Topological insulators (TI) are bulk insulating materials, which, however, exhibit metallic conductivity on their surfaces. This conductivity is guaranteed by the topology of the volume band structure — the surface possesses these states, while the symmetry determining the topological index remains unchanged.
In the so-called strong TIs, these states are protected and therefore present on all surfaces. However, in weak TIs, these properties are protected only on surfaces with a certain orientation. For example, the addition of two-dimensional TIs, i.e., QSHI, to form a three-dimensional crystal usually leads to a weak TI without protected states on the upper or lower surfaces of the crystal: there are metallic surface states inherited from the boundary states of the two-dimensional TI, as well as the plane of the insulating surface, which lies perpendicular to the laying direction.
Recent theoretical work, also conducted by MARVEL researchers, suggested, however, that this may not be the case for folded or voluminous jacquingite. The study suggested a more complex scenario – the material may be a topological crystalline insulator (TCI), as well as a weak TI. In TCI, the topology is defined by symmetry about the plane of the mirror, and metal surfaces can be detected on surfaces perpendicular to it. This state can be expected in the material due to its triple mirror symmetry. Jacutingaite also supports translational symmetry in layering, which means that it can also have weak TI properties. However, until now there have been no experimental results on the structure of volume zones.
Studies initiated by Theos EPFL and conducted in collaboration with the Department of Quantum Matter Physics at the University of Geneva and other groups, including a diamond light source in the UK, are currently describing the first synthesis of jacutite single crystals and used the sample to confirm their dual topological nature by comparing the volume and surface electronic structure determined from angular resolution photoemission experiments (ARPES) based on a synchrotron with DFT calculations. Paper, bulk and surface electronic structure of a semi-metal with a dual topology Pt2Hgse3was recently published in Physical Review Letters,
The work revealed topologically protected surface states in the plane of natural cleavage (001) of the material, which is unexpected, since it should rather support a weak topological phase, since it is a set of two-dimensional QSHI. The calculations of some topological invariants confirmed the weak phase of the topological insulator, usually characterized by gapless modes on the side surfaces, but completely jagged states on the upper and lower surfaces. Therefore, surface states found on surface 001 were considered a manifestation of a different topological phase.
<div data-thumb = "https://scx1.b-cdn.net/csz/news/tmb/2020/1-newexperimen.jpg" data-src = "https://scx2.b-cdn.net/ gfx / news / 2020/1-newexperimen.jpg "data-sub-html =" The crystal structure of bulk yakutaite (Pt2Hgse3), in red and blue, is one of two maximally localized Wannier functions that underlie the J3KM tight binding model. Courtesy: National Center for Research Competency (NCCR) MARVEL ">
Researchers have suggested that this may indicate a TCI phase associated with the symmetry of a triple crystal mirror. In this case, topologically protected surface states on crystalline surfaces are expected that retain mirror symmetry, and this was the case for a cleaved surface (001).
Using first-principle calculations, the researchers were able to identify this surface state as the signature of the TCI phase, which coexists with the common WTI phase found in the same calculations. Thus, the results indicate a predicted dual topology Pt2Hgse3, What remains unclear, however, is the mechanism behind the status of jakutait as a double topological insulator.
This topic was examined in the theoretical work developed at THEOS EPFL, studies that complemented the experimental and computational work done in another article. In the article Emergent dual topology in three-dimensional Kane-Mele Pt2Hgse3Researchers Antimo Marrazzo, Nicola Marzari, and his colleague Marco Gibertini from the University of Geneva, who previously worked at THEOS, extended the two-dimensional Kane-Mele (KM) model used to describe topological materials to volumetric jacquingite. This document was recently published in the journal Physical Review Research.
They showed that the unexpected topology in bulk Yakutaite comes from strong interlayer hybridization, which leads to a three-dimensional generalization of the CM model. While the nearest layers are almost separated from each other, there is a large, special hopping term that indicates a strong connection between layers that are two layers apart. The even and odd layers in this case are more or less independent and can be separately described using the three-dimensional KM model, referred to in the article J3KM, which includes the inversion of the band due to this new term of hopping. This results in a nodal line that breaks. spin-orbit coupling and a nonzero Chern number, i.e., protected surface states corresponding to TCI. When the bond between the even and odd layers is restored, the material again acts as a WTI.
This understanding provides a microscopic understanding of the emerging dual material topology. The J3KM model predicts the presence of surface states and nodal lines latched by spin-orbit interactions in accordance with ARPES measurements and first-principle modeling performed in another article. The model is applicable to all other layered materials from folded honeycomb lattices and offers an attractive strategy for breaking the standard weakness paradigm. topological insulators,
Finally, a combination of experimental data, first-principle simulations, and theoretical models on three-dimensional Yakuttingite confirms the earlier TEOS prediction that two-dimensional Yakuttingite is a Kane-Mele quantum spin spin insulator of the Kane-Mele type (graphene-like type).
I. Cucchi et al. The bulk and surface electronic structure of a semimetal with a double topology Pt2HgSe3, Physical Review Letters (2020). DOI: 10.1103 / PhysRevLett.124.106402
National Research Competency Center (NCCR) MARVEL
New experimental theoretical evidence identifies jacquingitis as a double-top insulator (2020, March 16)
restored March 16, 2020
This document is protected by copyright. Other than honest deals for private study or research, no
Part may be reproduced without written permission. Content is provided for informational purposes only.