Measurement of the Casimir force between a spherical gold tip and Si(111)-(7 x 7) surfaces

We have performed the measurement of Casimir force between a spherical Au tip and an atomically flat Si(111)-(7 x 7) surface at tip-sample distances ranging from 15 to 50nm in an ultrahigh vacuum of 1.5 x 10(-8)Pa by frequency-modulation atomic force microscopy. Atomically flat Si(111) surfaces provided by the ultrahigh-vacuum condition and a degassed Au tip reduce the contact potential difference that must be compensated. These experimental conditions led to the elucidation of the distance dependence of the Casimir force down to the distance of 15 nm. The observed distance dependence still follows a theory provided by Chen et al. [Phys. Rev. A 74, 022103 (2006)] within these distances. (C) 2016 The Japan Society of Applied Physic