Papers by Keyword: Superconductivity

Abstract: We study asymmetrical three-layers F1F2S and F1SF2 structures in an external parallelmag-netic field. Assuming that all F and S layers are dirty, we solve the boundary value problem for theUsadel function. We calculate the critical temperature of in the F1F2S and F1SF2 systems as functionof the F layers thicknesses and external magnetic field.

Abstract: We report the results of a detailed study of EuRh₄B₄ compound synthesized by an un-common method under high pressure temperature conditions for the first time. Synthesized sample has been initially characterized by X-ray diffraction, macroscopic electrical resistivity and ac-susceptibility methods. The ferromagnetic and superconducting phases were found below 83K and 5.8K respectively. Element-and shell-selective XANES and XMCD techniques have been further applied to check the possible valence transition of Eu ions and their magnetic states. XANES spectra recorded at the Eu L_2,3 absorption edges clearly indicate the presence of either magnetic Eu²⁺ or non-magnetic Eu³⁺ ions in the sample. The XMCD studies have also shown that local magnetic moments of Eu²⁺ ions exist in the superconducting EuRh₄B₄, at least above the transition.

Abstract: Superconductor samples Mg(B_1-2x(SiC)_x)₂ (x=0, 5%, 10%) are synthesized from nano SiC, Mg and amorphous boron powders by microwave direct synthesis in a short time. Powder X-ray diffraction (XRD) analysis indicates that the phases of the synthesis sample are MgB₂ (major phase) and a small amount of MgO and Mg₂Si. The main peaks of MgB₂, (100), (101), (002) and (110) are shift to the higher diffraction angle position and the width of half height of the diffraction plane is broaden for the SiC doping Mg(B_1-2x(SiC)_x)₂, which show that the B positions of MgB₂ are partly substituted and the grains of MgB₂ are fine. Scanning electron microscope (SEM) observation shows that the MgB₂ grain size is very small and the sample is tightness (compact). The onset superconducting transition temperature of the Mg(B_1-2x(SiC)_x)₂ (x=0, 5%, 10%) samples measured by magnetization measurement are about 37.6 K, 37.0 K, 36.8 K respectively. The critical current density J_c are calculated according to the Bean model from the magnetization hysteresis loop of the slab Mg(B_1-2x(SiC)_x)₂ (x=0, 5%, 10%) samples. The critical current density Jc of nano SiC doping Mg(B_1-2x(SiC)_x)₂ samples are greatly enhanced. In higher external magnetic field, the J_c of 10% SiC doped sample is the highest; in lower external magnetic field, the Jc of 5% SiC doped sample is the highest; while in the whole external magnetic field, the J_c of undoped sample is the lowest.

Abstract: Our Recent Works on the Search for New Superconductors Using Soft-Chemical Techniques Are Reviewed. We Have Found New Mg-Intercalated Superconductors of Mg_xZrNCl and Mg_xHFNCl with Tc ~ 13 K and 25 K, Respectively. Moreover, we Have Succeeded in Preparing Bulk Samples of Hole-Doped (La,Ca)₂CuO₄ with the Nd₂CuO₄-Type, Namely, the so-Called T’-Type Structure by Reduction of (La,Ca)₂CuO₄ with the K₂NiF₄-Type, Namely, the so-Called T-Type Structure Using CaH₂ at a Temperature as Low as 300°C and Subsequent Oxidation. This Result Is in Contrast with the Synthesis of (La,Ca)₂CuO₄ with the K₂NiF₄-Type Structure by the Conventional Solid-State Reaction Method at a Temperature as High as ~1000°C.

Abstract: The superconducting Tl_0.8Bi_0.2Sr₂Ca_0.8Y_0.2Cu₂O₇ powder prepared from chemical route was mixed with varying amounts of nano-sized MgO powders (x=0 to 0.50 wt. %) to form bulk superconducting composites. XRD results show that the addition of 0.15 wt.% nano-MgO is accompanied by an increase in 1212-phase and the disappearance of SrCO₃ impurity. SEM investigations revealed more regular-shaped and smaller grains microstructure for MgO added samples in contrast to MgO-free sample which revealed irregular shaped grains. EDS showed homogenous distribution of Mg for all MgO-added samples with the exception of x=0.50 where agglomeration of MgO particles were observed. However, MgO added samples exhibit lower zero-resistance critical temperature, T_{c zero} than the MgO-free sample.

Abstract: The effect of Fe doping on the structure, local electron density and electronic transport properties of DyBa₂Cu_3-XFe_xO_7-δ (x = 0.00- 0.30) superconductors have been investigated by X-ray diffraction (XRD), positron annihilation and other experimental methods. XRD results indicate that the compound undergoes an Orthorhombic–Tetragonal (O-T) phase transition at about x = 0.10. The electrical transport measurements show that the superconductivity was strongly suppressed by Fe doping, and the superconducting transition temperature decreases with the increase of Fe doping content. The results of local electron density calculated using positron annihilation data show that the local electron density of the systems is strongly changed by the Fe doping, and the local electron density vary noticeably at the range of O-T phase transition. From the results of local electron density and the superconductivity, it can be found that the superconducting transition temperature decreases with decreasing the local electron density. These investigations reveal that the crystal phase transition and the local electron density could strongly affect the superconductivity in the Fe doped DyBa₂Cu_3-XFe_xO_7-δ systems.

Abstract: In the age of technology, with smaller and smaller electronic components being used in a growing number of applications, one pertinent application of mathematics and physics is the study of superconductivity. Superconductive materials are capable of conducting electricity without any resistance and were first discovered by Kamerlingh Onnes in 1911 in a compound He at 4.2 K in what was to prove to be one of the most significant scientific breakthroughs of the 20th Century. Superconductivity and Magnetism are mutually exclusive to each other but the coexistence of both the phenomena leads to very interesting research of superconductivity at room temperature. In this paper, we have studied the effect of ferromagnetic and antiferromagnetic interactions on the superconducting transition temperature.

Abstract: NMR spin shift data of La1:85Sr0:15CuO4, HgBa2CuO4+ at ambient pressure, andof YBa2Cu4O8 at pressure up to 63 kbar are discussed that fail the traditional single-electronicuid picture used for the discussion of NMR data. Instead the results point to a commonexplanation for all three systems in terms of two electronic components. One of them hasa temperature-dependent magnetic susceptibility while the second component, as well as itscoupling to the rst component, are constant above the superconducting transition temperatureTc. All susceptibilities vanish below Tc.

Abstract: We report on structural and superconducting properties of La3-xRxNi2B2N3- where La is substituted by the magnetic rare-earth elements Ce, Pr, Nd. The compounds Pr3Ni2B2N3- and Nd3Ni2B2N3- are characterized for the first time. Powder X-ray diffraction confirmed all samples R3Ni2B2N3- with R = La, Ce, Pr, Nd and their solid solutions to crystallize in the body centered tetragonal La3Ni2B2N3 structure type. Superconducting and magnetic properties of La3-xRxNi2B2N3- were studied by resistivity, specific heat and susceptibility measurements. While La3Ni2B2N3- has a superconducting transition temperature Tc ~ 14 K, substitution of La by Ce, Pr, and Nd leads to magnetic pair breaking and, thus, to a gradual suppression of superconductivity. Pr3Ni2B2N3- exibits no long range magnetic order down to 2 K, Nd3Ni2B2N3- shows ferrimagnetic ordering below TC =17 K and a spin reorientation transition to a nearly antiferromagnetic state at 10 K.

Abstract: We performed experimentally the electron-density mapping on the crystal structure of the iron-based superconductor Lu2Fe3Si5 which revealed the Fe-Si covalent-bonding network with the fully ionized Lu-sites. And, for Lu2Fe3Si5, we investigated the effect of Fe- and Si-site substitutions on the superconductivity. The Fe-site-substitution study revealed that, in addition to the disorder-sensitive superconductivity, the additionally-provided 3d electrons or holes tend to be itinerant rather than localized. Taking into account that the localized magnetic moments introduced on the Lu-sites are inactive as magnetic pair breakers, the present results suggest that, in Lu2Fe3Si5, the Fe-Si bonding network should be the playground for the exotic superconductivity which is characterized by the sign-reversed superconducting gap function.