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Here, we use KPFM to directly distinguish AB-BLG (BLG), ABA-TLG (TLG), and twisted bilayer graphene (tBLG). Furthermore, we have explored the relationship between the surface potential and the different twist angles of tBLG.

Landau quantization associated with the quantized cyclotron motion of electrons under magnetic field provides the effective way to investigate topologically protected quantum states with entangled degrees of freedom and multiple quantum numbers. Here we report the cascade of Landau quantization in a strained type-II Dirac semimetal NiTe2 with spectroscopic-imaging scanning tunneling microscopy. The uniform-height surfaces exhibit single-sequence Landau levels (LLs) at a magnetic field originating from the quantization of topological surface state (TSS) across the Fermi level. Strikingly, we reveal the multiple sequence of LLs in the strained surface regions where the rotation symmetry is broken. Firstprinciples calculations demonstrate that the multiple LLs attest to the remarkable lifting of the valley degeneracy of TSS by the in-plane uniaxial or shear strains. Our findings pave a pathway to tune multiple degrees of freedom and quantum numbers of TMDs via strain engineering for practical applications such as high-frequency rectifiers, Josephson diode and valleytronics., Nano Letters 23, 3274 (2023)

As a ferromagnetic topological semimetal with a strong anomalous Hall effect (AHE), Co3Sn2S2 has been extensively explored for manipulating the topological state via the changed magnetic order as well as device applications connecting dissipationless spin and charge transport1-3. However, the main challenge is the improvement of the crystal quality, which is crucial for enhancing physical properties including AHE, carrier mobility, and magnetoresistance4,5. Here, we report the synthesis of ultra pure Co3Sn2S2 single crystals with an ultra low impurity density by an innovative crystal-sourced chemical vapor transport (CS-CVT) approach, achieving ultrahigh anomalous Hall angle (AHA) of 40%, carrier mobility and magnetoresistance (MR) of 10490 cm2V-1s-1 and 2500%, respectively. These values represent a huge improvement on previously reported highest values of either pristine or doped Co3Sn2S2 (of 33%, 2600 cm2V-1s-1, and 180% respectively)4,6. Equally importantly, the anomalous Hall conductivity (AHC) of our crystals reached 1600 Ω-1cm-1, which is larger than the theoretically predicted value of 1310 Ω-1cm-1 from the integral of Berry curvature7. Observation of an ultranarrow flat band and topological surface states near the Fermi level provides good microscopic understanding of this gigantic anomalous Hall effect. Our high-quality magnetic topological materials are expected to facilitate a full understanding of the strong electronic correlation and may perhaps lead to the discovery of a quantum AHE in the 2D limit.

Water electrolysis is a promising technique for carbon neutral hydrogen production. A great challenge remains at developing robust and low-cost anode catalysts. Many pre-catalysts are found to undergo surface reconstruction to give high intrinsic activity in the oxygen evolution reaction (OER). The reconstructed oxyhydroxides on the surface are active species and most of them outperform directly synthesized oxyhydroxides. The reason for the high intrinsic activity remains to be explored. Here, a study is reported to showcase the unique reconstruction behaviors of a pre-catalyst, thiospinel CoFe2 S4 , and its reconstruction chemistry for a high OER activity. The reconstruction of CoFe2 S4 gives a mixture with both Fe-S component and active oxyhydroxide (Co(Fe)Ox Hy ) because Co is more inclined to reconstruct as oxyhydroxide, while the Fe is more stable in Fe-S component in a major form of Fe3 S4 . The interface spin channel is demonstrated in the reconstructed CoFe2 S4 , which optimizes the energetics of OER steps on Co(Fe)Ox Hy species and facilitates the spin sensitive electron transfer to reduce the kinetic barrier of O-O coupling. The advantage is also demonstrated in a membrane electrode assembly (MEA) electrolyzer. This work introduces the feasibility of engineering the reconstruction chemistry of the precatalyst for high performance and durable MEA electrolyzers. Ministry of Education (MOE) National Research Foundation (NRF) Submitted/Accepted version The authors thank the support from the Singapore Ministry of Education Tier 2 Grant (MOE-T2EP10220-0001) and the Singapore National Research Foundation under its Campus for Research Excellence and Technological Enterprise (CREATE) programme, through the Cambridge Center for Carbon Reduction in Chemical Technology (C4T) and eCO2EP programmes.