@article{KlemzKubinaOstermeyeretal.2001, author = {Klemz, Guido and Kubina, P. and Ostermeyer, Martin and Menzel, Ralf}, title = {Diode pumped high power TEM_00 Nd:YAG rod laser with birefringence compensation}, year = {2001}, language = {en} } @article{RakosDominisSteindling2001, author = {Rakos, K. and Dominis, Dijana and Steindling, S.}, title = {Intermediate-band photometry of a rich cluster of galaxies : A2218}, year = {2001}, abstract = {This paper presents four color intermediate-band photometry of the cluster A2218 carried out using the Wise Observatory 1 m telescope. A2218 is one of the richest clusters in the Abell catalogue, with richness class 4. We find that A2218 has an unusually low fraction of blue galaxies for its redshift (z=0.171), and is populated mainly by E/S0 galaxies, while starburst systems are very scarce. The few starbursting galaxies present populate the faint end of the cluster luminosity function, close to the limiting magnitude of our observations. The difference between A2218 and other clusters at similar redshifts is very remarkable, underlining the influence of cluster richness on the evolution of its member galaxies.}, language = {en} } @article{DominisButlerSimonetal.2001, author = {Dominis, Dijana and Butler, Keith and Simon, Klaus and Clausen, Jens Viggo and Pritchard, John}, title = {Magellanic clouds' binaries as distance indicators}, issn = {0934-4438}, year = {2001}, language = {en} } @article{MemolaFendtBrinkmann2001, author = {Memola, Elisabetta and Fendt, Christian and Brinkmann, W.}, title = {Relativistic Jets from Blazars and Microquasars}, year = {2001}, language = {en} } @article{GoicoecheaGilMerinoSerraRicartetal.2001, author = {Goicoechea, Luis Julian and Gil-Merino, Rodrigo and Serra-Ricart, M. and Mediavilla, E. and Oscoz, Alejandro and Alcalde, D.}, title = {The nature of dark matter in elliptical (cD) galaxies : main lens galaxy of Q0957+561}, isbn = {1-583-81074-9}, year = {2001}, language = {en} } @article{JahnkeKuhlbrodtOerndahletal.2001, author = {Jahnke, Knud and Kuhlbrodt, B. and {\"O}rndahl, E. and Wisotzki, Lutz}, title = {QSO host galaxy star formation history from multicolour data}, isbn = {0-306-46662-7}, year = {2001}, language = {en} } @article{Wisotzki2001, author = {Wisotzki, Lutz}, title = {Luminosity and mass functions of active galactic nuclei}, isbn = {3-540-41581-5}, year = {2001}, language = {en} } @article{PugaAlcaldeBarrenaetal.2001, author = {Puga, E. and Alcalde, D. and Barrena, R. and Mediavilla, E. and Motta, V. and Munoz, J. A. and Oscoz, Alejandro and Serra-Ricart, M. and Gil-Merino, Rodrigo and Goicoechea, Luis Julian and Lewis, G. f and Libbrecht, C. and Moreau, O. and Surdej, J. and Wambsganß, Joachim}, title = {Daily monitoring of the gravitational lens QSO 2237+0305 at the Nordic Optical Telescope}, isbn = {0-7923-6974-2}, year = {2001}, language = {en} } @article{SchmidtWambsganssPenetal.2001, author = {Schmidt, Robert W. and Wambsganß, Joachim and Pen, U.-L. and Turner, E. L.}, title = {APO monitoring of Q2237+0305 in 1995-97 : evidence for microlensing}, isbn = {1-583-81074-9}, year = {2001}, language = {en} } @article{ReimersWisotzki2001, author = {Reimers, Dieter and Wisotzki, Lutz}, title = {The Hamburg all sky bright QSO surveys}, isbn = {1-5838-1127-3}, year = {2001}, language = {en} } @article{HaenischHeuerMenzel2001, author = {H{\"a}nisch, Christoph and Heuer, Axel and Menzel, Ralf}, title = {Threshold reduction of stimulated Brillouin scattering (SBS) using fiber loop schemes}, year = {2001}, language = {en} } @article{BeckStielLeupoldetal.2001, author = {Beck, Michael and Stiel, H. and Leupold, Dieter and Winter, Bernd and Pop, D. and Vogt, U. and Spitz, Christian}, title = {Evaluation of the energetic position of the lowest excited singlet state of ß-carotene by NEXAFS and photoemission spectroscopy}, year = {2001}, language = {en} } @article{SpitzDaehneOuartetal.2001, author = {Spitz, Christian and D{\"a}hne, Siegfried and Ouart, Andre and Abraham, Hans-Werner}, title = {Proof of Chirality of J-Aggregates Spontaneously and Enantioselectivily Generated from Achiral Dyes}, year = {2001}, language = {en} } @article{AfshaarvahidHeuerMenzeletal.2001, author = {Afshaarvahid, Shahraam and Heuer, Axel and Menzel, Ralf and Munch, Jesper}, title = {Temporal structure of stimulated-Brillouin-scattering reflectivity considering transversal-mode development}, year = {2001}, language = {en} } @article{WisotzkiChristliebLiuetal.2001, author = {Wisotzki, Lutz and Christlieb, N. and Liu, M. C. and Maza, J. and Morgan, N. D. and Schechter, P. L.}, title = {Discovery of a New Multiple Gravitationally Lensed QSO}, isbn = {1-583-81074-9}, year = {2001}, language = {en} } @article{KoopmansBruynWambsganssetal.2001, author = {Koopmans, L. V. E. and Bruyn, A. G. and Wambsganß, Joachim and Fassnacht, C. D.}, title = {A radio-microlensing caustic crossing in B1600+434?}, isbn = {1-538-81076-5}, year = {2001}, abstract = {First, we review the current status of the detection of strong `external' variability in the CLASS gravitational B1600+434, focusing on the 1998 VLA 8.5-GHz and 1998/9 WSRT multi-frequency observations. We show that this data can best be explained in terms of radio-microlensing. We then proceed to show some preliminary results from our new multi-frequency VLA monitoring program, in particular the detection of a strong feature (~30\%) in the light curve of the lensed image which passes predominantly through the dark-matter halo of the lens galaxy. We tentatively interpret this event, which lasted for several weeks, as a radio-microlensing caustic crossing, i.e. the superluminal motion of a micro-arcsec-scale jet-component in the lensed source over a single caustic in the magnification pattern, that has been created by massive compact objects along the line-of-sight to the lensed image.}, language = {en} } @article{KuhlbrodtWisotzkiJahnke2001, author = {Kuhlbrodt, B. and Wisotzki, Lutz and Jahnke, Knud}, title = {Two-dimensional modeling of AGN host galaxies}, isbn = {0-306-46662-7}, year = {2001}, language = {en} } @article{WisotzkiKuhlbrodtJahnke2001, author = {Wisotzki, Lutz and Kuhlbrodt, B. and Jahnke, Knud}, title = {The luminosity function of QSO host galaxies}, isbn = {0-306-46662-7}, year = {2001}, language = {en} } @article{ChristliebWisotzkiReimers2001, author = {Christlieb, N. and Wisotzki, Lutz and Reimers, Dieter}, title = {Mining the digital Hamburg / ESO objective prism survey}, isbn = {3-540-42468-7}, year = {2001}, language = {en} } @article{ChristliebWisotzkiReimersetal.2001, author = {Christlieb, N. and Wisotzki, Lutz and Reimers, Dieter and Homeier, D. and Koester, D. and Heber, Ulrich}, title = {The stellar content of the Hamburg/ESO survey : I. Automated selection of DA white dwarfs}, year = {2001}, abstract = {We describe automatic procedures for the selection of DA white dwarfs in the Hamburg/ESO objective-prism survey (HES). For this purpose, and the selection of other stellar objects (e.g., metal-poor stars and carbon stars), a flexible, robust algorithm for detection of stellar absorption and emission lines in the digital spectra of the HES was developed. Broad band (U-B, B-V) and intermediate band (Str{\"o}mgren c_1) colours can be derived directly from HES spectra, with precisions of sigma U-B=0.092 mag; sigma B-V=0.095 mag; sigma c_1=0.15 mag. We describe simulation techniques that allow one to convert model or slit spectra to HES spectra. These simulated objective-prism spectra are used to determine quantitative selection criteria, and for the study of selection functions. We present an atlas of simulated HES spectra of DA and DB white dwarfs. Our current selection algorithm is tuned to yield maximum efficiency of the candidate sample (minimum contamination with non-DAs). DA candidates are selected in the B-V versus U-B and c_1 versus W_lambda (Hbeta +Hgamma +Hdelta ) parameter spaces. The contamination of the resulting sample with hot subdwarfs is expected to be as low as ~ 8\%, while there is essentially no contamination with main sequence or horizontal branch stars. We estimate that with the present set of criteria, ~ 80\% of DAs present in the HES database are recovered. A yet higher degree of internal completeness could be reached at the expense of higher contamination. However, the external completeness is limited by additional losses caused by proper motion effects and the epoch differences between direct and spectral plates used in the HES. Based on observations collected at the European Southern Observatory, La Silla and Paranal, Chile.}, language = {en} } @article{WolfDyeKleinheinrichetal.2001, author = {Wolf, C. and Dye, S. and Kleinheinrich, M. and Meisenheimer, Klaus and Rix, Hans-Walter and Wisotzki, Lutz}, title = {Deep BVR photometry of the Chandra Deep Field South from the COMBO-17 survey}, year = {2001}, abstract = {We report on deep multi-color imaging (R5sigma = 26) of the Chandra Deep Field South, obtained with the Wide Field Imager (WFI) at the MPG/ESO 2.2 m telescope on La Silla as part of the multi-color survey COMBO-17. As a result we present a catalogue of 63 501 objects in a field measuring 31farcm5 x 30arcmin with astrometry and BVR photometry. A sample of 37 variable objects is selected from two-epoch photometry. We try to give interpretations based on color and variation amplitude.}, language = {en} } @article{Wambsganss2001, author = {Wambsganß, Joachim}, title = {Gravitational lensing as a universal astrophysical tool}, isbn = {1-563-96986-6}, year = {2001}, language = {en} } @article{GoicoecheaGilMerinoSerraRicartetal.2001, author = {Goicoechea, Luis Julian and Gil-Merino, Rodrigo and Serra-Ricart, M. and Oscoz, Alejandro and Alcalde, D. and Mediavilla, E.}, title = {IAC gravitational lenses monitoring program : difference signals from 2.5 years of QSO 0957+561 observations}, isbn = {0-7923-6974-2}, year = {2001}, language = {en} } @article{Wambsganss2001, author = {Wambsganß, Joachim}, title = {Gravitational lensing as a universal astrophysical tool}, year = {2001}, language = {en} } @article{Wambsganss2001, author = {Wambsganß, Joachim}, title = {Cosmological microlensing}, isbn = {1-583-81076-5}, year = {2001}, language = {en} } @article{GreggSchechterBeckeretal.2001, author = {Gregg, M. D. and Schechter, P. and Becker, Rolf and Wisotzki, Lutz and Maza, J. and Laurent-Muehleisen, S. and White, R.}, title = {Two new high resolution surveys for lensed quasars}, isbn = {1-583-81074-9}, year = {2001}, language = {en} } @article{Wambsganss2001, author = {Wambsganß, Joachim}, title = {Quasar Microlensing}, isbn = {1-583-81074-9}, year = {2001}, language = {en} } @article{Wisotzki2001, author = {Wisotzki, Lutz}, title = {The evolution of optically selected QSOs}, isbn = {1-5838-1127-3}, year = {2001}, language = {en} } @article{MukhopadhyayDattaSanyaletal.2001, author = {Mukhopadhyay, M. K. and Datta, A. and Sanyal, M. K. and Geue, Thomas and Pietsch, Ullrich}, title = {Synchrotron Studies of Melting of Langmuir-Blodgett Films}, year = {2001}, language = {en} } @article{EichhornBruceGuillonetal.2001, author = {Eichhorn, Holger and Bruce, Duncan W. and Guillon, D. and Gallani, Jean Louis and Fischer, Thomas and Stumpe, Joachim and Geue, Thomas}, title = {Metal ion mediated mesomorphism and thin film behaviour of amphitropic tetraazaporphyrin complexes}, year = {2001}, language = {en} } @article{MarwanKurths2001, author = {Marwan, Norbert and Kurths, J{\"u}rgen}, title = {Nonlinear analysis of bivariate data with cross recurrence plots}, year = {2001}, abstract = {We use the extension of the method of recurrence plots to cross recurrence plots (CRP) which enables a nonlinear analysis of bivariate data. To quantify CRPs, we develop further three measures of complexity mainly basing on diagonal structures in CRPs. The CRP analysis of prototypical model systems with nonlinear interactions demonstrates that this technique enables to find these nonlinear interrelations from bivariate time series, whereas linear correlation tests do not. Applying the CRP analysis to climatological data, we find a complex relationship between rainfall and El Nino data.}, language = {en} } @article{EllingPinnowDanzetal.2001, author = {Elling, B. and Pinnow, M. and Danz, Rudi and Wegener, Michael and Wirges, werner and Gerhard, Reimund}, title = {Coating of porous polytetrafluoroethylene films with other polymers for electret applications}, isbn = {0-7803-7053-8}, year = {2001}, language = {en} } @article{Herwig2001, author = {Herwig, Falk}, title = {Internal mixing and surface abundance of [WC]-CSPN}, year = {2001}, abstract = {Recent advances in constructing stellar evolution models of hydrogen-deficient post-asymptotic giant branch (AGB) stars are presented. Hydrogen-deficiency can originate from mixing and subsequent convective burning of protons in the deeper layers during a thermal pulse on the post-AGB (VLTP). Dredge-up alone may also be responsible for hydrogen- deficiency of post-AGB stars. Models of the last thermal pulse on the AGB with very small envelope masses have shown efficient third dredge-up. The hydrogen content of the envelope is diluted sufficiently to produce H-deficient post-AGB stars (AFTP). Moreover, dredge-up alone may also cause H-deficiency during the Born-again phase (LTP). During the second AGB phase a convective envelope develops. A previously unknown lithium enrichment at the surface of Born-again stellar models may be used to distinguish between objects with different post-AGB evolution. The observed abundance ratios of C, O and He can be reproduced by all scenarios if an AGB starting model with inclusion of overshoot is used for the post- AGB model sequence. An appendix is devoted to the numerical methods for models of proton capture nucleosynthesis in the He-flash convection zone during a thermal pulse.}, language = {en} } @article{SpahnPetzschmannSchmidtetal.2001, author = {Spahn, Frank and Petzschmann, Olaf and Schmidt, J{\"u}rgen and Sremcevic, Miodrag and Hertzsch, Jan-Martin}, title = {About the viscosity of granular gases : the force-free case versus granular gases under Keplarian differential rotation}, isbn = {3-540-41458-4}, year = {2001}, language = {en} } @article{FruebingBlischkeSalahKhaliletal.2001, author = {Fr{\"u}bing, Peter and Blischke, D. and Salah Khalil, M. and Gerhard, Reimund}, title = {Dielectric relaxations in titanium-dioxide-filled polyethylene: Filler-induced chemical modifications of the polymer provide the dipolar response}, isbn = {0-7803-7053-8}, year = {2001}, language = {en} } @article{GilMerinoGoicoecheaSerraRicartetal.2001, author = {Gil-Merino, Rodrigo and Goicoechea, Luis Julian and Serra-Ricart, M. and Oscoz, Alejandro and Alcalde, D. and Mediavilla, E.}, title = {Short time-scale fluctuations in the difference light curves of QSO 0957+561A,B: microlensing or noise?}, year = {2001}, abstract = {From optical R-band data of the double quasar QSO 0957+561A,B, we made two new difference light curves (with an overlap of about 330d between the time-shifted light curve for the A image and the magnitude-shifted light curve for the B image). We observed noisy behaviours around the zero line and no short time-scale events (with a duration of months), where the term `event' refers to a prominent feature that may be a result of microlensing or another source of variability. Only one event lasting two weeks and rising -33mmag was found. Measured constraints on the possible microlensing variability can be used to obtain information on the granularity of the dark matter in the main lensing galaxy and the size of the source. In addition, one can also test the ability of the observational noise to cause the rms averages and the local features of the difference signals. We focused on this last issue. The combined photometries were related to a process consisting of an intrinsic signal plus a Gaussian observational noise. The intrinsic signal has been assumed to be either a smooth function (polynomial), a smooth function plus a stationary noise process, or a correlated stationary process. Using these three pictures without microlensing, we derived some models totally consistent with the observations. We finally discussed the sensitivity of our telescope (at Teide Observatory) to several classes of microlensing variability.}, language = {en} } @article{SchindlerCastilloMoralesDeFilippisetal.2001, author = {Schindler, Sabine and Castillo-Morales, A. and De Filippis, E. and Schwope, A. D. and Wambsganß, Joachim}, title = {Discovery of depressions in the X-ray emission of the distant galaxy cluster RBS797 in a CHANDRA observation}, year = {2001}, abstract = {We present CHANDRA observations of the X-ray luminous, distant galaxy cluster RBS797 at z=0.35. In the central region the X-ray emission shows two pronounced X-ray minima, which are located opposite to each other with respect to the cluster centre. These depressions suggest an interaction between the central radio galaxy and the intra-cluster medium, which would be the first detection in such a distant cluster. The minima are symmetric relative to the cluster centre and very deep compared to similar features found in a few other nearby clusters. A spectral and morphological analysis of the overall cluster emission shows that RBS797 is a hot cluster (T=7.7+1.2-1.0 keV) with a total mass of Mtot(r500)= 6.5+1.6-1.2 *E14Msun.}, language = {en} } @article{HamannBrownFeldmeieretal.2001, author = {Hamann, Wolf-Rainer and Brown, John C. and Feldmeier, Achim and Oskinova, Lida}, title = {On the wavelength drift of spectral features from structured hot star winds}, year = {2001}, abstract = {Spectral lines formed in stellar winds from OB stars are observed to exhibit profile variations. Discrete Absorption Components (DACs) show a remarkably slow wavelength drift with time. In a straightforward interpretation, this is in sharp contradiction to the steep velocity law predicted by the radiation-driven wind theory, and by semi- empirical profile fitting. In the present paper we re-discuss the interpretation of the drift rate. We show that the Co- rotating Interaction Region (CIR) model for the formation of DACs does not explain their slow drift rate as a consequence of rotation. On the contrary, the apparent acceleration of a spectral CIR feature is even higher than for the corresponding kinematical model without rotation. However, the observations can be understood by distinguishing between the velocity field of the matter flow, and the velocity law for the motion of the patterns in which the DAC features are formed. If the latter propagate upstream against the matter flow, the resulting wavelength drift mimics a much slower acceleration although the matter is moving fast. Additional to the DACs, a second type of recurrent structures is present in observed OB star spectra, the so-called modulations. In contrast to the DACs, these structures show a steep acceleration compatible with the theoretically predicted velocity law. We see only two possible consistent scenarios. Either, the wind is accelerated fast, and the modulations are formed in advected structures, while the DACs come from structures which are propagating upstream. Or, alternatively, steep and shallow velocity laws may co-exist at the same time in different spatial regions or directions of the wind.}, language = {en} } @article{BergstedtHornMichelskisetal.2001, author = {Bergstedt, Christel and Horn, Martin Erik and Michelskis, Helmut and Winter, Rolf}, title = {Energie}, series = {Naturwissenschaften : Biologie, Chemie, Physik}, volume = {8}, journal = {Naturwissenschaften : Biologie, Chemie, Physik}, publisher = {Volk-und-Wissen-Verl.}, address = {Berlin}, isbn = {3-06-020935-9}, pages = {64 S.}, year = {2001}, language = {de} } @article{ChristliebGreenWisotzkietal.2001, author = {Christlieb, N. and Green, P. J. and Wisotzki, Lutz and Reimers, Dieter}, title = {The stellar content of the Hamburg/ESO survey : II. a large, homogeneously-selected sample of high latitude carbon stars}, year = {2001}, language = {en} } @article{Wambsganss2001, author = {Wambsganß, Joachim}, title = {Gravity's kaleidoscope : four consequences of gravitational light deflection}, year = {2001}, language = {en} } @article{Wambsganss2001, author = {Wambsganß, Joachim}, title = {Microlensing of Quasars}, year = {2001}, abstract = {Microlens-induced variability in multiple quasars can be used to study two cosmological issues of great interest, the size and brightness profile of quasars on one hand, and the distribution of compact (dark) matter along the line of sight on the other. Here a summary is given of recent theoretical progress as well as observational evidence for quasar microlensing, plus a discussion of desired observations and required theoretical studies.}, language = {en} } @article{KoopmansWambsganss2001, author = {Koopmans, L. V. E. and Wambsganß, Joachim}, title = {On the probability of microlensing in gamma-ray burst afterglows}, year = {2001}, abstract = {The declining light curve of the optical afterglow of gamma-ray burst (GRB) GRB000301C showed rapid variability with one particularly bright feature at about t-t0=3.8d. This event was interpreted as gravitational microlensing by Garnavich, Loeb \& Stanek and subsequently used to derive constraints on the structure of the GRB optical afterglow. In this paper, we use these structural parameters to calculate the probability of such a microlensing event in a realistic scenario, where all compact objects in the universe are associated with observable galaxies. For GRB000301C at a redshift of z=2.04, the a posteriori probability for a microlensing event with an amplitude of m>=0.95mag (as observed) is 0.7 per cent (2.7 per cent) for the most plausible scenario of a flat -dominated Friedmann- Robertson-Walker (FRW) universe with m=0.3 and a fraction f*=0.2 (1.0) of dark matter in the form of compact objects. If we lower the magnification threshold to m>=0.10mag, the probabilities for microlensing events of GRB afterglows increase to 17 per cent (57 per cent). We emphasize that this low probability for a microlensing signature of almost 1mag does not exclude that the observed event in the afterglow light curve of GRB000301C was caused by microlensing, especially in light of the fact that a galaxy was found within 2arcsec from the GRB. In that case, however, a more robust upper limit on the a posteriori probability of ~5 per cent is found. It does show, however, that it will not be easy to create a large sample of strong GRB afterglow microlensing events for statistical studies of their physical conditions on microarcsec scales.}, language = {en} } @article{WellsteinLangerBraun2001, author = {Wellstein, Stephan and Langer, Norbert and Braun, H.}, title = {Formation of contact in massive close binaries}, year = {2001}, language = {en} } @article{AbelCelaniVergenietal.2001, author = {Abel, Markus and Celani, A. and Vergeni, D. and Vulpiani, A.}, title = {Front propagation in laminar flows}, year = {2001}, abstract = {The Problem of front propagation in flowing media is addressed for laminar velocity fields in two dimensions. Three representative cases are discussed: stationary cellular flow, stationary shear flow, and percolating flow. Production terms of Fisher-Kolmogorov-Petrovskii-Piskunov type and of Arrhenius type are considered under the assumption of no feedback of the concentration on the velocity. Numerical simulations of advection-reaction-diffusion equations have been performed by an algorithm based on discrete-time maps. The results show a generic enhancement of the speed of front propagation by the underlying flow. For small molecular diffusivity, the front speed Vf depends on the typical flow velocity U as a power law with an exponent depending on the topological properties of the flow, and on the ratio of reactive and advective time scales. For open-streamline flows we find always Vf~U, whereas for cellular flows we observe Vf~U1/4 for fast advection and Vf~U3/4 for slow advection.}, language = {en} } @article{GreffetCarminatiJoulainetal.2001, author = {Greffet, Jean-Jacques and Carminati, R{\´e}mi and Joulain, Karl and Mulet, J. P. and Henkel, Carsten}, title = {Coherence properties of thermal near fields : implications for nanometer-scale radiative transfer}, year = {2001}, abstract = {With the recent development of local (optical and thermal) probe microscopy and the advent of nanotechnology, it seems necessary to revisit the old subject of coherence properies of thermal sources of light. Concerning temporal coherence, we show that thermal sources may produce quasi-monochromatic near fields. In light of this result, the possibility of perfoming near-field solid-state spectroscopy and of designing near-field infrared sources is discussed. The problem of radiative transfer between two thermal sources held at subwavelength distance is studied. The radiative flux may be enhanced by several orders of magnitude due to the excitation of resonant surface waves, and this may occur at particular frequencies. Finally, we study the spatial coherence of thermal sources and the substantial influence of the near field. Surface waves may induce long-range spatial correlation, on a scale much larger than the wavelength. Conversely, quasi-static contributions, as well as skin-layer currents, induce arbitary small correlations. With the recent development of local (optical and thermal) probe microscopy and the advent of nanotechnology, it seems necessary to revisit the old subject of coherence of thermal fields.}, language = {en} } @article{AudenaertEisertJaneetal.2001, author = {Audenaert, Katrien and Eisert, Jens and Jane, E. and Plenio, M. B. and Virmani, S. and Moor, B. D.}, title = {The asymptotic relative entropy of entanglement}, year = {2001}, abstract = {We present an analytical formula for the asymptotic relative entropy of entanglement for Werner states of arbitrary dimensionality. We then demonstrate its validity using methods from convex optimization. This is the first case in which the value of a subadditive entanglement measure has been obtained in the asymptotic limit. This formula also gives the sharpest known upper bound on the distillable entanglement of these states.}, language = {en} } @article{IvanovNuenesAmaralGoldbergeretal.2001, author = {Ivanov, Plamen Ch. and Nuenes Amaral, Lu{\´i}s A. and Goldberger, Ary L. and Havlin, Shlomo and Rosenblum, Michael and Stanley, H. Eugene and Struzik, Zbigniew R.}, title = {From 1/f noise to multifractal cascades in heartbeat dynamics}, issn = {1054-1500}, year = {2001}, language = {en} } @article{AvilovLepeshovPietschetal.2001, author = {Avilov, Anatoly S. and Lepeshov, G. G. and Pietsch, Ullrich and Tsirelson, Vladimir G.}, title = {Multipole analysis of the electron density and electrostatic potential in Germanium by high-resolution electron diffraction}, issn = {0022-3697}, year = {2001}, language = {en} } @article{DavaasambuuDanielStahnetal.2001, author = {Davaasambuu, Jav and Daniel, A. and Stahn, Jochen and Pietsch, Ullrich}, title = {Comparison of experimental and theoretical valence charge densites in cubic ZnSe}, issn = {0022-3697}, year = {2001}, language = {en} } @article{GrenzerDarowskiGeueetal.2001, author = {Grenzer, J{\"o}rg and Darowski, Nora and Geue, Thomas and Pietsch, Ullrich and Daniel, A. and Rennon, Siegfried and Reithmaier, Johann-Peter and Forchel, Alfred}, title = {Strain analysis and quantum well intermixing of a laterally modulated multiquantum well system produced by focused ion beam implantation}, year = {2001}, language = {en} } @article{ZeimerGrenzerPietschetal.2001, author = {Zeimer, Ute and Grenzer, J{\"o}rg and Pietsch, Ullrich and Bugge, F. and Smirnitzki, V. and Weyers, Markus}, title = {Investigation of strain-modulated InGaAs-nanostructures by grazing-incidence x-ray diffraction and photoluminescence}, year = {2001}, language = {en} } @article{ZeimerBuggeGramlichetal.2001, author = {Zeimer, Ute and Bugge, F. and Gramlich, S. and Smirnitzki, V. and Weyers, Markus and Tr{\"a}nkle, G. and Grenzer, J{\"o}rg and Pietsch, Ullrich and Cassabois, G. and Emiliani, V. and Linau, Christoph}, title = {Evidence of strain-induced lateral carrier confinement in InGaAs-quantum wells by low-temperature near-field spectroscopy}, year = {2001}, language = {en} } @article{PietschGrenzerGeueetal.2001, author = {Pietsch, Ullrich and Grenzer, J{\"o}rg and Geue, Thomas and Neißendorfer, Frank and Brezesinski, Gerald and Symietz, Christian and M{\"o}hwald, Helmuth and Gudat, Wolfgang}, title = {The energy dispersive reflectometer at BESSY II : a challenge for thin film analysis}, issn = {0167- 5087}, year = {2001}, language = {en} } @article{SchmidtSaloSpahn2001, author = {Schmidt, J{\"u}rgen and Salo, H. and Spahn, Frank}, title = {Viscous overstability in Saturn's B ring : II. Hydrodynamic theory and comparison to simulations}, year = {2001}, language = {en} } @article{Rosenblum2001, author = {Rosenblum, Michael}, title = {Comment on "Intermittency in chaotic rotations"}, year = {2001}, language = {en} } @article{PietschZeimerHofmannetal.2001, author = {Pietsch, Ullrich and Zeimer, Ute and Hofmann, L. and Grenzer, J{\"o}rg and Gramlich, S.}, title = {Strain induced compositional modulations in AlGaAs overlayers induced by lateral surface gratings}, issn = {0272-9172}, year = {2001}, language = {en} } @article{KuhlbrodtTitzFeudeletal.2001, author = {Kuhlbrodt, Till and Titz, Sven Holger and Feudel, Ulrike and Rahmstorf, Stefan}, title = {A simple model of seasonal open ocean convection. Part II: Labrador Sea stability and stochastic forcing}, issn = {1616-7341}, year = {2001}, abstract = {Aspects of open ocean deep convection variability are explored with a two-box model. In order to place the model in a region of parameter space relevant to the real ocean, it is fitted to observational data from the Labrador Sea. A systematic fit to OWS Bravo data allows us to determine the model parameters and to locate the position of the Labrador Sea on a stability diagram. The model suggests that the Labrador Sea is in a bistable regime where winter convection can be either ?on? or ?off?, with both these possibilities being stable climate states. When shifting the surface buoyancy forcing slightly to warmer or fresher conditions, the only steady solution is one without winter convection. We then introduce short-term variability by adding a noise term to the surface temperature forcing, turning the box model into a stochastic climate model. The surface forcing anomalies generated in this way induce jumps between the two model states. These state transitions occur on the interannual to decadal timescale. Changing the average surface forcing towards more buoyant conditions lowers the frequency of convection. However, convection becomes more frequent with stronger variability in the surface forcing. As part of the natural variability, there is a non-negligible probability for decadal interruptions of convection. The results highlight the role of surface forcing variability for the persistence of convection in the ocean.}, language = {en} } @article{DavaasambuuDanielStahnetal.2001, author = {Davaasambuu, Jav and Daniel, A. and Stahn, Jochen and Pietsch, Ullrich}, title = {Harmonic and anharmonic thermal vibrations in cubic ZnSe}, issn = {0044-2968}, year = {2001}, language = {en} } @article{WegenerWirgesRichteretal.2001, author = {Wegener, Michael and Wirges, Werner and Richter, Kristin and K{\"u}nstler, Wolfgang and Gerhard, Reimund}, title = {Charge stability and piezoelectric properties of porous fluoro-polymer space-charge electrets in layer systems}, year = {2001}, language = {en} } @article{TsirelsonAvilovLepeshovetal.2001, author = {Tsirelson, Vladimir G. and Avilov, Anatoly S. and Lepeshov, G. G. and Kuligin, A. K. and Stahn, Jochen and Pietsch, Ullrich and Spence, J. C. H.}, title = {Quantitative Analysis of the Inner-Crystal Electrostatic Potential of several Rock-Salt-Structure Crystals Using Accurate Electron-Diffraction Data}, issn = {1089-5647}, year = {2001}, language = {en} } @article{KrivovBanaszkiewicz2001, author = {Krivov, Alexander V. and Banaszkiewicz, Marek}, title = {Dust Influx to Titan from Hyperion}, isbn = {0-7923-6946-7}, year = {2001}, language = {en} } @article{PaajanenWegenerGerhard2001, author = {Paajanen, Mika and Wegener, Michael and Gerhard, Reimund}, title = {Understanding the role of the gas in the voids during corona charging of cellular electret films : a way to enhance their piezoelectricity}, year = {2001}, language = {en} } @article{ZaikinKurths2001, author = {Zaikin, Alexei A. and Kurths, J{\"u}rgen}, title = {Additive noise in noise-induced nonequilibrium transitions}, issn = {1054-1500}, year = {2001}, language = {en} } @article{KrivovBanaszkiewicz2001, author = {Krivov, Alexander V. and Banaszkiewicz, Marek}, title = {Unusual origin, evolution and fate of icy ejecta from Hyperion}, issn = {0032-0633}, year = {2001}, language = {en} } @article{FruebingBlischkeSalahKhaliletal.2001, author = {Fr{\"u}bing, Peter and Blischke, D. and Salah Khalil, M. and Gerhard, Reimund}, title = {Complete relaxation map of polyethylene : filler-induced chemical modifications as dielectric probes}, year = {2001}, language = {en} } @article{LandaZaikinSchimanskyGeier2001, author = {Landa, Polina S. and Zaikin, Alexei A. and Schimansky-Geier, Lutz}, title = {Effect of the potential shape and of a Brownian particle mass on noise-induced transport}, year = {2001}, language = {en} } @article{ZaikinMuraliKurths2001, author = {Zaikin, Alexei A. and Murali, K. and Kurths, J{\"u}rgen}, title = {Simple electronic circuit model for doubly stochastic resonance}, year = {2001}, abstract = {We have recently reported the phenomenon of doubly stochastic resonance [Phys. Rev. Lett. 85, 227 (2000)], a synthesis of noise-induced transition and stochastic resonance. The essential feature of this phenomenon is that multiplicative noise induces a bimodality and additive noise causes stochastic resonance behavior in the induced structure. In the present paper we outline possible applications of this effect and design a simple lattice of electronic circuits for the experimental realization of doubly stochastic resonance.}, language = {en} } @article{SaloSchmidtSpahn2001, author = {Salo, H. and Schmidt, J{\"u}rgen and Spahn, Frank}, title = {Viscous overstability in Saturn's B ring : I. Direct simulations and measurements of transport coefficients}, year = {2001}, language = {en} } @article{EisertBriegel2001, author = {Eisert, Jens and Briegel, Hans J.}, title = {Schmidt measure as a tool for quantifying multiparicle entanglement}, year = {2001}, abstract = {We present a measure of quantum entanglement which is capable of quantifying the degree of entanglement of a multi-partite quantum system. This measure, which is based on a generalization of the Schmidt rank of a pure state, is defined on the full state space and is shown to be an entanglement monotone, that is, it cannot increase under local quantum operations with classical communication and under mixing. For a large class of mixed states this measure of entanglement can be calculated exactly, and it provides a detailed classification of mixed states.}, language = {en} } @article{PikovskijPoliti2001, author = {Pikovskij, Arkadij and Politi, Antonio}, title = {Dynamic localization of Lyapunov vectors in Hamiltonian lattices}, year = {2001}, language = {en} } @article{MellingerWegenerWirgesetal.2001, author = {Mellinger, Axel and Wegener, Michael and Wirges, Werner and Gerhard, Reimund}, title = {Thermally stable dynamic piezoelectricity in sandwich films of porous and non-porous amorphous fluoropolymer}, year = {2001}, language = {en} } @article{MarwanNowaczykKurthsetal.2001, author = {Marwan, Norbert and Nowaczyk, Norbert R. and Kurths, J{\"u}rgen and Thiel, Marco}, title = {Cross recurrence plot based rescaling of geological time series}, issn = {1029-7006}, year = {2001}, abstract = {The rescaling of geological data series to a geological reference time series is of major interest in many investigations. For example, geophysical borehole data should be correlated to a given data series whose time scale is known in order to achieve an age-depth function or the sedimentation rate for the borehole data. Usually this synchronization is performed visually and by hand. Instead of using this wiggle matching by eye, we present the application of cross recurrence plots for such tasks. Using this method, the synchronization and rescaling of geological data to a given time scale is much easier and faster than by hand.}, language = {en} } @article{RaabKurths2001, author = {Raab, Corinna and Kurths, J{\"u}rgen}, title = {Estimation of Large-Scale Dimension Densities}, year = {2001}, abstract = {We propose a technique to calculate large-scale dimension densities in both higher-dimensional spatio-temporal systems and low-dimensional systems from only a few data points, where known methods usually have an unsatisfactory scaling behavior. This is mainly due to boundary and finite size effects. With our rather simple method we normalize boundary effects and get a significant correction of the dimension estimate. This straightforward approach is basing on rather general assumptions. So even weak coherent structures obtained from small spatial couplings can be detected with this method, what is impossible by using the Lyapunov-dimension density. We demonstrate the efficiency of our technique for coupled logistic maps, coupled tent maps, the Lorenz-attractor and the Roessler-attractor.}, language = {en} } @article{Koesterke2001, author = {Koesterke, Lars}, title = {Spectral analyses of WR-type central stars of Planetary Nebulae}, year = {2001}, abstract = {So far, the evolution of post-AGB stars is not fully understood. In particular the formation of hydrogen- deficient and hydrogen-free Central Stars of Planetary Nebulae (CSPN) is unsettled. New evolution models, which allow for the consistent treatment of the physics of late thermal pulses, promise new insights to the formation of these stars. In this paper we summarize the results of non-LTE analyses of CSPN with wind. By comparing these results with the predictions of the new evolution models, open questions concerning the evolution of the stars might be answered. In addition we discuss the driving mechanism of the winds of Wolf-Rayet CSPN. New models, which account for millions of iron lines, support the assumptions that these winds are driven by radiation.}, language = {en} } @article{Wambsganss2001, author = {Wambsganß, Joachim}, title = {Gravitational Lensing : a universal astrophysical tool}, year = {2001}, abstract = {In the roughly 20 years of its existence as an observational science, gravitational lensing has established itself as a valuable tool in many astrophysical fields. In the introduction of this review we briefly present the basics of lensing. Then it is shown that the two propagation effects, lensing and scintillation, have a number of properties in common. In the main part various lensing phenomena are discussed with emphasis on recent observations.}, language = {en} } @article{PopovychMaistrenkoMosekildeetal.2001, author = {Popovych, Orest and Maistrenko, Yu and Mosekilde, Erik and Pikovskij, Arkadij and Kurths, J{\"u}rgen}, title = {Transcritical riddling in a system of coupled maps}, year = {2001}, abstract = {The transition from fully synchronized behavior to two-cluster dynamics is investigated for a system of N globally coupled chaotic oscillators by means of a model of two coupled logistic maps. An uneven distribution of oscillators between the two clusters causes an asymmetry to arise in the coupling of the model system. While the transverse period-doubling bifurcation remains essentially unaffected by this asymmetry, the transverse pitchfork bifurcation is turned into a saddle-node bifurcation followed by a transcritical riddling bifurcation in which a periodic orbit embedded in the synchronized chaotic state loses its transverse stability. We show that the transcritical riddling transition is always hard. For this, we study the sequence of bifurcations that the asynchronous point cycles produced in the saddle-node bifurcation undergo, and show how the manifolds of these cycles control the magnitude of asynchronous bursts. In the case where the system involves two subpopulations of oscillators with a small mismatch of the parameters, the transcritical riddling will be replaced by two subsequent saddle-node bifurcations, or the saddle cycle involved in the transverse destabilization of the synchronized chaotic state may smoothly shift away from the synchronization manifold. In this way, the transcritical riddling bifurcation is substituted by a symmetry-breaking bifurcation, which is accompanied by the destruction of a thin invariant region around the symmetrical chaotic state.}, language = {en} } @article{StahnPietschBlahaetal.2001, author = {Stahn, Jochen and Pietsch, Ullrich and Blaha, Pawel and Schwarz, K. H.}, title = {Electric field induced charge-density variations in covalently bonded compounds}, year = {2001}, language = {en} } @article{PoloucekPietschGeueetal.2001, author = {Poloucek, P. and Pietsch, Ullrich and Geue, Thomas and Symietz, Christian and Brezesinski, Gerald}, title = {X-ray reflectivity analysis of thin complex Langmuir-Blodgett films}, year = {2001}, language = {en} } @article{AhlersZillmerPikovskij2001, author = {Ahlers, Volker and Zillmer, R{\"u}diger and Pikovskij, Arkadij}, title = {Lyapunov exponents in disordered chaotic systems : avoided crossing and level statistics}, year = {2001}, abstract = {The behavior of the Lyapunov exponents (LEs) of a disordered system consisting of mutually coupled chaotic maps with different parameters is studied. The LEs are demonstrated to exhibit avoided crossing and level repulsion, qualitatively similar to the behavior of energy levels in quantum chaos. Recent results for the coupling dependence of the LEs of two coupled chaotic systems are used to explain the phenomenon and to derive an approximate expression for the distribution functions of LE spacings. The depletion of the level spacing distribution is shown to be exponentially strong at small values. The results are interpreted in terms of the random matrix theory.}, language = {en} } @article{WesselMarwanMeyerfeldtetal.2001, author = {Wessel, Niels and Marwan, Norbert and Meyerfeldt, Udo and Schirdewan, Alexander and Kurths, J{\"u}rgen}, title = {Recurrence quantification analysis to characterise the heart rate variability before the onset of ventricular tachycardia}, year = {2001}, abstract = {Ventricular tachycardia or fibrillation (VT) as fatal cardiac arrhythmias are the main factors triggering sudden cardiac death. The objective of this recurrence quantification analysis approach is to find early signs of sustained VT in patients with an implanted cardioverter-defibrillator (ICD). These devices are able to safeguard patients by returning their hearts to a normal rhythm via strong defibrillatory shocks; additionally, they are able to store at least 1000 beat-to-beat intervals immediately before the onset of a life-threatening arrhythmia. We study the}, language = {en} } @article{RosenblumKurthsPikovskij2001, author = {Rosenblum, Michael and Kurths, J{\"u}rgen and Pikovskij, Arkadij}, title = {Comment on "Phase synchronization in discrete chaotic systems"}, year = {2001}, abstract = {Chen et al. [Phys. Rev. E 61, 2559 (2000)] recently proposed an extension of the concept of phase for discrete chaotic systems. Using the newly introduced definition of phase they studied the dynamics of coupled map lattices and compared these dynamics with phase synchronization of coupled continuous-time chaotic systems. In this paper we illustrate by two simple counterexamples that the angle variable introduced by Chen et al. fails to satisfy the basic requirements to the proper phase. Furthermore, we argue that an extension of the notion of phase synchronization to generic discrete maps is doubtful.}, language = {en} } @article{PikovskijRosenblum2001, author = {Pikovskij, Arkadij and Rosenblum, Michael}, title = {Detecting direction of coupling in interacting oscillators}, year = {2001}, abstract = {We propose a method for experimental detection of directionality of weak coupling between two self-sustained oscillators from bivariate data. The technique is applicable to both noisy and chaotic systems that can be nonidentical or even structurally different. We introduce an index that quantifies the asymmetry in coupling.}, language = {en} } @article{PikovskijNeumann2001, author = {Pikovskij, Arkadij and Neumann, Eireen}, title = {Comment on "Simple approach to the creation of a strange nonchaotic attractor in any chaotic system"}, year = {2001}, abstract = {We address the problem of existence of strange nonchaotic attractors (SNAs) in quasiperiodically forced dynamical systems. Recently, Shuai and Wong [Phys. Rev. E 59, 5338 (1999)] suggested a universal method for constructing a SNA in an arbitrary system possessing chaos. We demonstrate here that, in general, this method fails. For arbitrary systems, it gives a SNA only in a vicinity of transition to chaos. We discuss also a special example, where the method by Shuai and Wong indeed produces a SNA.}, language = {en} } @article{PikovskijRosenblum2001, author = {Pikovskij, Arkadij and Rosenblum, Michael}, title = {Comment on "Intermittency in chaotic rotations"}, year = {2001}, abstract = {Lai et al. [Phys. Rev. E 62, R29 (2000)] claim that the angular velocity of the phase point moving along the chaotic trajectory in a properly chosen projection (the instantaneous frequency) is intermittent. Using the same examples, namely the R{\"o}ssler and the Lorenz systems, we show the absence of intermittency in the dynamics of the instantaneous frequency.This is confirmed by demonstrating that the phase dynamics exhibits normal diffusion. We argue that the nonintermittent behavior is generic.}, language = {en} } @article{PikovskijPopovychMaistrenko2001, author = {Pikovskij, Arkadij and Popovych, Orest and Maistrenko, Yu}, title = {Resolving Clusters in Chaotic Ensembles of Globally Coupled Identical Oscillators}, year = {2001}, abstract = {Clustering in ensembles of globally coupled identical chaotic oscillators is reconsidered using a twofold approach. Stability of clusters towards "emanation" of the elements is described with the evaporation Lyapunov exponents. It appears that direct numerical simulations of ensembles often lead to spurious clusters that have positive evaporation exponents, due to a numerical trap. We propose a numerical method that surmounts the spurious clustering. We also demonstrate that clustering can be very sensitive to the number of elements in the ensemble.}, language = {en} } @article{TopajKyePikovskij2001, author = {Topaj, Dmitri and Kye, W.-H and Pikovskij, Arkadij}, title = {Transition to Coherence in Populations of Coupled Chaotic Oscillators: A Linear Response Approach}, year = {2001}, abstract = {We consider the collective dynamics in an ensemble of globally coupled chaotic maps. The transition to the coherent state with a macroscopic mean field is analyzed in the framework of the linear response theory. The linear response function for the chaotic system is obtained using the perturbation approach to the Frobenius-Perron operator. The transition point is defined from this function by virtue of the self-excitation condition for the feedback loop. Analytical results for the coupled Bernoulli maps are confirmed by the numerics.}, language = {en} } @article{TsimringPikovskij2001, author = {Tsimring, L. S. and Pikovskij, Arkadij}, title = {Noise-Induced Dynamics in Bistable Systems with Delay}, year = {2001}, abstract = {Noise-induced dynamics of a prototypical bistable system with delayed feedback is studied theoretically and numerically. For small noise and magnitude of the feedback, the problem is reduced to the analysis of the two-state model with transition rates depending on the earlier state of the system. Analytical solutions for the autocorrelation function and the power spectrum have been found. The power spectrum has a peak at the frequency corresponding to the inverse delay time, whose amplitude has a maximum at a certain noise level, thus demonstrating coherence resonance. The linear response to the external periodic force also has maxima at the frequencies corresponding to the inverse delay time and its harmonics.}, language = {en} } @article{HenkelPoetting2001, author = {Henkel, Carsten and P{\"o}tting, Sierk}, title = {Coherent transport of matter waves}, year = {2001}, abstract = {A transport theory for atomic matter waves in low-dimensional waveguides is outlined. The thermal fluctuation spectrum of magnetic near fields leaking out of metallic microstructures is estimated. The corresponding scattering rate for paramagnetic atoms turns out to be quite large in micrometer-sized waveguides (approx. 100/s). Analytical estimate for the heating and decoherence of a cold atom cloud are given. We finally discuss numerical and analytical results for the scattering from static potential imperfections and the ensuing spatial diffusion process.}, language = {en} } @article{Henkel2001, author = {Henkel, Carsten}, title = {Coherent transport}, issn = {1296-2147}, year = {2001}, abstract = {We discuss the transport of matter waves in low-dimensional waveguides. Due to scattering from uncontrollable noise fields, the spatial coherence gets reduced and eventually lost. We develop a description of this decoherence process in terms of transport equations for the atomic Wigner function. We outline its derivation and discuss the special case of white noise where an analytical solution can be found.}, language = {en} } @article{KoesterkeHamannUrrutia2001, author = {Koesterke, Lars and Hamann, Wolf-Rainer and Urrutia, Tanya}, title = {Line-Profile Variability in the Wolf-Rayet Stars WR 135 and WR 111}, year = {2001}, abstract = {We have obtained time-resolved observations of line-profile variations of the two Wolf-Rayet stars WR 135 and WR 111. The spectra, taken during two consecutive nights, cover a broad range from 4470 to 6590 Ang. The profile variability of the C iii emission line at 5696 Ang in WR 135 is shown in detail. The principal difficulties to constrain the velocity law from the frequency drift of discrete spectral features is discussed, emphasizing the crucial dependence on the adopted location of the line-emission region, and the possible necessity to distinguish between the motion of structures and the flow of the matter. - Full access to the observational data is provided via anonymous file transfer.}, language = {en} } @article{WegenerSeissGerhard2001, author = {Wegener, Michael and Seiß, Martin and Gerhard, Reimund}, title = {Prepariation and assessment of piezo- and pyroelectric poly- (vinylidene fluoride-hexafluoropropylene) copolymer films}, year = {2001}, language = {en} } @article{PietschStahnDavaasambuuetal.2001, author = {Pietsch, Ullrich and Stahn, Jochen and Davaasambuu, Jav and Pucher, Andreas}, title = {Electric field induced charge density variations in partially-ionic compounds}, issn = {0022-3697}, year = {2001}, language = {en} } @article{HennebergChiGeueetal.2001, author = {Henneberg, Oliver and Chi, Li Feng and Geue, Thomas and Saphiannikova, Marina and Pietsch, Ullrich and Rochon, Paul and Natansohn, Almeria}, title = {Atomic force microscopy inspection of the early state of formation of polymer surface relief grating}, year = {2001}, language = {en} } @article{PaajanenWegenerGerhard2001, author = {Paajanen, Mika and Wegener, Michael and Gerhard, Reimund}, title = {Charging of cellular space-charge electret films in various gas atmospheres}, isbn = {0-7803-7053-8}, year = {2001}, language = {en} } @article{HennebergGeueSaphiannikovaetal.2001, author = {Henneberg, Oliver and Geue, Thomas and Saphiannikova, Marina and Pietsch, Ullrich and Rochon, Paul and Natansohn, Almeria}, title = {Formation and dynamics of polymer surface relief gratings}, issn = {0378-5963}, year = {2001}, language = {en} } @article{RzazewskiGoralCironeetal.2001, author = {Rzazewski, Kazimierz and Goral, K. and Cirone, M. A. and Wilkens, Martin}, title = {Bose-Einstein Condensation of two interacting particles}, year = {2001}, abstract = {We investigate the notion of Bose-Einstein condensation of interacting particles. The definition of the condensate is based on the existence of the dominant eigenvalue of the single-particle density matrix. The statistical properies and the characteristics temperature are computed exactly in the soluble models of two interacting atoms.}, language = {en} } @article{FeudelRuedigerSeehafer2001, author = {Feudel, Fred and R{\"u}diger, Sten and Seehafer, Norbert}, title = {Bifurcation phenomena and dynamo effect in electrically conducting fluids}, year = {2001}, abstract = {Electrically conducting fluids in motion can act as self-excited dynamos. The magnetic fields of celestial bodies like the Earth and the Sun are generated by such dynamos. Their theory aims at modeling and understanding both the kinematic and dynamic aspects of the underlying processes. Kinematic dynamo models, in which for a prescribed flow the linear induction equation is solved and growth rates of the magnetic field are calculated, have been studied for many decades. But in order to get consistent models and to take into account the back-reaction of the magnetic field on the fluid motion, the full nonlinear system of the magnetohydrodynamic (MHD) equations has to be studied. It is generally accepted that these equations, i.e. the Navier-Stokes equation (NSE) and the induction equation, provide a theoretical basis for the explanation of the dynamo effect. The general idea is that mechanical energy pumped into the fluid by heating or other mechanisms is transferred to the magnetic field by nonlinear interactions. For two special helical flows which are known to be effective kinematic dynamos and which can be produced by appropriate external mechanical forcing, we review the nonlinear dynamo properties found in the framework of the full MHD equations. Specifically, we deal with the ABC flow (named after Arnold, Beltrami and Childress) and the Roberts flow (after G.~O. Roberts). The appearance of generic dynamo effects is demonstrated. Applying special numerical bifurcation-analysis techniques to high-dimensional approximations in Fourier space and varying the Reynolds number (or the strength of the forcing) as the relevant control parameter, qualitative changes in the dynamics are investigated. We follow the bifurcation sequences until chaotic states are reached. The transitions from the primary flows with vanishing magnetic field to dynamo-active states are described in particular detail. In these processes the stagnation points of the flows and their heteroclinic connections play a promoting role for the magnetic field generation. By the example of the Roberts flow we demonstrate how the break up of the heteroclinic lines after the primary bifurcation leads to a complicated intersection of stable and unstable manifolds forming a chaotic web which is in turn correlated with the spatial appearance of the dynamo.}, language = {en} } @article{DemircanSeehafer2001, author = {Demircan, Ayhan and Seehafer, Norbert}, title = {Dynamos in rotating and nonrotating convection in the form of asymmetric squares}, year = {2001}, abstract = {We study the dynamo properties of asymmetric square patterns in Boussinesq Rayleigh-B'enard convection in a plane horizontal layer. Cases without rotation and with weak rotation about a vertical axis are considered. There exist different types of solutions distinguished by their symmetry, among them such with flows possessing a net helicity and being capable of kinematic dynamo action in the presence as well as in the absence of rotation. In the nonrotating case these flows are, however, always only kinematic, not nonlinear dynamos. Nonlinearly the back-reaction of the magnetic field then forces the solution into the basin of attraction of a roll pattern incapable of dynamo action. But with rotation added parameter regions are found where the Coriolis force counteracts the Lorentz force in such a way that the asymmetric squares are also nonlinear dynamos.}, language = {en} } @article{SeehaferDemircanFeudel2001, author = {Seehafer, Norbert and Demircan, Ayhan and Feudel, Fred}, title = {Fluid helicity and dynamo effect}, year = {2001}, abstract = {Using the incompressible magnetohydrodynamic equations, we have numerically studied the dynamo effect in electrically conducting fluids. The necessary energy input into the system was modeled either by an explicit forcing term in the Navier-Stokes equation or fully selfconsistently by thermal convection in a fluid layer heated from below. If the fluid motion is capable of dynamo action, the dynamo effect appears in the form of a phase transition or bifurcation at some critical strength of the forcing. Both the dynamo bifurcation and subsequent bifurcations that occur when the strength of the forcing is further raised were studied, including the transition to chaotic states. Special attention was paid to the helicity of the flow as well as to the symmetries of the system and symmetry breaking in the bifurcations. The magnetic field tends to be accumulated in special regions of the flow, notably in the vicinity of stagnation points or near the boundaries of convection cells.}, language = {en} }