arXiv subjects

H. Kishan

Publications and source records attributed to H. Kishan.

66 records · Page 4Linked to original sources

arXiv2004

Revival of Superconductivity by Y3+/Ca2+ substitution in YBa2Cu2.7Co0.3O7 without reported phase transformation

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.

cond-mat.supr-con↗

arXiv2004

High pressure high temperature (HPHT) synthesis and magnetization of Magneto-Superconducting RuSr2(LnCe2)Cu2O12.25 (Ru-1232) compounds (Ln = Y and Dy)

RuSr2(LnCe2)Cu2O12.25 (Ru-1232) compounds with Ln = Y and Dy being synthesized by high pressure high temperature (6GPa, 12000C) solid state synthesis route do crystallize in space group P4/mmm in near single phase form with small quantities of SrRuO3 and RuSr2(RE1.5Ce0.5)Cu2O10 (Ru-1222). Both samples exhibit magnetic transitions (Tmag.) at ~90 K with significant branching of zfc (zero-field-cooled) and fc (field-cooled) magnetization and a sharp cusp in zfc at ~ 70 K, followed by superconducting transitions at ~ 30 K. Both compounds show typical ferromagnetic hysteresis loops in magnetic moment (M) versus field (H) magnetization right upto Tmag. i.e. < 90K. To our knowledge these are the first successfully synthesized Ru-1232 compounds in near single phase with lanthanides including Y and Dy. The results are compared with widely reported Gd/Ru-1222 and Ru-1212 (RuSr2GdCu2O8) compounds. In particular, it seems that the Ru moments magnetic ordering temperature (Tmag.) scales with the c-direction distance between magnetic RuO6 octahedras in Ru-1212/1222 or 1232 systems.

cond-mat.supr-con↗

arXiv2004

Resistivity and Thermoelectric power of NaxCoO2 (x =1.0, 0.7 and 0.6) system

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.

cond-mat.str-el↗

arXiv2004

Induction of superconductivity in Y0.4Pr0.6Ba2-xSrxCu3O7 system with increasing Sr substitution

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.

cond-mat.supr-con↗

arXiv2004

Possible Competition between superconductivity and magnetism in RuSr2Gd1.5Ce0.5Cu2O10-d (Ru-1222) Rutheno-cuprate compounds

The RuSr2Gd1.5Ce0.5Cu2O10-d (Ru-1222) compounds, with varying oxygen content, crystallize in a tetragonal crystal structure (space group I4/mmm). Resistance (R) versus temperature (T) measurements show that the air-annealed samples exhibit superconductivity with superconducting transition temperature (Tc) onset at around 32 K and R=0 at 3.5 K. On the other hand, the N2-annealed sample is semiconducting down to 2 K. Magneto-transport measurements on air-annealed sample in applied magnetic fields of 3 and 6 Tesla show a decrease in both Tc onset and TR=0. Magnetoresistance of up to 20% is observed in N2-annealed sample at 2 K and 3 T applied field. The DC magnetization data (M vs. T) reveal magnetic transitions (Tmag.) at 100 K and 106 K, respectively, for both air- and N2-annealed samples. Ferromagnetic components in the magnetization are observed for both samples at 5K and 20 K. The superconducting transition temperature (Tc) seems to compete with the magnetic transition temperature (Tmag.). Our results suggest that the magnetic ordering temperature (Tmag.) of Ru moments in RuO6 octahedra may have direct influence/connection with the appearance of superconductivity in Cu-O2 planes of Ru-1222 compounds. Key words: Ruthenocuprates, Superconductivity and Magnetism, Magneto transport and Magneto resistance. PACS: 74.25. Ha, 74.72. Jt, 75.25. +z, 75.30. Cr.

cond-mat.supr-con↗

arXiv2004

Micro-structure and Magnetization of the 80-K Superconductor, TbSr2Cu2.7Mo0.3O7+d

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.

cond-mat.supr-con↗

arXiv2004

Systematic induction of superconductivity in Y1-xCaxBa2Cu3O6.3 system

Compounds of series YBa2Cu3O7-d (d = 0.7), being synthesized by solid-state synthesis route with x = 0.0, 0.10, 0.15 and 0.20, crystallize in single-phase form with tetragonal structure (space group P4/mmm). The c-lattice parameter increases with increasing x, indicating successful substitution of Y3+ by bigger Ca2+ ion. Resistance versus temperature (R vs. T) measurements show that pristine sample (x = 0.0) is semiconducting down to 5 K. Induction of superconductivity is seen with an increase in x. The x = 0.10 sample exhibit the onset of superconducting transition (Tconset) at around 42 K without attaining zero resistance superconducting transition temperature (TcR=0) state down to 5 K. For x = 0.15 and 0.20 samples TcR=0 is observed at 25 K and 35 K with Tconset of 52 K and 61 K respectively. Thermo-electric power (S) measurements in temperature range of 5 - 300 K, exhibited TcS=0 at around 25 K and 35 K respectively for x = 0.15 and 0.20 samples. Further a sign change in S from +ve to -ve is observed at low temperatures for x = 0.0 sample. Room temperature +ve S value decreases with increase in x, indicating enhanced number of mobile holes. The Magneto-transport measurements under applied magnet ic fields of 3 and 6 Tesla show the broadening of superconducting transition for x = 0.15, and 0.20 samples. For x = 0.10 sample the superconducting transition is suppressed in applied fields of 3 and 6 Tesla. Our results will help in constructing the complete phase diagram of Y1-xCaxBa2Cu3O7-d compounds with various x and d.

cond-mat.supr-con↗

arXiv2003

Effect of Sr substitution on the stability of CuO chains in oxygen and nitrogen annealed YBa2-xSrxCu3O(x = 0.0, 0.50, 1.0) superconducting system

Samples of YBa2-xSrxCu3O7-d (x = 0.0, 0.50, 1.0) series were synthesized by solid-state reaction and first annealed in flowing O2 gas. All samples crystallized in orthorhombic structure, where orthorhombic distortion was found to decrease with an increase in x. Four probe resistivity measurements showed that superconducting transition temperature (Tc) decreases monotonically with x at a rate of dTc/dx = 1.3 K/at%. The sample with x=0 had a Tc = 91 K. The samples were further annealed in flowing N2 gas at 450 0C. The Tc of all samples decreased by nitrogen annealing. Interestingly, the decrease in Tc(x) became nearly three times larger at a rate of dTc/dx = 3.7 K/at%. The sample with x=0 had a Tc = 42 K. We discuss possible reasons and implications of the Tc(x) suppression vis stability of CuO chains. Key words: Y-123, Sr substitution, Superconducting transition

cond-mat.supr-con↗

arXiv2003

Comparison of the superconducting critical transition temperature of LaBaCaCu3O7-d and NdBaCaCu3O7-d

LaBaCaCu3O7 (La:1113) and NdBaCaCu3O7 (Nd:1113) compounds are synthesized by solid state reaction route. Both compounds crystallize in tetragonal structure. Four probe resistivity measurements showed that superconducting transition temperature (Tc) is 73 K and 41 K respectively for La:1113 and Nd:1113 compounds. Considering the fact that for RE:123 (REBa2Cu3O7) compounds the Tc hardly depends upon the choice of RE (Rare earth, except Ce, Pr and Tb), the current nearly two fold increase in Tc of La:1113 when compared to Nd:1113, warrants serious discussion.

cond-mat.supr-con↗

arXiv2003

Structural and magnetic properties of MSr2Y1.5Ce0.5Cu2Oz (M-1222) compounds with M = Fe and Co

MSr2Y1.5Ce0.5Cu2Oz (M-1222) compounds, with M = Fe and Co, have been synthesized through a solid-state reaction route. Both compounds crystallize in a tetragonal structure (space group I4/mmm). A Rietveld structural refinement of room-temperature neutron diffraction data for Fe-1222 reveals that nearly half the Fe remains at the M site, while the other half goes to the Cu site in the CuO2 planes. Existence of Fe at two different lattice sites, is also confirmed by 57Fe Mossbauer spectroscopy from which it is inferred that nearly 50% of the total Fe occupies the Cu site in the CuO2 planes as Fe3+, whereas the other 50 % is located at the M site with nearly 40 % as Fe4+ and around 10% as Fe3+. For the M = Co compound, nearly 84 % of Co remains at its designated M site, while the rest occupies the Cu site in the CuO2 planes. The oxygen content, z, based on oxygen occupancies refined from the neutron diffraction data, comes close to 9.0 for both the samples The ZFC and FC magnetization curves as a function of temperature show a complex behavior for both Fe-1222 and Co-1222 compounds.

cond-mat.str-el↗

arXiv2003

Superconductivity of up to 80 K for Tb-123 - TbSr2Cu2.7Mo0.3O7

The Tb-123 phase has been synthesized in single-phase form with the composition TbSr2Cu2.7Mo0.3O7 by solid-state reaction route and its phase purity is confirmed by neutron powder diffraction experiments. As-synthesized sample does not show superconductivity down to 5 K. An unusually high antiferromagnetic ordering temperature (TN) of around 7 K is seen for Tb. After 120-atm-O2 post-annealing, bulk superconductivity is achieved in the compound at around 30 K, without any significant effect on TN. To achieve higher oxygen content and higher Tc, the as-synthesized sample is subjected to high-pressure oxygenation (HPO) carried out in a closed cell at 5 GPa and 400 oC in the presence of AgO as an excess-oxygen source. This sample exhibited superconductivity onset at around 80 K with a Meissner fraction of larger than 10 % at 5 K. Our observation of superconductivity at 80 K is the highest Tc to-date among the Tb-123 compounds.

cond-mat.supr-con↗

arXiv2003

Spin Glass Behavior in the Ru-1222 and Ru-1212 rutheno-cuprate families: a comparative study

Strong evidences of a spin glass phase on polycrystalline RuSr2Ln1.5Ce0.5Cu2O10-x (LnRu-1222, Ln = Y, Ho and Dy) is provided by ac susceptibility and dc magnetization measurements. The fingerprint of a spin glass transition, the frequency-dependent cusp in ac susceptibility vs. T measurements at low magnetic field, is observed for all LnRu-1222 samples. The strong supression of the cusp in the ac susceptibility measurement in the presence of magnetic fields as low as 500 Oe, and the results of thermoremanent magnetization (TRM) and isothermal remanent magnetization (IRM) measurements are also evidences of a spin glass behavior. By comparing these results with measurements on a YRu-1212 sample we conclude that the glassy behavior is characteristic of the Ru-1222 phase, not being observed for the studied YRu-1212 sample. We suggest that the spin glass transition occur only in the Ru-1222 phase possibly due to the larger number of oxygen vacancies present in these compounds.

cond-mat.supr-con↗