Electronic Phase Transitions in Suspended Graphene Multilayers - Local and Correlated Studies of Humidity-mediated Ferroelectric Thin Film Surface Charge Dynamics

03.12.2019 13:00 – 15:00

It is very well understood that multilayer graphene of dierent thicknesses exhibits distinct electronic
properties. Nonetheless, it has been always assumed that its properties would converge very rapidly
to the semi-metallic behavior of graphite.
Contrary to these expectations, we will show electrical transport experiments in Bernal-stacked
multilayer graphene devices that display the occurrence of nite-temperature electronic phase
transitions with an impressive systematic evolution upon increasing the number of layers. Moreover,
an even-odd eect is observed at low temperatures. All even layer devices become insulating while
all odd layers remain conducting due to the contribution of a single Dirac band.
Finally, we will explain how these phenomena seen in multilayer graphene is very well described by
means of a second order phase transition, in which the order parameter is a mean-eld staggered
potential whose sign changes from one layer to the next.

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Surface water is present on all materials exposed to ambient environmental conditions, inherently
modifying the ground state in fundamental studies as well as aecting the operation of bare-chip
devices. By virtue of its polar nature, water strongly interacts with domains and domain walls in
ferroelectric materials: it in
uences polarisation switching dynamics in Pb(Zr0:2Ti0:8)O3 (PZT) thin
lms, and plays a key role (together with redistribution of oxygen vacancies) in the reversible control
of electrical transport at 180 domain walls in this material. Previous studies on this material
performed by functional scanning probe microscopy (SPM) and near-ambient x-ray photoelectron
spectroscopy (NA-XPS) have demonstrated a complex interplay between the initial lm polarisation,
applied voltage polarity and overall materials switching history.
Here, we present a study of the eect of relative humidity and polarisation switching history on the
surface charge dissipation by means of Kelvin probe force microscopy (KPFM). The results conrm
the previous observation of a drastic change in behaviour above 60% relative humidity, possibly
related to the formation of liquid water on the surface, which accelerates the dissipation of charges
along the surface. Through the combined use of physically constrained unsupervised machine
learning and reaction-diusion modeling, additional insights can be gained for the physical processes
governing charge dissipation.

Lieu

Bâtiment: Ecole de Physique

Auditoire Stückelberg

entrée libre