The occurrence of bulk superconductivity at ~22 K is reported in polycrystalline samples 1.INTRODUCTIONOver the last two years much progress has been made in establishing superconductivity unambiguously in MFe 2 As 2 (M=Ba,Sr,Ca,Eu) systems [1][2][3][4][5][6]. The pristine sample that has a Spin Density Wave (SDW) ground state is nudged into a superconducting (SC) state by electron/ hole doping and application of pressure [2,7,8,9,10]. Band structure calculations point to the fact that SDW state arises on account of the special 2D geometry of Fermi surface that is unstable to nesting [11,12]. Also associated with or preceding the magnetic transition is a tetragonal to orthorhombic structural transition, which is suppressed in the superconducting state. The strong interplay of structure, magnetism and electronic structure have been investigated recently in the Co substituted BaFe 2-x Co x As 2 system [13].The temperature composition phase diagrams determined for the different chemical substitutions at different sites in BaFe 2 As 2 [14,15,16] show a generic behaviour as a function of the concentration of the substituent, viz., a systematic suppression of the SDW transition, followed by co-existence of SDW and SC and the occurrence of a superconducting dome. Several transition metal (TM) substitutions with electrons in excess of Fe forming, BaFe 2-x TM x As 2 have been studied but the maximum T C has remained at ~25 K [17]. A much higher T C of 38 K and ~35 K, were however observed by optimal hole doping in the Ba 1-x K x Fe 2 As 2 [2] system and in BaFe 2 As 2 by application of high pressure [10]. A systematic investigation on the role of hydrostaticity, in the pressure dependent resistivity study of BaFe 2 As 2 , has revealed that uniaxial pressure favours the occurrence of high T C at 36 K whereas a lower T C of 29 K occurs under truly hydrostatic pressure [18]. Consistent with this finding are results that indicate that strained crystals of BaFe 2 As 2 and SrFe 2 As 2 display superconductivity at ambient pressure [19]. A compilation of structural data from several compounds of the related ReOFeAs (Re=rare-earth) superconducting family, indicates that T C is optimized at a particular Fe-As distance [20] and/or at a particular Fe-As tetrahedral angle [21], indicating that the local structure of the FeAs 4 tetrahedra plays a crucial role in determining T C . Devising schemes to effect structural distortions by chemical substitution that would lead to higher T C in the BaFe 2 As 2 system will be useful.Thus motivated, we examine the effect of Ru substitution at the Fe site in BaFe 2 As 2 . At the outset, it is clear that Ru is isoelectronic to Fe and being larger in size should 3 introduce steric effects, affecting the Fe-As bond length leading to distortions of the FeAs 4 tetrahedral motifs. In addition, the larger radius of the 4d electron shell should increase the metal-metal overlap in the Fe/Ru layer and increase the hybridization of metal atom with As leading to significant alterations in the electr...

Phonons and magnetic and ferroelectric ordering in La-substituted (Bi1−xLax)0.5Pb0.5Fe0.5Ti0.5O3 for samples with 0.0 ≤ x ≤ 0.5 are investigated using Raman, magnetization, and polarization measurements as a function of temperature. The system is tetragonal for pure Bi0.5Pb0.5Fe0.5Ti0.5O3 with a large c/a ratio. The anisotropy is reduced when Bi is partially replaced by La (0 ≤ x ≤ 0.5), and it turns cubic for x ≥ 0.4. All the properties are found to change significantly with changes in the c/a ratio. Evidence of spin-glass-like magnetic ordering at low temperature is found in the case of x = 0.2. A mechanism for the systematic change of magnetic ordering temperature as a function of doping is also discussed. The phonon frequencies and line widths exhibit discontinuous changes across the tetragonal-cubic transition. Large polarization and forbidden Raman scattering in the cubic phase are explained on the basis of symmetry breaking due to the formation of a polar nano region, which leads to relaxor behavior.

We are aware of the metallization of G-type antiferromagnetic BaMn 2 As 2 by light doping of K at the Ba site 1 and the possibility of pressure-induced metallicity at 16 GPa in the related correlated insulator LaMnPO. 2 It has also been seen recently that metallization can also be achieved in insulating LaMnAsO by chemical substitution of Sr at the La site, for a Sr fraction x 0.08. 3 We regret that these papers were not cited in our own.

The upper critical field was measured upto 12 T for three BaFe 2−x Co x As 2 single crystals with estimated Co concentrations of x = 0.082, x = 0.117 and x = 0.143. H C2 versus temperature was measured from temperature dependent resistivity, for various applied magnetic fields, H || ab and H || c. The [dH C2 /dT] T=Tc , normalized with the corresponding T C , decreases with increasing Co content, for both directions. The anisotropy γ defined as H C2 || ab / H C2 || c shows a distinct increase with Co content, and its temperature dependence shows a peak close to the T C . Magneto transport measurements, in the spin density wave regions, showed significant negative MR for H || ab and positive MR of H || c in the x = 0.082 sample. The implications of these results are discussed.

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