Fermi Surface Topology and Low-Lying Quasiparticle Dynamics of Parent ${\mathrm{Fe}}_{1+x}\mathrm{Te}/\mathrm{Se}$ Superconductor

We report the first photoemission study of ${\mathrm{Fe}}_{1+x}\mathrm{Te}$—the host compound of the newly discovered iron-chalcogenide superconductors (maximum $T_c\sim 27\mathrm{K}$). Our results reveal a pair of nearly electron-hole compensated Fermi pockets, strong Fermi velocity renormalization, and an absence of a spin-density-wave gap. A shadow hole pocket is observed at the “$X$” point of the Brillouin zone which is consistent with a long-range ordered magnetostructural ground state. No signature of Fermi surface nesting instability associated with $\mathbf{Q}=(\pi /2,\pi /2)$ is observed. Our results collectively reveal that the ${\mathrm{Fe}}_{1+x}\mathrm{Te}$ series is different from the undoped phases of the high $T_c$ pnictides and likely harbor an unusual mechanism for superconductivity and magnetic order.

Figure 1

Magnetostructural transition and long-range order in ${\mathrm{Fe}}_{1+x}\mathrm{Te}$. (a) Temperature dependence of the magnetic susceptibility [6]. (b) The spins in ${\mathrm{SrFe}}_2{\mathrm{As}}_2$ are collinear and ${\mathbf{Q}}_{\mathrm{AFM}}$ points along the $(\pi ,0)$ direction. (c) The ordering vector, ${\mathbf{Q}}_{\mathrm{AFM}}$ in FeTe, is rotated by 45° and points along the $(\frac{\pi }{2},\frac{\pi }{2})$ direction [15].

Figure 2

Fermi surface topology. (a) Fermi surface topology of FeTe. (a)–(d) Connection to the underlying band structure is revealed by considering the evolution of the density of states $n(k)$ as the chemical potential is rigidly lowered by 20, 40, and 60 meV. The Fermi surface consists of hole pockets centered at $\Gamma$ and electron pockets centered at $M$. An additional holelike feature is observed at $M$ in (b)–(d), which is attributed to a band lying below $E_F$ [see schematic in (e)]. (e) presents a schematic of dispersions in the $\Gamma \mathrm{\text{−}}M$ direction forming the electron and hole pockets. A qualitative agreement is observed between (f) the experimentally measured FS topology and (g) the local-density approximation (LDA) calculated [10] FS although measured bands are renormalized.

Figure 3

Polarization-resolved study reveals the low-lying band topology. ARPES spectra along the $\Gamma \mathrm{\text{−}}M$ direction in the (a) $\sigma$- and (b) $\pi$-scattering geometries. $v_F$ of the quasiparticle band forming the $\Gamma$ hole pocket is calculated from the dashed line. (c) The EDC of a high momentum resolution scan through $\Gamma$ shows two holelike bands crossing the $E_F$. (d)–(f) Cuts along the $M\mathrm{\text{−}}\Gamma$ direction through the electron FS pocket show that the hole band near $M$ remains at least 10 meV below $E_F$. For each scan direction the corresponding EDC is also presented, within the momentum range marked by green dashed lines. Comparison with (g) a band dispersion schematic of the LDA result [10]. (h) The directions of the six scans are identified in the first BZ.

Figure 4

Electronic structure and Magnetic ordering mechanism. While (a) the electron and hole pockets in ${\mathrm{SrFe}}_2{\mathrm{As}}_2$ can be kinematically nested by ${\mathbf{Q}}_{\mathrm{AFM}}=(\pi ,0)$, there exists (b) no FS pocket which can be nested by ${\mathbf{Q}}_{\mathrm{AFM}}=(\frac{\pi }{2},\frac{\pi }{2})$ in the FeTe BZ. A FS pocket observed at $X$ in ${\mathrm{SrFe}}_2{\mathrm{As}}_2$ has been attributed to the (c) $2\times 1$ surface order of the Sr atomic layer (red), which occurs in addition to a weak bulk distortion (blue) across the SDW transition. However, there are no additional Sr atoms between the Fe layers in FeTe, so no Sr order is possible, although a weak bulk or surface distortion is not excluded. (d) The EDCs measured at near the $\Gamma$ and $M$ pockets exhibit no evidence of energy gaps. Nevertheless, (e) a weak FS pocket is observed at $X$, (f) corresponding to two holelike bands dispersing towards the chemical potential.