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First of all I want to apologize if this is a stupid question. I'm a layman who's merely very interested in physics, without a degree to my name.

I was trying to research electric superconductors lately (in parts due to the news regarding LK-99, to nobody's surprise) which prompted a question in my head regarding the feasibility of thermal superconductors.

From my very limited understanding, thermal energy transfers occur either through the emission and absorption of photons given off an excited electron as it moves to a lower energy state, or electric field interactions of phonons. However, what I was wondering about was:

Since thermal energy transfer always moves in the direction from higher to lower energy states, what is the physical limitation that causes thermal superconductors not to be feasible (assuming they are not)?

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    $\begingroup$ This might help - Thermal conductivity of superconducting material $\endgroup$
    – mmesser314
    Commented Aug 3, 2023 at 14:36
  • $\begingroup$ @mmesser314 very interesting, thank you so much! $\endgroup$
    – user3106891
    Commented Aug 3, 2023 at 14:59
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    $\begingroup$ Have you looked at the thermal conduction of heat pipes? Not thermal superconductor.. but pretty close to one. $\endgroup$
    – Questor
    Commented Aug 4, 2023 at 0:51

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In an electrically insulating crystaline solid, heat is mostly transferred by phonons rather than by photons. What provides thermal resistance is phonon scattering via impurities or umklapp processes . Diamond has very little scattering (and isotopically pure diamond even less) and hence high thremal conductivity. This means that diamonds feel cold to the touch and are often called "ice". Even in the absence of impurities and crystal defects umklapp still occurs and prevents perfect thermal conductivity.

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  • $\begingroup$ I see, so the scattering causes the phonons to bounce back limiting the "flow" of thermal transfer? $\endgroup$
    – user3106891
    Commented Aug 3, 2023 at 14:57
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    $\begingroup$ Exactly. In the absence of backscattering you have ballistic transport. There is a nice book "Imaging Phonons: Acoustic Wave Propagation in Solids" by Jim Wolfe with pictures of what happens to low-energy phonons which are almost ballistic. $\endgroup$
    – mike stone
    Commented Aug 3, 2023 at 15:34
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    $\begingroup$ Thank you so much for your explanation and the book suggestion, mike! This is super helpful :) $\endgroup$
    – user3106891
    Commented Aug 3, 2023 at 16:26

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