Biomedical subjects

J Berakdar

Publications and source records attributed to J Berakdar.

16 recordsLinked to original sources

PubMed2006

Controlling the spin polarization of nanostructures on magnetic substrates.

It is shown that, by utilizing spin-selective quantum interference, the spin polarization of nanostructures deposited on a magnetic substrate with a surface state can be strongly modulated locally and energetically by an appropriate structural design. This finding is deduced from state-of-the-art ab initio calculations and interpreted within an analytical model. We present results for hexagonal Cu corrals and mesoscopic triangular Co islands on Co-covered Cu(111). These systems are experimentally feasible, and the effect should be detectable with current technology.

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PubMed2006

Tunable conductance of magnetic nanowires with structured domain walls.

We show that in a magnetic nanowire with double magnetic domain walls, quantum interference results in spin-split quasistationary states localized mainly between the domain walls. Spin-flip-assisted transmission through the domain structure increases strongly when these size-quantized states are tuned on resonance with the Fermi energy, e.g., upon varying the distance between the domain walls which results in resonance-type peaks of the wire conductance. This novel phenomenon is shown to be utilizable to manipulate the spin density in the domain vicinity. The domain wall parameters are readily controllable, and the predicted effect is hence exploitable in spintronic devices.

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PubMed2005

Mapping out electron-electron interactions at surfaces.

Using a high resolution coincidence technique, we measured for the first time the angular and energy correlation of an electron pair emitted from the valence band of a single crystal upon the impact of an electron with a specified momentum. We observe a hole in the measured two-particle correlation function when the two excited electrons have comparable momentum vectors, a fact traced back to exchange and repulsion among the electrons. We find the hole is not isotropic, has a finite extension, and is strongly suppressed when decoherence is operating.

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PubMed2005

Photoinduced charge currents in mesoscopic rings.

The temporal and spatial controllability of charge distribution in submicron structures opens new avenues for potential applications and for the understanding of nonequilibrium processes. Here we suggest a novel way to trigger and control within picoseconds charge currents and magnetic moments in nanoscopic and mesoscopic ring structures by applying two shaped, time-delayed light pulses. Our quantum dynamic calculations show that the magnitude and direction of the induced currents are tunable by varying the time delay and strengths of the pulses. Furthermore, in an array of rings desirable magnetic orders are generated depending on the ring sizes and particle number.

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PubMed2002

Spectroscopy of the electron-electron interaction in solids.

The spectrum of a photoexcited electron pair carries detailed information on the electron-electron interaction in metals. This is deduced from the results of a theoretical model presented here for the treatment of the double-photoelectron emission from surfaces. Main features in the two-particle spectra are assigned to (a) the exchange-correlation interaction, (b) the electronic band structure, (c) the photoelectron diffraction, and (d) the specific experimental setup. Comparison with experiments is made and common features and differences to the atomic case are pointed out.

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PubMed2001

Manifestation of charge-density fluctuations in metal clusters: suppression of the ionization channel.

The fluctuations in the electronic charge density of metallic clusters in response to an approaching electron suppress the single-ionization channel. This conclusion is made on the basis of numerical calculations for the total ionization cross sections using the random-phase approximation with exchange to describe the particle-hole (de)excitations. The general trends can be understood by means of the Thomas-Fermi model. The present theory explains, for the first time, the behavior of the measured total ionization cross section for C(60). The interplay between finite size and non-local screening effects is studied by tracing the changes in the ionization cross sections for Li clusters with an increasing cluster radius.

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PubMed2001

Magnetic and orbital dichroism in (e,2e) ionization of sodium.

We present the first measurement of (e,2e) ionization cross sections for a laser oriented atomic target by spin polarized electrons. Cross sections are presented as a function of target orientation and polarization direction of the incident electron beam. This study provides insight into mechanisms by which angular momentum is transferred from the valence electron to the two final-state continuum electrons in both singlet and triplet spin channels, by comparing measurement with distorted wave Born approximation and the dynamically screened three Coulomb wave calculations.

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PubMed2000

Incremental approach to strongly correlated many-body finite systems.

The transition and the Green operators of an interacting N body system are obtained from the solutions of the N-M body problem where M = 1,2,ellipsis,N-2. This is achieved via the development of a cumulative, nonperturbative approach that makes use of existing knowledge on the system when the number of interacting particles is reduced. The method is applied to four interacting Coulomb particles where the Green operator is expressed as a sum of Green operators of all three body subsystems that can be combined within the four body system. The calculated four particle continuum spectrum is in a remarkable agreement with recent experimental findings.

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PubMed2000

Visualizing spin-dependent electronic collisions In ferromagnets

This work demonstrates experimentally and theoretically that the coincident two-electron emission from a ferromagnetic surface, upon the impact of a polarized electron, carries detailed information on the spin-dependent electronic collisions in ferromagnets. The analysis of the calculated and the measured two-electron spectra reveals the potential of the electron-pair emission technique for the study of (a) surface magnetism and (b) spin-dependent electron scattering dynamics in ferromagnets.

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PubMed1996