Ionic liquid gating of the heavy fermion compound LiV2O4 (Dennis Hwang)
The Department of Quantum Materials seeks to synthesize compounds in which the electrons behave collectively in the most exotic of ways, and to elucidate the reason behind such behaviors. Some of these exotic electronic phases have proven so challenging to understand that they remain subjects of active research for decades.
The spinel oxide LiV2O4 is a decades-old puzzle. When cooled down to low temperatures (below 30 K), the electrons start becoming “heavy” – they behave as if their effective mass were many times the normal electron mass. In the case of LiV2O4, the effective mass reaches over 100 times the normal electron mass at the lowest temperature of 300 mK. While such “heavy fermion” behavior is found in compounds containing rare earth elements with f electrons, it is elusive for transition metal compounds with d electrons, with LiV2O4 being a notable exception. How LiV2O4 enters into a heavy fermion phase remains unknown, and new experiments are needed.
In this project, we will build a primitive electrochemical cell and electronic device out of LiV2O4. Starting with a thin (100 nm) film of LiV2O4 grown on a 5 x 5 mm substrate, we will deposit metal electrodes, then place a small droplet of ionic liquid in a defined region of the sample. By applying a voltage between the LiV2O4 film and one of the electrodes, positive and negative charges from the ionic liquid accumulate on the two surfaces. We seek to observe how the addition of charges modifies the heavy fermion phase of LiV2O4, which may yield clues to the underlying microscopic origin. We will perform experiments in a PPMS (physical property measurement system), which allows us to measure electrical resistance between 300 and 2 K while changing the concentration of charges in LiV2O4 through ionic liquid gating.
In a best-case scenario, we will obtain some results that offer new insights into the LiV2O4 puzzle. In a more likely (but equally valuable) scenario, you will learn the background and motivation behind your experiment (why you are doing it), learn some basic lab skills and how to operate complicated instruments, struggle through some technical difficulties (but not get too frustrated), make some preliminary measurements that lay the foundation for further investigations, and get a sense for what research is like. Prior relevant skills and experience are helpful, but most important are a good character and a willingness to learn.
1. https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.85.1052 (LiV2O4 reference, a bit more technical)
2. https://www.nature.com/articles/s41563-018-0246-7 (general review of ionic liquid gating)