Surface-driven electronic structure in LaFeAsO studied by angle-resolved photoemission spectroscopy

We measured the electronic structure of an iron arsenic parent compound LaFeAsO using angle-resolved photoemission spectroscopy (ARPES). By comparing with a full-potential linear augmented plane wave calculation we show that the extra large $\Gamma$ hole pocket measured via ARPES comes from electronic structure at the sample surface. Based on this we discuss the strong-polarization dependence of the band structure and a temperature-dependent holelike band around the $M$ point. The two phenomena give additional evidences for the existence of the surface-driven electronic structure.

Figure 1

(Color online) (a) ARPES Fermi-surface maps of LaFeAsO, integrated within $\pm 10\text{meV}$ with respect to the chemical potential. Bright areas indicate high intensity. Data are taken with a polarized monochromatic synchrotron beam at $T=10\text{K}$. Incident photon energies are indicated at the top of each panel. Inset of the 45 eV panel shows the labeling of the high-symmetry points. Note that $M_1$ and $M_2$ represent two zone corners showing different band structures due to different polarization arrangements. (b) Top: $k_z$ dispersion maps of LaFeAsO taken with incident photon energies $30<h\nu <70\text{eV}$ for the $\Gamma \text{−}{M}_1$ (left) and $\Gamma \text{−}\Gamma$ (right) directions, respectively. Locations of each Fermi crossing band are indicated by their symbols and orange arrows. Red arrows show the locations of the four maps in Fig. 1a. Bottom: peak extraction of the corresponding momentum distribution curves(MDCs) at the top panel. Different colors represent different bands.

Figure 2

(Color online) Results of the full-potential linear augmented plane wave calculation for the surface-driven electronic state of NdFeAsO. The results of LaFeAsO will be essentially the same (see discussion in the text). (a) Calculational setup of the crystal lattice. The red, yellow, lilac, and purple spheres represent the Fe, As, Nd, and O atoms, respectively. Labels such as Fe1 and Nd2 indicate atoms at different locations. The 12 a.u. distance between Nd2 and Fe4 layers is added manually to imitate the existence of the crystal surface. (b) Calculation result of the three-dimensional Fermi surface for the crystal in (a). (c) Fermi-surface sheets generated from different atomic layers.

Figure 3

(Color online) Band-structure analysis for the 45 eV data in Fig. 1. (a) Measured band structure for two diagonal cuts $\Gamma \text{−}{M}_1$ and $\Gamma \text{−}{M}_2$. Strong polarization dependence on the electronic structure in both the $\Gamma$ and $M$ pockets are clearly visible. (b) Constant energy maps for binding energies $E_b=0$, 30, and 60 meV in the vicinity of $M_1$ and $M_2$ points. Binding energies are indicated to the top of each column. Cuts #1 and #2 indicate the locations of the band-structure maps in panel (d). (c) Schematics of the experimental setup. Note that the electric-field vector of the incoming light is polarized along the $k_{\left(1,-1,0\right)}$ direction. The entrance slit of the electron analyzer is along the mirror plane.

Figure 4

(Color online) (a) Notations of the Fermi pockets and bands of LaFeAsO measured by ARPES. The existence of these pockets/bands is proved by the data from the synchrotron (Figs. 1, 3). The three hole pockets around $\Gamma$ are labeled as ${\alpha }_1$, ${\alpha }_2$, and ${\alpha }_3$; ${\beta }_{\text{e}1}$ $\left({\beta }_{\text{e}2}\right)$ is the $M$ electron pocket with the long axis along (perpendicular to) the $\Gamma \text{−}M$ direction; ${\beta }_{\text{h}}$ represents the intensity from a holelike band located right below the chemical potential $\mu$ at the vicinity of $M$. (b) ARPES intensity map at $\mu$ measured with a helium lamp $\left(h\nu =21.2\text{eV}\right)$, along with locations of the ARPES band structure cuts shown in Figs. 5, 6. The direction of these cuts is defined as $k_n$. Arrows in the cuts indicate the $k_n$ ranges in the corresponding panels in Figs. 5, 6.

Figure 5

(Color online) Band-structure analysis for the $\Gamma$ hole pockets. Locations of the cuts #1, #2, and #3 are indicated in Fig. 4b. Top row: MDCs integrated within $\pm 10\text{meV}$ with respect to the chemical potential $\mu$ for the three cuts. $\mu$ is determined by fitting a Fermi function to the spectra of polycrystalline gold. The MDCs are fitted with several Lorenzians for extracting the peak positions which are used to determine the Fermi crossing momenta $\left(k_F\text{s}\right)$. Middle row: band-dispersion maps obtained by ARPES measurements at $T=12\text{K}$. The incident photon energy is 21.2 eV. Bottom row: EDCs for the corresponding maps in the middle row. Each EDC is integrated within a momentum range of $0.011\pi /a$ and is symmetrized with respect to $\mu$. Insets show an expended region at the vicinity of the $k_F\text{s}$.

Figure 6

(Color online) Band-structure analysis for the $M$ hole band ${\beta }_{\text{h}}$ at three different temperatures. Location of the cut #4 is indicated in Fig. 4b. Top row: band-dispersion maps obtained by ARPES measurements at $T=12$, 120, and 200 K. The incident photon energy is 21.2 eV. Vertical dashed lines indicate the location of the high-symmetry point $M$. Bottom row: EDCs at the $M$ point for the three temperatures. Data are integrated within a momentum range of $0.011\pi /a$, symmetrized with respect to $\mu$, and normalized for high binding energies.