Letter abstract


Nature Materials 9, 26 - 30 (2010)
Published online: 8 November 2009 | doi:10.1038/nmat2568

Subject Categories: Semiconductors | Nanoscale materials

Quasi-ballistic thermal transport from nanoscale interfaces observed using ultrafast coherent soft X-ray beams

Mark E. Siemens1, Qing Li1, Ronggui Yang2, Keith A. Nelson3, Erik H. Anderson4, Margaret M. Murnane1 & Henry C. Kapteyn1

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Fourier theory of thermal transport considers heat transport as a diffusive process where energy flow is driven by a temperature gradient. However, this is not valid at length scales smaller than the mean free path for the energy carriers in a material, which can be hundreds of nanometres in crystalline materials at room temperature. In this case, heat flow will become ‘ballistic’—driven by direct point-to-point transport of energy quanta 1. Past experiments have demonstrated size-dependent ballistic thermal transport through nanostructures such as thin films, superlattices, nanowires and carbon nanotubes1, 2, 3, 4, 5, 6, 7, 8. The Fourier law should also break down in the case of heat dissipation from a nanoscale heat source into the bulk. However, despite considerable theoretical discussion and direct application to thermal management in nanoelectronics2, nano-enabled energy systems9, 10 and nanomedicine11, this non-Fourier heat dissipation has not been experimentally observed so far. Here, we report the first observation and quantitative measurements of this transition from diffusive to ballistic thermal transport from a nanoscale hotspot, finding a significant (as much as three times) decrease in energy transport away from the nanoscale heat source compared with Fourier-law predictions.

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  1. JILA, University of Colorado at Boulder, Boulder, Colorado 80309, USA
  2. Department of Mechanical Engineering, University of Colorado, Boulder, Colorado 80309, USA
  3. Department of Chemistry, MIT, Cambridge, Massachusetts 02139, USA
  4. Lawrence Berkeley Labs and Center for X-ray Optics, Berkeley, California 94720, USA

Correspondence to: Mark E. Siemens1 e-mail: siemens@colorado.edu



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