Invited Paper SP+AS+EM+GR+MI+NS+SS-FrM3
Electric Field Tuning of 2-dimensional Electrons in Graphene and Topological Insulators

Friday, November 1, 2013, 9:00 am, Room 202 C

The recent advances in classification of matter in terms of topological band theory have spurred a great deal of interest in synthesizing new materials demonstrating new topologically related properties. In a large class of these materials there are robust surface states on the spatial boundaries with vacuum. These surface states possess linear dispersive bands with chiral properties, similar to graphene. In this talk I will review our scanning tunneling spectroscopy measurements of graphene in applied electric field and magnetic fields [1-4] and compare them to some new results of applying electric fields in tunneling spectroscopy measurements of topological insulators [5,6].

Gate mapping tunneling spectroscopy has proved to be a powerful probe of the 2-dimensional electron system in graphene. In the presence of moderate disorder the charging of graphene quantum dots localized in the disorder potential has been observed with graphene on SiO₂ [1]. Intrinsic many body effects were observed in the renormalization of the dispersion velocity when substrate disorder was reduced using boron nitride spacer layers between graphene and SiO₂ [4]. In contrast, removing the substrate and creating suspended graphene membranes was seen to generate pseudomagnetic fields localizing the carriers in response to the strain generated from the forces between the probe and graphene membrane [3].

In the topological insulator Sb₂Te₃, we achieved gate tunable devices which are suitable for low temperature scanning tunneling microscopy (STM) studies by designing sample holders with back gating capability [5]. Thin films are epitaxially grown on pre-patterned SrTiO₃ substrates which are mounted on the specially designed sample holders. This allows in-situ gating on epitaxial films without any ex-situ processing of the sample [5]. In 3 QL thick Sb₂Te₃ films we observe a gap opening at the Dirac point due to the coupling of the top and bottom surface states [6]. More importantly, the gap is found to be tunable by the gate field, indicating the possibility of observing a topological phase transition in this system. A comparison of the data with an effective model of 3D topological insulators suggests that 3QL Sb₂Te₃ belongs to the quantum spin Hall insulator class.

[1] S. Jung et al., Nature Physics7, 245 (2011).

[2] G. M. Rutter et al., Nature Physics7, 649 (2011).

[3] N. N. Klimov et al., Science336, 1557 (2012).

[4] J. Chae et al., Phys. Rev. Lett. 197, 116802 (2012).

[5] T. Zhang et al., Phys. Rev. B 87, 115410 (2013).

[6] T. Zhang et al., arXiv:1304.3661.