Conventional mechanisms for exotic superconductivity

Abstract

We consider the pairing state due to the usual BCS mechanism in substances of cubic and hexagonal symmetry where the Fermi surface forms pockets around several points of high symmetry. We find that the symmetry imposed on the multiple pocket positions could give rise to a multidimensional nontrivial superconducting order parameter. The time reversal symmetry in the pairing state is broken. We suggest several candidate substances where such ordering may appear, and discuss means by which such a phase may be identified.Most conventional superconductors are described very well by the BCS theory [1]. The electron-phonon interaction mediates an attraction between electrons that is stronger than the Coulomb repulsion. This gives rise to the Cooper instability of the normal state leading to the appearance of a condensate of pairs. The order parameter (the anomalous function F [2]) in this case belongs to the "s-wave" type, i.e., it is invariant with respect to the transformations of G ⊗ R, where G is the crystal point group and R is time reversal operation. As a result the quasiparticle spectrum has a gap, which leads to well-known experimental consequences. We show below that exotic superconductivity can be a much more common phenomenon and does not require unusual mechanisms. The electron-phonon and Coulomb interactions are enough to give rise to an multidimensional order parameter which would have lower symmetry than the normal state -including the breaking of timereversal invariance. The effects we consider are possible in metals with several pockets which are centered at or around some symmetry points of the Brillouin Zone (BZ). A BCS approximation generalized to the the multi-band case (see, e.g., [12]) will be used. The new point here is that since the form of the interaction parameters describing the two electron scattering on and between the different pockets of the Fermi surface (FS) is fixed by symmetry, the resulting superconducting state need not be s-wave. Below we consider three cases in detail: a) three FS pockets centered about the X-points of a simple cubic lattice; b) three FS pockets at the M-points of the hexagonal lattice; c) four FS pockets at the L-points in the face-centered cubic lattice. A complete analysis of all other high symmetry points is possible, and will be published elsewhere [13].We emphasize that this FS structure is not unusual. Indeed, superconductivity with pockets as in case (a) and T c ∼ 0.1K is found in

Evolution of the neutron resonances inAFe2Se2

“…Within spin fluctuation theory, the consequences of this Fermi surface topology was already discussed by Kuroki et al, 5 who pointed out that this band structure is a 2D version of "Agterberg-BarzykinGor'kov" nodeless d-wave superconductivity 6 , since the symmetry-enforced nodal lines fall between the Fermi surfaces. This idea was adopted in the context of the new systems by several authors.…”

Mechanism for a pairing state with time-reversal symmetry breaking in iron-based superconductors

“…A recent theoretical proposal suggests that the system can resolve the frustration by a pairing state with s + id pairing symmetry which spontaneously breaks time-reversal (TR) symmetry [6]. The possibility of a TR-symmetry breaking pairing state due to frustrating pairing interactions among three or more Fermi pockets has also been investigated in several other contexts [7][8][9][10][11][12][13]. In general, time reversal breaking pairing states have rather accessible experimental signatures, and several proposals have been suggested in the context of iron based SC [6].…”

Josephson systems based on ballistic point contacts between single-band and multi-band superconductors

“…In this regard, there appeared models based on the assumption of a multi-component chiral structure of the order parameter with the symmetry of the with the symmetry of types s+id [15], s ± +is ++ [16], or with conventional s-wave symmetry with three or more gaps. Under certain conditions, the presence of chirality leads to the appearance of frustration, when the time-reversal symmetry in a superconductor is broken [17][18][19][20][21][22][23][24][25][26]. This means that the phases of the order parameter cannot simultaneously satisfy the minimum energy condition, hence creating a two or more fold degenerate ground state.…”