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.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
customersupport@researchsolutions.com
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
This site is protected by reCAPTCHA and the Google Privacy Policy and Terms of Service apply.
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.