The idea that the crystalline structure plays a large role in the bulk thermal conductivity of the material is kind of mind-blowing at first and then retrospectively obvious.
Let's see how well I can explain this (haven't read the article, yet, sorry! Waiting for a plane...)
So you're no doubt familiar with the physics of a vibrating string; it resonates at wavelengths (length of string, 2 * length of string, 3 * length of string... n as n->inf). So you can express any vibrational state of the string as sum(intensity * wavelength); so you can represent the state of the string as a vector of intensities on a basis of allowable vibrations of the string.
Let's call these _vibrational modes_. Let's assume occupancy of these modes is quantized. It's (sort of...) the same as energy levels of atomic orbitals in electronic structure, if you remember that from chemistry classes; the way it's not the same is important (bosons vs fermions) but not at this level of handwaving :-)
So this is how solids store energy, and we call this energy "heat".
A reasonable approximation for a crystalline structure is balls – point masses – connected by springs, where the springs are covalent bonds, plus electrostatic effects between point charges. Intuitively, you can follow that the same kind of _vibrational spectrum_ will arise from this arrangement (in the same kind of way; the solutions of the differential equation of this system of forces under periodic boundary conditions). So materials have resonant frequencies in the same way guitar strings do.
Therefore, this vibrational spectrum defines the thermal behavior of a material; heat capacity, thermal conductance, etc etc etc etc. Each of these vibrational modes is also tied to a collective motion of the particles in the material, which (if sufficiently violent) will tear the structure apart – there's the solid-to-liquid phase transition – or, more subtly, if lost will lower the symmetry of the crystal structure, which gives rise to solid-to-solid phase transitions (an example would be alpha to beta quartz, which will crack your crockery if you leave it in the oven on a cleaning cycle; https://en.wikipedia.org/wiki/Quartz_inversion).
There's a lot of depth here, as you can imagine!
https://www.youtube.com/watch?v=CiHN0ZWE5bk
The incoming light wave causes an avalanche of secondary waves through electromagnetically perturbing the individual atoms, and the superposition of all these results in a wave that seems to travel slower than the speed of light.
Thanks! This is a great analogy.
Also, (and I may be totally wrong about this!), the concept of Annealing from metallurgy -- seems related to that of Quartz Inversion:
Any suggestion to read ?
To you the simpleness of the final result is surprising, but that simple result was discovered after a long and exhaustive search. A search into a wide and shallow space can be just as impressive and difficult as a search into a narrow and deep space.
Engineer here btw.
We don’t have good theory for materials science. Our understanding is closer to a list of observations (“effects”) than anything unifying. That almost ensures there will be surprises in the gaps.
At the same time, popular disillusionment with repeated claims of wonder materials has led to—in my opinion—underinvestment in basic research.