KIT - Campus South - Wolfgang-Gaede-Str.1
Seminar Room 10.01, Bldg. 30.23 (Physikhochhaus)

08.09.2009

Prof. Alexander Savchenko
School of Physics
University of Exeter, U.K.

14:00

Abstract: In the experimental conditions when quantum interference is important, electron transport in graphene is very different from that in conventional two-dimensional structures.  In graphene charge carriers are chiral. They possess an additional quantum number (the pseudo-spin) and a Berry phase of π.  As a result, quantum interference becomes dependent not only on inelastic (phase breaking) scattering but also on a number of elastic scattering mechanisms: by defects and imperfections. This makes quantum interference a sensitive tool for studying such defects.
Using mechanically exfoliated structures on Si/SiO2 substrates, we investigate the specifics of graphene’s quantum interference in the effects of weak localisation [1,2] and universal conductance fluctuations [3]. A comparative study of mono-layer [1] and bi-layer [2] graphene is performed – although electrons are massless in monolayer and massive in bi-layer, both systems have a common feature: the chirality of charge carriers.
We show that by changing experimental conditions (the temperature and electron density) a transition can be observed from localisation to anti-localisation – an increase of electron conduction which is a direct consequence of the Berry phase π . We also show that quantum interference in graphene can be seen at unusually high temperatures, up to T~200 K, due to weak electron-phonon scattering in graphene [4].
Using ‘air-bridge’ top gates, we have realised high-mobility p-n-p graphene structures [5] and shown that graphene p-n junctions have selective quantum transmission for chiral carriers.

1.  F.V.Tikhonenko et al.,  Phys. Rev. Lett. 100, 056802 (2008).
2.  R.V. Gorbachev et al.,  Phys. Rev. Lett. 98,  176805 (2007).
3.  K. Kechedzhi et al., Phys. Rev. Lett.  102, 066801 (2009).
4.  F.V.Tikhonenko et al., arXiv:0903.4489.
5.  R.V. Gorbachev et al., Nano Lett. 8 , 1995 (2008).