bluuebunny/crossref_metadata_2025_split huggingface dataset:
Influence of magnetic impurities on the de Haas‐van Alphen oscillations
Authors: Stephan F. , Keiter H. 
Published: May 2007

AbstractUsing a simple extension of Shiba's formula for the de Haas‐van Alphen‐oscillations, the influence of scattering by magnetic impurities is studied. Already for a classical spin model one obtains a minimum in the dHvA amplitudes as function of the magnetic‐field, which is influenced by normal scattering. The change of this behavior for the Anderson model is studied within the Non‐Crossing‐Approximation, and raises questions for a comparison between experiment and theory.
Metastable phase formation and structural change characteristics of vapor deposited semiconductor films
Authors: Fuxi Gan , Songsheng Xue , Zhengxiu Fan 
Published: May 2007

AbstractReported here are studies about the thermal and laser induced metastable phase formation in amorphous GeSb2Te4 thin films prepared by RF‐magnetron sputtering. The general structural properties of this most promising optical phase change recording material are discussed from the point of view of fast structural phase transformation.
Unbiased estimate of forces from measured correlation functions, including the case of strong multiplicative noise
Authors: Borland Lisa , Haken Hermann 
Published: May 2007

AbstractStarting from appropriate short‐time correlation function measurements, we propose a dynamical “learning” method to derive the deterministic and stochastic forces underlying an observed process, even if this process contains strong multiplicative noise. To do this we extend the ideas of our previous paper [1] to establish mathematical relationships in this more general case between the joint distribution function of the process and its corresponding Ito‐Langevin equation. A numerical example for a simulated process containing strong multiplicative noise shows good agreement with the theory.
Resistivity, fluctuation‐enhanced conductivity, and irreversibility line of high‐quality Tl2Ba2Ca2Cu3O10 single crystals
Authors: Winzer K. , Kumm G. , Maaß P. , Thomas H. , Schwarzmann E. , Aghaie A. , Ladenberger F. 
Published: May 2007

AbstractWe present measurements of the resistivity tensor components ρa (T) and ρc (T) of high‐quality Tl2Ba2Ca2Cu3O10 single crystals wit Tc = 118 … 121 K. The in‐plane resistivity ρa as well as the out‐of‐plane resistivity ρc show a metal‐like temperature dependence with an anisotropy ratio ρc/ρa of up to 103. The coherence length ξc (0) = 0.15 nm could be determined from an analysis of the fluctuation conductivity above Tc. From measurements of the ac‐susceptibility in magnetic fields up to 10 Tesla the temperature dependence of the depinning line could be obtained. The diffusion coefficient of the flux lines obtained from the frequency dependence of the depinning temperature shows a thermally activated behaviour of the flux motion in the field range 0.1 T ⩽ B ⩽ 10 T.
Some notes on stationary vacuum solutions of the Einstein equations with shearing nongeodesic eigenrays
Authors: Horváth I. , Lukács B. 
Published: May 2007

AbstractThe stationary vacuum solutions of the Einstein equations of General Relativity give the external space‐time around stationary mass distributions, as e.g. final states of stellar evolution. The Kerr solution has shear‐free geodesic eigenrays and describes all black hole configurations with good asymptotic behaviour at infinity. Other solutions of this class are unphysical. Classes with shearing geodesic or shearfree nongeodesic eigenrays do not contain physical solutions at all, so for other physical configurations one must turn to the generic case of shearing nongeodesic eigenrays. For the stationary axisymmetric case Ansätze for solutions can be formulated in form of a specific functional dependence between the parameters of shear and nongeodesicity, unless they both are constants. Here we investigate the second subcase. Our result is that there is no solution of the vacuum Einstein equations in this subclass, except when both parameters vanish.
Second order Hamiltonian formalism and constraints algebra for non‐polynomial supergravities
Authors: Foussats A. , Repetto C. , Zandron O. P. , Zandron O. S. 
Published: May 2007

AbstractThe second order Hamiltonian formalism for a non‐polynomial N = 1D = 10 supergravity coupled to super Yang‐Mills theory is developed. This is done by starting from the first order canoncial covariant formalism on group manifold. The Hamiltonian, generator of time evolution, is found as a functional of the first class constraints of this coupled system. These contraints close the constraint algebra and they are the generators of all the Hamiltonian gauge symmetries.
Lyapunov exponents from quantum dynamics
Authors: Haake Fritz , Wiedemann Harald , Życzkowski Karol 
Published: May 2007

AbstractWe extract classical Lyapunov exponents from the time dependence of quantum mechanical expectation values. Classical chaos is revealed as a quantum transient with a liftetime ∼︁ ln ħ. Our strategy is shown to work for the example of a periodically kicked top.
Scanning tunneling microscopy/spectroscopy, etching and modification of surfaces of YbBa2Cu3O7‐δ single crystals using an STM
Authors: Yang X. , Rosner K. , Winzer K. , von Minnigerode G. 
Published: May 2007

AbstractThe surface of YbBa2Cu3O7‐δ single crystals was investigated using a scanning tunneling microscope at room temperature in air. Growth steps with step heights equal to the lattice parameter c as well as steps with step heights of multiples of 1.35 c were obtained. On smooth surfaces atoms arranged in a 3.8 Å square lattice could be resolved. We observed a tunneling‐induced etching effect leading to an etch pit on the surface. Furthermore, by applying voltage pulses with amplitude > 5.0 V and duration > 100 μs holes with diameters of ∼︁ 150 nm could be produced. Scanning tunneling spectroscopic measurements were performed yielding I‐V and ln I‐s characteristics. The extremely small effective barrier height of ϕ = 0.3 eV inferred from d ln I/ds may be caused by dirt between the surface and the tip.
RPA in wavefunction representation
Authors: Reinhard P.‐G. , Gambhir Y. K. 
Published: May 2007

AbstractThe RPA is formulated in subspaces of coordinate‐like and momentum‐like 1 ph operators. This allows to embed a large class of approximative schemes into a generalized RPA treatment. We give a detailed formulation in terms of wavefunctions in coordinate space which is ideally suited to practical programming. In particular, we work out the reduction to spherical tensors in the case of spherical symmetry which is most often the starting point in finite Fermion systems.
From sum rules to RPA: 1. Nuclei
Authors: Reinhard P.‐G. 
Published: May 2007

AbstractWe take up the flexible formulation of RPA in subspaces of coordinate‐like and momentum‐like 1 ph operators which was developed in the preceeding paper. We draw the connection to many of the known collective approximations to RPA by proper choice of the subspace of operators. We finally apply the newly developed scheme to comparative studies of a hierarchy of approaches for the RPA spectra of doubly magic nuclei described with Skyrme forces.
Pressure effects on 2p5 3p ‐ 2p5 3d transitions in neon
Authors: Trawiński R. S. , Bielski A. , Ciuryło R. , Szudy J. 
Published: May 2007

AbstractThe collisional broadening and shift of four spectral lines of neon (747.2, 748.8, 753.5, 754.4 nm) arising from 2p5 3p ‐ 2p5 3d transitions, emitted from a low‐pressure glow discharge in pure neon and neon‐helium mixtures have been measured using a Fabry‐Perot interferometer. The values of pressure broadening and shift coefficients are determined and analyzed in terms of the impact theory.
The anomaly of the specific heat of La2‐xSrxCuO4+δ near Tc
Authors: Ernst P. , Schnelle W. , Breuer M. , Borowski R. 
Published: May 2007

AbstractThe specific heat of three polycristalline samples of La2‐xSrxCuO4+δ (x = 0.15, 0.15, 0.16) near Tc was measured with a resolution better than 0.1%. The anomaly at Tc is analysed for fluctuations for the first time. It is well described by a superposition of a mean field step and a contribution due to gaussian fluctuations. By a fit of a Lawrence‐Doniach type of model to the specific heat near Tc we estimate ΔCMF and the anisotropic coherence lengths ξ and ξ⊥ of the superconductor.
Surface phonons and elastic surface waves
Authors: Büscher H. , Klein‐Heßling W. , Ludwig W. 
Published: May 2007

AbstractTheoretical investigations on the dynamics of the (001), (110) and (111) surfaces of some cubic metals (Ag, Cu, Ni) will be reviewed. Both, lattice dynamical and continuum theoretical results are obtained via a Green's function formalism. The main attitude of this paper is the comparison of our results with experiments and with results obtained via slab‐calculations. The calculation of elastic surface waves has been performed using a modified surface‐green‐function‐matching method. We have used two different approaches of calculation the bulk Green's function (a) using the spectral representation and (b) a method, what works on residues. The investigations are carried out using shortrange phenomenological potentials. The atomic force constants in the first surface layers are modified to describe surface phonon anomalies, observed by experiments. In the case of Ag (100) and Ag(110) we conclude that the detection of odd symmetry shear modes by Erskine et al. [1 a, b] was not very accurate.
Stochastic fluctuations of persistent currents in mesoscopic rings
Authors: Eckern Ulrich , Schmid Albert 
Published: May 2007

AbstractWe investigate systematically the effect of the electronic Coulomb interaction on the stochastic fluctuations (from sample to sample) in the persistent currents I(Φ) of mesoscopic rings threaded by a magnetic flux Φ. In contrast to our recent publication [Europhys. Lett. 18 (1992) 457], we avoid separation of impurity and interaction scattering events and we present here a comprehensive view. By this alternative procedure, we are able to confirm our earlier result, namely that 〈I2〉1/2 ∼ evF/L, which is a much larger quantity than the average current 〈I〉 ∼ evFl/L2.
Collective diffusion of Brownian particles with square well and hydrodynamic interaction
Authors: Vogel Jürgen 
Published: May 2007

AbstractThe low wavenumber collective diffusion coefficient of a semi‐dilute suspension of spherical Brownian particles interacting via square well potential and hydrodynamic pair interaction is considered. The first two nonvanishing terms of an expansion in powers of the wavenumber are calculated. Analytical expressions are found for extremely narrow wells and in the limit of large well diameters.
Metastable phases in vapour‐quenched Al78Cu22 film
Authors: Yan Haibo , Gan Fuxi , Wang Lixin 
Published: May 2007

AbstractThe structure and structural changes of an Al78Cu22 (at%) alloy film produced by coevaporation onto a cooled substrate have been investigated with transmission electron microscopy. A series of metastable phase transitions were observed during the crystallization from the as‐deposited amorphous phase to the final equilibrium state. The deposit structure at room temperature consists of a mixture of fine grained α' crystallites (fcc, α = 4.81 Å) with an amorphous phase, and some welldefined crystals with an fcc lattice (a = 7.18 Å, namely α1). The subsequent annealing led to the grown crystals transforming to an ordered α′ phase. It is argued that the development of such a transition is the result of a gradual atomic rearrangement – first topologically then a chemically predominant disorder‐order transition.
The astrophysical S‐factor of the reaction 7Be (p, γ)8B in the direct capture model
Authors: Krauss H. , Grün K. , Rauscher T. , Oberhummer H. 
Published: May 2007

AbstractThe astrophysical S‐factor for the reaction 7Be(p, γ)8B up to an energy of 2 MeV (c.m.) and the capture cross section of 7Li(n,γ)8Li up to 1 MeV (c.m.) are calculated using the Direct Capture model (DC). Both calculations are in good agreement with experimental data.
Specific heat of a BCS superconductor
Authors: Ferrell Richard A. 
Published: May 2007

AbstractBy expressing the free energy as an appropriate temperature integration over the square of the BCS energy gap function and by substituting simple but accurate analytic expressions for the latter, corresponding expressions are obtained for the specific heat in the superconducting state. In the upper portion of the temperature range, the ratio of the specific heats in the superconducting and normal states is 2.43 (1 + 0.936 in t), to better than one per cent accuracy, where t is the reduced temperature. Thus, the semilog plot of temperature versus the ratio of specific heat to temperature is a straight line over a large portion of one decade of specific heat. Corresponding expressions of similar simplicity and accuracy are obtained for the middle and low temperature ranges.
On the composition‐dependent isotope effect of HTSC in the two‐band model
Authors: Konsin P. , Kristoffel N. , Örd T. 
Published: May 2007

AbstractThe oxygen isotope effect in high‐TC superconductors has been investigated on the basis of a two‐band model where superconducting phase transition is induced by interband (mainly Coulombic) interaction. The isotope shift of TC appears due to the dependence of averaged interband electronphonon coupling constant on oxygen mass. This coupling has a repulsive nature and gives a relatively small contribution to the total interaction inducing superconductivity. The calculated isotope effect exponent depending on the carrier concentration has been compared with the experimental one as a function of x for La2‐xSrxCuO4.
Statistical properties of Lyapunov exponents and of quantum conductance of random systems in the regime of hopping transport
Authors: Markoš P. , Kramer B. 
Published: May 2007

AbstractThe statistics of the zero‐temperature conductance and the Lyapunov exponents of one‐, two‐ and three‐dimensional disordered systems in the regime of strong localization is studied numerically. In one dimension, the origin of the universality of the moments of the conductance is explained. The relation between the most probable value of the conductance and its configurational average is discussed. The relative fluctuations of the conductance (and of the resistance) are shown to grow exponentially with the system length. In higher dimensions the conductance is almost entirely determined by the smallest of the Lyapunov exponents. The statistics of the conductance is therefore the same as in the one dimensional case. A model is proposed for the treatment of the fluctuations in hopping transport at finite temperatures. An exponential dependence of the relative fluctuations of the conductance/resistance on the temperature is predicted, log (δg/g) ∞ T−a with α = 1/(d+1). It is concluded that the presently available experimental data on the temperature dependence of the conductance fluctuations in the hopping regime can be understood by replacing the system size in the zerotemperature result for the fluctuations of the conductance by the hopping length.
Linear response, persistent currents and related problems
Authors: Janßen Martin , Gummich Ute , Hajdu János , Viehweger Olaf 
Published: May 2007

AbstractBased on a general linear response approach we provide a systematic and unified survey of existing theories on persistent currents. The central notions in this context are equilibrium and dynamic persistent currents which are analyzed with respect to their similarities and differences in the canonical and grand canonical ensemble. We present criteria which relate the existence of persistent currents to the equipartition law and ergodicity for current correlators. We find that in additive Fermion systems at low temperatures both kinds of persistent currents coincide in the canonical ensemble whereas they differ in the grand canonical ensemble. Comparing different works on averaged persistent currents in diffusive mesoscopic rings within our framework and discussing several methods of calculating canonical currents with the help of grand canonical ensembles, we clarify some misunderstandings which have arisen in methodologically different approaches to the phenomenon of persistent currents. Finally, we relate the presence of dynamic persistent currents to the Hall conductivity on a finite cylinder and the center coordinate Kubo formula for the Hall conductivity.
Ferromagnetism in the Hubbard model: instability of the Nagaoka state on the square lattice
Authors: Hanisch Th. , Müller‐Hartmann E. 
Published: May 2007

AbstractThe instability of the fully polarized ferromagnetic ground state (Nagaoka state) of the Hubbard model on the square lattice is investigated. We use single spin flip variational wave functions including majority spin correlation effects and calculate spin flip energies in the thermodynamic limit. With very local wave functions and with moderate numbers of variational parameters we reproduce the best known estimate for the critical hole density δcr = 0.29 and we obtain an estimate of Ucr = 63 t for the critical coupling which is considerably better than the best estimate of Ucr = 42 t previously known. The simplicity of our wave functions makes the physical origin of the various aspects of the instability particularly transparent.
Electronic band structure of Cu3Au: An angle‐resolved photoemission study along the [111]‐direction
Authors: Lau Markus , Löbus Stefan , Courths Ralf , Halilov Samed , Gollisch Herbert , Feder Roland 
Published: May 2007

AbstractHigh‐resolution normal photoemission (ARPE) spectra have been recorded for Cu3Au(111) with the use of polarized synchrotron and rare‐gas resonance radiation in the photon energy range from 9 to 27 eV. It is for the first time that dispersions of the gold‐like bands have been found experimentally. Using a fully relativistic layer‐KKR photoemission formalism, occupied and unoccupied bands as well as one‐step‐model photoemission spectra have been calculated. The comparison of calculated spectra with experimental ones and the observation of direct‐transition resonances upon photon energy near the Brillouin zone‐center reveal a shift of the unoccupied ground‐state bands by about +2.5 eV (self‐energy shift). The direct‐transition structures in the experimental spectra have been exploited to determine the dispersions of the occupied bands along the [111] direction (A line in k space). In order to determine the wave vector of the experimental direct transitions we used as final state that calculated unoccupied band along [111], which also exists in pure copper and gold up to about 20 eV above the Fermi energy (“unfolded” band structure), shifted by + 2.5 eV. The experimental occupied bands with Cu character are in very good agreement with theory after shifting the latter by about 0.3 eV to lower energy, whereas somewhat bigger discrepancies exist for the gold‐like bands.
Complete determination of the transition amplitudes from a comprehensive data analysis of the fusion reactions D(d,p)3H and D(d,n)3He for Ed<500 keV
Authors: Lemaître S. , Paetz gen. Schieck H. 
Published: May 2007

AbstractAll relevant low‐energy transition amplitudes for the D(d,n)3He and D(d,p)3H reactions were determined from a fit to Legendre expansion coefficients of the available experimental data. A simple barrier penetrability model was used. Quintet S‐wave transitions are found to contribute strongly thus obliterating the idea of neutron‐lean “polarized” fusion energy production. The D+D interaction radius was determined with good accuracy for both reactions individually. The astrophysical S functions show a small S‐wave enhancement and P‐wave suppression of the D(d,p)3H branch.
The superconducting penetration depth from the semiclassical model
Authors: Chandrasekhar B. S. , Einzel D. 
Published: May 2007

AbstractThe semiclassical approach of Onsager and Pippard has been very successful in relating the electronic properties of a normal metal to its band structure and fermi surface. This paper extends the method to a superconductor. A generalized London equation relating the supercurrent density to the vector potential is obtained, in terms of band and fermi surface parameters and an energy gap which may be anisotropic. The results allow an interpretation of measured penetration depths directly in terms of the electronic and gap structures of the superconductor. They lead to easy physical visualization of the origin of anisotropies and other features of measured penetration depths.
Dynamics of a quantum particle in a random potential
Authors: Groß M. , Bosse J. , Gabriel H. 
Published: May 2007

AbstractThe motion of a quantum particle in a random potential with short‐range fluctuations is analyzed using a generalized mode‐coupling approximation. The original theory of Götze was extended by introducing a wave‐vector dependent current relaxation kernel. Here we apply the theory to the special case of exciton dynamics in a binary molecular crystal. We find a localization–delocalization transition in three dimensions with a critical exponent of 1 for the inverse localization length. In the intermediate time regime, our theory predicts a region of anomalous diffusion. This can be interpreted as a precursor to localization. At the transition point it leads to the known asymptotic low‐frequency behaviour D″(ω) α ω1/3.
Formal power series representation for gauge boson stars
Authors: Schunck Franz E. 
Published: May 2007

AbstractWe represent spherically symmetric, static, and non‐singular solutions of the Einstein SU(2)‐Yang‐Mills and the Yang‐Mills‐dilaton system by means of formal power series expansions. Their coefficients are algebraically expressed in terms of new recursion relations. The solutions of Bartnik and McKinnon, found by numerical integration, are contained in our solution manifold.
From sum rules to RPA: 2. 3He droplets
Authors: Weisgerber S. , Reinhard P.‐G. 
Published: May 2007

AbstractWe investigate the excitation spectra of closed‐shell 3He droplets using a flexible formulation of the RPA which has been presented in preceeding papers. The calculations are based on an energy functional which thorougly employs finite‐range interactions. We compare the results for different stages in a hierarchy of approximations to the full RPA. We furthermore discuss the consequences of taking the zero‐range limit of the present energy functional, i.e. of using Skyrme‐type interactions.
Diffusion controlled growth of hydrogen clusters in metals
Authors: Nowakowski B. , Frisch H. L. , Borzi Carlos , Mijovilovich Ana 
Published: May 2007

AbstractWe show that a simple diffusion controlled growth, with subsequent aggregation, mean field theory accounts numerically for the growth of hydrogen clusters in certain metals (e.g. V, Nb, Zr, etc.). We also calculate the percolation threshold for the formation of an “infinite” aggregate both analytically and by a Monte Carlo simulation in a cubic lattice. The results are comparable and seem to correspond to the observed physical behavior.
Complex conductivity in high‐Tc single crystal superconductors
Authors: Ciccarello I. , Fazio C. , Guccione M. , Li Vigni M. 
Published: May 2007

AbstractThe electromagnetic surface impedance of single crystal high‐Tc superconductors has been examined within the framework of the two‐fluid model and the hypothesis that the e m field modulates the partial concentrations of both normal and condensate fluids. A comparison with experimental data is reported.