Optical investigations of the normal and superconducting states reveal two electronic subsystems in iron pnictides

Wu, Dan; Barišić, N.; Kallina, P.; Faridian, Ahmad; Gorshunov, B.; Drichko, Natalia; Li, L. J.; Lin, Xin; Cao, Guanghan; Xu, Z. A.; Wang, N. L.; Dressel, Martin

doi:10.1103/physrevb.81.100512

Cited by 144 publications

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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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“…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. 3.…”

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“…This reduction of reflectance below T N is due to a formation of SDW gap, and has been analyzed and discussed by published works, including similar results on Ba122. [17][18][19] With increasing P , in Figs. 2(b)-2(d), the reduction of reflectance becomes smaller, and it is absent at P =6 GPa.…”

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confidence: 99%

“…[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. At low temperatures, a narrow Drude peak due to free carriers was observed even after the SDW gap was formed in σ(ω).…”

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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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“…3(a). As in most iron-pnictides, the optical response of CaKFe 4 As 4 below 8 000 cm −1 can be described by a narrow Drude (a coherent response), a broad Drude (an incoherent response), and a Lorentz term (interband transition) [25,26], reflecting the multiband nature of FeSCs. With this fitting, we extrapolate the optical conductivity to the zero fre- quency and find that the extrapolation agree well with the resistivity (Fig.…”

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Anomalous phonon behavior in superconducting CaKFe4As4 : An optical study

Yang

Dai

et al. 2017

Phys. Rev. B

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The temperature dependence of ab-plane optical conductivity of CaKFe4As4 has been measured below and above its superconducting transition temperature Tc ≃ 35.5 K. In the normal state, analysis with the two-Drude model reveals a T -linear scattering rate for the coherent response, which suggests strong spin-fluctuation scattering. Below the superconducting transition, the optical conductivity below 120 cm −1 vanishes, indicating nodeless gap(s). The Mattis-Bardeen fitting in the superconducting state gives two gaps of ∆1 ≃ 9 meV and ∆2 ≃ 14 meV, in good agreement with recent Angle-resolved photoemission spectroscopy (ARPES) results. In addition, around 255 cm −1 , we observe two different infrared-active Fe-As modes with obvious asymmetric lineshape, originating from strong coupling between lattice vibrations and spin or charge excitations. Considering a moderate Hund's rule coupling determined from spectral weight analysis, we propose that the strong fluctuations induced by the coupling between itinerant carriers and local moments may affect the phonon mode, and the electron-phonon coupling through the spin channel is likely to play an important role in the unconventional pairing in iron-based superconductors.

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Optical properties of superconducting EuFe₂ (As_1‐xP_x)₂

Neubauer

Pronin

Zapf

et al. 2016

Physica Status Solidi (b)

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We present a broadband optical-conductivity study of superconducting single-crystalline EuFe2(As1-xPx)2 with three different substitutional levels. We analyze the normal-state electrodynamics by decomposing the conductivity spectra using a Drude-Lorentz model with two Drude terms representing two groups of carriers with different scattering rates. The analysis reveals that the scattering rate of at least one of the Drude components develops linearly with temperature for each doping level. This points towards strong electron-electron correlations and a non-Fermi-liquid behavior in the P-substituted superconducting Eu-122 pnictides. We also detect a transfer of the spectral weight from mid-infrared to higher frequencies and assign it to the Hund's-rule coupling between itinerant and localized carriers. The conductivity spectra below the superconducting transition show no sharp features to be associated with the dirty-limit superconducting BCS gaps. We interpret these results in terms of clean-limit superconductivity in EuFe2(As1-xPx)2. The best parametrization fit can be achieved using a two-gap model. We find that the larger gap at the hole pockets of the Fermi surface is likely to be isotropic, while the smaller gap at the electron pockets is anisotropic or even nodal.Comment: to be published in Physica Status Solidi B - Special Issue on Iron Pnictide

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Optical investigations of the normal and superconducting states reveal two electronic subsystems in iron pnictides

Cited by 144 publications

References 32 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

Anomalous phonon behavior in superconducting CaKFe4As4 : An optical study

Optical properties of superconducting EuFe₂ (As_1‐xP_x)₂

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Optical investigations of the normal and superconducting states reveal two electronic subsystems in iron pnictides

Cited by 144 publications

References 32 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

Anomalous phonon behavior in superconducting CaKFe4As4 : An optical study

Optical properties of superconducting EuFe2 (As1‐xPx)2

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Product

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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

Optical properties of superconducting EuFe₂ (As_1‐xP_x)₂