KAIST

(Professor Han (far right) and his research team)

A KAIST research team observed a new quantum mechanical magnetic state ‘Jeff = 3/2.’ This first observation of ‘Jeff=3/2’ could be the foundation for future research on superconductivity and quantum magnetism.

In quantum mechanics, total angular momentum is defined as the sum of spin and orbital angular momenta and is denoted with the ‘J.’ The newly identified magnetic moment can be described as a kind of angular momentum that occurs when specific conditions are met and has been denoted ‘Jeff’ with the meaning ‘effective angular momentum’ in the field.  Jeff=3/2 has been a topic of discussion but was yet to be observed.

The research was co-led by Professor Myung Joon Han of the Department of Physics at Chung-Ang University in  Korea, RIKEN in Japan, and the Argonne National Laboratory in the US. This research was  published in Nature Communications on October 14, 2017.

In academia, spin-orbital coupling was known to lead to a unique quantum state and has been an active area of recent research. In contrast to magnetic moment by electron spin and orbital, the effective magnetic moment Jeff, formed from the coupling of the two, shows a unique ground state and interaction patterns, which could lead to new phenomena and properties.

Most studies in the last decade focused on ‘Jeff=1/2’, but there has not been any observation of ‘Jeff=3/2’, which led to slow progress. In 2014, the research team led by Prof. Han theoretically predicted the possibility of  the ‘Jeff=3/2’ state  in a certain type of materials  based on molecular orbital, instead of atomic orbital. In the current study, the team applied the Selection Rule of quantum mechanics for the ‘Jeff=3/2’ state, which differs to the general spin moment, in order to experimentally detect this moment.

When electrons near the atomic nucleus are excited by X-rays, the excited electrons can be absorbed or re-emitted through interactions with other electrons. Here, the Selection Rule is applied to electrons. According to quantum mechanics, this rule is very unique in the ‘Jeff=3/2’ state and ‘Jeff=3/2’ is predicted to be distinguishable from general spin states. The prediction that was made using this idea was verified through the experiment using electrons extracted from tantalum at two different energy levels. In this material, the  unique quantum mechanical interference by the ‘Jeff=3/2’ moment can be taken as direct evidence for its existence.

The new quantum state is very unique from any of the previously known magnetic states and this study could be the starting point for future research on the ‘Jeff=3/2’ moment. Further, this finding could contribute to future research on various properties of the magnetic states and its interactions.

(Figure 1: Crystal structure, MO levels, and RIXS process in GaTa₄Se₈.)

   (Figure 2: Cluster model calculations of the L₃ and L₂ RIXS spectra)