New Quantum State Discovered in Hidden Order

Strong interaction between electrons leads to many unusual phenomena such as unconventional superconductivity and unusual phases. Uranium ruthenium silicate, URu2Si2, which was first synthetised in 1985, is a material that shows all these phenomena.

Interaction between uranium’s electrons means that the material is a so called heavy- electron metal and at low temperatures below 1.5 kelvin, unconventional superconductivity arises. The most amazing thing, however, is the transition to an unknown phase below 17 kelvin. The electrons form a spontaneous and collective mystical new order in the material. But what this hidden order consists of is not yet fully known, despite more than 35 years of intensive research within the field. It is viewed as one of the greatest mysteries within condensed matter physics. [1].

Jan Rusz and Peter Oppeneer at the Department of Physics and Astronomy have now together with researchers in an international collaboration discovered that a new quantum state arises on the surface of URu2Si2 at temperatures close to absolute zero, at temperatures of just a few millikelvin.

On the surface of the URu2Si2 crystal there are small crystalline terraces which are of atomic layer thickness and approximately 10-20 nanometres wide. Due to their strong interactions, the electrons form on the two-dimensional surface an extremely thin, and thereby heavy, electron band, which could be shown by detailed quantum structure calculations by the two Uppsala researchers.

With the help of scanning tunneling microscopy, the researchers Edwin Herrera and Hermann Suderow from Bogota and Madrid, discovered that the heavy two-dimensional electron state goes through an unexpected transition to another quantized state close to absolute zero, at the millikelvin level.

The new quantum state which arises may be compared with a quantum particle in a box, where space limitations lead to quantised waves. The terraces on the surface give a natural space limitation in two dimensions, but that a quantum state for such heavy electrons could be quantised further is an astonishing discovery.

"The interesting thing is that the new quantum state appears within the hidden order and thereby can contribute with previously unknown information about what is happening with the electrons within the hidden order, says Peter Oppeneer, Professor at the Department of Physics and Astronomy."

Scanning with tunneling microscopy measurements reveals that the lateral symmetry is spontaneously broken on the surface when superconducting within the hidden order. A square terrace shows not a fourfold, but instead a twofold symmetry which becomes visible when scanning in different quantum states. This research spreads new light over the hidden order and paves the way to generate previously unknown quantum phases in strongly correlated and superconducting materials.

The research has been carried out in an international collaboration between researchers from Bogota (Colombia), Madrid (Spain), Uppsala, Grenoble (France), and Oarai (Japan).

[1] J.A. Mydosh and P.M. Oppeneer, Colloquium: Hidden order, superconductivity, and magnetism: The unsolved case of URu2Si2. Reviews of Modern Physics 83, 1301 (2011).

Article reference

Quantum-well states at the surface of a heavy-fermion superconductor. Edwin Herrera, Isabel Guillamón, Víctor Barrena, William Herrera, Jose Augusto Galvis, Alfredo Levy Yeyati, Jan Rusz, Peter M. Oppeneer, Georg Knebel, Jean Pascal Brison, Jacques Flouquet, Dai Aoki and Hermann Suderow, Nature (2023). Publication date: 23 March, 2023.

DOI:10.1038/s41586-023-05830-1

Contact

Jan Rusz, Senior Lecturer/Associate Professor at the Department of Physics and Astronomy, jan.rusz@physics.uu.se

Peter Oppeneer, peter.oppeneer@physics.uu.se

Camilla Thulin

English translation: Johan Wall