Measurement of thec-Axis Optical Reflectance ofAFe2As2(A=Ba,

Abstract

We present the c-axis optical reflectance measurement on single crystals of BaFe2As2 and SrFe2As2, the parent compounds of FeAs based superconductors. Different from the ab-plane optical response where two distinct energy gaps were observed in the SDW state, only the smaller energy gap could be seen clearly for E c-axis. The very pronounced energy gap structure seen at a higher energy scale for E ab-plane is almost invisible. We propose a novel picture for the band structure evolution across the SDW transition and suggest different driving mechanisms for the formation of the two energy gaps.PACS numbers: 74.25. Gz, 74.70.Xa, 75.30.Fv For quasi-two dimensional layered materials, striking differences could exist in the in-plane and out-ofplane charge transport and dynamics. For example, in some high-T c cuprates, metallic in-plane charge transport coexists with nonmetallic conductivity along the caxis [1,2]. The contrasting behavior violates the conventional concept of band electron transport, and has been the subject of intensive study. Fe-pnictide superconducting materials also crystalize in the layered structure with Fe-As layers separated by alkaline metal ions or other insulator-like layers. Band structure calculations based on the local-density approximation (LDA) or generalized gradient approximations (GGA) indicate dominantly two-dimensional (2D) cylinder-like Fermi surfaces (FSs) along the c-axis [3][4][5]. It is important to see whether or not the Fe-pnictides share similar anisotropic charge dynamical properties with cuprates.Optical spectroscopy is a powerful technique to investigate charge dynamics and band structure of a material as it probes both free carriers and interband excitations. In particular, it yields direct information about the energy gap formation in the broken symmetry state. Optical spectroscopy studies on the ab-plane properties of different Fe-pnictides and chalcogenides systems have been reported by several groups. [6][7][8][9][10][11][12][13][14][15][16][17][18] For the parent compounds of Fe-pnictides, the measurements provide clear evidence for the formation of the partial energy gaps in the magnetic phase, supporting the itinerant picture that the energy gain for the antiferromagnetic ground state is achieved through the opening of a spin-density-wave (SDW) gap on the FSs. [7,10,[14][15][16] For the superconducting samples, the superconducting pairing gaps were also detected by the technique. [6,17,18] However, optical investigations have not been carefully done on the c-axis response of Fe-pnictide materials. There is only one work in the literature containing optical data along the c-axis.[10] Unfortunately, the data were limited to the high frequencies, above 700 cm −1 . Because of this limitation, neither the free-carrier response nor any feature related to the SDW gap were observed. In fact, the reported reflectance data appear to have extraordinarily low values. As information about the anisotropic charge dynamics is extremely important for understanding the materials,...

Three-dimensionality of band structure and a large residual quasiparticle population in Ba0.67K0.33<

“…The difference was explained naturally by assuming the existence of two types of FSs in the system: 2D cylinder like FSs and a large-size 3D ellipsoid like FS. 6 Here the data could be again well understood from the same electronic structures, as schematically shown in Fig. 5.…”

“…This observation suggests that the band structure of Fe-pnictide should be quite threedimensional (3D), in contrast to the expectation based on its layered crystal structure. An anisotropy ratio of optical conductivities at the low frequency limit is less than 4, which is close to the value of the undoped compound 6 , suggesting that the anisotropy does not show significant The c-axis R(ω) and σ1(ω) at 300 K for the Ba0.67K0.33Fe2As2 over a broad frequency up to 15000 cm −1 . The ab-plane spectra were also included for a purpose of comparison.…”

“…The c-axis polarized optical data are available only for parent compounds where the spin-density-wave gap structure was found to be substantially different from that of E ab-plane. 6 In this work we present the c-axis optical spectroscopy measurements on superconducting (Ba,K)Fe 2 As 2 crystals. We show that, in contrast to the high-T c cuprates, the c-axis Josephson plasmon is completely absent in the superconducting state.…”

Two-Dimensional Massless Dirac Fermions in Antiferromagnetic AFe2As2 Chen1, Wang2, Song3 et al. 2017Phys. Rev. Lett.276220View full textAdd to dashboardCite

“…Largely due to the strong (next) nearest neighbor exchange coupling [45], an ultrahigh magnetic field (B > 500 T) along the crystalline c axis is estimated to destroy the low-temperature collinear AFM order in PCIS [46]. The applied magnetic field (B ≤ 17.5 T) here is, therefore, too low to dramatically change the AFM order at T ∼ 4.5 K. In addition, the AFM-order-related spectral-weight transfer in the reflectance spectra of BaFe 2 As 2 takes place in the energy range up to about 250 meV [23][24][25][26][27], which is much wider than that observed in Fig. 1(b) (∼55 meV).…”

“…After the AFM phase transition, the paramagnetic Brillouin zone of PCIS would be folded along the AFM wave vector connecting the hole-type Fermi surfaces (FSs) surrounding the Γ point and the electron-type FSs at the M point, which leads to the intersection between the electron and hole bands and results in the band gap opening at the band crossing points near Fermi energy (E F ) [23][24][25][26][27]. However, theoretical studies suggest that for PCIS, some band crossing points are topologically protected by (i) the collinear AFM order, (ii) inversion symmetry, and (iii) a combination of timereversal and spin-reversal symmetry [19,20].…”