Polarized neutron scattering fromCeCu2Si2in applied magnetic fields

Polarized neutron scattering from the heavy-fermion system ${\mathrm{CeCu}}_2$ ${\mathrm{Si}}_2$ has been measured in horizontal magnetic fields in the range 0–20 kG. The scattering due to the electron excitation from up (down) (relative to the magnetic field) spin states to the down (up) spin states has been observed separately at T=2.5 K. The difference between intensities for the two opposite spin-flip processes, $I_{\uparrow \downarrow }$-$I_{\downarrow \uparrow }$, agrees well with the value expected for a narrow resonance of width (HWHM) Γ∼1 meV at the Fermi level. This work introduces a new way of determining the very small energy scales for spin fluctuations in heavy-fermion systems.