Probing magnetic states of quantum materials in proximity to superconductivity
Jungpil Seo (서정필)
Department of Physics and Chemistry, DGIST, Korea 42988

The spin-polarized scanning tunneling microscopy (SPSTM) technique, which is based on scanning tunneling microscopy (STM), has long been recognized for its exceptional resolution in studying the magnetic properties of quantum materials. However, its widespread use has been hindered by the challenge of maintaining a stable magnetic tip. In this presentation, I will introduce a novel experimental technique for measuring the magnetic moments of quantum materials. Unlike SPSTM, this technique does not rely on intricate magnetic tips, yet it achieves an even higher magnetic resolution. The new technique requires s-wave superconductors that are coupled in proximity to quantum materials. When the quantum materials are placed next to s-wave superconductors, the quantum materials turn into superconductors within the length scale of superconducting coherence length, known as a superconducting proximity effect. When the quantum materials are magnetic, their magnetism disrupts the Cooper pairs in the superconductors, giving rises to the Yu-Shiba-Rusinov (YSR) bound states in the superconducting gap. By monitoring the YSR bound states in quantum materials, we visualize their magnetic properties. During the talk, I will present several examples of quantum materials grown on a Pb(111) substrate, including individual Co atoms, Mn-phthalocyanine (MnPC) molecules, and FeSe. Furthermore, I will also demonstrate that this technique provides an ideal platform for studying the competition between superconductivity and the Kondo effect as the strength of the superconductivity can be controlled by changing the experimental temperature.