Quantum theory states that certain physical quantities can only have specific values, and the values in between are not allowed. This principle is key to how materials behave. For example, electrons in a material can only have certain energy levels, which explains why metals conduct electricity and insulators do not.
Sometimes, unexpected connections can form between these allowed energy levels, letting electrons switch from one level to another. In 2007, scientists found such a transition in copper-containing materials called cuprates.
Scientists at TU Wien (Vienna) have shown that these connections are not rare. They occur when the interaction between electrons is strong enough. This means there’s an additional state between being a metal and an insulator.
They discovered that an “umbilical cord” exists between different quantum states in some materials, a common phenomenon in many materials.
Prof Karsten Held from TU Wien explained that electrons in an atom can only have specific energy values, and switching between these values is called a quantum jump. In solids, electrons can have entire ranges of energy values called energy bands.
Both the energy and speed of electrons are essential. Electrons can have different speeds, changing their energy, but only within a specific range. Electrons need a significant energy boost to jump from one allowed energy level to the next.
In insulators, energy bands are separated by a wide “forbidden” range, which prevents electrons from moving to higher energy levels and keeps them bound to their atomic nuclei. This prevents electric current from flowing. There is no forbidden range in conductive materials so that electrons can move freely.
The arrangement of these energy bands depends on the material and electron interactions, which can be adjusted by adding certain atoms, a technique used in semiconductor production.
If the strength of electron interaction changes gradually, one energy range can split into two separate ranges. Initially, these two bands stay connected by a quantum “umbilical cord.”
Held also mentioned that during the separation into two allowed energy bands, these two bands initially stay connected by a kind of quantum “umbilical cord.”
At most speeds, an electron must be in either the higher or lower energy band. However, an electron can have a wide range of energy values at one specific speed, connecting both bands. This anomaly has been observed before, but the cause is unclear.
Held and Juraj Krsnik from TU Wien showed that this “umbilical cord effect” happens when the interaction strength between electrons falls within a specific range. This discovery adds a new category of states in solid materials.
This is similar to a 2016 Nobel Prize-winning discovery of “topological states” in superconductors, defined by a specific relationship between energy and momentum.
Held found the results surprising. They show that the umbilical cord-like connection naturally occurs when one energy band splits from another. This discovery opens new perspectives on materials science, revealing more complexity between electrical conductors and insulators than previously thought.
Journal Reference:
- Juraj Krsnik et al, Local correlations necessitate waterfalls as a connection between quasiparticle band and developing Hubbard bands, Nature Communications (2025). DOI: 10.1038/s41467-024-55465-7
Source: Tech Explorist
