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

TL;DR

This optical spectroscopy study investigates the electronic structure changes in P-doped CaFe$_2$As$_2$ during the transition to the collapsed tetragonal phase, revealing band reconstruction and suppressed electron correlations.

Contribution

It provides experimental evidence of band structure reconstruction and correlation suppression in the collapsed tetragonal phase of P-doped CaFe$_2$As$_2$, supported by theoretical calculations.

Findings

Abrupt reduction of low-frequency spectral weight across transition

Emergence of a new optical feature near 0.4 eV

Suppression of electron correlation effects in the cT phase

Abstract

We present an optical spectroscopy study on P-doped CaFe$_2$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 emergence of a new feature near 3200 \cm (0.4 eV) in optical conductivity across the transition, indicating an abrupt reconstruction of band structure. The appearance of new feature is related to the interband transition arising from the sinking of hole bands near $\Gamma$ 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 electron correlation effect.