URhGe

URhGe is a uranium-based heavy-fermion compound that exhibits the coexistence of itinerant ferromagnetism and superconductivity at ambient pressure, making it one of the notable ferromagnetic superconductors studied in condensed-matter physics.

Crystal structure and composition: URhGe crystallizes in an orthorhombic TiNiSi-type structure (space group Pnma). The material comprises uranium, rhodium, and germanium atoms arranged in a layered, anisotropic network that strongly influences its electronic and magnetic properties.

Magnetic properties: The compound orders ferromagnetically below a Curie temperature of about 9.5 K, with a relatively small ordered moment oriented along the crystallographic c-axis (the easy axis). The ferromagnetic state coexists with superconductivity at low temperatures, indicating a delicate balance between magnetic order and superconducting pairing.

Superconductivity: Superconductivity in URhGe appears below roughly 0.25 K and persists within the ferromagnetic phase. The coexistence suggests an unconventional pairing mechanism, with the prevailing interpretation favoring spin-triplet pairing enabled by strong ferromagnetic spin fluctuations and the internal exchange field.

Magnetic field effects and unusual phenomena: When a magnetic field is applied along the b-axis, ferromagnetism can be suppressed at low temperatures, while superconductivity can be enhanced, producing a field-induced or reentrant superconducting state at higher fields. This behavior highlights the intimate coupling between magnetic fluctuations and superconducting pairing in URhGe.

Research context: URhGe is part of a family of uranium-based ferromagnetic superconductors that includes UGe2 and UCoGe. It has been investigated through transport, specific-heat, magnetization, neutron scattering and related techniques to probe the interplay between magnetism and superconductivity in heavy-fermion systems.