Hidden order and 5f electron density
URu2Si2 is a mysterious quantum material. Thirty years ago an ordering transition was found at 17 degrees above absolute zero temperature from specific heat (entropy) measurements. Since then, this order has been investigated by many experimental and theoretical physicists worldwide. However, so far, there is no satisfying answer to the question of what orders and how - although the existence of ordering effects at 17 K are observable in many physical properties. This quantum phenomenon is therefore called hidden order.
Solving the hidden order puzzle is not only an intellectual challenge; it also advances theoretical concepts that are relevant for designing new materials with exotic properties which may find applications in daily life. URu2Si2 is therefore a model material for solid state physicists in a sense that continuously new theories and new experiments are being developed for solving the hidden order. It can be compared with the fruit flies which have solved so many puzzles for biologists.
By now it is generally accepted that the distribution of the electronic density of the uranium atoms at low temperatures is an important key for understanding the hidden order. However, measuring these densities has so far not been possible. This is where the team of the Max-Planck Institute for Chemical Physics of Solids in Dresden in collaboration with the group of Dr. Andrea Severing from the Institute of Physics II at the University of Cologne and scientists from the Van-der-Waals-Zeeman Institute at the University of Amsterdam in the Netherlands approaches the hidden order problem. The result of this work has now been published under the title Direct bulk sensitive probe of 5f symmetry in URu2Si2 in the Proceedings of the National Academy of Sciences.
Thanks to modern inelastic scattering methods with x-rays (IXS), that are available e.g. at the European Synchrotron Radiation Facility (ESRF) in Grenoble in France, the relevant 5f electron density of uranium in URu2Si2 could be measured. The experimental results limit the number of hidden order scenarios and are therefore an important touch-stone for theoretical models.