Magnetism and superconductivity in ferromagnetic heavy-fermion system UCoGe under in-plane magnetic fields

Abstract

We study the ferromagnetic superconductor UCoGe at ambient pressure under ab-plane magnetic fields H, which are perpendicular to the ferromagnetic easy axis. It is shown that, by taking into account the DyaloshinskiiMoriya interaction arising from the zigzag chain crystal structure of UCoGe, we can qualitatively explain the experimentally observed in-plane anisotropy for critical magnetic fields of the paramagnetic transition. Because of this strong dependence on the magnetic field direction, upper critical fields of superconductivity, which is mediated by ferromagnetic spin fluctuations, also become strongly anisotropic. The experimental observation of S-shaped H c2 b axis is qualitatively explained as a result of enhancement of the spin fluctuations due to decreased Curie temperature by the b-axis magnetic field. We also show that the S-shaped H c2 is accompanied by a rotation of the d vector, which would be a key to understand the experiments not only at ambient pressure but also under pressure.

Symmetry-Protected Line Nodes in Non-symmorphic Magnetic Space Groups: Applications to UCoGe and UPd2Al3

“…Theoretically, in UPd 2 Al 3 , it was discussed that a spin-singlet superconductivity with horizontal line nodes occurs via the virtual exchange of magnetic excitons [7,8]. In the ferromagnetic superconductors, it has been considered that the Ising-like ferromagnetic fluctuation can lead to a spin-triplet pairing state [9,10], and many fascinating phenomena including the odd H − T phase diagram have been studied [11][12][13][14][15][16]. However, in spite of the growing interest, the properties characteristic of the coexisting phase is less well understood systematically.…”

Pairing interaction in superconducting UCoGe tunable by magnetic field

“…Mineev discussed the triplet pairing triggered by the exchange of magnetic fluctuations with a phenomenological fluctuation spectrum [23][24][25]. Tada and Fujimoto showed that the experimentally observed anomalous H c2 behaviors against T and the angle in the bc plane are well reproduced by the theoretical model with appropriately taking into account the critical FM fluctuations observed in experiments [12,26,27]. Hattori and Tsunetsugu discussed the RSC in URhGe by strong attractive interactions generated by soft magnons in the Ising systems with transverse fields [28,29].…”

“…As mentioned above, we have shown the relationship between the Ising-type FM fluctuations and the superconductivity in UCoGe based on the 59 Co NMR measurements. From a theoretical perspective, a theoretical models of spin-triplet superconductivity observed in U-based ferromagnets were discussed by Mineev [23][24][25], and Tada and Fujimoto [12,26,27], Hattori and Tsunetsugu [28], independently. Mineev discussed the triplet pairing triggered by the exchange of magnetic fluctuations with a phenomenological fluctuation spectrum [23][24][25].…”

Non-unitary superconductivity in the monolayer of orthorhombic CoSb

“…2 onto the lowest two spin-dependent bands around the Fermi level 59,60 in the momentum space, and∆ 0 ( k) = ∆ 0∆ ( k) ⊗ iτ y represents the pairing potential with a pairing amplitude of ∆ 0 . In the tensor products, the first sector represents the spin channels σ =↑, ↓ shown in the caption of Table I while of 3 B 3u is energetically unfavorable rather than that of 3 B 2u , to avoid the short-range repulsion caused by the antiferromagnetic superexchange coupling along thex axis [60][61][62][63] , which can also been seen clearly in Fig. S3 in SI.…”