| dc.description.abstract | Electron transport at low temperatures in two-dimensional electron systems is
governed by two quantum corrections to the conductivity: weak localisation and
electron-electron interaction in the presence of disorder. We present the first experimental
observation of these quantum corrections in graphene, a single layer of
carbon atoms, over a temperature range 0.02 - 200 K. Due to the peculiar properties
of graphene, weak localisation is sensitive not only to inelastic, phase-breaking
scattering events, but also to elastic scattering mechanisms. The latter includes
scattering within and between the two valleys (intra- and inter-valley scattering,
respectively). These specifics make it possible, for example, to observe a transition
from weak localisation to antilocalisation. Our work reveals a number of surprising
features. First of all the transition occurs not only as the carrier density is varied,
but also as the temperature is tuned. The latter has never been observed in any
other system studied before. Second, due to weak electron-phonon interaction in
graphene, quantum interference of electrons survives at very high temperatures, up
to 200 K. For comparison, in other two-dimensional (2D) systems the weak localisation
effect is only seen below 50 K.
The electron-electron interaction correction is also affected by elastic scattering.
In a two-valley system, there are two temperature regimes of the interaction correction
that depend on the strength of inter-valley scattering. In both regimes the
correction has its own expression. We show that because of the intra-valley scattering,
a third regime is possible in graphene, where the expression for the correction
takes a new form. The study of weak localisation demonstrates that the third regime
is realised in our experiments. We use the new expression to determine the Fermiliquid
parameter, which turns out to be smaller than in other 2D systems due to
the chirality of charge carriers.
At very low temperatures (below 100 mK) we observe a saturation of the electron
dephasing length. We study different mechanisms that could be responsible for the
saturation and discuss in detail one of them – spin-orbit interaction. We determine
the spin coherence length from studies of weak localisation and the temperature
dependence of the conductivity and found good agreement between the two types
of experiments. We also show the way to tune the spin coherence length by an order of magnitude by controlling the level of disorder. However, experiment shows
contradictions with theory both in values of the spin coherence length and the type
of spin relaxation. We speculate about another spin-related mechanism, spin flip by
vacancies, which to some extent could also explain our observations.
We also present electron transport in graphene irradiated by gallium ions. Depending
on the dosage of irradiation the behavior of electrons changes. Namely,
electron localisation can be tuned from weak to strong. At low dosages we observe
the weak localisation regime, where the mentioned quantum corrections to the conductivity
dominate at low temperatures. We found the electron scattering between
the valleys to be enhanced, attributing it to atomically sharp defects (kicked out
carbon atoms) produced by ion irradiation. We also speculate that gallium ions
can be embedded in the substrate or trapped between silica and graphene. We draw
this conclusion after investigation of the spin-orbit interaction in irradiated samples.
At high dosages electrons become strongly localised and their transport occurs via
variable-range hopping. | en_GB |
