Charge dynamics of Co-doped BaFe<sub>2</sub>As<sub>2</sub>

Lucarelli, Andrea; Dusza, A.; Pfuner, F.; Lerch, Philippe; Analytis, James; Chu, J.-H.; Fisher, I. R.; Degiorgi, L.

doi:10.1088/1367-2630/12/7/073036

Cited by 85 publications

(106 citation statements)

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“…The formation of incomplete gap in σ(ω) below T N has been observed and discussed for both Sr122 and Ba122 by many authors. [17][18][19]24,25 Note that, as previously discussed, the Drude response remains even at 8 K. From these results, it has been concluded that a gapping of Fermi surface occurs at certain portions of the Brillouin Zone due to the SDW formation, and that the electronic subsystem responsible for the narrow Drude response is distinct from that for the SDW gap opening. 18,25 At P =2.5 GPa, in Fig.…”

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“…1 In particular, the optical conductivity [σ(ω)] technique has provided much information. [17][18][19][20][21][22][23][24][25][26][27] As for the stoichiometric 122 compounds, a clear depletion of σ(ω) below ∼ 1200 cm −1 was observed below T N . [17][18][19] This was attributed to a gapping of the Fermi surface at certain portions of the Brillouin Zone, which was caused by spindensity-wave (SDW) formation.…”

mentioning

confidence: 99%

“…With chemical doping into the 122 compounds, the SDW gap in σ(ω) was progressively suppressed, and in the SC phase, a SC gap was clearly observed in σ(ω). [21][22][23][24][25][26][27] In this work, σ(ω) of Sr122 has been measured under external pressure (P ) to 6 GPa and at temperature (T ) to 8 K in order to study the pressure evolution of microscopic electronic structures. The advantage of σ(ω) technique is fully taken here since a pressure study is technically difficult for other spectroscopic techniques such as photoemission and tunneling.…”

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confidence: 99%

“…In previous works with chemically doped samples, measurements were made to much lower frequencies, generally down to 50 cm −1 or lower. 21,24,25,27 In the previous optical studies of 122 compounds, Drude-Lorentz spectral fitting 29,30 was used to analyze the σ(ω) spectra. [17][18][19]24,25 Results of similar fitting to our high pressure data at 8 K are indicated by the broken curves in Fig.…”

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Pressure Suppression of Spin-Density-Wave Gap in the Optical Conductivity of SrFe₂As₂

et al. 2013

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Optical reflectance R(ω) of a pressure-induced superconductor SrFe2As2 has been measured under external pressure to 6 GPa and at temperatures to 8 K. Optical conductivity σ(ω) has been derived from the measured R(ω). At ambient pressure, in the antiferromagnetic state below TN=198 K, a pronounced feature develops in σ(ω) due to the opening of a spin density wave (SDW) gap, as already reported in the literature. With increasing pressure, the SDW gap feature in σ(ω) is progressively suppressed. At 4 GPa, where the sample is superconducting, the SDW gap feature in σ(ω) is strongly reduced than that at ambient pressure, but is still clearly observed. At 6 GPa, the SDW gap is completely suppressed. The pressure evolutions of the SDW gap magnitude and the spectral weight closely follow the pressure evolution of TN. PACS numbers:Regarding the superconductivity (SC) exhibited by Fe-based compounds, 1 the pressure-induced SC exhibited by the stoichiometric "122" compounds AFe 2 As 2 (A=Ba, Ca, Sr, Eu) have attracted much interest. 2-13 They exhibit SC with external pressure only, although many other families of Fe based superconductors require some form of chemical doping or deviation from stoichiometry to exhibit SC. 1 In particular, SrFe 2 As 2 (Sr122) and BaFe 2 As 2 (Ba122) show pressure-induced SC at relatively high temperatures of T c =34 K 5,7-9 and 28 K, 4,7,9,11 , respectively. Interestingly, the 122 compounds also show SC with chemical doping, as shown by (Sr, K)Fe 2 As 2 , 14 Sr(Fe,Co) 2 As 2 , 15 and SrFe 2 (As, P) 2 . 16 At ambient pressure, the stoichiometric 122 compounds are antiferromagnetic (AFM), poor metals. For Sr122 and Ba122, the AFM transition temperature (T N ) is 198 K and 136 K, respectively. At T N , the resistivity [ρ(T )] shows a kink and rapid decrease with cooling below T N . With increasing pressure, T N is gradually lowered, and the kink in ρ(T ) becomes broadened. Above a critical pressure, the SC appears. Similar suppression of AFM and emergence of SC have also been observed with chemical doping. [14][15][16] In this regard, both methods have provided a lot of insight, but one advantage of the pressure technique compared with chemical doping is that the former does not cause disorder in the crystal lattice.However, there is an additional complication regarding the pressure-induced SC in the 122 compounds. Namely, it has been shown that the pressure evolutions of the AFM and SC states strongly depend on the hydrostaticity. 8,11,12 Here, hydrostaticity refers to how isotropic the pressure acting on the sample is. The hydrostaticity in a high pressure experiment depends on the types of pressure cell and pressure transmitting medium used. Figure 1 summarizes the results of high pressure studies on Sr122. 4,5,7-9 As discussed in detail later, these results show that the AFM state is suppressed and SC appears at lower pressure when the applied pressure is less hydrostatic (more uniaxial). This may indicate that a uniaxial stress promotes SC in the 122 compounds.To probe the microscopic elect...

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Pressure Suppression of Spin-Density-Wave Gap in the Optical Conductivity of SrFe₂As₂

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“…Consequently, a number of experimental studies have been performed on the optical properties of the iron pnictides, in the normal state, the SDW state, as well as in the SC state. 5,11,[29][30][31][32][33]58 The SDW state is characterized by (i) the appearance of a peak in the optical conductivity at the SDW gap frequency, (ii) an anisotropic dc response and (iii) an almost isotropic response in the infrared and optical region of the spectrum. On the other hand, while several theoretical works based on local density approximation (LDA) 28 and LDA+DMFT 24,34 have been done on the paramagnetic phase, there is still a lack of DFT calculations for the optical conductivity in the SDW state.…”

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Analysis of spin-density wave conductivity spectra of iron pnictides in the framework of density functional theory

et al. 2010

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The optical conductivity of LaFeAsO, BaFe2As2, SrFe2As2, and EuFe2As2 in the spin-density wave (SDW) state is investigated within density functional theory (DFT) in the framework of spinpolarized generalized gradient approximation (GGA) and GGA+U. We find a strong dependence of the optical features on the Fe magnetic moments. In order to recover the small Fe magnetic moments observed experimentally, GGA+U eff with a suitable choice of negative on-site interaction U eff = U − J was considered. Such an approach may be justified in terms of an overscreening which induces a relatively small U compared to the Hund's rule coupling J, as well as a strong Holstein-like electron-phonon interaction. Moreover, reminiscent of the fact that GGA+U eff with a positive U eff is a simple approximation for reproducing a gap with correct amplitude in correlated insulators, a negative U eff can also be understood as a way to suppress magnetism and mimic the effects of quantum fluctuations ignored in DFT calculations. With these considerations, the resulting optical spectra reproduce the SDW gap and a number of experimentally observed features related to the antiferromagnetic order. We find electronic contributions to excitations that so far have been attributed to purely phononic modes. Also, an orbital resolved analysis of the optical conductivity reveals significant contributions from all Fe 3d orbitals. Finally, we observe that there is an important renormalization of kinetic energy in these SDW metals, implying that the effects of correlations cannot be neglected.

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Weak and Strong Correlations in Fe Superconductors

Medici

2014

Iron-Based Superconductivity

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In this chapter the strength of electronic correlations in the normal phase of Fesuperconductors is discussed. It will be shown that the agreement between a wealth of experiments and DFT+DMFT or similar approaches supports a scenario in which strongly-correlated and weakly-correlated electrons coexist in the conduction bands of these materials. I will then reverse-engineer the realistic calculations and justify this scenario in terms of simpler behaviors easily interpreted through model results. All pieces come together to show that Hund's coupling, besides being responsible for the electronic correlations even in absence of a strong Coulomb repulsion is also the origin of a subtle emergent behavior: orbital decoupling. Indeed Hund's exchange decouples the charge excitations in the different Iron orbitals involved in the conduction bands thus causing an independent tuning of the degree of electronic correlation in each one of them. The latter becomes sensitive almost only to the offset of the orbital population from half-filling, where a Mott insulating state is invariably realized at these interaction strengths. Depending on the difference in orbital population a different 'Mottness' affects each orbital, and thus reflects in the conduction bands and in the Fermi surfaces depending on the orbital content. arXiv:1506.01678v1 [cond-mat.supr-con]

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Charge dynamics of Co-doped BaFe₂As₂

Cited by 85 publications

References 24 publications

Pressure Suppression of Spin-Density-Wave Gap in the Optical Conductivity of SrFe₂As₂

Pressure Suppression of Spin-Density-Wave Gap in the Optical Conductivity of SrFe₂As₂

Analysis of spin-density wave conductivity spectra of iron pnictides in the framework of density functional theory

Weak and Strong Correlations in Fe Superconductors

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Charge dynamics of Co-doped BaFe2As2

Cited by 85 publications

References 24 publications

Pressure Suppression of Spin-Density-Wave Gap in the Optical Conductivity of SrFe2As2

Pressure Suppression of Spin-Density-Wave Gap in the Optical Conductivity of SrFe2As2

Analysis of spin-density wave conductivity spectra of iron pnictides in the framework of density functional theory

Weak and Strong Correlations in Fe Superconductors

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Charge dynamics of Co-doped BaFe₂As₂

Pressure Suppression of Spin-Density-Wave Gap in the Optical Conductivity of SrFe₂As₂

Pressure Suppression of Spin-Density-Wave Gap in the Optical Conductivity of SrFe₂As₂