Recent advances in scanning tunneling microscopy involving magnetic probes and samples

We report on recent developments in the field of STM performed with magnetic probes and samples. The choice of appropriate magnetic sensors and their in situ preparation will be described. We further focus on the information obtained in spin-polarized scanning tunneling microscopy (SPSTM) as well as on possible modes of operation for simultaneous acquisition of topographic and magnetic data. The prospects for SPSTM and related magnetic sensitive SXM techniques will be discussed.Permanent address: Forschungszentrum Jülich, ISI, W-5170 Jülich 1, Fed. Rep. Germany     

Search for collective transverse flow using particle transverse momentum spectra in relativistic heavy-ion collisions

The particle transverse momentum spectra recently measured in relativistic heavy-ion collisions at CERN and BNL are analysed within an expanding fireball model. All the particle spectra at a given beam energy can be reproduced simultaneously with a single set of intensive parameters for the initial state of the fireball. As typical freeze-out parameters in this beam energy region we find a freeze-out temperatureT _f?110 MeV for most hadrons, and an average transverse expansion velocity at freeze-out of 〈v/c〉?0.4–0.45. The striking enhancement at transverse momentap _T<200 MeV/c in the CERN pion data cannot be fully explained by the existence of transverse flow.     

Interlayer pairing in high temperature superconductors: effect of nonmagnetic impurities

The influence of impurity scattering is studied theoretically in a two-layer model for the high-T _c superconductor Y₁Ba₂Cu₃O_7– with intra- and inter-layer pairing. Two types of impurities are considered: (I) impurities which conserve the reflection symmetry of the two layers and (II) impurities which break it. Impurities of type (I) have no influence on the critical temperature. Type (II) impurities have strong influence onT _c as well as onH _c2 if there is a pairing interaction between carriers of different layers. The treatment of type (II) impurities is generalized to a periodic layer model appropriate for La_2–x Ba _x CuO₄. Available experiments on impurities in Y₁Ba₂Cu₃O_7– and La_2–x Ba _x CuO₄ are interpreted with our theory.     

Magnetic circular dichroism in the luminescence ofF andF A centres in alkali halides

Summary The magnetic circular dichroism has been in the past an excellent tool to clarify the optical cycle of theF centres in alkali halides and the nature of the relaxed excited state. We have used such technique to study the weak doubleF emission found recently in NaI and the luminescence ofF andF _A centres (of both type I and II) in KCl. We have found that the long-wavelengthF emission in NaI and theF _A (I) emission in KCl∶Na⁺ behave like the well-known emission of theF centres, confirming the attribution of the above luminescence in NaI to the normal relaxed excited state. A completely different behaviour is displayed by the short-wavelengthF emission in NaI and by theF _A (II) emission in KCl∶Li⁺. The experimental data have been compared with the Ham and Grevsmuhl vibronic theory of the relaxed excited state.

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Riassunto Gli effetti di dicroismo magnetico circolare hanno costituito in passato un ottimo mezzo per chiarire il ciclo ottico dei centriF in alogenuri alcalini e la natura dello stato eccitato rilassato. Abbiamo usato tale tecnica per studiare la debole doppia emissioneF trovata recentemente in NaI e la luminescenza dei centriF edF _A (di tipo I e II) in KCl. Si è trovato che l'emissioneF a minore energia in NaI e l'emissioneF _A (I) in KCl∶Na⁺ si comportano come la ben nota emissione dei centriF, il che conferma l'attribuzione di tale luminescenza in NaI allo stato eccitato rilassato normale. L'emissioneF a energia maggiore in NaI e l'emissioneF _A (II) in KCl∶Li⁺ mostrano un comportamento completamente diverso. I dati sperimentali sono stati confrontati con la teoria vibronica di Ham e Grevsmuhl dello stato eccitato rilassato.

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Резюме Магнитный круговой дихроизм ранее применялся как инструмент для выяснения оптического циклаF центров в щелочных галогенидах и природы релаксирующего возбужденного состояния. В этой работе мы используем эту технику для исследования слабого двойного F излучения, обнаруженного недавно в NaI и при люминесценцииF иF _A центров (типа I и типа II) в KCl. Мы обнаружили, что длинноволновоеF излучения в NaI иF _A (I) излучение в KCl∶Na⁺ ведет себя как хорошо известное излучениеF центров, что подтверждает приписывание вышеуказанной люминесценции в NaI нормальному релаксирующему возбужденному состоянию. Полностью отличное поведение обнаруживает коротковолновоеF излучение в NaI иF _A (II) излучение в KCl∶Li⁺. Полученные экспериментальные данные сравниваются с вибрационной теорией Гама и Гревсмула для релаксирующего возбужденного состояния.

     

Numerical studies on the anderson localization problem

The electron localization is studied for Anderson's tight-binding model with diagonal and off-diagonal disorder for a very large square lattice (10,000 sites) and diamond lattice (27,000 sites). The numerical investigations are based on the Lanczos recursion method. The convergence of the recursion coefficientsa _n ,b _n is discussed with regard to the electron localization.From Anderson's criterion and an exact real space renormalization method the energy of the localization edge is found as a function of the degree of disorder. Also the dependence of the spatial decay rate of localized wave functions on the energy and the degree of disorder is evaluated. Near the Anderson transition, where all states become localized, we get two critical exponentsv _E andv _W , which lead us to the tentative suggestion of multicritical scaling laws for this transition.     

The (d,p) Reactions below 3.0 MeV

Excitation functions for a number of proton groups for the reactions ²³Na(d, p)²⁴Na and ²⁷Al(d, p)²⁸Al were measured in the energy range E_d = 1.5 to 3.0 MeV. The angular distributions for a number of proton groups were measured at a number of deuteron energies below 3.0 MeV for the three reactions ¹²C(d, p)¹³C, ²³Na(d, p)²⁴Na and ²⁷Al(d, p)²⁸Al. The theory of deuteron stripping reactions at incident energies below the Coulomb barrier has been considered. A closed analytical form for the differential cross-section has been obtained considering three cases according to the incident deuteron and outgoing proton energies. An attempt has been made to fit the angular distribution measurements at a number of deuterons bombarding energies accordingly.     

Maximal operators of Fejér means of double Walsh-Fourier series

The main aim of this paper is to prove that the maximal operator of the Fejér mean of the double Walsh-Fourier series is not bounded from the Hardy space H _1/2 to the space weak-L _1/2. This paper was written during the visit of the author at the College of Nyíregyháza in Hungary.     

Behavior of concrete cylinders confined by carbon-composite tapes and prestressed yarns 1. Experimental data

The results of a comprehensive test program for the mechanical behavior of round concrete specimens confined by carbon-fiber epoxy tapes and prestressed carbon yarns are reported. Five different concrete batches of compressive strength from 20 to 100 MPa and the confinement of various thickness and pretension level were investigated. The specimens, which were subjected to monotonic or repeated compressive loadings, showed a nonlinear stress-strain behavior with a significant ductility and increased ultimate strength, in contrary to the brittle behavior of plain concrete. The limit of linearity on the stress-strain diagrams of the confined concrete roughly coincided with the ultimate strength of plain concrete. Above this limit, the damage accumulation and plastic deformations proceeded in the confined specimens. This fact was evidenced by the increasing slope of deformation diagrams in unloadings and repeated loadings and by the pronounced residual strains. The limit of linearity could be raised significantly by pretension of the carbon yarns during their winding. Russian translation published in Mekhanika Kompozitnykh Materialov, Vol. 42, No. 1, pp. 21–44, January–February, 2006.     

Elastic capsules in shear flow: Analytical solutions for constant and time-dependent shear rates

We investigate the dynamics of microcapsules in linear shear flow within a reduced model with two degrees of freedom. In previous work for steady shear flow, the dynamic phases of this model, i.e. swinging, tumbling and intermittent behaviour, have been identified using numerical methods. In this paper, we integrate the equations of motion in the quasi-spherical limit analytically for time-constant and time-dependent shear flow using matched asymptotic expansions. Using this method, we find analytical expressions for the mean tumbling rate in general time-dependent shear flow. The capsule dynamics is studied in more detail when the inverse shear rate is harmonically modulated around a constant mean value for which a dynamic phase diagram is constructed. By a judicious choice of both modulation frequency and phase, tumbling motion can be induced even if the mean shear rate corresponds to the swinging regime. We derive expressions for the amplitude and width of the resonance peaks as a function of the modulation frequency.