Near-field optical probes of quantum phase transitions in the Mott insulator Ca2RuO4
Many-body effects in correlated materials give rise to exciting quantum phenomena and phases of matter. In this thesis, I study calcium ruthenate (Ca2RuO4), a Mott insulator at room temperature, because it exhibits interesting magnetic, transport and structural properties. The delicate balance in its ground state is easily perturbed by temperature, pressure and small electric field (40 V/cm) which all induce an insulator-to-metal transition (IMT). However, distinctly different metallic phases in a temperature- or current-driven case are being stabilized.
One goal is to distinguish the different mechanisms driving the transition, especially to understand the current-driven IMT which remains to be fully explained. To this end I probe the changes in the optical properties of Ca2RuO4 during the current- and temperature-driven IMT using far-field infrared spectroscopy and scattering-type scanning near-field optical microscopy. Probing the nanoscale optical response throughout the transition allows for identifying the Mott insulating and metallic phases. I present imaging of the two different phases of Ca2RuO4 coexisting and evolving in a manner, characteristic to the driving mechanism. The distinct spectroscopic fingerprints of the temperature- and current-driven IMT were also explored.
ResearchGate | DPG Regensburg 2019
Anyone interested in the topic is most welcome to contact me. At the moment there isn’t much clarity on publishing openly the thesis itself, sorry. I remain confident any physicists stumbling upon this would recognise if it’s something they desperately need to read (ha-ha!). For everyone curious enough to get the gist of it, here is my attempt at a demystified version of the abstract (with some broader context).
Images of metal and insulator zones on the surface of a special crystal (with nanometre resolution)
In some unusual materials, the electrons of their atoms are acting as a team, which breaks the predictions of conventional physical models. As a result, these materials can do new and exciting things, like superconductivity (zero electrical resistance) or being a different kind of insulators (for a physical comparison to semiconductors see here). It is yet unclear how these effects are happening and what is possible inside the materials, so we need to investigate them closely for the advancement of new devices like resistive RAM, smart windows, magnetic levitation and whatnot.
I studied one such crystal (Ca2RuO4), which can switch from a special insulator to a metal. The switching is done by heating the crystal (in contrast, usually you need to cool to very low temperatures to make the material change) or by applying a small voltage (this is also very rare, even unique, and might find uses in electronics). When the crystal is switched, the change is not homogeneous, rather some small zones (a few nanometres) of metal and insulator are forming, a bit like at 0°C there can be ice crystals swimming in liquid water. Oh, but since no one had done the experiment before, actually we didn’t know if the change is homogeneous, if we’ll see any zones at all, let alone their shape and size. It was kind of a shot in the dark and much more dramatic—now I tell you all this with hindsight.
I used a scanning microscope capable of measuring areas of a few nm (AFM) together with a laser that “photographs” changes in the surface (for example, bright metal zones and dark insulating zones). Thus, I could take images of the crystal surface at different temperatures and different voltages. That way I could follow the switching of the material and its zones, compare how it looks before/after and see the difference between using heating and voltage. I did some additional stuff (like FTIR) to learn even more, but this is already going into detail.
In the end, are we any closer to memristors or levitation or grasping the magic of many-body quantum physics? Not really, but you know how the saying about Rome goes. And science is more like building Rome atom by atom.