A sufficiently large supercurrent can close the energy gap in a superconductor and create gapless quasiparticles through the Doppler shift of quasiparticle energy caused by finite Cooper pair momentum. In this gapless superconducting state, zero-energy quasiparticles reside on a segment of the normal state Fermi surface, whereas the remaining Fermi surface is still gapped. Here we use quasiparticle interference to image the field-controlled Fermi surface of Bi2Te3 thin films proximitized by the superconductor NbSe2. A small applied in-plane magnetic field induces a screening supercurrent, which leads to finite-momentum pairing on the topological surface states of Bi2Te3. We identify distinct interference patterns that indicate a gapless superconducting state with segmented Fermi surface. Our results reveal the strong impact of finite Cooper pair momentum on the quasiparticle spectrum.

A two-dimensional topological insulator features (only) one bulk gap with nontrivial topology, which protects one-dimensional boundary states at the Fermi level. We find a quantum phase of matter beyond this category: a multiple topological insulator. It possesses a ladder of topological gaps; each gap protects a robust edge state. We prove a monolayer of van der Waals material PtBi 2 as a two-dimensional multiple topological insulator. By means of scanning tunneling spectroscopy, we directly visualize the one-dimensional hot electron (and hole) channels with nanometer size on the samples. Furthermore, we confirm the topological protection of these channels by directly demonstrating their robustness to variations of crystal orientation, edge geometry, and sample temperature. The discovered topological hot electron materials may be applied as efficient photocatalysts in the future.

YbMnBi2 is a recently discovered time-reversal-symmetry breaking type-II Weyl semimetal. However, as a representation of the new category of topological matters, the scanning tunneling microcopy (STM) results on such important material are still absent. Here, we report the STM investigations on the morphology of vacuum cleaved single crystalline YbMnBi2 samples. A hill and valley type of topography is observed on the YbMnBi2 surface, which is consistent with the non-layer nature of its crystal structure. Analysis of STM images yields the information of the index of the vicinal surface. Our results here lay a playground of future atomic scale research on YbMnBi2.

Breaking time reversal symmetry in a topological insulator may lead to quantum anomalous Hall effect and axion insulator phase. MnBi4Te7 is a recently discovered antiferromagnetic topological insulator with TN ~12.5 K, which is constituted of alternatively stacked magnetic layer (MnBi2Te4) and non-magnetic layer (Bi2Te3). By means of scanning tunneling spectroscopy, we clearly observe the electronic state present at a step edge of a magnetic MnBi2Te4 layer but absent at non-magnetic Bi2Te3 layers at 4.5 K. Furthermore, we find that as the temperature rises above TN, the edge state vanishes, while the point defect induced state persists upon temperature increasing. These results confirm the observation of magnetism induced edge states.Our analysis based on an axion insulator theory reveals that the nontrivial topological nature of the observed edge state.

The Mn-Bi-Te class of compounds are recently discovered topological insulators with broken time-reversal-symmetry, which host unique quantum anomalous Hall and axion insulator states. Their key characteristics are believed to be sufficiently understood by models in a single-particle picture. Here, we apply scanning tunneling microscopy to study the electronic properties of MnBi2Te4 and MnBi4Te7. Unexpectedly, our quasiparticle interference (QPI) results demonstrate that rotational symmetry of the crystal breaks, i.e. a nematic-like pattern arises, in certain energy range but persists in others. Moreover, our data in the presence of an external magnetic field rule out the possibility of the material magnetism as an origin of the C2 symmetric QPI pattern. This study reveals that the interaction in the Mn-Bi-Te class of topological materials may play an essential role in their electronic states, and thus opens a new path for investigating the interplay between wavefunction topology and symmetry breaking phases.