Strange material state connects insulator and metal
In some materials, electrons switch between energy ranges that are actually strictly separated from each other. Now it turns out that this behaviour is not an exotic special case, but occurs quite naturally and points to a new class of materials.
Materials are usually either metal or insulator, which means that the charge carriers can move freely in them or not. Although the electrons sometimes appear to be in a mysterious intermediate region, such observations were considered anomalies that were difficult to explain. As the solid-state physicists Juraj Krsnik and Karsten Held from TU Wien report, such transition states are by no means outsiders. On the contrary, they occur regularly. According to the researchers, this reveals a whole new class of materials.
It depends on the energy of the electrons whether they flit around between the atoms of the material or whether they remain bound to individual atoms. This happens in areas with certain possible energies, known as energy bands. The charge carriers then also have typical speeds or momentum there. States in between are forbidden - actually.
However, some substances showed a strange connection around 20 years ago: At certain, narrowly defined values for the momentum, more than just one energy level is suddenly open to the electrons; they can then reach both the higher and the lower energy band. A narrow "waterfall" appears to bridge the two. The process occurs in oxides of so-called transition metals, such as cuprates and nickelates, which have also been attracting attention for years as high-temperature superconductors. Experts have developed various complicated explanations, such as couplings to hidden particles or quasiparticles in the material.
Krsnik and Held investigated the phenomenon in more detail. To do so, they modelled the interactions between the electrons and in the material in detail and compared their theoretical results with the results of past experiments. Their surprising realisation: the process is not as unusual as they thought. On the contrary, it even seems to occur regularly - whenever one energy band splits off from another. In their publication, the physicists therefore coined the metaphor of an umbilical cord instead of a waterfall, which continues to connect the mother and daughter bands for a while after the birth of an energy band.
In any case, her explanation brings the phenomenon closer to everyday life, not only conceptually but also physically. It does not require any exotic effects, but merely depends on the interaction between the electrons. This points to another class of states in solids, as Held states in a press release: "There is more to materials science between electrical conductors and insulators than previously thought after all."
Spectrum of science
We are partners of Spektrum der Wissenschaft and want to make well-founded information more accessible to you. Follow Spektrum der Wissenschaft if you like the articles.
Original article on Spektrum.de
Experts from science and research report on the latest findings in their fields – competent, authentic and comprehensible.