Article: 1D charge density wave in the hidden order state of URu2Si2. Published in Communications Physics.
A collaboration between IFIMAC, Grenoble and Colombia discovers a new 1D modulation in the so-called hidden order state of URu2Si2. At 17.5 K, this system enters the hidden order state through a second order phase transition, with a significant reduction of entropy. The origin of the hidden order low temperature phase is completely unknown. There are no obvious charge, structural or magnetic transitions accompanying the reduction of entropy. Since its discovery more than 30 years ago, experiments have shown that the hidden order is associated to the onset of incommensurate spin excitations with a fluctuating magnetic moment of order of the Bohr magneton, which are located within the plane of the tetragonal crystal structure. But these are fluctuating and do not establish magnetic order. Furthermore, the Fermi surface suffers a reconstruction below 17.5 K, which leads nearly to a doubling of the unit cell along the c-axis. But there is no structural transition associated to it. How these spin and charge degrees of freedom combine to produce the reduction of entropy observed in the experiment is still a mistery. Some researchers state that the hidden order problem in URu2Si2 is for condensed matter physics what the “dark matter problem” is for high energy physics, suggesting that the not yet fully determined degrees of freedom producing the low temperature state could be found in other compounds too.
In the article, researchers find, using very careful millikelvin Scanning Tunneling Microscopy experiments, a new 1D charge density wave at very low temperatures. They observe an incommensurate pattern with a very long wavelength that can be explained by a moiré construction between the crystal lattice and a modulation having the same wavevector as the fluctuating spin excitations. Researchers make a careful analysis of the cryogenic cleaving process used to prepare the surface of the sample and find that the travelling fracture front provides a one-dimensional symmetry breaking field. This suggests that dynamic excitations can be quenched in a cleaving process, opening the path to use fracture to modify properties of solids. The experiment shows the prominent role of charge excitations in the hidden order state. [Full article]