employ monomer-resolved molecular dynamics simulations and theoretical considerations to analyze the conformations of multiarm polyelectrolyte stars close to planar, uncharged walls. We identify three mechanisms that contribute to the emergence of a repulsive star-wall force, namely, the confinement of the counterions that are trapped in the star interior, the increase in electrostatic energy due to confinement as well as a novel mechanism arising from the compression of the stiff polyelectrolyte rods approaching the wall. The latter is not present in the case of interaction between two polyelectrolyte stars and is a direct consequence of the impenetrable character of the planar wall.
Geometric phase effects in the coherent control of the branching ratio of photodissociation products of phenol
Authors: Abe Mayumi , Ohtsuki Yukiyoshi , Fujimura Yuichi , Lan Zhenggang , Domcke Wolfgang
Published: June 2006

Optimal control simulation is used to examine the control mechanisms in the photodissociation of phenol within a two-dimensional, three-electronic-state model with two conical intersections. This model has two channels for H-atom elimination, which correspond to the π2 and σ2 states of the phenoxyl radical. The optimal pulse that enhances σ2 dissociation initially generates a wave packet on the S1 potential-energy surface of phenol. This wave packet is bifurcated at the S2−S1 conical intersection into two components with opposite phases because of the geometric phase effect. The destructive interference caused by the geometric phase effect reduces the population around the S1−S0 conical intersection, which in turn suppresses nonadiabatic transitions and thus enhances dissociation to the σ2 limit. The optimal pulse that enhances S0 dissociation, on the other hand, creates a wave packet on the S2 potential-energy surface of phenol via an intensity borrowing mechanism, thus avoiding geometric phase effects at the S2−S1 conical intersection. This wave packet hits the S1−S0 conical intersection directly, resulting in preferred dissociation to the π2 limit. The optimal pulse that initially prepares the wave packet on the S1 potential-energy surface (PES) has a higher carrier frequency than the pulse that prepares the wave packet on the S2 PES. This counterintuitive effect is explained by the energy-level structure and the S2−S1 vibronic coupling mechanism.