Time-resolved photoemission electron microscopy of magnetic field and magnetisation changes

Owing to its parallel image acquisition, photoemission electron microscopy is well suited for real-time observation of fast processes on surfaces. Pulsed excitation sources like synchrotron radiation or lasers, fast electric pulsers for the study of magnetic switching, and/or time-resolved detection can be utilised. A standard approach also being used in light optical imaging is stroboscopic illumination of a periodic (or quasi-periodic) process. Using this technique, the time dependence of the magnetic field in a pulsed microstrip line has been imaged in real time exploiting Lorentz-type contrast. Similarly, the corresponding field-induced changes in the magnetisation of cobalt microstructures deposited on the microstrip line have been observed exploiting magnetic X-ray circular dichroism as a contrast mechanism. The experiment has been performed at the UE 56/1-PGM at BESSY II (Berlin) in the single-bunch mode. Received: 2 September 2002 / Accepted: 2 September 2002 / Published online: 5 March 2003 RID="*" ID="*"Corresponding author. Fax: +49-6131/392-3807, E-mail: krasyuk@mail.uni-mainz.de     

Precession and Interference in the Aharonov–Casher and Scalar Aharonov–Bohm Effects

The ideal scalar Aharonov–Bohm (SAB) and Aharonov–Casher (AC) effect involve a magnetic dipole pointing in a certain fixed direction: along a purely time dependent magnetic field in the SAB case and perpendicular to a planar static electric field in the AC case. We extend these effects to arbitrary direction of the magnetic dipole. The precise conditions for having nondispersive precession and interference effects in these generalized set ups are delineated both classically and quantally. Under these conditions the dipole is affected by a nonvanishing torque that causes pure precession around the directions defined by the ideal set ups. It is shown that the precession angles are in the quantal case linearly related to the ideal phase differences, and that the nonideal phase differences are nonlinearly related to the ideal phase differences. It is argued that the latter nonlinearity is due to the appearance of a geometric phase associated with the nontrivial spin path. It is further demonstrated that the spatial force vanishes in all cases except in the classical treatment of the nonideal AC set up, where the occurring force has to be compensated by the experimental arrangement. Finally, for a closed space-time loop the local precession effects can be inferred from the interference pattern characterized by the nonideal phase differences and the visibilities. It is argued that this makes it natural to regard SAB and AC as essentially local and nontopological effects.     

Modelling of magnetic effects in near-field optics

Green's dyadic technique represents a powerful tool for calculations in electrodynamics, especially in modelling optical properties of nanoscopic objects. The method does not only provide field distributions, but also maps of susceptibilities and densities of states. Whereas the formalism is well established for dielectrics and electric fields, I present here a straight forward extension to tensors of both electric and magnetic type as well as mixed ones and furthermore to the situation where objects with dielectric and magnetic permeabilities are present together. As examples, characteristic field patterns are compared for elementary dielectric and magnetic perturbations. Green's tensors calculated for a coral structure reveal that mixed susceptibilities can exhibit other symmetries than pure electric or magnetic ones. Maps of all tensor components can thus give essential clues to the interpretation of near-field images. Received 15 December 2002 Published online 20 June 2003 RID="a" ID="a"Files “maths.ps” and “tensors.ps” are only available in electronic form at http://www.edpsciences.org RID="b" ID="b"e-mail: Ursula.Schroeter@uni-konstanz.de     

Determination of pore space shape and size in porous systems using NMR diffusometry. Beyond the short gradient pulse approximation

The influence of finite length gradient pulses on NMR diffusion experiments on liquids confined to diffuse between two parallel planes is investigated. It is experimentally verified that the pore size decreases when determined using finite gradient pulses if the results are analyzed within the short gradient pulse approximation. The results are analyzed using the matrix formulation. The observed minima in the echo decay profiles are considerably less sharp than theoretical analysis would indicate and we suggest that this is due to the presence of a distribution of pore sizes in the sample. In addition, effects due to the presence of background gradients are discussed. It is argued that effects due to the finite length gradient pulses are relatively minor and in realistic applications the effects due to inhomogeneities in pore sizes and effects due to background gradients will constitute more serious problems in pore size determinations by means of NMR diffusometry.     

Predictions of pulsed field gradient NMR echo-decays for molecules diffusing in various restrictive geometries. Simulations of diffusion propagators based on a finite element method

Pulsed field gradient NMR diffusometry is a promising tool for investigating structures of porous material through determinations of dynamic displacements of molecules in porous systems. A problem with this approach is the lack of closed analytical expressions for echo-decays in anything but idealized pore geometries. We present here an approach based on calculating the appropriate diffusion propagator by means of finite element calculations. The suggested method is quite general, and can be applied to arbitrary porous systems. The protocol for the calculations is outlined and we show results from some different cases: diffusion in confined geometries and in systems that are spatially inhomogeneous with respect to concentration.     

Narrow α + <Superscript>28</Superscript>Si elastic-scattering states at high excitation in <Superscript>32</Superscript>S

The excitation function and angular distributions of elastic α-particle scattering on ²⁸Si have been measured in the laboratory energy range 6-28 MeV using a backscattering technique on a thick target, yielding a continuous energy distribution. More than 200 narrow states are observed, with widths in the range ∼ 30-100 keV at excitation energies E ^* = 13-32 MeV. Angular distributions at backward angles were measured, and angular momentum values of more than 83 states have been deduced. The analysis gives spin-parities J ^π, α-partial widths Γ_α and reduced widths of the narrow high-lying resonant states in ³²S. The experimentally observed states display both the negative- and the positive-parity rotational-like sequences with seemingly no parity splitting, a finding which is at variance with most potential-model predictions. The deduced effective moment of inertia indicates a more extended structure than the ground-state configuration. The observed strength of each ℓ-value is analyzed in terms of an underlying split doorway state of Lorentzian form, which yields an interpretation as fragmented rotational α + ²⁸Si states. Received: 26 June 2000 / Accepted: 16 September 2002 / Published online: 4 February 2003 RID="a" ID="a"e-mail: kkallman@abo.fi RID="b" ID="b"Present address: Swedish Polytechnic, FIN-65200 Vasa, Finland. Communicated by D. Guerreau     

Spectroscopy of neutron deficient 65Ge

Excited states of ⁶⁵Ge were populated via the ¹²C + ⁵⁸Ni (261 MeV) reaction using the NORDBALL detector array equipped with charged-particle and neutron detector systems for reaction channel separation. On the basis of γγ-coincidence relations and angular distribution ratios a significantly extended level scheme was constructed up to E _x = 9 MeV and J ^π = (33/2^?). The low-energy states of the nucleus are discussed in the framework of the interacting boson-fermion model.     

Single-particle enhancement of heavy-element production

Fusion barriers are calculated in a macroscopic-microscopic model for several cold-fusion heavy-ion reactions leading to heavy and superheavy elements. The results obtained in such a picture are very different from those obtained in a purely macroscopic model. For reactions on ²⁰⁸Pb targets, shell effects in the entrance channel result in fusion-barrier energies at the touching point that are only a few MeV higher than the ground state for compound systems near Z = 110. The entrance-channel fragment-shell effects remain far inside the touching point, almost to configurations only slightly more elongated than the ground-state configuration, where the fusion barrier has risen to about 10 MeV above the ground-state energy. Calculated single-particle level diagrams show that few level crossings occur until the peak in the fusion barrier very close to the ground-state shape is reached, which indicates that dissipation is negligible until very late in the fusion process. Whereas the fission valley in a macroscopic picture is several tens of MeV lower in energy than is the fusion valley, we find in the macroscopic-microscopic picture that the fission valley is only about 5 MeV lower than the fusion valley for cold-fusion reactions leading to compound systems near Z = 110. These results show that no significant “extra-extrapush” energy is needed to bring the system inside the fission saddle point and that the typical reaction energies for maximum cross section in heavy-element synthesis correspond to only a few MeV above the maximum in the fusion barrier.     

Lip kinematics in long and short stop and fricative consonants

This paper examines lip and jaw kinematics in the production of labial stop and fricative consonants where the duration of the oral closure/constriction is varied for linguistic purposes. The subjects were speakers of Japanese and Swedish, two languages that have a contrast between short and long consonants. Lip and jaw movements were recorded using a magnetometer system. Based on earlier work showing that the lips are moving at a high velocity at the oral closure, it was hypothesized that speakers could control closure/constriction duration by varying the position of a virtual target for the lips. According to this hypothesis, the peak vertical position of the lower lip during the oral closure/constriction should be higher for the long than for the short consonants. This would result in the lips staying in contact for a longer period. The results show that this is the case for the Japanese subjects and one Swedish subject who produced non-overlapping distributions of closure/ constriction duration for the two categories. However, the peak velocity of the lower lip raising movement did not differ between the two categories. Thus if the lip movements in speech are controlled by specifying a virtual target, that control must involve variations in both the position and the timing of the target.     

Pulsed-field gradient nuclear magnetic resonance study of transport properties of fluid catalytic cracking catalysts

Pulsed-field gradient nuclear magnetic resonance (PFG NMR) has been applied to study molecular diffusion in industrial fluid catalytic cracking (FCC) catalysts and in USY zeolite for a broad range of molecular displacements and temperatures. The results of this study have been used to elucidate the relevance of molecular transport on various displacements for the rate of molecular exchange between catalyst particles and their surroundings. It turned out that this rate, which may determine the overall rate and selectivity of FCC process, is primarily related to the diffusion mode associated with displacements larger than the size of zeolite crystals located in the particles but smaller than the size of the particles. This conclusion has been confirmed by comparative studies of the catalytic performance of different FCC catalysts.