The interplay between strong correlations and topology can lead to the emergence of intriguing quantum states of matter. One well-known example is the fractional quantum Hall effect, where exotic electron fluids with fractionally charged excitations form in partially filled Landau levels. The emergence of topological moiré flat bands provides exciting opportunities to realize the lattice analogs of both the integer and fractional quantum Hall effects without the need for an external magnetic field. These effects are known as the integer and fractional quantum anomalous Hall (IQAH and FQAH) effects. Here, we present direct transport evidence of the existence of both IQAH and FQAH effects in small-angle-twisted bilayer ${\mathrm{MoTe}}_2$. At zero magnetic field, we observe well-quantized Hall resistance of $h/e^2$ around moiré filling factor $\nu =-1$ (corresponding to one hole per moiré unit cell), and nearly quantized Hall resistance of $3h/2e^2$ around $\nu =-2/3$, respectively. Concomitantly, the longitudinal resistance exhibits distinct minima around $\nu =-1$ and $-2/3$. The application of an electric field induces topological quantum phase transition from the IQAH state to a charge transfer insulator at $\nu =-1$, and from the FQAH state to a topologically trivial correlated insulator, further transitioning to a metallic state at $\nu =-2/3$. Our study paves the way for the investigation of fractionally charged excitations and anyonic statistics at zero magnetic field based on semiconductor moiré materials.

Recently, theory has predicted a Majorana zero mode (MZM) to induce spin selective Andreev reflection (SSAR), a novel magnetic property which can be used to detect the MZM. Here, spin-polarized scanning tunneling microscopy or spectroscopy has been applied to probe SSAR of MZMs in a topological superconductor of the ${\mathrm{Bi}}_2{\mathrm{Te}}_3/{\mathrm{NbSe}}_2$ heterostructure. The zero-bias peak of the tunneling differential conductance at the vortex center is observed substantially higher when the tip polarization and the external magnetic field are parallel rather than antiparallel to each other. This spin dependent tunneling effect provides direct evidence of MZM and reveals its magnetic property in addition to the zero energy modes. Our work will stimulate MZM research on these novel physical properties and, hence, is a step towards experimental study of their statistics and application in quantum computing.

Majorana fermions have been intensively studied in recent years for their importance to both fundamental science and potential applications in topological quantum computing. They are predicted to exist in a vortex core of superconducting topological insulators. However, it is extremely difficult to distinguish them experimentally from other quasiparticle states for the tiny energy difference between Majorana fermions and these states, which is beyond the energy resolution of most available techniques. Here, we circumvent the problem by systematically investigating the spatial profile of the Majorana mode and the bound quasiparticle states within a vortex in ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ films grown on a superconductor ${\mathrm{NbSe}}_2$. While the zero bias peak in local conductance splits right off the vortex center in conventional superconductors, it splits off at a finite distance $\sim 20\mathrm{nm}$ away from the vortex center in ${\mathrm{Bi}}_2{\mathrm{Te}}_3$. This unusual splitting behavior has never been observed before and could be possibly due to the Majorana fermion zero mode. While the Majorana mode is destroyed by the interaction between vortices, the zero bias peak splits as a conventional superconductor again. This work provides self-consistent evidences of Majorana fermions and also suggests a possible route to manipulating them.

We propose a new type of spin-valley locking (SVL), named C-paired SVL, in antiferromagnetic systems, which directly connects the spin/valley space with the real space, and hence enables both static and dynamical controls of spin and valley to realize a multifunctional antiferromagnetic material. The new emergent quantum degree of freedom in the C-paired SVL is comprised of spin-polarized valleys related by a crystal symmetry instead of the time-reversal symmetry. Thus, both spin and valley can be accessed by simply breaking the corresponding crystal symmetry. Typically, one can use a strain field to induce a large net valley polarization/magnetization and use a charge current to generate a large noncollinear spin current. We predict the realization of the C-paired SVL in monolayer V₂Se₂O, which indeed exhibits giant piezomagnetism and can generate a large transverse spin current. Our findings provide unprecedented opportunities to integrate various controls of spin and valley with nonvolatile information storage in a single material, which is highly desirable for versatile fundamental research and device applications.

All Set for Majoranas: When put in the proximity of a superconductor, topological insulators (TIs) are expected to support Majorana fermions, exotic particles that are their own antiparticles. For this to be realized, the interface between the TI and superconductor layers has to be atomically sharp but electronically transparent. Wang et al. (p. 52 , published online 15 March) fabricated this heterostructure by growing a film of the TI material Bi ₂ Se ₃ on the superconductor NbSe ₂ covered with a Bi bilayer. Scanning tunneling spectroscopy revealed a superconducting gap on the TI surface of the heterostructure with varying thickness of the Bi ₂ Se ₃ film. This coexistence of superconductivity and topological order should now allow observation of exotic phenomena such as Majorana fermions.

Topological superconductors (TSCs), featuring fully gapped bulk and gapless surface states as well as Majorana fermions, have potential applications in fault-tolerant topological quantum computing. Because TSCs are very rare in nature, an alternative way to study the TSC is to artificially introduce superconductivity into the surface states of a topological insulator through the proximity effect [X. L. Qi, T. L. Hughse, S. Raghu, and S. C. Zhang, Phys. Rev. Lett. 102, 187001 (2009); L. Fu and C. L. Kane, Phys. Rev. Lett. 100, 096407 (2008); J. Alicea, Rep. Prog. Phys. 75, 076501 (2012); C. W. J. Beenakker, Annu. Rev. Condens. Matter Phys. 4, 113 (2013)]. Here we report the experimental realization of the proximity effect\char21{}induced TSC in ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$ thin films grown on a ${\mathrm{NbSe}}_{2}$ substrate, as demonstrated by the density of states probed using scanning tunneling spectroscopy. We observed Abrikosov vortices and Andreev lower energy bound states on the surface of the topological insulator, with the superconducting coherence length depending on the film thickness and the magnetic field. These results also indicate that the topological surface states of ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$ thin films are superconducting and, thus, that the ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}/{\mathrm{NbSe}}_{2}$ is an artificial TSC. The feasibility of fabricating a TSC with an individual Majorana fermion bound to a superconducting vortex for topological quantum computing is discussed.