Seminar: Irradiation induced doping of topological insulators, route to electronic transport via surface and tuning of magnetic interactions

Prof. M. Konczykowski  

Laboratoire des Solides Irradiés, CNRS & CEA, Ecole Polytechnique, 91128 Palaiseau, France

Time: 2pm, April 10, 2019 (Wednesday)
Place: East Bulg. 4, Room 242, Zijingang Campus

Abstract
Native defects retained in the crystal during the growth process of topological insulators (TI) provide free carriers and large bulk conductivity masking electronic transport via topologically protected surface conduction channels. Attempts to reduce the contribution of bulk carriers, such as nanostructured synthesis/growth, chemical doping or compositional tuning, did not give satisfactory results. We introduced alternative method using irradiation with energetic particles to drive topological insulator to charge neutrality point (CNP) in the bulk [1]. We apply this method to two classes of TI’s: time reversal symmetry protected (Bi2Te3 and Bi2Se3) and crystal symmetry protected Pb1-xSnxSe. We focused on the low temperature (20K), 2.5 MeV electron irradiation producing mainly vacancies on Bi or Pb sites acting as donors in Bi2Te3 and Bi2Se3 and as acceptors in Pb1-xSnxSe. Thus, starting from p-type in the first case or with n-type in the second, we can drive system to metal insulator transition followed by conductivity type inversion. To obtain stable condition close to CNP we proceed in two steps, irradiation far above type inversion followed by partial annealing.

We perform electronic transport measurements and ARPES spectroscopy on irradiation-modified Bi2Te3 and Bi2Se3 crystals, leading to conclusions listed below.
– Dirac cones of surface states were not altered by irradiation-induced disorder, ARPES reveals only shift of the Fermi energy, reversible after annealing.
– Evolution of Shubnikov-de Hass oscillations reflects shrinking/expansion of the Fermi sphere with irradiation dose. Close to the CNP contribution of the surface states appears via Berry phase.

- Semiconducting temperature dependence of resistivity, exhibiting various regimes from thermally activated to variable range hopping is observed in samples close to CNP. 

- Weak antilocalization type magnetoresistance was observed close to CNP, it fits perfectly Hikami-Larkin-Nagaoka formula with prefactor corresponding to two conduction channels.

This method, fine-tuning of the Fermi level to charge neutrality point by low temperature electron irradiation doping, without altering the surface state, applies to wider class of materials, in particular to crystalline TI’s of Pb1-xSnxSe family. Magnetic dopants (Mn, Cr) introduced to TI’s lead to the variety of novel phenomena but not yet demonstrated. Tuning of the Fermi level by irradiation/annealing procedure reveals substantial contribution of free carrier mediated interactions between magnetic dopants.

Ref. [1] L. Zhao, M.K et al, Nature Comm. 7, 10957 (2016)

About the Speaker

Education and training:
Warsaw Technical University    — Mathematics —   MSA 1971
Institute of Physics, Polish Academy of Sciences  — Physics —   PhD 1975
Nuclear Research Center, Fontenay aux Roses Postdoctoral Associate 1976-1977
  
Appointments:
2015 –      :  Emerited Research Director, Centre National de Recherche Scientifique
2008-2015:  Research Director 1 class, Centre National de Recherche Scientifique, 
1994-2008: Research Director 2 class, Centre National de Recherche Scientifique
1983-1994: Research staff member, Centre National de Recherche Scientifique
1982-1983: Professor of Physics, University of Grenoble
1981-1982: Foreign Collaborator of French Atomic Commission
1978-1981: High Pressure Physics Research Center, Polish Academy of Sciences, Research Staff, Head of Laboratory of Crystal Growth
Research Areas: Disorder effects in condensed matter, superconductivity, vortex dynamics, irradiation effects, topological insulators, impurities in semiconductors.
Publications:
~ 311 in peer-reviewed journals (8099 citations, h-index 44, according to WEB of Sciences)