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

Abstract

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...

Spin-density-wave-induced anomalies in the optical conductivity ofAFe2As2, (Charnukha1, Pröpper2, Larkin3 et al. 2013Phys. Rev. B2717531View full textAdd to dashboardCite

“…15,29,40 Our own ellipsometric measurements on the Sr-and Ba-based compounds, shown in Fig. 2(b),2(e) and 2(c),2(f), fully reproduce and confirm previously reported data, [15][16][17][18]29,31,40 provide additional information due to the independently obtained real and imaginary parts of the optical conductivity, and allow for a systematic investigation of the electronic properties of the ThCr 2 Si 2 -type iron-based materials as a function of the intercalating atom.…”

“…Below the spin-densitywave transition temperature of 150 K this itinerant response experiences a dramatic suppression due to the opening of an energy gap in the quasiparticle excitation spectrum, as previously observed in this and other 122-type iron-based materials. [15][16][17][18] Due to the conservation of the total number of electrons, the missing area under the conductivity curve at low frequences is transferred to higher energies, which results in a formation of a "hump" structure. 54,55 In the most common case when the spin-density-wave energy gap does not cover the entire Fermi surface of a material and, therefore, the optical conductivity remains non-zero at all frequencies, determination of the optical quasiparticle excitation gap 2∆ from the conductivity spectra is difficult but it can be approximated by the energy, at which the low-temperature conductivity spectrum crosses the one at the Néel temperature for the first time.…”

“…2 Investigations of the optical conductivity of this and other compounds have been instrumental in obtaining the value of the energy gap and characterising the energetics of the spin-density-wave order. [15][16][17][18] Although the growth of CaFe 2 As 2 (Ref. 19) and its phase diagram with respect to cobalt doping 20 were first reported several years ago, the interest in this material started to build up upon the observation of its surprising sensitivity not only to doping and pressure but also to annealing.…”

Optical study of BaFe2As2 under pressure: Coexistence of spin-density-wave gap and superconductivity

“…The pressure-induced SC phase in the 122 class of pnictides was mostly studied by transport probes [10][11][12][13][14][15][16][17] : Pressure causes the gradual suppression of the SDW state and the emergence of superconductivity in the parent compounds, whereas for the doped iron pnictides pressure increases T c [18][19][20] . In contrast, very limited work on the iron pnictides has been done with infrared spectroscopy under pressure 21,22 , focusing mainly on the SDW state. None of these studies was able to demonstrate the existence of the SC state.…”

Piston pressure cell for low-temperature infrared investigations

“…6,7 To reach high pressures, the tips of the diamonds are usually only several 100 µm in diameter, while the diamonds themselves may be several millimeters thick. There are only a small number of reports on successful infrared investigations utilizing a diamond anvil cell combined with a conventional infrared spectrometer [8][9][10][11][12][13][14][15] because the extremely weak light intensity makes experiments really challenging and-with very few exceptions-possible only at room temperature. Due to the higher brilliance of the radiation and better focus of the beam, synchrotron light sources are the method of choice when diamond anvil cells are required to reach very high pressure.…”