When we think about to what extent physics has changed our lives, the first examples that come to our minds are the transistor, that allowed processors, kick-starting the microelectronics industry, giant magnetoresistance for storing huge amounts of data, or the photoelectric effect that allowed solar energy conversion. All these effects appear at room temperature. Actually, this is a prerequisite for real-life applications. However, physicists usually look at phenomenas at much lower temperatures to understand physical properties of materials. Why is it needed? One often questions me why I do experiments at low temperatures, where I could do this at room temperature, where real life applications could exist — and at least, we would be able to survive such temperatures to observe the phenomena. For those interested in the photovoltaic effects, one may not care about what happens at 1K (~-274°C, -458°F), because we wouldn’t survive in such conditions. However, without low temperature physics, we may not have had experimental evidences of quantum physics theory, that is the basis of all the phenomenas I’ve cited so far.

Let’s first define what temperature is. To be short, we may think about individual atoms in a lattice, vibrating around their sites, occasionally colliding with their neighbours pushing them into activity with their excess of energy. The vibrational (and rotational) energy of the collection of particles is the temperature. It is then easy to understand that at low temperature, the movement of atoms becomes very weak. At the theoretical temperature of 0K (-273.15°C, -479.67°F), the atoms can’t vibrate. An interesting phenomena in physics is the understanding of transport. Trying to get the picture of electrons, excitons or quasiparticles flowing into a crystal is of uttermost importance in modern fundamental science.

Now, there are two ways to represent an electron being transported through a crystal. The first picture is the one of “fast projectiles” (as JM Ziman called them in his seminal “Electrons and phonons” book). In the crystal, an electron would penetrate the closely packed array of atomic hard spheres. “It is as if one could play cricket in the jungle” Ziman says. It cannot really propagate. Therefore, seeing the electron as a particle fails at explaining electronic conduction. On the other side, we may think about electrons as waves, flowing inside a crystal. Bloch states notably describe the fact the electron wave function may have the same periodicity as the one of the lattice, thus ensuring the electron flows inside a crystal without actually seeing it. If temperature is too high, the vibrating atoms would create scattering centres, on which the electrons would bump. Particularly, when the temperature becomes low enough, the probability to encounter scattering processes between atoms and electrons decreases, and the mean free path increases up to the dimensions of the macroscopic specimen.

Finally, looking at low temperature physics allows us to isolate the electron (or any charge carrier, that can be excitons, polaritons, etc.) behaviour, by reducing the vibrational noise (or the phonon modes) that prevents us from seeing quantum phenomena. At these temperatures, quantum phenomena can also naturally emerge, if the energy distribution of particles involved is very narrow. That is notably the case at low temperature where the width of the Fermi-Dirac distribution becomes narrower as the temperature decreases.

If you wanted a response to the question “how cold is low temperature?”, I’d respond that it depends on what you want to observe. Typically, Bloch oscillation can resist up to 200K (~-73°C, -100°F) in pure system, but in most of the cases, Aharonov-Bohm oscillations or Shubnikov-de Haas quantum oscillations do not persist up to 4K.