Strong Electron Correlations in the Normal State of the Iron-Based ${\mathrm{FeSe}}_{0.42}{\mathrm{Te}}_{0.58}$ Superconductor Observed by Angle-Resolved Photoemission Spectroscopy

We investigate the normal state of the “11” iron-based superconductor ${\mathrm{FeSe}}_{0.42}{\mathrm{Te}}_{0.58}$ by angle-resolved photoemission. Our data reveal a highly renormalized quasiparticle dispersion characteristic of a strongly correlated metal. We find sheet dependent effective carrier masses between $\approx 3\mathrm{\text{and}}16m_e$ corresponding to a mass enhancement over band structure values of $m^{*}/m_{\mathrm{band}}\approx 6–20$. This is nearly an order of magnitude higher than the renormalization reported previously for iron-arsenide superconductors of the “1111” and “122” families but fully consistent with the bulk specific heat.

Figure 1

(a) STM topograph of a cleaved surface taken at $T=6\mathrm{K}$ in a 40 nm-square field of view with a 12 nm square inset showing the atomically resolved (Se, Te) surface layer ($V=146\mathrm{mV}$, $I=0.44\mathrm{nA}$). (b) LEED pattern taken with a primary beam energy of 234 eV. The reciprocal lattice vectors of the orthorhombic phase are indicated by black arrows. (c), (d) Schematic of the crystal structure and Brillouin zone (not to scale). In (c), the conventional orthorhombic and a primitive unit cell of the low temperature phase are indicated by grey (green) and black solid lines respectively.

Figure 2

(a) Fermi surface mapping of ${\mathrm{FeSe}}_{0.42}{\mathrm{Te}}_{0.58}$ measured with 21.2 eV excitation energy and $p$ polarization at $T=12\mathrm{K}$. The photoemission intensity has been integrated over 10 meV about the chemical potential. At this photon energy, $k_z$ is approximately 2.7 Å, which is near the Z point in the third BZ. In (b), we compare the extracted contours with the DFT Fermi surface of FeSe. Dotted and solid lines correspond to the basal plane and midplane FS, respectively.

Figure 3

(a), (b) Band dispersion along $X\Gamma M$ (21.2 eV, $s$ polarization, $T=12\mathrm{K}$). The inset shows data around the $M$ point, divided by a smooth function to enhance the contrast ($h\nu =36\mathrm{eV}$). The white line is a guide to the eye. (c) DFT band structure calculation for FeSe. The dispersion of the low-energy excitations extracted from the data in (a) is overlaid.

Figure 4

(a), (b) Orbital character of the DFT wave functions for FeSe and FeTe in a coordinate system defined by the Fe lattice. (c), (d) Polarization dependence of the ARPES intensity along the $\Gamma M$ mirror plane [see Fig. 1c for a definition of the scattering planes].