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Register a new userPhonon-roton modes of liquid 4He beyond the roton in MCM-41
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We present neutron scattering measurements of the phonon-roton (P-R) mode of superfluid 4He confined in 47 AA MCM-41 at T = 0.5 K at wave vectors, Q, beyond the roton wave vector (Q_R =1.92 AA$^{-1}$). Measurements beyond the roton require access to high wave vectors (up to Q = 4 AA$^{-1}$) with excellent energy resolution and high statistical precision. The present results show for the first time that at T = 0.5 K the P-R mode in MCM-41 extends out to wave-vector Q$simeq$3.6 AA$^-1$ with the same energy and zero width (within precision) as observed in bulk superfluid 4He. Layer modes in the roton region are also observed. Specifically, the P-R mode energy, $omega_Q$, increases with Q for Q > QR and reaches a plateau at a maximum energy Q = 2{Delta} where {Delta} is the roton energy, {Delta} = 0.74 $pm$ 0.01 meV in MCM-41. This upper limit means the P-R mode decays to two rotons when its energy exceeds 2{Delta}. It also means that the P-R mode does not decay to two layers modes. If the P-R could decay to two layer modes, $omega_Q$ would plateau at a lower energy, $omega_Q$ = 2{Delta}L where {Delta}L = 0.60 meV is the energy of the roton like minimum of the layer mode. The observation of the P-R mode with energy up to 2{Delta} shows that the P-R mode and the layer modes are independent modes with apparently little interaction between them.
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