Concealed d -wave pairs in the s ^± condensate of iron-based superconductors

Abstract

A central question in iron-based superconductivity is the mechanism by which the paired electrons minimize their strong mutual Coulomb repulsion. In most unconventional superconductors, Coulomb repulsion is minimized through the formation of higher angular momentum Cooper pairs, with Fermi surface nodes in the pair wavefunction. The apparent absence of such nodes in the iron-based superconductors has led to a belief they form an s-wave (s ± ) singlet state, which changes sign between the electron and hole pockets. However, the multiorbital nature of these systems opens an alternative possibility. Here, we propose a new class of s ± state containing a condensate of d-wave Cooper pairs, concealed by their entanglement with the iron orbitals. By combining the d-wave (L = 2) motion of the pairs with the internal angular momenta I = 2 of the iron orbitals to make a singlet (J = L + I = 0), an s ± superconductor with a nontrivial topology is formed. This scenario allows us to understand the development of octet nodes in potassium-doped Ba 1−x K X Fe 2 As 2 as a reconfiguration of the orbital and internal angular momentum into a high spin (J = L + I = 4) state; the reverse transition under pressure into a fully gapped state can then be interpreted as a return to the low-spin singlet. The formation of orbitally entangled pairs is predicted to give rise to a shift in the orbital content at the Fermi surface, which can be tested via laser-based angle-resolved photoemission spectroscopy. superconductivity | iron-based superconductors | unconventional superconductivity | strongly correlated electrons T he family of iron-based high-temperature superconductors exhibits a marked absence of nodes in the superconducting gap function (1-13). This stands in stark contrast with almost all other strongly correlated superconductors and superfluids, including the cuprates, heavy fermions, ruthenates, and 3 He (14-28), where the repulsive interaction between fermions drives the formation of higher angular momentum pairs with nodes in the pair wavefunction. Iron-based superconductors are generally believed to have an isotropic s ± gap structure (29) with a sign change between gap function on the electron and hole pockets. Signatures of this sign reversal have been observed in scanning tunneling microscopy (7) and half-integer flux experiments (8). Remarkably, this fully gapped state survives with no apparent impact on the transition temperature in systems where the hole pockets are absent, such as A x Fe 2 Se 2 (30) and single-layer FeSe (31) where transition temperatures comparable to the cuprate family of hightemperature superconductors have recently been measured (32), even though the phase cancellation between the electron and hole pockets that minimizes the on-site Coulomb repulsion is now compromised. The underlying concern, voiced by Lev D. Landau (see ref. 33), that one cannot repeal Coulomb's law must therefore find a new resolution in the Fe-based systems and other unconventional superconductors with multiple orbitals. In this...

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“…22,23,28 Such orbital selectivity has also been the subject of extensive recent theoretical studies. [29][30][31] All these aspects make it natural to study orbital-dependent [32][33][34] and -related 35 superconducting pairing. We are thus motivated to address the hitherto unexplored question, viz.…”

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