Ultrafast Photovoltaic Response in Ferroelectric Nanolayers Page: 2 of 17
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We show that light drives large-amplitude structural changes in thin films of the prototypical
ferroelectric PbTiO3 via direct coupling to its intrinsic photovoltaic response. Using time-
resolved x-ray scattering to visualize atomic displacements on femtosecond timescales,
photoinduced changes in the unit-cell tetragonality are observed. These are driven by the motion
of photogenerated free charges within the ferroelectric and can be simply explained by a model
including both shift and screening currents, associated with the displacement of electrons first
antiparallel to and then parallel to the ferroelectric polarization direction.
Light couples to atomic-scale degrees of freedom in complex materials, offering new
avenues for engineering functionality in nanoelectronic devices and enhancing material
properties. The key functionality of ferroelectrics-a thermodynamically stable, switchable
polarization that persists even in the absence of an applied electric field-itself destabilizes the
ferroelectric phase in thin films, due to the existence of an internal depolarization field [1-7],
associated with the surface charge density a = P - n (where P is the electric polarization and n is
the unit normal vector), that points opposite to the direction of the polarization. This field can be
screened by surface adsorbates or free charges [1,5], as in the simple case of ferroelectric
capacitors with metallic electrodes, and can lead to the formation of stripe domains: periodic
nanometer-scale domains of alternating polarization that minimize the free energy of the
system [3,4,8]. Intensive research has been aimed at bypassing the intrinsic size limits imposed
by the depolarization field on the ferroelectric phase, including epitaxial strain
engineering [2,9,10] and the use of electrodes to screen the depolarization field [6,11].
At the same time, ongoing work over several decades [12-21] has sought to elucidate
photovoltaic effects in ferroelectrics and multiferroics and the role of the depolarization field in
these phenomena [15,19]. These are associated with the splitting of electron-hole pairs within a
noncentrosymmetric crystal and lead to the generation of open-circuit voltages much larger than
the bandgap. A number of mechanisms have been implicated in these effects, including the
presence of the internal depolarization field in thin films , internal fields at domain
walls , and second-order nonlinear optical responses in the field of the incident light in the
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Daranciang, Dan. Ultrafast Photovoltaic Response in Ferroelectric Nanolayers, article, February 15, 2012; United States. (digital.library.unt.edu/ark:/67531/metadc836961/m1/2/: accessed February 23, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.