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Secu M.,National Institute of Materials Physics Bucharest
Journal of Nanoparticle Research

Oxyfluoride glass-ceramic in the system SiO2-Al 2O3-CaF2-SmF3 containing Sm 3+-doped CaF2 nanocrystals in the range from 15 to 150 nm size were produced by using the controlled ceramization of the precursor glass. The incorporation of the Sm3+-dopant ion in the glass ceramic creates new electron-trapping centers and thermoluminescence (TL) method has been used in order to trace their evolution during glass ceramization. The 370 °C TL peak observed in precursor glass has been assigned to the recombination of the electrons released from the Sm2+-traps in the amorphous glass network. In the glass-ceramic sample containing nanocrystals with about 15 nm size the new weak TL peaks at 270, 290, and 310 °C were attributed to the recombination of the electrons released from the Sm2+-traps located mainly at the surface of the CaF2 nanocrystals. In the glass-ceramic sample containing nanocrystals with about 150 nm size, the new TL peaks at 232, 270, and 302 °C size have been assigned to the recombination of the electrons released from the Sm2+-traps located inside the CaF 2 nanocrystals. © Springer Science+Business Media B.V. 2010. Source

Gartner P.,University of Bremen | Gartner P.,National Institute of Materials Physics Bucharest
Physical Review A - Atomic, Molecular, and Optical Physics

The problem of the two-level laser is studied analytically. The steady-state solution is expressed as a continued fraction and allows for accurate approximation by rational functions. Moreover, we show that the abrupt change observed in the pump dependence of the steady-state population is directly connected to the transition to the lasing regime. The condition for a sharp transition to Poissonian statistics is expressed as a scaling limit of vanishing cavity loss and light-matter coupling, κ→0, g→0, such that g2/κ stays finite and g2/κ>2γ, where γ is the rate of nonradiative losses. The same scaling procedure is also shown to describe a similar change to the Poisson distribution in the Scully-Lamb laser model, suggesting that the low-κ, low-g asymptotics is of more general significance for the laser transition. © 2011 American Physical Society. Source

Miu L.,National Institute of Materials Physics Bucharest
Physica C: Superconductivity and its Applications

Standard zero-field cooling (zfc) magnetization relaxation measurements over a large temperature T interval were performed for YB 2Cu 3O 7 films with BaZrO 3 nanorods parallel to the c axis having the matching field B Φ ∼ 2.3 T. A rich non-monotonous T dependence of the normalized magnetization relaxation rate S at low external magnetic fields H (oriented along the nanorods) was observed. For H = 2 kOe, for example, S(T) clearly exhibits two maxima, located around T = 30 K and T = 62 K, as well as a pronounced S(T) deep at T ∼ 77 K. It is shown that the first S(T) maximum may have an extrinsic origin, related to the occurrence of thermo-magnetic instabilities in the low-T domain, the second one is due to a crossover between the non-glassy vortex creep regime involving double vortex kink formation and variable range vortex hopping, whereas the S(T) deep at high T is generated by the accommodation of nearly isolated vortices to the nanorods in the presence of the T dependent macroscopic current induced in the specimen. © 2012 Elsevier B.V. All rights reserved. Source

Sandu V.,National Institute of Materials Physics Bucharest
Modern Physics Letters B

We present the scaling law of the pinning force F P in intermediate and high temperature superconductors, the factors which influence its field and temperature dependence, the limitations imposed by the enlarged anisotropy and high operating temperatures, giant creep included. The theoretical developments which incorporate the new features of these classes of superconductors, most of them based on the AndersonKim theory, prove to be a useful source of information relative to the nature of the pinning and the characteristic fields. The use of models based on thermal activation integrates into scaling the tail of pinning force and also substantiates the use of the irreversibility field H irr as scale field. Finally, the data on scaling law in the two class of superconductors are presented and discussed. © 2012 World Scientific Publishing Company. Source

Ganea C.P.,National Institute of Materials Physics Bucharest
Central European Journal of Physics

The influence of mobile ions on the results of impedance spectroscopy (dielectric spectroscopy) measurements performed on a liquid crystal cell using the new mathematical model recently described was investigated. This mathematical model reformulates the fundamental equation system of continuity for mobile charge carriers and the Poisson equation using new variables. One makes the following assumptions: ions have different mobilities and diffusion coefficients, there is no generation-recombination process, the equilibrium carrier concentrations are uniform and equal each other, the electrodes are either completely blocking or blocked with adsorption-desorption processes. The final result is the analytical expression of the equivalent admittance for the system, allowing to have a clearer picture of the mobile ions and of the processes that occur at the electrode interface influencing the dielectric behavior. © 2013 Versita Warsaw and Springer-Verlag Wien. Source

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