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One of the most important properties of very recently reported FeSe based superconductors is the robustness of their superconductivity under applied magnetic field. The synthesis and control of superconductivity in FeSe based compounds is rather a difficult task. Synthesis and physical property characterization for optimized superconductivity of FeSe1/2Te1/2 at 13 K is reported here. The compound crystallized in a tetragonal structure with lattice parameters a=3.8015(2) and c =6.0280(4) Å. Magnetization measurements indicated bulk superconductivity with lower critical field (Hc1) of around 180 Oe. By applying Ginzburg-Landau theory, the Hc2(0) value is estimated to be ∼1840 kOe for the 90% of resistive transition. A heat capacity measurement revealed bulk superconductivity by a hump at Tc near 13 K and an expected decrease in the same was observed under an applied magnetic field. [ABSTRACT FROM AUTHOR]

The superconductivity of MgB2, AlB2, NbB2+x, and TaB2+x is intercompared. The stretched c-lattice parameter (c=3.52 Å) of MgB2 in comparison to NbB2.4 (c=3.32 Å) and AlB2 (c=3.25 Å) decides empirically the population of their π and σ bands and as a result their transition temperature Tc values, respectively, at 39 and 9.5 K for the first two and no superconductivity for the later. The nonstoichiometry induces an increase in c parameter with Boron excess both in NbB2+x and TaB2+x. Magnetization (M-T) and resistivity measurements (ρ-T) in case of niobium boride samples show the absence of superconductivity in stoichiometric NbB2 sample (c=3.26 Å) while a clear diamagnetic signal and a ρ=0 transition for boron excess NbB2+x samples. On the other hand, superconductivity is not achieved in TaB2+x case. The probable reason behind is the comparatively lesser or insufficient stretching of c parameter. [ABSTRACT FROM AUTHOR]

We report superconductivity in the SmFe0.9Co0.1AsO system being prepared by most easy and versatile single-step solid-state reaction route. The parent compound SmFeAsO is non-superconducting but shows the spin density wave ( SDW) like antiferromagnetic ordering at around 140 K. To destroy the antiferromagnetic ordering and to induce the superconductivity in the parent system, the Fe2+ is partially substituted by Co3+. Superconductivity appears in SmFe0.9Co0.1AsO system at around 14 K. The Co doping suppresses the SDW anomaly in the parent compound and induces the superconductivity. Magnetization measurements show clearly the onset of superconductivity with T at 14 K. The isothermal magnetization measurements exhibit the lower critical fields ( Hc1) to be around 200 Oe at 2 K. The bulk superconductivity of the studied SmFe0.9Co0.1AsO sample is further established by open diamagnetic M( H) loops at 2 and 5 K. Normal-state (above Tc) linear isothermal magnetization M( H) plots excluded the presence of any ordered magnetic impurity in the studied compound. [ABSTRACT FROM AUTHOR]

We have performed a systematic study on the occurrence of superconductivity in Nb1− x Mg x B2 (0.0≤ x≤0.40). X-ray diffraction and magnetization measurements are carried out to determine the changes in lattice parameters, superconducting transition temperature ( T c ) and critical field ( H c1). The substitution of Mg at Nb site results in considerable stretching of c-parameter with only a slight change in a parameter. Rietveld analysis on X-ray diffraction patterns gives a=3.11 Å and c=3.26 Å for pure NbB2 while a=3.10 Å and c=3.32 Å for Nb0.60Mg0.40B2. This increased c-parameter introduces superconductivity in niobium diboride. Magnetization measurements though indicate the absence of superconductivity in NbB2, the same shows a clear diamagnetic signal at about 10 K for Nb0.60Mg0.40B2 sample. The magnetization M( H) plots exhibit weak superconductivity like hysteresis loops. The stretching of c-parameter from around 3.26 to 3.32 i.e. by 0.06 cannot be explained solely by substitution of Nb by Mg in the lattice. It seems that some Nb deficiencies are introduced in the Nb1− x Mg x B2 as Mg is not substituted completely at the vacant Nb sites. This could be seen from XRD results, where one can clearly notice the presence of small amount of MgO in Nb1− x Mg x B2 samples. [ABSTRACT FROM AUTHOR]

Abstract: We report the synthesis, magnetic susceptibility and crystal structure analysis for samples. The study helps find a correlation among the lattice parameters, chemical composition and the superconducting transition temperature . Rietveld analysis is done on the -ray diffraction patterns of all synthesized samples to determine the lattice parameters. The parameter decreases slightly and has a random variation with increasing , while parameter increases from 3.26 for pure NbB2 to 3.32 for i.e. NbB2.4. With higher Boron content the parameter decreases slightly. The stretching of lattice in direction induces superconductivity in the non-stoichiometric niobium boride. Pure NbB2 is non-superconductor while the other samples show diamagnetic signal in the temperature range 8.9–11 K. Magnetization measurements () at a fixed temperature of 5 K are also carried out in both increasing and decreasing directions of field. The estimated lower and upper critical fields ( & ) as viewed from plots are around 590 and 2000 Oe, respectively, for NbB2.6 samples. In our case, superconductivity is achieved in NbB2 by varying the Nb/B ratios, rather than changing the processing conditions as reported by others. [Copyright &y& Elsevier]

Abstract: We report the synthesis and variation of superconductivity parameters such as transition temperature , upper critical field , critical current density , irreversibility field and flux pinning parameter for the MgB2−x C x system with nano-carbon doping up to . Carbon substitutes successfully on boron site and results in significant enhancement of and . Resistivity measurements reveal a continuous decrease in under zero applied field, while the same improves remarkably at higher fields with an increase in nano-carbon content for MgB2−x C x system. The irreversibility field value is 7.6 and 6.6 T at 5 and 10 K respectively for the pristine sample, which is enhanced to 13.4 and 11.0 T for sample at same temperatures. Compared to undoped sample, critical current density for the nano-carbon-doped sample is increased by a factor of 24 at 10 K at 6 T field. [Copyright &y& Elsevier]

Bulk polycrystalline samples of nano-Mo doped MgB2 were synthesized by Fe tube encapsulation at ambient pressure under argon annealing (850°C). Mo substitution takes place successfully at the Mg site in Mg1-xMoxB2 only till x=0.2. For higher (x>0.2) Mo content the same did not enter the MgB2 lattice but rather forms an isomorphic lattice in the host with decreased c but an increased a-parameter. The ρ(T) measurements showed superconducting transition temperature (Tc) of around 36 K for all the samples till x=0.3 and slightly decreased values of 35 and 34 K for x=0.4 and 0.5 samples, respectively. Resistivity under magnetic field [R(T)H] experiments showed distinct single peaks in dρ/dT for all applied fields up to 8 Tesla. The estimated upper critical field Hc2 is 8 Tesla for pristine and x=0.2 samples at 15.6 and 19 K, respectively. Thus Hc2 increases up to x=0.20 samples and decreases afterwards. Magnetic susceptibility measurements exhibited sharp transition to superconducting state with a sizeable diamagnetic signal at 38 K (Tc) in zero field-cooled measurements. Commendable current density (Jc) of up to 105 A/cm2 in 1–2 T (Tesla) fields at temperatures (T) of up to 10 K is seen from magnetization measurements invoking the Bean's critical state model for pristine samples. For higher fields above 2.5 Tesla the Jc(H) characteristics of x=0.1 and 0.2 samples were found to be slightly superior to that for pristine samples with enhanced Hirr (irreversibility field). [ABSTRACT FROM AUTHOR]

Abstract: We report synthesis and superconductivity of Pb-1212–Cu1− x Pb x Sr2Y0.6Ca0.4Cu2O7 (x =0.5–0.9) compounds. These compounds were synthesized through solid-state reaction route with optimized sintering temperatures and conditions. In particular, one needs to employ reducing atmosphere conditions to achieve superconductivity in higher Pb content samples. The X-ray diffraction (XRD) patterns reveals that all the compounds are crystallized in space group P4/mmm RE-123 structure. Superconductivity at 56K (onset) is achieved for Pb content as high as 90%. Our study reveals that superconductivity and structure stabilization in Pb-based are more critical to synthesizing conditions than other cuprates. It is concluded that superconductivity can be introduced in Pb-1212 compounds by synthesizing the same in reducing atmosphere and thus the Pb in lower (<+4) valence state. This ensures the replacement of Cu–O x chains of RE-123 by Pb–O x sheets acting as charge reservoir carrier donating blocks. [ABSTRACT FROM AUTHOR]