Doping induced band renormalization and orbital selectivity in 122-type iron based superconductor

Chemical substitution is an effective parameter for tuning the physical properties of the correlated materials including the high temperature superconductors which continues to be an outstanding issue in material science. The chemical doping in Fe-based pnictide/chalcogenides suppresses the structural and magnetic transitions of the parent compound and induces superconducting phase at a critical doping range [1]. For instance, CaFe$_2$As$_2$ (Ca122) is a parent compound in a Fe-based superconducting (FeSCs) family which undergoes both magnetic and structural transition at about 170 K from tetragonal paramagnetic to orthorhombic AFM phase [2]. Replacement of Fe with Co leads to suppression of both the transitions and superconductivity appears within at a suitable doping range [3]. Since, the dopants usually have less/more no of electrons relative to the atom at the doped site, such processes change the charge carrier concentration leading to a rigid shift of the Fermi energy ($E_F$) of the compound. Interestingly, the electronic structure studies carried out on FeSCs using angle resolved photoemission spectroscopy (ARPES) technique shows a significant orbital dependent band renormalization [4] and increased spin-orbit coupling (SOC) strength [5] with doping which dominates over the expected $E_F$-shift.

In order to probe the underlying physics, we studied the electronic structure of the superconducting composition of Ca122 achieved by Fe-replacement with Co-atoms (CaFe$_{1.9}$Co$_{0.1}$As$_2$, TC $\sim$15 K), employing high resolution ARPES technique. The experimental results are discussed with the spectra of the parent compound measured in similar conditions to investigate the effect of Co-substitution. The ARPES spectra reveals significant orbital dependent band renormalization and unequal energy shift of the bands along with an enhancement of SOC gap between the bands at the center of Brillouin zone. Considering the bond lengths of both the compounds and their evolution with temperature [6] in theoretical band structure calculation, we observe that the changes observed in the electronic structure with doping can be linked to the change in structural parameters. We observe significant change in the dispersion of the hole-band corresponding to the Fermi pockets with the change of $k_z$-momentum suggesting a possibility of $k_z$-dependent superconducting gap properties in this system.

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[2] N. Kumar et al., Phys. Rev B, 79, 012504 (2009); A. I. Goldman et al., Phys. Rev. B, 78, 100506 (R) (2008).
[3] L. Harnagea et al., Phys. Rev. B, 83, 094523 (2011).
[4] Z. K. Liu et al., Phys. Rev. B, 92, 235138 (2015); T. Sudayama et al., J. Phys. Soc. Jpn., 80, 113707 (2011).
[5] H. Miao et al., Phys. Rev. B, 89, 22050 ® (2014).
[6] R. P. Pandeya et al., J. Phys.: Condens. Matter, 33, 19LT01 (2021).

E-mail for corresponding author: [email protected]