Superconductor UGe2 Research Articles

Magnetic form factor of UGe2 under high pressure

The magnetic form factor in the pressure-induced ferromagnetic superconductor UGe2 has been measured with polarized neutrons. The experiments were performed on single crystal samples at 2K by applying a magnetic field of 2T along the easy magnetization axis a under pressures of 0 and 1.4GPa. In spite of the itinerant nature of the 5f electrons, the shapes of the observed magnetic form factors at both 0 and 1.4GPa are very close to the theoretical 5f form factor of the free U3+ or U4+ ions calculated by the dipole approximation. The effect of pressure on the 5f form factor seems to be small in spite of the reduction of the magnetic moment.

Magnetic Properties of a Pressure-induced Superconductor UGe2

We performed the DC-magnetization and neutron scattering experiments under pressure {\it P} for a pressure-induced superconductor UGe$_2$. We found that the magnetic moment is enhanced at a characteristic temperature {\it T}$^{*}$ in the ferromagnetic state, where {\it T}$^{*}$ is smaller than a Curie temperature {\it T}$_{\rm C}$. This enhancement becomes remarkable in the vicinity of {\it P}$_{\rm C}^{*}$ = 1.20 GPa, where {\it T}$^{*}$ becomes 0 K and the superconducting transition temperature {\it T}$_{\rm SC}$ shows a maximum. The characteristic temperature {\it T}$^{*}$, which decreases with increasing pressure, also depends on the magnetic field.

Coexistence of superconductivity and ferromagnetism in the d-band metal ZrZn2

It has generally been believed that, within the context of the Bardeen-Cooper-Schrieffer (BCS) theory of superconductivity, the conduction electrons in a metal cannot be both ferromagnetically ordered and superconducting. Even when the superconductivity has been interpreted as arising from magnetic mediation of the paired electrons, it was thought that the superconducting state occurs in the paramagnetic phase. Here we report the observation of superconductivity in the ferromagnetically ordered phase of the d-electron compound ZrZn2. The specific heat anomaly associated with the superconducting transition in this material appears to be absent, and the superconducting state is very sensitive to defects, occurring only in very pure samples. Under hydrostatic pressure superconductivity and ferromagnetism disappear at the same pressure, so the ferromagnetic state appears to be a prerequisite for superconductivity. When combined with the recent observation of superconductivity in UGe2 (ref. 4), our results suggest that metallic ferromagnets may universally become superconducting when the magnetization is small.

Phenomenological theory of ferromagnetic superconductivity.

It is argued that the pairing symmetry realized in a ferromagnetic superconductor UGe(2) must be a nonunitary triplet pairing. This particular state is free from the Pauli limitation and can survive under a huge internal molecular field. To check our identification we examine its basic properties and several experiments are proposed. In particular, the external field is used to raise T(c) by controlling the internal spontaneous dipole field.