Wydarzenia

Seminarium Oddziału Badań Magnetyków

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

prof. Marcin Konczykowski

École Polytechnique w Palaiseau we Francji

Native defects retained in the crystal during the growth process of topological insulators (TI) provide free carrier 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 Pb_1-xSn_xSe. 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 Pb_1-xSn_xSe. 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.

• 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 Pb_1-xSn_xSe 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.

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