Class Of Topological Materials Research Articles

Not trivial to realize

In 2009, two papers provided the first unambiguous examples of three-dimensional topological insulators — bulk insulators boasting metallic surface states with massless Dirac electrons. These now form just one of many classes of topological materials.

First Principles Prediction of Topological Phases in Thin Films of Pyrochlore Iridates.

While the theoretical and experimental study of topological phases of matter has experienced rapid growth over the last few years, there remain a relatively small number of material classes that have been experimentally shown to host these phases. Most of these materials contain bismuth, and none so far are oxides. In this work we make materials-specific predictions for topological phases using density functional theory combined with Hartree-Fock theory that includes the full orbital structure of the relevant iridium d-orbitals and the strong but finite spin-orbit coupling strength. We find Y2Ir2O7 bilayer and trilayer films grown along the [111] direction can support topological metallic phases with a direct gap of up to 0.05 eV, which could potentially bring transition metal oxides to the fore as a new class of topological materials with potential applications in oxide electronics.

Repulsive Casimir force between Weyl semimetals

Weyl semimetals are a class of topological materials that exhibit a bulk Hall effect due to time-reversal symmetry breaking. We show that for the idealized semi-infinite case, the Casimir force between two identical Weyl semimetals is repulsive at short range and attractive at long range. Considering plates of finite thickness, we can reduce the size of the long-range attraction even making it repulsive for all distances when thin enough. In the thin-film limit, we study the appearance of an attractive Casimir force at shorter distances due to the longitudinal conductivity. Magnetic field, thickness, and chemical potential provide tunable nobs for this effect, controlling the Casimir force: whether it is attractive or repulsive, the magnitude of the effect, and the positions and existence of a trap and antitrap.

Mapping the unconventional orbital texture in topological crystalline insulators

The newly discovered topological crystalline insulators (TCIs) harbor a complex band structure involving multiple Dirac cones. These materials are potentially highly tunable by external electric field, temperature or strain and could find future applications in field-effect transistors, photodetectors, and nano-mechanical systems. Theoretically, it has been predicted that different Dirac cones, offset in energy and momentum-space, might harbor vastly different orbital character, a unique property which if experimentally realized, would present an ideal platform for accomplishing new spintronic devices. However, the orbital texture of the Dirac cones, which is of immense importance in determining a variety of materials properties, still remains elusive in TCIs. Here, we unveil the orbital texture in a prototypical TCI Pb$_{1-x}$Sn$_x$Se. By using Fourier-transform (FT) scanning tunneling spectroscopy (STS) we measure the interference patterns produced by the scattering of surface state electrons. We discover that the intensity and energy dependences of FTs show distinct characteristics, which can directly be attributed to orbital effects. Our experiments reveal the complex band topology involving two Lifshitz transitions and establish the orbital nature of the Dirac bands in this new class of topological materials, which could provide a different pathway towards future quantum applications.

Single Crystalline Nanostructures of Topological Crystalline Insulator SnTe with Distinct Facets and Morphologies

Topological crystalline insulators (TCIs) are a new class of topological materials that possess unique metallic surface states protected by crystalline mirror symmetry. Their topological surface properties are expected to strongly depend on the surface orientation. By combining density functional theory (DFT) calculations and synthesis experiments, we demonstrate the controlled growth of single crystalline nanostructures of the prototypical TCI SnTe with distinct facets and morphologies. Our calculations suggest that the excess energy of the {111} surfaces can be either higher or lower than that of the {100} surfaces, depending on the stoichiometry, while the {110} is always higher than the {100}. In our synthesis experiment, we qualitatively controlled the stoichiometry by tailoring the growth temperature and obtained two types of single crystalline nanowires: smooth nanowires dominated by {100} facets at high temperatures and zigzag nanowires composed of both {100} and {111} surfaces at low temperatures. Notably, there is no {110} facet in our nanostructures, strongly supporting the DFT calculations. Our device fabrication and electrical characterizations suggest that both types of nanowires are suitable for transport studies of topological surface states.