Orbital-dependent electron dynamics in Fe-pnictide superconductors (Angle Resolved Photoelectron Spectroscopy (ARPES) and time- and angle-resolved Photoelectron spectroscopy (trARPES) study)

The discovery of High-Tc superconductivity in Fe-based compounds has generated high curiosity in the condensed matter community. Although extensive research has already been carried out, many unconventional properties of these compounds still remain unexplained [1]. We used Angle Resolved Photoelectron  Spectroscopy (ARPES) to probe different bands [2] and orbital selective time resolved ARPES measurements (trARPES) on EuFe2As2, a parent compound of Fe-pnictide superconductors to probe different excitations and coupling phenomena of the system [3]. We observed different relaxation time-scales for dxz/dyz and dxy electrons, selectively excited by changing the polarization of the pump beam [4]. The relaxation time-constant for dxy electrons is approximately 2 ps, whereas dxz/dyz electron relaxes faster within approximately 800 fs. The time-constant also increases below the spin density wave transition at 190 K due to the opening of gap at the Fermi level. The different time-constants can be explained by taking into account the degree of localization of different orbitals. Previous ARPES results show that dxy orbitals are localized, whereas dxz/dyz electrons have higher degree of itineracy because of their hybridization with As p states [5-7]. Although electron effective masses for different orbitals show small differences in the parent compound, the differentiation increases with hole-doping. The contrast between the relaxation dynamics of dxy and dxz/dyz orbitals therefore indicates that the photo-induced excited state of EuFe2As2 behaves like a hole-doped composition. Our results support orbital-selective Mott behaviour in Fe-pnictide systems, which can explain many exotic properties of these compounds [5].

References:
[1] 	Q. Si, R. Yu, and E. Abrahams, Nature Rev. Mater. 1, 16017 (2016)
[2] 	Electronic structure of EuFe2As2 Ganesh Adhikary, Nishaina Sahadev, Deepnarayan Biswas, R. Bindu, Neeraj Kumar, A. Thamizhavel, S. K. Dhar, and Kalobaran Maiti J. Phys.: Condens. Matter 25, 225701-225708 (2013).
[3] 	Orbital-dependent electron dynamics in Fe-pnictide superconductors Ganesh Adhikary, Barbara Ressel, Matija Stupar, Primož Rebernik Ribič, Jurij Urbančič, A. Thamizhavel, Kalobaran Maiti and Giovanni De Ninno Communicated in physical review letters arXiv:1711.05464 (https://arxiv.org/pdf/1711.05464.pdf)
[4] 	A. Damascelli, Z. Hussain, and Z.-X. Shen, Rev. Mod. Phys. 75, 2 (2003)
[5] 	L. de’ Medici, G. Giovannetti, and M. Capone, Phys. Rev. Lett. 112, 177001 (2014)
[6] 	Hidden order in Fe-pnictide superconductors Khadiza Ali, Ganesh Adhikary, Sangeeta Thakur, Swapnil Patil, Sanjoy K. Mahatha, A. Thamizhavel, Giovanni De Ninno, Paolo Moras, Polina M. Sheverdyaeva, Carlo Carbone, Luca Petaccia and Kalobaran Maiti Accepted in Phys. Rev. B (in press)
[7] 	Complex temperature evolution of the electronic structure of CaFe2As2 Ganesh Adhikary, Deepnarayan Biswas, Nishaina Sahadev, R. Bindu, Neeraj Kumar, S. K. Dhar, A. Thamizhavel, and Kalobaran Maiti J. Appl. Phys. 115, 123901-123906 (2014).