New quantum states on the surface of a mysterious material

An international team of scientists has observed unexpected phenomena on the surface of a new superconducting material.

The team includes experts from the Autonomous University of Madrid (UAM) in Spain, and from institutions in Colombia, Spain, France, Japan and Sweden.

In physics, some of the most spectacular images of the phenomenon of quantum interference have been obtained by studying free electrons on the surface of simple and well-known metals, such as copper or gold.

Now, the new observations demonstrate for the first time the phenomenon of electron quantization in heavy fermions and, furthermore, reveal a type of electronic ordering whose origin is totally unknown.

In laboratories, for more than three decades, it is possible to obtain atomically flat surfaces on simple metals. In the steps that can form between these surfaces, electrons can be trapped and are very sensitive to any change; for example, a step of atomic size constitutes an important barrier for its propagation.

“On a rung between two rungs, 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 maximums varies with the energy in integral 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 the electrons behave almost as if they were independent particles. In quantum materials, the electrons are not independent and, as a consequence of the entanglement due to the interactions, they carry a heavy armor with them”, explains Isabel Guillamón, a researcher from the same Department and co-author of the study.

Alfredo Levy Yeyati, a member of the Department of Theoretical Physics of Condensed Matter and co-author, adds, “Our results clearly show highly correlated quantized states of electrons and we demonstrate in detail the interaction between these states and superconductivity. Theoretical calculations that support the observations suggest that such states could also be seen in many other quantum materials. In addition, they provide new experimental evidence that would serve to understand the origin of the mysterious hidden order, showing the absence of a certain symmetry near the surface”.

Atomic size steps on a surface. Image taken just 0.1 degrees above absolute zero (the lowest temperature allowed by the laws of physics) with a microscope built entirely at UAM. The color indicates the height. The difference in height between two steps is only a few atoms. In the upper left part you can see the atomic network. The arrows indicate the crystallographic directions. Results obtained in the Laboratory of Low Temperatures and High Magnetic Fields of the Department of Physics of Condensed Matter of the Faculty of Sciences. (Image: Edwin Herrera / UAM)

The mysterious hidden order

Scientifically speaking of hidden order to refer to a mysterious ordering phenomenon, different from any other type of known electronic order in solids (for example magnetism).

As part of the results, the quantized states observed on the surface of the URu2Si2 superconductor show that the electrons have a mass 17 times greater than that found in simple metals. This additional mass, according to the authors, explains the fact that in their experiments the temperature had to be lowered extremely in order to be able to observe the new quantum states.

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

“These are collective phenomena in which large numbers of particles band together to display counterintuitive behaviors that defy imagination, such as dissipationless motion, circulation quantization, or topological protection.”

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

The study is titled “Quantum-well states at the surface of a heavy-fermion superconductor”. And it has been published in the academic journal Nature. (Source: UAM)