Optical spectroscopy study of the collapsed tetragonal phase of ${\mathrm{CaFe}}_2\left({\mathrm{As}}_{0.935}{\mathrm{P}}_{0.065}\right){}_2$ single crystals

We present an optical spectroscopy study on P-doped ${\mathrm{CaFe}}_2{\mathrm{As}}_2$ which experiences a structural phase transition from tetragonal to collapsed tetragonal (cT) phase near 75 K. The measurement reveals a sudden reduction of low-frequency spectral weight and the emergence of a feature near $3200 \mathrm{cm}{}^{-1}$ (0.4 eV) in optical conductivity across the transition, indicating an abrupt reconstruction of band structure. The appearance of the feature is related to the interband transition arising from the sinking of hole bands near the $\Gamma$ point below Fermi level in the cT phase, as expected from the density function theory calculations in combination with the dynamical mean field theory. However, the reduction of Drude spectral weight is at variance with those calculations. The measurement also indicates an absence of the abnormal spectral weight transfer at high energy (near 0.5–0.7 eV) in the cT phase, suggesting a suppression of the electron correlation effect.

Figure 1

Temperature dependence of normalized resistivity of ${\mathrm{CaFe}}_2{\mathrm{As}}_2$ and ${\mathrm{CaFe}}_2\left({\mathrm{As}}_{0.935}{\mathrm{P}}_{0.065}\right){}_2$. The inset plots an enlarged region for superconducting transition in ${\mathrm{CaFe}}_2\left({\mathrm{As}}_{0.935}{\mathrm{P}}_{0.065}\right){}_2$.

Figure 2

Temperature dependence of magnetic susceptibility of ${\mathrm{CaFe}}_2\left({\mathrm{As}}_{0.935}{\mathrm{P}}_{0.065}\right){}_2$ with H$\parallel$$ab$-plane at 3000 Oe. The inset shows the zero-field-cooled and field-cooled data at 30 Oe.

Figure 3

Left panel: $R\left(\omega \right)$ for (a) ${\mathrm{CaFe}}_2{\mathrm{As}}_2$ and (b) ${\mathrm{CaFe}}_2\left({\mathrm{As}}_{0.935}{\mathrm{P}}_{0.065}\right){}_2$ below $8000 \mathrm{cm}{}^{-1}$ . Middle panel: $\sigma {}_1\left(\omega \right)$ for (c) ${\mathrm{CaFe}}_2{\mathrm{As}}_2$ and (d) ${\mathrm{CaFe}}_2\left({\mathrm{As}}_{0.935}{\mathrm{P}}_{0.065}\right){}_2$ below $4000 \mathrm{cm}{}^{-1}$ . The Drude-Lorentz fit for $T=10$ K is shown at the bottom. Inset: The optical conductivity over a much wider frequency range. Right panel: Temperature dependence of ${\omega }_p^2$ normalized to the value of 300 K and scattering rate 1/$\tau$ of the two Drude terms of ${\mathrm{CaFe}}_2\left({\mathrm{As}}_{0.935}{\mathrm{P}}_{0.065}\right){}_2$.

Figure 4

Ratio of the integrated spectral weight as a function of $\omega$ at 10 K in ${\mathrm{CaFe}}_2{\mathrm{As}}_2$ and ${\mathrm{CaFe}}_2\left({\mathrm{As}}_{0.935}{\mathrm{P}}_{0.065}\right){}_2$. The low-temperature integrated spectral weight is normalized to the room-temperature data.