National Institute Of Materials Physics - Romania

Publications

5781 articles found

4051. Transport through a quantum ring, dot, and barrier embedded in a nanowire in magnetic field

Authors: Gudmundsson, V; Lin, YY; Tang, CS; Moldoveanu, V; Bardarson, JH; Manolescu, A

Published: JUN 2005, PHYSICAL REVIEW B, 71, DOI: 10.1103/PhysRevB.71.235302

We investigate the transport through a quantum ring, a dot, and a barrier embedded in a nanowire in a homogeneous perpendicular magnetic field. To be able to treat scattering potentials of finite extent in a magnetic field we use a mixed momentum-coordinate representation to obtain an integral equation for the multiband scattering matrix. For a large embedded quantum ring we are able to obtain Aharonov-Bohm type of oscillations with superimposed narrow resonances caused by interaction with quasibound states in the ring. We also employ the scattering matrix approach to calculate the conductance through a semiextended barrier or well in the wire. The numerical implementations we resort to in order to describe the cases of weak and intermediate magnetic field allow us to produce high resolution maps of the "near field" scattering wave functions, which are used to shed light on the underlying scattering processes.

4052. Investigations by X-ray topography of quartz and langasite resonators

Authors: Mateescu, I; Capelle, B; Detaint, J; Johnson, G; Tsoi, E; Bran, C

Published: JUN 2005, JOURNAL DE PHYSIQUE IV, 126, 6, DOI: 10.1051/jp4:2005126001

This paper presents the results of the X-ray topography investigations of mass-loading influence on electrical parameters of AT-cut quartz resonators and Y-cut langasite resonators. This study reveals that the mass-loading effect on electrical parameters of Y-cut langasite resonators is smaller than in the case of AT-cut quartz resonators. The results of the X-ray topography investigations are in good agreement with electrical measurements.

4053. Magnetically modulated microwave absorption investigation of Bi1.8Pb0.4Sr1.8Ba0.2Ca2Cu3Ox/(LiF)(y) superconducting system

Authors: Velter-Stefanescu, M; Duliu, OG; Mihalache, V

Published: JUN 2005, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 7, 1561, DOI:

The effect of LiF content on the 2223 phase of the high temperature superconducting system Bi1.8Pb0.4Sr1.8Ba0.2Ca2Cu3Ox/(LiF)(y) (y = 0.02 to 0.15) was investigated by means of magnetically modulated microwave absorption (MAMMA). The experimental results confirmed a two fold increase of the superconducting phase content together with a slight rise (around 2 %) of the critical temperature for y=0.07. At the same time, for y between 0.05 and 0.12 the -Delta T-c(mw)/Delta B ratio was practically constant and equal to (18.5 +/- 1.5) KT-1, a typical value for 2223 superconducting phase.

4054. Correlated photon-pair emission from a charged single quantum dot - art. no. 235308

Authors: Ulrich, SM; Benyoucef, M; Michler, P; Baer, N; Gartner, P; Jahnke, F; Schwab, M; Kurtze, H; Bayer, M; Fafard, S; Wasilewski, Z; Forchel, A

Published: JUN 2005, PHYSICAL REVIEW B, 71, DOI: 10.1103/PhysRevB.71.235328

The optical creation and recombination of charged biexciton and trion complexes in an (In,Ga)As/GaAs quantum dot is investigated by microphotoluminescence spectroscopy. Under the condition of quasiresonant pulsed excitation into the p shell we find nearly background-free photoluminescence from single quantum dots where multiple-photon (> 1) emission is fully suppressed. This reflects almost perfect triggered single-photon emission. Photon cross-correlation measurements demonstrate the temporally correlated decay of charged biexciton and trion states. Our calculations provide strong evidence for radiative decay from the excited trion state, thus indicating a long-lived p-electron spin configuration.

4055. Raman spectra of AsxSe100-x glasses doped with metals

Authors: Iovu, MS; Kamitsos, EI; Varsamis, CPE; Boolchand, P; Popescu, M

Published: JUN 2005, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 7, 1221, DOI:

Chalcogenide glasses in the system AsxSe100=x, as well as As40Se60 and As50Se50 glasses doped with metals (Bi, Ge, Cd, Ag, Sn, Mn, Dy and Sm) were investigated by Raman spectroscopy. The Raman spectra of AsxSe100-x with 0 <= x <= 10 suggest that the majority building units are Se chains and See closed rings. At higher As contents, new structural units are formed from As and Se atoms. The Raman profiles for x > 50 are characterized by the presence of strong narrow bands indicating the high ordering in the structure of these glassy materials. The doped samples with small amounts of metals, 0.1 - 1.0 at.%, do not show appreciable changes in their short-range order structure, with the exception of the 0.5 at.% Dy-doped sample. For the sample As50Se50+7.5 at.% Sn, there is direct evidence for the formation of new Sn-based structural units, such as Sn(Se-1/2)(4) tetrahedra.

4056. Micro-structure and magnetic properties of Fe-Cu nanocomposites for anisotropic permanent magnets

Authors: Kuncser, V; Valeanu, M; Lifei, F; Predoi, D; Jianu, A; Kappel, W; Codescu, M; Patroi, E; Pasuk, I; Bulinski, M; Filoti, G

Published: MAY 31 2005, JOURNAL OF ALLOYS AND COMPOUNDS, 395, 6, DOI: 10.1016/j.jallcom.2004.11.059

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.

4057. Vanadium aluminium oxynitride catalysts for propane ammoxidation reaction - Effect of the V/Al ratio on the structure and catalytic behviour

Authors: Florea, M; Silvy, RP; Grange, P

Published: MAY 26 2005, , DOI: 10.1016/j.apcata.2005.02.032

4058. Space-charge-limited current involving carrier injection into impurity bands of high-k insulators

Authors: Goldenblum, A; Pintilie, I; Buda, M; Popa, A; Botila, T; Dimoulas, A; Vellianitis, G

Published: MAY 16 2005, APPLIED PHYSICS LETTERS, 86, DOI: 10.1063/1.1935045

Photoelectrical measurements have shown that the current flow through La2Hf2O7 and LaAlO3 high-k insulator layers deposited on silicon takes place via impurity channels. A space charge limited current is evidenced, for different insulator thicknesses and temperatures, by the square law dependence of current versus voltage. The analysis demonstrates that this space charge limited (SCL) current in thin insulator films can be explained only by the presence of impurity channels situated near the Fermi level of the injecting contact. Many other aspects related to the SCL current behavior were found. (c) 2005 American Institute of Physics.

4059. Structural and magnetic properties of NiZn and Zn ferrite thin films obtained by laser ablation deposition

Authors: Sorescu, M; Diamandescu, L; Swaminathan, R; McHenry, ME; Feder, M

Published: MAY 15 2005, JOURNAL OF APPLIED PHYSICS, 97, DOI: 10.1063/1.1854416

sLaser ablation deposition has been used to synthesize nanoscale ferrite structures. Our investigations were performed on NiZn and Zn ferrite films deposited on silicon(100) substrates. Films produced by laser ablation at room temperature were annealed at 550 degrees C for 1 h. Other films were deposited directly at a 550 degrees C substrate temperature without subsequent annealing. Complementary x-ray diffraction and superconducting quantum interference device magnetometry measurements helped identify the optimum laser ablation deposition conditions for obtaining the desired nanoferrite structures. From the hysteresis loops at 300 and 10 K we. identified the paramagnetic or ferromagnetic behavior of the films. The zero field cooled-field cooled (ZFC-FC) magnetization, M(T), curves yielded the value of the blocking temperature in both NiZn and Zn ferrite systems. (c) 2005 American Institute of Physics.

4060. Evidence for the participation of lattice nitrogen from vanadium aluminum oxynitrides in propane ammoxidation

Authors: Olea, M; Florea, M; Sack, I; Silvy, RP; Gaigneaux, EM; Marin, GB; Grange, P

Published: MAY 15 2005, , DOI: 10.1016/j.jcat.2005.02.020