Stock, C.; Broholm, C.; Zhao, Y.; Demmel, F.; Kang, H.J.; Rule, K.C.; Petrovic, C.: Magnetic Field Splitting of the Spin Resonance in CeCoIn5. Physical Review Letters 109 (2012), p. 167207/1-5
10.1103/PhysRevLett.109.167207

Abstract:
Neutron scattering in strong magnetic fields is used to show the spin resonance in superconducting CeCoIn5 (Tc ¼ 2:3 K) is a doublet. The underdamped resonance (@ ¼ 0:069  0:019 meV) Zeeman splits into two modes at E ¼ @0  B0H with  ¼ 0:96  0:05. A linear extrapolation of the lower peak reaches zero energy at 11:2  0:5 T, near the critical field for the incommensurate ‘‘Q phase.’’ Kenzelmann et al. [Science 321, 1652 (2008)] This, taken with the integrated weight and polarization of the low-energy mode (E), indicates that the Q phase can be interpreted as a Bose condensate of spin excitons.