566 research outputs found
Polar phonons in some compressively stressed epitaxial and polycrystalline SrTiO3 thin films
Several SrTiO3 (STO) thin films without electrodes processed by pulsed laser
deposition, of thicknesses down to 40 nm, were studied using infrared
transmission and reflection spectroscopy. The complex dielectric responses of
polar phonon modes, particularly ferroelectric soft mode, in the films were
determined quantitatively. The compressed epitaxial STO films on (100)
La0.18Sr0.82Al0.59-Ta0.41O3 substrates (strain 0.9%) show strongly stiffened
phonon responses, whereas the soft mode in polycrystalline film on (0001)
sapphire substrate shows a strong broadening due to grain boundaries and/or
other inhomogeneities and defects. The stiffened soft mode is responsible for a
much lower static permittivity in the plane of the compressed film than in the
bulk samples.Comment: 11 page
Central mode and soft mode behavior in PbMg1/Nb2/3O3 relaxor ferroelectric
The relaxor ferroelectric PbMg1/Nb2/3O3 was investigated by means of
broad-band dielectric and Fourier Transform Infrared (FTIR) transmission
spectroscopy in the frequency range from 1 MHz to 15 THz at temperatures
between 20 and 900 K using PMN films on infrared transparent sapphire
substrates. While thin film relaxors display reduced dielectric permittivity at
low frequencies, their high frequency intrinsic or lattice response is shown to
be the same as single crystal/ceramic specemins. It was observed that in
contrast to the results of inelastic neutron scattering, the optic soft mode
was underdamped at all temperatures. On heating, the TO1 soft phonon followed
the Cochran law with an extrapolated critical temperature equal to the Burns
temperature of 670 K and softened down to 50 cm-1. Above 450 K the soft mode
frequency leveled off and slightly increased above the Burns temperature. A
central mode, describing the dynamics of polar nanoclusters appeared below the
Burns temperature at frequencies near the optic soft mode and dramatically
slowed down below 1 MHz on cooling below room temperature. It broadened on
cooling, giving rise to frequency independent losses in microwave and lower
frequency range below the freezing temperature of 200 K. In addition, a new
heavily damped mode appeared in the FTIR spectra below the soft mode frequency
at room temperature and below. The origin of this mode as well as the
discrepancy between the soft mode damping in neutron and infrared spectra is
discussed.Comment: 7 pages with 7 figures, submitted to Phys. Rev.
Origin of the "Waterfall" Effect in Phonon Dispersion of Relaxor Perovskites
Inelastic neutron scattering study of the perovskite relaxor ferroelectric
PZN:8%PT elucidates the origin of the previously reported unusual kink on the
low frequency transverse phonon dispersion curve (known as "waterfall" effect).
We show that its position depends on the choice of the Brillouin zone and that
the relation of its position to the size of the polar nanoregions is highly
improbable. The observation is explained in the framework of a simple model of
coupled damped harmonic oscillators representing the acoustic and optic phonon
branches.Comment: 4 pages, 6 figures, LaTe
Peculiar Bi-ion dynamics in Na1/2Bi1/2TiO3 from terahertz and microwave dielectric spectroscopy
Dynamics of the main dielectric anomaly in Na1/2Bi1/2TiO3 (NBT) was studied
by time-domain THz and microwave spectroscopy, using also previously published
data and their new overall fits. Above the dielectric maximum temperature Tm ~
600 K, the response consists of coupled sub-THz oscillator and a relaxation
mode, assigned to strongly anharmonic Bi-ion vibrations and hopping, whose
slowing down explains the paraelectric-like permittivity increase to Tm. Below
Tm, the main relaxation continues slowing down and additional relaxation,
assigned to quasi-Debye losses, appears in the 10^11 Hz range. The oscillator
hardens on cooling and takes over the whole oscillator strength. The
permittivity decrease below Tm is caused by the reduced strength of the
relaxations due to dominance of the rhombohedral phase within the coexistence
region with the tetragonal phase. The anharmonic dynamics of Bi is supported by
previous structural studies. NBT represents a hybrid between standard and
relaxor ferroelectric behaviour
Pressure-induced changes in the optical properties of quasi-one-dimensional -NaVO
The pressure-induced changes in the optical properties of
-NaVO single crystals at room temperature were studied
by polarization-dependent Raman and far-infrared reflectivity measurements
under high pressure. From the changes in the Raman- and infrared-active phonon
modes in the pressure range 9 - 12 GPa a transfer of charge between the
different V sites can be inferred. The importance of electron-phonon coupling
in the low-pressure regime is discussed.Comment: 7 pages, 5 figure
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