Electronic correlations and unusual superconducting response in the optical properties of the iron chalcogenide ${\text{FeTe}}_{0.55}{\text{Se}}_{0.45}$

The in-plane complex optical properties of the iron-chalcogenide superconductor ${\text{FeTe}}_{0.55}{\text{Se}}_{0.45}$ have been determined above and below the critical temperature $T_c=14\text{K}$. At room temperature the conductivity is described by a weakly interacting Fermi liquid; however, below 100 K the scattering rate develops a frequency dependence in the terahertz region, signaling the increasingly correlated nature of this material. We estimate the dc conductivity ${\sigma }_{\text{dc}}\left(T\gtrsim T_c\right)\simeq 3500\pm 400{\Omega }^{-1}{\text{cm}}^{-1}$ and the superfluid density ${\rho }_{s0}\simeq 9\pm 1\times {10}^6{\text{cm}}^{-2}$, which places this material close to the scaling line ${\rho }_{s0}/8\simeq 8.1{\sigma }_{\text{dc}}{T}_c$ for a BCS dirty-limit superconductor. Below $T_c$ the optical conductivity reveals two gap features at ${\Delta }_{1,2}\simeq 2.5$ and 5.1 meV.

Figure 1

(Color online) The reflectance of ${\text{FeTe}}_{0.55}{\text{Se}}_{0.45}$ in the far-infrared region for light polarized in the Fe-Te planes at several temperatures above and below $T_c$. Inset: the extended unit cell of FeTe in the tetragonal $P4/nmm$ space group showing the tetrahedrally coordinated Te above and below the Fe planes.

Figure 2

(Color online) The real part of the in-plane optical conductivity for ${\text{FeTe}}_{0.55}{\text{Se}}_{0.45}$ at several temperatures above and below $T_c$ in the infrared region. Inset: the conductivity in the far-infrared region compared with a model calculation for a single isotropic gap (${\Delta }_0=4.5\text{meV}$, $1/\tau =4{\Delta }_0$) superimposed on the Lorentzian contribution.

Figure 3

(Color online) The in-plane frequency-dependent scattering rate of ${\text{FeTe}}_{0.55}{\text{Se}}_{0.45}$ for several temperatures above and below $T_c$ in the far-infrared region. The values for $1/{\tau }_D$ are shown at 295 (●), 200 (◼), and 100 K (▲), respectively, where the scattering rate displays little temperature dependence. For $T\gtrsim T_c$ (18 K) at low frequency $1/\tau \left(\omega \right)\propto \omega$ while for $T<T_c$ large changes in the scattering rate are observed in response to the formation of superconducting gap(s).

Figure 4

(Color online) The in-plane optical conductivity of ${\text{FeTe}}_{0.55}{\text{Se}}_{0.45}$ shown at 6 K (solid line). The calculated optical conductivity with gaps of $2{\Delta }_1\simeq 5\text{meV}$ and $2{\Delta }_2\simeq 10.2\text{meV}$ for $T⪡T_c$ (short-dashed lines) is superimposed on the contribution from the bound excitations in the mid-infrared (long-dashed line); the smoothed linear combination of the three curves (dot-dash line) is in good agreement with the measured data below $200{\text{cm}}^{-1}$.