Bulk electronic structure of optimally dopedBa(Fe1−xCox)2

Abstract

We report high-resolution, bulk Compton scattering measurements unveiling the Fermi surface of an optimally doped iron-arsenide superconductor, Ba͑Fe 0.93 Co 0.07 ͒ 2 As 2 . Our measurements are in agreement with first-principles calculations of the electronic structure, revealing both the X-centered electron pockets and the ⌫-centered hole pockets. Moreover, our data are consistent with the strong three dimensionality of one of these sheets that has been predicted by electronic structure calculations at the local-density-approximation-minimum As position. Complementary calculations of the noninteracting susceptibility, 0 ͑q , ͒, suggest that the broad peak that develops due to interband Fermi-surface nesting, and which has motivated several theories of superconductivity in this class of material, survives the measured three dimensionality of the Fermi surface in this family.The discovery of superconductivity in LaO 1−x F x FeAs, a member of the so-called "1111" family of iron-pnictide materials, 1 has triggered intense research into its nature and origin. Tempted by the proximity of the superconductivity in these compounds to an antiferromagnetic spin-density-wave ͑SDW͒ state, many theoretical models have focused on spin fluctuations as the key, 2,3 supported by first-principles calculations of the electronic structure. Such calculations predict four Fermi-surface ͑FS͒ sheets: two hole sheets at the center of the Brillouin zone ͑BZ͒ and two electron sheets centered at its corner. 4,5 In the undoped ͑and lightly doped͒ "1111" compounds the hole and electron sheets are well nested, leading to a broad peak in the noninteracting susceptibility. 2 The spin fluctuations that are thought to develop from this FS instability have already been observed in neutronscattering measurements, 6 and theoretical models suggest that they promote a strong pairing interaction, provided that the order parameter changes phase between the hole and electron FS sheets ͑the so-called s Ϯ model͒. 2,3 Central to such models, however, is a detailed understanding of the topology of the FS; here we present a bulk measurement of the FS of an optimally doped iron-arsenide superconductor, Ba͑Fe 0.93 Co 0.07 ͒ 2 As 2 , alongside complementary calculations of the noninteracting susceptibility, 0 ͑q , ͒. We demonstrate the data are most consistent with a three-dimensional ͑3D͒ FS but that the nesting that is required for the s Ϯ model survives, lending its availability to all iron-pnictide compounds.Ba͑Fe 1−x Co x ͒ 2 As 2 is a member of the "122" family of iron-pnictide superconductors, with a maximum T c ϳ 24 K for x ϳ 0.06. 7,8 Unlike the "1111" family, whose FS is quasitwo-dimensional ͑2D͒, the "122" compounds are predicted by electronic structure calculations to have substantial 3D warping of one of the hole sheets, 5 the degree of which is sensitive to the position of the As with respect to the Fe plane. Experimental studies have not yet adequately resolved whether the electronic structure is better described by the experimental As position or...

The puzzle of high temperature superconductivity in layered iron pnictides and chalcogenides

“…41, 140 which shows a peak at Q AF , which is at the corner M point of the primitive tetragonal Brillouin zone in the bottom panel of Fig. 14 above. A similar enhancement of χ(Q) at the M point was found for LaFeAsO in more refined calculations by Monni et al 264 Utfeld et al have calculated that the noninteracting χ(Q) for optimally doped Ba(Fe 0.93 Co 0.07 ) 2 As 2 still has a pronounced peak near the nesting wave vector, in spite of significant 3D dispersion of one of the hole Fermi pockets, 158 and Yaresko et al also found peaks in the noninteracting χ(Q) for both doped and undoped LaFeAsO 1−x F x and (Ba 1−x K x )Fe 2 As 2 , 266 thus supporting the s ± supercon-ducting pairing model (see below). An early unbiased numerical investigation of a two orbital model gave robust evidence for the stability of the stripe state in the undoped iron arsenides.…”

“…158 These bulk data indicate substantial dispersion of one or both hole Fermi surfaces along k z that are in agreement with LDA band calculations in the virtual crystal approximation with the optimized As position, with the best agreement obtained after rigidly shifting the electron and hole bands in energy in opposite directions. 158 …”

Directional point-contact Andreev-reflection spectroscopy of Fe-based superconductors: Fermi surface topology, gap symmetry, and electron–boson interaction

“…In this concern, it is worth mentioning that also the relative weight of the two terms in the conductance (let's say, w 1 and (1 − w 1 )) is the same irrespective of the direction of the current. This seems to indicate that all the FS sheets have a similar degree of three-dimensionality, as also shown by ARPES [61] and x-ray Compton scattering [62]. Thick lines in figure 9(c) represent the two-gap 2D BTK fit of the curves, while thin lines indicate the single-gap 2D BTK fit.…”

Probing the Fermi surface by positron annihilation and Compton scattering

“…This careful comparison further showed that a rigid upward shift of the Fermi energy by 11mRy ($0.15 eV) was necessary to bring the theoretical calculation for LaRu 2 Si 2 into agreement with experiment. This kind of shift (albeit for individual bands, rather than a global shift) was found to be necessary recently in Fermi surface measurements in the Fe-pnictide superconductors on the basis of quantum oscillations 70 and Compton scattering, 71 and has also been found to be necessary to accurately reproduce Fermi surfaces in other materials. 72,73 As Monge et al pointed out, however, if such shifts are necessary even without the complexities introduced by f electrons, it does call into question the assertion, based on such LDA calculations and dHvA data, 69,74 that the f electrons are itinerant in the heavy fermion state.…”