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Friday, 14 March 2014

Optical phonons limit electron transport in topological insulators

Physical Review Letters published the first study to show the impact of optical phonons on electron transport in topological insulators. This study, led by Dr. Marius V. Costache and ICREA Prof. Sergio O. Valenzuela of the ICN2 Physics and Engineering of Nanodevices Group, explores the limitations of transport in these unique materials.

Topological insulators are a new class of materials that have an insulating bulk and a highly conductive surface layer. One of the most unique properties of these materials is that the spins of the electrons flowing across the surface are locked at right angles to their direction of motion.  Physicists expect that this spin-momentum locking will prevent backscattering and protect the surface transport, even in topological insulators at room temperature. Because their remarkable properties are believed to prevail at room temperature, researchers and engineers hope to make devices that use not only the charge but also the spin of electrons to carry and store energy and information, opening doors to new technologies that were inaccessible using conventional materials.

However, the exploration of topological insulators is in its infancy, and scientists do not understand in detail the factors that impact the transport of electrons in the surfaces of the materials.  Through theoretical and experimental studies of topological insulators, researchers identified a crystal lattice vibration, known as an optical-phonon mode, with a characteristic energy of 6 to 8 milli-electron volts (meV) and the potential to scatter electrons.  In their recent publication in Physical Review Letters, Dr. M. Costache and ICREA Prof. S. Valenzuela, of the ICN2 Physics and Engineering of Nanodevices Group, in collaboration with ICREA Prof. S. Roche of the ICN2 Theory and Computational Nanoscience Group and colleagues of the Bulgarian Academy of Science, unveil the first study ever done on the impact of this optical-phonon mode on electron transport in topological insulators.

After performing temperature- and voltage-dependent transport measurements in bismuth selenide (Bi2Se3) thin crystals, a common topological insulator, the authors found compelling evidence that strong electron-phonon interaction is the main source of the scattering they observed in the crystals.  At low temperatures (T < 90 K), the electrical resistance in the crystals is greatly affected by thermal fluctuations; as temperature increases, optical phonons are activated and their interaction with electrons increases.  Furthermore, current in the crystals increases linearly with voltage but then starts to saturate at a threshold voltage value.  All of these phenomena can be explained by an optical-phonon mode of ? 8 meV acting on the surface of the topological insulator.  Researchers have found similar effects of optical phonons on electrical transport in graphene, corroborating the results found in this study.

Even after being exposed to air, the crystal surface exhibited the 8-meV optical-phonon mode.  At room temperature and in air – the conditions under which most electronic devices are used, this phonon mode becomes the dominant source of scattering and decreases conductivity in topological insulators.  Costache and Valenzuela’s study is an important and ground-breaking exploration of the limitations of transport in these unique materials.  Their findings also serve as a reality check: using topological insulators in practical devices will be more challenging than scientists initially predicted.  The key to the development of electronic applications is to understand the properties that limit charge transport in topological insulators, and so research in this area must continue to forge ahead.

Article Reference:

M. V. Costache, I. Neumann, J. F. Sierra, V. Marinova, M. M. Gospodinov, S. Roche, and S. O. Valenzuela. Fingerprints of Inelastic Transport at the Surface of the Topological Insulator Bi2Se3: Role of Electron-Phonon Coupling. Phys. Rev. Lett. 112, 086601 – Published 25 February 2014