2013/07/03 by R. T. Azuah, Azuah, R. T., S. O. Diallo +7
Physics and Astronomy · #FOS: Physical sciences #Other Condensed Matter (cond-mat.other) #cond-mat.other
paper · pdf · doi:10.48550/arxiv.1307.1052
8 Pages, 7 Figures, to appear in Phys. Rev. B
arxiv created 2013/07/03 · arxiv updated 2013/07/04
We present neutron scattering measurements of the phonon-roton (P-R) mode of superfluid 4He confined in 47 ÅMCM-41 at T = 0.5 K at wave vectors, Q, beyond the roton wave vector (QR =1.92 Å-1). Measurements beyond the roton require access to high wave vectors (up to Q = 4 Å-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≃3.6 Å-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, ωQ, increases with Q for Q > QR and reaches a plateau at a maximum energy Q = 2Δ where Δ is the roton energy, Δ = 0.74 ± 0.01 meV in MCM-41. This upper limit means the P-R mode decays to two rotons when its energy exceeds 2Δ. 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, ωQ would plateau at a lower energy, ωQ = 2ΔL where ΔL = 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 2Δ shows that the P-R mode and the layer modes are independent modes with apparently little interaction between them.