Coexistence of superconductivity and magnetism in spin-fermion model of ferrimagnetic spinel in an external magnetic field

Abstract

A two-sublattice spin-fermion model of ferrimagnetic spinel, with spin-1/2 itinerant electrons at the sublattice A site and spin-s localized electrons at the sublattice B site is considered. The exchange between itinerant and localized electrons is antiferromanetic. As a result the external magnetic field, applied along the magnetization of the localized electrons, compensates the Zeeman splitting due to the spin-fermion exchange and magnon-fermion interaction induces spin anti-parallel p-wave superconductivity which coexists with magnetism. We have obtained five characteristic values of the applied field (in units of energy) Hcr1 < H3 < H0 < H4 < Hcr2. At H0 the external magnetic field compensates the Zeeman splitting. When Hcr1 < H < Hcr2 the spin antiparallel p-wave superconductivity with T1u configuration coexists with magnetism. The superconductor to normal magnet transition at finite temperature is second order when H runs the interval (H3, H4). It is an abrupt transition when Hcr1 < H < H3 or H4 < H < Hcr2. This is proved calculating the temperature dependence of the gap for three different values of the external magnetic field Hcr1 < H < H3, H4 < H < Hcr2 and H = H0. In the first two cases the abrupt fall to zero of the gap at superconducting critical temperature shows that the superconductor to normal magnet transition is first order. The Hubbard term (Coulomb repulsion), in a weak coupling regime, does not affect significantly the magnon induced superconductivity. Relying on the above results one can formulate a recipe for preparing a superconductor from ferrimagnetic spinel: i) hydrostatic pressure above the critical value of insulator-metal transition. ii) external magnetic field along the sublattice magnetization with higher amplitude.PACS numbers: 75.50. Gg,74.20.Mn,74.20.Rp Electron-phonon mechanism of superconductivity [1-4] has been developed to explain the pairing in a large variety of materials, from Hg and Al to recently discovered M gB 2 [5]. The discovery of superconductivity in LaBaCuO [6] and in other cuprates, as well as discovery of superconductivity in F e-based pnictides [7] established another direction of research in this field. The bosonfermion models still remained respected but bosons are not lattice vibrations.Alternatively, the possibility of an electronic pairing mechanism in systems with rotational invariance was put forward in a seminal paper by . Although the bare interaction among electrons is repulsive, there is an effective attractive interaction that arise at higher order of perturbation theory. The KohnLuttinger instability of a three-dimensional rotationally invariant system results in the formation of a unconventional superconducting ground state due to the peak in the particle-hole susceptibility near zero wave vector. The works [11][12][13][14][15] have made significant progress in our understanding of superconductivity from repulsive interaction.There are many experiments addressing external magnetic field induced, enhanced or reentered superconductivity. Expe...

Superconductivity induced by interfacial coupling to magnons

“…Paramagnonic [16] or magnonic [17] coupling may explain experimental observations of superconductivity coexisting with ferromagnetism in bulk materials [18][19][20]. Paramagnons [16,21] and magnons [17,22] are predicted to mediate triplet p-wave pairing with equal and antiparallel spins, respectively.…”