Decoupling of lattice and orbital degrees of freedom in an iron-pnictide superconductor

The interplay between structural and electronic phases in iron-based superconductors is a central theme in the search for the superconducting pairing mechanism. While electronic nematicity is competing with superconductivity, the effect of purely structural orthorhombic order is unexplored. Here, using x-ray diffraction and angle-resolved photoemission spectroscopy, we reveal a structural orthorhombic phase in the electron-doped iron-pnictide superconductor ${\mathrm{Pr}}_4{\mathrm{Fe}}_2{\mathrm{As}}_2{\mathrm{Te}}_{0.88}{\mathrm{O}}_4$ ($T_{\mathrm{c}}=25$ K), which is distinct from orthorhombicity in the nematic phase in underdoped pnictides. Despite the high electron doping we find an exceptionally high orthorhombic onset temperature ($T_{\mathrm{ort}}\sim 250$ K), no signatures of phase competition with superconductivity, and absence of electronic nematic order as the driving mechanism for orthorhombicity. Combined, our results establish a high-temperature phase in the phase diagram of iron-pnictide superconductors and impose strong constraints for the modeling of their superconducting pairing mechanism.

Figure 1

High-temperature orthorhombicity in ${\mathrm{Pr}}_4{\mathrm{Fe}}_2{\mathrm{As}}_2{\mathrm{Te}}_{0.88}{\mathrm{O}}_4$ (PFATO). (a) Schematics for structural ${\mathrm{C}}_4$ symmetry breaking at $T<T_{\mathrm{ort}}$ in iron pnictides. (b) Schematics of orbital splitting in the nematic phase of underdoped iron pnictides. Below $T_{\mathrm{nem}}$, the $d_{xz}$ and $d_{yz}$ are no longer degenerate and split in energy, with largest splitting at the M points of the Brillouin zone [17]. Here, $E_{\mathrm{xz}}$ ($E_{\mathrm{yz}}$) denote the maximum (minimum) of the holelike (electronlike) bands at ${\mathrm{M}}_{\mathrm{y}}$. (c) Room-temperature unit cell of PFATO [19]. (d)–(e) High-resolution transverse scans through the (1, 1, 0) Bragg reflection (see inset) at temperatures as indicated. A splitting of the diffraction peak is observed below $T_{\mathrm{ort}}\sim 250$ K. (f) Magnetic susceptibility at 1 mT cooled in zero field. (g) Temperature dependence of the orthorhombic order parameter $\delta$. Lower inset: $\delta$ across $T_{\mathrm{c}}$ measured in finer temperature steps. Upper inset: Resistivity vs temperature, displaying a saturation above the onset of orthorhombicity.

Figure 2

Electronic band structure of ${\mathrm{Pr}}_4{\mathrm{Fe}}_2{\mathrm{As}}_2{\mathrm{Te}}_{0.88}{\mathrm{O}}_4$. (a) Symmetrized Fermi surface map recorded on PFATO around zone corners and displayed in false color scale with color saturation as indicated (maximum intensity is normalized to 1). The Fermi surface contours of the two electron pockets are shown schematically, with solid lines in the top right zone corner. The two underlying Fermi pockets amount to a combined filling of $\sim 0.11\pm 0.01$ electrons per Fe [23]. (b)–(d) Low-energy electronic band structure along cut 0 (${\mathrm{\Gamma }}_0-{\mathrm{M}}_0$) measured with circular, $\sigma$, and $\pi$ polarized light ($h\nu =80$ eV). Overlaid lines are shifted and renormalized DFT calculated bands. All bands are visible with circular polarized light, while linear polarization probes bands depending on their orbital character, (see Supplemental Material, Fig. S1). All spectra were recorded at 5.5 K. (e) DFT-calculated Fermi surface and (f) band structure of undoped ${\mathrm{Pr}}_4{\mathrm{Fe}}_2{\mathrm{As}}_2{\mathrm{TeO}}_4$ in which the two electron pockets around M compensate the three hole pockets around $\mathrm{\Gamma }$.

Figure 3

Absence of electronic nematicity in ${\mathrm{Pr}}_4{\mathrm{Fe}}_2{\mathrm{As}}_2{\mathrm{Te}}_{0.88}{\mathrm{O}}_4$ (PFATO). (a) Schematics of observed band dispersion in PFATO. (b) Schematics of band dispersion expected for the electronic nematic phase with a finite nematic order parameter (energy splitting between $d_{xz}$ and $d_{yz}$ states) of ${\delta }_{\mathrm{nem}}\sim 20$ meV. (c)–(h) Diagonal cuts through ${\mathrm{M}}_0$ and ${\mathrm{M}}_1$, defined in panel (o), measured at 5.5 K with photon energy and polarization as indicated, plotted in false-color scale with color saturation as indicated (intensity normalized to 1). (e), (f), (h) Curvature plots of the raw spectra (c), (d), and (g) calculated as described in Ref. [30] (minimum intensity renormalized to –1). All expected bands with $d_{xy}$, $d_{xz}$, and $d_{yz}$ orbital character (blue, red, and green lines) are identified. (i)–(n) Similar spectra as (c)–(h) but for cuts through ${\mathrm{\Gamma }}_0$ and ${\mathrm{\Gamma }}_1$, as indicated in (o). Spectra in (j) and (l) are symmetrized around ${\mathrm{\Gamma }}_1$. (o) Sketch of the Brillouin zone and electronlike Fermi surface sheets indicated in gray colors and cut 0 and cut 1 indicated in orange and blue. Horizontal dashed lines in (a)–(n) indicate the Fermi level $E_{\mathrm{F}}$.

Figure 4

Comparison between ${\mathrm{Pr}}_4{\mathrm{Fe}}_2{\mathrm{As}}_2{\mathrm{Te}}_{0.88}{\mathrm{O}}_4$ and $\mathrm{Ba}{({\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x)}_2{\mathrm{As}}_2$. The nematic and spin-density wave (SDW) phases in underdoped ${\mathrm{Ba}\left({\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x\right)}_2{\mathrm{As}}_2$ follow a similar doping dependence and are intersecting the superconducting dome close to optimal doping at $\sim 0.06$ electrons per Fe. At roughly twice the doping ($x=0.12$), ${\mathrm{Pr}}_4{\mathrm{Fe}}_2{\mathrm{As}}_2{\mathrm{Te}}_{0.88}{\mathrm{O}}_4$ hosts a high-temperature onset of orthorhombicity at $T_{\mathrm{ort}}\sim 250$ K [100 K above $T_{\mathrm{nem}}$ of ${\mathrm{BaFe}}_2{\mathrm{As}}_2$], and a superconducting transition at $T_c\sim 25$ K [similar to optimally doped $\mathrm{Ba}({{\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x)}_2{\mathrm{As}}_2$]. Inset: Orthorhombic order parameter versus temperature for $\mathrm{Ba}{\left({\mathrm{Fe}}_{0.941}{\mathrm{Co}}_{0.059}\right)}_2{\mathrm{As}}_2$ and PFATO. The partial (absence of) suppression below $T_c$ evidences phase competition (“friendly” coexistence) between orthorhombicity/nematicity and superconductivity. Blue line is a linear fit to $\delta$.