Review of U-based Ferromagnetic Superconductors: Comparison between UGe2, URhGe, and UCoGe

Abstract

The discovery in 2000 that the ferromagnetic (FM) compound UGe2 (T_Curie = 52 K at ambient pressure) becomes superconducting under a pressure of P = 1.1 GPa until it enters the paramagnetic (PM) phase above Pc = 1.6 GPa was a surprise. Successive searches for new materials (URhGe and UCoGe) led to the discovery of the coexistence of superconductivity (SC) and ferromagnetism at ambient pressure. Furthermore in UCoGe, it was found that SC survives in the PM regime from P_c = 1.1 to 4 GPa. Focus has been on low-temperature experiments under extreme conditions of magnetic field (H), pressure, and uniaxial stress. In UGe2, strong interplay exists between Fermi surface (FS) reconstructions in the cascade of different FM and PM ground states and their magnetic fluctuations. Similar phenomena occur in URhGe and UCoGe but, at first glance, the SC seems to be driven by the FM fluctuations. In UCoGe, a longitudinal field scan leads to a drastic decrease in the FM fluctuations, while a transverse field scan leads to suppression of the Curie temperature, T_Curie; the consequence is a boost in FM fluctuations, leading to a reinforcement of SC. The singularity in URhGe is the weakness of the anisotropy between c- and b-axes; the most noteworthy feature is the detection of reentrant SC near H_R. All the experimental results give evidence that the SC in these three materials originates from the FM fluctuations, which are amplitude modes of magnetic excitations in the FM state. Spin-triplet pairing has been anticipated in the FM superconductors and was actually observed by Knight-shift measurements in the SC state of UCoGe. Their fascinating (p, T, H) phase diagrams are now well established. Discussion is presented on how different theoretical approaches can describe the various phenomena discovered by experimentalists.

30 pages, 59 figures, accepted for publication in J. Phys. Soc. Jpn as an invited review paper

References in corpus (26)

• Superconductivity on the border of weak itinerant ferromagnetism in UCoGe
• Tricritical point and wing structure in the itinerant ferromagnet UGe2
• Unusual upper critical field of the ferromagnetic superconductor UCoGe
• Quantum Oscillations in Hole-Doped Cuprates
• Field Re-entrant Superconductivity Induced by the Enhancement of Effective Mass in URhGe
• Anisotropic magnetic fluctuations in the ferromagnetic superconductor UCoGe studied by angle-resolved ^{59}Co NMR
• Topologically stable gapless phases in nonsymmorphic superconductors
• High-field moment polarization in the ferromagnetic superconductor UCoGe
• Properties of Ferromagnetic Superconductors
• Symmetry protected line nodes in non-symmorphic magnetic space groups: Applications to UCoGe and UPd${}_2$Al${}_3$
• Collapse of ferromagnetism and Fermi surface instability near reentrant superconductivity of URhGe
• Lifshitz Transitions in the Ferromagnetic Superconductor UCoGe
• Absence of Meissner State and Robust Ferromagnetism in the Superconducting State of UCoGe: Possible Evidence of Spontaneous Vortex State
• Dimensionality driven enhancement of ferromagnetic superconductivity in URhGe
• Reentrant superconductivity in URhGe
• Quantum Criticality in the Ferromagnetic Superconductor UCoGe under Pressure and Magnetic Field
• Wing structure in the phase diagram of the Ising Ferromagnet URhGe close to its tricritical point investigated by angle-resolved magnetization measurements
• Relationship between Ferromagnetic Criticality and the Enhancement of Superconductivity Induced by Transverse Magnetic Fields in UCoGe
• Fermi Surface Instabilities in Ferromagnetic Superconductor URhGe
• Itinerant magnetism in URhGe revealed by angle resolved photoelectron spectroscopy
• Pressure Induced Quantum Phase Transitions
• Superconductivity in Uranium Compounds
• Spatially Inhomogeneous Superconducting State near $H_{c2}$ in UPd${}_2$Al${}_3$
• Low magnetic field phase diagram of UCoGe
• Pairing gaps near ferromagnetic quantum critical points
• Superconductivity in the Ferromagnet URhGe under uniaxial pressure

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