An international team has observed an astonishing effect in a nickel-oxide material when it cools: Instead of freezing, certain fluctuations even increase with falling temperature. Nickel oxide is a model system that is structurally similar to high-temperature superconductors. The experiment shows once again that the behavior of this class of material always has surprises in store.
In practically all matter, lower temperatures mean less movement of its microscopic components. The less heat is available as energy, the less often atoms change their location or magnetic moments change direction: They freeze. An international team led by scientists from HZB and DESY has now for the first time observed the opposite behavior in a nickel-oxide material that is closely related to high-temperature superconductors. Fluctuations in this nickelate do not freeze when it cools, but become faster.
We used the innovative technique of X-ray correlation spectroscopy for their observation: We were able to follow the order of elementary magnetic moments (spins) in space and time by means of coherent soft X-rays. As it cools down, the spins are arranged in a striped pattern. This order is not perfect at higher temperatures, but consists of a random arrangement of small, locally ordered areas. We found that this arrangement is not static but fluctuates on time scales of a few minutes. As the cooling continues, these fluctuations initially become slower and the individual structured areas grow. To this extent, this behavior corresponds to what a large number of materials show: the less thermal energy is available, the more fluctuations freeze and order increases.
However, it was completely unusual and never before observed that, with further cooling, the fluctuations became faster again, while the ordered areas shrank. The stripe order decays at low temperatures both spatially and through ever faster fluctuations and thus shows a kind of anti-freezing.
This observation may help to better understand the high temperature superconductivity in copper oxides (cuprates). In cuprates it is assumed that a stripe order similar to that in nickelates competes with superconductivity. There, too, the stripe order disintegrates at low temperatures, which was previously explained by the fact that the superconductivity that forms replaces the stripe order. Since there is no superconductivity in nickelates, but the stripe order still disintegrates at low temperatures, an important aspect seems to be missing from the previous description of cuprate superconductivity. It is possible that the stripe order in cuprates is not simply displaced, but also disintegrates for intrinsic reasons, thus clearing the field for the development of superconductivity.
The study shows the potential that coherent soft X-rays have for studying materials that are spatially inconsistent, especially those materials where this spatial inconsistency gives rise to new functionality. Correlation spectroscopy with lasers has been used for many decades, for example to study the movement of colloids in solutions. Transferred to soft X-rays, the technology can be used to track fluctuations in magnetic and, for example, electronic and chemical disorder in space and time.
The experiments described here were performed at the Advanced Light Source ALS, California.
With future X-ray sources such as BESSY III, which will generate coherent X-rays that are many orders of magnitude more intense than today’s sources, it will be possible to extend this technology to faster fluctuations and shorter length scales and thus to observe effects that have not previously been achievable.
Featured image: The development of this pattern of spots over time shows microscopic fluctuations in the sample. © 10.1103 / PhysRevLett.127.057001
Reference: Alessandro Ricci, Nicola Poccia, Gaetano Campi, Shrawan Mishra, Leonard Müller, Boby Joseph, Bo Shi, Alexey Zozulya, Marcel Buchholz, Christoph Trabant, James CT Lee, Jens Viefhaus, Jeroen B. Goedkoop, Agustinus Agung Nugroho, Markus Braden, Sujoy Roy, Michael Sprung, and Christian Schüßler-Langeheine, “Measurement of Spin Dynamics in a Layered Nickelate Using X-Ray Photon Correlation Spectroscopy: Evidence for Intrinsic Destabilization of Incommensurate Stripes at Low Temperatures”, Phys. Rev. Lett. (2021): DOI: 10.1103 / PhysRevLett.127.057001
Provided by Helmholtz Association of German Research Centres