Advent calendar - December 8th - Xiaojing Liu

In the Zernike Institute Advent Calendar, we are presenting 24 short spotlights in December. In these specials, we highlight PhD students, postdocs, support staff and technicians of our research groups and team - providing a glimpse in their typical day at work. In Episode 8 meet Xiaojing Liu, PhD student in the Surfaces and Thin Films Group.

I am Xiaojing Liu, and I am a PhD student in the Surfaces and Thin Films Group. My research aims to uncover the hidden nature of two-dimensional (2D) materials by investigating the behaviour of electrons, and to do this I use angle-resolved photoemission spectroscopy (ARPES)-a powerful technique that allows us to directly probe the electronic properties of solids and surfaces.

In general, materials are categorised as conductors, semiconductors, or insulators, but certain insulators exhibit conducting surface which makes them special. There are also materials that we expect to be conductors, but they are insulators due to strong correlation; and things get even more unusual when we look into materials that lack one dimension, 2D materials that consist of just a surface. Many exotic states and materials like these remain undiscovered, making this an exciting field to explore.

Besides 2D and bulk materials, I am also interest in what happens when a material goes from bulk (being 3D) to a surface (being 2D). Does the material retain its electronic properties? Or do new, exotic phenomena emerge? To answer these questions, I peel thin films or even monolayers, off bulk materials using KISS method. Since ARPES is highly surface sensitive, every sample is prepared in an ultra-high vacuum chamber to ensure the surface stays super clean during measurements. Therefore, much of my work involves operating a magnetic transfer arm and joystick, monitoring the process through a circular glass window in the chamber.

After the measurements, we generate thousands of images that tell us where the electronic bands are locate in a 3D (or 4D for bulk samples!) reciprocal space. Yes, electronic bands actually do not exist in real but reciprocal (or k) space! Though beamtime only lasts few days, you end up with a lot of data (quite a few GB). I have spent over a year adjusting and optimizing parameters in my data analysis to find ones that fit my results the best. To fully understand in depth how the electronic bands change, I analyse and compare 3D datasets pixel by pixel. We also get help from the theory - combining ARPES with and density functional theory (DFT) calculations allows us to not only clarify material’s electronic properties but also explain the mechanisms behind these properties.

In order to make beamtime as efficient as possible we always work as a team, taking turns on day and night shifts. For safety reasons, at least two people always work at same time, which means I am never alone. Collaboration is essential to our work, and we discuss every step in the plan together before proceeding to the next step. We even made a joke that if anything goes wrong, I will blame Crystalla, Crystalla will blame Giovanna, Giovanna will blame Dan, and Dan will blame me-because all decisions are made together.

I feel very fortunate to have many opportunities to work at different institutes and collaborate with many outstanding researchers as these experiences enrich my research journey.

See all Advent Calendar items 2024 here!