A unique feature of quantum materials is their rich electronic phase diagram featuring different electronic/magnetic states. These electronic phases are often close in energy leading to a phase competition in quantum materials. A prime example is the electronic phase separation (EPS) in manganites in which ferromagnetic metallic phase and antiferromagnetic insulating phase coexist spatially at microscopic scale. Such EPS is responsible for the colossal magnetoresistance in manganites. On the other hand, many macroscopic electronic/magnetic phenomena are rooted in a spatial order of electronic states. For example, the magnetic hysteresis loop of a ferromagnet is a manifestation of local ferromagnetic (FM) domain evolution. The transport behavior of quantum Hall effect (quantized and zero ) is closely tied to the existence of an insulating bulk and a conductive edge. With traditional bulk measurements, one can only infer the existence of such spatial order; but with modern imaging methods, one now can see it, quantify it and even manipulate it.
In Shen lab, we employ various scanning probe microscopies (SPM) to visualize and investigate different electronic/magnetic structures of quantum materials on length scales ranging from micrometer to atomic scale. We directly image and manipulate a magnetic nano-skyrmion phase in Fe/Ir(111) system with a spin-polarized scanning tunneling microscopy (SP-STM)1. We use magnetic force microcopy (MFM) to directly visualize EPS state in manganites to study its evolution under different external stimuli2,3. We also use MFM to characterize FM domain structures of magnetic topological materials in which magnetism plays a vital role in determining the topological properties4. Recently, we acquired a new SPM, i.e., scanning microwave impedance microscopy (sMIM), allowing us to characterize local transport properties (conductivity, dielectric constant) of a material at nanometer scale. We adopt sMIM to study magnetic topological insulator MnBi2Te4 and discover a time-reversal symmetry broken quantum spin Hall state in even-layer MnBi2Te4 thin flake5,6. We also use sMIM to study the metal-insulator transition in a canonical system V2O37. These SPM techniques greatly empower our study of correlated and topological quantum materials by revealing information related to the underlying mechanism of a phenomenon or a functionality. Looking ahead, we would like to invest our SPM more in the study of quantum devices when the Moore law ultimately shrink the size of such devices down to the nanometer scale.
References:
1. Huang, H. et al. Cryogen free spin polarized scanning tunneling microscopy and magnetic exchange force microscopy with extremely low noise. Rev. Sci. Instrum. 93, 073703, (2022).
2. Liu, H. et al. Reversibility of magnetic field driven transition from electronic phase separation state to single-phase state in manganites: A microscopic view. Phys. Rev. B 96, (2017).
3. Li, Q. et al. Electronically phase separated nano-network in antiferromagnetic insulating LaMnO3/PrMnO3/CaMnO3 tricolor superlattice. Nat. Commun. 13, (2022).
4. Li, Q. et al. Magnetic domain structure and domain-wall bound states of the topological semimetal EuB6. Phys. Rev. B 106, (2022).
5. Lin, W. et al. Direct visualization of edge state in even-layer MnBi2Te4 at zero magnetic field. Nat. Commun. 13, 7714, (2022).
6. Feng, Y. et al. Helical Luttinger Liquid on the Edge of a Two-Dimensional Topological Antiferromagnet. Nano Lett. 22, 7606-7614, (2022).
7. Lin, W. Y. et al. Direct visualization of percolating metal-insulator transition in V2O3 using scanning microwave impedance microscopy. Sci. China Phys. Mech. 65, 297411, (2022).
