Forum DQMP

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Date(s) - 26/04/2016
13 h 00 min - 14 h 00 min

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Tuesday, April 26th 2016 – 13h00

Auditoire Stückelberg
Ecole de physique

Coffee and tea will be available from 12h50 at the entrance of the Auditoire


Switchable mechanical properties of ferroelectrics due to flexoelectricity
Kumara Cordero (group of Prof. Paruch)

The mechanical properties of materials are believed to be invariant with respect to space inversion,
even for non-centrosymmetric materials, such as ferroelectrics, because all the magnitudes involved
(stress, strain, and elastic constants) are described by even parity tensors. The standard theory,
however, does not take into account the effect of flexoelectricity on structural properties.
Flexoelectricity is a coupling between polarization and strain gradients, and it can be huge around
crack tips, so it should be important in fracture phenomena. Our recent work, using the nanoindentation
technique and PFM images, provide evidences that this spatial inversion symmetry is broken in
ferroelectric materials. The nanoindentation technique enables us to mobilize the flexoelectric effect
around the sharp indenter tip while simultaneously probing the mechanical properties of the material.
The measurements on a stoichiometric single crystal Lithium Niobate (SLN) and a periodic poled
Lithium Niobate (PPLN) indicate that the energy dissipation, fracture toughness, and mechanical
properties are asymmetric in respect to the sign of the polarization. The PFM images of the indented
areas show domain switching in positively poled regions, resulting in different polarization patterns in
180° antiparallel domains. We show that this new physical phenomenon is enabled by the interplay
between ferroelectricity and flexoelectricity. Aside from the fundamental importance of this new insight,
the predicted asymmetry may also find uses in smart nanodevices and coatings with switchable
mechanical properties.

 

Origin of the conduction in polar/non-polar heterostructures: the case of SrTiO3 capped LaAlO3/SrTiO3 interfaces
Zhenping Wu (group of Prof. Triscone)

The two-dimensional electron liquid that forms at the interface between the two insulating oxides
LaAlO3 (LAO) and (001) SrTiO3 (STO) has stimulated enormous research interest, owing to its
extraordinary tunable electronic properties, such as high carrier mobility, large spin-orbit coupling, and
superconductivity. One of the hallmarks of the system is the transition from an insulating to a
conducting state that occurs when the LAO film exceeds 3 unit cells. Such a critical thickness is also
observed at LAO/STO interfaces grown along the (110) and (111) orientations.
In the (001) case, the metallic state and the presence of a critical thickness have for origin the polar
discontinuity that is found at the interface. The electric field inside the LAO layer can lead to either a
Zener breakdown or the creation of oxygen vacancies at the LAO surface.
In this study, we report on the observation of metallic conduction in LAO/STO interfaces capped with
STO crystalline films. Heterostructures with LAO layers as thin as 1 unit cell are found to be metallic
and superconducting. We discuss these results considering a model based on the polar discontinuity
and leading to surface oxygen vacancies.

 

Forum Committee : L. Foini, C. Lichtensteiger, N. Ubrig (18.04.2016)

Réalisation : Sur Mesure concept