Development of topological insulator and topological crystalline insulator nanostructures

Localization and Directionality of Surface Transport in Bi2Te3 Ordered 3D Nanonetworks

The resistance of an ordered 3D-Bi2Te3 nanowire nanonetwork was studied at low temperatures. Below 50 K the increase in resistance was found to be compatible with the Anderson model for localization, considering that conduction takes place in individual parallel channels across the whole sample. Angle-dependent magnetoresistance measurements showed a distinctive weak antilocalization characteristic with a double feature that we could associate with transport along two perpendicular directions, dictated by the spatial arrangement of the nanowires. The coherence length obtained from the Hikami-Larkin-Nagaoka model was about 700 nm across transversal nanowires, which corresponded to approximately 10 nanowire junctions. Along the individual nanowires, the coherence length was greatly reduced to about 100 nm. The observed localization effects could be the reason for the enhancement of the Seebeck coefficient observed in the 3D-Bi2Te3 nanowire nanonetwork compared to individual nanowires.

Two-atom-thin topological crystalline insulators lacking out of plane inversion symmetry

A two-dimensional topological crystalline insulator (TCI) with a single unit cell (u.c.) thickness is demonstrated here. To that end, one first shows that tetragonal (C₄in-plane) symmetry is not a necessary condition for the creation of zero-energy metallic surface states on TCI slabs of finite-thicknesses, because zero-energy states persist even as all the in-plane rotational symmetries-furnishing topological protection-are completely removed. In other words, zero-energy levels on the model are not due to (nor are they protected by) topology. Furthermore, effective two-fold energy degeneracies taking place at few discretek-points away from zero energy in the bulk Hamiltonian-that are topologically protected-persist at the u.c. thickness limit. The chiral nature of the bulk TCI Hamiltonian permits creating a2×2square Hamiltonian, whose topological properties remarkably hold invariant at both the bulk and at the single u.c. thickness limits. The identical topological characterization for bulk and u.c.-thick phases is further guaranteed by a calculation involving Pfaffians. This way, a two-atom-thick TCI is deployed hereby, in a demonstration of a topological phase that holds both in the bulk, and in two dimensions.

In-Situ Chemical Thinning and Surface Doping of Layered Bi2Se3

As a promising topological insulator, two-dimensional (2D) bismuth selenide (Bi₂Se₃) attracts extensive research interest. Controllable surface doping of layered Bi₂Se₃ becomes a crucial issue for the relevant applications. Here, we propose an efficient method for the chemical thinning and surface doping of layered Bi₂Se₃, forming Se/Bi₂Se₃ heterostructures with tunable thickness ranging from a few nanometers to hundreds of nanometers. The thickness can be regulated by varying the reaction time and large-size few-layer Bi₂Se₃ sheets can be obtained. Different from previous liquid-exfoliation methods that require complex reaction process, in-situ and thickness-controllable exfoliation of large-size layered Bi₂Se₃ can be realized via the developed method. Additionally, the formation of Se nanomeshes coated on the Bi₂Se₃ sheets remarkably enhance the intensity of Raman vibration peaks, indicating that this method can be used for surface-enhanced Raman scattering. The proposed chemical thinning and surface-doping method is expected to be extended to other bulk-layered materials for high-efficient preparation of 2D heterostructures.

Advances and Prospects in Topological Nanoparticle Photonics

Topological nanophotonics is a new avenue for exploring nanoscale systems from visible to THz frequencies, with unprecedented control. By embracing their complexity and fully utilizing the properties that make them distinct from electronic systems, we aim to study new topological phenomena. In this Perspective, we summarize the current state of the field and highlight the use of nanoparticle systems for exploring topological phases beyond electronic analogues. We provide an overview of the tools needed to capture the radiative, retardative, and long-range properties of these systems. We discuss the application of dielectric and metallic nanoparticles in nonlinear systems and also provide an overview of the newly developed topic of topological insulator nanoparticles. We hope that a comprehensive understanding of topological nanoparticle photonic systems will allow us to exploit them to their full potential and explore new topological phenomena at very reduced dimensions.

Systematic Investigation of the Coupling between One-Dimensional Edge States of a Topological Crystalline Insulator

The interaction of spin-polarized one-dimensional (1D) topological edge modes localized along single-atomic steps of the topological crystalline insulator Pb_{0.7}Sn_{0.3}Se(001) has been studied systematically by scanning tunneling spectroscopy. Our results reveal that the coupling of adjacent edge modes sets in at a step-to-step distance d_{ss}≤25  nm, resulting in a characteristic splitting of a single peak at the Dirac point in tunneling spectra. Whereas the energy splitting exponentially increases with decreasing d_{ss} for single-atomic steps running almost parallel, we find no splitting for single-atomic step edges under an angle of 90°. The results are discussed in terms of overlapping wave functions with p_{x}, p_{y} orbital character.