The electronic structure in the nematic phases of FeSe and NaFeAs from detwinned ARPES measurements

Many of the Fe-based superconductors exhibit a “nematic” phase where four-fold symmetry of the lattice is spontaneously broken without magnetic order. Angle-resolved photoemission spectroscopy (ARPES) measurements of FeSe are able to give a unique insight into the evolution of the electronic structure in the nematic phase. ARPES measurements have revealed substantial shifts of both the electron and hole bands associated with the onset of the nematic order parameter, however the interpretation of these data has caused some controversy due to the formation of twin orthorhombic domains in samples. Our high-resolution ARPES results from the I05 beamline at Diamond Light Source obtained on FeSe crystals “detwinned” by application of mechanical strain, reveal remarkable anisotropies hidden in the measurements of twinned samples. We find that the low temperature Fermi surface of FeSe consists of one elliptical hole pocket and only one orthogonally-oriented electron pocket. Finally, we find that in detwinned NaFeAs samples in nematic phase (below the structural transition at T_S= 54 K but above the antiferromagnetic transition at T_N= 43 K) spectral weight is also detected on only one elliptical electron pocket.