A. Ruyter

Angle-resolved transport measurements in a Bi-2212 crystal with inclined columnar defects: study of the directional effects

Abstract Columnar defects at 45° with respect to the ab planes have been installed in a Bi-2212 single crystal by 5.8 GeV Pb ion irradiation. Transport measurements have been carried out in the so-called “top” and “cross” configurations, as a function of the temperature T , the magnetic field B , the current i and θ, the angle between B and the ab planes. In a restricted area of the B - T plane, the R ( θ ) curves exhibit a marked dip centered along the direction of the columnnar defects, for B lower or close to the fluence equivalent field B φ t which corresponds to a vortex density equal to the planar density of the columnar defects. The directional features become less pronounced as the field or the temperature increase, and disappear completely above a crossover line which extrapolates at low fields to a temperature lower than T c . This crossover is discussed in terms of a decorrelation of line vortices induced by thermal fluctuations.

Investigation of the domain structure and hierarchy in potassium–sodium niobate lead-free piezoelectric single crystals

Recently, many techniques have been used to grow large K0.5Na0.5NbO3 (KNN) based single crystals. However, most of them required the use of a crucible, fluxes in the melt at high temperature and long process times that could lead to alkali volatilization or inclusion of impurities in the crystals. In this study, the floating zone method, which is especially suitable for compounds that melt incongruently or present volatile elements, is employed to study the microstructure of [011] oriented KNN crystals. Then, the domain structure and their relationships with piezoelectric properties in KNN are investigated using scanning electron microscopy (SEM), transmission electron microscopy (TEM) and piezo force microscopy (PFM). This study reveals self-organized and hierarchized domain structures on various length scales ranging from micrometer to nanometer scale in KNN crystals. First, parallel stripe-like microdomains of 6–7 μm wide, which contain themselves parallel stripes (1–4 μm wide), are observed using SEM. These domains have been confirmed by TEM. TEM observations have also shown the presence of additional zigzag sub-microdomains with 90 and 120° orientations, which are arranged inside these observed stripes. Moreover, high resolution TEM highlighted the presence of regular antiphase boundaries at the location of the domain walls, which are caused by the small misfits between the parameters of the different structures. Split spots on the FFT image attests of these phenomena. Additionally, PFM images showed also parallel stripe-like microdomains (3–4 μm wide), which contain sub-microdomains. These sub-microdomains consist of parallel stripes of 200–400 nm and zigzag nanodomains with 60, 90, 120 and 180° orientations. The local polarization switching test by PFM emphasized that the observed domain structures correspond well with ferroelectric domains, confirming through microstructural observations the ferroelectricity of KNN. The combination of SEM, TEM and PFM observations of orthorhombic KNN crystals have shown the correlation between microstructure, sub-microstructure and domain structures at different scales. The [011] oriented KNN crystal exhibited interesting piezoelectric properties such as kt of 0.47, d33 ∼ 60–80 pm V−1, eSr,33 of 123, dielectric losses (δe) of 0.07 but also high mechanical losses (δm) of 0.21 that can be induced by 90° domain walls moving under the influence of an electric field during poling.

Magnetocontrollability of Fe7C3@C superparamagnetic nanoparticles in living cells.

BackgroundA new type of superparamagnetic nanoparticles with chemical formula Fe7C3@C (MNPs) showed higher value of magnetization compared to traditionally used iron oxide-based nanoparticles as was shown in our previous studies. The in vitro biocompatibility tests demonstrated that the MNPs display high efficiency of cellular uptake and do not affect cyto-physiological parameters of cultured cells. These MNPs display effective magnetocontrollability in homogeneous liquids but their behavior in cytoplasm of living cells under the effect of magnetic field was not carefully analyzed yet.ResultsIn this work we investigated the magnetocontrollability of MNPs interacting with living cells in permanent magnetic field. It has been shown that cells were capable of capturing MNPs by upper part of the cell membrane, and from the surface of the cultivation substrate during motion process. Immunofluorescence studies using intracellular endosomal membrane marker showed that MNP agglomerates can be either located in endosomes or lying free in the cytoplasm. When attached cells were exposed to a magnetic field up to 0.15 T, the MNPs acquired magnetic moment and the displacement of incorporated MNP agglomerates in the direction of the magnet was observed. Weakly attached or non-attached cells, such as cells in mitosis or after cytoskeleton damaging treatments moved towards the magnet. During long time cultivation of cells with MNPs in a magnetic field gradual clearing of cells from MNPs was observed. It was the result of removing MNPs from the surface of the cell agglomerates discarded in the process of exocytosis.ConclusionsOur data allow us to conclude for the first time that the magnetic properties of the MNPs are sufficient for successful manipulation with MNP agglomerates both at the intracellular level, and within the whole cell. The structure of the outer shells of the MNPs allows firmly associate different types of biological molecules with them. This creates prospects for the use of such complexes for targeted delivery and selective removal of selected biological molecules from living cells.

On voltage-current characteristics and critical current in Bi-2212

Abstract.V(I) characteristics have been performed in a monocrystalline microbridge ofnBi[ Pb

Peak effect in an irradiated microbridge of Bi-2212 single crystal: evidence for a Bragg Bose glass? ☆

]-2212

Universality of glass scaling in Bi2Sr2Ca1-xYxCu2O8 single crystals in the presence of correlated disorder

. The vortex phase diagram has been greatly investigated. Linear butnnon-ohmic Voltage (Current) (V(I)) curves with a well defined critical current have beennobserved. A departure from this behavior is observed near the peak effect where an out ofnequilibrium high threshold current can be stabilized. At high temperature, the criticalncurrent persists in the “liquid” state despite the dissipation at the lowest bias. Somenimplications of these results are discussed. In particular, it is proposed that thensurface disorder, rather than the bulk disorder, is responsible for the vortex pinning innthis sample.nn

Bose-glass behaviour in Bi2Sr2Ca1−xYxCu2O8 crystals with columnar defects: experimental evidence for variable-range hopping

We have investigated I–V characteristics at very low temperature on a micro-bridge in a Bi-2212 single crystal with columnar defects patterned by laser ablation. We have shown that the critical current IC presents a maximum when increasing the applied magnetic field. This maximum is located below the matching field at a filling fraction f=BMax/BΦ=0.8. The transverse Meissner effect (TME) has also been checked by tilting the magnetic field away from the tracks. Contrary to the low field regime (f<0.8) where the vortex are strongly localized, the transverse component of the magnetic field is not screened anymore when the parallel one is higher than BMax. It suggests that the vortex system could be considered as a weak Bose glass with interstitial vortices. It suggests that the vortex system could be considered as a weak Bose glass with interstitial vortices. Finally, our results strongly support the existence of a typical vortex phase at low temperature and low magnetic field.

Current density dependence of the activation energy in Tl-2212 thin films at weak magnetic fields

Abstract Current–voltage characteristics are investigated in BiSr 2 Ca 1− x Y x Cu 2 O 8 crystals irradiated parallel to the c -axis with 5.8xa0GeV Pb ions. Over a wide range of filling fractions 0.026≤ f ≤1, the I – V isotherm curves near the superconducting transition are consistent with the Bose-glass scaling theory. Field and matching field independent critical exponent values z ′=5.27±0.05 and ν ′=1.30±0.07 have been found. This result is in good agreement with the well-established universality rules for the critical exponents at standard second-order transition. The glass line, derived from scaling analysis, is well described by the Lindemann criterion that accounts for the contribution of correlated disorder. Two characteristic temperatures, beyond which the vortex system is increasingly dominated by vortex–vortex interactions or thermal fluctuations, respectively, are identified. In these domains, the Bose-glass melting line merges the pristine melting line.

Dimensional crossover vortex fluctuations in irreversibility line of high-quality Tl2Ba2CaCu2O8 thin film

Abstract We report on vortex transport in Bi 2 Sr 2 Ca 1− x Y x Cu 2 O 8 crystals irradiated at different doses of heavy ions. We show evidence of a flux-creep resistivity typical of a variable-range vortex hopping mechanism as predicted by Nelson and Vinokur (Phys. Rev. B. 48 1993 13060).

VORTEX PINNING AND COLUMNAR DEFECTS IN SUPERCONDUCTING OXIDES

Abstract The flux motion has been investigated in a Tl 2 Ba 2 CaCu 2 O 8 thin film by transport measurements with a weak applied magnetic field perpendicular to the ( a , b ) plane ranging from 20 to 300 mT. We have found a well defined line T ∗ (H) separating ohmic from non ohmic behaviour in the H-T ∗ diagram. This line is well described by 1 - t ∗ ≈ H 2 3 where t ∗ = T ∗ /T c and T c is the zero magnetic field transition temperature. Above T ∗ (H) , a thermally activated resistance is observed. Our experimental results clearly indicate that the activation energy is well approximated by U(T, H) ∼ [l-(T/T c ] 3 2 /H both below and above the T ∗ (H) line in the H-T ∗ diagram. The nonlinear current dependence of the activation energy U ( T , H , I ) is extracted from I – V characteristics, and it is found that U ( T , H , I )= U ( T , H ) ln( I 0 / I ). Satisfactory agreement is found between our results and recent models which take into account a periodic logarithmic-like pinning potential.