Observation of polarization density waves in SrTiO3
arxiv(2024)
Abstract
The nature of the "failed" ferroelectric transition in SrTiO3 has been a
long-standing puzzle in condensed matter physics. A compelling explanation is
the competition between ferroelectricity and an instability with a mesoscopic
modulation of the polarization. These polarization density waves, which should
become especially strong near the quantum critical point, break local inversion
symmetry and are difficult to probe with conventional x-ray scattering methods.
Here we combine a femtosecond x-ray free electron laser (XFEL) with THz
coherent control methods to probe inversion symmetry breaking at finite momenta
and visualize the instability of the polarization on nanometer lengthscales in
SrTiO3. We find polar-acoustic collective modes that are soft particularly at
the tens of nanometer lengthscale. These precursor collective excitations
provide evidence for the conjectured mesoscopic modulated phase in SrTiO3.
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