Superconductivity on the border of itinerant-electron ferromagnetism in UGe2

Abstract

The absence of simple examples of superconductivity adjoining itinerant-electron ferromagnetism in the phase diagram has for many years cast doubt on the validity of conventional models of magnetically mediated superconductivity. On closer examination, however, very few systems have been studied in the extreme conditions of purity, proximity to the ferromagnetic state and very low temperatures required to test the theory definitively. Here we report the observation of superconductivity on the border of ferromagnetism in a pure system, UGe2, which is known to be qualitatively similar to the classic d-electron ferromagnets. The superconductivity that we observe below 1 K, in a limited pressure range on the border of ferromagnetism, seems to arise from the same electrons that produce band magnetism. In this case, superconductivity is most naturally understood in terms of magnetic as opposed to lattice interactions, and by a spin-triplet rather than the spin-singlet pairing normally associated with nearly antiferromagnetic metals.

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.…”

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

“…For example, the system may be threedimensional in terms of single-electron diffusion physics, but fall into the category of one-dimensional superconductors when it comes to the fluctuation analysis. We believe that such is the case with the W electrodes that host superconductivity and most of the fluctuation physics in the experiment under study involving L (1) ⊥ ∼ 250nm wide and L (2) ⊥ ∼100nm thick W strips.…”

“…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.…”

Coexistence of Superconductivity and Ferromagnetism in Intermetallic Uranium-Based Superconductors UCoGe, UIr and UGe2

“…Since the discovery of superconductivity in itinerant ferromagnet UGe2 under pressure in a limited range (1 ≤ P ≤ 1.5 GPa) below 1 K, a lot of interest has been generated in the study of coexistence of these two cooperative phenomena of superconductivity and ferromagnetism [1] which have been considered until very recently hostile and incompatible.…”

“…well below the Curie temperature T C , without expelling magnetic order. The superconducting ferromagnets discovered hitherto are UGe2 (under pressure) [7], URhGe [8], UIr (under pressure) [9], and UCoGe. In these uranium intermetallics magnetism has a strong itinerant character and both ordering phenomena are carried by the same 5f electrons.…”