Coexistence of superconductivity and ferromagnetism in URhGe

Abstract

The discovery of superconductivity at high pressure (albeit over a restricted range) in the ferromagnetic material UGe2 raised the possibility that bulk superconductivity might be found in other ferromagnets. The exact symmetry of the paired state and the dominant mechanism responsible for the pairing, however, remain unidentified. Meanwhile, the conjecture that superconductivity could occur more generally in ferromagnets has been fuelled by the recent observation of a low-temperature transition that suggests an onset of superconductivity in high-quality crystals of the itinerant-ferromagnet ZrZn2 (ref. 2), although the thermodynamic signature of this transition could not be detected. Here we show that the ferromagnet URhGe is superconducting at ambient pressure. In this case, we find the thermodynamic signature of the transition-its form is consistent with a superconducting pairing of a spin-triplet type, although further testing with cleaner samples is needed to confirm this. The combination of superconductivity and ferromagnetism may thus be more common and consequently more important than hitherto realized.

Effect of Pressure on Magnetism of UIrGe

“…Of particular interest are compounds located close to the empirical Hill criterion, 7) which defines the limit of the distance of nearest uranium ions as dU-U  3.5 Å, at which 5f electrons change from a localized to itinerant character. Isostructural compounds URhGe and UCoGe, which crystallize in a TiNiSi-type structure naturally fulfill this condition, and the coexistence of ferromagnetism (FM) and SC has been discovered here 8,9) . Both FM and SC can be affected by alloying [10][11][12][13][14] , the application of a magnetic field 15,16) , and the external pressure [17][18][19] .…”

Ferromagnetic order in U(Rh, Co)Ge

“…Evidence is at hand, however, that in ferromagnetic superconductors an alternative pairing mechanism is at work: magnetic fluctuations associated with a magnetic quantum critical point mediate spin-triplet Cooper pairs [4]. The family of ferromagnetic superconductors is small: UGe 2 (under pressure) [5], URhGe [1], UIr (under pressure) [6] and UCoGe [2]. In these materials ferromagnetic order has a strong itinerant character.…”

“…The intermetallic compounds URhGe [1] and UCoGe [2] belong to the family of ferromagnetic superconductors, which attracts much attention. Metallic ferromagnetism is observed upon cooling to liquid helium temperatures, and at even lower temperatures superconductivity sets in, which coexists with ferromagnetic order.…”

Microscopic theory for a ferromagnetic nanowire/superconductor heterostructure: Transport, fluctuations, and topological superconductivity

“…36,41 An alternative approach to realizing a one-dimensional topological superconductor is to deposit a ferromagnetic semiconductor wire on top of a ferromagnetic superconductor. [1][2][3][4] A particularly attractive candidate ferromagnetic semiconductor is europium oxide (EuO), which is known to possess nearly spin-polarized bands. 85,86 EuO becomes ferromagnetic below 70K under ambient pressure and the Curie temperature is known to increase with pressure reaching 200K under 1.5 × 10 5 atmospheres.…”

“…Following the first experimental realization a decade ago there are now four such compounds, [1][2][3][4] two of which exhibit the phenomena at ambient pressures. 2,3 We propose that depositing URhGe electrodes on the EuO wire, in the arrangement shown in Fig. 1(a), would be conceptually the simplest structure to realize Majorana fermions that would require neither topological insulators nor control over spin-orbit couplings.…”