Photons have revealed the heavy electrons that are behind the transition to the so-called 'hidden order' phase in the superconductor URu2Si2, researchers report online this week in Nature Physics. Although the microscopic nature of the hidden-order phase remains unknown, the experiment will help guide future studies of this enigma.

For nearly 25 years, the low-temperature phase of URu2Si2 has been a mystery. Although many other uranium-based superconductors contain heavy electrons, there has not been any direct evidence for heavy electrons in URu2Si2. Santander-Syro and colleagues use spectroscopic techniques to show that they exist and that they take part in the hidden-order change of state.

At the point which this phase change occurs, the Fermi surface separating the filled and unfilled electron states that define the properties of a given solid undergoes instability. The team detected a band of heavy-electron states above the transition, which then migrates into the hidden-order state, where the electrons develop a gap in their excitation energy. None of the present theories on hidden order have predicted this kind of behaviour.