Multiple Coexisting Dirac Surface States in Three-Dimensional Topological Insulator PbBi6Te10

Topological insulators (TIs) are a new class of materials, which have generated a considerable amount of fundamental interest as well as speculation over the possibility of new electronic devices. Typically, TIs are three-dimensional materials composed by binary or ternary quintuple-layer (QL) or septuple-layer (SL) structured compounds. TIs display insulator behavior in the bulk but also exotic metallic states (topological surface states: TSSs) confined at the surface and localized in the relatively large bulk energy gap. Importantly, the surface states of the TIs disperse linearly with momentum, that is their energy bands form Dirac cones described by the Dirac-Weyl equations for massless relativistic fermions. One remarkable difference between the surface of a topological insulator and that of graphene is that the topological insulator has only one Dirac point an no spin degeneracy, whereas graphene has two Dirac points and is spin degenerate. In addition, the TI’s Dirac cones are protected as long as the TI’s symmetry is preserved remaining gapless in nonmagnetic environments. This means that the massless Dirac behavior is robust against many ordinary perturbations such as surface disorder or defect, chemical doping and external electric fields.
Recently we proved that the number of TSSs can be engineered by means of alternating non-equivalent QL and SL blocks that compose the crystal, as opposed to conventional layered TIs made of equivalent QLs or SLs, only. In particular we provided evidence that, nonequivalent QL- and SL-surface terminations, host distinct Dirac states confined at the surface of the 3D crystal.
In a recent experiment carried out at the APE-IOM beamline of the Elettra synchrotron radiation facility we have unveiled the electronic band structure and the atomic structure of the three-dimensional topological insulator PbBi6Te10 by means of angle-resolved photoemission spectroscopy (ARPES) and scanning tunneling microscopy (STM) investigations. For such a crystal three cleavage planes are possible: two nonequivalent QL and one SL blocks. Careful scanning tunneling microscopy measurements on PbBi6Te10 confirmed three different alternating surface terminations (Figure 1e) with distinct apparent heights. High-resolution ARPES investigations confirmed that the presence of different type of terraces results in the formation of distinct and coexisting TSSs (Figure 1a), displaying distinct electronic band dispersion, constant energy contours (Figure 1b-d) and Dirac point energies.
Furthermore, we proved the presence of a largely two-dimensional Rasba-type split states close to the Fermi level, and characteristic M- and V-shaped band at deeper energy.  The experimental findings are supported by the predictions from density functional theory calculations, which also evidence different spatial charge density distributions of the topological surface states.
These results represent an important step in the understanding on the diversity of topological surface states in TIs and may pave a way for the application of the topological insulator to the real devices.

Figure 1. (a) ARPES band structure for PbBi6Te10. (b) Costant energy ARPES maps measured at the Fermi level (EF), (c) 0.20 and (d) 0.28 eV below EF. (e) Constant current STM image on fresh cleaved PbBi6Te10 (size 400 nm x 200 nm).

This research was conducted by the following research team:

M. Papagno1, S. V. Eremeev2,3,4, J. Fujii5, Z. S. Aliev6,7, M. B. Babanly6, S. K. Mahatha8, I. Vobornik5, N. T. Mamedov7, D. Pacilé1, and E. V. Chulkov3,9,10

1 Dipartimento di Fisica, Università della Calabria, Italy
2 Institute of Strength Physics and Materials Science, Tomsk, Russia
3 Tomsk State University, Tomsk, Russia
4 Saint Petersburg State University, Saint Petersburg, Russia
5 CNR-IOM, TASC Laboratory,  Trieste, Italy
6 Institute Catalysis and Inorganic Chemistry, Azerbaijan National Academy of Science, Baku, Azerbaijan
7 Institute of Physics, Azerbaijan National Academy of Science, Baku, Azerbaijan
8 Istituto di Struttura della Materia, Consiglio Nazionale delle Ricerche, Trieste, Italy
9 Donostia International Physics Center, San Sebastián/Donostia, Basque Country, Spain
10 Departamento de Física de Materiales UPV/EHU, Centro de Física de Materiales CFM - MPC and Centro Mixto CSIC-UPV/EHU, San Sebastián/Donostia, Basque Country, Spain 

Contact person:

Marco Papagno, email:
Ivana Vobornik, email:



Marco Papagno, Sergey V. Eremeev, Jun Fujii, Ziya S. Aliev, Mahammad B. Babanly, Sanjoy Kr. Mahatha, Ivana Vobornik, Nazim T. Mamedov, Daniela Pacilé, and Evgueni V. Chulkov, "Multiple Coexisting Dirac Surface States in Three-Dimensional Topological Insulator PbBi6Te10", ACSNano (Accepted); DOI: 10.1021/acsnano.5b07750.

Last Updated on Monday, 11 April 2016 12:09