Session H13: Rahman Prize Lecture and Computational Design of New Topological Materials

Designing topologicality using oxides

In this talk we will describe a series of ab intio calculations carried out on different oxide-based systems and their nanostructures that show emerging non-trivial topological properties or nodal Fermi surfaces. We will show that various well-known oxide structures with the appropriate filling host Dirac points at the Fermi level that could eventually respond to spin-orbit coupling. In particular, we will focus on the results obtained in rutile multilayers[1], perovskite bilayers[2] grown along the polar (111) direction and corundum-based multilayers[3]. Topologically non-trivial phases occur in various limits of spin-orbit coupling strength and on-site Coulomb repulsion, using different fillings of the d-shell for various 3d and 5d elements in the active layers. The different systems will be discussed and compared to try to understand the key ingredients that lead to non-trivial topological properties in oxides and how these can be enhanced or tuned. \newline [1] V. Pardo, W.E. Pickett, Phys. Rev. Lett. 102, 166803 (2010). \newline [2] J.L. Lado, V. Pardo, D. Baldomir, Phys. Rev. B 88, 155119 (2013). \newline [3] J.F. Afonso, V. Pardo, arxiv/1507.08813 (2015).   

Designing topological states by pressure, strain, and functionalization

Various examples of the design of topological states by means of first-principles calculations are discussed. The presentation focusses on the design parameters (1) pressure, (2) strain, and (3) functionalization. TiTe$_2$ is found to be unusually accessible to strain effects and the first compound that under hydrostatic pressure (up to experimentally reasonable 30 GPa) is subject to a series of four topological phase transitions, which are related to band inversions at different points of the Brillouin zone. Therefore, TiTe$_2$ enables experimental access to all these transitions in a single compound. Phase transitions in TlBiS$_2$ and TlSbS$_2$ are identified by parity analysis and by calculating the surface states. Zero, one, and four Dirac cones are found for the (111) surfaces of both TlBiS$_2$ and TlSbS$_2$ when the pressure grows, which confirms trivial-nontrivial-trivial phase transitions. The Dirac cones at the ${\rm \overline{M}}$ points are anisotropic with large out-of-plane component. TlBiS$_2$ shows normal, topological, and topological crystalline insulator phases under hydrostatic pressure, thus being the first compound to exhibit a phase transition from a topological to a topological crystalline insulator. While monolayer arsenic and arsenic antimonide are semiconductors (direct band gap at the $\Gamma$ point), fluorination results for both compounds in Dirac cones at the K points. Fluorinated monolayer arsenic shows a band gap of 0.16 eV due to spin-orbit coupling and fluorinated arsenic antimonide a larger band gap of 0.37 eV due to inversion symmetry breaking. Spin-orbit coupling induces spin splitting similar to monolayer MoS$_2$. Phonon calculations confirm that both materials are dynamically stable. Calculations of the edge states of nanoribbons by the tight-binding method demonstrate that fluorinated arsenic is topologically nontrivial in contrast to fluorinated arsenic antimonide.   

High-Throughput Computational Design of Advanced Functional Materials: Topological Insulators and Two-Dimensional Electron Gas Systems

As a rapidly growing area of materials science, high-throughput (HT) computational materials design is playing a crucial role in accelerating the discovery and development of novel functional materials. In this presentation, I will first introduce the strategy of HT computational materials design, and take the HT discovery of topological insulators (TIs) as a practical example to show the usage of such an approach. Topological insulators are one of the most studied classes of novel materials because of their great potential for applications ranging from spintronics to quantum computers. Here I will show that, by defining a reliable and accessible descriptor, which represents the topological robustness or feasibility of the candidate, and by searching the quantum materials repository aflowlib.org, we have automatically discovered 28 TIs (some of them already known) in five different symmetry families. Next, I will talk about our recent research work on the HT computational design of the perovskite-based two-dimensional electron gas (2DEG) systems. The 2DEG formed on the perovskite oxide heterostructure (HS) has potential applications in next-generation nanoelectronic devices. In order to achieve practical implementation of the 2DEG in the device design, desired physical properties such as high charge carrier density and mobility are necessary. Here I show that, using the same strategy with the HT discovery of TIs, by introducing a series of combinatorial descriptors, we have successfully identified a series of candidate 2DEG systems based on the perovskite oxides. This work provides another exemplar of applying HT computational design approach for the discovery of advanced functional materials.   

Rahman Prize Talk: Pushing the frontier in the simulation of correlated quantum many body systems

Amazing progress in the simulation of correlated quantum many body systems has been achieved in the past two decades by combining significant advances in new algorithms with efficient implementations on ever faster supercomputers. This has enabled the accurate simulation of an increasing number of problems and helped settle many open questions. I will review a selection of results that my collaborators and I have worked on, from quantum phase transitions in quantum magnets, over supersolidity of bosons in lattice models and Helium-4 to recent simulations of correlated fermions and quantum gases. I will then provide an outlook to the future and discuss how in the short term analog quantum simulators can help tackle problems for which no efficient simulation algorithms exist and how in the longer term quantum computers can be used to solve many of the still open questions in the field. I will finally connect to the topic of the remainder of this symposium by touching on how the design of new topological materials will help in the construction of these quantum computers.