Observation of Dirac Cone Electronic Dispersion in ${\mathrm{BaFe}}_2{\mathrm{As}}_2$

We performed an angle-resolved photoemission spectroscopy study of ${\mathrm{BaFe}}_2{\mathrm{As}}_2$, which is the parent compound of the so-called 122 phase of the iron-pnictide high-temperature superconductors. We reveal the existence of a Dirac cone in the electronic structure of this material below the spin-density-wave temperature, which is responsible for small spots of high photoemission intensity at the Fermi level. Our analysis suggests that the cone is slightly anisotropic and its apex is located very near the Fermi level, leading to tiny Fermi surface pockets. The bands forming the cone show an anisotropic leading edge gap away from the cone that suggests a nodal spin-density-wave description.

Figure 1

(a) FS mapping (25 K) obtained by integrating the photoemission intensity in a 20 meV window centered at $E_F$. The FS is described in terms of the unreconstructed BZ. The double arrow indicates light polarization. (b) Corresponding second derivative intensity plot. ${\tilde{Q}}_{\mathrm{SDW}}$ is the in-plane projection of the SDW wave vector. (c),(d) ARPES intensity plot and corresponding second momentum derivative intensity plot, respectively, along the $\Gamma (0,0)\mathrm{\text{−}}M(\pi ,0)$ symmetry line. The dotted lines are guides for the eye. The solid lines in (c) correspond to our LDA calculated bands renormalized by a factor of 3. The vertical dashed lines in (b)–(d) indicate the 2D low-$T$ BZ. (e) EDCs around $\Gamma$. The arrow indicates the bottom of an $e$-like dispersion that hybridizes with the $\alpha$ and ${\beta }_1$ bands. (f)–(j) $T$ evolution of the FS around $M$. The angle $\theta$ is defined in (j).

Figure 2

ARPES intensity plots (25 K) for cuts going trough $\Lambda (075\pi ,0)$ are displayed in the top row and indexed after the $\theta$ angle defined in Fig. 1j. The corresponding 2EIPs and 2MIPs are shown in the second and third rows, respectively. The direction of the straight arrow in Fig. 1j defines the momentum increase, which is towards $M$ for $\theta =0$.

Figure 3

(a) ARPES intensity spectra of ${\mathrm{BaFe}}_2{\mathrm{As}}_2$ at the $\Lambda$ point ($\theta \sim 90°$) recorded at 100 K, after division by the Fermi-Dirac function convoluted with the instrumental resolution function. (b) Corresponding 2MIP. Long dashed lines are guides for the eye. The corresponding EDCs are given in  (c), where the bold EDC refers to the $\Lambda$ point. (d) Polar representation of $v_F$ around the $\Lambda$ point (25 K). Open circles and closed circles represent data measured and data obtained by reflection with respect to the $\Gamma \mathrm{\text{−}}M$ symmetry line, respectively. The large filled circle represents the average value of $v_F$ while the thick line is a fit of the data to the two-ellipse model described in the text, with parameters $a$, $b$, and $c$. (e) Contour plot of the electronic dispersion below $E_F$ around the $\Lambda$ point, as calculated from our model. The small filled circle represents the FS associated with the cone.

Figure 4

(a) 3D representation of the Dirac cone at $\Lambda$. The color scale indicates the distance from $\Lambda$. (b) Schematic FS around the $M$ point. The folded $\alpha$ band, which is barely touching $E_F$, is not indicated. Shaded areas indicate gapped regions and the dashed arrow indicates the orientation of the ARPES cut associated with (d) and (e). (c) MGL of EDCs (25 K) as a function of the angle $\omega$ defined in (b). The inset shows the LEG as a function of $\omega$ after a 3.5 meV shift (see the text). (d) Symmetrized EDCs (25 K) along the cut indicated in (b). (e) 2EIP (25 K) along the cut indicated in (b). The vertical dashed line indicates the $M$ point.