Novel topological insulator and superconducting materials

Compounds are usually classified as metals, semiconductors or insulators, depending on the features of their band structure. However, a new fascinating quantum state of matter was recently discovered, which is insulator in the bulk, but presents metallic electrical conductivity at the edges or surfaces: the topological insulators (TIs) [1-4]. The TIs special conducting surface electron states are “topologically protected” from localization and back-scattering due to a strong spin-orbit coupling, as long as the interacting scattering potential does not violate time-reversal symmetry. Moreover, the surface electron states of TIs are spin- polarized, providing a spin-current that can be used in spintronics, most notably for applications like super-fast quantum computing. However, most three dimensional (3D) TIs have still fairly large bulk electrical conductivities: a low crystalline quality, impurities and defects can put the Fermi level back from the insulating gap into the conduction or valence bands, mixing the surface and bulk states, and masking the observation of many of the predicted novel phenomena. Consequently, much research effort has been done in the fields of solid-state chemistry and materials science to obtain good samples. The physical and chemical characteristics of the best known TI compounds (as HgTe, Bi2Se3, or Bi2Te3) complicate their preparation as high-quality samples. The main scientific goal of the present project is the identification, preparation and study of high-quality Tis, with an emphasis on novel compounds that have recently been theoretically suggested as very promising [5], namely 5f (uranium) compounds that are semiconductors, non-magnetic and feature a large spin-orbit coupling favouring band inversion at specific points of the Brillouin zone, a key ingredient for TI behaviour. In addition, recent exciting developments connecting TI behaviour and superconductivity (superconducting proximity effect, topological superconductivity, huge enhancement of Tc possibly due to topological carrier confinement of a single layer of FeSe) will be exploited in this project [6,7]. It will be achieved through the execution of the following main tasks: (i) Synthesis and single crystal growth of high-quality 3D bulk Tis based on traditional Bi chalcogenides compounds and novel uranium semiconducting compounds; (ii) Preparation of 3D TI nanostructured materials by wet chemical synthesis methods and hybrid materials with Fe based superconductors; (Iii) Structural, morphologic, electronic and magnetic characterization of Tis and novel superconducting materials (Fe based and exotic 5f systems); (iv) Investigation of TIs behaviour in uranium compound systems (small and wide gap semiconductors) and superconductivity in Fe- based materials using low-temperate characterization of transport properties; (v) Characterization of surface electronic states by ARPES, Raman scattering and muSR; (vi) Modelling of the electronic structure of the bulk and surface states using state of the art DFT and GW methods. The work will start with the research on the phase diagrams in order to grow high-quality single crystals of TIs. This task will be guided by DFT calculations to select the most promising systems. In parallel, new synthesis methods will be developed based on wet chemical routes to grow 3D TI nanostructured materials, starting from the doped prototype binary compounds. The synthesis conditions will be optimized to obtain the aimed particle size, morphology, dopant and defect concentration for a systematic characterization of the relationship between composition and processing conditions (temperature, solvents, surfactants, etc.) on the electronic properties. A detailed structural characterization (XRD), composition (XRF, ICP-MS), morphology (SEM, TEM, AFM), electronic and magnetic properties, transport and thermoelectric properties) will be done. The electronic characterization of surface states will be performed via Raman Scattering and ARPES. The exciting possibility of finding TI behaviour in uranium compounds (due to the strong spin-orbit coupling of the 5f electrons) will be performed through a study of selected uranium layered, non- magnetic and semiconducting compounds. Finally, modelling the bulk and surface electronic structure with state of the art methods will allow a systematization of the results and rationalization of the properties. Sample preparation, DFT calculations and measurements will be performed using the equipment available at UC (CFsUC.) and IST (C2TN), but specific characterizations (such as ARPES or muSR) will be done through external international collaborations.

Status: Running

Starting date: 1/Jan/2016

End date: 31/Dec/2018

Financing: 194680 Euros

Financing entity: FCT

Project ID: PTDC/FIS-NAN/6099/2014

Person*month: 0

Group person*month: 0