Evidence of Mott physics in iron pnictides from x-ray spectroscopy

Abstract

X-ray emission and absorption spectroscopies are jointly used as fast probes to determine the evolution as a function of doping of the fluctuating local magnetic moments in K-and Cr-holedoped BaFe 2 As 2 . An increase in the local moment with hole-doping is found, supporting the theoretical scenario in which a Mott insulating state that would be realized for half-filled conduction bands has an influence throughout the phase diagram of these iron-pnictides.A scenario has recently been proposed in the field of Fe-based superconductors, where the low energy physics is described in terms of a coexistence of strongly and weakly correlated electrons [1]. Such a picture provides a framework to understand a wealth of experiments on quasiparticle mass enhancements (specific heat, low-frequency optical conductivity, angleresolved photoemission spectroscopy, quantum oscillations etc.) in electron-and hole-doped A direct method to probe the local magnetic moment in transition metal compounds is Kβ coreto-core (CTC) x-ray emission spectroscopy (XES) that is sensitive to the local 3d spin because of the intra-atomic 3p-3d exchange interaction [5,6] The unoccupied density of states around Fe can be probed by means of K edge x-ray absorption spectroscopy (XAS) providing information that is complementary to Kβ spectroscopy. We have recorded high-energy resolution fluorescence detected XAS on all K-and Cr-doped samples.All samples were plate-like single crystals grown by the self-flux technique as detailed elsewhere (111) monochromator crystals. The inelastically scattered photons were analyzed using a set of five spherically bent Ge(620) crystals that were arranged with the sample and detector (avalanche photodiode) in a vertical Rowland geometry (R = 1 m). The surface of the samples was aligned to 45º with respect to the incident beam direction, i.e. the angle between the electric field of the linearly polarized x-rays and the sample [001] direction. Non-resonant Fe Kβ XES measurements were recorded at incident energy 7200 eV. High-energy resolution fluorescence-detected x-ray absorption near edge structure (HERFD-XANES) spectra were obtained by setting the emission energy to the maximum of the Kβ 1,3 line while scanning the incoming energy through the absorption edge. The resulting spectrum is an approximation to the 1s photoabsorption cross section with sharper spectral features than in standard XAS [5,6]. The total experimental broadening, determined as full width at half maximum of the elastic profiles, was 1.3 eV. All the measurements were performed at room temperature. Both XES and HERFD-XANES spectra have been normalized in area using the ranges 7025-7080 eV and 7105-7190 eV respectively.Non-resonant Fe Kβ CTC XES spectra have been measured as a function of doping in Ba 1-x K x Fe 2 As 2 (0 ≤ x ≤ 1) and Ba(Fe 1-x Cr x ) 2 As 2 (0.026 ≤ x ≤ 0.47). Figure 1( [20] where J is the exchange integral between the electrons in the 3p and 3d shells. It becomes visible in the difference spectra of Figure 1 and is about 10-...

Orbital Selectivity in Electron Correlations and Superconducting Pairing of Iron-Based Superconductors

“…Spectroscopy measurements have provided ample evidence that, for the parent compounds of the FeSCs, the incoherent part of the electron spectral weight (1 − w) is larger than the coherent counterpart w, which provides a microscopic definition of a bad metal. The full force of the electron correlations in the FeSCs has now become quite apparent [13,[81][82][83][84][85][86][87][88][89][90][91][92][93][94][95][96][97][98]. In leading toward this understanding, the orbital-selective aspects of the correlations and pairing have played an important role.…”

Anisotropic magnetic excitations and incipient Néel order in Ba(Fe1−xMnx)2As2

“…It is known that the composition versus temperature (x vs T ) phase diagram of the transition metal (T M ) substituted Ba(Fe 1−x T M x ) 2 As 2 systems presents an unexpected asymmetry, which concerns the fact that SC is not observed for Cr and Mn substituted samples, that are on the hole-doped side of the phase diagram [6]. The unique aspect related to Mn, or Cr substitution, is the presence of strongly localized magnetic moments at the dopant site [7][8][9], contrasting with the moments observed for Co and K substituted phases, which still present a high degree of itinerancy [10]. Indeed, magnetic moments within the FeAs layers seem to be detrimental to SC, which in other cases has been shown to persist when magnetism is formed between these planes [11,12].…”

Spin-wave excitations in the spin-density wave state of doped iron pnictides

“…This is expected to suppress the density of states at the Fermi level and the electron movement in the lattice leading to the departure from metallicity. 36,37 Despite that the FSs exist for the all the dopings considered here because of a small ratio U /W ≈ 0.25 with W as the electron bandwidth, and consequently reconstructed bandstructure plays a crucial role in the SDW excitations. For instance, the particle-hole continuum in the bare-spin susceptibility shifts towards low energy near M, which should play a very important role in the damping and disappearance of the spin-wave excitations along X-M in the hole-doped region.…”