PHONONS 2007
Journal of Physics: Conference Series 92 (2007) 012177
IOP Publishing
doi:10.1088/1742-6596/92/1/012177
Ultrafast structure and polarization dynamics in
nanolayered perovskites studied by femtosecond
X-ray diffraction
C v Korff Schmising1 , M Bargheer2 , M Kiel2 , N Zhavoronkov1 , M
Woerner1 , T Elsaesser1 , I Vrejoiu3 , D Hesse3 and M Alexe3
1
Max-Born-Institut für Nichtlineare Optik und Kurzzeitspektroskopie, 12489 Berlin, Germany
Institut fr Physik, Universität Potsdam, Am Neuen Palais 10, 14469 Potsdam, Germany
3
Max-Planck-Institut für Mikrostrukturphysik, Weinberg 2, D-06120, Halle, Germany
2
E-mail: [email protected]
Abstract. The polarization and lattice dynamics in a metal/ferroelectric/metal nanolayer
system is studied by femtosecond x-ray diffraction. Two Bragg reflections provide information
on the coupled dynamics of the two relevant phonon modes for ferroelectricity in perovskites,
the tetragonal distortion and the ion displacement. Optical excitation of the metallic SrRuO3
(SRO) layers generates giant stress, compressing the ferroelectric PbTr0.2 Ti0.8 O3 (PZT) layers
by up to 2%. The maximum elongation of the tetragonal mode is reached after 1.3 ps. With a
slight delay the ferroelectric polarization P is reduced by up to 100 percent that is due to the
anharmonic coupling of the two modes.
Nanolayers with a perovskite crystal structure are important ingredients of future microelectronics, since not only metals (SRO) and dielectrics but also ferroelectrics (PZT) can be
incorporated into novel devices on the nanoscale with epitaxial accuracy [1, 2, 3]. The static
dielectric and structural properties of such nanostructures have been analyzed in great detail [4],
while experimental data about their dynamical properties and in particular their ferroelectricity
has remained limited. In ferroelectric materials the lattice energy displays a double-minimum
potential along the soft-mode distortion, i.e., the relative displacement of anions and cations ξ
within the unit cell. This goes along with a tetragonal distortion η in the ferroelectric phase
[5] (Fig. 1A). Here we present an ultrafast time resolved X-ray structure analysis to directly
measure the polarization dynamics of PZT in a PZT/SRO superlattice, which is triggered by
optically induced uniaxial stress in the metallic SRO layers [6].
The PZT/SRO superlattice (SL) studied here was fabricated by pulsed laser deposition [7]
and consists of 15 periods of 4 nm PZT and 6 nm SRO. The measured stationary x-ray reflectivity (red line) and a simulation based on dynamic x-ray diffraction theory (black line, Darwin
formalism [8]) are shown in Fig. 2A as a function of the diffraction angle. Together with the
assumption that the lattice constant of the SRO layers in the SL agrees with the value for a
pseudocubic thin film [9], we determine the exact parameters describing the equilibrium structure of the sample. The SL period dSL = dP ZT + dSRO gives rise to maxima (0 0 ℓ) at Bragg
angles Θ corresponding to multiples G = ℓ · gSL of the reciprocal SL vector gSL = 2π/dSL . The
Fourier transforms of a single PZT layer (green, dotted line) and of a single SRO layer (blue,
c 2007 IOP Publishing Ltd
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PHONONS 2007
Journal of Physics: Conference Series 92 (2007) 012177
IOP Publishing
doi:10.1088/1742-6596/92/1/012177
dashed line) are envelope functions, which determine the intensity of the individual reflections.
The positions of the envelopes are given by the average lattice constants (cSRO and cPZT ) within
the respective layer. An expansion of the SRO layer and a compression of the PZT layer shift
the envelope functions to smaller and larger angles, respectively. For the (0 0 56) reflection,
which is located on the steepest slope of the two envelope functions, this implies that changes
of η result in an increased reflected intensity. Differently the (0 0 55) reflection is located at the
maximum of the PZT envelope function and its intensity is particularly sensitive to the ionic
displacement ξ within the PZT unit cell.
Figure 1. A) Tetragonal distortion η = c/a and ferroelectric ion displacements ξPb and ξTi/Zr
in the unit cell of PZT. B) TEM image of a thinned PZT/SRO SL.
In the femtosecond experiments, the sample is excited by a 50 fs pump pulse at 800 nm
which interacts exclusively with the SRO layers. The resulting lattice response is probed by an
ultrashort hard x-ray pulse (Cu Kα , photon energy 8.05 keV, λ = 0.154 nm) which is diffracted
from the excited sample. Changes of the diffracted intensity are measured as a function of
pump-probe delay. The experimental setup has been described in detail elsewhere [10, 11].
In Fig. 2B we show the transient reflectivity change of the (0 0 56) reflection for a pump
fluence of 2 mJ/cm2 . After 1.5 ps we measure an increase of ∆R/R0 = 0.4 which is followed by
intensity oscillations with a period of 2.2 ps. Fig. 2C compares the transient change of the x-ray
reflectivity ∆R/R0 of the (0 0 56) (black squares) and (0 0 55) (red dots) Bragg reflections of the
PZT/SRO sample for a moderate pump fluence of 7.5 mJ/cm2 . Fig. 3A shows the reflectivity
change of the (0 0 56) SL-Bragg peak on a logarithmic time scale up to 200 ps (pump fluence 9
mJ/cm2 ). After 1.5 ps the maximal intensity change reaches 180% and drops to approximately
100% for later times. The simultaneously measured change of the SL period ∆dSL /d0 as a
function of the time delay is plotted in Fig. 3B. The expansion of the SL reaches a maximum
after 30 ps and remains constant up to 200 ps.
The 800 nm pump pulse generates an electronic excitation in the SRO layers with a spatial periodicity 1/dSL . Electron-phonon coupling results in the generation of coherent acoustic
phonon motions with wavevector g = 2π/dSL . Such elongations along a SL mode periodically
modulate the SRO and PZT layer thicknesses (i.e., tetragonal distortions ηPZT and ηSRO ) with
a period of 2.2 ps, determined by dSL and the respective velocity of sound. The expansion of the
entire SL structure originates from strain fronts starting from the interfaces of the SL to air and
to the substrate, where the stress is not balanced [12]. The shift of the Bragg peak positions
occur on a timescale T = N · dSL /vph ≈ 30ps, where N is the number of SL layers and vph ≈ 5
nm/ps is the phonon group velocity.
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PHONONS 2007
Journal of Physics: Conference Series 92 (2007) 012177
IOP Publishing
doi:10.1088/1742-6596/92/1/012177
1
A
0055 0056
R0
∆R / R0
0.1
0.01
0.5
1.2
0 0 55
D
Θ (deg)
B
0056
0.2
1.044
1.0
0.8
1.040
0.6
1.036
0.4
0.0
0
2
4
time (ps)
6
8
0
1
2
time (ps)
ηPZT
21.5 22.0 22.5 23.0 23.5
ξ/ξ0=P/P0
1E-4
∆R/R0
0 0 56
1.0
0.0
1E-3
0.4
C
1.5
1.032
3
Figure 2. (color online) A) Measured (red solid line) and simulated (black dash-dotted line)
stationary x-ray reflectivity of the SL as a function of the Bragg angle Θ. Substrate Peak at
23.30 . B) Oscillatory reflectivity change of the (0 0 56) SL-Bragg peak for a pump fluence of 2
mJ/cm2 . C) Comparison of the transient change of the (0 0 56) (black squares) and (0 0 55)
(hollow red dots) reflectivities for a moderate fluence of 7.5 mJ/cm2 . D) Derived transient
change of the tetragonality η(t) and polarization ξ/ξ0 = P/P0 .
The anharmonic coupling of the directly driven tetragonal distortion η and the soft mode
coordinate ξ results in a simultaneous elongation of the latter and a change of the polarization
P. Our measurements for two different Bragg peaks allow for a quantitative analysis of the
microscopic lattice dynamics, i.e., the time-dependent elongations along the two coordinates
ξ and η [6]. The intensity of the two SL peaks behaves differently as a function of the two
coordinates and allows us to calculate the time-dependence of ηP ZT and ξ/ξ0 ∼ P/P0 as plotted
in Fig. 2D. The tetragonal distortion of PZT reaches a maximum after 1.3 ps. The relative
soft mode coordinate ξ/ξ0 shows an ultrafast and slightly delayed reduction, corresponding to
decrease of the polarization P/P0 of approximately 60%. For the highest excitation fluence the
measured ∆Rmax /R0 = 3 of the (0 0 56) peak determines a peak strain ∆η/η0 = 2.3% (cf.
Fig. 3C). This corresponds to a complete switch-off of the polarization P.
Simultaneous analysis of the intensity change and change of the superlattice period ∆dSL /d0 of
the (0 0 56) Bragg reflection allows to determine the tetragonal distortion η of the PZT and SRO
layer separately. Fig. 3C compares ∆η/η0 after 1.5 ps and 200 ps and shows a linear dependence
on the excitation fluence. After 200 ps the metallic SRO layers are still expanded nearly 50%
of the peak value, which is an order of magnitude larger than the estimated thermal expansion
(αSRO = 1.5 × 10−5 ). The calculated change of η due to an estimated temperature rise of 30
K is plotted in Fig. 3C as the dashed, green line. In contrast, the PZT shows a much smaller
compression which can be explained by the induced temperature rise and the negative thermal
expansion coefficient of PZT (αP ZT ≈ −7 × 10−5 , dashed, blue line in Fig. 3C). This implies
that the tetragonal distortion η0 is restored after 30 ps and the recovery of the PZT polarization
(ξP b−T i (30 ps) ≈ ξ0 ) is directly connected to the expansion of the entire SL.
In conclusion we have presented a time-resolved ultrafast X-ray structure analysis of lattice and
3
PHONONS 2007
Journal of Physics: Conference Series 92 (2007) 012177
2.0
IOP Publishing
doi:10.1088/1742-6596/92/1/012177
0.02
A
C
(0 0 56)
∆R/R0
1.5
0.01
1.0
SRO
∆η/η0
0.5
-3
∆dSL/d0 [10 ]
0.0
0
B
(0 0 56)
0.00
PZT
-0.01
1
2
-0.02
3
1
10
100
0
time (ps)
3
6
9
12 15
2
excitation fluence (mJ/cm )
Figure 3. A) Transient change of the (0 0 56) reflectivitiy on a logarithmic timescale (Pump
fluence 9 mJ/cm2 ) B) Simultaneously measured change of the SL period ∆dSL /d0 as a function
of the time delay. The expansion of the SL reaches its maximum after 30 ps. C) ∆η/η0 of the
SRO and PZT layers after 1.5 ps (hollow symbols) and 200 ps (filled symbols). The solid lines
are guides to the eye. The calculated changes of η of the SRO and PZT layers due to thermal
expansion and compression are shown as dashed lines.
polarization dynamics in a ferroelectric nanolayer. Analysis of complementary Bragg reflections
allows to derive the ultrafast dynamical behavior of the two coupled modes (η and ξ) that
are relevant for the ferroelectric polarization P . Our work shows the feasibility of active
ultrafast manipulation of electric fields in nanostructures by optically generated mechanical
stress and exemplifies how ultrafast x-ray diffraction can be used to monitor terahertz strain
waves, which will be highly relevant in the construction of novel terahertz devices based on
perovskite technology.
We gratefully acknowledge the financial support by the Deutsche Forschungsgemeinschaft
(SPP1134).
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