Electronic and Magnetic Properties of Superconducting Sr4V2Fe2As2O6 Versus Sr4Sc2Fe2As2O6

First principles of FLAPW-GGA calculations have been performed with the purpose to understand the peculiarities of band structure and Fermi surface topology for recently discovered 37 K superconductor: Sr₄V₂Fe₂As₂O₆—in comparison with isostructural phase Sr₄Sc₂Fe₂As₂O₆. Our main finding is that the replacement of Sc with V leads to drastic transformation of electronic, magnetic and conductive properties of these materials: as against non-magnetic Sr₄Sc₂Fe₂As₂O₆ which is formed from non-magnetic conducting [Fe₂As₂] and insulating [Sr₄Sc₂O₆] blocks, Sr₄V₂Fe₂As₂O₆ consists of non-magnetic conducting [Fe₂As₂] blocks and [Sr₄V₂O₆] blocks which exhibit magnetic half-metallic properties.     

Growth and characterizations of Ba2Ti2Fe2As4O single crystals

Abstract

Single crystals of a new iron-based superconductor Ba₂Ti₂Fe₂As₄O have been grown successfully via a Ba₂As₃-flux method in a sealed evacuated quartz tube. Bulk superconductivity with T_c ～ 21.5 K was demonstrated in resistivity and magnetic susceptibility measurements after the as-grown crystals were annealed at 500 °C in vacuum for a week. X-ray diffraction patterns confirm that the annealed and the as-grown crystals possess the identical crystallographic structure of Ba₂Ti₂Fe₂As₄O. Energy-dispersive x-ray spectra indicate that partial Ti/Fe substitution exists in the [Fe₂As₂] layers and the annealing process redistributes the Ti within the Fe-plane. The ordered Fe-plane stabilized by annealing exhibits superconductivity with magnetic vortex pinned by Ti.     

Synthesis and Magnetic Properties of Fe3O4 Nanoparticles

Magnetic Fe₃O₄ nanoparticles with size below 10 nm have been prepared by the aqueous phase coprecipitation method. The Fe₃O₄ nanoparticles show typical superparamagnetism. Comparison is made between the dispersed sample and the powder sample, and the results are discussed.     

Assembly of Fe3O4 nanoparticles on SiO2 monodisperse spheres

The assembly of superparamagnetic Fe₃O₄ nanoparticles on submicroscopic SiO₂ spheres have been prepared by an in situ reaction using different molar ratios of Fe³⁺/Fe²⁺ (50–200%). It has been observed that morphology of the assembly and properties of these hybrid materials composed of SiO₂ as core and Fe₃O₄ nanoparticles as shell depend on the molar ratio of Fe³⁺/Fe²⁺.     

NH4Zr2V3O12 proton conductor

NH₄Zr₂V₃O₁₂, a new proton conductor, has been synthesized by flux, melt and hydrothermal methods. The crystals were subjected to X-ray diffraction, differential thermal analysis, infrared spectroscopy and impedance measurements.     

Piezoelectric properties of Sr3Ga2Ge4O14 single crystals

A new piezoelectric single crystal, Sr₃Ga₂Ge₄O₁₄ (SGG), has been grown successfully by the vertical Bridgman method with crucible-sealing technique. SGG crystal up to 2″ in diameter has been obtained. The relative dielectric constants, the piezoelectric strain constants, elastic compliance constants and electromechanical coupling factors have been determined with resonance and anti-resonance frequencies method by using the impedance analyzer (Agilent 4294A). The results show that the piezoelectric strain constants and electromechanical coupling factors of SGG single crystal are higher than those of LGS single crystals making it a potential substrate material for surface-acoustic wave applications.     

Some thermodynamic aspects of the high-temperature transition in doped V2O3

Measurements of the entropy change are reported for the high-temperature metal-insulator (MI) transitions in the (V_1–xCr_x)₂O₃ and (V_1–xAl_x)₂O₃ systems. It is emphasized that the entropy of the I phase exceeds that of the M phase. Evidence is presented to show that the M and I phases coexist over a narrow temperature range. The transformation is attended by enormous hysteresis effects; these indicate that the lattice plays an important role in the transition. The probable role of Cr³⁺ and Al³⁺ as a dopant in the V₂O₃ lattice is briefly discussed. A phase diagram for the dilute V₂O₃-Al₂O₃ alloy system is presented.     

Incommensurate fluctuations in Bi2Sr2CaCu2O8

A special neutron scattering technique has been used to discover an incommensurate fluctuation in Bi₂Sr₂CaCu₂O₈ that appears below T_c. The fluctuation is identified as a dynamic charge density wave since its scattering intensity appears to increase with increasing momentum transfer. The fluctuation is found at a wavevector near 2k_F and could be associated with a dynamic stripe phase.     

Theoretical Consideration on Triplet Superconductivity in UPt3 and Sr2RuO4

Two representative superconductors, UPt₃ and Sr₂RuO₄, are comparatively studied on the basis of a general symmetry argument. The existing experimental data set for both systems strongly and commonly points to a triplet pairing. For the former, heavy fermion material, UPt₃, a non-unitary bipolar state is the most possible candidate, whereas for the latter system, Sr₂RuO₄, determination of a precise pairing symmetry class, either unitary or non-unitary, is debated.     

Superconductivity in Na-doped Bi2Sr2CaCu2O y system

Samples with the nominal composition Bi₂Sr₂Ca_1−x Na _x Cu₂O _y (x=0, 0·1, 0·2 and 0·3) were prepared by solid-state reaction of the individual compounds. X-ray diffraction patterns indicate that the samples have a majority 2212 phase with 2223 also being present. From the DC four-probe resistance data, we have observed that the furnace-cooled samples show metallic behaviour while the quenched samples show superconductivity up to 97 K.     

磁性纳米四氧化三铁在生物医学中的应用

近年来,由于磁性纳米粒子在实际应用中发挥越来越重要的作用,有关磁性纳米粒子的应用受到科学界广泛关注,特别是生物医学领域。由于磁性纳米Fe_3O_4粒子制作简单且晶体对细胞无毒,在生物医药领域大量应用,磁性纳米Fe_3O_4粒子主要通过表面包覆成为免疫磁性微球进行使用。简述了磁性纳米Fe_3O_4粒子的制备方法,重点综述了近些年磁性纳米Fe_3O_4粒子在生物医学上的应用,包括磁共振成像技术、磁分离技术、靶向药物载体技术、肿瘤热疗技术、造影剂技术,并且阐述了磁性纳米Fe_3O_4粒子的发展前景。     

Structural and optical properties of spray pyrolytically prepared Fe2O3 thin films

Fe₂O₃ thin film was prepared using aqueous solution of FeCl₃ by spray pyrolysis. The substrate temperature was 450°C. The lattice parametersa andc for different concentrations were calculated from X-ray diffraction study. Hexagonal structure of the Fe₂O₃ thin film was confirmed. Band gap values of Fe₂O₃ prepared from different concentrations were determined from optical transmission data.     

High temperature resistivity characteristics of nonstoichiometric V2O3

The resistivity anomaly encountered in stoichiometric V₂O₃ is gradually suppressed with increasing density of cation vacancies. These observations are discussed in terms of related findings reported in the literature. Research supported by NSF-MRL Grant DMR 77-23798.     

Fe-EDTA Thermal Decomposition,a Route to Highly Crystalline Hematite (Alpha Fe2O3) Nanoparticle Synthesis

In this study, we develop an experimental procedure to synthesize hematite nanoparticles by hydrothermal decomposition of Fe-EDTA complex in the presence of hydrogen peroxide, starting from ferric ammonium sulfate and Na ₄ EDTA as main precursors. The product was investigated by X-ray diffraction, scaning electron microscopy, dispersive X-ray spectroscopy, magnetic measurements, and UV-vis optical absorption measurements. The size of nanoparticles was determined to be 42 nm evaluated by XRD patterns using the Scherrer equation. This method allowed the formation of pure hematite nanoparticles with good and stable crystallographic characteristics. This procedure can be an effective method for synthesizing hematite nanoparticles exhibiting good crystallinity, stoichiometry, magnetic, and optical band gap properties. A possible mechanism for the formation of hematite nanocrystals was suggested.     

Preparation and characterisation of PPy/NanoGs/Fe3O4 conductive and magnetic nanocomposites

Nanocomposites composed of polypyrrole (PPy), graphite nanosheets (NanoGs), magnetite (Fe₃O₄) nanoparticles, have been successfully synthesised with a two-step process. First, we prepared NanoGs/Fe₃O₄ powder via wet chemical co-precipitation method. Next, pyrrole was polymerised in the suspension of NanoGs/Fe₃O₄ and then PPy/NanoGs/Fe₃O₄ nanocomposites were produced. The products were characterised by Fourier-transform infrared spectroscopy, Transmission electron microscopy, Thermogravimetric, conductivity and magnetisation analysis. The result showed that the conductivity of the PPy/NanoGs/Fe₃O₄ composites, compared with pure PPy, increased dramatically. And the saturation magnetisation of nanocomposites increased with the increase of the volume fraction of the Fe₃O₄ particles. In addition, according to the thermal gravimetric analysis, compared with PPy, nanocomposites exhibited enhanced thermal stability due to the introduction of NanoGs/Fe₃O_4. The PPy/NanoGs/Fe₃O₄ composites show potential applications in electric–magnetic shield materials.     

Evidence for supercooling across a vortex-matter phase transition: studies on CeRU2 and Bi2Sr2CaCu2O8

We present DC magnetization data indicating a first-order phase transition in the vortex state of CeRu₂, with the higher entropy phase exhibiting enhanced pinning. Minor hysteresis loops show evidence of supercooling of the higher entropy phase as the phase boundary is crossed both isothermally as well as at constant field. These features are shown to be absent across the Bragg-glass to vortex-glass transition in Bi₂Sr₂CaCu₂O₈. The supercooling is more persistent in the constant field case.     

Characteristic analysis of Fe3O4 nanoparticles modified by oleic acid and undecylenic acid

Carbonization of magnetic polymer microspheres is one of the methods for the preparation of magnetic carbon materials. Fe₃O₄ magnetic particle characteristics considerably influence the magnetic content and size distribution of magnetic polymer microspheres. The characteristics of Fe₃O₄ nanoparticles modified by oleic acid (OA) and undecylenic acid (UA) were analyzed by X-ray diffraction, Fourier transform infrared, scanning electron microscopy, dynamic laser light scattering, thermogravimetry/differential thermogravimetry, vibrating sample magnetometer, and water contact angle. Fe₃O₄ nanoparticles modified by OA and UA are nearly spherical and exhibit superparamagnetism. Fe₃O₄ particle size and saturation magnetization are slightly influenced by the OA and UA composition. OA and UA both are chemically adsorbed onto Fe₃O₄ as bidentate chelates. OA shows easier adsorption onto Fe₃O₄ than UA. OA groups have an expanded arrangement on OA@Fe₃O₄, whereas UA groups have a condensed arrangement on UA@Fe₃O₄. Particle lipophilicity decreases and particle clustering increases with decreasing OA content and increasing UA content on OA-UA@Fe₃O₄ nanoparticles.     

The Specific Heat and the Fermi Surface Anisotropy in Sr2RuO4

The influence of the Fermi surface anisotropy on the specific heat jump is studied for the p-wave superconductor with a gap function d = z^(k _x ± ik _y ) in connection with Sr₂RuO₄. It is shown that the normalized specific heat jump, C/T _c , is reduced from its universal value, 1.43, in accordance with experimental findings. We show that this behavior fits well into our present understanding of the quasiparticle spectrum and the impurity effects.     

Structural and magnetic study of nitrided Co74Fe8B12Si6 and Co74Fe4Mn4B12Si6 ribbons

The nitriding thermochemical treatment (NTT) is commonly used for steels. In this paper, the experimental conditions required for NTT, and the influence of such treatments on the structure and hysteresis loops of Co₇₄Fe₈B₁₂Si₆ and Co₇₄Fe₄Mn₄B₁₂Si₆ ribbons are reported. The results have been compared with those obtained with ribbons treated according to conventional thermal treatment (CTT) as well.     

Superconducting Properties of the 3-K Phase of Sr2RuO4 near Hc2

The superconducting transition temperature, T _c , of the impurity-free, intrinsic Sr₂RuO₄ is as high as 1.50 K. However, we recently showed that T _c is remarkably increased up to 3 K in the Sr₂RuO₄–Ru eutectic system, in which plate-like microdomains of Ru metal are embedded in the primary-phase Sr₂RuO₄. The phase diagram of the anisotropic upper critical field of the 3-K phase indicates that H _c2 for the field parallel to the RuO₂ plane is strongly suppressed at low temperatures. We argue that the reorientation of the Cooper-pair spin direction near the Sr₂RuO₄–Ru interface may be responsible for this suppression. In addition, we observed unusual hysteresis in the out-of-plane resistivity, _c , at low temperatures and near H _c2, only when the field was applied parallel to the RuO₂ plane.