FORUM_DQMP

30.01.2018 13:00 – 15:00

Impurity state coupled to a lattice with disorder
Anne-Maria Visuri (group of Prof. Giamarchi)

A discrete state coupled to a continuum is a well-known model in quantum optics and a number
of results have been derived for the decay of occupation of the initially occupied discrete state.
When the coupling is weak compared to the width of the continuum, the decay is exponential
with a decay rate given by the Fermi Golden Rule. A finite width of the continuum modifies the
decay law. We study the case of coupling to an n-dimensional lattice, corresponding to a finitewidth
continuum, relevant for impurities in condensed-matter systems. We show how different
densities of states lead either to a decay to zero, saturation to a constant value, or persistent
oscillations in the long-time limit.
We further calculate the infinite-time occupation probability (return probability) in the case of
coupling to a disordered lattice, and investigate the possibility of using the return probability of
a particle in a discrete state to characterize localization.


Local probe studies of switching and dynamics in
Pb(Zr0.2Ti0.8)O3 thin films
Philippe Tückmantel (group of Prof. Paruch)

Ferroelectric materials are characterized by the presence of at least two spontaneous
switchable electric polarization states, which can form domains with different polarisation
orientations. In these materials, defects and electrostatic boundary conditions have been shown
to greatly impact the intrinsic configuration, geometry and growth of these domains. Indeed,
defects can induce different switching dynamics, where the polarization reversal can be
dominated by the nucleation of new domains or by the motion of existing domain walls. Defects
such as oxygen vacancies can also play an important role in controlling the electrical conduction
at ferroelectric domain walls and, in conjunction with electrostatic boundary conditions can even
allow fully reversible control of this phenomenon.
Here, we present our results on Pb(Zr0.2Ti0.8)O3 thin films showing different switching dynamics
in samples showing different defect densities, grown by pulsed laser deposition and off-axis RF
magnetron sputtering. Using piezoresponse force microscopy (PFM) in ultra-high vacuum, we
study the nanoscale nucleation and motion of domains as a function of applied tip voltage and
time and their relation to the corresponding defect densities. We find a complex behaviour
affected by the overall relative defect densities, applied bias polarity and domain writing history.
We also study the nucleation and growth of single domains at the nanoscale and observe their
stochastic nucleation and creep motion of the domain walls, following a logarithmic time
dependence.
Forum Committee: C. Lichtensteiger, N. Ubrig, A. Tamai (22.01.2018)

Lieu

Bâtiment: Ecole de Physique

Auditoire Stückelberg

entrée libre