Uranium is a naturally occurring radioactive element, as its nucleus decays into nuclei of lighter elements. In the process, it emits what scientists call the ‘alpha particle’, the nucleus of a helium atom. Scientists have successfully designed methods for using its radioactivity to generate nuclear power, which can solve the world’s energy demands. However, the electronic and thermal properties of uranium are not well understood. An example of electronic properties includes understanding how the element behaves like a superconductor at temperatures close to the absolute zero temperature, or -273˚C.
Researchers often use a technique called the ‘Fourier transform’, named after its inventor Joseph Fourier, to simplify studying properties of systems. For example, while tracing how a physical quantity changes with time, they study it in frequency, which is called the ‘Fourier space’ of time. Similarly, the Fourier transform of any physical quantity existing in space is how it varies with momentum, the Fourier space of length. When scientists look at the implications of quantum mechanics in the Fourier transform of the atomic vibrations of some solids, something they call the ‘Kohn anomaly’ emerges. It is an aberration or problem in the solid’s mathematical description in the Fourier space. The variation of the energy in the ‘momentum space’ affects how solids behave as their atoms carry out small vibrations around their average positions. Uranium exhibits multiple such Kohn anomalies. Although this has been known since 1979, the anomalies have defied explanation.
‘Phonons’ are the quanta of the vibrational modes of solids, which interact with the electrons. Strong interactions between phonons and electrons lead to the Kohn anomaly. A recent study by researchers from the Indian Institute of Technology Bombay (IIT Bombay) and the Bhabha Atomic Research Centre (BARC), Mumbai, has explained why uranium exhibits multiple Kohn anomalies. Their study, funded by the Industrial Research and Consultancy Centre of IIT Bombay, the Department of Atomic Energy, and the Ministry of Human Resource Development (now Ministry of Education), Government of India, was published in the journal Physical Review Letters.
The researchers carried extensive computer simulations using the quantum mechanical laws to study how the electrons and phonons interact in the material and what effect the interaction has on the data in the Fourier space. They used supercomputing facilities located at IIT Bombay and BARC, on which the simulations ran for ten days each.
“The anomaly is the strongest manifestation of electron-phonon interaction,” says Prof Dipanshu Bansal of IIT Bombay, one of the authors of the study.
Superconductors also exhibit such strong interactions between electrons and phonons. The explanation of the Kohn anomaly in uranium is a step towards understanding its superconductive behaviour at near-zero temperatures.
“Our work resolves the five-decade-old mystery of this important nuclear material,” asserts Prof Bansal.
Currently, the researchers are investigating the same anomaly of other nuclear materials like isotopes of uranium and thorium.
Editor's Note: An earlier version of the title was published and the correct title of the story has been updated. The error is regretted.