Role of the orbital degree of freedom in iron-based superconductors

Abstract

Almost a decade has passed since the serendipitous discovery of the iron-based high temperature superconductors (FeSCs) in 2008. The fact that, as in the copper oxide high temperature superconductors, long-range antiferromagnetism in the FeSCs arises in proximity to superconductivity immediately raised the question of the degree of similarity between the two. Despite the great resemblance in their phase diagrams, there exist important differences between the FeSCs and the cuprates that need to be considered in order to paint a full picture of these two families of high temperature superconductors. One of the key differences is the multi-orbital multi-band nature of the FeSCs, which contrasts with the effective single-band nature of the cuprates. Systematic studies of orbital related phenomena in FeSCs have been largely lacking. In this review, we summarize angle-resolved photoemission spectroscopy (ARPES) measurements across various FeSC families that have been reported in literature, focusing on the systematic trends of orbital dependent electron correlations and the role of different Fe 3d orbitals in driving the nematic transition, the spin-density-wave transition, and superconductivity.

Nematic Energy Scale and the Missing Electron Pocket in FeSe

“…We utilize selection rules for different light polarizations [7,55] to identify the dominant orbital characters of bands observed on the detwinned crystal. Near the Γ point, three hole bands are resolved close to E F , dominated by d xz , d yz , and d xy characters, as expected for all FeSCs [56]. Amongst these three, the d xz and d yz hole bands have comparable band velocities while the d xy hole band is much flatter and does not cross E F .…”

Low work function in the 122-family of iron-based superconductors

“…The work function is expected to decrease with decreasing n 43 , which agrees with the trends of Φ Alk and Φ of the FeSCs presented in Table I. The difference between Φ and Φ Alk can thus be attributed to (3). In general, a rougher surface has a larger dipole due to the smoothing of the electron distribution, which reduces the work function 44 .…”

Orbital-selective Mott phase in multiorbital models for iron pnictides and chalcogenides

“…In this phase, the nondegenerate 3d xy orbital loses its coherent spectral weight at the Fermi energy, while this weight remains nonzero for the other orbitals (including the degenerate 3d xz/yz orbitals). These features have been clearly identified in the ARPES measurements [7][8][9][10] . From a theoretical perspective, it is important to stress that, the orbitals are kinetically coupled -i.e.…”

“…It can be defined in terms of the mass enhancement (m * /m b , the ratio of the effective mass observed experimentally to that of the non-interacting band dispersion) being different for the electronic states with predominantly different orbital contents. In practice, m * /m b for electronic states with predominantly 3d xy orbital character is much larger (reaching 10 − 20 in the iron selenides) than that for the electronic states with predominantly 3d xz/yz (typically 3 − 4) and other orbital characters 4,10 . Strictly speaking, the orbital selectivity is in itself not precisely defined, because the hybridization mixes the different orbitals in the electronic band states.…”

“…In the case of the iron-based superconductors (FeSCs) [1][2][3][4][5][6] , recent developments have further highlighted the importance of electron correlations. For instance, angle-resolved photoemission spectroscopy (ARPES) has found an orbitalselective Mott phase (OSMP) in the iron chalcogenides [7][8][9][10] . The OSMP phase arises in multiorbital Hubbard models of the FeSCs, which contain both Hubbard interactions and Hund's coupling 11,12 .…”