Absence of Y-pocket in 1-Fe Brillouin zone and reversed orbital occupation imbalance in FeSe

Abstract

The FeSe nematic phase has been the focus of recent research on iron-based superconductors (IBSs) due to its unusual properties, which are distinct from those of the pnictides. A series of theoretical/experimental studies were performed to determine the origin of the nematic phase. However, they yielded conflicting results and caused additional controversies. Here, we report the results of angle-resolved photoemission and X-ray absorption spectroscopy studies on FeSe detwinned by a piezo stack. We fully resolved band dispersions with orbital characters near the Brillouin zone (BZ) corner, and revealed an absence of any Fermi pocket at the Y point in the 1-Fe BZ. In addition, the occupation imbalance between d xz and d yz orbitals was the opposite of that of iron pnictides, consistent with the identified band characters. These results resolve issues associated with the FeSe nematic phase and shed light on the origin of the nematic phase in IBSs.

On the Remarkable Superconductivity of FeSe and Its Close Cousins

“…Those measurements could only observe one of the the two crossed "peanut-shaped" electron pockets at the X point [106,[109][110][111]; similar conclusions were reported recently using nano ARPES [112] within individual nematic domains. Various explanations for this dramatic consequence of nematicity were offered, including selection rules specific to ARPES [110], strong correlation effects rendering some of the electronic states less coherent [20], unusual band shifts and hybridization of bands [113,114]; some data of these experiments are shown in Figure 4e-g. Finally, evidence for an additional band splitting was reported, which could be due to magnetism or SOC on the surface [115].…”

“…Recently, evidence for the second pocket at the Y point has been reported in the literature as well [117,118] suggesting that a detailed understanding of this issue might also be related to shifts of the d xz and d yz orbital states in the orthorhombic state. Depending on the assignments of peaks in the measured spectral function to bands at the X-point, a large splitting of 50 meV [109,113,116,119] or much smaller splitting of 10 meV [107,110] has been deduced, while in both scenarios the sign of the splitting is reversed between the Γ point and the X point [106]-similar to findings in Ba122 [120]. The four branches of oscillation frequencies observed in quantum oscillations of the resistivity [92,121,122] correspond to the extremely small areas of the Fermi surface sheets covering only few percent of the Brillouin zone.…”

“…Whether the ARPES data imply a strongly differentiated coherence between the d xz and the d yz orbital [105], e.g., to understand the absence of the Y-pocket, is currently controversial. Experimental investigations have proposed as an alternative explanation strong shifts of the bands close to the Y point together with an orbital hybridization such that the Y-pocket is diminished or a gap opens [113,114]. Within a tight-binding approach, it is, however, difficult to construct a hybridization term that will lift the Y-pocket entirely away from the Fermi surface, while preserving all symmetries of the crystal.…”

“…Recently, alternative explanations of the latter two experimental results were brought forward in terms of effects of the three dimensional electronic structure giving rise to some averaging in the sum over k on the r.h.s of Equation (18) [212] or possible lifting of the Y pocket due to band hybridizations [113,114]. However, the latter explanation has been questioned by a theoretical model of the expected spectral functions accounting for an electronic structure that has a nematic order parameter in the d xz /d yz channel and the d xy channel, and additionally hybridizations due to spin-orbit coupling are taken into account [97].…”

Revealing the single electron pocket of FeSe in a single orthorhombic domain

“…Therefore conventional ARPES measurements will usually detect a superposition from both domains with approximately equal intensity. Several groups have attempted to overcome this limitation by aligning the orthorhombic domains along a preferential axis by applying uniaxial strain [17][18][19][20], commonly known as "detwinning." These measurements revealed a single elliptical holelike pocket at the Brillouin zone center, in line with theoretical predictions [21].…”