content_image/382702.jpg content_image/382713.jpg content_image/382704.jpg
<img src='content_image/382702.jpg'>

leads to a narrow residual Drude component at very low frequencies (see the inset of Fig. 2 (a) and (b)). Above T$_{SDW}$ , the Drude feature is not clear for E ‖ c-axis, being different from the spectra in the ab-plane. The data pro- vide clear evidence for the partial energy gap formation along the c-axis in the SDW ordered phase as well. In the ab-plane optical spectra,[7] we observed two energy gaps in the SDW state with the conductivity peak ener- gies near 360 cm − $^{1}$and 890 cm − $^{1}$. Here we found that the energy scale of the gap for E ‖ c is close to the smaller one seen in the ab-plane σ 1 ( ω ). The more pronounced gap structure at higher energy in the ab-plane becomes extremely weak for the c-axis polarization in both R( ω ) and σ 1 ( ω ), as indicated by an upward grey arrow in the figure. 

It is tempting to analyze the data in a more quanti- tative way. Since a residual narrow Drude component is present at very low temperature in the SDW state, it is relatively easy to isolate its spectral weight. We use a standard Drude-Lorentz model to decompose the spectra into different components: 

<img src='content_image/382713.jpg'>

Here, ϵ ∞ is the dielectric constant at high energy, the middle and last terms are the Drude and Lorentz com- ponents, respectively. The experimental R( ω ) and σ 1 ( ω ) spectra at 10 K could be well reproduced by using a Drude and several several Lorentz peaks, as shown in Fig. 3[22]. This analysis leads to ω$_{pc}$ =3600 cm − $^{1}$and 1/ τ$_{c}$ =47 cm − $^{1}$for the Drude component at 10 K. Our earlier study on the ab-plane properties indicates that those two parameters are 4660 cm − $^{1}$and 55 cm − $^{1}$at 10 K, respectively.[7] Then we find that the anisotropy ratio 

of the spectral weight, which is equal to the ratio of the square of plasma frequency ω 2 $_{pc}$/ ω 2 $_{pab}$, is about 60% at 10 K. The value indicates that the anisotropy of the band structure reconstructed in the SDW order state is not very strong. Since the Drude-like features above T$_{SDW}$ are not very clear, there is no unique or unambiguous way to isolate the Drude components from the overall spectra, we did not try to estimate those parameters at high temperatures. 

The substantial difference of low frequency gap struc- tures in the SDW state between the two polarizations has important implication for the electronic structure of Fe- pnictides. It is important to check whether the observa- tions are generic for 122-type parent compounds. For this purpose, we performed optical measurement on SrFe 2 As 2 single crystals. Because the obtained SrFe 2 As 2 crystals have smaller dimension along the c-axis, the signal-to- noise ratio at low frequencies is much smaller. Neverthe- less, we observed essentially the same spectral features for SrFe 2 As 2 as well. As shown in Fig. 4, only the smaller gap could be seen for E ‖ c-axis in the SDW state, the large energy gap is almost invisible. Those experiments indicate that the observed spectral features are generic for 122-type undoped compounds. 

Understanding why two distinct gaps could be ob- served for E ‖ ab-plane but only a smaller one could be clearly seen for E ‖ c-axis is a crucial issue here. It has 

<img src='content_image/382704.jpg'>

