Near-field radiative heat transfer and noncontact friction

TLDR

Fluctuating electromagnetic fields arise from thermal and quantum current fluctuations in all bodies, and near a surface at sub‑thermal wavelengths these fields become coherent via surface modes, driving phenomena such as radiative heat transfer, van der Waals forces, and friction. The authors aim to formulate a general power‑spectral‑density formalism for the fluctuating electromagnetic field and apply it to analyze radiative heat transfer and van der Waals friction using semiclassical and quantum field‑theoretic approaches. They derive the formalism and compute the power spectral density for moving bodies, accounting for fields generated by static surface charges or bias voltages, and use it to evaluate heat transfer and friction. The calculations show that radiative heat transfer and van der Waals friction are strongly enhanced at sub‑thermal separations, especially when surface modes are present, and that electrostatic friction can be greatly increased by two‑dimensional electron systems or adsorbed ion layers, with implications for scanning probe spectroscopy and a tentative explanation of experimental noncontact friction data.

Abstract

All material bodies are surrounded by a fluctuating electromagnetic field because of the thermal and quantum fluctuations of the current density inside them. Close to the surface of planar sources (when the distance $d⪡{\ensuremath{\lambda}}_{T}=c\ensuremath{\hbar}∕{k}_{B}T$), thermal radiation can be spatially and temporally coherent if the surface can support surface modes like surface plasmon polaritons, surface phonon polaritons, or adsorbate vibrational modes. The fluctuating field is responsible for important phenomena such as radiative heat transfer, the van der Waals interaction, and the van der Waals friction between bodies. A general formalism for the calculation of the power spectral density for the fluctuating electromagnetic field is presented and applied to the radiative heat transfer and the van der Waals friction using both the semiclassical theory of the fluctuating electromagnetic field and quantum field theory. The radiative heat transfer and the van der Waals friction are greatly enhanced at short separations $(d⪡{\ensuremath{\lambda}}_{T})$ between the bodies due to the evanescent electromagnetic waves. Particularly strong enhancement occurs if the surface of the body can support localized surface modes like surface plasmons, surface polaritons, or adsorbate vibrational modes. An electromagnetic field outside a moving body can also be created by static charges which are always present on the surface of the body due to inhomogeneities, or due to a bias voltage. This electromagnetic field produces electrostatic friction which can be greatly enhanced if on the surface of the body there is a two-dimensional electron or hole system, or an incommensurate adsorbed layer of ions exhibiting acoustic vibrations. Applications of radiative heat transfer and noncontact friction to scanning probe spectroscopy are discussed. The theory gives a tentative explanation for the experimental noncontact friction data.