1.4 Neutron Clarifies the Interplay of Superconductivity and Magnetism

Fig. 1-7

Temperature dependence of the neutron inelastic scattering profile in heavy fermion superconductor UPd2Al3, which contains information on magnetism. A peak appearing near delta E=0.4 meV below the superconducting transition temperature of 1.95 K was found for the first time in this experiment.

Fig. 1-8

The solid line shows the temperature dependence of the energy of the magnetic excitation gap and the broken line shows the temperature dependence of the superconducting energy gap expected from the BCS theory. The agreement between the temperature dependence of the two energy gaps indicates the strong interplay of magnetism and superconductivity.
It had once been considered that magnetism and superconductivity are not compatible with each other and, as a result, magnetic metals do not exhibit superconductivity. Later, however, heavy fermion superconductors were found, in which magnetism and superconductivity coexist. And now, the interplay of magnetism and superconductivity is one of the major issues in solid state physics.
A uranium compound, UPd2Al3, is one of the most typical heavy fermion supercon-ductors. UPd2Al3 exhibits antiferromagnetic ordering below the Neel temperature TN=14.5 K. Superconductivity occurs below the superconducting transition temperature Tc=1.95 K, where magnetism and super-conductivity coexist. Most of the previous neutron and magnetic X-ray scattering studies, however, have yielded no evidence of the interplay between magnetism and superconductivity in UPd2Al3. Therefore, magnetism and superconductivity have been believed to be independent in this material.
We had a particular interest in this material and performed a high-resolution neutron scattering experiment. The accuracy of the scattering intensity measurements was 0.1%, an order of magnitude better than previous ones. In addition, the sample was cooled to 0.3 K, far below Tc. As a result, we succeeded in observing an inelastic scattering peak with the excitation energy corresponding to the superconductivity gap at temperatures below Tc (Fig. 1-7). This is clear, direct evidence that magnetism and supercon-ductivity are strongly coupled in UPd2Al3 and is the first discovery in heavy fermion superconductors. The present results as well as a series of our other experimental results have shown that the superconductivity of this material is of magnetic origin.

Reference
N. Metoki et al., Superconducting Energy Gap Observed in the Magnetic Excitation Spectra of a Heavy Fermion Superconductor UPd2Al3, Phys. Rev. Lett., 80 (24), 5417 (1998).

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