Wednesday, July 06, 2016

Keeping your (samples) cool is not always easy.

Very often in condensed matter physics we like to do experiments on materials or devices in a cold environment.  As has been appreciated for more than a century, cooling materials down often makes them easier to understand, because at low temperatures there is not enough thermal energy bopping around to drive complicated processes.  There are fewer lattice vibrations.  Electrons settle down more into their lowest available states.  The spread in available electron energies is proportional to \(k_{\mathrm{B}}T\), so any electronic measurement as a function of energy gets sharper-looking at low temperatures.

Sometimes, though, you have to dump energy into the system to do the study you care about.  If you want to measure electronic conduction, you have to apply some voltage \(V\) across your sample to drive a current \(I\), and that \(I \times V\) power shows up as heat.  In our case, we have done work over the last few years trying to do simultaneous electronic measurements and optical spectroscopy on metal junctions containing one or a few molecules (see here).   What we are striving toward is doing inelastic electron tunneling spectroscopy (IETS - see here) at the same time as molecular-scale Raman spectroscopy (see here for example).   The tricky bit is that IETS works best at really low temperatures (say 4.2 K), where the electronic energy spread is small (hundreds of microvolts), but the optical spectroscopy works best when the structure is illuminated by a couple of mW of laser power focused into a ~ 1.5 micron diameter spot.

It turns out that the amount of heating you get when you illuminate a thin metal wire (which can be detected in various ways; for example, we can use the temperature-dependent electrical resistance of the wire itself as a thermometer) isn't too bad when the sample starts out at, say, 100 K.  If the sample/substrate starts out at about 5 K, however, even modest incident laser power directly on the sample can heat the metal wire by tens of Kelvin, as we show in a new paper.  How the local temperature changes with incident laser intensity is rather complicated, and we find that we can model this well if the main roadblock at low temperatures is the acoustic mismatch thermal boundary resistance.  This is a neat effect discussed in detail here.  Vibrational heat transfer between the metal and the underlying insulating substrate is hampered (like \(1/T^3\) at low temperatures) by the fact that the speed of sound is very different between the metal and the insulator.   There are a bunch of other complicated issues (this and this, for example) that can also hinder heat flow in nanostructures, but the acoustic mismatch appears to be the dominant one in our case.   The bottom line:  staying cool in the spotlight is hard.  We are working away on some ideas on mitigating this issue.  Fun stuff.

(Note:  I'm doing some travel, so posting will slow down for a bit.)

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