We report the experimental and theoretical study on magnetic nature of Bi 3 Ni system. The structure is found to be orthorhombic (Pnma) with lattice parameters a = 8.879Å b = 4.0998Å and c = 4.099Å. The title compound is synthesized via a solid state reaction route by quartz vacuum encapsulation of 5N purity stoichiometric ingredients of Ni and Bi. The superconducting transition temperature is found to be 4.1 K as confirmed from magnetization and specific heat measurements. The lower critical field (H c1 ) and irreversibility field (H irr ) are around 150 and 3000Oe respectively at 2K. Upper critical field (Hc 2 ) as determined from in field (up to 4 Tesla) ac susceptibility is found to be around 2 Tesla at 2K. The normal state specific heat is fitted using Sommerfeld-Debye equation C(T) = γT + βT 3 +δT 5 and the parameters obtained are γ= 11.08mJ/mol-K 2 , β= 3.73mJ/mol-K 4 and δ= 0.0140mJ/mol-K 6 . The calculated electronic density of states (DOS) at Fermi level N(E F ) and Debye temperature Θ D are 4.697 states/eV per formula unit and 127.7K respectively. We also estimated the value of electron phonon coupling constant (λ) to be 1.23, which when substituted in MacMillan equation gives T c = 4.5K. Density functional (DFT) based calculations for experimentally determined lattice parameters show that Ni in this compound is non-magnetic and ferromagnetic interactions seem to play no role. The Stoner condition I*N(E F ) = 0.136 per Ni atom also indicates that system cannot have any ferromagnetism. The fixed spin moment (FSM) calculations by fixing total magnetic moment on the unit cell also suggested that this system does not exhibit any signatures of ferromagnetism. Further it is concluded that ferromagnetic interactions play no role in superconductivity of Bi 3 Ni. This is in contrast to a recent report [14] related to possibility of coexistence of superconductivity and magnetism in Bi 3 Ni. Our results will surely attract more researchers to work on superconductivity of this and similar Ni containing compounds. determined from Reitveld analysis of the studied Bi 3 Ni is given in Fig. 1(b). Results and DiscussionThe DC and AC magnetic susceptibility plots of studied Bi 3 Ni are shown in Fig. 2 and 3 respectively. Namely, Fig. 2(a) depicts the DC magnetic susceptibility (χ) in both zero-fieldcooled (FC) and field-cooled (FC) situations in temperature range of 2K to 10K. The applied field is 10Oe. Superconductivity is observed at 4.1K with a sharp diamagnetic transition in magnetic susceptibility (χ) in both ZFC and FC situations. The superconducting volume fraction seems to be around 87.6% as calculated from FC (χ). This is slightly higher than as reported in ref.13.Though an estimated value is given, still we believe estimating superconducting volume 4 fraction without exactly knowing the pinning properties is not correct. What one can safely conclude from Fig. 2(a) is that the studied Bi 3 Ni is a bulk superconductor with superconducting transition temperature (T c ) at 4.1K. The AC susceptibility of Bi 3 Ni i...

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, 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(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 be satisfied. Force minimization done for FeTe x Se 1-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 E F 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.

No abstract

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

We scrutinize the enhanced superconducting performance of melt quench Bismuth based Bi 2 Sr 2 CaCu 2 O 8 (Bi-2212) superconductor. The superconducting properties of melt quenched Bi-2212 (Bi2212-MQ) sample are compared with non-melted Bi2212-NM and Bi 1.4 Pb 0.6 Sr 2 Ca 2 Cu 3 O 10 (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 (T c ) and upper critical field (H c2 ) 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 (J c ) 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. 2

The bulk polycrystalline sample FeSe 1/2 Te 1/2 is synthesized by solid state reaction route in an evacuated sealed quartz tube at 750 o C. The presence of superconductivity is confirmed through magnetization/thermoelectric/resistivity studies. It is found that the superconducting transition temperature (T c ) is around 12 K. Heat capacity (C p ) of superconducting FeSe 1-x Te x exhibited a hump near T c , 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 T c 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.

The results of phase transformation studies carried out in four homologues of 4-n-alkyl-4'cyanobiphenyls, using positron annihilation spectroscopy, are presented. The homologues investigated are, hexyl-, heptyl-, octyl- and decyl-cyanobiphenyls (6CB, 7CB, 8CB and 10CB). In these compounds, the positron lifetime measurements were performed as functions of temperature. The positron annihilation parameters are found to exhibit strong dependence on temperature. It was found that the ortho-positronium pick-off lifetime shows changes which strongly support (i) a gradual disappearance, instead of an abrupt one, of some memory of the more ordered solid phases on passing to the liquid crystalline phases, (ii) the strong tendency for the molecules in the mesophases to undergo anti-parallel bimolecular association and (iii) the formation of cyabotactic groups of a smectic phase in a nematic medium. Changes were also observed in the ortho-positronium formation probability which apparently indicate a systematic transformation of the solid phase from a close-pack structure to an open-pack structure as one goes from a lower to a higher homologue of the compounds investigated. Solid crystalline polymorphism has been observed in 8CB. A change in molecular packing in the solid phase of 10CB has been observed.

No abstract