Abstract: In this paper, the obtained experimental results concerning creation of bulk elementary excitations (BEEs) in isotopically pure liquid ⁴He at low temperatures ~60 mK are discussed. Positive rotons’ (R⁺-rotons) creation by a pulsed heater was studied. Signals were recorded for the following quantum processes: quantum evaporation of ⁴He-atoms from the free liquid-helium surface by the BEEs of the liquid helium-II, and BEEs reflection from the free surface back into the bulk liquid. Typical signals are shown, and ratios of signal amplitudes are evaluated. For long heater pulses from 5 to 10 μs, appearance of the second atomic cloud consisting of evaporated ⁴He-atoms was observed in addition to the first atomic cloud. It is thought that the first atomic cloud of the evaporated helium atoms consists of very fast ⁴He-atoms with energies ~35 K evaporated by positive rotons with the special energies ~17 K (~2E_R~2×8.6 K with E_R representing the roton minimum energy) corresponding to the third non-dispersive Zakharenko wave. The second cloud of slower ⁴He-atoms was created by surface elementary excitations (SEEs or ripplons) possessing the special energies ~7.15 K representing the binding energy. It was assumed that such SEEs can be created by phonons incoming to the liquid surface with special energies ~6.2 K corresponding to the first non-dispersive Zakharenko wave, which can interact at the liquid surface with the same phonons already reflected from the surface for long heater pulses. Also, some pulsed-heater characteristics were studied in order to better understand the features of such heaters in low temperature experiments.

Key words: Superfluid helium-II, Bulk elementary excitations (BEEs), Low temperatures, Cooper pairing phenomenon, Non-dispersive Zakharenko waves

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