Excitations and their temperature dependence in superfluid 4He beyond the roton

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The temperature dependence of the dynamic structure factor S(Q,ω) for superfluid 4He has been measured by inelastic neutron scattering for wave vectors between 2.3 and 2.6 Å−l. S(Q,ω) has two peaks: one sharp peak at low energies whose dispersion flattens out and whose strength decreases with increasing Q, and one broader peak at higher energies with a stronger dispersion. The first peak disappears gradually with increasing temperature, while only part of the second peak vanishes at Tλ. This indicates the existence of a third broad contribution, related to atoms above the Bose condensate. The two-peak structure can be interpreted in terms of a Bose-condensate induced coupling of the two-particle spectrum to the one-particle spectrum. The overall temperature dependence is consistent with the density-quasiparticle picture.

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