Optical experiments on 'bad metals' at TU Wien. Photo: TU Wien.
Optical experiments on 'bad metals' at TU Wien. Photo: TU Wien.

The term 'metals' brings to mind solid, unbreakable objects that conduct electricity and exhibit a typical metallic sheen. The behaviours of classical metals, such as their electrical conductivity, can be explained with well-known, well-tested physical theories.

But there are also more exotic metallic compounds that pose riddles: some alloys are hard and brittle, while special metal oxides can be transparent. There are even materials right at the border between metal and insulator: tiny changes in chemical composition can turn the metal into an insulator – or vice versa.

In such materials, metallic states with extremely poor electrical conductivity can occur, referred to as 'bad metals'. Up until now, it seemed these 'bad metals' simply couldn't be explained with conventional theories, but new measurements, reported in a paper in Nature Communications, show that these metals are not so 'bad' after all. Upon closer inspection, their behavior fits in perfectly with what we already knew about metals.

Andrej Pustogow and his research group at the Institute for Solid State Physics at the Vienna University of Technology (TU Wien) in Austria are conducting research into special metallic materials – small crystals that have been specially grown in the laboratory. "These crystals can take on the properties of a metal, but if you vary the composition just a little bit, we are suddenly dealing with an insulator that no longer conducts electricity and is transparent like glass at certain frequencies," says Pustogow.

Right at this transition, an unusual phenomenon occurs: the electrical resistance of the metal becomes extremely large – larger, in fact, than should be possible according to conventional theories.

"Electrical resistance has to do with the electrons being scattered by each other or by the atoms of the material," explains Pustogow. According to this view, the greatest possible electrical resistance should occur if an electron is scattered by every single atom on its way through the material – after all, there is nothing between an atom and its neighbour that could throw the electron off its path. But this rule does not seem to apply to so-called 'bad metals': they show a much higher resistance than this model would allow.

The key to solving this puzzle is that the material properties are frequency dependent. "If you just measure the electrical resistance by applying a DC voltage, you only get a single number – the resistance at zero frequency," says Pustogow. "We, on the other hand, made optical measurements using light waves with different frequencies."

This showed that the 'bad metals' are not so 'bad' after all. At low frequencies, they hardly conduct any current, but at higher frequencies they behave as one would expect from metals. The research team proposes that tiny amounts of impurities or defects in the material are a possible cause. These defects cause some areas of the crystal to stop conducting electricity, because in those areas the electrons remain localized in a certain place instead of moving through the material. If a DC voltage is applied to the material so that the electrons move from one side of the crystal to the other, then virtually every electron will eventually hit such an insulating region and current will hardly flow.

At high AC frequencies, on the other hand, every electron moves back and forth continuously – it does not cover a long distance in the crystal because it keeps changing direction. This means that, in this case, many electrons never come into contact with one of the insulating regions in the crystal.

"Our results show that optical spectroscopy is a very important tool for answering fundamental questions in solid-state physics," says Pustogow. "Many observations for which it was previously believed that exotic, novel models had to be developed could very well be explained by existing theories if they were adequately extended. Our measurement method shows where the additions are necessary." In earlier studies, Pustogow and his international colleagues gained important insights into the boundary region between metal and insulator using spectroscopic methods.

The metallic behavior of materials subject to strong correlations between electrons is also particularly relevant for so-called 'unconventional superconductivity' – a phenomenon that was discovered half a century ago but is still not fully understood.

This story is adapted from material from TU Wien, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.