In everyday life, we are familiar with heat being conducted through solid materials by diffusion, meaning that it spreads relatively slowly but steadily in all directions through random collisions. At very low temperatures and in extremely thin materials, heat can propagate in a wave-like, or coherent, manner, traveling in one direction at the characteristic speed of sound of the material. In this case, heat transport can be directly controlled through various wave phenomena, such as interference and resonance.
In this study, thermal conductivity was measured in a 300 nanometer thick silicon nitride membrane on which a so-called phononic crystal consisting of micrometer scale aluminum pillars was fabricated. A phononic crystal is a periodic structure that affects the propagation of phonons, the quanta of elastic vibrations (including those associated with heat). The measurements were performed near absolute zero, at temperatures below one kelvin, and the pillar-based phononic crystal was found to reduce the membrane's thermal conductivity by as much as a factor of ten. This reduction in thermal conductivity is based on a phenomenon known as local resonance. The measurements also agreed well with simulations based on coherent transport theory. However, as the size of the pillars was increased, it was observed that beyond a certain size, the experimental results no longer followed coherent transport theory, and heat transport became diffusive, or particle-like.
The significance of this study lies in the fact that it was the first experimental demonstration of controlling thermal conductivity through wave phenomena using pillar-based phononic crystals. Previously, this had only been achieved with structures in which the crystal was formed by holes patterned into a membrane.
One of the most promising applications of phononic crystals is improving the sensitivity of certain radiation detectors. If the thermal conductivity of the surroundings of a detector, which operates by measuring the temperature change caused by radiation, is reduced, it becomes possible to detect smaller temperature changes. This, in turn, makes it possible to detect lower-energy particles or smaller numbers of particles.
The public examination of Tatu Korkiamäki’s doctoral dissertation “Coherent and Incoherent Thermal Transport in Pillared Phononic Crystals at Low Temperatures” will be held on Friday, November 20, 2026 at 12 o'clock in the lecture hall KEM1 on Ylistönrinne. The Opponent is Professor Dr. Pierre-Olivier Chapuis (CETHIL - INSA de Lyon) and the Custos is Professor Ilari Maasilta (University of Jyväskylä). The defense will be held in English.
Further infromation: tatu.a.s.korkiamaki@jyu.fi
