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New quantum states on the surface of a mysterious material

New quantum states on the surface of a mysterious material

The study of free electrons on the surface of simple and known metals, such as copper or gold, has provided spectacular images of the phenomenon of interference and quantum wells.

Now, an international collaboration led by the Autonomous University of Madrid (UAM), which includes researchers from Colombia, Spain, France, Japan and Sweden, has investigated this field, but with a new material: the superconductor silicide of ruthenium and uranium (URu2Si2). .

A superconducting material made of silicon, ruthenium and uranium allowed to demonstrate for the first time the quantization of electrons in heavy fermions, with an electronic arrangement of unknown origin

The observations, published this month in the journal Naturedemonstrate for the first time the phenomenon of electron quantization in heavy fermions (a type of elementary particle) and, furthermore, reveal a type of electronic ordering whose origin is totally unknown.

For more than three decades, it has been possible to obtain atomically flat surfaces on simple metals. In the steps that can form between these surfaces, electrons can become trapped and are very sensitive to any change. For example, an atomic-sized step constitutes an important barrier to its propagation. The researchers focused on them.

“On a step between two steps, electrons can hardly escape and show their wave nature in full splendor. This gives rise to quantized states, which are characterized by the formation of waves whose distance between the maximums varies with the energy in integer multiples of a certain value”, explains Hermann Suderow, from the Department of Condensed Matter Physics at UAM.

“To date, these states have only been studied in systems in which electrons behave almost as if they were independent particles. In quantum materials, electrons are not independent and, as a consequence of entanglement due to their interactions, they carry heavy armor with them”, clarifies co-author Isabel Guillamón, from the same department.

The mysterious hidden order

In turn, another of the UAM authors, Alfredo Levy Yeyati, adds: “Our results clearly show highly correlated quantized states of electrons and we demonstrate the interaction between these states and superconductivity. The theoretical calculations supporting the observations suggest that these states can be seen in many other quantum materials as well. Furthermore, they provide new experimental evidence that would serve to understand the origin of the mysterious occult order, showing the absence of a certain symmetry near the surface.”

When scientists talk about hidden order refer to an enigmatic ordering phenomenon, unlike any other known type of electronic order in solids, such as magnetism.

As part of the results, the quantized states observed on the surface of the superconductor URu2Si2 show that electrons have a mass 17 times greater than that found in simple metals.

Temperatures close to absolute 0

This additional mass, according to the authors, explains the fact that in their experiments the temperature had to be extremely low in order to be able to observe the new quantum states. The authors obtained images with their own microscope at just 0.1 degrees above absolute zero (0 kelvins or -273.15 °C).

These behaviors allow us to dream of building devices that are insensitive to clutter, that transport energy without loss or that serve to design radically different computers.

Edwin Herrera (UCentral/Unal/UAM)

“The properties of the new quantum states helped us to characterize this mysterious material. Among others, we observed unexpected phenomena in the corners of the steps that may be related to the hidden order”, emphasizes the first author, Edwin Herrera, collaborator of the Central University and the National University of Colombia and postdoctoral researcher at UAM. .

“These are collective phenomena in which large numbers of particles come together to exhibit counter-intuitive behaviors that defy imagination, such as motion without dissipation, circulation quantization nod topological protection”, adds the researcher.

“These behaviors – he concludes – allow us to dream of building devices that are insensitive to disorder, that transport energy without loss or that serve to design radically different computers, such as quantum computers”.


Edwin Herrera, Hermann Suderow et al. “Quantum well states in the surface of a heavy fermion superconductor”. Nature, 2023

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