bluuebunny/crossref_metadata_2025_split huggingface dataset:
Scanning tunneling microscopy with atomic resolution on ReS2 single crystals grown by vapor phase transport
Authors: Friemelt K. , Akari S. , Lux‐Steiner M.‐Ch. , Schill T. , Bucher E. , Dransfeld K. 
Published: May 2007

AbstractAtomic resolution images of layered transition metal‐dichalcogenide ReS2 single‐crystals (n‐type semiconductor) were obtained using a scanning tunneling microscope with a positive tip. In most cases only unresolved clusters of four rhenium atoms could be seen. Occasional images with higher resolution showed that these bright structures consist of four separated atoms. The symmetry of the imaged atoms is identical to that of the rhenium sublattice but not to that of the sulfur atoms. We conclude therefore that the main contribution to the tunneling current is due to the rhenium atoms, although the sulfur atoms are placed by about 0.15 nm closer to the tip. Thus for our positive bias of the tip the tunneling electrons originate from occupied rhenium states in the valence band of the semiconductor.
Dissociation of doubly charged alkali metal clusters
Authors: López J. M. , Alonso J. A. , Garcias F. , Barranco M. 
Published: May 2007

AbstractWe have studied the competition experimentally observed between fission and neutral atom evaporation, as dissociation channels of excited doubly charged sodium clusters, using the Density Functional Theory and the jellium model. The fission barrier has been obtained from an Extended Thomas‐Fermi calculation including density gradient corrections to the kinetic energy of the electronic cloud. We discuss the influence of the coefficient of the density gradient term on the barrier height.
Localization lengths in disordered Peierls systems
Authors: Wolf M. , Fesser K. 
Published: May 2007

AbstractWe have studied the influence of bond‐ and site‐type impurities on the ground state properties of one‐dimensional Peierls systems. Using a functional integral formalism with both commuting and anticommuting variables we have calculated the averaged Green's function which determines the electronic density of states and localization length (Thouless formula). Some limiting cases can be solved analytically. To apply our model to doped polymers we derive the connection between doping concentration and disorder strengths Dj. For illustration we present the results with parameters appropriate for polyacetylene.
Desorption kinetics and surface diffusion of potassium, rubidium and cesium on a silicon(111)7 × 7‐surface
Authors: Storch R. , Stolz H. , Wassmuth H.‐W. 
Published: May 2007

AbstractUsing pulsed atomic beam technique and a surface ionization ion microscope, the desorption kinetics and the surface diffusion of the alkalis potassium, rubidium and cesium were investigated on a Si(111)7 × 7‐surface at extremely low alkali coverages.In the temperature range 1120 … 800 K, the mean adsorption lifetime τ(T) = τ0 · exp(Edesi/kT) and the mean diffusion length x(T) ‐ defined in the equilibrium between adsorption, diffusion and desorption ‐ were measured. From these data the diffusion constant D(T) = D0 · exp(‐Ediff/kT) was obtained as D = x−2/τ. For temperatures T ⩽ 750 K, the diffusion constant was calculated from nonstationary alkali concentration profiles using the Boltzmann‐Matano method.From the temperature dependence of these quantities the parameters of desorption (Edes,i τ0) and surface diffusion (Ediff, D0) for K, Rb and Cs on Si(111) were obtained. The values of Ediff and D0 are comparably high and may be interpreted by non‐localized diffusion according to a model proposed by Bonzel (Surf. Sci. 21 (1970) 45).
On electron screening effects in the low‐energy 3He(d, p)4He data
Authors: Langanke Karlheinz , Lukas Dirk 
Published: May 2007

AbstractWe have calculated the screening effects of the two electrons in the atomic 3He gas target on the low‐energy3He(d, p)4He data. The nuclear degrees of freedom have been described within the microscopic multi‐channel Resonating Group Method, while the two electrons have been treated in Born‐Oppenheimer approximation deriving the exact screening potential within the Path Integral Monte Carlo method. Our model underestimates the observed screening effects implying the need to consider the coupling of nuclear and electronic degrees of freedom.
Tunneling dynamics in stationary field emission
Authors: Gottlieb B. , Kleber M. 
Published: May 2007

AbstractStationary three‐dimensional field emission out of an ultrathin quantum well is analyzed in terms of the semiclassical Hamilton‐Jacobi theory. We compare the spatial quantum distribution of the tunneling electrons with the corresponding results obtained from real‐time tunneling trajectories. The classical‐limit trajectory method reproduces the exact quantum distribution for weak external fields. The Büttiker‐Landauer expression for the barrier crossing velocity is confirmed.
Localization and interaction in two dimensions: The disordered Tomonaga model
Authors: Fastenrath Ulrich 
Published: May 2007

AbstractThe effect of electron‐electron interactions on the localization behavior of two‐dimensional disordered systems is investigated in the framework of a generalized version of Tomonaga's model. The main result is a closed expression for the dynamical conductivity which allows to predict the critical electron concentration below which localization sets in as a function of microscopic parameters only. Experimental verification is possible, although difficult.
The one‐dimensional 1/4‐filled Hubbard model with alternating interactions
Authors: Japaridze G. , Khomskii D. , Müller‐Hartmann E. 
Published: May 2007

AbstractThe one‐dimensional Hubbard model with different on‐site interaction on the even (Ua) and odd (Ub) sites is considered within the framework of the weak coupling approach. In the case of a 1/4‐filled band the dynamical nonequivalence of sites leads to the appearance of Umklapp processes in the system and to the dynamical generation of a commensurability gap in the charge excitation spectrum for Ua ≠ Ub and Ua>0 or Ub>0. Depending on the relation between the bare coupling constants the system shows four different regimes of behaviour in the infrared limit corresponding to normal metal, nonmagnetic insulator, antiferromagnetic insulator and superconducting states. The extended model including interaction between particles on nearest and next‐nearest neighbour sites is also considered.
Near field optics and light confinement
Authors: Courjon D. , Bainier C. , Girard C. , Vigoureux J. M. 
Published: May 2007

AbstractBy exploring the surface of an illuminated object at a very short distance by means of a subwavelength detector, it is possible to reconstruct images whose resolution is beyond the diffraction limit. This opportunity due to the properties of near fields can be explained in terms of non radiating field detection. Such a detection is the basis of near field microscopy. In this communication emphasis will be put on some fundamental aspects of the new concept of non radiative optics such as the relation between Rayleigh criterion and Heisenberg principle and the notion of light confinement. Some proposals of non radiative transfers will be evoked.
Spin dynamics for 2d Heisenberg antiferromagnets with combined easy‐plane and Dzyaloshinsky interactions
Authors: Völkel A. R. , Mertens F. G. , Bishop A. R. , Wysin G. M. 
Published: May 2007

AbstractWe investigate the spin dynamics of the classical two‐dimensional easy‐plane Heisenberg antiferromagnet with an additional Dzyaloshinsky interaction, which serves as a simple model for Langmuir‐Blodgett films of Mn (C18H35O2)2. By mapping the system onto a pure easy‐plane model we discuss the corresponding spin wave and vortex dynamics. The additional Dzyaloshinsky interaction forces all spins to cant in a certain direction, which is the same for neighboring spins on different sublattices. This canting causes the presence of a second spin wave peak in the dynamical in‐plane correlation function below the Kosterlitz‐Thouless transition temperature TKT and a second vortex central peak above TKT. Using a vortex gas approach we explicitly calculate the contribution of the free vortices to several dynamical correlation functions. These results are compared to a combined Monte Carlo‐Molecular Dynamics simulation on square lattices with different sizes. We also discuss the relevance of this simple model for describing the spin dynamics of Mn (C18H35O2)2.
Far‐infrared spectrum and the soft mode of ferroelectric betaine phosphite
Authors: Koch G. , Happ H. 
Published: May 2007

AbstractThe far‐infrared reflection spectra of betaine phosphite single crystals in the three crystallographic directions are investigated in the range from 5 to 600 cm−1 at temperatures between 203 and 323 K using a dispersive Fourier spectrometer. The dielectric function is evaluated from the complex amplitude reflection coefficient measured. In the spectrum polarized along the ferroelectric direction a soft mode of relaxation type is observed which accounts for a large part of the relevant static dielectric constant at Tc = 210 K. Among the oscillator modes, which generally exhibit normal temperature dependence, there is a heavily overdamped mode at 58 cm−1, polarized along the crystallographia a1 axis, with anomalous damping behaviour. The damping constant decreases nearly linearly with temperature up to 323 K which is close to T = 355 K where a second phase transition is known to exist.
δ‐function perturbations and boundary problems by path integration
Authors: Grosche C. 
Published: May 2007

AbstractA wide class of boundary problems in quantum mechanics is discussed by using path integrals. This includes motion in half‐spaces, radial boxes, rings, and moving boundaries. As a preparation the formalism for the incorporation of δ‐function perturbations is outlined, which includes the discussion of multiple δ‐function perturbations, δ‐function perturbations along perpendicular lines and planes, and moving δ‐function perturbations. The limiting process, where the strength of the δ‐function perturbations gets infinitely repulsive, has the effect of producing impenetrable walls at the locations of the δ‐function perturbations, i.e. a consistent description for boundary problems with Dirichlet boundary‐condition emerges. Several examples illustrate the formalism.
Discrete resistance fluctuations in pressure‐type point contacts
Authors: Kokkedee J. A. , Thier C. , Jansen A. G. M. 
Published: May 2007

AbstractIn high ohmic pressure‐type metallic point contacts (resistance range 50 Ω to 3 k Ω) the point‐contact resistance is observed to switch randomly between two or more discrete levels. This effect can be explained by the motion or reorientation of single defects, thereby changing their cross section for electron scattering. From the temperature‐ and voltage‐dependence of the characteristic times of the fluctuations, electromigration parameters for a defect in silver are extracted.
Quantum trajectories for nonlinear optical processes
Authors: Goetsch Peter , Graham Robert 
Published: May 2007

AbstractThe stochastic Schrödinger equation, a novel method for describing the dissipative dynamics of open quantum systems interacting with their environment, is applied to some nonlinear optical processes, namely nonlinear absorption and second‐harmonic generation, the latter also in the range of instabilities and classical chaotic behaviour. Where possible the results are compared with numerical simulations of the positive P representation, which in some cases are found to give unreliable results.
Flow‐equations for Hamiltonians
Authors: Wegner Franz 
Published: May 2007

AbstractFlow‐equations are introduced in order to bring Hamiltonians closer to diagonalization. It is characteristic for these equations that matrix‐elements between degenerate or almost degenerate states do not decay or decay very slowly. In order to understand different types of physical systems in this framework it is probably necessary to classify various types of these degeneracies and to investigate the corresponding physical behavior.In general these equations generate many‐particle interactions. However, for an n‐orbital model the equations for the two‐particle interaction are closed in the limit of large n. Solutions of these equations for a one‐dimensional model are considered. There appear convergency problems, which are removed, if instead of diagonalization only a block‐diagonalization into blocks with the same number of quasiparticles is performed.
Electrons with correlated hopping interaction in one dimension
Authors: Japaridze G. , Müller‐Hartmann E. 
Published: May 2007

AbstractThe 1D system of correlated electrons characterized in addition to the usual on‐site (U) and nearest‐neighbour (V) repulsion by a correlated‐hopping term (t*) is considered. The ground state phase diagram is studied within the framework of the weak‐coupling continuum‐limit approach. At filling v the effective interaction originating from the correlated‐hopping term which appears in the continuum‐limit theory is given by t* cos (πv). Being repulsive for v < 1/2 and attractive for v > 1/2, this interaction leads to a characteristic band‐filling dependence of the phase diagram of the system. For v ≠ 1/2, depending on the relation between the bare values of the coupling constants (U, V, t* cos (π v)) and, hence, on the band‐filling, the system shows three different phases in the infrared limit: a Luttinger metal, a nonmagnetic metal and a singlet superconducting phase. For v = 1/2, the correlated‐hopping interaction is dynamically trivial, leading only to a renormalization of the oneelectron hopping amplitude and the phase diagram of the model coincides with that of the extended Hubbard model.
Fulde‐Ferrell‐Larkin‐Ovchinnikov state in layered superconductors
Authors: Burkhardt H. , Rainer D. 
Published: May 2007

AbstractThe phase diagram of a layered superconductor in a large parallel magnetic field is calculated. This includes a calculation of the lower critical field beyond which the superconductor is in the FFLO‐state and possesses a spatially modulated order parameter and spin polarization. The order parameter, spin polarization, free‐energy density, and phase boundaries of this unconventional superconducting phase are evaluated numerically in the complete B–T plane. The analysis suggests that the transition at the lower critical field is of second order, and not of first order, as previously assumed. The order parameter of the FFLO‐state merges continuously into the uniform superconducting state.
On the limitation imposed by gravity on measurements of the critical viscosity of a pure fluid
Authors: Ferrell Richard A. , Moldover Michael R. 
Published: May 2007

AbstractMeasurements of the viscosity of a fluid by means of a parallel plate torsional oscillator are conventionally limited to a plate separation that is much larger than the viscous penetration depth of the fluid contained between the plates. Here we demonstrate that, in principle, useful measurements can be carried out in the opposite regime where the thickness is smaller than the penetration depth. An alternative method, described in the last part of this paper, for alleviating the restriction imposed by the density stratification that results from the earth's gravity, is to use electrostriction, rather than the inertial force, to drive the fluid between the plates.
Path integrals for two‐ and three‐dimensional δ‐function perturbations
Authors: Grosche Christian 
Published: May 2007

AbstractThe incorporation of two‐ and three‐dimensional Δ‐function perturbations into the path integral formalism is discussed. Contrary to the one‐dimensional case, a regularization prescription is needed due to the divergence of the Green function corresponding to a potential V, G(v) (x,y;E) (x, y ϵ ℝ2, ℝ3) for x→y. The known procedure to define proper self‐adjoint extensions for Hamiltonians with point‐interactions can be exploited to define the incorporation of δ‐function perturbations in the path integral. Several examples illustrate the formalism.
Spheroidally deformed sodium clusters in the selfconsistent jellium model
Authors: Hirschmann Th. , Brack M. , Meyer J. 
Published: May 2007

AbstractWe present the first systematic study of potential energy curves and prolate‐oblate shape transitions of sodium clusters with 8 < N < 40 atoms. The Kohn‐Sham equations are solved in the local density approximation for the jellium model with spheroidal deformations. The ionic background density is taken to have a diffuse surface of Woods‐Saxon type. The quadrupole and hexadecupole moments of the electron and jellium densities are investigated, revealing a strong hexadecupole dependence for selected clusters. Collective dipole resonances are described in the simple surface plasmon model. Shape transitions are found to occur at particle numbers 12–14 (prolate‐oblate), 18–20–22 (oblate‐spherical‐prolate) and 30–32 (prolate‐oblate), which are in good agreement with experimental results; triaxiality is predicted for Na‐36. Comparing our results with those of molecular dynamics calculations, we confirm the scheme of Kohn‐Sham levels and the gross behaviour of potentials and densities.
Event‐enhanced quantum theory and piecewise deterministic dynamics
Authors: Blanchard Ph. , Jadczyk A. 
Published: May 2007

AbstractThe standard formalism of quantum theory is enhanced and definite meaning is given to the concepts of experiment, measurement and event. Within this approach one obtains a uniquely defined piecewise deterministic algorithm generating quantum jumps, classical events and histories of single quantum objects. The wave‐function Monte Carlo method of Quantum Optics is generalized and promoted to the level of a fundamental process generating all the real events in Nature. The already worked out applications include SQUID‐tank model and generalized cloud chamber model with GRW spontaneous localization as a particular case. Differences between the present approach and quantum measurement theories based on environment‐induced master equations are stressed. Questions: what is classical, what is time, and what observers are addressed. Possible applications of the new approach are suggested, among them connection between the stochastic commutative geometry and Connes' noncommutative formulation of the Standard Model, as well as potential applications to the theory and practice of quantum computers.
Physical properties of dynamic force microscopies in contact and noncontact operation
Authors: Krüger D. , Anczykowski B. , Fuchs H. 
Published: May 2007

AbstractIn the field of Scanning Force Microscopy several dynamical contact and noncontact modes have been introduced increasing the range of detectable surface and interface properties, and allowing to detect material properties such as elasticity and mass density on the nanometer scale. A detailed understanding of tip/surface interactions and the dynamic processes involved is required to understand the origin of a material contrast using these techniques. Here a general method to solve the equation of motion of a vibrating SFM cantilever/tip system in an external force field is presented. Contact modes as well as intermittent contact modes are discussed using a single set of equations describing the cantilever/tip motion, and by varying the size of amplitudes of the vibrating cantilever/tip system. To quantitatively describe the oscillation behavior of the SFM cantilever at large amplitudes the computer simulations are based on the MYD/BHW model providing a realistic contact model with respect to the contact area, the size of the contact forces as well as the transition from repulsive to attractive forces. The results are compared with the experiment and with different approaches based on analytical and numerical models.
Discrete chaotic cryptography
Authors: Kotulski Zbigniew , Szczepański Janusz 
Published: May 2007

AbstractIn the paper we propose a new method of constructing cryptosystems utilising a nonpredictability property of discrete chaotic systems. We formulate the requirements for such systems to assure their safety. We also give examples of practical realisation of chaotic cryptosystems, using a generalisation of the method presented in [7]. The proposed algorithm of encryption and decryption is based on multiple iteration of a certain dynamical chaotic system. We assume that some part of the initial condition is a plain message. As the secret key we assume the system parameter(s) and additionally another part of the initial condition.
On the problem of divergences in Quantumelectrodynamics
Authors: Sünkel W. , Wildermuth K. 
Published: May 2007

AbstractIn a first step we have tried to show that very probably in Quantumelectrodynamics (QED) due to vacuum polarization effects the original bare charges of electrons and positrons are smeared out so much that their original divergent energy distances from the vacuum energy become finite. Furthermore, we have sketched how one has to generalize the calculations to refine the considerations presented in this paper.
The Vlasov equation for Coulomb systems and the Husimi picture
Authors: Domps A. , L'Eplattenier P. , Reinhard P. G. , Suraud E. 
Published: May 2007

AbstractWe explore the validity of the Vlasov equation as a semi‐classical approximation of time‐dependent Hartree‐Fock and time‐dependent LDA theories. We discuss the h → 0 limit for the propagation of quantal wavefunctions in terms of classical densities. The h → 0 limit is studied formally by means of its Wigner and Husimi phase‐space representations. We consider an application to the valence electron cloud of metal clusters and show a comparison between quantal and Vlasov dynamics in this case.
Differences between women and men in surgical treatment and case fatality rates for ruptured aortic abdominal aneurysm in England
Authors: Filipovic M , Seagroatt V , Goldacre M J 
Published: May 2007

Abstract

Background
Differences between women and men in treatment and outcome after admission with a ruptured abdominal aortic aneurysm (AAA) in England were studied.

Methods
Routinely collected data in Hospital Episode Statistics, linked to death records, for emergency admissions for ruptured AAA in England were analysed. The percentage of patients who underwent surgical repair was calculated, together with 30-day case fatality rates and age-adjusted odds ratios (ORs), comparing women with men.

Results
A total of 2463 women and 7615 men were admitted with a primary diagnosis of ruptured AAA (mean age 79·8 and 74·9 years respectively); 39·6 per cent of women and 66·4 per cent of men underwent surgical repair (OR 0·47 (95 per cent confidence interval 0·42 to 0·52)). Overall, 75·6 per cent of women and 61·7 per cent of men died within 30 days of admission (OR 1·36 (1·22 to 1·52)). The death rate for women and men who had surgery was similar (OR 1·01 (0·88 to 1·17)); when no operation was performed the mortality rate was higher in women, but not significantly so (OR 1·14 (0·91 to 1·42)).

Conclusion
Women with a ruptured AAA were less likely to be treated surgically than men, and their overall mortality rate was higher. Lower rates of surgery in women than in men may contribute to the higher mortality in women, but other explanations are possible.