Dramatic changes in the electronic structure upon transition to the collapsed tetragonal phase in CaFe${}_2$As${}_2$

We use angle-resolved photoemission spectroscopy and density functional theory calculations to study the electronic structure of CaFe${}_2$As${}_2$ in the collapsed tetragonal (CT) phase. This unusual phase of iron arsenic high-temperature superconductors was hard to measure as it exists only under pressure. By inducing internal strain, via the postgrowth thermal treatment of single crystals, we were able to stabilize the CT phase at ambient pressure. We find significant differences in the Fermi surface topology and band dispersion data from the more common orthorhombic-antiferromagnetic or tetragonal-paramagnetic phases, consistent with electronic structure calculations. The top of the hole bands sinks below the Fermi level, which destroys the nesting present in parent phases. The absence of nesting in this phase, along with an apparent loss of Fe magnetic moment, are now clearly experimentally correlated with the lack of superconductivity in this phase.

Figure 1

(a) Schematic phase diagram for the CaFe${}_2$As${}_2$ system based on data in Ref. [29]. Blue, green, and red circles mark the annealing condition of samples used in this study. The hatched area marks the boundary between two distinct phases. (b)–(d) Band dispersion along cuts No. 1 and No. 2 for the three samples measured at $T=15$ K and $k_z=15.3\pi /c$.

Figure 2

The band structure of CaFe${}_2$As${}_2$ in three different phases measured at $k_z=13.5\pi /c$. (a) Fermi surface map. (b) Band dispersion along cut No. 1 [marked in (a)]. (c) Band dispersion along cut No. 2 measured at 190 K in the tetragonal-paramagnetic phase for a sample that was postgrowth annealed at 400 ${}^{\circ }$C. (d)–(f) The same plots for the same sample measured at 15 K in the orthorhombic-antiferromagnetic phase. The arrow in (d) points to small pockets associated with AFM order, while arrows in (e) and (f) point to band splitting caused by AFM order. (g)–(i) The same plots for a sample that was postgrowth annealed at 960 ${}^{\circ }$C measured at 110 K again in the tetragonal-paramagnetic phase. (k)–(m) The same plots for the “960C” sample, but measured at $T=15$ K in the CT phase. (n) EDCs at the corner of the zone, $k=(1,1)$. The location of the peak marks the energy of the bottom of the electron pocket. (o) MDCs at the center of the zone (the bottom two curves) and the corner of the zone (the top two curves).

Figure 3

Results of the DFT calculation of the band structure in CT at $k_z=15.3\pi /c$ corresponding to ARPES data shown in Fig. 1. (a) The left column shows band dispersion at the zone center, and the right column corresponds to the zone corner. (b) Calculated band dispersion along the $k_z$ direction. Dashed lines mark the $k_z$ values of data in Figs. 1 and 2. Colors denote orbital contributions.

Figure 4

FS maps of the CaFe${}_2$As${}_2$ “960C” sample measured with (a) 35, (b) 45, (c) 49, and (d) 62 eV photon energies at $T_{\mathrm{S}}\approx 15$ K. (e) Intensity plots close to the center of the zone along cut No. 1 for photon energies from 30 to 80 eV. (f) EDCs at the center of the zone for photon energies from 30 to 80 eV, plotted every 1 eV. Thick (black) lines mark photon energies of 45 and 62 eV used in Figs. 1 and 2.