Abstract
Iron-based superconductors have been found to exhibit an intimate interplay of orbital, spin, and lattice degrees of freedom, dramatically affecting their low-energy electronic properties, including superconductivity. Albeit the precise pairing mechanism remains unidentified, several candidate interactions have been suggested to mediate the superconducting pairing, both in the orbital and in the spin channel. Here, we employ optical spectroscopy (OS), angle-resolved photoemission spectroscopy (ARPES), ab initio band-structure, and Eliashberg calculations to show that nearly optimally doped NaFe0.978Co0.022As exhibits some of the strongest orbitally selective electronic correlations in the family of iron pnictides. Unexpectedly, we find that the mass enhancement of itinerant charge carriers in the strongly correlated band is dramatically reduced near the Γ point and attribute this effect to orbital mixing induced by pronounced spin-orbit coupling. Embracing the true band structure allows us to describe all low-energy electronic properties obtained in our experiments with remarkable consistency and demonstrate that superconductivity in this material is rather weak and mediated by spin fluctuations.
Highlights
The comparison of the fitted effective masses to the theoretical band structure in Fig. 1c reveals a renormalization by a factor of 4.3 and 4.5 for the inner and middle hole bands of Fe-3dxz,yz orbital character, respectively
The outer hole band of Fe-3dxy orbital character in Fig. 1a shows a dramatic renormalization by a factor of more than 8, clearly demonstrating the existence of very strong and orbitally selective electronic correlations in NaFe0.978Co0.022As, similar to FeTe1−xSex[12,13,14]
This dramatic reduction of the effective mass at lower binding energies cannot be attributed solely to the effect of high-energy electronic correlations, which typically lead to an enhancement of the effective mass over the entire electronic band width
Summary
Angle-resolved photoemission measurements were performed using synchrotron radiation (“13–ARPES” end-station at BESSY) within the range of photon energies 20–90 eV and various polarizations on cleaved surfaces of high-quality single crystals of NaFe0.978Co0.022As. The overall energy and angular resolution were~5 meV and 0.3°, respectively, for the low temperature measurements. High-accuracy absolute measurements of the sample’s reflectance were carried out using the gold-overfilling technique. High-quality single crystals of superconducting NaFe0.978Co0.022As were synthesized by the self-flux technique and were characterized by x-ray diffraction, transport and magnetization measurements[41]. The latter revealed a superconducting transition temperature of about 18 K. Band structure calculations were performed for the experimental crystal structure of NaFeAs (ref. 42) using the PY LMTO computer code[43]
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