Unprecedented anisotropic metallic state in undoped iron arsenide BaFe2As2 revealed by optical spectroscopy

Abstract

An ordered phase showing remarkable electronic anisotropy in proximity to the superconducting phase is now a hot issue in the field of high-transition-temperature superconductivity. As in the case of copper oxides, superconductivity in iron arsenides competes or coexists with such an ordered phase. Undoped and underdoped iron arsenides have a magnetostructural ordered phase exhibiting stripe-like antiferromagnetic spin order accompanied by an orthorhombic lattice distortion; both the spin order and lattice distortion break the tetragonal symmetry of crystals of these compounds. In this ordered state, anisotropy of in-plane electrical resistivity is anomalous and difficult to attribute simply to the spin order and/or the lattice distortion. Here, we present the anisotropic optical spectra measured on detwinned BaFe(2)As(2) crystals with light polarization parallel to the Fe planes. Pronounced anisotropy is observed in the spectra, persisting up to an unexpectedly high photon energy of about 2 eV. Such anisotropy arises from an anisotropic energy gap opening below and slightly above the onset of the order. Detailed analysis of the optical spectra reveals an unprecedented electronic state in the ordered phase.

Fig. 1.

Temperature dependence of the in-plane resistivity of detwinned BaFe₂As₂, ρ_a (blue) and ρ_b (red). (A) For the as-grown crystal, anisotropy starts at approximately 220 K and persists down to the lowest measurement temperature. (B) Annealing decreases residual resistivity and increases magnetostructural transition temperature T_s (at the peak in ρ_b). (C) Annealing markedly decreases the overall anisotropy, both the magnitude and temperature ranges.

Fig. 2.

Temperature evolution of the optical conductivity spectrum of detwinned BaFe₂As₂ for polarization parallel to the a and b axes. The crystal is twin-free as long as compressive pressure is applied in one direction. (Inset) The crystalline a and b axes, as well as the spin alignment with respect to the pressure direction, are shown. Rapid suppression of low-energy conductivity is seen in both spectra below 140 K, just below T_s = 143 K. Spectra at temperatures well above T_s show weak T dependence showing no incipient gap feature. A precursory gap is observed only for the b-axis spectrum at T = 150 K. Singular features (a spike at 257 cm^-1 and a cusp at 334 cm^-1) appear only in σ_b(ω).

Fig. 3.

Comparison between the a- and b-axis optical conductivity spectra at 5 K above 50 cm^-1. Below 1,350 cm^-1, σ_b is lower than σ_a, whereas above 1,350 cm^-1 the anisotropy is reversed. The anisotropy persists up to an energy as high as 17,000 cm^-1 (approximately 2 eV). Note that the anisotropy decreases and approaches 1 toward ω = 0.

Fig. 4.

Decomposition of the low-energy conductivity spectrum below 1,500 cm^-1. (A) The spectrum at 150 K in the tetragonal-paramagnetic phase is decomposed into a broad Drude component and a narrow Drude component, both contributing to the dc conductivity. (B and C) In the orthorhombic-stripe AF phase (T = 5 K), the width of the narrow Drude component in A becomes extremely narrow, and the peak height reaches 30,000 Ω^-1 cm^-1 in both spectra. A gap opens in the broad Drude component (in gray), but the gap in σ_b is deeper than that in σ_a. In the gap region a small Drude component (yellow) and an absorption band (red) remain in σ_a, whereas in σ_b only a singular band (red) remains and a spike appears.