Dissimilarities between the electronic structure of chemically doped and chemically pressurized iron pnictides from an angle-resolved photoemission spectroscopy study

We have studied the electronic structure of EuFe${}_2$As${}_{2-x}$P${}_x$ using high-resolution angle-resolved photoemission spectroscopy. Upon substituting As with the isovalent P, which leads to chemical pressure and to superconductivity, we observe a nonrigid-band-like change of the electronic structure along the center of the Brillouin zone (BZ) in the form of an orbital and $k_z$-dependent increase or decrease in the size of the hole pockets near the $\Gamma -Z$ line. The diameter of the Fermi surface cylinders at the BZ corner which form electron pockets, increases at $K$ and changes in a nonmonotonous way at $X$. This is in stark contrast to $\mathrm{p-}$ and $n$-type doped iron pnictides where, on the basis of ARPES experiments, a more rigid-band-like behavior has been proposed. These findings indicate that there are different ways in which the nesting conditions can be reduced causing the destabilization of the antiferromagnetic order and the appearance of the superconducting dome.

Figure 1

DC susceptibility data of EuFe${}_2$As${}_{1.68}$P${}_{0.32}$. We show measurements under the conditions of zero field cooling (ZFC) and field cooling (FC) for an applied field of 50 Oe parallel to the $ab$ plane. The inset shows the ac susceptibility.

Figure 2

ARPES data of EuFe${}_2$As${}_{1.56}$P${}_{0.44}$ taken with a photon energy $h\nu$ $=$ 93 eV. (a) Fermi surface map along $\Gamma -X$ symmetry line measured with $s$-polarized light (indicated by a double arrow). Solid white lines indicate the directions of the cuts shown in (b), (c), and (f). (b) and (c) Energy distribution maps near $X$ and $\Gamma$, respectively. Solid red lines connect the maxima obtained by least squares fits to momentum distribution curves (MDC). Energy distribution curve fit results are shown in the immediate vicinity of the high-symmetry points. (d) and (e) Constant energy contours 50 meV below $E_F$ for two different polarizations in the $k$ range indicated by the white dashed rectangle in (a). Dashed circles are guides to the eye. (f) MDCs near $\Gamma$ at $E_F$ for s and $p$ polarization.

Figure 3

ARPES Fermi surface maps of EuFe${}_2$As${}_{1.56}$P${}_{0.44}$ in the $k_{\left[001\right]}-k_{[-110]}$ plane. Actually, the photon-energy dependence of the intensity is shown. (a) and (b) Data near the $\Gamma -Z$ line taken with $s$- and $p$-polarized light, respectively. (c) Data near the corner of the BZ measured with $s$-polarized light. The symbols indicate the band dispersion having different orbital character: open (closed) circles predominantly $xy$ ($xz/yz$) character, closed squares predominantly $z^2$ character.

Figure 4

ARPES energy distribution maps near the high-symmetry points $\Gamma$, $X$, $Z$, and $K$ of the parent compound EuFe${}_2$As${}_2$ taken in the paramagnetic state at $T=220$ K. The data near $Z$ and $\Gamma$ were recorded using both $s$- and $p$-polarized photons. The data near $K$ and $X$ were obtained using $s$-polarized photons.

Figure 5

ARPES of EuFe${}_2$As${}_{1.72}$P${}_{0.28}$. (a) A Fermi surface map for $k_z=0$. The $k_z$ dispersion at the zone center is given in (b). (c) and (d) Energy distribution maps taken near the $\Gamma$ and $X$ points, respectively.

Figure 6

ARPES of EuFe${}_2$As${}_{1.68}$P${}_{0.32}$. (a) A Fermi surface map for $k_z=1$. The white lines indicate the directions of the measurements shown in (b)–(e). (b) The $k_z$ dispersion at the zone center. (c), (d), and (e) EDMs taken near the $Z$, $\Gamma$, and $K$ points, respectively. The data were taken with circularly polarized photons.

Figure 7

Momentum distribution curves at the Fermi level integrated over 10 meV near the high-symmetry points $\Gamma$, $Z$, $X$, and $K$ for EuFe${}_2$As${}_{2-x}$P${}_x$. The P concentration $x$ increasing in all cases from bottom to top are given in the individual panels. The data taken with $s$, $p$, and circularly polarized photons are labeled with (s), (p), and (c), respectively. Dots (red): measured data; solid (blue) line: result from a least squares fit. Maxima derived from these fits are marked by symbols which are the same as described in the legend of Fig. 3, indicating the orbital character of the bands.

Figure 8

Fermi vectors plotted as a function of doping/substitution concentrations at $Z$, $\Gamma$, $K$, and $X$. (a) Isovalently substituted (chemically pressurized) system EuFe${}_2$As${}_{2-x}$P${}_x$. (b) Aliovalently substituted (electron doped) BaFe${}_{2-x}$Co${}_x$As${}_2$. The assignment of the symbols to the orbital character is the same as described in the legend of Fig. 3. Half-filled circles indicate $xy$ and $xz/yz$ character. The solid lines are least-squares fits assuming a linear dependence of $k_F$ on $x$ (see text).