We present measurements of the near-field heat transfer between the tip of athermal profiler and planar material surfaces under ultrahigh vacuumconditions. For tip-sample distances below 10-8 m our results differ markedlyfrom the prediction of fluctuating electrodynamics. We argue that thesedifferences are due to the existence of a material-dependent small length scalebelow which the macroscopic description of the dielectric properties fails, anddiscuss a corresponding model which yields fair agreement with the availabledata. These results are of importance for the quantitative interpretation ofsignals obtained by scanning thermal microscopes capable of detecting localtemperature variations on surfaces.
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