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MgB 2 superconductor has turned out to be an advantageous energy application material due to its cost, abundance, and promising properties at temperature ~20K. However, the challenge of connectivity, pinning, anisotropy-driven percolation and two band nature still plagues in reaching the optimized values of critical current density ( J C ) especially in presence of magnetic fields. In the present chapter we discuss these concepts in relevance to MgB 2 superconductors and illustrate their working in pure and doped samples synthesized both by in situ and ex situ methods. Series of polycrystalline MgB 2 bulk samples were prepared by solid state synthesis with excess Mg and further addition of nano-Ag and nano-SiC, nano-Ni, YBCO, nano-C, and nano-Fe 2 O 3 . Detailed investigations were carried out by X-ray diffraction, scanning electron microscope, and thermoelectric power S ( T ), resistivity ρ( T ), and magnetic ( J C ) between temperatures T =4–300K and applied magnetic fields B =0–14 T. The experimental results of R ( T , B ), B – T phase diagram and J C ( B , T ) in all these variety of samples are universally and quantitatively analyzed in terms of effective transport cross-section area, nature of pinning laws, and change in anisotropy, percolation threshhold and inter/intra-band scattering; which is shown to give a fair idea of how to go about optimizing the J C ( B , T ) of MgB 2 .

Doping effect of ZnO-nanoparticles on the superconducting properties of MgB 2 has been studied. The 2% nano-ZnO doped MgB 2 shows the excellent J c and H irr at all temperatures and magnetic fields amongst the doped and undoped sample. The lattice parameter- c shows the higher value for 2% ZnO doped MgB 2 sample, which clearly demonstrates the presence of the lattice strain in doped samples. The residual resistivity ratio was increased as the nano-ZnO doping level increased. Very slight variation in T c is observed from the temperature dependence of resistivity plot of nano-ZnO doped MgB 2 . In M ( H ) plot at low applied fields, we have observed large vortex instabilities (vortex-avalanches) associated with 2% and 4% doped samples. Vortex avalanche effect is diminishes with increasing temperature and disappears near 15 K. The results are discussed in terms of local-vortex instabilities caused by doping of ZnO nanoparticles. Scanning electron microscopy studies show that the synthesized samples are well adherent and grains are uniformly distributed with an average particle size of ∼5–10 μm.

Here, the superconducting and AC/DC magnetic properties of REBa2Cu3O7−δ (RE: Eu, Gd, Ho and Er) are investigated in order to understand the inter-granular and intra-granular behavior of these compounds. These compounds were prepared by standard solid-state reaction route. ErBa2Cu3O7−δ (Er-123) and HoBa2Cu3O7−δ (Ho-123) (consisting of heavier rare earth) show higher orthorhombic splitting in their XRD patterns than those observed for EuBa2Cu3O7−δ (Eu-123) and GdBa2Cu3O7−δ (Gd-123) (consisting of lighter rare earth). Even though Eu, Gd, Ho and Er are magnetic elements, substitution of these at Y site in Y-123 does not change the superconducting transition temperature (T c). Er-123 and Ho-123 exhibit very sharp superconducting transition while Eu-123 and Gd-123 exhibit relatively broad transition with little hump at the lower part of transition close to the long-range superconducting state with zero resistance. In addition to that, Eu-123 and Gd-123 show relatively higher normal state resistivity than those of Er-123 and Ho-123. The AC susceptibility measurements reveal that these compounds possess different strength of inter-granular couplings. The hump in the superconducting transition of Eu and Gd is not due to the lack of inter-grain couplings in these samples because of having better grain connectivity in the samples as compared to other rare earth elements in REBa2Cu3O7−δ system. Though intra-grain peak is insensitive to the applied magnetic field, inter-grain peak is sensitive to the applied magnetic field.

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.

Samples of MgB2 pure phase, with Ni nanoparticles addition, were prepared using a solid diffusion reaction method. Clearly, the Ni nanoparticles act as effective pinning centers and enhance the critical current density values, especially for a sample with 0.5%Ni. A negligible amount of Ni diffuses inside the MgB2 grains, thus having a small effect on the transition temperature, which remains around 37.5 K.

The structural, transport and magnetic properties of MgB2 superconductor heavily blended with Mg is studied. The samples are synthesized with a new approach in both, pressed carbon environment and in flowing argon. The excess magnesium used is observed to play dual role: one being the prevention of Mg losses during the synthesis process and hence maintaining the stoichiometry of MgB2 phase, and second being the formation of Mg milieu probably all around the MgB2 grains to give a ductile and dense structure. Excess Mg also improves the grain connectivity by going in to the pores and there by minimizing the insulating junctions., Comment: 21 pages Text + Figs, comments/auggestions (awana@mail.nplindia.ernet.in) (www.freewebs.com/vpsawana)

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)

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 A and c=3.26 A for pure NbB2 while a=3.10 A and c=3.32 A 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.

We report synthesis and magnetic characterization of variously processed magneto-superconducting (Rutheno-cuprates) RuSr2Eu1.5Ce0.5Cu2O10−δ . The compound crystallizes in I4/mmm tetragonal structure. Magnetization measurements showed bulk magnetic ordering and superconductivity at around 100 K and 30 K, respectively. Further, the careful examination of the low field magnetic susceptibility reveals two minor magnetic transitions at around 135 K and 200 K, in addition to the major transition at 100 K. When the samples are processed in different environments of air, O2 and slightly pressurized O2, the nature of magnetic transitions and the superconductivity changes dramatically. The highest superconducting transition is achieved for the high pressure O2 annealed samples and the lowest for the air annealed one. On the other hand, the minor magnetic transitions are more prominent in air-annealed samples. Though the minor magnetic transitions are intrinsic to all variously processed samples, they are not clearly seen for higher O2 content samples. Basically, the Ru spins order antiferro magnetically at around 200 K (first minor transition), and reorients themselves at 125 K (second minor transition) before finally ordering in canted ferromagnetic state or a spin glass structure. These results can be explained on the basis of fluctuating valance of Ru4+/Ru5+.

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

Superconducting (YBa2Cu3O7−d)1−x/(SiO2)x (i.e. Y123/S) composites with x = 0, 0.05, 0.1, 0.15 and 0.2 were prepared by solid state synthesis. Detailed investigations of structural and electrical transport and magnetization in applied magnetic fields (B = 0–9 T) were carried out. The typical double-step superconducting transitions observed both resistively (B = 0) and magnetically (B = 1 mT) reveal the granular nature of all the samples. The XRD, SEM and superconducting transitions show that addition of SiO2 mainly influences the inter-grain regions of the samples, leaving the pure Y123 phase in intra-grain regions practically intact. This leads to an interesting behaviour of M(T,B) and resistivity ρ(T,B) in the samples. The field-cooled (FC) magnetization M(T) measured at B = 1 mT shows a dip in the diamagnetic transition at a characteristic temperature that coincides with the onset of the second step in the zero-field-cooled (ZFC) M(T) measured in the same field. In the presence of B≥1 T, the broadening of the superconducting transition revealed by ρ(T) below the first step is found to be field-dependent, whereas below the second step at lower temperatures the broadening becomes field-independent. The width (ΔM) of the high field hysteresis loops (M–B) measured isothermally in the superconducting state initially varies as a power law of B. Depending on SiO2 content and temperature, the power law further deviates into either a shallow peak or a monotonically decreasing curve. We show that these results and a host of other considerations point towards an interaction of vortices in inter- and intra-grain regions.

We report the comparisons of the effect on the resistivity behaviors for the four different encapsulated MgB 2 samples where we optimized the pure MgB 2 , using the same standard solid-state synthesis technique, with different shielding materials. The encapsulations were done in Fe Tube, Ta Tube, Ta Foil and Mo Foil, respectively. For the overall comparison, Fe Tube encapsulation seems better sample than others. The lattice parameters ‘ a ’ and ‘ c ’ were found to be of the same order. The XRD patterns showed that the compound depicted the single-phase, which are crystallized in the hexagonal structure (space group P 6 / mmm ) at room temperature, exhibited T c of around 37–39 K. We could not completely avoid the inclusion of the foreign particles in our samples, despite having used several encapsulations, but the sample synthesized by this method holds the easiest way to attain better superconducting polycrystalline bulk MgB 2 .

Substitutional studies have been carried out on sintered bulk samples of YBCO to determine the effect of doping it with Pr, Ca (at Y site) and Ni (at Cu site) in small concentrations on the formation of effective pinning centres for jc enhancement. The results show Pr and Ca substitutions to be far more effective than Ni substitution. The former two give rise to formation of rows of oxygen vacancies with pronounced strain fields in CuO2 planes where superconductivity resides. The defects thus formed seem to be analogous to arrays of normal nanodots in CuO2 planes which are expected to have strong intragrain pinning interactions with pancake vortices of CuO2 planes forming the mixed state structure. Both these substitutions tend to marginalize the adverse effects of grain boundaries. The superconducting volume fraction of the pristine sample is significantly raised by both these substitutions. These special features are, however, absent with Ni substitution. This paper brings out the importance of Pr substitution in significantly enhancing jc(B).

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.

Nano-Al2O3 doped Mg1?xAlxB2 with 0?x?6% were synthesized by solid state reaction at 750??C in Fe tube encapsulation under a vacuum of 10?5?Torr. Resistance measurement shows that the Tc decreases with x and zero resistivity for x = 0 and 6% are obtained at 38 and 35?K, respectively. XRD measurement shows that the lattice parameter and cell volume also decrease monotonically with increasing doping levels. From this we infer that the Al has been substituted in the lattice of MgB2 at Mg sites. Resistivity measurement shows a systematic decrease in Tc with doping which also confirms the substitution of Al. Magnetization studies in the temperature range from 4 to 35?K and in the magnetic field up to 9?T shows a significant increase in the irreversibility field (Hirr), critical current density (Jc) and remanent magnetization (MR) with increasing concentration of the Al2O3 nanoparticle. At low fields we have observed large vortex instabilities (known as a vortex avalanche) associated with all doped samples. The vortex-avalanche effect is reduced with increasing temperature and vanishes near 20?K. The results are discussed in terms of local-vortex instabilities caused by doping of Al2O3 nanoparticles.

We report on the synthesis, phase formation, microstructure, and magnetization, of nano(n)-TiO2-added MgB2 polycrystalline compounds. The added n-TiO2 amounts are varied from 1% to 15% in weight (wt). All the studied samples are near single phase with small amounts of un-reacted Mg/MgO up to 10 wt%. The 15 wt% n-TiO2 added sample seems to be a multi-phase compound with unusual broadening of the main MgB2 reflection and additional unidentified lines in its X-ray diffraction (XRD) pattern. The superconducting transition temperature (Tc), as measured by magnetization experiments, decreases marginally with n-TiO2 addition, for example the Tcs are at 37.5 K and 35.5 K, respectively, for pristine and 10 wt% n-TiO2-added samples. This indicates that Ti has not significantly substituted into the host MgB2 lattice. The grain morphology of these compounds reveals porous regions and does not change much with TiO2 addition. High resolution transmission electron microscopy (HRTEM) studies revealed the presence of n-TiO2 in these samples. The critical current density (Jc) of the MgB2–n-TiO2 samples, as estimated using the Bean model, shows better performance under magnetic fields above 3 T than pristine MgB2 for up to 4 wt% of addition, and decreases rapidly for additions above 6 wt%. We conclude that n-TiO2 helps in enhancing the flux pinning centers in MgB2 superconductor and hence improves the Jc(H) performance for additions up to 4 wt% in fields above 3 T.

Bulk polycrystalline samples of nano- Mo doped MgB 2 were synthesized by Fe tube encapsulation at ambient pressure under argon annealing (850°C). Mo substitution takes place successfully at the Mg site in Mg 1-x Mo x B 2 only till x=0.2. For higher (x>0.2) Mo content the same did not enter the MgB 2 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 (T c ) 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 H c2 is 8 Tesla for pristine and x=0.2 samples at 15.6 and 19 K, respectively. Thus H c2 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 (T c ) in zero field-cooled measurements. Commendable current density (J c ) 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 J c (H) characteristics of x=0.1 and 0.2 samples were found to be slightly superior to that for pristine samples with enhanced H irr (irreversibility field).

We report fluctuation-induced conductivity (FIC) of the polycrystalline MgB2 superconductor in the presence of magnetic field. The results are described in terms of the temperature derivative of the resistivity, dρ/dT. The dρ/dT peak temperature observed for H = 0 Tesla at 39 K remains very distinct under applied fields of 6 Tesla and 8 Tesla at 22 and 20 K respectively. Aslamazov and Larkin (AL) equations are used to explain the anisotropic nature of the polycrystalline MgB2. The effective coherence length, ξp (0) determined experimentally is 55.17 A, which roughly matches with previously reported experimental work.

Raman spectroscopy studies are reported for the RuSr2Eu1.5Ce0.5Cu2O10 (Ru-1222) compound at various temperatures of 300, 250, 200 and 90 K. Three distinct vibrational bands: the first at 110, 140, and 160 cm−1, the second at 295 and 347 cm−1, and third one at 651 cm−1 are seen in Raman spectra of the compound at room temperature. These bands are attached to the Cu atoms’ c -direction, the Ru atoms’ ab-plane stretching and Ru atoms’ c -direction anti-stretching modes. Below 200 K, an extra vibrational mode is also seen at 260 cm−1. Also, with a decrease in temperature, though the Cu vibrational modes remain intact, the Ru atoms’ ab-plane stretching (295 cm−1) and c -direction anti-stretching (651 cm−1) modes shift gradually to higher wave number positions. The frequencies of modes at 260 and 651 cm−1 showed anomalous softening and line-width broadening below 100 K that corroborates well with the spin ordering seen in susceptibility studies. The studied compound is a ferromagnetic superconductor with magnetic ordering of the Ru spins at 200 K and superconductivity below 30 K. A magnetic and electrical transport characterization of the compound is also presented briefly.

Substitution of both Co and Mo at Ru -site in RuSr 2 Eu 1.5 Ce 0.5 Cu 2 O 10-δ system takes place iso-structurally in tetragonal structure (space group I4/mmm) with full solubility. The resistance versus temperature (R versus T) behavior of the pristine sample confirmed superconductivity [Formula: see text] around 25 K. The DC magnetization data (M versus T) for the same sample revealed magnetic transition (T mag ) at 140 K followed by superconducting transition [Formula: see text] at 32 K. With progressive Co substitution at Ru -site both [Formula: see text] and [Formula: see text] decrease monotonically and superconductivity is not observed for x ≥ 0.10 (10 at%). The magnetic behavior of materials with Co up to x = 0.2 preserves the magnetic structure of the parent RuSr 2 Eu 1.5 Ce 0.5 Cu 2 O 10-δ compound. For 0.8 ≥ x ≥ 0.2 a clear antiferromagnetic transition at TN=31 K is observed regardless of Co content. In the case of Mo -doped ruthenocuprates Ru 1-x Mo x Sr 2 Eu 1.5 Ce 0.5 Cu 2 O 10-δ, it has been found that the magnetic transition temperature decreases with x. The samples of x = 0.8 and 1.0 are found to have no magnetic or superconducting effects. The values of the superconducting transition temperature obtained from the resistivity versus temperature data increase by 4.5 and 7.0 K for x = 0.2 and 0.4 respectively, and then decrease by 17 K for x = 0.6. The observed variation of Tc with x in case of Mo has been explained in terms of a theory which combines the effects of weakening magnetic behavior and reducing carrier concentration in a phenomenological manner.

Samples of series Bi 1.6 Pb 0.4 Sr 2 Ca 2 Cu 3-x Zn x O 10-δ with x = 0.0, 0.015, 0.03, 0.06, 0.09 and 0.12 are synthesized by solid-state reaction route. All the samples crystallize in tetragonal structure with majority (> 90%) of Bi -2223( Bi 2 Sr 2 Ca 2 Cu 3 O 10) phase (c-lattice parameter ~ 36 Å). The proportion of Bi -2223 phase decreases slightly with an increase in x. The lattice parameters a and c of main phase ( Bi -2223) do not change significantly with increasing x. Superconducting critical transition temperature (Tc) decreases with x as evidenced by both resistivity [ρ(T)] and ac magnetic susceptibility [χ(T)] measurements. Interestingly the decrement of Tc is not monotonic and the same saturates at around 96 K for x > 0.06. In fact Tc decreases fast (~ 10 K/at%) for x = 0.015 and 0.03 samples and later nearly saturates for higher x values. Present results of Zn doping in Bi -2223 system are compared with other Zn -doped HTSC (high temperature superconducting) systems, namely the RE -123 ( REBa 2 Cu 3 O 7) and La -214 (( La, Sr )2 CuO 4).

Series of samples of the composition Cu (Y1−xCax)Ba2Cu3O6+δ () have been prepared by a solid-state synthesis route. Precise oxygen content was determined for all the samples by iodometric titration. The c lattice parameter increases with increasing x, indicating successful substitution of Y3+ by the bigger Ca2+ ion. Resistivity (R) versus temperature (T) measurements show that the pristine sample (x = 0.0) is semiconducting down to 5 K. Induction of superconductivity is seen with increasing x. The x = 0.10 sample exhibits the onset of superconducting transition (Tconset) at 42 K without attaining a zero resistance superconducting transition temperature (TcR = 0) state above 5 K. For the x = 0.15 and 0.20 samples TcR = 0 is observed at 25 and 35 K with Tconset at 52 and 61 K, respectively. DC magnetic susceptibility measurements corroborated the findings from the resistivity measurements. Thermoelectric power (S) measurements in the temperature range of 5–300 K exhibit TcS = 0 at 25 and 35 K respectively for x = 0.15 and 0.20 samples. The room-temperature S value decreases with increasing x, indicating an enhanced number of mobile holes. Magneto-transport measurements in applied magnetic fields of 3 and 6 T show a broadening of the superconducting transition. These results will be useful in constructing a more complete phase diagram of the Cu (Y1−xCax)Ba2Cu3O6+δ system in terms of x and δ.

We report systematic studies of structural, microstructural and transport properties of Y1-xCaxBa2Cu3O7-d bulk samples with 0.20 x 0.0. The partial substitution of Ca2+ at Y3+ site in YBa2Cu3O7-d leads towards slightly overdoped regime, which along with disorder in CuO2 planes is responsible for decrease in superconducting transition temperature (Tc) of the doped system. The microstructural variants being studied by transmission electron microscopy (TEM) technique in imaging and selected area diffraction modes revealed an increase in the density of twins with increase in Ca concentration despite the fact that there is slight decrease in the orthorhombicity. This is against the general conviction of decrease in twin density with decreasing orthorhombicity. An increase in twin density results in sharpening of twin boundaries with increasing Ca concentration. These sharpened twin boundaries may work as effective pinning centers. A possible correlation between microstructural features and superconducting properties has been put forward and discussed in the present investigation., Comment: 21 pages of TEXT and Figs. To appear in Physica C (2005)

Samples of Y0.4Pr0.6Ba2-xSrxCu3O7 have been synthesized by a solid- state reaction route. The samples with x = 0.60 crystallize in an orthorhombic structure (with orthorhombic distortion decreasing with increasing x), while the samples with x=0.80 and 1.0 crystallize in a pseudo tetragonal structure. Resistance (R) measurements as a function of temperature (T) show that x = 0.0 sample is highly semiconducting with R2K/R300K ratio of ~ 65. This ratio decreases to only ~23 and 5 for x=0.20 and x=0.4 samples, respectively. Further, the x=0.60 sample shows onset of a broad superconducting transition temperature (Tconset) at around 20 K without achieving the TcR=0 state down to 2 K. The TcR=0 state is observed in x = 0.80 and 1.00 samples at around 5 and 14 K, respectively. Thermo-electric power (S), also exhibits TcS~0 state at around 11 K and 16 K, respectively for x = 0.8 and 1.00 samples. Thermo electric power at room temperature is positive and decreases with increasing x, indicating enhanced number of mobile holes. These results demonstrate that substitution of Sr at Ba-site systematically induces metallicity and eventually superconductivity in Y0.4Pr0.6Ba2-xSrxCu3O7 system. The results are explained on the basis of reduced Pr(4f) orbital hybridization with O(2p) in supercondu cting Cu-O2 planes, resulting in delocalization of the mobile carriers. The role of decreased Pr/Ba intermixing disorder is also considered., Comment: 15 pages of Text and Figs. Accepted in Physica C

The magnetic behavior of polycrystalline samples of the magneto-superconductors RuSr 2 Ln 1.5 Ce 0.5 Cu 2 O 10− δ (LnRu-1222, Ln=Y, Ho and Dy) and RuSr 2 YCu 2 O 8− δ (YRu-1212) is analyzed and compared. Clear evidences for the occurrence of a spin glass phase on all LnRu-1222 samples are provided by ac susceptibility ( χ ac ) and dc magnetization measurements. A frequency-dependent peak in χ ac vs. T measurements, at low magnetic field, is observed. The strong suppression of this peak in the presence of magnetic fields as low as 500 Oe, as well as the results of thermoremanent magnetization (TRM) and isothermal remanent magnetization (IRM) measurements, suggest a spin glass behavior. Interestingly, the same measurements on the YRu-1212 sample show no indication of glassy behavior, contrasting with the results for LnRu-1222. We suggest that frustration of the Ru magnetic moments due to the presence of oxygen vacancies would be favored only in the LnRu-1222 compounds, thus causing the glassy behavior. This idea is supported by the observation of large oxygen deficiency that affects the RuO 2 planes in Ru-1222, but does not occur in Ru-1212.

A Tb-123 phase with the composition, TbSr 2 Cu 2.7 Mo 0.3 O 7+ δ , has been synthesized in single-phase form by the solid-state reaction route. Its phase purity has been confirmed from neutron powder diffraction studies. The as-synthesized Tb-123 sample does not show superconductivity down to 5 K. On the other hand, an unusually high antiferromagnetic ordering temperature ( T N ) of around 7 K is seen for the Tb moments. After 120-atm-O 2 post-annealing, bulk superconductivity is achieved in this compound with a superconducting transition temperature ( T c ) of about 30 K, without any significant effect on T N . To achieve higher oxygen content and higher T c , the as-synthesized sample was subjected to high-pressure oxygenation, carried out in a closed cell, at 5 GPa and 400 °C in the presence of AgO as an excess-oxygen source. This sample exhibited superconductivity onset at around 80 K with a Meissner fraction larger than 10% at 5 K. Our observation of superconductivity at 80 K is the highest T c to-date for the Tb-123 type compounds.

The mechanism of superconductivity in pnictides is discussed through direct doping in superconducting FeAs and also in charge reservoir REO layers. The un-doped SmFeAsO is charge neutral SDW (Spin Density Wave) compound with magnetic ordering below 150 K. The Superconducting FeAs layers are doped with Co and Ni at Fe site, whereas REO layers are doped with F at O site. The electron doping in SmFeAsO through Co results in superconductivity with transition temperature (Tc) maximum up to 15 K, whereas F doping results in Tc upto 47 K in SmFeAsO. All these REFe/Co/NiAsO/F compounds are iso-structural to ZrCuSiAs structure. The samples are crystallized in a tetragonal structure with space group P4/nmm. Variation of Tc with different doping routes shows the versatility of the structure and mechanism of occurrence of superconductivity. It seems doping in redox layer is more effective than direct doping in superconducting FeAs layer., Comment: 4 Pages text + Figs: (awana@mail.nplindia.ernet.in)

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 synthesis, structural details and complete superconducting characterization of very recently discovered Nb2PdS5 new superconductor. The synthesized compound is crystallized in mono-clinic structure. Bulk superconductivity is seen in both magnetic susceptibility and electrical resistivity measurements with superconducting transition temperature (Tc) at 6K. The upper critical field (Hc2), being estimated from high field magneto-transport measure-ments is above 240kOe. The estimated Hc2(0) is clearly above the Pauli paramagnetic limit. Heat capacity measurements show clear transition with well defined peak at Tc, but with lower jump than as expected for a BCS type superconductor. The Sommerfield constant and Debye temperature as determined from low temperature fitting of heat capacity data are 32mJ/moleK2 and 263K respectively. Hall coefficients and resistivity in conjugation with electronic heat capacity indicates multiple gap superconductivity signatures in Nb2PdS5. We also studied the impact of hydrostatic pressure on superconductivity of Nb2PdS5 and found nearly no change in Tc for the given pressure range., 14 pages text + Figs

Proximity coupling and magnetic switching dynamics are shown to be correlated in bilayers (B) of fully spin polarized ferromagnet (F) Co2FeSi and superconductor (S) NbN. The upper critical field derived from resistivity measurements shows a dimensional crossover with a reduced effective thickness of the S layer. At temperatures (T) << superconducting Tc, the measured M-H loops show two step switching; one at T-independent value ~ 7 Oe and other at strongly T-dependent value becoming very large ~ 1 kOe at 2 K. These results reveal induced ferromagnetism in S-layer at the S/F interface with vortex pinning influenced dynamics., 11 pages, 4 figures; Appl. Phys. Lett. to appear

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)

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)

The superconductivity of MgB2, AlB2, NbB2+x and TaB2+x is inter-compared. The stretched c-lattice parameter (c = 3.52 A) of MgB2 in comparison to NbB2.4 (c = 3.32 A) and AlB2 (c = 3.25 A) decides empirically the population of their p and s bands and as a result their transition temperature, Tc values respectively at 39K and 9.5K for the first two and no superconductivity for the later. Besides the electron doping from substitution of Mg+2 by Al+3, the stretched c-parameter also affects the Boron plane constructed hole type Sigma-band population and the contribution from Mg or Al plane electron type Pi band. This turns the electron type (mainly Pi-band conduction) non-superconducting AlB2 to hole type (mainly s-band conduction) MgB2 superconductor (39 K) as indicated by the thermoelectric power study. Keeping this strategy in mind that stretching of c-parameter enhances superconductivity, the NbB2+x and TaB2+x samples are studied for existence of superconductivity. The non-stoichiometry induces an increase in c parameter with Boron excess in both borides. Magnetization (M-T) and Resistivity measurements (R-T) in case of niobium boride samples show the absence of superconductivity in stoichiometric NbB2 sample (c = 3.26 A) while a clear diamagnetic signal and a R = 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., 14 pages Text+Figs contact (awana@mail.nplindia.ernet.in). ACCEPTED: J. APPL.PHYS. (MMM-2008 Proceedings)

We report the synthesis, magnetic susceptibility and crystal structure analysis for NbB2+x (x = 0.0 to 1.0) samples. The study facilitates in finding a correlation among the lattice parameters, chemical composition and the superconducting transition temperature Tc. Rietveld analysis is done on the X- ray diffraction patterns of all synthesized samples to determine the lattice parameters. The a parameter decreases slightly and has a random variation with increasing x, while c parameter increases from 3.26 for pure NbB2 to 3.32 for x=0.4 i.e. NbB2.4. With higher Boron content (x>0.4) the c parameter decreases slightly. The stretching of lattice in c direction induces superconductivity in the non- stoichiometric niobium boride. Pure NbB2 is non-superconductor while the other NbB2+x (x>0.0) samples show diamagnetic signal in the temperature range 8.9-11K. Magnetization measurements (M-H) at a fixed temperature of 5K are also carried out in both increasing and decreasing directions of field. The estimated lower and upper critical fields (Hc1 & Hc2) as viewed from M-H plots are around 590 and 2000Oe 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., 14 pages TEXT+Figs; comments/suggestions (awana@mail.nplindia.ernet.in). ACCEPTED: Solid State Communications (2008)

We report the synthesis and variation of superconductivity parameters such as transition temperature Tc, upper critical field Hc, critical current density Jc, irreversibility field Hirr and flux pinning parameter (Fp) for the MgB2-xCx system with nano-Carbon doping up to x=0.20. Carbon substitutes successfully on boron site and results in significant enhancement of Hirr and Jc(H). Resistivity measurements reveal a continuous decrease in Tc under zero applied field, while the same improves remarkably at higher fields with an increase in nano-C content for MgB2-xCx system. The irreversibility field value (Hirr) is 7.6 & 6.6 Tesla at 5 and 10K respectively for the pristine sample, which is enhanced to 13.4 and 11.0 Tesla for x = .08 sample at same temperatures. Compared to undoped sample, critical current density (Jc) for the x=0.08 nano-Carbon doped sample is increased by a factor of 24 at 10K at 6 Tesla field., 14 pages Text+Figs; Comments/suggestions 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)

We report on magnetic, resistivity, M\"ossbauer spectroscopy, and scanning tunneling spectroscopy studies of ceramic superconducting (SC) and non-SC $\mathrm{Ru}{\mathrm{Sr}}_{2}{\mathrm{Y}}_{1.5}{\mathrm{Ce}}_{0.5}{\mathrm{Cu}}_{2}{\mathrm{O}}_{10\ensuremath{-}\ensuremath{\delta}}$ $(x=0.5)$ and $\mathrm{Ru}{\mathrm{Sr}}_{2}\mathrm{Y}\mathrm{Ce}{\mathrm{Cu}}_{2}{\mathrm{O}}_{10}$ $(x=1)$. All materials studied were prepared at high oxygen pressure under the same conditions. Two magnetic transitions at ${T}_{2\mathrm{M}}$ and ${T}_{\mathrm{M}}$ $({T}_{2\mathrm{M}}l{T}_{\mathrm{M}})$ are observed. For the SC $x=0.5$ material, both ${T}_{2\mathrm{M}}$ and ${T}_{\mathrm{M}}$ and saturation moments are lower, but its coercive fields are higher as compared to the non-SC $x=0.5$ sample material. In the $x=1$ sample, a dramatic increase in the magnetoresistance occurs at ${T}_{\mathrm{M}}$.

The physical property characterization of Al doped Mg1-xAlxB2 system with x = 0.0 to 0.50 is reported. The results related to phase formation, structural transition, resistivity R(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., Comment: 17 Pages Text + Figs. To appear in Physica C

Magnetic measurements on MgB2 superconductors doped with various concentration (0 < x < 0.6%) of Al2O3 nano‐particles (∼20 nm) have been performed in the temperature range 4–40K and in a magnetic field of strength up to 9 Tesla. A significant increase in the irreversibility field (Hirr), critical current density (Jc) and remanent magnetization (MR) have been obtained with increasing the concentration of the Al2O3 nano‐particle. At low field we have observed large vortex instabilities (known as vortex‐avalanche) associated with all doped samples characterized with high critical current density. Vortex avalanche effect is reduced with increasing temperature vanishing near 20K. The results are discussed in terms of local‐vortex instabilities caused by doping of Al2O3 nano‐particles.

We report x-ray photoelectron spectroscopic investigation of $RuSr_2Eu_{1.5}Ce_{0.5}Cu_2O_{10}$ with ferromagnetic TC\sim100 K and a superconducting transition temperature of $\sim$30 K compared with $RuSr_2EuCeCu_2O_{10}$, which is a ferromagnetic (TC\sim 150 K) insulator. Our results show that the rare earths, Eu and Ce, are in 3+ and 4+ states, respectively. Comparing the Ru core level spectra from these compounds to those from two Ru reference oxides, we also show that Ru in these ruthenocuprates is always in 5+ state, suggesting that the doped holes in the superconducting compound arising from the substitution of $Ce^{4+}$ by $Eu^{3+}$ are primarily in the Cu-O plane, in close analogy to all other doped high-$T_C$ cuprates. Analysis of Cu 2p spectra in terms of a configuration interaction model provides a quantitative description of the gross electronic structures of these ruthenocuprates.

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)

Mo doped ruthenocuprates ${\mathrm{Ru}}_{1\ensuremath{-}x}{\mathrm{Mo}}_{x}{\mathrm{Sr}}_{2}{\mathrm{Eu}}_{1.5}{\mathrm{Ce}}_{0.5}{\mathrm{Cu}}_{2}{\mathrm{O}}_{10\ensuremath{-}\ensuremath{\delta}}$ are synthesized for $x=0.0$, 0.2, 0.4, 0.6, 0.8, and 1.0, and their magnetic and superconducting properties are studied. It has been found that the magnetic transition temperature $T_{\mathit{ZFC}}{}^{\text{peak}}$, which corresponds to the appearance of a weak ferromagnetic effect, decreases from its value of $75\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ for $x=0.0$ to 22, 25, and $18\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, respectively for the $x=0.2$, 0.4, and 0.6 samples. Another finding is that the magnetic susceptibility reduces at $T_{\mathit{ZFC}}{}^{\text{peak}}$ by a factor of about 6, 85, and 413 for $x=0.2$, 0.4, and 0.6, respectively. The samples of $x=0.8$ and 1.0 are found to have no magnetic or superconducting effects. The values of the superconducting transition temperature are obtained from the resistivity versus the temperature data. An important result is that ${T}_{c}$ increases by 4.5 and $7.0\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ for $x=0.2$ and 0.4, respectively, and then decreases by $17\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ for $x=0.6$. The observed variation of ${T}_{c}$ with $x$ has been explained in terms of a theory which combines the effects of weakening magnetic behavior and reducing carrier concentration in a phenomenological manner. The resulting theory is found to provide a good agreement with the observed value of ${T}_{c}$.

This is the first report of the observation of the onset of excess volume and also of the strain along the a-axis near the magnetic ordering temperature in Ru-1222 superconductor, and indicates a coupling between the lattice and the magnetism in this system. Magnetization, magneto transport and thermoelectric power measurements being carried out on the same sample are also reported., Comment: 15 Pages Text Plus Figs. Physica C (2006) accepted

The magnetic and superconducting properties of $\mathrm{Ru}{\mathrm{Sr}}_{2}{\mathrm{Gd}}_{1.5}{\mathrm{Ce}}_{0.5}{\mathrm{Cu}}_{2}{\mathrm{O}}_{10\ensuremath{-}\ensuremath{\delta}}$ polycrystalline samples with different oxygen-doping levels are presented. A strong suppression of the superconducting temperature $({T}_{c})$, as well as a reduction in the superconducting fraction, occurs as the oxygen content is reduced by annealing the samples in oxygen-deprived atmospheres. Drastic changes in the electrical resistivity are observed above ${T}_{c}$, possibly associated with oxygen removal, mainly from grain boundaries. However, the magnetic ordering is relatively less affected by the changes in oxygen content of the samples. The spin-glass transition is enhanced and shifted to higher temperatures with the reduction in oxygen content. This could be correlated with an increase in the spin disorder and frustration for the oxygen-depleted samples. Also, the same oxygen-vacancy-induced disorder could explain the reduction in the fraction of the sample showing antiferromagnetic order. We also report significant changes in the measured properties of the samples as a function of time.

Results of phase formation, resistivity (r), and thermo-electric power (S), are reported on Y1-xCaxBa2Cu2.7Co0.3O7 compounds with x = 0.1 and 0.2. Pristine compound i.e. without Co or Ca substitution crystallizes in orthorhombic structure with space group P/mmm. Cu-site Co substituted compound i.e. YBa2Cu2.7Co0.3O7 is tetragonal. With simultaneous doping of Ca at Y site in Co substituted compound i.e. Y1-xCaxBa2Cu2.7Co0.3O7 the tetragonal nature still remains. r(T) measurements showed superconducting transition temperature (Tc) to decrease from 90K (YBa2Cu3O7) to 33 K for YBa2Cu2.7Co0.3O7 which with further Ca substitution increases from 33K to 53K (Y0.9 Ca0.1Ba2Cu2.7Co0.3O7) and 67 K for Y0.8 Ca0.2Ba2Cu2.7Co0.3O7. Tc decreases first with Cu-site Co substitution by hole-filling and later recovers by simultaneous hole creation by Y site Ca substitution. Room temperature thermoelectric power S(300 K), which is an indirect measure of mobile carriers shows the decrease of carriers with Co doping and creation by Ca substitution. Our results demonstrate the hole filling by Co substitution is compensated by simultaneous Ca substitution., 10 pages of TEXT and Figs

Results of thermo-electric power (S) and electrical resistivity (r) measurements are reported on NaxCoO2 compounds with x = 1.0, 0.7 and 0.6. These are single-phase compounds crystallizing in the hexagonal structure (space group P63/mmc) at room temperature. Thermo-electric power values at 300K (S300K) are, 80mV/K, 39mV/K and 37mV/K for x = 1.0, 0.7 and 0.6 samples, respectively. The samples with x=0.7 and 1.0 are metallic down to 5 K, while the x = 0.6 sample is semiconducting. The value of r300K for x = 1.0 sample is \~0.895 mW-cm and the power factor (S2/r) is = 7.04 x 10-3 W/mK2 which qualifies it as a good thermo-electric material. In x =1.0 sample, S(T) is positive throughout 300-5K temperature range and decreases monotonically to zero as temperature T= 0. In contrast, S(T) of x = 0.7 and 0.6 samples changes sign and shows negative values between 90 K and 16 K before approaching zero as T = 0. Anomalous S(T) behavior of x = 0.6 and 0.7 samples, which are coincidentally the precursor materials to the reported superconductivity in this class of materials, indicates a dramatic change in the electronic structure of these compounds on lowering the Na content., Text With Figures (10 pages)

The results of micro-structural and detailed magnetization studies are reported here for our recently published TbSr2Cu2.7Mo0.3O7+d superconductor with a transition temperature (Tc) at as high as 80 K [1]. From XRD and EDX the sample was confirmed to be of single-phase and possess the nominal cation stoichiometry. The SEM images showed well-developed grains for this ceramic high-Tc superconductor. Magnetization measurements at various temperatures and fields revealed bulk superconductivity below 80 K. Magnetization versus applied field (M vs. H) loops at various temperatures below the Tc exhibited clear full penetration field HP. In normal state (> 100 K) the susceptibility follows the Curie-Weiss behavior with an effective paramagnetic moment of 9.83 mB., 11 pages of text and figs. Accepted in Physica C