Physical Review B, Volume 62, 9621 (2000)

4He liquid-vapor interface below 1 K studied using x-ray reflectivity
Konstantin Penanen, Masafumi Fukuto, Ralf K. Heilmann, Isaac F. Silvera, and Peter S. Pershan

The free surface of thin films of liquid helium adsorbed on a solid substrate has been studied using x-ray reflectivity. The film thickness and interfacial profile are extracted from the angular dependence of measured interference between signals reflected from the liquid-vapor and liquid-substrate interfaces. Polished silicon wafers, chemically cleaned and passivated, were used as substrates. Results are reported for measurements for $^4$He films 35 to 130 {\AA} thick in the temperature range 0.44 to 1.3 K. The 10\%/90\% interfacial width for temperature T=0.45 K varies from $5.3\pm0.5$ {\AA} for $36\pm1.5$ {\AA} thick films to $6.5\pm0.5$ {\AA} for $125\pm1.5$ {\AA} thick films. The profile width at zero temperature should not differ significantly from that measured at T=0.45 K. For T=1.22 K, the width is $7.8\pm1.0$ {\AA}.

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