From Incommensurate Correlations to Mesoscopic Spin Resonance in $\mathrmYbRh_2\mathrmSi_2$
Spin fluctuations are reported near the magnetic field—driven quantum critical point in YbRh2Si2. On cooling, ferromagnetic fluctuations evolve into incommensurate correlations located at q0=±(δ,δ), with δ=0.14±0.04 r.l.u. At low temperatures, an in-plane magnetic field induces a sharp intradoublet resonant excitation at an energy E0=gμBμ0H with g=3.8±0.2. The intensity is localized at the zone center, indicating precession of spin density extending ξ=6±2 Å beyond the 4f site.