Neutrons explore exotic insulator's quantum secrets
A recent study published in Nature Communications reveals an unexpected transition between two different insulator states. Neutron diffractometry experiments at the ILL, conducted on the D2B high-resolution, two-axis diffractometer, open up the path towards advanced technologies by providing vital insights into the complex electronic behavior of such materials.
The two-axis diffractometer instrument D2B © Max Alexander
Mott insulators are peculiar materials: theoretically, they are expected to be conductors, but in reality, they are insulators. This unexpected behaviour arises from the fact that their charge carriers are inactive due to so-called Coulomb repulsion - the fact that similar electric charges repel each other. Changing the properties of Mott insulators can lead to novel, exotic electronic states. A fundamental exploration of different types of these materials is therefore of fundamental interest.
Neutron spectroscopy allows to measure the energies of the relevant transitions. A good energy resolution is a key aspect for such studies. An example of a Mott insulator is the material CeMnAsO1-xFx. Its components are cerium (Ce), manganese (Mn), arsenic (As), oxygen (O) and fluorine (F). "x" is the fraction of fluorine, varying from 0 – 0.05 across different versions of the material. This compound was successfully investigated on ILL's high-resolution neutron diffractometer D2B at the ILL by a French-British collaboration.
The temperature-dependent resistivity of CeMnAsO1-x Fx , showing an insulator-insulator transition for x > 0.03. The inset shows the variation of log(resistivity) versus inverse temperature where the insulator-insulator transition is observed for x = 0.035, 0.050 and 0.075.
"Below a certain characteristic temperature, we observed samples of CeMnAsO1-xFx transition from a Mott insulator to a quantum one", says Abbie Mclaughlin, the principal investigator of the project. As opposed to Mott insulators, quantum insulators do allow for charge carriers to be mobile on their surface. Despite this, quantum insulators remain non-conductive on their inside.
"We also found that we were able to tune this temperature by slightly modifying the material's composition ratio", adds Mclaughlin. This tunability implies a strong interest for practical applications of this material in various functional devices. These may include highly advanced technology such as quantum computing and neuromorphic calculations, which rely on reproducing the computational pathways of the human brain.
Curiously, classical explanations for this transition, such as changes in crystal symmetry, were ruled out by precise, temperature-dependent neutron diffraction experiments performed by the researchers. A detailed explanation of the phenomena observed remains yet to be elucidated and is likely to be an exotic one which has not been observed to date. These experiments have therefore paved the way towards novel, exciting physics with a plethora of opportunities for both fundamental and applied research.
Reference : E. J. Wildman, G. B. Lawrence, A. Walsh, K. Morita, S. Simpson, C. Ritter, G. B. G. Stenning, A. M. Arevalo-Lopez & A. C. Mclaughlin - Observation of an exotic insulator to insulator transition upon electron doping the Mott insulator CeMnAsO. Nat Commun14, 7037 (2023). https://doi.org/10.1038/s41467-023-42858-3
ILL Instrument : High-resolution two-axis diffractometer D2B
ILL contact : C. Ritter