Optical spectroscopy of single-crystalline LaFeAsO

Millimeter-sized single crystals of LaFeAsO were grown from NaAs flux and the in-plane optical properties were studied over a wide frequency range. A sizable electronic correlation effect was indicated from the analysis of the free-carrier spectral weight. With decreasing temperature from 300 K, we observed a continuous suppression of the spectral weight near 0.6 eV. But a spin-density-wave gap formation at lower energy scale was seen only in the broken-symmetry state. We elaborate that both the itinerancy and local-spin interactions of $\text{Fe}3d$ electrons are present for the FeAs-based systems; however, the establishment of the long-range magnetic order at low temperature has a dominantly itinerant origin.

Figure 1

(Color online) The dc resistivity and magnetic susceptibility vs temperature for LaFeAsO single crystal. Two anomalies could be identified at the dash lines positions. Inset: the derivative of resistivity vs temperature. A crystal structure for LaFeAsO is also drawn.

Figure 2

(Color online) The room-temperature (a) optical reflectance and (b) conductivity for LaFeAsO single crystal over broad frequencies up to $25000{\text{cm}}^{-1}$. An analysis of the conductivity spectrum by the Drude-Lorentz model is displayed as the dashed curve.

Figure 3

(Color online) The $ab$-plane reflectance spectra for LaFeAsO below $6000{\text{cm}}^{-1}$ at several different temperatures. Inset: the reflectance spectra in the low-frequency region.

Figure 4

(Color online) The $ab$-plane conductivity spectra for LaFeAsO below $6000{\text{cm}}^{-1}$ at several different temperatures. Inset: an expanded plot of the conductivity spectra in the low-frequency region. The two arrows indicate two phonon peaks. The dc conductivity values at respective temperatures are also shown.