We report a low-temperature scanning tunneling microscopy study of the charge density wave (CDW) order in 1T -TiSe2 and Cu0.08TiSe2. In pristine 1T -TiSe2 we observe a long-range coherent commensurate CDW (C-CDW) order. In contrast, Cu0.08TiSe2 displays an incommensurate CDW (I-CDW) phase with localized C-CDW domains separated by domain walls. Density of states measurements indicate that the domain walls host an extra population of fermions near the Fermi level which may play a role in the emergence of superconductivity in this system. Fourier transform scanning tunneling spectroscopy studies suggest that the dominant mechanism for CDW formation in the I-CDW phase may be electron-phonon coupling.

Weyl semimetals display a novel topological phase of matter where the Weyl nodes emerge in pairs of opposite chirality and can be seen as either a source or a sink of Berry curvature. The exotic effects in Weyl semimetals, such as surface Fermi arcs and the chiral anomaly, make them a new playground for exploring novel functionalities. Further exploiting their potential applications requires clear understanding of their topological electronic properties, such as Weyl points and Fermi arcs. Here we report a Fourier transform scanning tunneling spectroscopy (FT-STS) study on a type-II Weyl semimetal candidate MoTe 2 whose Weyl points are predicated to be located above Fermi level. Although its electronic structure below the Fermi level have been identified by angle resolved photo emission spectroscopy (ARPES), by comparing our experimental data with first-principles calculations, we are able to identify the origins of the multiple scattering channels at energies both below and above Fermi level. Our calculations also show the existence of both trivial and topological arc like states above the Fermi energy. In the FT-STS experiments, we have observed strong signals from intra-arc scatterings as well as from the scattering between the arc-like surface states and the projected bulk states. A detailed comparison between our experimental observations and calculated results reveals the trivial and nontrivial scattering channels are difficult to distinguish in this compound. Interestingly, we find that the broken inversion symmetry changes the terminating states on the two inequivalent surfaces, which in turn changes the relative strength of the scattering channels observed in the FT-STS images on the two surfaces.

We have employed spin-polarized scanning tunneling microscopy and Monte-Carlo simulations to investigate the effect of lateral confinement onto the nanoskyrmion lattice in Fe/Ir(111). We find a strong coupling of one diagonal of the square magnetic unit cell to the close-packed edges of Fe nanostructures. In triangular islands this coupling in combination with the mismatching symmetries of the islands and of the square nanoskyrmion lattice leads to frustration and triple-domain states. In direct vicinity to ferromagnetic NiFe islands, the surrounding skyrmion lattice forms additional domains. In this case a side of the square magnetic unit cell prefers a parallel orientation to the ferromagnetic edge. These experimental findings can be reproduced and explained by Monte-Carlo simulations. Here, the single-domain state of a triangular island is lower in energy, but nevertheless multi-domain states occur due to the combined effect of entropy and an intrinsic domain wall pinning arising from the skyrmionic character of the spin texture.Magnetic skyrmions are particle-like states [1] which can occur in magnetic systems with broken inversion symmetry due to the Dzyaloshinskii-Moriya interaction [2,3]. Skyrmions can either form lattices at certain field ranges [4-10] or they can be created and manipulated individually [9], thereby offering great potential for data storage, transfer and processing [11,12]. Since the geometric layout is an essential part of a device, theoretical investigations explored the effect of boundaries and confinement in skyrmionic systems [13,14]. For instance, skyrmion movement in racetracks [12] and the effect of notches [15] on their trajectories were studied and lateral confinement was suggested as a means to stabilize skyrmions without an external magnetic field [16]. However, the relative orientation of skyrmion lattices in regard to the geometry of the boundary of magnetic systems is still an open question. Most experiments focused on extended films and on their temperature and fielddependence while finite size effects were explored only very recently, e.g. in nanostripes [17] or in disk-shaped structures [18,19].Here, we employ spin-polarized scanning tunneling microscopy (SP-STM) and Monte-Carlo (MC) simulations to investigate the effect of two types of boundaries onto the nanoskyrmion lattice in the Fe atomic monolayer on Ir(111). This skyrmion lattice has four-fold symmetry, a period of about 1 nm, and exists already in zero field due to the four-spin interaction [6]. At T = 4.2 K it is insensitive to fields of up to 9 Tesla [20], which shows that it has a vanishing net moment. In extended films, three rotational domains can be found [6], where a diagonal of the magnetic unit cell aligns with one of the three symmetry equivalent close-packed atomic rows of the hexagonal Fe layer. Our new data show that at a close-packed edge of an Fe nanostructure, one diagonal of the magnetic unit cell is preferentially oriented parallel to the edge, i.e. the edge selects one of the three rotational ...

In addition to novel surface states, topological insulators can also exhibit robust gapless states at crystalline defects.Step edges constitute a class of common defects on the surface of crystals. In this work we establish the topological nature of one-dimensional (1D) bound states localized at step edges of the [001] surface of a topological crystalline insulator (TCI) Pb0.7Sn0.3Se, both theoretically and experimentally. We show that the topological stability of the step edge states arises from an emergent particle-hole symmetry of the surface low-energy physics, and demonstrate the experimental signatures of the particle-hole symmetry breaking. We also reveal the effects of an external magnetic field on the 1D bound states. Our work suggests the possibility of similar topological step edge modes in other topological materials with a rocks-salt structure. PACS numbers:arXiv:1809.10689v3 [cond-mat.str-el]

Metal-to-insulator transitions (MIT) can be driven by a number of different mechanisms, each resulting in a different type of insulator—Change in chemical potential can induce a transition from a metal to a band insulator; strong correlations can drive a metal into a Mott insulator with an energy gap; an Anderson transition, on the other hand, due to disorder leads to a localized insulator without a gap in the spectrum. Here, we report the discovery of an alternative route for MIT driven by the creation of a network of narrow channels. Transport data on Pt substituted for Ti in 1T-TiSe2 shows a dramatic increase of resistivity by five orders of magnitude for few % of Pt substitution, with a power-law dependence of the temperature-dependent resistivity ρ(T). Our scanning tunneling microscopy data show that Pt induces an irregular network of nanometer-thick domain walls (DWs) of charge density wave (CDW) order, which pull charge carriers out of the bulk and into the DWs. While the CDW domains are gapped, the charges confined to the narrow DWs interact strongly, with pseudogap-like suppression in the local density of states, even when they were weakly interacting in the bulk, and scatter at the DW network interconnects thereby generating the highly resistive state. Angle-resolved photoemission spectroscopy spectra exhibit pseudogap behavior corroborating the spatial coexistence of gapped domains and narrow domain walls with excess charge carriers.