V. P. S. Awana

High field performance of nanodiamond doped MgB2 superconductor

The results from magnetotransport and magnetization of nanodiamond 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 nanodiamond 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 7.5T at 4K and the same is increased to 13.5T for 3wt% 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 3wt% nD gives the best Jc values at all 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 interband scattering for highly ...

Superconductivity at 5 K in NdO0.5F0.5BiS2

We report appearance of superconductivity at 5 K in NdO0.5F0.5BiS2 and supplement the discovery [Demura et al., e-print arXiv:1207.5248] of the same in layered sulfide based compound. Detailed structural analysis showed that the studied compound is crystallized in tetragonal P4/nmm space group with lattice parameters a = 3.9911(3) A and c = 13.3830(2) A. Bulk superconductivity is established in NdO0.5F0.5BiS2 at 5 K by both transport and magnetic measurements. Electrical transport measurements showed superconducting Tc onset at 5.2 K and Tc (ρ = 0) at 4.7 K. Under applied magnetic field, both Tc onset and Tc (ρ = 0) decrease to lower temperatures and an upper critical field [Hc2(0)] of above 23 kOe is estimated. Isothermal magnetization exhibited typical type-II behavior with lower critical field (Hc1) of around 25 Oe. Specific heat [Cp(T)] is investigated in the temperature range of 1.9–50 K in zero external magnetic field. A Schottky-type anomaly is observed at low temperature below 7 K.

Structural, electrical, magnetic, and thermal studies of Cr-doped La0.7Ca0.3Mn1−xCrxO3 (0≤x≤1) manganites

We report detailed structural, electrical, magnetic and specific heat studies on La0.7Ca0.3Mn1-xCrxO3 manganites. Rietveld analysis of fitted and observed XRD patterns exhibited the single-phase nature of all the studied materials, which crystallize in Pbnm space group. Successive substitution of Cr at Mn site in La0.7Ca0.3Mn1-xCrxO3 manganites increases the electrical resistivity and decrease the characteristic insulator-metal transition temperature (TIM) of the parent compound along with a hump like feature for higher Cr-content (x > 0.06) samples. The hump structure basically signifies the onset of anti-ferromagnetic (AFM) interactions as inferred by both the magnetic and infra-red (IR) spectroscopy studies. The systematic suppression of FM state results in a spin glass (SG) like behavior. IR studies revealed that the vibration mode at 413 cm -1 being associated with internal bending of MnO6 octehedra, becomes softer, indicating an increase in distortion and hence the possible spin glass behavior. The critical exponents (and  are calculated from the heat capacity (CP) data near the TIM/TFM. The same exhibited variations of their values with doping. In particular the value of increases from 0.37(x = 0.0) to 0.43(x = 0.04), clearly indicating the coexistence of both long and short range magnetic orders i.e. tendency towards SG state for Cr doped samples. On the basis of present results, it is suggested that Cr dilutes double-exchange (DE) based FM and rather promotes the AFM based super exchange interactions (SE) via Cr 3+ /Mn 4+ ions. Substitution of CrWe report detailed structural, electrical, magnetic and specific heat studies on La0.7Ca0.3Mn1-xCrxO3 manganites. Rietveld analysis of fitted and observed XRD patterns exhibited the single-phase nature of all the studied materials, which crystallize in Pbnm space group. Successive substitution of Cr at Mn site in La0.7Ca0.3Mn1-xCrxO3 manganites increases the electrical resistivity and decrease the characteristic insulator-metal transition temperature (TIM) of the parent compound along with a hump like feature for higher Cr-content samples. The hump structure basically signifies the onset of anti-ferromagnetic (AFM) interactions as inferred by both the magnetic and infra-red (IR) spectroscopy studies. The systematic suppression of FM state results in a spin glass (SG) like behavior. IR studies revealed that the vibration mode at 413 cm-1 being associated with internal bending of MnO6 octehedra, becomes softer, indicating an increase in distortion and hence the possible spin glass behavior. The critical exponents alpha, beta and gama are calculated from the heat capacity (CP) data near the TIM/TFM. The same exhibited variations of their values with doping. In particular the value of beta increases from 0.37 to 0.43, clearly indicating the coexistence of both long and short range magnetic orders i.e. tendency towards SG state for Cr doped samples. On the basis of present results, it is suggested that Cr dilutes double-exchange (DE) based FM and rather promotes the AFM based super exchange interactions (SE) via Cr3+/Mn4+ ions. Substitution of Cr systematically destroys both the metallic state and long range ferromagnetic (FM) order.

Superconductivity above 90 K in low Tc phase Bi2Ca1Sr2Cu2Ox

Abstract Superconductivity in the low T c Bi-based 2-1-2-2 cuprate has been studied as a function of processing parameters. T c ( R =0) above 91 K has been realized through the optimization of process conditions. XRD of the samples indicate subtle changes in respect of intensity and splitting of certain peaks. The results are compared with those on Bi-based 2-2-2-3 and Y-based 1-2-3 cuprates. The enhanced T c and the reduced δT c of the 2-1-2-2 compound have been suggested to be the consequence of improved atomic ordering and orthorhombicity.

High resolution STM/STS studies of the role of oxygen in superconductivity of Bi-2122 and optimization of Tc

The presence of excess oxygen intercalated in Bi-O redox layers has been found to be detrimental to the Tc of the Bi-2122 phase. High resolution STM has been effectively used to locate intercalated oxygen and STS studies have revealed Bi-O layers becoming more insulating with intercalation. By suitable nitrogenation when the excess oxygen is depleted, more conducting redox layers give rise to an improved coupling between Cu-O planes of the neighbouring demi-unit cells in the c-direction, and it is this improved coupling that is responsible for enhanced Tc. In this way it has been possible to optimize Tc(R=0) for the single phase Bi-2122 cuprate at 96 K with a transition width of 3 K, which is the best value reported to date for the bulk ceramic sample of this material.

Superconductivity and resistivity studies in Bi2Sr2Ca1−xMxCu2O8+y (M=Eu, Dy, Tm and 0⩽x⩽0.6)

Abstract Electrical resistivity measurements have been performed on the Bi 2 Sr 2 Ca 1−x M x Cu 2 O 8+y (M=Eu, Dy, Tm) system with various heat treatments. A gradual change in normal state resistivity ( ϱ n ) and T c has been observed with increasing dopant concentration. The results provide experimental evidence for a transition from a metallic behaviour to a variable range hopping regime at low temperatures. In the insulator regime (highly semiconducting-like) the resistivity behaviour is found to turn from 2D to 3D variable range hopping for x ⩾0.6. The results are discussed in terms of a theoretical model which determines the intrinsic competition between superconductivity and localization in an essentially localized system. T c data are compared with the earlier reported findings.

High resolution STM/STS studies of Bi-O redox layers and superconductivity in pure and substituted Bi-2122 high-Tc cuprate

Abstract Effects arising from the intercalation of excess oxygen in Bi-O redox layers of the Bi-2122 compound have been studied with a STM/STS technique which has been successfully used for obtaining atomically resolved images. The excess oxygen is incorporated by substituting Dy 3+ at the Ca 2+ -site as well as by oxygenating the pure sample. It has been possible to detect and directly observe the excess oxygen atoms in the Bi-O layer which is in agreement with the information inferred from neutron diffraction and HRTEM studies of other workers. The STS studies show that there is a gradual increase in its room temperature energy gap and a decrease in the LDOS, indicating the Bi-O layer to become insulating with intercalation of excess oxygen. Depletion of excess oxygen through nitrogenation or Pb substitution at the Bi-site produces the reverse effect. The importance of improving the metallicity of the redox layer is elaborated and it is demonstrated that the T c of the bulk Bi-2122 phase can be significantly sharpened and raised to 96 K by depleting the excess oxygen from the layer. The redox reaction and the influence of the resistive state of the Bi-O layers in relation to the observed superconductivity results are discussed.

Fluxoid jump coupled high critical current density of nano-Co3O4 doped MgB2

Polycrystalline MgB2 samples, with added 0, 2, 4 and 6% nano-Co3O4, synthesized by vacuum (10−5 Torr) annealing at 750 °C for two and a half hours each, are found to be nearly single phase with the presence of only a small quantity of Mg/MgO in the pristine sample in addition to the Co2O3 in the doped compounds. All the samples exhibited clear and sharp diamagnetic transitions at around 38 K, in zero-field-cooled (ZFC) magnetic susceptibility measurements with a sizeable signal. The field-cooled (FC) measurements, though having sharp transitions, showed a very small signal, indicating a high level of pinning centres in these samples. Further, some of the doped samples exhibited the paramagnetic Meissner effect (PME) in an applied field of 5 Oe. The critical current density (Jc), estimated by invoking Beans model for the pristine compound, increases by nearly an order of magnitude for 2% and 4% nano-Co3O4 doping and then decreases sharply for the 6% sample at nearly all studied temperatures and applied fields. Further, the increased Jc (~108 A cm−2) is coupled with fluxoid jumps (T≤20 K and H≤1 T). Fluxoid jumps are not seen in the relatively low Jc pristine or 6% sample. This means that the fluxoid jumps are intrinsic only to the high-Jc samples.

Room temperature magneto-resistance and temperature coefficient of resistance in La0.7Ca0.3−xAgxMnO3 thin films

We investigate the magnetotransport properties of La0.7Ca0.3−xAgxMnO3 (0⩽x⩽0.3) polycrystalline films prepared by spray pyrolysis. The virgin La0.7Ca0.3MnO3 (LCMO) (x=0.0) film shows paramagnetic-ferromagnetic [insulator-metal (IM)] transition at TC∼260K (TIM∼245K). Ag doping induces appreciable enhancement in TC∕TIM and interestingly TIM of Ag doped films is higher than the corresponding TC value. In the 30% Ag doped film IM transition occurs at ∼325K which is ∼10K higher than its TC∼315K. Strong irreversibility is observed in zero field cooled–field cooled magnetization of all the films and the irreversibility temperature Tirr>TC increases with Ag doping. This suggests the formation of magnetic clusters above TC. The pure LCMO film exhibits small magnetoresistance (MR) ∼20% and rather poor temperature coefficient of resistance (TCR) in the vicinity of the room temperature. In contrast, around room temperature the Ag doped films show large MR which is ∼60% for x=0.15 and decreases to ∼30% for the pure Ag...

57Fe Mossbauer spectroscopy and magnetic measurement studies of oxygen deficient LaFeAsO

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.