Incoherent $c$-Axis Interplane Response of the Iron Chalcogenide ${\mathrm{FeTe}}_{0.55}{\mathrm{Se}}_{0.45}$ Superconductor from Infrared Spectroscopy

We report on the interplane $c$-axis electronic response of ${\mathrm{FeTe}}_{0.55}{\mathrm{Se}}_{0.45}$ investigated by infrared spectroscopy. We find that the normal-state $c$-axis electronic response of ${\mathrm{FeTe}}_{0.55}{\mathrm{Se}}_{0.45}$ is incoherent and bears significant similarities to those of mildly underdoped cuprates. The $c$-axis optical conductivity ${\sigma }_c(\omega )$ of ${\mathrm{FeTe}}_{0.55}{\mathrm{Se}}_{0.45}$ does not display well-defined Drude response at all temperatures. As temperature decreases, ${\sigma }_c(\omega )$ is continuously suppressed. The incoherent $c$-axis response is found to be related to the strong dissipation in the $ab$-plane transport: a pattern that holds true for various correlated materials as well as ${\mathrm{FeTe}}_{0.55}{\mathrm{Se}}_{0.45}$.

Figure 1

$T$-dependent $c$-axis reflectance $R_c(\omega )$ of ${\mathrm{FeTe}}_{0.55}{\mathrm{Se}}_{0.45}$ with $T_c=14\mathrm{K}$. The $ab$-plane reflectance $R_{ab}(\omega )$ at 295, 18, and 6 K is also plotted for comparison [16]. The inset shows $R_{ab}(\omega )$ and $R_c(\omega )$ over broad frequency up to $25000{\mathrm{cm}}^{-1}$.

Figure 2

(a) $T$-dependent $c$-axis optical conductivity ${\sigma }_c(\omega )$. The peak at about $268{\mathrm{cm}}^{-1}$ is due to the infrared-active phonon mode [57]. ${\sigma }_c(\omega )$ in the far-infrared region becomes clearly depressed with decreasing $T$, and the suppressed SW seems not to be recovered by $5000{\mathrm{cm}}^{-1}$, as shown in the inset. (b) $T$-dependent $ab$-plane optical conductivity ${\sigma }_{ab}(\omega )$. The inset shows the frequency-dependent $ab$-plane scattering rate ${\Gamma }_{ab}(\omega )$ (Ref. 16). The dotted line corresponds to $\Gamma (\omega )=\omega$.

Figure 3

$ab$-plane scattering rate ${\Gamma }_{ab}({\omega }_0)$ divided by ${\omega }_0$. ${\omega }_0$ is the frequency that accounts for 30% of the intraband SW. The gray bar denotes the region close to ${\Gamma }_{ab}({\omega }_0)\sim {\omega }_0$. The left and right sides of this region represent the weak and strong dissipation regimes, respectively. The majority of the materials with strong $ab$-plane dissipation exhibit incoherent $c$-axis response (red symbols). The latter is defined by the suppression of the low-energy SW in ${\sigma }_c(\omega )$ at low $T$ concomitant with $d{\rho }_c/dT<0$. The majority of the materials with weak $ab$-plane dissipation reveal coherent $c$-axis behavior (blue symbols): the enhancement of the low-energy SW in ${\sigma }_c(\omega )$ and $d{\rho }_c/dT>0$. Data points: ${\mathrm{FeTe}}_{0.55}{\mathrm{Se}}_{0.45}$ (Ref. 16), ${\mathrm{BaFe}}_2{\mathrm{As}}_2$ (Refs. 38, 39), $2H\mathrm{\text{−}}{\mathrm{NbSe}}_2$ (Ref. 27), $(\mathrm{BEDT}\mathrm{\text{−}}\mathrm{TTF}{)}_2\mathrm{Cu}(\mathrm{NCS}{)}_2$ (Refs. 29, 37), cuprates (Refs. 33, 34, 35, 36, 41, 42, 43, 44). (UD, underdoped; OpD, optimally doped; OD, overdoped. The number denotes $T_c$ of each compound.)