Combining STM, AFM, and Magnetotransport Measurements for In-Operando Studies of Quantum Materials*

Research in new quantum materials require multi-mode measurements spanning length scales, correlations of atomic scale variables with macroscopic functions, and with ultimate spectral resolution only obtainable at ultra-low temperatures. In this talk I describe a multi-mode instrument achieving µeV tunneling resolution with in-operando measurement capabilities of scanning tunneling microscopy (STM), atomic force microscopy (AFM), and magnetotransport inside a dilution refrigerator operating at 10 mK. I illustrate the capabilities of this new instrument in the study of quantum Hall edge states in graphene devices. The edge states, a set of alternating compressible and incompressible strips, are formed at the electrostatic pn junction boundary geometrically defining the Hall bar. To comprehensively characterize these microscopic objects, we apply all capabilities of the new instrument using modalities of AFM, STM, and magnetotransport measurements at mK temperatures. The Kelvin probe force microscopy (KPFM) mode of AFM detects the chemical potential transitions when Landau levels are being filled or emptied as a function of back gate potential and show the same fidelity for Landau level spectroscopy as STS measurements. In particular, symmetry breaking states can be resolved at filling factors ν = ±1 inside the N=0 Landau level manifold, showing the lifting of the graphene four-fold degeneracy due to spin and valley. With KPFM we can map the dispersion of the Landau levels across the quantum Hall edge boundary as a function of density and spatial position, including resolving the ν = ±1 edge modes. The microscopic properties of quantum Hall edge states can now be correlated with macroscopic magnetotransport measurements.

*In collaboration with S. Kim, J. Schwenk, D. Walkup, F. Ghahari, M. Slot, Y. Zeng, C. Dean, J. Berwanger, F. Giessibl, K. Watanabe, T. Taniguchi, and N. Zhitenev.