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We report on the superconducting performance of the ex-situ SiC doped MgB2 monofilamentary tapes. Polycrystalline powders of MgB2 doped with 5 and 10 wt% SiC were synthesized by conventional solid-state reaction route and characterized for their superconducting performance. It is found that superconducting parameters i.e. upper critical field (Hc2), irreversibility field (Hirr) and critical current density (Jc) are all improved significantly with SiC addition. Also it was found that relatively lower synthesis temperature (700 C) resulted in further improved superconducting parameters. As synthesized powders are used for ex-situ powder-in-tube (PIT) monofilamentary tapes and superconducting parameters are determined. Albeit the superconducting transition temperature (Tc) is decreased slightly (2K) for SiC doped tapes, the superconducting performance in terms of critical current density (Jc), being determined from both magnetization and transport measurements, is improved significantly. In particular the SiC doped and 700 {\deg}C synthesized MgB2 tapes exhibited the transport Jc of nearly 10^4 A/cm2 under applied fields of as high as 7 Tesla. Further it is found that the Jc anisotropy decreases significantly for SiC doped tapes. Disorder due to substitution of C at B site being created from broken SiC and the presence of nano SiC respectively in SiC added ex-situ MgB2 tapes are responsible for decreased anisotropy and improved Jc(H) performance., Comment: 15 pages, 7 figures

In the present study, we report an intercomparison of various physical and electronic properties of MgB2 and AlB2. In particular, the results of phase formation, resistivity ρ(T), thermoelectric power S(T), magnetization M(T), heat capacity (C P ), and electronic band structure are reported. The original stretched hexagonal lattice with a=3.083 A, and c=3.524 A of MgB2 shrinks in c-direction for AlB2 with a=3.006 A, and c=3.254 A. The resistivity ρ(T), thermoelectric power S(T) and magnetization M(T) measurements exhibited superconductivity at 39 K for MgB2. Superconductivity is not observed for AlB2. Interestingly, the sign of S(T) is +ve for MgB2 the same is −ve for AlB2. This is consistent with our band structure plots. We fitted the experimental specific heat of MgB2 to Debye–Einstein model and estimated the value of Debye temperature (Θ D) and Sommerfeld constant (γ) for electronic specific heat. Further, from γ, the electronic density of states (DOS) at Fermi level N(E F) is calculated. From the ratio of experimental N(E F) and the one being calculated from DFT, we obtained value of λ to be 1.84, thus placing MgB2 in the strong coupling BCS category. The electronic specific heat of MgB2 is also fitted below T c using α-model and found that it is a two gap superconductor. The calculated values of two gaps are in good agreement with earlier reports. Our results clearly demonstrate that the superconductivity of MgB2 is due to very large phonon contribution from its stretched lattice. The same two effects are obviously missing in AlB2, and hence it is not superconducting. DFT calculations demonstrated that for MgB2, the majority of states come from σ and π 2p states of boron on the other hand σ band at Fermi level for AlB2 is absent. This leads to a weak electron phonon coupling and also to hole deficiency as π bands are known to be of electron type, and hence obviously the AlB2 is not superconducting. The DFT calculations are consistent with the measured physical properties of the studied borides, i.e., MgB2 and AlB2.

Temperature dependence behavior of resistivity for nano diamond doped MgB 2 superconductor was measured in different magnetic field 0 T ⩽ H ⩽ 8 T. Irreversibility ( H irr ) is enhanced by nano diamond doping. We reveal that flux-creep activation energy is a nonlinear function of current density U 0 ∝ J c - 0.2 . In order to determine the activation energy of thermally activated flux flow, Arrhenius law has been taken into account. The magnetic field dependence of flux flow activation energy of nano diamond doped MgB 2 superconductors was calculated from the slope of the linear part of the Arrhenius plot and found to be decrease corresponding with applied field. Therefore, the critical current density of this sample will need to be enhanced for practical applications.

We report an easy and versatile route of synthesis for ReFeAsO (Re = La, Gd, Pr and Sm) by both single step and precursor based double step solid-state reaction routes. The studied ground state compounds of the new FeAs based superconductor family crystallize in tetragonal P4/nmm structure, with lattice parameters variation following their ionic radii. The resistivity measurements exhibit clearly the appearance of a metallic step below say 150-130 K, which is reminiscent of the SDW (Spin Density Wave) magnetic character. Also, It is observed that oxygen stoichiometric ReFeAsO is not formed and rather the same is slightly oxygen deficient. The Mossbauer spectroscopy (MS) carried out on some of them at 290 K and 90 K i.e. both above and below the SDW temperature exhibits the Fe ordered state at 90 K. Further the MS is proved to be the most efficient technique in picking up the FeAs based impurities in these materials. The heat capacity measurements on SmFeAsO, clearly demonstrates the SDW temperature with a hump in Cp(T) at around 130 K. Further Sm spins are ordered anti-ferro-magnetically (AFM) at 4.5 K. Superconductivity is also induced in SmFeAsO with Co substitution at Fe site with Tc = 14 K. The general behavior of the ReFeAsO is discussed and it is concluded that (a) they can be synthesized by an easy and versatile methods, (b) their ground state is not oxygen stoichiometric, but deficient and (c) MS is the most efficient technique to pick up the Fe based impurities in these materials, that is yet rampant in this new class of superconductors.

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 Tcdia 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.

A reasonable cause of absence of hump structure in thermal conductivity of MgB2 below the superconducting transition temperature (Tc) lies in the appearance of multigap structure. The gaps of lower magnitude can be suppressed by defects so that this system becomes effectively a single gap superconductor. When such a situation is created, it is hoped that thermal conductivity will show hump below Tc. Proceeding along these lines, a sample of MgB2 with a relatively higher residual resistivity (33.3 mili-Ohm-cm)has been found to show a hump structure below Tc. The actual electronic thermal conductivity kel of this sample is less than that expected from the Wiedeman- Franz law by more than a factor of 2.6 in the considered temperature range. Modifying the Wiedeman- Franz law for the electronic contribution by replacing the Lorenz number by an effective Lorenz number Leff (L0) we have obtained two sets of kel, namely those with Leff = 0.1L0 and 0.2L0. Corresponding to these two sets of kel, two sets of the phonon thermal conductivity kph are obtained. kph has been analyzed in terms of an extended Bardeen- Rickayzen- Tewordt theory. The main result of this analysis is that the hump structure corresponds to a gap ratio of 3.5, and that large electron-point defect scattering is the main source of drastic reduction of the electronic thermal conductivity from that given by the usual Wiedeman- Franz law., Comment: 18 pages of Text + Figs: comments welcome (awana@mail.nplindia.ernet.in)

The enhancement of the critical current density (Jc(H)) of carbon and nano-SiC doped MgB2 is presented and compared. The upper critical field (Hc2) 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-xCx carbon is substituted for boron that induces disorder in the boron network and acts as internal pinning centres. The optimal Jc(H) values are obtained for x = 0.1 sample . In case of nano-SiC doped in MgB2, the Jc(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., Comment: 17 Pages text + Figs

The synthesis and characterization of PVA (Poly Vinyl Acetate) doped bulk MgB2 superconductor is reported here. PVA is used as a Carbon source. PVA doping effects made two distinguishable contributions: first enhancement of Jc field performance and second an increase in Hc2 value, both because of carbon incorporation into MgB2 crystal lattice. The susceptibility measurement reveals that Tc decreased from 37 to 36 K. Lattice parameter a decreased from 3.085 A to 3.081 A due to the partial substitution of Carbon at Boron site. PVA doped sample exhibited the Jc values greater than 10^5 A/cm2 at 5 & 10 K at low fields; which is almost 3 times higher than the pure one, while at high fields the Jc is increased by an order of magnitude in comparison to pure MgB2. From R(T)H measurements we found higher Tc values under magnetic field for doped sample; indicating an increase in Hc2. Also the magnetization measurements exhibited a significant enhancement in Hirr value. The improved performance of PVA doped MgB2 can be attributed to the substitution of carbon at boron site in parent MgB2 and the resulting impact on the carrier density and impurity scattering. The improved flux pinning behavior could easily be seen from reduced flux pinning force plots., 14 Pages of Text + Figs. To appear in Physica C

We report the synthesis, high-resolution micro-structure, magneto-transport and magnetization of nano-diamond doped MgB2?nDx with x = 0.0?0.1. The superconducting transition temperature (Tc) is not affected by x up to x = 0.05, indicating that the added nano-diamond (1) does not decompose to C and (2) does not partially substitute for B in MgB2. R(T) versus H measurements show higher Tc values under the same applied magnetic field for the nano-diamond added samples, resulting in higher estimated Hc2 values. Isothermal magnetization measurements show that above 2?T, the critical current density (jc) is of the order of 105?A?cm?2 for the pristine sample. jc is further increased to three times for 3% nano-diamond doped samples. High-resolution transmission electron microscopy (HRTEM) observations clearly show the dispersion of nano-diamond particles, with an average particle size of 8?10?nm, in the MgB2 matrix. It seems likely that the dispersed nano-diamond particles of below 10 nm in size are acting as effective pinning centres responsible for improving the superconducting performance of the parent MgB2.

We report synthesis, structural details and transport measurements on Sr4V2O6Fe2As2. Namely, the stoichiometric amounts of V2O5+1/2×SrO2+7/2×Sr+2×FeAs are weighed mixed, ground thoroughly and palletized in rectangular form in a glove box in high purity Ar atmosphere. The pellet is further sealed in an evacuated (10−5 torr) quartz tube and put for heat treatments at 750 and 1150°C in a single step for 12 and 36 hours respectively. Finally the quartz ampoule is allowed to cool naturally to room temperature. The as-synthesized sample is black in color. The compound crystallized in P4/nmm space group with lattice parameters a=b=3.925 A and c=15.870 A. Also seen are some small impurity lines. The compound did not exhibit superconductivity but instead a spin density wave (SDW) like metallic step at around 175 K is seen in R(T) measurements. Principally in [FeAs]−1{Sr4V2O6}C[FeAs]−1 the net value of blocking layer charge C must be either less or more than 2, to let it be electron or hole type superconductor respectively. Efforts are under way to achieve superconductivity in the studied system.

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.

We report the experimental and theoretical study on magnetic nature of Bi3Ni system. The structure is found to be orthorhombic (Pnma) with lattice parameters a = 8.879{\AA} b = 4.0998{\AA} and c = 4.099{\AA}. The title compound is synthesized via a solid state reaction route by quartz vacuum encapsulation of 5N purity stoichiometric ingredients of Ni and Bi. The superconducting transition temperature is found to be 4.1 K as confirmed from magnetization and specific heat measurements. The lower critical field (Hc1) and irreversibility field (Hirr) are around 150 and 3000Oe respectively at 2K. Upper critical field (Hc2) as determined from in field (up to 4 Tesla) ac susceptibility is found to be around 2 Tesla at 2K. The normal state specific heat is fitted using Sommerfeld-Debye equation C(T) = {\gamma}T + {\beta}T3+{\delta}T5 and the parameters obtained are {\gamma}= 11.08mJ/mol-K2, {\beta}= 3.73mJ/mol-K4 and {\delta}= 0.0140mJ/mol-K6. The calculated electronic density of states (DOS) at Fermi level N(EF) and Debye temperature {\Theta}D are 4.697 states/eV per formula unit and 127.7K respectively. We also estimated the value of electron phonon coupling constant ({\lambda}) to be 1.23, which when substituted in MacMillan equation gives Tc = 4.5K. Density functional (DFT) based calculations for experimentally determined lattice parameters show that Ni in this compound is non-magnetic and ferromagnetic interactions seem to play no role. The Stoner condition I*N(EF) = 0.136 per Ni atom also indicates that system cannot have any ferromagnetism. The fixed spin moment (FSM) calculations by fixing total magnetic moment on the unit cell also suggested that this system does not exhibit any signatures of ferromagnetism., Comment: 22 pages text+figs; comments/suggestions(awana@mail.nplindia.ernet.in & http://www.freewebs.com/vpsawana)

We report an easy single step synthesis route of title compound NdFeAsO0.80F0.20 superconductor having bulk superconductivity below 50 K. The title compound is synthesized via solid-state reaction route by encapsulation in an evacuated (10-3 Torr) quartz tube. Rietveld analysis of powder X-ray diffraction data shows that compound crystallized in tetragonal structure with space group P4/nmm. R(T)H measurements showed superconductivity with Tc (R=0) at 48 K and a very high upper critical field (Hc2) of up to 345 Tesla. Magnetic measurements exhibited bulk superconductivity in terms of diamagnetic onset below 50 K. The lower critical field (Hc1) is around 1000 Oe at 5 K. In normal state i.e., above 60 K, the compound exhibited purely paramagnetic behavior and thus ruling out the presence of any ordered FeOx impurity in the matrix. In specific heat measurements a jump is observed in the vicinity of superconducting transition (Tc) along with an upturn at below T=4 K due to the AFM ordering of Nd+3 ions in the system. The Thermo-electric power (TEP) is negative down to Tc, thus indicating dominant carriers to be of n-type in NdFeAsO0.80F0.20 superconductor. The granularity of the bulk superconducting NdFeAsO0.8F0.2 sample is investigated and the intra and inter grain contributions have been individuated by looking at various amplitude and frequencies of the applied AC drive magnetic field., 26pages text + Figures: comments/suggestions welcome (awana@mail.nplindia.ernet.in & http://www.freewebs.com/vpsawana)

The bulk polycrystalline sample FeSe1/2Te1/2 is synthesized by solid state reaction route in an evacuated sealed quartz tube at 750 oC. The presence of superconductivity is confirmed through magnetization/thermoelectric/resistivity studies. It is found that the superconducting transition temperature (Tc) is around 12 K. Heat capacity (Cp) of superconducting FeSe1-xTex exhibited a hump near Tc, instead of well defined Lambda transition. X-ray Photo electron spectroscopy (XPS) studies revealed well defined positions for divalent Fe, Se and Te but with sufficient hybridization of Fe (2p) and Se/Te (3d) core levels. In particular divalent Fe is shifted to higher BE (binding energy) and Se and Te to lower. The situation is similar to that as observed earlier for famous Cu based HTSc (High Tc superconductors), where Cu (3d) orbital hybridizes with O (2p). We also found the satellite peak of Fe at 712.00 eV, which is attributed to charge carrier localization induced by Fe at 2c site., Comment: 12 pages text + Figs contact Author-VPS Awana (www.freewebs.com/vpsawana)

We report synthesis, structural details and magnetization of SmFe1-xCoxAsO with x ranging from 0.0 to 0.30. It is found that Co substitutes fully at Fe site in SmFeAsO in an iso-structural lattice with slightly compressed cell. The parent compound exhibited known spin density wave (SDW) character below at around 140 K. Successive doping of Co at Fe site suppressed the SDW transition for x = 0.05 and later induced superconductivity for x = 0.10, 0.15 and 0.20 respectively at 14, 15.5 and 9K. The lower critical field as seen from magnetization measurements is below 200Oe. The appearance of bulk superconductivity is established by wide open isothermal magnetization M(H) loops. Superconductivity is not observed for higher content of Co i.e. x = 0.30. Clearly the Co substitution at Fe site in SmFe1-xCoxAsO diminishes the Fe SDW character, introduces bulk superconductivity for x between 0.10 and 0.20 and finally becomes non-superconducting for x above 0.20. The Fe2+ site Co3+ substitution injects mobile electrons to the system and superconductivity appears, however direct substitution introduces simultaneous disorder in superconducting FeAs layer and thus superconductivity disappears for higher content of Co., Comment: 14 Pages Text + Figs comments (awana@mail.nplindia.ernet.in)

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 = 3.8008(10) and c = 6.0187 (15) A. Magnetization measurements indicated bulk superconductivity with lower critical field (Hc1) of around 180 Oe. By applying Ginzburg Landau (GL) 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 was observed under an applied magnetic field., Comment: 13 pages text + Figs: commenta (awana@mail.nplindia.ernet.in)

We study the effect of nano(n)-SiC addition on the crystal structure, critical temperature (Tc), critical current density (Jc) and flux pinning in MgB2 superconductor. X-ray diffraction patterns show that all the samples have MgB2 as the main phase with very small amount of MgO, further with n-SiC addition the presence of Mg2Si is also noted and confirmed by SEM & EDS. The Tc value for the pure MgB2 is 18.9K under 8 Tesla applied field, while is 20.8K for the 10-wt % n-SiC doped sample under the same field. This points towards the increment in upper-critical field value with n-SiC addition. The irreversibility field (Hirr) for the 5% n-SiC added sample reached 11.3, 10 and 5.8 Tesla, compared to 7.5, 6.5, and 4.2 Tesla for the pure MgB2 at 5, 10 and 20K respectively. The critical current density (Jc) for the 5-wt % n-SiC added sample is increased by a factor of 35 at 10K and 6.5 Tesla field and by a factor 20 at 20K and 4.2 Tesla field. These results are understood on the basis of superconducting condensate (sigma band) disorder and ensuing intrinsic pining due to B site C substitution clubbed with further external pinning due to available n-SiC/Mg2Si pins in the composite system., 19 pages of Text + Figs. comments/suggestions are welcome (awana@mail.nplindia.ernet.in)

We report on the magnetic behavior of oxygen deficient LaFeAsO1-x (x-0.10) compound, prepared by one-step synthesis, which crystallizes in the tetragonal (S.G. P4/nmm) structure at room temperature. Resistivity measurements show a strong anomaly near 150 K, which is ascribed to the spin density wave (SDW) instability. On the other hand, dc magnetization data shows paramagnetic-like features down to 5 K, with an effective moment of 0.83 mB/Fe. 57Fe Mossbauer studies (MS) have been performed at 95 and 200 K. The spectra at both temperatures are composed of two sub-spectra. At 200 K the major one (88%), is almost a singlet, and corresponds to those Fe nuclei, which have two oxygen ions in their close vicinity. The minor one, with a large quadrupole splitting, corresponds to Fe nuclei, which have vacancies in their immediate neighborhood. The spectrum at 95 K, exhibits a broadened magnetic split major (84%) sub-spectrum and a very small magnetic splitting in the minor subspectrum. The relative intensities of the subspectra facilitate in estimating the actual amount of oxygen vacancies in the compound to be 7.0(5)%, instead of the nominal LaFeAsO0.90. These results, when compared with reported 57Fe MS of non-superconducting LaFeAsO and superconducting LaFeAsO0.9F0.1, confirm that the studied LaFeAsO0.93 is a superconductivity-magnetism crossover compound of the newly discovered Fe based superconducting family., Comment: 7 pages text + Figs : Comments/suggestions welcome (awana@mail.nplindia.ernet.in)

Polycrystalline MgB2-nDx (x= 0 to 0.1) samples are synthesized by solid-state route with ingredients of Mg, B and n-Diamond. The results from magneto-transport and magnetization of nano-diamond doped MgB2-nDx are reported. Superconducting transition temperature (Tc) is not affected significantly by x up to x = 0.05 and latter decreases slightly for higher x > 0.05. R(T) vs H measurements show higher Tc values under same applied magnetic fields for the nano-diamond added samples, resulting in higher estimated Hc2 values. From the magnetization measurements it was found that irreversibility field value Hirr for the pristine sample is 7.5 Tesla at 4 K and the same is increased to 13.5 Tesla for 3-wt% nD added sample at the same temperature. The Jc(H) plots at all temperatures show that Jc value is lowest at all applied fields for pristine MgB2 and the sample doped with 3-wt% nD gives the best Jc values at all fields. For the pure sample the value of Jc is of the order of 105 A/cm2 at lower fields but it decreases very fast as the magnetic field is applied and becomes negligible above 7 Tesla. The Jc is 40 times higher than pure MgB2 at 10 K at 6 Tesla field in case of 3%nD doped sample and its value is still of the order of 103 A/cm2 at 10 Tesla for the same sample. On the other hand at 20K the 5%nD sample shows the best performance at higher fields. These results are discussed in terms of extrinsic pinning due to dispersed n-Diamond in the host MgB2 matrix along with the intrinsic pinning due to possible substitution of C at Boron site and increased inter-band scattering for highly doped samples resulting in extraordinary performance of the doped system., 12 PAGES (TEXT+FIGS). ACCEPTED: J. APPL.PHYS. (MMM-2007 Proceedings)

We report phase formation, and detailed study of magnetization and resistivity under magnetic field of MgB2 polycrystalline bulk samples prepared by Fe-tube encapsulated and vacuum (10-5 torr) annealed (750 0C) route. Zero-field-cooled magnetic susceptibility (cZFC) measurements exhibited sharp transition to superconducting state with a sizeable diamagnetic signal at 39 K (Tc). The measured magnetization loops of the samples, despite the presence of flux jumps, exhibited a stable current density (Jc) of around 2.4 x 105 A/cm2 in up to 2 T (Tesla) field and at temperatures (T) up to 10 K. The upper critical field is estimated from resistivity measurements in various fields and shows a typical value of 8 T at 21 K. Further, cFC measurements at an applied field of 0.1 T reveal paramagnetic Meissner effect (PME) that is briefly discussed., 13 pages text + figs. accepted: solid state commun. (2006)

The effect of ruthenium (Ru) doping in SmFe1−xRuxAsO (x = 0.0–0.50) compound is studied here. The samples synthesized are mostly phase pure and not contaminated with various FeAs phases. It is found that with increasing Ru concentration x, the system turns from weak ferromagnetic (spin density wave (SDW)) to ferromagnetic state at x = 0.20–0.30 without any sign of superconductivity.

Here, effect of single-walled carbon nano-tube (CNT) doping on transition temperature, lattice parameters, critical current density and flux pinning in MgB2 superconductor was studied. The carbon substitution for B was found to enhance critical current density Jc in magnetic fields but depress critical temperature Tc. The depression of Tc is caused by the carbon substitution for B, which is also confirmed by Rietveld analysis of room temperature XRD patterns of doped samples. We achieved enhancement of critical current density (Jc) and flux pinning by almost an order of magnitude, particularly in higher magnetic fields.

We report synthesis, structural details and transport measurements on SmFe0.5Ru0.5AsO and GdFe0.5Ru0.5AsO. The samples are synthesized at normal pressure with vacuum encapsulation technique. The parent REFeAsO (RE = Sm and Gd) crystallizes in tetragonal P4/nmm space group having their lattice parameters as a = 3.9375(6) A, c = 8.5021(4) A for Sm and a = 3.9152(4) A, c = 8.4546(1) A for Gd. With 50% Ru substitution at Fe site i.e. for SmFe0.5Ru0.5AsO and GdFe0.5Ru0.5AsO the lattice parameter a is increased and c is decreased to as a = 4.0113(55) A, c = 8.3129(17) A for Sm and a = 3.9795(64) A, c = 8.2787 (20) A for Gd samples. Resistivity measurements on pristine samples though exhibited clearly the step like metallic transition at around 150 K, the Fe site 50% Ru substituted samples show all through a metallic behavior without the characteristic spin density wave (SDW) step. The samples with lower content of Ru at Fe site i.e. 10% and 30% are also studied. Though 10% Ru doped samples exhibited weak superconductivity like transition below 15 K, the 30% doped samples are metallic. It seems that the Fe2+ site Ru4+/5+ substitution provides electron carriers for conduction process. For achieving bulk superconductivity in Fe-site Ru substituted REFeAsO (1111) systems the efforts are still underway.

We study the effect of synthesis temperature on the phase formation in nano(n)-SiC added bulk MgB2 superconductor. In particular we study: lattice parameters, amount of carbon (C) substitution, microstructure, critical temperature (Tc), irreversibility field (Hirr), critical current density (Jc), upper critical field (Hc2) and flux pinning. Samples of MgB2+(n-SiC)x with x=0.0, 0.05 & 0.10 were prepared at four different synthesis temperatures i.e. 850, 800, 750, and 700oC with the same heating rate as 10oC/min. We found 750oC as the optimal synthesis temperature for n-SiC doping in bulk MgB2 in order to get the best superconducting performance in terms of Jc, Hc2 and Hirr. Carbon (C) substitution enhances the Hc2 while the low temperature synthesis is responsible for the improvement in Jc due to the smaller grain size, defects and nano-inclusion induced by C incorporation into MgB2 matrix, which is corroborated by elaborative HRTEM (high-resolution transmission electron microscopy) results. We optimized the the Tc(R=0) of above 15K for the studied n-SiC doped and 750 0C synthesized MgB2 under 140 KOe field, which is one of the highest values yet obtained for variously processed and nano-particle added MgB2 in literature to our knowledge., 32 pages Text + Figs: comments/suggestions welcome (awana@mail.nplindia.ernet.in)/www.freewebs.com/vpsawana/

We report synthesis, structure, electrical transport and heat capacity of SmFeAsO. The title compound is synthesized by one-step encapsulation of stoichiometric FeAs, Sm, and Sm2O3 in an evacuated (10-5 Torr) quartz tube by prolong (72 hours) annealing at 1100oC. The as synthesized compound is crystallized in tetragonal structure with P4/nmm space group having lattice parameters a = 3.93726(33) A and c = 8.49802(07) A. The resistance (R-T) measurements on the compound exhibited ground state spin-density-wave (SDW)-like metallic steps below 140 K. Heat capacity CP(T) measurements on the title compound, showed an anomaly at around 140 K, which is reminiscent of the SDW ordering of the compound. At lower temperatures the CP(T) shows a clear peak at around 4.5 K. At lower temperature below 20 K, Cp(T) is also measured under an applied field of 7 Tesla. It is concluded that the CP(T) peak at 4.5 K is due to the anti-ferromagnetic(AFM) ordering of Sm3+ spins. These results are in confirmation with ordering of Sm in Sm2-xCexCuO4., 9 pages Text + Figs Contact Author (awana@mail.nplindia.ernet.in)

Sample of Y1-xPrxBa2Cu3O7 with x = 0.0 to 0.80 are synthesized by solid- state reaction route. All the samples crystallized in orthorhombic structure with space group pmmm. Slight decrease in orthorhombicity is seen with an increase in Pr content. Superconductivity is destroyed above 50% Pr content. Non-superconducting samples are highly semi conducting. Detailed high field magnetization (M-H) measurements showed that the lower critical field (Hc1) improves slightly with Pr content of up to 4% and the same later decreases for higher Pr contents. The critical current density calculated by invoking Bean's critical state model improves dramatically until x=0.04 and decrease later for higher Pr contents in comparison to the pristine sample. These results are explained on the basis of Pr4f and O2p hybridization induced disorder in superconducting CuO2 planes. For low Pr content the mild disorder in superconducting condensate (CuO2 plane) induces intrinsic pinning and hence resulting in an increased Jc(H) and Hc1.

The superconductivity of MgB2, Mg1−xAlxB2 and NbB2+x is compared. The stretched c-lattice parameter (c = 3.52 A) of MgB2 in comparison to NbB2.8 (c = 3.32 A) and AlB2 (c = 3.25 A) decides empirically the population of their π and σ bands and, as a result, their Tc values at 39 and 11 K, respectively, for the first two and no superconductivity for the latter. Besides stretching of the c-lattice parameter not only the density of the carriers but also their signs change in these isostructural di-borides. The thermoelectric power of these compounds clearly demonstrates their changing π and σ band contributions and the ensuing appearance/disappearance of superconductivity. An increased c parameter increases the boron plane constructed hole type σ band population and decreases the contribution from the Mg or Al plane electron type π band. This turns the hole type (mainly σ band conduction) MgB2 superconductor (39 K) into the electron type (mainly π band conduction) non-superconducting AlB2. The importance of hole type σ band conduction dominating the superconductivity of the various borides is further established by the high performance of intrinsically pinned MgB2−xCx. Our results on MgB2 added with nano-diamond, nano-SiC and various organics such as glucose, PVA and adipic acid, when compared with MgB2−xCx, clearly demonstrate that the main role is played by C substitution at the B site in the host MgB2 and the ensuing σ plane disorder and vortex pinning. The best strategy could be to add (

We report the effect of adipic acid (C6H10O4) doping on lattice parameters, microstructure, critical temperature (Tc), current density (Jc), and irreversibility field (Hirr) for MgB2 superconductor. Actual carbon (C) substitution level for boron (B) is estimated to be from 0.40 percent to 2.95 percent for different doping levels. A reduction in Tc from 38.43 to 34.93 K and in lattice parameter a from 3.084(3) A to 3.075(6) Ais observed for the10 wt percent C6H10O4 doped sample in comparison to pristine MgB2. This is an indication of C substitution at boron sites, with the C coming from the decomposition of C6H10O4 at the time of reaction. Interestingly the doped samples have resulted in significant enhancement of Jc and Hirr. All the doped samples exhibit the Jc value of the order of 10^4 A/cm2 at 5 K and 8 T, which is higher by an order of magnitude as compared to undoped sample. This result indicates that C6H10O4 is a promising material for MgB2 for obtaining the excellent Jc values under higher magnetic fields., Comment: 22 pages of text + Figs comments (awana@mail.nplindia.ernet.in)