Low-Energy Magnetic Excitation of Kondo Semiconductor

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The energy gap in the magnetic excitation of the Kondo semiconductor CeNiSn was shown by NMR measurements. Complementary and more direct evidence of the energy gap can be obtained by neutron scattering, which is the purpose of the present investigation. However, the magnetic signal is very difficult to observe, because it is very weak and hidden in larger noise. To resolve this problem we planned to increase the intensity of a cold-guide triple-axis spectrometer by preparing a horizontally focusing analyzer which utilizes a larger solid angle. The geometry of the analyzer was optimized using a computer simulation. Test experiments of the analyzer showed that it gives a large gain factor of 14 in intensity compared to the previous collimator condition with the same energy resolution. Reduction of background is also important for weak signals, and this was achieved by inserting a shield between the analyzer and the detector. By these efforts of the focusing analyzer, the performance of the triple-axis spectrometer was substantially improved in terms of both intensity and noise. Using this instrument we measured low-energy magnetic excitation on a polycrystal-like sample which is made of an isotope ^<58>Ni. It has been becoming clear that the magnetic gap really exists in an energy range below 1.5 meV.Although the energy scale of 1.5 meV is larger than the value expected from specific heat and NMR by a factor two or three, the spectral shape and the temperature dependence is definitely consistent with the NMR results. We conclude that the gap of the magnetic excitation is proved by the present work.