Publications (9)8.14 Total impact
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Article: Enhanced superconducting performance of melt quenched Bi2Sr2CaCu2O8 (Bi-2212) superconductor
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ABSTRACT: We scrutinize the enhanced superconducting performance of melt quench Bismuth based Bi2Sr2CaCu2O8 (Bi-2212) superconductor. The superconducting properties of melt quenched Bi-2212 (Bi2212-MQ) sample are compared with non-melted Bi2212-NM and Bi1.4Pb0.6Sr2Ca2Cu3O10 (Bi-2223). Crystal structure and morphology of the samples are studied using X-ray diffraction and Scanning Electron Microscopy (SEM) techniques. The high field (14T) magneto-transport and DC/AC magnetic susceptibility techniques are extensively used to study the superconducting properties of the investigated samples. The superconducting critical temperature (Tc) and upper critical field (Hc2) as well as thermally activated flux flow (TAFF) activation energy are estimated from the magneto-resistive [R(T)H] measurements. Both DC magnetization and amplitude dependent AC susceptibility measurements are used to determine the field and temperature dependence of critical current density (Jc) for studied samples. On the other hand, the frequency dependent AC susceptibility is used for estimating flux creep activation energy. It is found that melt quenching significantly enhances the superconducting properties of granular Bi-2212 superconductor. The results are interpreted in terms of better alignment and inter-connectivity of the grains along with reduction of grain boundaries for Bi2212-MQ sample.11/2012; -
Article: Chalcogen Height Dependence of Magnetism and Fermiology in FeTe_xSe_{1-x}
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ABSTRACT: FeTexSe1-x (x=0, 0.25, 0.50, 0.75 and 1) system has been studied using density functional theory. Our results show that for FeSe, LDA seems better approximation in terms of magnitude of magnetic energy whereas GGA overestimates it largely. On the other hand for FeTe, GGA is better approximation that gives experimentally observed magnetic state. It has been shown that the height of chalcogen atoms above Fe layers has significant effect on band structure, electronic density of states (DOS) at Fermi level N(EF) and Fermi surfaces. For FeSe the value of N(EF) is small so as to satisfy Stoner criteria for ferromagnetism, (I\timesN(EF)\geq1) whereas for FeTe, since the value of N(EF) is large, the same is close to be satisfied. Force minimization done for FeTexSe1-x using supercell approach shows that in disordered system Se and Te do not share same site and have two distinct z coordinates. This has small effect on magnetic energy but no significant difference in band structure and DOS near EF when calculated using either relaxed or average value of z for chalcogen atoms. Thus substitution of Se at Te site decreases average value of chalcogen height above Fe layers which in turn affect the magnetism and Fermiology in the system. By using coherent-potential approximation for disordered system we found that height of chalcogen atoms above Fe layer rather than chalcogen species or disorder in the anion planes, affect magnetism and shape of Fermi surfaces (FS), thus significantly altering nesting conditions, which govern antiferromagnetic spin fluctuations in the system.06/2012; -
Article: Significant Improvement in Superconductivity by Substituting Pb at Bi-site in Bi2−xPbxSr2CaCu2O8 with x=0.0 to 0.40
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ABSTRACT: Here we report the synthesis and superconducting properties of bulk Bi2−x Pb x Sr2CaCu2O8 (x=0.0 to0.4) compound. Though the superconducting transition temperature (T c) decreases marginally, the critical current density under magnetic field J c(H) increases with Pb content. An optimization is observed for x=0.16 with J c(H) (7.894×103A/cm2) that is nearly doubled in comparison to the pristine compound. It seems that controlled substitution of Pb at Bi-site in bulk Bi-2212 (Bi2Sr2CaCu2O8) system can enhance the superconducting critical parameters. These results are explained on the basis of possible improved inter- and intra-granular properties with Pb substitution in Bi2−x Pb x Sr2CaCu2O8 system. KeywordsBi-2212 (Bi2Sr2CaCu2O8) system-Superconducting transition temperature (T c)-Critical current density and magnetizationJournal of Superconductivity and Novel Magnetism 04/2012; 23(4):493-499. · 0.65 Impact Factor -
Article: Superconductivity in the vicinity of ferromagnetism in oxygen freeperovskite MgCNi3: An experimental and DFT (Density Functional Theory) study
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ABSTRACT: We report synthesis, structural, magnetic, specific heat and Density Functional Theory (DFT) studies on MgCNi3 superconductor. Polycrystalline MgCNi3 samples are synthesized through standard solid state reaction route and found to crystallize in cubic perovskite structure with space group Pm3m, without any detectable trace of Ni impurity. Both AC and DC magnetization exhibited superconducting transition (Tc) at around 7.25 K. The lower critical field (Hc1) and irreversibility field (Hirr) are around 140 Oe and 11 kOe respectively at 2 K. The upper critical field (Hc2) being determined from in-field AC susceptibility measurements is 11.6 kOe and 91.70 kOe with 50% and 90% diamagnetism criteria respectively. Heat capacity (Cp) measurements are carried out under applied field of up to 140 kOe and down to 2 K. The Sommerfeld constant ({\gamma}) and Debye temperature ({\Theta}D) as determined from low temperature fitting of Cp(T) data to Sommerfeld-Debye model are 36.13 mJ/mole-K2 and 263.13 K respectively. The Bardeen-Cooper-Schrieffer (BCS) parameter (2{\Delta}/KBTc) is around 3.62, suggesting MgCNi3 to be an intermediate coupled BCS superconductor with value {\lambda} = 0.69. Although the density functional theory (DFT) calculations exhibited the compound to be non-magnetic but with spin fluctuations, the experimental isothermal magnetization MH loops at 20, 50, 100, 200 and 300 K showed some ferromagnetic nature in this compound with coercive field (Hc) of around 50 Oe at 20 K. The Ni3d states play dominant roles near the Fermi levels and there is strong hybridization between Ni3d and C2p states. It seems that MgCNi3 is superconducting in close proximity of ferromagnetism.Journal of Applied Physics. 02/2012; 111:033907. -
Article: Chalcogen height dependence of magnetism and Fermiology in FeTe x Se 1−x Chalcogen height dependence of magnetism and Fermiology in FeTe x Se 1−x
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ABSTRACT: The FeTe x Se 1−x (x = 0, 0.25, 0.50, 0.75 and 1) system has been studied using density functional theory. Our results show that for FeSe, the local density approximation (LDA) seems a better approximation in terms of magnitude of magnetic energy whereas the generalized gradient approximation (GGA) greatly overestimates it. On the other hand for FeTe, GGA is a better approximation that gives the experimentally observed magnetic state. It has been shown that the height of chalcogen atoms above Fe layers has a significant effect on band structure, electronic density of states (DOS) at the Fermi level N(E F) and Fermi surfaces. For FeSe the value of N(E F) is small so as to satisfy Stoner criteria for ferromagnetism, (I × N(E F) ≥ 1) whereas for FeTe, since the value of N(E F) is large, the same is close to being satisfied. Force minimization for FeTe x Se 1−x using the supercell approach shows that in a disordered system Se and Te do not share the same site and have two distinct z co-ordinates. This has a small effect on magnetic energy but no significant difference in band structure and DOS near E F when calculated using either the relaxed or average value of z for chalcogen atoms. Thus substitution of Se at the Te site decreases the average value of chalcogen height above Fe layers, which in turn affects the magnetism and Fermiology in the system. By using the coherent potential approximation for the disordered system, we found that height of chalcogen atoms above the Fe layer, rather than chalcogen species or disorder in the anion planes, affects the magnetism and shape of Fermi surfaces (FSs), thus significantly altering nesting conditions, which govern antiferromagnetic spin fluctuations in the system. (Some figures may appear in colour only in the online journal)Superconductor Science and Technology 01/2012; 25:95002-11. · 2.66 Impact Factor -
Article: Physical property and electronic structure characterization of bulk superconducting Bi3Ni
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ABSTRACT: We report the experimental and theoretical study of the magnetic nature of the Bi3Ni system. The structure is found to be orthorhombic (Pnma) with lattice parameters a = 8.879 Å, b = 4.0998 Å and c = 4.099 Å. The title compound is synthesized via a solid state reaction route by quartz vacuum encapsulation of 5 N 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 3000 Oe respectively at 2 K. Upper critical field (Hc2), as determined from in-field (up to 4 T) ac susceptibility, is found to be around 2 T at 2 K. The normal state specific heat is fitted using the Sommerfeld–Debye equation C(T) = γT + βT3 + δT5 and the parameters obtained are γ = 11.08 mJ mol − 1 K − 2, β = 3.73 mJ mol − 1 K − 4 and δ = 0.0140 mJ mol − 1 K − 6. The calculated electronic density of states (DOS) at the Fermi level N(EF) and Debye temperature ΘD are 4.697 states/eV/f.u. and 127.7 K respectively. We also estimated the value of the electron–phonon coupling constant (λ) to be 1.23, which when substituted in the MacMillan equation gives Tc = 4.5 K. Density functional theory (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 IN(EF) = 0.136 per Ni atom also indicates that the 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.Superconductor Science and Technology 06/2011; 24(8):085002. · 2.66 Impact Factor -
Article: Comparative experimental and Density Functional Theory (DFT) study of the physical properties of MgB2 and AlB2
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ABSTRACT: In present study, we report an inter-comparison of various physical and electronic properties of MgB2 and AlB2. Interestingly, the sign of S(T) is +ve for MgB2 the same is -ve for AlB2. This is consistent our band structure plots. We fitted the experimental specific heat of MgB2 to Debye Einstein model and estimated the value of Debye temperature (theta) and Sommerfeld constant (gamma) for electronic specific heat. Further, from gamma the electronic density of states (DOS) at Fermi level N(EF) is calculated. From the ratio of experimental N (EF) and the one being calculated from DFT, we obtained value of Lembda to be 1.84, thus placing MgB2 in the strong coupling BCS category. The electronic specific heat of MgB2 is also fitted below Tc using pi-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 Sigma and Pi 2p states of boron on the other hand Sigma band at Fermi level for AlB2 is absent. This leads to a weak electron phonon coupling and also to hole deficiency as Pi 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 AlB202/2011; -
Article: Negative thermoelectric power of over-doped Bi2Sr2CaCu2O8 superconductor
Physica C: Superconductivity. 12/2010; 470:S203-S204. -
Article: Anomalous thermoelectric power of overdoped Bi2Sr2CaCu2O8 superconductor
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ABSTRACT: All the samples possess single-phase character and their superconducting transition temperatures (T<sub>c</sub><sup>R=0</sup>) are 85, 90, and 72 K, respectively for Bi2212-MQ, Bi2212- N <sub>2</sub> , and Bi2212- O <sub>2</sub> . Though the Bi2212-MQ and Bi2212- N <sub>2</sub> samples are in near optimum doping regime, the Bi2212- O <sub>2</sub> is an overdoped sample. T<sub>c</sub><sup>S=0</sup> values obtained through S(T) data are also in line with those determined from the temperature dependence of resistance (T<sub>c</sub><sup>R=0</sup>) and dc magnetization (T<sub>c</sub><sup> dia </sup>) . Interestingly, S(T) behavior of the optimally doped Bi2212-MQ and Bi2212- N <sub>2</sub> samples is seen to be positive in whole temperature range, the same is found negative for the overdoped Bi2212- O <sub>2</sub> sample above T<sub>c</sub><sup>S=0</sup> . This anomalous S(T) behavior is seen in the light of the recent band structure calculations and the ensuing split Fermi surface as determined by angle-resolved photoelectron spectroscopy.Journal of Applied Physics 12/2009; · 2.17 Impact Factor
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Institutions
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2011–2012
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Himachal Pradesh University
- Department of Physics
Shimla, State of Himachal Pradesh, India
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