Physicists have discovered a quantum "umbilical cord" linking metals and insulators.

According to quantum theory, electrons in atoms can only occupy strictly defined energy levels, with forbidden zones existing between them. In solids composed of numerous atoms, the situation becomes more complex: electrons form allowed and forbidden energy bands.

In metals, where electrons can move freely, the allowed and forbidden bands overlap. In insulators, however, wide forbidden zones block the movement of electrons, preventing the material from conducting electricity.

Both energy and momentum (or velocity) of electrons are important: an electron can take on various momentum values. This means its energy also varies, but only within a specific range. To transition from one allowed energy range to another, an electron requires additional energy from external sources.

The distribution of these energy bands depends on the material and the strength of the interactions between electrons. This interaction strength can be controlled by altering the material's composition through alloying — the introduction of impurities of a different type of atoms.

Researchers from the Vienna University of Technology (TU Wien) have discovered a universal phenomenon related to transitions between quantum states in solids. They found that when the interaction strength between electrons is altered, one allowed energy band can split into two.

However, during the splitting process, these two bands remain connected by a unique quantum "umbilical cord." In most cases, an electron must choose one of the bands, but there exists a particular momentum value for the electron where a wide range of energy values can connect both bands.

The "quantum umbilical cord" effect has been observed before, but its underlying reasons remained unclear. Now, scientists have managed to prove that this phenomenon is systematic and that the conditions under which the cord arises can be identified. The physicists' discovery introduces a new category of states in solids that lie between conductors and insulators. The scientific work has been published in the journal Nature Communications.

“Our results showed that the 'cord' should naturally arise when energy bands separate. This opens a new perspective on the properties of materials that could be useful for developing new technologies,” noted Professor of Physics Karsten Held (Karsten Held).