All (2002-Present)  SS21  WS21  SS22  WS22  SS23  WS23  SS24

 Karl Franzens University Graz

Graz University of Technology 

New Developments in the area of topological insulators
Dr. Claudia Felser
MPI CPFS Dresden
17:00 - 18:00 Tuesday 05 May 2015 TUG P2

Topological insulators are a hot topic in condensed matter physics. The excitement in the physics community is comparable with the excitement when a new superconductor is discovered. HgTe and other systems such as Heusler compounds have an s-p-band inversion at the Fermi edge that makes this family suitable for the Quantum Spin Hall effect and topological insulators [1]. The diversity of Heusler materials opens wide possibilities for tuning the bandgap and setting the desired band inversion by choosing compounds with appropriate hybridization strength (by the lattice parameter) and magnitude of spin-orbit coupling (SOC, by the atomic charge). Based on first-principle calculations we demonstrate that around 50 Heusler compounds show band inversion similar to that of HgTe [1]. The topological state in these zero-gap semiconductors can be created by applying strain or by designing an appropriate quantum well structure, similar to the case of HgTe. Many of these ternary zero-gap semiconductors (LnAuPb, LnPdBi, LnPtSb and LnPtBi) contain the rare-earth element Ln, which can realize additional properties ranging from superconductivity (for example LaPtBi) to magnetism (for example GdPtBi) and heavy fermion behavior (for example YbPtBi). These properties can open new research directions in realizing the quantized anomalous Hall effect and topological superconductors. C1b Heusler compounds have been grown as single crystals and as thin films. The control of the defects, the charge carriers and mobilities can be optimized [2]. The band inversion is proven by ARPES [3]. The combination of a piezoelectric Heusler compounds and compounds at the borderline between trivial and topological insulators offers the possibility of a switchable device. It is also possible to design new topological insulators with strong correlations. In AmN and PuTe a band gap is opened by correlation effects. In a family of semiconductors with the simple NaCl structure band gaps up to 0.4 eV were found [4]. This is not so surprising since the SOC is large in Actinides. Heusler compounds are similar to a stuffed diamond, correspondingly, it should be possible to find the “high Z”' equivalent of graphene in a graphite-like structure or in other related structure types with 18 valence electrons and with inverted bands [5]. Indeed the ternary compounds, such as LiAuSe and KHgSb with a honeycomb structure of their Au-Se and Hg-Sb layers feature band inversion very similar to HgTe which is a strong precondition for existence of the topological surface states [6,7]. LiAuSe is a strong TI, whereas KHgSb a weak TI. We will discuss the necessary and sufficient conditions for new TI materials, based in symmetry and bonding arguments [8]. A new topological insulators was identified in cerium-filled skutterudite (FS) compounds. We find that two compounds, CeOs4As12 and CeOs4Sb12, are zero gap materials with band inversions between Os-d and Ce-f orbitals. Both compounds are predicted to become topological Kondo insulators at low temperatures, which are Kondo insulators in the bulk but with robust Dirac surface states on the boundary [9]. Up to now there are no oxides which were identified to be topological insulators. BaBiO3 is an oxide which shows a band inversion similar to HgTe. The superconductor BaKBiO3 (BKBO) with Tc nearly 30 K emerges as a TI in the electron-doped region, whereas it is a superconductor in the hole-doped region. BBO exhibits a large topological energy gap of 0.7 eV [4]. We will discuss the necessary and sufficient conditions for new TI materials, based in symmetry and bonding arguments [10].


1. S. Chadov, X. Qi, J. Kübler, G. H. Fecher, C. Felser, S.-C. Zhang, Nature Mater. 2010, 9, 541 “Tunable multifunctional topological insulators in ternary Heusler compounds”


2. Chandra Shekhar, Siham Ouardi, Ajaya K. Nayak, Gerhard H. Fecher, Walter Schnelle, and Claudia Felser Phys. Rev. B 86 (2012) 155314 “Ultrahigh mobility and nonsaturating magnetoresistance in Heusler topological insulators”


3. C. Shekhar et al. to be published 


4. D. Ebke et al. to be published

5. X. Zhang, HJ. Zhang, J. Wang, C. Felser, S.-C. Zhang, Science 2012, 335, 1464 “Actinide Topological Insulator Materials with Strong Interaction”


6. B. Yan, L. Müchler, C. Felser, Phys. Rev. Lett. 2012, 109, 116406 “Prediction of weak topological insulators in layered semiconductors”


7. H.-J. Zhang, S. Chadov, L. Müchler, B. Yan, XL. Qi, J. Kübler, SC. Zhang, C. Felser, Phys. Rev. Lett. 2011, 106, 156402 “Topological Insulators in Ternary Compounds with a Honeycomb lattice”


8. L.Müchler, HJ. Zhang, S. Chadov, B. Yan, F. Casper, J. Kübler, SC. Zhang, C. Felser, Angew. Chem. Int. Ed. 2012, 51, 7221 “Topological Insulators from a Chemist’s Perspective”


9. B. Yan, L. Müchler, X.-L. Qi, S.-C. Zhang, C. Felser, Phys. Rev. B 2012, 85, 165125 “Topological insulators in filled skutterudites”


10. Binghai Yan, Martin Jansen, Claudia Felser, A large energy-gap oxide topological insulator based on the superconductor BaBiO3, Nature Phys. 9 (2013) 709.