Publications

When a quantum dot is embedded into a superconducting environment, it leads to the formations of localized Shiba states inside the gap. If the Coulomb interaction is sufficiently small, the Shiba states consist of a pair of singlet states and a doublet, that compete for the ground state and induce a quantum phase transition. In the presence of an external microwave field, the Shiba energy spectrum is significantly modified. Moreover, the transmission of the cavity inherits features that can pinpoint the exact location of the quantum critical point. In terms of methods used, our analytical calculations are supplemented by state of the art numerical renormalization group calculations.

The electrospinning technique was employed for the preparation of SnO2 fibers starting from a precursor solution consisting of a tin salt, polyvinylpyrrolidone as carrier polymer and N,N-dimethylformamide as dispersion medium. In order to achieve single-phase crystalline structures, the as-spun fibers were calcined at 500, 700 or 900 degrees C, with two different heating rates of 1 or 10 degrees C/min. The thermally treated samples were characterized in terms of structure, morphology and bandgap by employing X-ray diffraction, scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy and UV-Vis spectroscopy. A fine tuning of the bandgap width was attained through the selection of different values for the electrospinning and calcination parameters.

The electrical and photosensing properties correlated with structure and morphology of TiO2/(GeSi/TiO2)(2) multilayers are investigated. The multilayers are prepared by magnetron sputtering followed by rapid thermal annealing. Studies of Raman spectroscopy, transmission electron microscopy and X-ray diffraction are carried out. Measurements of dark current versus voltage and temperature are done. The photosensing properties are studied by measuring photocurrent spectra at different temperatures. We obtain multilayers with 10 - 15 nm Ge0.6Si0.4 nanocrystals (NCs) by annealing at 800 degrees C. We evidence the tunneling mechanism between neighbor NCs (T-1/2 law) in the dark current-temperature dependence. The photocurrent spectrum has a maximum with position shifting from 940 to 980 nm when the measurement temperature increases from 150 to 300 K, being due to the GeSi NCs.

The aim of this study was to investigate the antifungal activity of samarium doped hydroxyapatite (Ca10-xSmx(PO4)(6)(OH)(2), x(Sm) = 0.02 and x(Sm) = 0.05) layers deposited on titanium substrate by a thermal evaporation technique. The morphology of the samarium doped hydroxyapatite layers deposited on a Ti (Ti-Sm:HAp_2 and Ti-Sm: HAp_5) substrate were investigated using scanning electron microscopy (SEM). The antifungal activity of the Sm:HAp layers was assessed using Candida albicans ATCC 10231 fungal strain. The biofilm development of the C. albicans on the Sm: HAp layers was investigated by confocal laser scanning microscopy (CLSM). The results have evidenced a significant inhibition of the fungal cells adherence and biofilm development on the Sm: HAp layers.

The results of the surface morphology studies as well as optical and electrical properties of nano -dimensional vanadium oxide composite structures deposited on various substrates- silica, glass substrates covered by conducting layer of SnO2, pure pyroceramics plates and the same covered by Al(2)O3, SiO2, Ta2O5 layers are presented. According to Raman spectroscopy results the thin film structure obtained on quartz corresponds to the a, -V2O5 orthorhombic phase. In contrast with this the Raman spectra of V2O5 films obtained on pyroceramics substrates, passivated by Al2O3, Ta2O5, SiO2 oxides, do not appear clearly marked narrow lines which indicate their amorphous character. The investigation of the optical properties of the fabricated layers in the spectral range of 200-1000 nm had revealed the presence of both allowed direct and indirect electron transitions. In the frequency range of 10(3)-10(7) Hz the real Z' and imaginary Z '' part of the total impedance of pyroceramics/Al2O3/Al/V2O5/AI structure were studied. The applied direct bias leads to a considerable Z', as well as Z '' decrease. The analysis of the obtained dependencies is carried out by using the method of equivalent circuits.

Commercial MgB2 powder was loaded into a Fe-tube, by plastic deformation a tape of similar to 0.5 mm in thickness and 6.9 mm in width was obtained. Short pieces were processed by Spark Plasma Sintering (SPS) at 950, 1050 and 1150 degrees C for 3 min. The optimum sintering temperature is 1050 degrees C. From magnetic/electrical measurements, the onset critical temperature and the irreversibility field at 5 K were 38.7 / 38.9 K and 6.2 / 13.5 T, respectively. The pinning-force-related parameters indicate that the dominant flux pinning mechanism is of point pinning type. Contribution of grain boundary pinning is stronger at lower temperatures.

We describe a new concept to encode information in magnetic films that goes beyond the conventional way of digital magnetic recording. In our approach the information is stored via a continuous variable, namely the remanent coupling angle between two magnetic films that are separated by a nonmagnetic spacer layer. Using the technique of nuclear resonant scattering (NRS) [1, 2] we show with good precision, how this coupling angle can be conveniently adjusted with high degree of remanence by shortly applied external magnetic fields. Moreover this effect is explained using a micromagnetic model [3, 4]. Extremely important for future applications of this concept, we demonstrate, that the remanent coupling angles can be read out via magneto-optical or magneto-resistance effects. In principle, this approach allows to design novel memory cells for advance data storage devices, where multiple states per unit cell can be generated and recorded.

The "avalanche" up-conversion properties of the glass-ceramics containing (Ho3+, Yb3+)doped LiYF4 nanocrystals have been studied by comparison to the corresponding nanocrystals and polycrystalline powder samples. Under 810 nm laser light pumping, all the samples showed "green" ((H-2(11/2), 4S(3/2)) -> I-4(15/2)) up-conversion luminescence. The pump power threshold for the avalanche mechanism is about 75mW for the nanocrystalline or polycrystalline powder samples indicating a weak influence of the nanosize related effects is but smaller of about 43mW for the nano-glass ceramic due to the high "local" Yb3+ concentrations.

Two methods were used for the synthesis of composites based on reduced graphene oxide (RGO) and poly(para-phenylene vinylene) (PPV) in un-doped and doped state, respectively, i.e. the annealing conversion (AC) of the PPV precursor solution (PS) at 300 degrees C and the electropolymerization (E) of alpha, alpha, alpha', alpha'-tetrabromo-p-xylene. In the case of the E method, a decrease in the distyrylbenzene (DSB) weight onto the RGO sheet surface in favor of PPV in doped state is demonstrated by the s and p polarized IR spectroscopy. The change of the orientation angle of the transition dipole moment vector for the IR bands at 835 and 964 cm(-1), in the case of the AC of the PPV PS, is a result of the p-p* interactions between the phenyl groups of PPV and the RGO sheets. The photoluminescence (PL) bands at 2.42 and 2.26 eV indicate the formation of PPV macromolecular chains (MCs) with lengths of 5 and 710 repeating units (RUs), respectively. The anisotropic PL studies performed on the composites RGO/PPV in un-doped and doped state, respectively, highlight an increase of the wrapping angle of RGO sheets with PPV MCs with lengths of 5 RUs, when graphene concentration increases.