Andrei T. Matveev

Dramatic Role of Critical Current Anisotropy on Flux Avalanches in MgB2 Films

Anisotropic penetration of magnetic flux in MgB(2) films grown on vicinal sapphire substrates is investigated using magneto-optical imaging. Regular penetration above 10 K proceeds more easily along the substrate surface steps, the anisotropy of the critical current being 6%. At lower temperatures the penetration occurs via abrupt dendritic avalanches that preferentially propagate perpendicular to the surface steps. This inverse anisotropy in the penetration pattern becomes dramatic very close to 10 K where all flux avalanches propagate in the strongest pinning direction. The observed behavior is fully explained using a thermomagnetic model of the dendritic instability.

Boron Nitride Nanoparticles with a Petal-Like Surface as Anticancer Drug-Delivery Systems

Nanoparticles (NPs) have a great potential as nanosized drug-delivery carriers. Such systems must safely deliver the drug to the site of the tumor without drug leakage, effectively penetrate inside cancer cells, and provide intracellular drug release. Herein we developed an original and simple method aimed at the fabrication of spherical boron nitride NPs (BNNPs), 100-200 nm in diameter, with peculiar petal-like surfaces via chemical vapor deposition. Such structures were found to be able to absorb a large amount of antitumor drug-killing tumor cells. They revealed low cytotoxicity and rapid cellular uptake. BNNPs were saturated with doxorubicin (DOX) and then dispersed. The BNNPs loaded with DOX (BNNPs-DOX) were stable at neutral pH but effectively released DOX at pH 4.5-5.5. MTT assay and cell growth testing showed that the BNNPs-DOX nanocarriers had been toxic for IAR-6-1 cells. BNNPs loaded with DOX penetrated into the neoplastic IAR-6-1 cells using endocytic pathways, and then DOX released into the cytoplasm and cell nuclei and resulted in cell death.

Epitaxial growth of NaxCoO2 thin films by pulsed-laser deposition

Single-phase thin films of NaxCoO2 have been grown epitaxially by pulsed-laser deposition technique. The growth conditions were studied based on the logpO2−1∕T phase diagram of Co–O2 using different types of substrate materials. For Na0.58CoO2, metallic behavior is found down to 4.2K.

Boron nitride nanotube growth via boron oxide-assisted chemical vapor transport-deposition process using LiNO3 as a promoter

High-purity straight and discrete multiwalled boron nitride nanotubes (BNNTs) were grown via a boron oxide vapor reaction with ammonia using LiNO3 as a promoter. Only a trace amount of boron oxide was detected as an impurity in the BNNTs by energy-dispersive X-ray (EDX) and Raman spectroscopies. Boron oxide vapor was generated from a mixture of B, FeO, and MgO powders heated to 1,150 °C, and it was transported to the reaction zone by flowing ammonia. Lithium nitrate was applied to the upper side of a BN bar from a water solution. The bar was placed along a temperature gradient zone in a horizontal tubular furnace. BNNTs with average diameters of 30–50 nm were mostly observed in a temperature range of 1,280–1,320 °C. At higher temperatures, curled polycrystalline BN fibers appeared. Above 1,320 °C, the number of BNNTs drastically decreased, whereas the quantity and diameter of the fibers increased. The mechanism of BNNT and fiber growth is proposed and discussed.

Bending of magnetic avalanches in MgB2 thin films

The penetration of magnetic flux into a superconductor above the lower critical field does not necessarily happen in a regular way. It is found that under particular conditions, a chaotic penetration in form of magnetic avalanches occurs. These avalanches are closely related to the so-called thermomagnetic instability, which identifies local heating due to flux line movement as the origin of the effect. In case of MgB2 thin films, these avalanches are found only below T=10K and are suppressed by a covering metallic layer with high thermal conductivity and sufficient thickness. These avalanches are observed in the case of partly gold covered MgB2 films by the magneto-optical Faraday effect. The investigation of avalanches propagating into a gold-covered region revealed a change of the propagation direction depending on the incident angle of these avalanches.

Homologous series of high-Tc superconductors (Cu,C)Sr2Can−1CunOy (n=2,5) and (Cu,N,C)Sr2Can−1CunOy (n=3–6) synthesized under high pressure

Abstract The Sr–Ca–Cu–C–O and Sr–Ca–Cu–N–C–O systems were investigated under high pressure/high temperature conditions. The former includes (Cu,C)Sr 2 Ca n −1 Cu n O y [(Cu,C)-12( n −1) n -Sr, n =2,5] phases, the latter, (Cu,N,C)Sr 2 Ca n −1 Cu n O y [(Cu,N,C)-12( n −1) n -Sr, n =3–6] phases. In the former series, the n =3,4 members could not be synthesized in bulk form. A small level of nitrogen-doping was found to be highly effective in stabilizing the members with n =3–6. All these phases present superconductivity, with the highest T c of 113 K in (Cu,N,C)-1234-Sr. Two-types of superstructures with 4 a × b ×2 c and 2 a × b ×2 c were observed in the phases with n ≤4, depending on the system and n -value.

Synthesis of MgB2 films in Mg vapour flow and their characterization

Surface morphology, element distribution and superconducting properties of MgB2 films prepared by ex situ annealing of multilayer B/Mg precursors in a flow of Mg vapour have been studied. The composition of the films and distribution of the elements have been analysed by both Auger electron spectroscopy (AES) and x-ray photoelectron spectroscopy (XPS). The films were found to be composed of a metallic Mg and an Mg-deficient Mg1−xB2 phase. Al and O have also been detected across the whole film, suggesting a reaction of the substrate with the adjacent Mg layer of the precursor. The films are nano-crystalline or even amorphous, which is ascribed to precipitation of impurities at grain boundaries. The critical current density of the MgB2 films extracted from both magneto-optical and magnetization measurements is Jc~1 × 107 A cm−2 at 10 K and self-field, and Jc~3 × 104 A cm−2 at 2 T and 5 K. The upper critical field is Hc 2 = 6.5 T at 5 K. Additionally, an unknown phase is found in the Mg–Al2O3 system after annealing of Mg films on sapphire in conditions similar to the synthesis process.

Phase stability limit of RuSr2GdCu2O8 at various partial oxygen pressures

Abstract The thermal stability limit of the RuSr 2 GdCu 2 O 8 phase has been determined at oxygen partial pressures of 0.35 and 0.85 bar by thermogravimetry combined with powder X-ray analysis. Decomposition of the phase was observed at 1040 °C in 0.35 bar of oxygen, and at 1060 °C in 0.85 bar of oxygen. The decomposition starts as solid phase process leading to cation redistributions in the parent phase. The peritectic decomposition occurs above 1085 °C in 35% of O 2 and above 1110 °C in 85% of O 2 .

Substitution of Ru by Cu in the synthesis of ceramic RuSr2GdCu2O8

Our research targets the study of the interaction and coupling between the two states, magnetic and superconducting, coexisting in the hybrid ruthenocuprates RuSr2GdCu2O8 (Ru-1212). While the magnetic phase arises from RuO2 layers, and the superconducting (SC) phase originates in the CuO2 planes, it is worth observing and investigating how the properties of these phases change under the substitution of Ru by Cu in this material. In order to study the effects of the annealing parameters and the substitution of Ru by Cu on the magnetic and SC properties of Ru-1212, five Cu-doped polycrystalline ceramic samples Ru1−xSr2GdCu2+xO8−δ (x = 0–0.4) were synthesized and annealed under oxygen pressure at temperatures of 1039 and 1058 °C respectively.

High-pressure synthesis and properties of a new oxycarbonitrate superconductors in the Sr-Ca-Cu-N-C-O system

Recently, oxycarbonitrate superconductors (Cu, N, C)Sr2Can-1CunOy were prepared under high pressure. They have M-12(n-1)n type structures with the M sites occupied by mixed atoms of (Cu, N, C). The phases with n = 3-6 have been isolated, identified and characterized. However, the n = 1 and 2 members have not been reported and it is even unknown whether they exist or not. In the present study, we performed systematic high-pressure synthesis experiments for the Sr-Ca-Cu-N-C-O system to find the n = 1 and 2 members. The n = 1 phase has a somewhat different characteristic, i.e. its M site is occupied only by C and N without partial replacement of Cu, having a composition of (N, C)Sr2CuOy. On the other hand, in the n = 2 phase the M site is occupied by the three atoms Cu, N and C, as in the n = 3-6 members. These two new phases show superconductivity below 33 and 91 K, respectively. The superstructure due to an ordered arrangement of Cu, C and N was studied using a high-resolution electron microscope. A superstructure with 4a×b×2c representing a simple tetragonal cell with a, b, c was observed in the n = 2 phase.