Seminário "Two-dimensional topological insulators: Graphene vs Bi bilayers"
Nesta quinta-feira, 26/09/2013, às 15:00 h, no Auditório do PPGFis/CCE, teremos o seminário "Two-dimensional topological insulators: Graphene vs Bi bilayers", a ser ministrado pelo Prof. Dr. Juan Jose Palacios (U.A. de Madrid/Spain). Todos estão convidados.
Resumo:
The non-trivial band structure of bidimensional topological insulators, also known as quantum spin Hall insulators (QSHI), is expected to manifest itself through a robust universal quantum of conductance G = 2e²/h, which is supported by helical edge states. In the first part of my presentation I will explore the consequences of time-reversal symmetry preserving edge reconstructions in nominally single-branched quantum spin Hall systems (QSHI's). To this end we consider a prototypical zigzag graphene nanoribbon with enhanced spin-orbit coupling. We show that a well-known edge reconstruction increases the number of pairs of helical edge states in the gap from 1 to 3. While disorder opens the way to intra-edge backscattering, it also induces inter-edge backscattering in one of the new channels, which largely extends into the bulk. This turns the conductance sample-dependent, masking the most direct experimental manifestation of the topological invariant.
This result contrast to the one in another proposed QSHI, a Bi(111) bilayer, which also features three pairs of helical branches, but presents strong edge confinement and very short localization lengths. In this case the quantized conductance becomes a topological invariant even for very small sample sizes. In the second part of my talk I will report conductance measurements of Bi nanocontacts created by repeated tip-surface indentation using a scanning tunneling microscope at temperatures of 4 K and 300 K. As a function of the elongation of the nanocontact we measure stable, nanometer-long conductance plateaus at 2e²/h at room temperature. This observation can be accounted for by the mechanical exfoliation of a Bi(111) bilayer in the retracing process following a tip-surface contact (see figure).
This is further supported by the additional observation of conductance steps below 2e²/h before breakup at both temperatures. Our finding is extraordinary on three accounts, namely, it provides the first experimental evidence of the possibility of mechanical exfolation of Bi bilayers, of the existence of the QSHI phase in a two-dimensional crystal, and, most importantly, of the observation of this novel phase at room temperature [1].
