IITB researchers unravel the anomalies in uranium

image

Researchers have explained how the electronic properties and atomic vibrations of uranium are
linked.

An electronic instability destabilizes the lattice, triggering charge-density wave and inducing
Kohn anomaly. [Image Credits: Aditya Prasad Roy, Department of Mechanical Engineering, IIT
Bombay, an author of the study]

Uranium is a naturally occurring radioactive element, whose nucleus decays into other elements.
It emits what scientists call the „alpha particle‟, the nucleus of a helium atom. Scientists have
successfully designed methods for using its radioactivity to create nuclear power, which has the
potential to solve the world‟s energy demands. However, the electronic and thermal properties of
uranium are not very 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, an anomaly
known as 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 its atoms carry out small vibrations around their average positions.

„Phonons‟ are the quanta of the vibrational modes of solids, which interact with the electrons of
the solid. Strong interactions between phonons and electrons lead to the Kohn anomaly. A 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 re-analysed the data from inelastic neutron scattering experiments on the
uranium carried out in 1979. These experiments probed uranium‟s atomic vibrations in the
Fourier space, which they were aiming to use to understand its heat dissipation under an extreme
nuclear environment. However, on re-analysis, they discovered Kohn anomalies in multiple
atomic vibrations. These anomalies were theoretically proposed to exist in one-dimensional
systems, but their observation in three-dimensional materials was rare.

To understand this peculiar observation, the researchers carried out extensive computer
simulations using the laws of quantum mechanics to study how the electrons and phonons
interact in the material, and what effect the interaction has on the data in the Fourier space. “The
simulations were computationally intensive, and we had to use supercomputing facilities located
at IIT Bombay and BARC, on which the simulations ran for ten days each,” says Aditya Prasad
Roy of IIT Bombay, first-author of the study.

“The anomaly is the strongest manifestation of electron-phonon interaction,” explains 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 absolute 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 in
other uranium and thorium-based nuclear materials.