National Institute Of Materials Physics - Romania

We investigate theoretically in a tight-binding model the transport properties of the Aharonov-Bohm interferometer (ABI) with one dot embedded in each of its arms. For weak interdot coupling the model Hamiltonian describes the system considered in the experiments of Holleitner et al. [Phys. Rev. Lett. 87, 256802 (2001)]. The electronic transmittance of the interferometer is computed within the Landauer-Buttiker formalism while the coexistence of resonant and coherent transport is explicitly emphasized by using the Feschbach formula. The latter produces effective Hamiltonians whose spectral properties describe the tunneling processes through each dot. We reproduce numerically the stability charging diagrams reported in the experiments of Holleitner et al. When the magnetic flux is fixed and one dot is set to resonance the interferometer transmittance shows Fano, lineshapes as a function of the gate voltage applied to the other dot. Our model includes the effect of the magnetic field on the dot levels and explains the change of the asymmetric tail as the magnetic flux is varied. The transmittance assigned to the Fano, dips located in the almost crossing point of the charging diagrams shows Aharonov-Bohm oscillations.

We propose a general formalism for describing the coexistence of coherent and resonant transport in hybrid mesoscopic structures. The approach is based on Landauer- Buttiker formula for the electronic transmittance and on an old formula of Feshbach. The latter gives the complete Green function of coupled subsystems in terms of effective Green functions of the disconnected parts and provides informations about the individual contributions of each subsystem to transport. Motivated by the experiments of Kobayashi et al.(Phys. Rev. Lett. 88, 256806 (2002)) and Holleitner et al. (Phys. Rev. Lett. 87, 256802 (2001)) we apply the formalism to study transport in Aharonov-Bohm interferometers (ABI) containing one or two coupled two-dimensional quantum dots (QD). In the single dot case, we reproduce and explain the magnetic field control of the Fano interference and investigate the interaction effects in a self-consistent approach. In the double dot case, we obtain the charging diagrams and establish precise criteria for the observation of mesoscopic Fano effect and AB oscillations. (c) 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Giant magnetoresistive (GMR) coatings on silicon, glass and Kapton were prepared by simultaneous thermionic vacuum are (TVA) discharges in high vacuum conditions. The magnetic metal or alloy (Fe, Co, Ni, Permalloy) together with the noble non magnetic metal were deposited on substrates having different positions relative to the discharges. Local magnetic interactions and Fe-phase composition were obtained by Mossbauer spectroscopy whereas the magnetoresistance effects were measured by a dc method using a four-point configuration with perpendicular to plane magnetic fields up to 0.8 T. The surface morphology of the coatings was characterized by atomic force microscopy (AFM) in contact mode.

The effects produced by heavy ion irradiation in KBr:Ag and KCl:Ag crystals are investigated using optical spectroscopy. For KCl, the results are compared with the effects obtained in the pure crystals. After irradiation with heavy ions up to 11.2 MeV/u specific energy, the absorption bands suffer a series of changes, related to the structure of defects and also to their dimensions. In the high Ag concentrated samples, the Ag+ ions are changing their valence to Ag- while in the samples containing Ag nanoclusters the irradiation induces changes in the nanocluster size.

Aluminophosphate glasses doped with Fe, Mn, and Cr have been obtained by a wet non-conventional method. Structural information was provided by IR absorption spectra in the range 2000-500 cm(-1). The optical behaviour (transmission and refractive index) of the samples has been studied by UV-VIS-NIR spectroscopy. The Fe valence state and the local coordination were also analysed via Fe-57 Mossbauer spectroscopy, whose data revealed the redox equilibrium in the Fe-doped glasses according to the redox potentials of the transition ions.

A layer of doped CdS nanocrystals was deposited on the surfaces of nanopores of polyethyleneterephtalate (PET) membranes. Optical absorption measurements were used to determine the bandgaps of the nanocrystals; CdS:Mn2+ and CdS:Mn2+:Cu+ show a blue and red shift, respectively, relative to the band gap of nanostructurated CdS. Photoluminescence spectra of the doped and undoped CdS samples exhibit wide bands covering the visible spectrum range.

Single-walled carbon nanotubes (SWNTs) under a moderate non-hydrostatic pressure, of 0.58 GPa, undergo a non-reversible transformation. Due to the plastic deformations, structural defects and carbon nanotube fragments of different size are produced. Short fragments of spherical or ellipsoidal form, behaving as closed-shell fullerenes are observed both by Raman spectroscopy and photoluminescence (PL). The main vibrational indicative of the fragments of shorter length is the band at similar to 1458 cm(-1) that is regularly observed in the Raman spectra of fullerene. Similar to fullerenes self assemblies, the interaction between the nanotube fragments is noticed in the Raman spectrum by the band at similar to 94 cm(-1) that reveals an inter-particle vibration mode. When the nanotubes are dispersed into host matrix, as aromatic hydrocarbons (AHs) with isolated or condensed phenyl rings, supplementary mechanico-chemical reactions take place. For AHs with isolated phenyl rings, like biphenyl or p-terphenyl, a chemical functionalization of SWNT fragments is demonstrated by the appearance of a new Raman band at 1160 cm(-1). In PL spectra, the interaction of SWNTs with AHs, is noticed by a detailed vibronic structure appearing in the high energy side of the emission spectrum, that increase with the weight of SWNTs in the AHs/SWNTs mixtures.

Correlated studies of Surface Enhanced Raman Scattering (SERS) spectroscopy, photoluminescence (PL) and viscosity on aggregation of C-60 fullerene in N-methyl-2-pyrrolidinone (NMP) solutions were performed. The aggregation is a result of storing of the solutions at the room temperature for different periods of time. The modified phonon spectrum in the aggregate phase is a consequence of the intermolecular interactions that lead to a large reduction in parent I-h C-60 symmetry. The Raman range below 800 cm(-1) is the most diagnostic for aggregate assignment. Substitution of the original of H, (1) band from similar to 269 cm(-1) with a new one at similar to 255 cm(-1) relates the formation of (C-60, NMP) complexes. In the second stage of aggregation, this band shifts at similar to 246 cm(-1) and a new band, associate to the interball interaction, is formed at similar to 94 cm(-1). PL emission of the C-60 solutes in NMP presents two components, one in red (1.92-2.00 eV) and another in green (2.18-2.3 eV), which originate in two successive processes. Their weights depend on C-60 concentration and on storage time. In the beginning are formed quickly molecular complexes (C-60, NMP) by charge transfer, which on associate slowly in stable aggregates. A dependence of specific viscosity on C-60 solution concentration is observed too.

We investigate the properties of the electron spin-transmission through an Aharonov-Bohm interferometer with an embedded two-dimensional (2D) multilevel quantum dot containing magnetic impurities. A suitable formalism is developed. The amplitude and the phase of the flip- and non-flip-transmittances are calculated numerically as function of the magnetic field. The effects induced by the exchange interaction and those arising from the 2D character of the dot are shown. (c) 2005 Elsevier B.V. All rights reserved.

Melt spun ribbons of FexCu100-x were obtained by different procedures, in order to maximize the dispersion of the Fe atoms in the Cu matrix. Subsequent thermal and mechanical treatments were used for a controlled crystallization process of the soft magnetic phase. A detailed analysis of structural aspects and crystallization dynamics and their relation with the magnetic behavior of the Fe-Cu ribbons has been obtained via Mossbauer spectroscopy, X-ray diffraction (XRD) and magnetic measurements. In order to obtain composite permanent magnets with shape anisotropy, the optimal conditions and the suitable processing were established. © 2004 Elsevier B.V. All rights reserved.