Dissipation of ripplon flow at the surface of superfluid 4He

Low-Temperature Mobility of Surface Electrons and Ripplon-Phonon Interaction in Liquid Helium

“…The surface (ripplons) and volume (phonons) quasiparticles of liquid helium interact only to the extent of a small compressibility of helium. The rate of the dominant process of momentum transfer at low temperatures (the phonon creation with the annihilation of two ripplons [7]) is proportional to T 20/3 [5]. Consequently, the ripplon subsystem in the Письма в ЖЭТФ T → 0 limit appears to be practically isolated from the liquid bulk and has to follow the regular motion of the electrons.…”

“…The electron momentum-relaxation time τ e is related to the SE mobility µ with respect to ripplons as τ e = m * µ/e (see below) and weakly depends on temperature in the range under consideration. According to the experimental data on the electron mobility [12] and effective mass [13], the electron-ripplon relaxation time at T = 70 ÷ 80 mK and n s = 1.3 × 10 9 cm −2 is τ e ∼ 5 × 10 −7 s. Thus, at T 0.1 K the ripplon-phonon momentum relaxation time τ RP = 8.8×10 −9 T −5 K 5 s [5] is much greater than τ e . Note, that the comprehensive kinetic calculation of τ e for free electrons is also based on the equilibrium of the electron and ripplon subsystem [1].…”

“…Temperature dependence of the SE mobility measured via the longitudinal ac conductivity σxx of the twodimensional system in the Corbino geometry at (•)ω/2π = 10 kHz, E ⊥ = 15 V/cm, ne = 1.4 × 10 7 cm −2 [9]; ( ) ω/2π = 10 kHz, E ⊥ = 92.5 V/cm, ne = 1.08 × 10 8 cm −2[8], and (•) via the damping of the mixed phonon-ripplon modes of a Wigner solid at ω/2π = 5 ÷ 10 MHz, ne = 1.3 × 10 9 cm −2[12]. The straight lines show the dc mobility(5) for the respective values of ne (from left to right in the order of increasing density). Some experimental points are omitted for better visibility.…”

Low-temperature mobility of surface electrons and ripplon-phonon interaction in liquid helium

“…The rate of the dominant process of momentum transfer at low temperatures (the phonon creation with the annihilation of two ripplons [7]) is proportional to T 20/3 [5]. Consequently, the ripplon subsystem in the T → 0 limit appears to be practically isolated from the liquid bulk and has to follow the regular motion of the electrons.…”

“…On the other hand, in the case of films, an important role is played by the surface smoothness, as the ripplons can be scattered not merely by phonons but also by surface imperfections caused by substrate defects (roughness). Such a one-body elastic scattering of ripplons has a much weaker temperature dependence than the ripplonphonon interaction and therefore dominates at low temperature [5]. Guaranteed exclusion of this one-body elastic channel of the ripplon flow dissipation imposes quite severe requirements on the surface smoothness over a large area.…”