Yihua Wang, Department of Physics, Fudan University

Thursday June 13 at 2:00 pm (Paris time)

Utilizing scanning SQUID microscopy to explore 2D quantum materials

Quantum phases such as superconductivity, magnetism and topological orders manifest themselves quite differently in the 2-dimensional (2D) materials than in their bulk counterpart. The famed Berezinskii-Kosterlitz-Thouless (BKT) transition is a prime example : it only occurs in the 2D limit of superconductors and ferromagnets. Experimental methods to study quantum properties of 2D materials are largely based on charge sensing techniques such as electrical transport and tunnelling spectroscopy. Magnetic measurements are just as useful. However, conventional volumetric magnetic probes are not sensitive enough for measuring the weak magnetic signal of 2D materials.

Nano-fabricated superconducting quantum interference devices (nano-SQUID) are very sensitive magnetic probes that are suitable for magnetic imaging of quantum materials on the mesoscopic scale. We have developed scanning SQUID microscopes working under different environments [1,2] to study superconductors, magnets and topological antiferromagnets in the 2D limit. We utilize both magnetometry and susceptometry to image superconducting vortices, magnetization, susceptibility and topological edge current in these systems. In this talk, I will report our recent work on the observations of chiral edge current in magnetic topological insulators [3], BKT transition in cuprates [4] and emergent quantum orders [5,6] in iron-based superconductors. If times allows, I will discuss future opportunities at the interface between superconducting nano-devices and quantum materials.

[1] Pan, Y. P. et al. 3D nano-bridge-based SQUID susceptometers for scanning magnetic imaging of quantum materials. Nanotechnology 30, 305303 (2019). https://doi.org:10.1088/1361-6528/ab1792

[2] Xiang, B. K. et al. Flux focusing with a superconducting nano-needle for scanning SQUID susceptometry. Microsystems and Nanoengineering 9, 78 (2023).

[3] Zhu, J. et al. Direct observation of chiral edge current at zero magnetic field in odd-layer MnBi2Te4. arXiv 2307.10150 (2023).

[4] Wang, S. Y. et al. Oscillating paramagnetic Meissner effect and Berezinskii-Kosterlitz-Thouless transition in Bi2Sr2CaCu2O8+δ monolayer. National Science Review (2023). https://doi.org:doi.org/10.1093/nsr/nwad249

[5] Lin, Y. S. et al. Direct Observation of Quantum Anomalous Vortex in Fe(Se,Te). Physical Review X 13, 011046 (2023). https://doi.org:10.1103/PhysRevX.13.011046

[6] Xiang, B. K. et al. Observation of long-range ferromagnetism via anomalous supercurrents in a spin-orbit coupled superconductor. arXiv 2307.10722 (2023).

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