Adsorbate induced enhancement of electrostatic non-contact friction

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We study the non-contact friction between an atomic force microscope tip and a metal substrate in the presence of bias voltage. The friction is due to energy losses in the sample created by the electromagnetic field from the oscillating charges induced on the tip surface by the bias voltage. We show that the friction can be enhanced by many orders of magnitude if the ads orbate layer can support acoustic vibrations. The theory predicts the magnitude and the distance dependence of friction in a good agreement with recent puzzling non-contact friction experiment Stipe. We demonstrate that even an isolated adsorbate can produce high enough friction to be measured experimentally.