bluuebunny/crossref_metadata_2025 huggingface dataset:
Modular Invariants for Theta‐Functions with Characteristics and the Twisted N = 2 Superconformal and SU(2) Kac‐Moody Algebras
Authors: Dobrev V. K. , Ganchev A. H.
Published: May 2007

We classify the modular invariants for θ‐functions with characteristics relevant for the twisted n = 2 super‐Virasoro and the twisted su(2) Kac‐Moody algebras. Detailed and complete proofs are given. Based on this we give the classification of the modular invariants for the above algebras.
Derivation of Attractive Electron‐Electron Interactions for Superconductivity via Canonical Transformations
Authors: Kolley E. , Kolley W.
Published: May 2007

Two canonical transformations, related to the theory of conventional and hightemperature superconductivity are performed in detail. The BCS Hamiltonian is deduced from the Fröhlich Hamiltonian, and the t ‐ J model with attraction from the Hubbard model in the limit of large on‐site Coulomb repulsion. The underlying approximations are pointed out.
To Probe into B.R.S.T. Transformations
Authors: Łopuszański Jan
Published: May 2007

We discuss the extention of the B.R.S.T. formalism obtained by assuming nonvanishing curvatures in ghost and mixed gauge‐ghost sectors.
Beispiele von Algebren mit gleicher dynamischer Entropie
Authors: Narnhofer H.
Published: May 2007

Bei gegebener dynamischer Entropie wird versucht, einen Überblick über die möglichen zugehörigen Algebren zu finden. Überabzählbare, nicht äquivalente Beispiele werden konstruiert. Im besonderen wird untersucht, ob sie Quanten‐K‐Systeme sind und wie weit sich Clustereigenschaften ableiten lassen.
On the Trotter Product Formula for Gibbs Semigroups
Authors: Neidhardt H. , Zagrebnov V. A.
Published: May 2007

The trace‐norm convergence of the Trotter product formula is proved for Gibbs semigroups.
Homogeneous Isotropic Cosmology on the Basis of a New 5‐Dimensional Projective Unified Field Theory
Authors: Schmutzer E.
Published: May 2007

The previously elaborated Projective Unified Field Theory (PUFT) of the author is applied to a homogeneous isotropic cosmological model generalising the Friedman model. The following special cases are treated: dust model and electromagnetic radiation model.
Analysis und Physik
Authors: Wehrl A.
Published: May 2007

Es wird ein kurzer Abriß über die Wechselwirkung von Analysis und Physik präsentiert, beginnend vom Jahre 7000 v.u.Z. über Euklid, Archimedes, Leibniz, Newton bis zu modernen Entwicklungen (Sobolew‐Ungleichungen, Nicht‐Standard‐Analysis).
On Generating Functionals for Interacting Disordered Fermi Systems without Denominators
Authors: Weller W. , Souleiman M. , Kolley W. , Kasner M.
Published: May 2007

For a disordered Fermi System the generating functional for Green functions in Keldysh technique is represented as a functional integral in a time/frequency lattice. The normalization denominator is rigorously shown to be equal to 1. Rules for the diagram analysis are given reducing the multi‐frequency scheme to the external frequencies.
On the Accessibility of States for Systems with Dissipative Dynamics
Authors: Zylka Ch.
Published: May 2007

We investigate certain dissipative dynamical systems from the aspect of the accessibility of states. Here the set of the n‐dimensional probability vectors serves as state space, while it is assumed that the dynamics can be modelled by generalized doubly stochastic matrices. We clarify what on these assumptions the set of states looks like starting from which a given state is accessible.
Die Annalen im Wandel ihrer Aufgabe. Zweihundert Jahre
Authors: Hund F.
Published: May 2007

Die „Annalen der Physik”︁ erscheinen jetzt seit zweihundert Jahren. In diesem Zeitraum haben sich Träger und Inhalte der Forschung und die Form der Übermittlung von Wissenschaft gewandelt. Der Beitrag überblickt das Wirken der Annalen in diesem Wandel.
Bemerkungen zur Theorie des Himmelsblaus
Authors: Richter G.
Published: May 2007

Die blaue Farbe des Himmelslichtes und dessen beachtliche Intensität werden bekanntlich auf die Streuung der Sonnenstrahlen (nach Lord Rayleigh) an den Gasmolekülen der Atmosphäre, bzw. (nach M. v. Smoluchowski und A. Einstein) an den thermisch‐statistischen Schwankungen der Luftdichte in gegenüber einem Wellenlängenkubus sehr kleinen Volumelementen zurückgeführt. Über das Verhältnis beider Streuprozesse in der Gesamtstrahlung besteht jedoch in der Literatur keine einheitliche Meinung.

Im folgenden wird daher die Frage erneut diskutiert und festgestellt, daß ‐ vor allem bei verdünnten Gasen ‐ beide Streuprozesse bei der Aufsummierung der von den einzelnen Molekülen ausgesandten Streuwellen zu einer Gesamtstrahlung zu beachten sind und beide Betrachtungsweisen zu gleichen Ergebnissen führen.

Einen gewissen Beitrag zu der im Rayleighschen Bild wesentlichen Inkohärenz der von den verschiedenen Molekülen emittierten Streuwellen leistet auch der Dopplereffekt der thermischen Bewegung der Streuzentren, sowie die Inkohärenz der primären „weißen”︁ Sonnenstrahlung.
Ab initio vel ex eventu. II
Authors: Thiessen P. A. , Treder H.‐J.
Published: May 2007

Jedes „initium”︁ wird durch „experimenta crucis”︁ zum „eventus”︁. Jedes theoretisch interpretierbare ex‐eventu‐Resultat führt auf ein neues Initium. Gerade dies ist die gemeinsame Aussage von Atomistik, Quantenmechanik und Relativitätstheorie.
Some Remarks on Unsharp Quantum Measurements, Quantum Non‐Demolition, and All That
Authors: Busch Paul , Lahti Pekka J.
Published: May 2007

The theory of unsharp quantum measurements is reviewed as a generalization of the von Neumann‐Lüders measurement theory and applied to measurements of continuous quantities. A generalized notion of repeatability is proposed which is applicable even to intrinsically unsharp continuous observables. As an illustration a precise formulation of a quantum non‐demolition (QND) measurement performed on a harmonic oscillator is given.
Gruppenwirkung und Differentialgeometrie
Authors: Burghardt R.
Published: May 2007

Der Zusammenhang zwischen Gruppentheorie und Differentialgeometrie soll vertieft werden in Hinblick auf eine vollständig kovariante Formulierung der höherdimensionalen Feldtheorien.
Electron Emission from Clean Solid Surfaces by Fast Ions
Authors: Hasselkamp D. , Hippler S. , Scharmann A. , Schmehl T.
Published: May 2007

Total backward electron yields from 27 elemental, non‐crystalline, clean solids were measured during bombardment by H+‐, H‐, H‐, He+‐ and Ar+‐ions in the energy range from 100 keV to 800 keV. The yields were found to exhibit an oscillatory dependence on the atomic number of the target material correlated with the periods of the periodic system. These Z2‐oscillations are relatively insensitive to the type of projectile and the impact energy at the high projectile energies of this experiment. Present theories of electron emission cannot explain the main experimental results. The reasons for this failure are discussed.
Diffusion by Turbulence
Authors: Grossmann Siegfried
Published: May 2007

The turbulent diffusivity K of the atmosphere has first been studied by Richardson 1926 who empirically found that K depends on the scale l, K = Alα, with α = 4/3 and A = 0.6 cm2/3 s−1. This empirical scaling law is derived here from a unified theory (based on the Navier‐Stokes equation) together with an explicit result for the prefactor, A = 2.4 ϵ1/3. The mean atmospheric dissipation rate compatible with this is ϵ = 0.016 cm2 s−3. For windtunnels with typical dissipation rates ϵ ≅ 1 m2 s−3 the turbulent diffusion coefficient should be K/cm2 s−1 = 52 (l/cm)4/3.
Integrabilität, Irreversibilität und Kosmogonie
Authors: Treder Hans‐Jürgen
Published: May 2007

In der berühmten Diskussion über die Begründbarkeit der statistischen Thermodynamik stand hinter Zermelos Angriffen auf Boltzmann Planck selbst. Zermelos Einwände gegen Boltzmanns Atomistik und Boltzmanns Antwort sind heute wieder aktuell. Boltzmann begründete die statistische Thermodynamik letztendlich kosmogonisch.
Strahlteilungs‐ und Interferenzexperimente mit Photonenpaaren
Authors: Brendel J. , Lange R. , Mohler E. , Martienssen W.
Published: May 2007

Wir führen eine Reihe von elementaren optischen Experimenten mit einem nichtklassischen Lichtfeld durch, das über parametrische Fluoreszenz erzeugt wird und die Besonderheit aufweist, daß die Photonen ausschließlich paarweise auftreten. Beim Auftreffen auf einen verlustfreien, teildurchlässigen Spiegel werden die Photonen eines Paares gemäß einer Binomialverteilung auf das reflektierte und das transmittierte Bündel verteilt. Dieses Ergebnis ist unabhängig vom Kohärenzgrad. Die beobachtete Invarianz der Verteilung kann auf eine Balance zwischen quantenmechanischen Stimulations‐ und Interferenzeffekten in den Ein‐ und Ausgangsarmen des Strahlteilers zurückgeführt werden. Beim zweimaligen Durchlaufen eines Strahlteilers in einem Michelson‐Interferometer treten Interferenzen zwischen den verschiedenen möglichen Zweiphotonenzuständen im Interferometer auf. Das Korrelations‐Interferogramm für Photonenpaare besitzt eine höhere Auflösung als das Intensitäts‐Interferogramm für die Transmission einzelner Photonen. Wir deuten dieses Verhalten als quantenmechanische Interferenz zwischen der größeren Zahl von Möglichkeiten, die ein Photonenpaar im Vergleich zu einzelnen Photonen hat, das Interferometer zu durchlaufen. Die Anwendung dieser Überlegungen auf Lichtfelder mit Paaren von Photonen unterschiedlicher Energie führt zu Korrelations‐Interferogrammen, die deutlich von den mit klassischen Feldern aufgenommenen Interferogrammen abweichen.
Derivation of Composite Particle Dynamics by Weak Mapping of Quantum Fields
Authors: Kerschner R. , Stumpf H.
Published: May 2007

Alternatively to path integrals quantum fields can be characterized by functional equations. Weak mappings of quantum fields are defined by mappings between such functional equations. This is exemplified for a regularized nonlinear spinor field theory and a boson theory of bound states. A corresponding weak mapping theorem is derived. In the appendix strong mappings, i.e. operator mappings are critically discussed.
Macroscopic State of Gases in a Light Field
Authors: Nienhuis G. , van Enk S. J.
Published: May 2007

We consider a gas or a gas mixture in a radiation field. The microscopic evolution equations for the distribution functions for the various particle states are separated into a rapid and a slow time scale. This separation defines the relevant quantities that specify the macroscopic state of the system. The evolution equations for the macroscopic variables take the form of a modified diffusion equation in the case of a dilute vapor in a buffer gas, and of generalized Navier‐Stokes equations for a single‐component gas. The equations contain light‐induced fluxes as source terms, and the transport coefficients are modified by the light.
Pion Interferometry in High Energy Hadron and Nuclear Collisions: Radial Expansion and Freeze‐Out
Authors: Stock R.
Published: May 2007

It is shown that the transverse radius, obtained from analysis of two‐pion correlation functions in the mid‐rapidity domain, should reflect the radius of the pion emitting volume. Emission is interpreted as the decoupling from strong interaction, during expansion of the initial interaction volume. Assuming an isotropic expansion without radial flow, and a simultaneous freeze‐out of pions throughout the source volume we show that the transverse radius from interferometry should be proportional to the square root of the hadron rapidity density, dN/dy, observed at mid‐rapidity. Recent data from hadron‐hadron and nucleus‐nucleus collisions at or above √s ≈︁ 20 GeV per nucleon are well interpolated by this result.
Why Are Most Nuclei Axially Symmetric?
Authors: Zickendraht W.
Published: May 2007

A previous publication was concerned with the average prolateness of nuclei [1]. With the same methods as there it is shown here that axial symmetry should be predominant in nuclei, which it is indeed. The tools for the proof are special collective and single particle coordinates. The nucleon‐nucleus interaction is approximated by a shell model potential plus Nilsson coupling; strong coupling is used. The minimum of the total energy is found for axial symmetry.
Chiral Soliton Model with Dynamical Restoration of Chiral Symmetry from Quark Flavour Dynamics
Authors: Reinhardt H. , Kämpfer B.
Published: May 2007

The Nambu‐Jona‐Lasinio model as a particular model of quark flavour dynamics is bosonized in the low‐energy regime using a generalized heat kernel expansion with local mass scale. The resulting effective meson Lagrangian defines a chiral soliton model, the solutions of which show a partial restoration of chiral symmetry inside the soliton.
A Connecting Link between Lorentz and Dirac Theories of Electrons
Authors: Thattey Sudhir
Published: May 2007

A theory, that is an initial step towards bridging the gap between Lorentz and Dirac theories of electrons, is presented. An electromagnetic Lagrangian density is postulated, such that, the theory can be cast into a form, similar to Dirac theory. Electromagnetic interpretation of conserved currents, and of usual bilinear covariants in the Dirac theory, is obtained. A counterpart of Abraham‐Lorentz equation, with oscillatory solutions is derived. Self‐energy of electrons is expressed in terms of their self‐potentials and self‐currents.
Interference in Phase Space
Authors: Dowling J. P. , Schleich W. P. , Wheeler J. A.
Published: May 2007

A central problem in quantum mechanics is the calculation of the overlap, that is, the scalar product between two quantum states. In the semiclassical limit (Bohr's correspondence principle) we visualize this quantity as the area of overlap between two bands in phase space. In the case of more than one overlap the contributing amplitudes have to be combined with a phase difference again determined by an area in phase space. In this sense the familiar double‐slit interference experiment is generalized to an interference in phase space. We derive this concept by the WKB approximation, illustrate it by the example of Franck‐Condon transitions in diatomic molecules, and compare it with and contrast it to Wigner's concept of pseudo‐probabilities in phase space.
Quantum General Relativity, Torsion and Uncertainty Relations
Authors: De Sabbata V. , Sivaram C. , v. Borzeszkowski H.‐H. , Treder H.‐J.
Published: May 2007

It is shown that in gravitational theories with torsion one is led to commutation rules corresponding to Landau‐Peierls type uncertainty relations.
The Jump Problem in Fourth‐Order Gravity
Authors: Giering K.
Published: May 2007

Using a shock wave ansatz, first, the fourth‐order jump problem in gravitational theories containing fourth derivatives of the metric is investigated and, second, the obtained results are generalized for arbitrary n‐order jumps. For fourth‐order jumps, in particular, the dynamic modes of the field are analysed.
Higher Derivative versus Second Order Field Equations
Authors: Müller‐Hoissen F.
Published: May 2007

The first part of this work reviews problems of (particle and field) theories with higher than second time derivatives. The second part gives an overview about the known results concerning the structure of bosonic field theories with at most second order equations.
Forward‐Backward Coupling in Superfluorescence
Authors: Steudel H.
Published: May 2007

It is shown that — within the framework of an idealized model with taking into account backscattering at density fluctuations — there is a number NK of atoms such that for an atomic number N ≫ NK forward‐backward independence will be found while N ≪ NK leads to synchronized superfluorescence pulses in both directions.
Zur Reflexion und Transmission elektromagnetischer Wellen bei senkrechtem Einfall auf ebene Grenzflächen zwischen unterschiedlich absorbierenden Medien
Authors: Richter G.
Published: May 2007

Born und Ladenburg bemerkten bereits 1911, daß bei der Reflexion und Transmission elektromagnetischer Wellen durch eine Grenzfläche zwischen zwei verschieden stark absorbierenden Medien außer den mit den laufenden Wellen verbundenen Poyntingvektoren noch ein zur Erfüllung der Energiebilanz an der Grenzfläche notwendiger, sog. „gemischter”︁ Poyntingvektor zu beachten ist. Die mit diesem Vektor beschriebene Energieströmung ist stets dorthin gerichtet, wo aus derselben mehr Energie absorbiert wird. Sie entsteht im Zusammenhang mit einer räumlichen Modulation des Energieverbrauchs im partiell stehenden Interferenzfeld der einlaufenden und der reflektierten Welle, wobei die Energieabsorption bei den meisten Substanzen durch das elektrische Feld erfolgt, während das Magnetfeld daran praktisch unbeteiligt ist.

Da jedoch die Mitberücksichtigung der magnetischen Effekte zum besseren Verständnis des gemischten Poyntingvektors beitragen kann. wird im folgenden zu ihrer quantitativen Beschreibung in die zweite Maxwellsche Gleichung ein dem Stromglied in der ersten Maxwell‐Gleichung analoger Zusatzterm eingeführt. Die Amplitude des „gemischten”︁ Poyntingvektors erweist sich dann als wesentlich abhängig vom Verhältnis der elektrischen zu den magnetischen Verlusten. Dies wird durch Betrachtung eines Mediums verdeutlicht, in welchem außer den Verlusten durch das elektrische Feld auch hiermit vergleichbare Verluste über das Magnetfeld auftreten, wobei beide Verlustarten zusammen eine an die räumliche Verteilung der Feldenergie besser angepaßte Absorption ergeben. Bei gleichen Energiedichten und Energieverlusten des elektrischen und des magnetischen Feldes verschwindet der „gemischte”︁ Poyntingvektor.
Intracavity Multi‐Photon‐Absorption and the Quantum Noise Spectrum of Light
Authors: Herzog Ulrike
Published: May 2007

The intensity noise spectrum is investigated quantum‐mechanically for incident coherent light passing through a resonator which is filled with a k‐photon‐absorber. For k⩽ 2 the noise of the outgoing light is reduced below the shot noise level the reduction being maximal for frequencies that are small in comparison to the cavity bandwidth. It turns out that the highest possible value of this low‐frequency noise reduction is obtained when the resonator losses due to transmission through the outcoupling mirror are 2k – 1 times as large as the k‐photon absorption losses. In this case the noise at zero frequency is reduced with respect to the shot noise level by a factor of k/(2k – 1).
Crossover from BCS‐superconductivity to Bose‐condensation
Authors: Drechsler M. , Zwerger W.
Published: May 2007

Starting from a model of free Fermions in two dimensions with an arbitrary strong effective interaction, we derive a Ginzburg‐Landau theory describing the crossover from BCS‐superconductivity to Bose‐condensation. We find a smooth crossover from the standard BCS‐limit to a Gross‐Pitaevski type equation for the order parameter in a Bose superfluid. The mean field transition temperature exhibits a maximum at a coupling strength, where the behaviour crosses over from BCS to Bose like with corresponding values of 2 Δ0/Tc ≈ 5 which are characteristic for high Tc superconductors.
Strong linear amplification of quantum fields
Authors: Bandilla A.
Published: May 2007

Going ahead with investigations started by ideas of H. Paul it is examined how different quantum properties in the initial field state show up in the strongly amplified output field. As examples of extreme quantum fields the number state and the recently introduced phase optimized state are used. For the linear laser amplifier the inevitable noise is fully taken into account. These results are compared with those for a coherent input field. The characteristic differences in the macroscopic output fields allow unique conclusions onto the initial field. For the calculations the positive definite Glauber's P‐representation after strong amplification is applied.
Investigations on macroscopic intrinsic stress in amorphous binary‐alloy films
Authors: Dina S. , Geyer U. , v. Minnigerode G.
Published: May 2007

Macroscopic tensile and compressive mechanical stresses in the film plane are observed during the growth of amorphous binary‐alloy films of ZrCo and TbCo prepared by the condensation of the metal vapours from two electron‐beam evaporators. The mechanical stress is measured by a capacitive cantilever beam technique. Both kinds of stresses are affected by composition and substrate temperature, but the compressive stress additionally depends on the film thickness, evaporation rate and substrate material. The results are discussed in terms of a model for the growth of amorphous films published earlier.
Dynamik einer Anordnung aus zwei Tunnelkontakten ultrakleiner Kapazität mit supraleitenden Elektroden
Authors: Seume F. , Krech W.
Published: May 2007

Dynamics of an Array of two Tunnel Junctions with Ultrasmall Capacitances and Superconducting Electrodes

The dynamics of a series array of two voltage‐biased SET junctions with superconducting electrodes is investigated in the frame of a master equation formalism. Explicit expressions for the number of electrons on the central electrode and the electric current are given in the zero temperature case. Situations without current are analyzed in detail and using current‐voltage characteristics, special properties of the system in the stationary regime are discussed.
Response of finite many‐electron systems beyond the time‐dependent local density approximation: Application to small metal clusters
Authors: Pacheco J. M. , Ekardt W.
Published: May 2007

A linear response formalism is developed which is based on density functional theory within the local density approximation, but which is now corrected for its spurious self‐interaction errors, in the way originally proposed by Perdew and Zunger for ground state calculations. The original formulation is extended to incorporate self‐interaction corrections in the scrrening terms. The general formalism is then applied to the calculation of the static and dynamic response of the metal clusters {Na8, Na9+}, {Na20, Na} and {Na40 Na} within the jellium model. Comparison with experimental data and with other theoretical calculations indicates that the present formalism accounts for the overall (and most of the fine) features of the photoabsorption spectrum of these systems, providing a systematic improvement with respect to previous approaches. The remaining discrepancies are rationalized in terms of the effects to be expected by correctly accounting for the discrete structure of the ionic cores.
Mean values and fluctuations in the fragmentation of mesoscopic systems using an exactly solvable model with strict particle conservation and quantum symmetry
Authors: Gross D. H. E. , Ecker A. , Deangelis A. R.
Published: May 2007

A detailed discussion of self‐similarities in fragment‐size distributions and fluctuations is presented using an exactly solvable model of fragmentation (the “chain model”). The effects of particle‐number conservation and quantum symmetry can rigorously be considered in systems ranging from microscopic to macroscopic. Due to the analyticity of the model the various scalings can be studied free of any statistical noise. Using a tuning parameter we can generate self‐similar distributions with realistic power‐laws and/or fluctuations which show intermittency. Finite‐size effects neither destroy nor cause intermittency. The relation of self‐similarity in both the averages and the fluctuations can be studied analytically. It is found that they are unlinked ‐ there are cases where the size‐distribution is a power‐law with realistic exponents τ between −2 and −3 but no intermittency. Two cases will even be shown which have indistinguishable fragment distributions but very different factorial moments. We also discuss the interpretation of both the size and slope of the factorial moments in terms of multiplicity and bin mixing. We show that while either is sufficient to produce large moments, one must have bin mixing to produce large slopes. The two types of mixing are necessarily linked in constrained systems such as described by our model.
Fragmentation, dissipative expansion, and freeze‐out in medium energy heavy‐ion collisions
Authors: Gross D. H. E. , Li Bao‐An , DeAngelis A. R.
Published: May 2007

The collision dynamics of 96Mo + 96Mo at 55 A MeV is simulated by solving numerically the Boltzmann‐Uehling‐Uhlenbeck (BUU) transport equation for the one‐body phase‐space distribution‐function of nucleons with and without Coulomb interaction. A scatter‐plot of the one‐body density distribution shows an initial compression, subsequent homogeneous expansion, a breaking into “fragments”, a very slow creeping expansion up to a freeze‐out and in the case of included Coulomb‐interaction a Coulomb‐explosion. In the calculation which included Coulomb‐interaction the overall shape of the ensemble of dense fragments is spherical. The fragments are created over the entire volume of the dense part of the source and not at the surface only. In the simulation without Coulomb interaction a doughnut‐like shape may develop.
Observation of sharp resistance anomalies near Tc in granular superconductors
Authors: Rubin S. , Schimpfke T. , Weitzel B. , Voßloh C. , Micklitz H.
Published: May 2007

Granular films prepared from well‐defined In clusters embedded in Kr (⋍ 10‐40 at % Kr) show sharp resistance anomalies very close to the superconducting transition temperature. The observed resistance peak(s), which go(es) above the normal state resistance RN and which can be as high as ⋍ 1.5 RN, can be explained by a “mesoscopic” effect: Superconducting percolation aggregates of size ξρ, larger than the superconducting coherence length ξS, but smaller than the quasiparticle relaxation length λQ*, have a superconducting gap, but are not yet able to carry a supercurrent.
Positons of the modified Korteweg de Vries equation
Authors: Stahlhofen A. A.
Published: May 2007

The concept of positons, i.e. certain multiparametric solutions of the Korteweg de Vries equation with new properties, is extended to the modified Korteweg de Vries equation. It is shown that the essential features of positons carry over to this case; the collision of positons, the solitary‐wave‐positon interaction and simple generalizations are discussed in detail. Suggestions for future research and possible applications of the present work are sketched.
Measurement of the impedance of very thin superconducting YBaCuO films at 10 GHz with a cavity perturbation method between 4 and 300 K
Authors: Schaumburg G. , Helberg H. W. , Berberich P. , Kinder H.
Published: May 2007

We have measured the complex film impedance 1/σd (σ conductivity, d film thickness) of three YBaCuO thin films with d = 44, 115, and 168 nm on MgO substrates at 10.2 GHz in the temperature range between 300 and 4 K. Below Tc, the experimental results are discussed in terms of the two‐fluid model and the BCS theory. The residual resistance decreases with the film thickness. The thinnest film has a residual surface resistance of 3 · 10−4 Ω. For this film, the complex microwave conductivity is calculated and compared with the models. Apart from the residual resistance, the measured conductivity is in agreement with the peak caused by the energy gap of the BCS theory. All measurements were performed with a cavity perturbation method which we have to our knowledge applied for the first time to superconducting thin films. The method allows to determine the complex impedance of films with arbitrary thickness. In particular, films with thicknesses small compared to the skin depth δ or the London penetration depth λ can be measured. Therefore, we are able to measure the impedance both in the normal and superconducting state.
Tunnelling in periodically driven systems
Authors: Goetsch P. , Graham R.
Published: May 2007

Tunnelling in periodically driven bistable symmetric potential wells is investigated in an analytical approximation in a domain where the driving frequency is large compared to the tunnelling frequency and only the four lowest lying levels contribute significantly. The influence of finite level widths is taken into account, and a smooth variation of the amplitude of the driving field is allowed for.
On the clumpy structure of molecular clouds
Authors: Yue Z. Y. , Zhang B. , Winnewisser G. , Stutzki J.
Published: May 2007

Two‐dimensional hydrodynamic calculations for a self‐gravitating, magnetic and compressible medium are carried out. The results based on numerical simulations show that the joint effect of the nonlinearity of the hydrodynamic equations and self‐gravity is the mechanism for generating the observed complex structure of molecular clouds (e.g. clumps and filaments). In addition, parameters such as the evolution time of molecular clouds, the density contrast, fractal dimension, and the velocity probability distribution are derived. Their possible relation to the observed properties of molecular clouds are discussed.
Dynamic nonlinear σ‐model of electron localization
Authors: Horbach M. L. , Schön G.
Published: May 2007

We review the nonlinear σ‐model approach to the localization problem. However, different from usual, we do not make use of replicas or superfields in order to perform the averaging over the realizations of the disorder. Instead, we pursue the derivation of the nonlinear σ‐model as an effective theory for the disordered electron system within the framework of the so‐called time‐path formalism, which is a real‐time finite‐temperature quantum field theory. This formalism is especially suitable for the description of the disordered electron system due to the automatic normalization of real‐time quantities, thereby avoiding the denominator problem. We shall compare this dynamic method with the replica‐ and superfield‐methods as the perturbative level of the calculation of physical quantities.
Effects of geometry on transfer matrices, spin chains and critical behaviour
Authors: Davies Brian , Peschel Ingo
Published: May 2007

We consider two‐dimensional Ising models bounded by general parabolic curves and study their transfer matrices and associated quantum spin chains. We derive their eigenvalue spectra numerically and analytically, both at the critical point and in its vicinity. From this we find how the geometrical form of the system is reflected in the spectrum and how it influences the critical behaviour near the tip.
Monte Carlo simulation of cluster kinetics in three‐dimensional Ising model
Authors: Behrens M. , Stauffer D.
Published: May 2007

Simulations in lattices of size 1003 at the critical point of the three‐dimensional Glauber kinetic Ising model indicate that the 1935 Becker‐Doring equation no longer works there: The growth rates decay in time. These conclusions confirm those in two dimensions.
Anisotropic coherence lengths of YBa2Cu3O7‐δ from specific heat measurements near Tc
Authors: Schnelle W. , Ernst P. , Wohlleben D.
Published: May 2007

We present a detailed analysis of high resolution measurements of the specific heat anomalies at the Tc of three YBa2Cu3O7‐δ‐samples. The anomalies can be described by the sum of a mean field (MF) step and a contribution from gaussian fluctuations of the order parameter. These fluctuations are threedimensional (3D) near Tc and show a crossover to 2D‐behaviour some K away from Tc. By means of a Lawrence‐Doniach type description of the fluctuations we estimate the anisotropic coherence lengths ξ and ξ⊥. The values are consistent with data of the fluctuation contribution to the susceptibility above Tc. We give data for ΔCMF and discuss the relevance of critical fluctuations. The influence of the oxygen content δ on the broadening and splitting of the transition is shown.
Theory of light rectification in scanning tunneling microscopy in the presence of an adsorbed molecule
Authors: Yeyati A. Levy , Flores F.
Published: May 2007

We consider the problem of the rectified current induced by laser radiation in the STM junction when the tip is placed above a small molecule like CO or NO. This is calculated assuming a simple tight‐binding model for the tunneling junction including the adsorbate and using nonequilibrium Green's functions techniques. The coupling between tunneling electrons and the molecule vibrational modes is taken into account by a local electron‐phonon interaction term. In a second step we estimate the excitation rate of the molecule vibrations for a given laser power. This value is then used to obtain the relative change in the rectified current when the laser is in resonance with a molecule vibration. For a moderate laser power of 2 kW/cm2 a relative change of 1 to 3% is predicted.
Isochromat spectroscopy of photons emitted from metal surfaces in an STM
Authors: Berndt Richard , Gimzewski James K.
Published: May 2007

The pronounced variation of the intensity of photons emitted from the tunneling gap of an STM with respect to the applied bias voltage Vt is studied experimentally using simultaneous measurements of tunneling characteristics and photon emission. We show that the structure in isochromat photon spectra are determined by the following: bias‐dependent changes in tunneling characteristics, density of initial and final states, and modifications of tip‐induced plasmon modes. It is demonstrated that isochromat spectra provide a conclusive test for the inelastic tunneling mechanism. Coupling between tunneling electrons and tip‐induced plasmon modes which gives rise to the intense photon emission observed is discussed within this model.
Photovoltaic characterization of WSe2 with the scanning tunneling microscope
Authors: Akari S. , Lux‐Steiner M. Ch. , Glöckler K. , Schill T. , Heitkamp R. , Koslowski B. , Dransfeld K.
Published: May 2007

In the present study we use the Scanning Tunneling Microscope (STM) as an instrument to investigate the photovoltaic properties of semiconducting materials. The surfaces of the layered semiconductor WSe2 were optically illuminated during the tunneling process. The resulting photo‐induced tunneling current (PITC) was measured as a function of the wavelength. Microscopic information on the energy dependent generation and recombination of the photo‐electrons in the vicinity of the tunneling tip was obtained by this method without the necessity of covering the surface with a conducting electrode. The analysis of the wavelength dependence of the PITC points at the existence of excitonic excitations. Compared to the spectral response of conventional photosensitive heterodiodes our PITC spectra exhibit a more structured response and an increase of the signal in the UV region. Finally, PITC studies on differently treated WSe2 surfaces show an improved photoactivity for WSe2 crystals treated with NaI/I2 solutions.
Influence of giant angular resonances on the electromagnetic characteristics of low‐lying states
Authors: Mikhailov I. N. , Usmanov Ph. N.
Published: May 2007

The two‐rotor model0 with the Feshbach projection operator method is applied for investigating the properties of the positive parity low‐lying collective states in the rare earth nuclei. The calculations of the energy spectra, E2‐transition probability and magnetic properties of the states of β‐ and γ‐bands are carried out for the isotopes 164, 166, 168Er. The B(M1) values from 1+ states to the ground band are estimated.
Magnetic and other normal properties of Y‐Ba‐Cu‐O caused by Cu‐O chain fragments
Authors: Uimin G. , Stepanov V.
Published: May 2007

We discuss the questions related to modifications of the orthorhombic phase of YBCO. A general approach of statistical mechanics allows to determine numerically such quantities as the paramagnetic Curie constant, concentration of holes doped into Cu(2) planes from chain fragments, the fractions of copper ions in differently coordinated positions. Where the experimental information is available, we discuss a correspondence between our theoretical results end experimental ones.
Anomalies of the photo‐response and thermal boundary resistance of a YBaCuO/YSZ structure
Authors: Bonch‐Osmolovskii M. M. , Galkina T. I. , Golovashkin A. I. , Dovydenko K. Yu. , Klokov A. Yu. , Krasnosvobodtsev S. I. , Oktyabrskii S. R. , Romanov E. G.
Published: May 2007

The photoresponse of a YBaCuO/ZrO2 bolometric structure was measured under modulated (λ = 630 nm) and pulsed (τ ≈ 7 – 8 ns; λ = 337 nm) laser excitation. The shape of the measured photoresponse was interpreted by a thermal model; nevertheless, the pulse amplitude for vanishing YBaCuO film resistance was 5–6 times greater than predicted; the thermal boundary resistance RBd between YBaCuO and YSZ was evaluated ⋍ 10−2 K × cm2/Watt, which is considerably larger than estimated theoretically for the similar situation of YBaCuO/MgO [5].
Dilatometrische und dielektrische Anomalien bei den Phasenumwandlungen in Ba(Ti1‐xSnx)O3
Authors: Kajtoch Czesław
Published: May 2007

The temperature dependence of the thermal expansion, the dielectric constant, the pyroelectric current and the remanent polarization in polycrystalline samples of Ba(Ti1‐xSnx)O3 was measured. The influence of the external electric field and Sn‐substitution on the character of all phase transitions was investigated.
Classical Becker‐Döring cluster equations: Rigorous results on metastability and long‐time behaviour
Authors: Kreer Markus
Published: May 2007

We consider the classical Becker‐Döring cluster equations with constant monomer concentration c1 = z > 0 and

equation image

as a model which describes the kinetics of a first‐order phase transition. For a large class of positive coefficients al and bl (including the ones commonly used in physics and chemistry) we prove the following:
(i) When the monomer concentration z is slightly greater than zs = liml→∞ bl/al then all initial states ‐ containing only subcritical clusters of size l < l* (where l* denotes the critical size of a nucleus and depends on the supersaturation z‐zs > 0) ‐ converge within a fairly short time towards a metastable state. In this metastable state only subcritical clusters are present. The “metastable equilibrium” has an exponentially long lifetime TM ∼ exp (C(z‐zs)−ω) (where C and ω are some positive constants).

(ii) For times greater than the lifetime TM this metastable state breaks down in the following sense: as t→∞ each of the cl(t) converges towards the Becker‐Döring steady‐state solution fl(z) like cl(t)‐fl(z) = O (exp (‐ | λ1 | t)) (where λ1 < 0 is the eigenvalue closest to 0 of a certain infinite transition matrix) and the total mass of supercritical clusters (i.e. clusters of size l > l*) diverges in this limit. For large times the cluster‐number increases linearly in time in the sense that limt→∞ n (t)/t = J(z), where J(z) > 0 is the Becker‐Döring steady‐state current. For the average cluster size l = Σ lcl(t)/Σt=1 c(t), we find for sufficiently large times algebraic growth in time t, that is, μ1t1/(1‐α) < l(t) < μ2 t1(1‐α) (where 0 < α < 1 is the algebraic growth exponent of the al ∼ lα and μ1, μ2 are suitable positive constants). This bound covers previous suggestions due to computer simulations and heuristic calculations.
Frequency shifts in a cesium atomic clock due to Majorana transitions
Authors: Bauch Andreas , Schröder Roland
Published: May 2007

This paper deals with a theoretical model which describes the effect of Majorana transitions on the frequency of a cesium atomic clock. In contrast to the explanations given in the past, the phase correlation between the individual successive excitations in inhomogeneous magnetic fields and the microwave resonator are for the first time taken into account.

The π transitions with the selection rule ΔF = 1, Δm = ± 1 are of particular importance. The theory allows experimental results obtained in the past with the CSX apparatus, in particular the large frequency shifts observed, to be interpreted. The effect on the determination of the uncertainty of primary atomic clocks will be discussed.
Theoretical study of the temperature dependence of the width of magnetic domain walls
Authors: Jensen P. J. , Bennemann K. H.
Published: May 2007

The temperature dependence of magnetic domain walls in ferromagnetic systems with strong exchange coupling and weak lattice anisotropy is studied assuming that the thermal influence results mainly from the temperature dependence of the magnetization. We obtain that in lattices with an uniaxial symmetry like Co the wall width increases with temperature, but stays finite up to the Curie temperature Tc. In contrary, for a cubic lattice like Fe the wall width diverges for T → Tc, if only the lattice anisotropy is taken into account. The shape of the domain walls is not conserved, since at Tc the wall is determined only by the lowest order of anisotropy. In addition, the temperature dependence of a domain wall width for a thin magnetic film is determined. Using a special symmetry, we obtain a diverging wall width at a temperature markedly lower than Tc. However, the consideration of additional domain wall modes should modify this result.
Grad theory and extended thermodynamics
Authors: Nettleton R. E.
Published: May 2007

Grad‐type approaches introduce an ansatz involving tensor Hermite functions with coefficients expresed in terms of moments of the ansatz. This formalism in usual form yields terms linear in first‐order spatial derivatives in kinetic equations for the moments. Such terms disagree with alternative statistical derivations and phenomenological arguments. This disagreement is removed if different ansatzes are used to calculate entropy and moment equations. These are non‐unique, and so Grad theory, while providing theoretical expressions for transport coefficients, does not serve uniquely to determine the structure of phenomenological equations.
Aging of supersaturated Al‐Cu‐Co decagonal phase
Authors: Grushko B.
Published: May 2007

Aging of the supersaturated Al68Cu11Co21 decagonal phase was studied by transmission and scanning electron microscopy. Changes in the twofold [12100] electron diffraction patterns are reported. The decagonal phase aged at 700–800°C exhibited a reduction of the intensities of the quasiperiodic odd‐n reflections and the appearance of extra‐reflections along quasiperiodic directions.
Dynamics of radiation induced quasiparticles in superconducting tunnel junction detectors
Authors: Jochum J. , Kraus H. , Gutsche M. , Kemmather B. , v. Feilitzsch F. , Mössbauer R. L.
Published: May 2007

In order to achieve a high resolution in the spectroscopy of low energy X‐rays, detectors based on superconducting tunnel junctions as sensors are presently investigated. The knowledge of the processes affecting the signal generation in such sensors is essential for the interpretation of the detector response. Starting from a diffusion model including decay and tunneling of excess quasiparticles in the metal layers of a superconducting tunnel junction detector, the detector response is determined as a function of absorption position and of rate constants. Model predictions agree very well with experimental data. The advantages of a detector employing quasiparticle trapping are pointed out and the parameters determining the signal gain are deduced. The linearity of the detector signal is much more affected by pair recombination of the quasiparticles during their tunneling rather than during their diffusive propagation into the tunneling region.
Cluster equations for the Glauber kinetic Ising ferromagnet: I. Existence and uniqueness
Authors: Kreer Markus
Published: May 2007

The infinite set of cluster equations, proposed by Binder and Müller‐Krumbhaar for a Glauber kinetic Ising ferromagnet in 1974, generalize the Becker‐Döring equations used in classical nucleation theory. For positive symmetric transition rates satisfying certain growth conditions and a detailed balance condition we prove for sufficiently fast decaying initial cluster distributions the existence of a positive cluster distribution with finite density for all finite times solving the cluster equations. Uniqueness is proven under some further conditions on the transition rates. Our existence and uniqueness results apply e.g. for a Glauber kinetic Ising ferromagnet in two dimensions.
Light scattering and small angle neutron scattering experiments with an aggregating binary liquid mixture of critical composition
Authors: Schmitz J. , Belkoura L. , Woermann D.
Published: May 2007

Results of light scattering and small angle neutron scattering experiments with a 2‐butoxyethanol (2‐C4E1)/D2O mixture of critical composition (stoichiometric mole fraction of 2‐C4E1x = 0.0598; critical temperature Tc (visual) = 42.714 °C) are reported. They are carried out with the aim to find out whether the temperature dependence of the correlation length ξ characterizing the extension of concentration fluctuations near Tc is different from that characterizing the size of aggregates of the 2‐C4E1 molecules which appear to exist at temperatures away from Tc. The experiments show that the temperature dependence of ξ near Tc is the same as that away from Tc within the uncertainty of the measurements. – As a by‐product of this study, neutron scattering data were collected corresponding to a wide range of values of the scaling variable x (3 < x < 2100; x = qξ q is the absolute value of the scattering vector). They are analysed in terms of the limiting form of the universal correlation scaling function gξ(x) = C1x−(2‐η) to determine the value of the universal critical exponent η. A value of η = 0.039 ± 0.004 is found. It is considered consistent with the theoretically expected values.
On local and global currents in mesoscopic conductors
Authors: Loosen Roland , Menne Rolf , Hajdu János
Published: May 2007

Using linear response theory we show that, in a quasi‐stationary state, the local multiprobe conductance of a mesoscopic system of non‐interacting electrons with a time reversal invariant Hamiltonian does not depend on the local shape of the driving self‐consistent potential and thus is entirely determined by the asymptotic values of the potential in the leads. In the ballistic limit, the local conductance in the lateral direction exhibits oscillations depending on the occupation of channels. Scattering by a point impurity leads to softening of the quantized global conductance steps. In addition to that for an attractive scattering potential, a dip occurs in each plateau regime the shape of which is calculated for different values of the potential strength. We also investigate the local conductance for both a point scatterer and a finite scattering region.
Metallic solutions of the continuum model for conducting polymers
Authors: Streitwolf H. W. , Puff H.
Published: May 2007

In the continuum approximation of the Su‐Schrieffer‐Heeger model for degenerate and nondegenerate ground state systems, an exhaustive survey of solutions of the standard gap equations is given. With special emphasis on the electronic energy spectrum, the pertinent results of Takahashi [1] are extended to the whole μ, k‐plane. Periodic metallic phases and their grand potentials are discussed.
Cluster radioactivities of odd‐mass nuclei
Authors: Poenaru D. N. , Hourani E. , Greiner W.
Published: May 2007

The partial half‐lives of the hypothetical even‐even equivalent of an odd‐mass nucleus for cluster transitions toward various excited states of the daughter, used as a reference to find the hindrance factor, can be calculated within analytical superasymmetric fission model, by taking into account the angular momentum of the emitted cluster. Detailed tables are presented for 14C radioactivity of 221Fr, 221,223Ra, 225Ac; 24Ne radioactivity of 233U, 231Pa, and 23F decay of 231Pa, showing that, except for 225Ac, the existing experimental evidences, do not exclude (moderate) hindered transitions to the ground states of the daugther nuclei.
Beschleunigungsreaktionen von Neutronen
Authors: Petrov Yuri V.
Published: May 2007

Acceleration of neutrons by isomeric nuclei.

The inelastic neutron acceleration (INNA) by long living isomeric nuclei is studied. The theoretical estimates of the INNA cross‐sections for thermal neutrons for a number of isomers give the values in the range 0.1 ‐ 102b. Inelastic acceleration of thermal neutrons was observed by 152mEu and 180mHf, and confirmed the high value of the INNA cross‐sections. As a result of the high probability of acceleration some isomeric nuclei 113mIn, 115mIn, are not usual moderators but accelerators of neutrons. The use of the INNA reaction for the study of high excited levels and the nature of various nuclear hindrances are discussed.