Orbital-selective pairing and superconductivity in iron selenides

Abstract

An important challenge in condensed matter physics is understanding iron-based superconductors. Among these systems, the iron selenides hold the record for highest superconducting transition temperature and pose especially striking puzzles regarding the nature of superconductivity. The pairing state of the alkaline iron selenides appears to be of d-wave type based on the observation of a resonance mode in neutron scattering, while it seems to be of s-wave type from the nodeless gaps observed everywhere on the Fermi surface. Here we propose an orbital-selective pairing state, dubbed sτ 3 , as a natural explanation of these disparate properties. The pairing function, containing a matrix τ 3 in the basis of 3d-electron orbitals, does not commute with the kinetic part of the Hamiltonian. This dictates the existence of both intraband and interband pairing terms in the band basis. A spin resonance arises from a d-wave-type sign change in the intraband pairing component, whereas the quasiparticle excitation is fully gapped on the FS due to an s-wave-like form factor associated with the addition in quadrature of the intraband and interband pairing terms. We demonstrate that this pairing state is energetically favored when the electron correlation effects are orbitally selective. More generally, our results illustrate how the multiband nature of correlated electrons affords unusual types of superconducting states, thereby shedding new light not only on the iron-based materials but also on a broad range of other unconventional superconductors such as heavy fermion and organic systems.

Experimental consequences of Bogoliubov Fermi surfaces

“…The pairing of electrons with additional internal degrees of freedom resulting, e.g., from different orbitals or basis sites, can lead to qualitatively new pairing states. Such pairing states have for example been proposed for iron-based superconductors [2][3][4][5][6][7][8][9], Cu x Bi 2 Se 3 [10,11], cubic systems such as half-Heusler compounds [12][13][14][15][16][17][18][19][20][21], UPt 3 [22,23], transition-metal dichalcogenides [24,25], and twisted bilayer graphene [26][27][28]. It has been shown that in centrosymmetric multiband superconductors that break TRS, point and line nodes are generically "inflated," by interband pairing, into Fermi surfaces of Bogoliubov quasiparticles [29,30].…”

Anisotropic spin fluctuations in detwinned FeSe

“…In addition to having a weak peak around E ≈ 3.2 meV, we find that the scattering changes from well-defined commensurate peaks centered around Q AF below E = 3.625 ± 0. Although these results on twinned FeSe suggest that spin fluctuations play an important role in the superconductivity of FeSe, they provide no information on the possible orbital selective nature of the fluctuations that may lead to a highly anisotropic electron pairing state [19,31,[35][36][37][38]. From STM quasiparticle interference measurements on a single domain (detwinned) FeSe, where the Fermi surface geometry of electronic bands can be determined in the nematic phase, sign-reversed superconducting gaps are found at the hole [Γ or Q = (0, 0)] and electron [X or Q AF = (1, 0)] Fermi surface states derived from d yz orbitals of the Fe atoms along the orthorhombic a o -axis direction [Figs.…”

Bogoliubov Fermi surfaces: General theory, magnetic order, and topology

“…However, motivated in part by developments in topological materials [4][5][6][7][8], it has recently been realized that the internal electronic degrees of freedom (i.e., orbital or sublattice) which give rise to the multiband structure can also appear in the Cooper pair wavefunction. Pairing states involving a nontrivial dependence on these internal degrees of freedom, which we refer to as "internally anisotropic" states, have been proposed for many multiband systems, such as the iron-based superconductors [9][10][11][12][13][14][15], nematic superconductivity in Cu x Bi 2 Se 3 [16,17], j = 3/2 pairing in cubic materials motivated by the half-Heusler compounds [18][19][20][21][22][23][24][25], and j = 5/2 pairing and topological superconductivity in UPt 3 [26][27][28]. These pairing states have also attracted attention as a way to generate oddfrequency pairing [29][30][31] and an intrinsic ac Hall conductivity that is responsible for the polar magneto-optical Kerr effect in superconductors with broken time-reversal symmetry (TRS) [32,33].…”

“…Candidates for superconductors that break TRS arXiv:1806.03773v1 [cond-mat.supr-con] 11 Jun 2018 have been experimentally identified through muon-spinrotation and polar-Kerr-effect measurements, and include UPt 3 [40,41], Th-doped UBe 13 [42], PrOs 4 Sb 12 [43,44], Sr 2 RuO 4 [45,46], URu 2 Si 2 [47], SrPtAs [48], and Bi/Ni bilayers [49]. In addition, theory has predicted additional possibilities such as graphene [50,51], twisted bilayer graphene [52,53], the half-Heusler compound YPtBi [18], water-intercalated sodium cobaltate Na x CoO 2 ·yH 2 O [54,55], Cu-doped TiSe 2 [56], and monolayer transition-metal dichalcogenides [57].…”