Effects of Transition Metal Substitutions on the Incommensurability and Spin Fluctuations inBaFe2As2by Elastic and Inelastic Neutron Scattering

Abstract

The spin fluctuation spectra from nonsuperconducting Cu-substituted, and superconducting Co-substituted, BaFe2As2 are compared quantitatively by inelastic neutron scattering measurements and are found to be indistinguishable. Whereas diffraction studies show the appearance of incommensurate spin-density wave order in Co and Ni substituted samples, the magnetic phase diagram for Cu substitution does not display incommensurate order, demonstrating that simple electron counting based on rigid-band concepts is invalid. These results, supported by theoretical calculations, suggest that substitutional impurity effects in the Fe plane play a significant role in controlling magnetism and the appearance of superconductivity, with Cu distinguished by enhanced impurity scattering and split-band behavior. PACS numbers: 74.70.Xa, 75.30.Fv, 75.30.Kz The role of chemical substitution and its effects on structure, magnetism and superconductivity have become central issues in studies of the iron-pnictide superconductors. [1][2][3][4] This is particularly true for transition-metal (M ) substitution on Fe sites, resulting, nominally, in electron doping of the FeAs layers. When low concentrations of Co, [5,6] Ni, [7,8] Rh,[9, 10] Pt [11] and Pd [9,10] replace Fe, the structural transition temperature (T S ) and the antiferromagnetic (AFM) transition temperature (T N ) are both suppressed to lower values and split with T S > T N . [5-7, 9, 12-14] When the structural and magnetic transitions are suppressed to sufficiently low temperatures, superconductivity emerges below T c and coexists with antiferromagnetism over some range of concentration. Moreover, for Co, Rh and Ni substitutions in BaFe 2 As 2 , neutron diffraction measurements manifest a distinct suppression of the magnetic order parameter in the superconducting regime (T < T c ), which clearly indicates competition between AFM order and superconductivity. [13][14][15][16][17] Cu substitution in BaFe 2 As 2 , in contrast, suppresses the magnetic and structural transitions, but does not support superconductivity [2,8] except, perhaps, below 2 K over a very narrow range in composition.[18] This dichotomy between Co and Ni substitutions and that of Cu is also realized in quaternary fluoroarsenides.[19] However, for Co/Cu co-substitutions in BaFe 2 As 2 , at a fixed non-superconducting Co concentration, the addition of Cu first promotes and then suppresses T c .[18] It has been suggested that previously neglected impurity effects play an important role in this behavior. [8,20] The effects of impurity scattering are also neglected in a simple rigid-band picture for M substitutions, which, at least for Co substitution in BaFe 2 As 2 , seems to adequately account for the evolution of angleresolved photoemission spectroscopy (ARPES) [21], Hall effect, and thermoelectric power (TEP) measurements with concentration.[22] The rigid-band model has also been used successfully to model the suppression of the AFM transition temperature and ordered moment in Ba(Fe 1−x Co x ) 2 As 2 for "u...

Doping evolution of antiferromagnetism and transport properties in nonsuperconductingBaFe2−2xNixCrxAs2

“…In particular, the short-ranged incommensurate AF ordered phase near optimal superconductivity is consistent with a cluster spin glass phase in the matrix of the superconducting phase 18 . Similar conclusions are also reached from muon spin relaxation (µSR) 19 and Nuclear magnetic resonance (NMR) measurements 20 on BaFe 2−x Co x As 2 family of materials 21,22 , while a separate NMR measurement on BaFe 2−x Ni x As 2 suggests the presence of a magnetic QCP at x= 0.10 and a structural QCP at x= 0.14, both associated with the non-Fermi-liquid behavior 23 . For iso-valently doped BaFe 2 (As 1−x P x ) 2 , the quantum critical behavior has been reported near optimal superconductivity around x = 0.3 from transport and superfluid density measurements [24][25][26][27] .…”

Spin polarization of Ru in superconducting Ba(Fe0.795Ru0.205)2As

“…For relatively small x, both the structural (tetragonal-to-orthorhombic) and the antiferromagnetic (AFM) transition temperatures (T S , T N ) are suppressed with T S > T N , and superconductivity emerges over a small compositional range as doping x increases. [1][2][3][4][8][9][10][11][12][13][14][15][16][17][18][19] In the case of Ba(Fe 1−x Cu x ) 2 As 2 , Cu appears to manifest strong impurity scattering effects 7,20 and superconductivity is not observed, although T S and T N are progressively suppressed. Nominal hole-doping through TM substitutions, including Cr 21 and Mn 22 also suppresses T N and T S , but superconductivity is not realized for any level of substitution.…”

Local breaking of fourfold rotational symmetry by short-range magnetic order in heavily overdoped Ba(Fe1−xCux)2As2Wang1, Song2, Hu3 et al. 2017Phys. Rev. B112101View full textAdd to dashboardCite

“…For BaFe 2−x T M x As 2 (T M = Co, Ni, Cu), while Co and Ni doping result in superconducting domes with optimal T c ∼ 20 K, optimal T c ∼ 2 K or no superconductivity is observed in AFe 2−x Cu x As 2 (A = Ba,Sr) [8][9][10]. This contrast points to the inadequacy of a simple rigid band picture [11][12][13] and highlights differences between dopants [14,15].…”

“…For BaFe 2−x T M x As 2 (T M = Co, Ni, Cu), while Co and Ni doping result in superconducting domes with optimal T c ∼ 20 K, optimal T c ∼ 2 K or no superconductivity is observed in AFe 2−x Cu x As 2 (A = Ba,Sr) [8][9][10]. This contrast points to the inadequacy of a simple rigid band picture [11][12][13] and highlights differences between dopants [14,15].Compared to AFe 2−x Cu x As 2 , superconductivity with optimal T c = 11.5 K is observed in NaFe 1−x Cu x As [16,17]. With increasing Cu concentration (x 10%), insulating-like transport and short-range magnetic order develop, evolving towards an insulator with long-range magnetic order and Fe-Cu ordering near x ≈ 50% [18].…”