Which metals work badly together

Mystery solved: why good metals become "bad"

About 80 percent of all known elements are considered metals in chemistry, although there are of course flowing transitions at the edges. Liquid or solid metallic substances have four characteristics: they have a high thermal conductivity and metallic sheen, are malleable and usually have a high electrical conductivity. In the case of some metallic materials, however, even tiny changes in the chemical composition are sufficient to make a decision about conduction or insulation. Scientists know such metallic states with low electrical conductivity as "bad metals".

Where the electrical conductivity comes from can be explained with well-tested physical theories. Conventional theories, on the other hand, have not yet been able to break down what is behind the "bad metals". Viennese physicists are now showing, together with international colleagues, that "bad metals" are not that "bad" at all: If you look closely, their behavior fits in well with what we already knew about metals.

From conductor to insulator

Special metallic materials, small crystals grown in the laboratory, are currently the research interests of Andrej Pustogow and his working group at the Institute for Solid State Physics at the Vienna University of Technology. "These crystals can take on the properties of a metal, but if you vary the composition minimally, we are suddenly confronted with an insulator that no longer conducts electricity and is transparent like glass at certain frequencies," says Pustogow.

Directly at this transition one encounters an unusual phenomenon: the electrical resistance of the metal becomes extremely high - and indeed greater than it should be possible at all according to conventional theories. "Electrical resistance has to do with the fact that the electrons are scattered on each other or on the atoms of the material," explains Pustogow.

More resistance than possible

According to this approach, the greatest possible electrical resistance would have to be measured if the electron is scattered on its way through the material on every single atom - after all, there is nothing between an atom and its neighbor that could throw the electron out of its orbit. But this rule does not seem to apply to the "bad metals": They show a significantly 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 direct voltage, you only get a single number - the resistance for the frequency 0," says Andrei Pustogow. "On the other hand, we carried out optical measurements and used light waves with very different frequencies."

Tiny defects

It turned out that the "bad metals" are not that "bad" at all: At low frequencies they hardly conduct any electricity, but at higher frequencies they behave as one would expect of metals. The research team names tiny amounts of impurities or imperfections in the material as a possible cause, which can no longer be adequately shielded by a metal at the border to an insulator.

These defects can mean that some areas of the crystal no longer conduct electricity because the electrons remain localized there instead of moving further. If you apply a direct voltage to the material so that the electrons can migrate from one side of the crystal to the other, then practically every electron hits such an insulating region at some point, and electricity can hardly flow.

At a high alternating current frequency, on the other hand, every electron moves back and forth without interruption - it does not cover a long distance in the crystal because it keeps changing direction. This means that in this case many electrons do not come into contact with one of the insulating regions in the crystal, the researchers report in the journal "Nature Communications".

Mysterious "unconventional superconductivity"

"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 with known theories if they are adequately supplemented. Our measurement method shows where the additions are necessary."

The metallic behavior of materials in which there are strong correlations between the electrons is also particularly relevant for what is known as "unconventional superconductivity" - a phenomenon that was discovered half a century ago but is still not fully understood today. (red, 03/15/2021)