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The possible realization of Majorana fermions as quasiparticle excitations in condensed matter physics has created much excitement. Most recent studies have focused on Majorana bound states which can serve as topological qubits. More generally, akin to elementary particles, Majorana fermions can propagate and display linear dispersion. These excitations have not yet been directly observed, and can also be used for quantum information processing. One route to realizing this is in a line junction between two phase-shifted superconductors coupled to topological surface states. Recent theory indicates that in iron-based superconductors, a particular type of crystalline defect, i.e., a domain wall (DW) between two regions with a halfunit cell shift between them, should create a π-phase shift in the superconducting order parameter. Combined with recent data showing topological surface states in FeSexTe1-x we find that this is the ideal system to realize helical 1D-dispersing Majorana modes. Here we report scanning tunneling spectroscopic (STS) measurements of crystalline DWs in FeSe0.45Te0.55. By analyzing large-area superconducting gap maps, we identify the gap in the topological surface state, demonstrating that our sample is an effective Fu-Kane proximitized topological system. We further locate DWs across which the atoms shift by half a unit cell. STS data on these DWs reveal a flat density of states inside the superconducting gap, a hallmark of linearly dispersing modes in 1D. This unique signature is absent in DWs in the related superconductor, FeSe which is not in the topological phase. Our combined are consistent with the observation of dispersing Majorana states at a π-phase shift DW in a proximitized topological material.

The electronic and magnetic properties of transition metal dichalcogenides are known to be extremely sensitive to their structure. In this paper we study the effect of structure on the electronic and magnetic properties of mono-and bilayer VSe 2 films grown using molecular beam epitaxy. VSe 2 has recently attracted much attention due to reports of emergent ferromagnetism in the two-dimensional (2D) limit. To understand this compound, high-quality 1T and distorted 1T films were grown at temperatures of 200°C and 450°C, respectively, and studied using 4 K scanning tunneling microscopy and spectroscopy. The measured density of states and the charge density wave (CDW) patterns were compared to band structure and phonon dispersion calculations. Films in the 1T phase reveal different CDW patterns in the first layer compared to the second. Interestingly, we find the second layer of the 1T film shows a CDW pattern with 4a × 4a periodicity which is the 2D version of the bulk CDW observed in this compound. Our phonon dispersion calculations confirm the presence of a soft phonon at the correct wave vector that leads to this CDW. In contrast, the first layer of distorted 1T phase films shows a strong stripe feature with varying periodicities, while the second layer displays no observable CDW pattern. Finally, we find that the monolayer 1T VSe 2 film is weakly ferromagnetic, with ∼3.5 μ B per unit similar to previous reports.

The physical realization of Chern insulators is of fundamental and practical interest, as they are predicted to host the quantum anomalous Hall (QAH) effect and topologically protected chiral edge states which can carry dissipationless current. Current realizations of the QAH state often require complex heterostructures and sub-Kelvin temperatures, making the discovery of intrinsic, high temperature QAH systems of significant interest. In this work we show that time-reversal symmetry breaking Weyl semimetals, being essentially stacks of Chern insulators with inter-layer coupling, may provide a new platform for the higher temperature realization of robust chiral edge states. We present combined scanning tunneling spectroscopy and theoretical investigations of the magnetic Weyl semimetal, Co3Sn2S2. Using modeling and numerical simulations we find that depending on the strength of the interlayer coupling, chiral edge states can be localized on partially exposed kagome planes on the surfaces of a Weyl semimetal. Correspondingly, our dI/dV maps on the kagome Co3Sn terraces show topological states confined to the edges which display linear dispersion. This work provides a new paradigm for realizing chiral edge modes and provides a pathway for the realization of higher temperature QAH effect in magnetic Weyl systems in the two-dimensional limit.

Significance There is an intense ongoing search for two-level quantum systems with long lifetimes for applications in quantum communication and computation. Much research has been focused on studying isolated spins in semiconductors or band insulators. Mott insulators provide an interesting alternative platform but have been far less explored. In this work we use a technique capable of resolving individual spins at atomic length scales, to measure the two-level switching of spin states in 1T-TaS 2 . We find quasi-1D chains of spin-1/2 electrons embedded in 1T-TaS 2 which have exceptionally long lifetimes. The discovery of long-lived spin states in a tractable van der Waal material opens doors to using Mott systems in future quantum information applications.