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.

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