Three- to Two-Dimensional Transition of the Electronic Structure in ${\mathrm{CaFe}}_2{\mathrm{As}}_2$: A Parent Compound for an Iron Arsenic High-Temperature Superconductor

We use angle-resolved photoemission spectroscopy (ARPES) to study the electronic properties of ${\mathrm{CaFe}}_2{\mathrm{As}}_2$—parent compound of a pnictide superconductor. We find that the structural and magnetic transition is accompanied by a three- to two-dimensional (3D-2D) crossover in the electronic structure. Above the transition temperature ($T_s$) Fermi surfaces around $\Gamma$ and $X$ points are cylindrical and quasi 2D. Below $T_s$, the $\Gamma$ pocket forms a 3D ellipsoid, while the $X$ pocket remains quasi 2D. This finding strongly suggests that low dimensionality plays an important role in understanding the superconducting mechanism in pnictides.

Figure 1

ARPES data of ${\mathrm{CaFe}}_2{\mathrm{As}}_2$ low temperature orthorhombic phase ($T=12\mathrm{K}$) for a few photon energies. (a)–(d) ARPES intensity integrated within 10 meV about $\mu$ for $h\nu =35$, 40, 50, and 60 eV, respectively. Bright areas mark the location of the Fermi surfaces. (e)–(f) Band dispersion data along the $\Gamma –X$ direction for $h\nu =40$ and 50 eV. $\alpha$, $\beta$, $\gamma$ correspond to different bands that cross $\mu$. (g)–(h) Energy distribution curves (EDCs) for data in panels (e)–(f) over the same $k$ range.

Figure 2

$k_z$ dispersion data for ${\mathrm{CaFe}}_2{\mathrm{As}}_2$ low temperature orthorhombic phase ($T=40\mathrm{K}$). (a) $k_z$ dispersion data obtained by plotting ARPES intensity integrated within 10 meV about $\mu$ as a function of $k_{(110)}$ and the energy of the incident photons (which corresponds to different values of $k_z$). The photon energy range used was from 35 to 105 eV. (b) Fermi momentum $k_F$ was extracted from the data in panel (a) using the peak positions of the MDCs. (c)–(e) Band dispersion data along the $\Gamma \mathrm{\text{−}}X$ direction for $k_z=18$, 16, and $14\pi$ at $k_{(110)}=0$ (corresponding to photon energies of $h\nu =80$, 58 and 41 eV, respectively). The locations of these cuts are marked by green (gray) arrows in panel (a).

Figure 3

$k_z$ dispersion data for ${\mathrm{CaFe}}_2{\mathrm{As}}_2$ in the high-temperature tetragonal phase ($T=200\mathrm{K}$). (a) FS maps for $h\nu =100\mathrm{eV}$. (b) Brillouin zone structure in the $k_x\mathrm{\text{−}}{k}_y$ plane for the measured range. (c) Same as (b) but in the $k_x\mathrm{\text{−}}{k}_z$ ($\Gamma \mathrm{\text{−}}Z$) plane. (d) $k_z$ dispersion data parallel to $\Gamma \mathrm{\text{−}}M$ [marked by magenta (gray) arrow in panel (a)]. The corresponding photon energy range was 80 to 190 eV. (e) Fermi crossing momenta $k_{F}s$ extracted from the data in panel (d). (f)–(i) Band dispersion data along a direction parallel to $\Gamma \mathrm{\text{−}}M$ for $k_z=18$, 19, 20, and $21\pi$ at $k_x=0$ (corresponding to incident photon energies of $h\nu =80$, 89, 99, and 110 eV). The data was divided by the resolution convoluted Fermi function to reveal the dispersion in the vicinity of $\mu$.

Figure 4

Direct comparison of band dispersions for low and high-temperature phase of ${\mathrm{CaFe}}_2{\mathrm{As}}_2$. Each panel shows the EDCs in vicinity of the $\Gamma$ point (black solid curve) along $k_{(110)}$ direction. High-temperature data was divided by the resolution convoluted Fermi function to reveal dispersion in the vicinity of $\mu$. Insets show the corresponding plot of band dispersion. Data was obtained at temperatures and momenta indicated in the panels.