Session M41: Novel Phenomena in Multiorbital Superconductors

Superconductivity in multiorbital systems: Role of Fermi surface geometry and repulsive interactions in Hund's pairing

Unconventional superconductivity is often referred to as originating from a pairing mechanism different from electron-phonon interactions and connected to an anisotropic superconducting order parameter with sign change of the Cooper pair wavefunction. A common microscopic mechanism is the spin-fluctuation mediated pairing where the formation of Cooper pairs is possible by avoiding the repulsive Coulomb interaction in space. For multiband systems, there is the possibility of a superconducting instability even when all local bare interactions are repulsive. This mechanism is referred to as Hund’s pairing and the specific channels are dominantly off-diagonal in orbital space resulting in exotic interband pair states. This mechanism and its relation to the exchange of spin fluctuations together with Fermi surface geometry is examined for multiband models relevant for Fe-based superconductors and Sr2RuO4 [1-2]. Additional players for superconducting pairing in these systems are strong correlations [3] and electron-phonon interactions [4]. Implications of these on the dominating superconducting instability will be discussed.

[1] Rømer, et al, Phys. Rev. Research 4, 033011 (2022)

[2] Roig, et al, Phys. Rev. B 106, L100501 (2022)

[3] Kreisel, et al, Phys. Rev. Lett. 129, 077002 (2022)

[4] Biswas, et al., Phys. Rev. B 108, L020501 (2023)   

Microscopic origins of ultranodal states in spin-1/2 systems

Several unconventional superconductors show indications of zero-energy excitations in the superconducting state consistent with the existence of a so-called Bogoliubov Fermi surface (BFS). In particular, FeSe doped with S seems to acquire a nonzero density of states at zero energy at low temperatures when doped into the tetragonal phase, consistent with a previously proposed phenomenological theory assuming an anisotropic spin singlet pairing gap coexisting with a nonunitary interband triplet component. Here we search for a microscopic model that can support the coexistence of singlet pairing with other orders, including interband nonunitary triplet pairing, and discuss several candidates that indeed stabilize ground states with Bogoliubov Fermi surfaces. We show that with proper choice of the coupling strength of the various orders in our model, spontaneous breaking of C₄ rotational symmetry is realized at low temperatures, in accordance with recent angle-resolved photoemission experiments in Fe(Se,S) in the tetragonal phase.   

Nematic Bogoliubov Fermi surfaces from magnetic toroidal order: application to FeSe1-x Sx

Superconducting Bogoliubov Fermi surfaces require broken time-reversal symmetry to be realized. Recently, this has been examined  in multiorbital superconductors that preserve inversion symmetry [1]. In this case, the resultant Bogoliubov Fermi surfaces are topologically protected. However, despite the lack of topological protection, they can also appear when both time-reversal and inversion symmetries are broken. Here we examine the development of Bogoliubov Fermi surfaces due to magnetic toroidal order, magnetic order that breaks both time-reversal and inversion symmetries but preserve their product [2,3]. We show that the nematic Bogoliubov Fermi surfaces observed in tetragonal FeSe_1-xS_x [4] are naturally explained if such a toroidal magnetic order coexists with superconductivity [3]. We further show that Néel (checkerboard) magnetic order or pair density wave superconductivity both give rise to magnetic toroidal order consistent with the observed nematic Bogoliubov Fermi surfaces.  

 

    

Bogoliubov Fermi surfaces in the superconducting state of tetragonal FeSe1-xSx

Recently, topologically protected ultranodal pairing states with so-called Bogoliubov Fermi surfaces have been predicted in time-reversal symmetry-breaking (TRSB) superconductors [1,2]. The iron-chalcogenide superconductors FeSe_1-xS_x are among the most promising candidates for such exotic superconducting states. In FeSe_1-xS_x, nematic order (tetragonal-to-orthogonal structural transition) can be completely suppressed at x = 0.17, above which the superconducting properties change drastically and the residual density of states in the superconducting state is significantly enhanced [3-5]. Our recent muon spin rotation (μSR) measurements reveal that TRSB occurs just below the superconducting transition temperature and a significant fraction of electrons remains unpaired in the superconducting state of the tetragonal phase [6]. These results are consistent with the ultranodal pairing state with BFSs. I will review recent studies of FeSe_1-xS_x superconductors, showing exotic superconducting states, including recent direct observations of BFSs in angle-resolved photoemission spectroscopy (ARPES) measurements [7] and a strong upturn of the nuclear magnetic relaxation (NMR) rate in the superconducting state associated with possible nesting properties between BFSs [8] as well as a possible impurity-induced disappearance of BFSs revealed by penetration depth measurements in the tetragonal FeSe_1-xS_x [9].

[1] D. F. Agterberg, P. M. R. Brydon, and C. Timm, Phys. Rev. Lett. 118, 127001 (2017).

[2] C. Setty et al, Nat. Commun. 11, 523 (2020).

[3] Y. Sato et al., Proc. Natl. Acad. Sci. USA 115, 1227 (2018).

[4] T. Hanaguri et al., Sci. Adv. 4, eaar6419 (2018).

[5] Y. Mizukami et al., Commun. Phys. 6, 183 (2023).

[6] K. Matsuura et al., Proc. Natl. Acad. Sci. USA 120, e2208276120 (2023).

[7] T. Nagashima, T. Hashimoto et al., preprint (2023).

[8] Z.-Y. Yu et al., Commun. Phys. 6, 175 (2023).

[9] T. Nagashima, K. Ishihara et al., preprint (2023).   

Superconducting vortices carrying a temperature-dependent fraction of the flux quantum

Vortices in superconductors typically carry exactly one quantum of magnetic flux. Here we use SQUID microscopy to investigate isolated vortices in the hole-overdoped Ba_1−xK_xFe₂As₂ (x = 0.77). In many locations, we observed vortices that carried only part of a flux quantum, with a magnitude that varied continuously with temperature. These features may be quantum vortices with non-universally quantized (fractional) magnetic flux whose magnitude is determined by the temperature-dependent parameters of a multiband superconductor.