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The high field magnetization and magnetotransport measurements are carried out to determine the critical superconducting parameters of MgB2-xCx system. The synthesized samples are pure phase and the lattice parameter evaluation is carried out using the Rietveld refinement. The R-T(H) measurements are done up to a field of 140 kOe. The upper critical field values, Hc2, are obtained from these data based on the criterion of 90% of normal resistivity, i.e., Hc2=H at which ρ=90%ρN, where ρN is the normal resistivity, i.e., resistivity of about 40K in our case. The Werthamer–Helfand–Hohenberg prediction of Hc2(0) underestimates the critical field value even below the field up to which measurement is carried out. After this model, the Ginzburg–Landau theory is applied to the R-T(H) data which not only calculate the Hc2(0) value but also determine the dependence of Hc2 on temperature in the low temperature high field region. The estimated Hc2(0)=157.2 kOe for pure MgB2 is profoundly enhanced to 297.5 kOe for the x=0.15 sample in MgB2-xCx series. Magnetization measurements are done up to 120 kOe at different temperatures and the other parameters such as irreversibility field Hirr and critical current density Jc(H) are also calculated. The nano carbon doping results in substantial enhancement of critical parameters such as Hc2, Hirr, and Jc(H) in comparison to the pure MgB2 sample. [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 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]

Abstract: The physical property characterization of Al doped Mg1− x Al x B2 system with x =0.0 to 0.50 is reported. The results related to phase formation, structural transition, resistivity ρ(T) and magnetization M(T) measurements are discussed in detail. It is shown that the addition of electrons to MgB2 through Al results in loss of superconductivity. Also seen is a structural transition associated with the collapse of boron layers reflected by the continuous decrease in the c parameter. The main emphasis in this paper is on slow scan X-ray diffraction (XRD) results, which confirm the existence of a superstructure along the c-direction for the x =0.50 sample. The appearance of some additional peaks, viz. [103], [004], [104] and [112], results in doubling of the lattice parameter along the c-axis. This possibly indicates the alternative ordering of Al and Mg in MgAlB4 separated by hexagonal boron layers but still maintaining the same hexagonal AlB2 type structure. [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]

Dc magnetization studies were carried out on a polycrystalline MgB2 superconductor, which was synthesized at 750 °C in Fe tube encapsulation under a vacuum of 10-5 Torr. Zero resistances at H=0 and 8 T are obtained at 38 and 22 K, respectively. The material possesses a sharp diamagnetic transition in the zero-field-cooled branch, whereas the field-cooled branch exhibits the paramagnetic-Meissner-effect-like phenomenon below Tc, indicating strong pinning in this compound. The critical current density at 20 K and 2 T, estimated by the Bean model is higher than 105 A/cm2. Due to the strong pinning, we observed the presence of flux avalanches below H=2 T at temperatures below 20 K. The magnetization M(H) grows (as usual) slowly with H and falls sharply to near zero moment value, and further grows again in a common way. The flux avalanches were seen quite symmetric in both increasing/decreasing the field in all four quadrants of the M(H) loops. The dynamics behavior of sinusoidal-like symmetric reproducible flux avalanches is discussed. [ABSTRACT FROM AUTHOR]

Abstract: This paper deals with the determination of critical properties of MgB2 along with the impact of carbon substitution on critical parameters. The change in lattice parameters and decrease of transition temperature, Tc confirms the successful substitution by carbon at boron site. The magneto transport measurements up to 140kOe are carried out to determine upper critical field (H c2). The upper critical field values, H c2 are obtained from ρ–T(H) data based upon the criterion of 90% of normal resistivity. The Ginzburg Landau theory (GL equation) is applied to the ρ–T(H) data which accounts for the temperature dependence behavior of Hc 2 in the low temperature high field region along with the determination of Hc (0) value. The Hc (0) value of about 300kOe is obtained for the carbon doped sample while the same is just near to 160kOe for the pure MgB2 sample. [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]

Abstract: In this paper we investigate the properties of polycrystalline series of Ru1− x Cr x Sr2Eu1.5Ce0.5Cu2O10− δ (0.0⩽ x ⩽0.40) by resistivity, XRD and dc magnetization measurements. EuRu-1222 is a reported magneto superconductor with Ru spins magnetic ordering at temperatures near 100K and superconductivity occurs in Cu–O2 planes below T c ≅40K. The exact nature of Ru spins magnetic ordering is still being debated and no conclusion has been reached yet. In this work, we found the superconducting transition temperature T c =20K from resistivity and dc magnetization measurements for pristine sample. DC magnetization measurements exhibited ferromagnetic like transition for all samples. [ABSTRACT FROM AUTHOR]

Abstract: Synthesis and structural details of MSr2RECu2O z (M-1212) compounds with M=Ga, Nb, Fe, Al and Co) and RE=Eu, Y are reported. Reitveld refinement of X-ray diffraction (XRD) patterns shows that all compounds are crystallized in single phase. Nb-, Fe- and Al-1212 possess tetragonal P4/mmm space group structure while the Ga-1212 and Co-1212 are crystallized in orthorhombic Ima2 space group. The change of space group from P4/mmm to Ima2 indicating towards the doubling of unit cell. The buckling angle [Cu(2)–O(2)–Cu(2) angle] shows that most of the studied samples are heavily under doped and hence they could not exhibit superconductivity. Thermogravimetric (TGA) analysis shows the M-1212 compounds to be more stable than widely studied 90K superconductor Cu-1212 (RE-123). [ABSTRACT FROM AUTHOR]

Abstract: The thermoelectric power S(T) studies of Bi2Sr2CaCu2O8 (Bi2212) samples, including as-processed Partially Melted (Bi2212-PM) and 800°C O2-annealed (Bi2212-O2) are reported here. While Bi2212-PM sample is near optimum doping regime, Bi2212-O2 is an over-doped sample. values obtained through S(T) data match very well with those deduced from the R(T). Interestingly, S(T) behavior of the optimally-doped Bi2212-PM sample is seen to be positive in whole temperature range, it is found negative for the over-doped Bi2212-O2 sample above . The observed negative S(T) dependence of the over-doped Bi2212-O2 is in accord with the recent band structure calculations and is in line with the theoretical investigations on a van Hove singularity in electronic density of states in HTSc compounds. [ABSTRACT FROM AUTHOR]

Abstract: The enhancement of the critical current density (J c(H)) of carbon and nano-SiC doped MgB2 is presented and compared. The upper critical field (H c2) being determined from resistivity under magnetic field experiments is though improved for both C substitution and nano-SiC addition the same is more pronounced for the former. In MgB2− x C x carbon is substituted for boron that induces disorder in the boron network and acts as internal pinning centres. The optimal J c(H) values are obtained for x =0.1 sample. In case of nano-SiC doped in MgB2, the J c(H) improves more profoundly and two simultaneous mechanisms seems responsible to this enhancement. Highly reactive nano-SiC releases free carbon atom, which gets easily incorporated into the MgB2 lattice to act as intrinsic pinning centres. Further enhancement is observed for higher nano-SiC concentrations, where the un-reacted components serve as additional extrinsic pinning centres. [Copyright &y& Elsevier]

We report an easy and versatile one-step synthesis route for a newly discovered Fe based LaFeAsO1− δ compound with 0.0≤ δ≤0.15. Instead of widely used high-pressure-high-temperature ( HPHT) synthesis, we applied the normal atmosphere solid-state reaction route. The stoichiometric mixtures of Fe, La2O3, La, and As in ratio LaFeAsO1− δ with 0.0≤ δ≤0.15 are sealed in an evacuated quartz tube and further heated at 500, 850, and 1,100 °C in Ar for 12, 12, and 33 hours, respectively, in a single step. The resulting compounds are single phase LaFeAsO crystallized in tetragonal P4/ nmm structure. These samples showed the ground state spin density wave ( SDW) like metallic behavior below around 150 K. In conclusion, the ground state (non-superconducting) of newly discovered Fe based superconductor is synthesized via an easy one-step solid-state reaction route. [ABSTRACT FROM AUTHOR]

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]

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]