Physics

Research reveals that a quantum ‘umbilical cord’ connects the metal and insulator states of many materials

DFT+DMFT calculations for copper oxide waterfalls. Credit: Nature Communications (2025). DOI: 10.1038/s41467-024-55465-7

Some materials exhibit a kind of umbilical cord between different quantum states. Researchers at the Vienna University of Technology have shown that this “umbilical cord” is common to many materials.

This is the basic principle of quantum theory. Certain physical quantities may only assume very specific values. All values ​​in between are not allowed in physics. This fact plays a decisive role in the behavior of the material. Certain energy ranges are possible for the material’s electrons, while others are not. Among other things, this explains the difference between conductive metals and non-conductive insulators.

However, in some cases, surprising connections can occur between tolerance ranges, causing electrons to switch from one range to the other. One such unusual transition region was discovered in 2007 in certain copper-containing materials known as copper oxides.

Scientists at TU Wien (Vienna) were able to prove that these are not rare special cases. In fact, this effect always occurs if the interactions between the electrons are large enough. This means that an additional state exists between the metal and the insulator. The findings will be published in the journal Nature Communications.

Quantum jump between allowed energies

“An electron moving around the nucleus can only take on very specific energy values. Anything in between is prohibited. From one allowed energy value to another. You can only switch. This is known as a quantum jump,” says the professor. Karsten Held, from the Institute of Solid State Physics at the Vienna University of Technology. “For electrons in solids, it’s a little more complicated; instead of just specific energy values, whole energy ranges (we call them energy bands) are allowed.”

Both the electron’s energy and momentum (or speed) play a role here. Electrons can have different momentum values. In other words, its energy also changes, but only within a certain range. Moving from one allowed energy range to the next requires more additional energy.

Insulators and conductive metals

In an insulator, these allowed energy bands are separated from each other by a wide “forbidden” energy range. This prevents the electrons from switching from a low-energy band, where each electron remains bound to the nucleus, to a high-energy band, where it can move from atom to atom through the material. This means that all the electrons remain in place and no current can flow. On the other hand, in conductive materials, such forbidden energy regions do not exist, so electrons can move easily.

“How these allowed and forbidden energy bands are arranged depends on the material and, in particular, the strength of the electron interactions in that material,” Held says. The strength of this electronic interaction can be tuned by doping the material with a certain number of different types of atoms. This technology is routinely used in semiconductor manufacturing.

A new energy band is born and remains connected by an “umbilical cord”

If the strength of this interaction changes continuously, one allowed energy range can be split into two separate allowed energy ranges. “In this case, it’s particularly interesting to see what kind of structure arises here and what combinations of energy and momentum are possible from it,” Held says.

“In the process of separating into two permissible energy bands, we found that these two bands initially remained connected to each other by a kind of quantum umbilical cord,” Held says.

For most momentum values, the electron must make a decision. This means that electrons can only exist in either the upper or lower energy band. However, there is one momentum value that can have a wide range of energy values, and it connects both bands. This anomaly, in which there is one momentum value but many energy values, had been discovered in previous experiments, but the cause was initially unknown.

Held and Juraj Krusnik of the Vienna University of Technology believe that this phenomenon is not a special case, but that the “umbilical cord effect” always occurs when the strength of the interaction between electrons is within a certain range. succeeded in showing that. This means that additional state classes must be considered when classifying solids.

This is not new in solid state physics. For example, in 2016 the Nobel Prize in Physics was awarded for the so-called “topological states” of superconductors. This is another new set of states, which is also defined by a very special relationship between superconductors. Energy and momentum values.

Nevertheless, the results were quite surprising. “We were able to show very clearly that this umbilical cord-like connection must occur very naturally when one energy band separates from another,” Held says. “This opens up a whole new perspective on a technically very interesting class of materials and shows that there is more to materials science than previously thought between conductors and insulators. ”

Further information: Juraj Krsnik et al, Local correlation requires waterfall as connection between quasiparticle band and developing Hubbard band, Nature Communications (2025). DOI: 10.1038/s41467-024-55465-7

Provided by Vienna University of Technology

Citation: Quantum ‘umbilical cord’ connects metallic and insulating states of many materials, research results (January 20, 2025) https://phys.org/news/2025-01-quantum-umbilical-cord- Retrieved January 20, 2025 from links-metal.html

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