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Register a new userUltrafast terahertz-field-driven ionic response in ferroelectric BaTiO$_3$
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The dynamical processes associated with electric field manipulation of the polarization in a ferroelectric remain largely unknown but fundamentally determine the speed and functionality of ferroelectric materials and devices. Here we apply sub-picosecond duration, single-cycle terahertz pulses as an ultrafast electric field bias to prototypical BaTiO$_3$ ferroelectric thin films with the atomic-scale response probed by femtosecond x-ray scattering techniques. We show that electric fields applied perpendicular to the ferroelectric polarization drive large amplitude displacements of the titanium atoms along the ferroelectric polarization axis, comparable to that of the built-in displacements associated with the intrinsic polarization and incoherent across unit cells. This effect is associated with a dynamic rotation of the ferroelectric polarization switching on and then off on picosecond timescales. These transient polarization modulations are followed by long-lived vibrational heating effects driven by resonant excitation of the ferroelectric soft mode, as reflected in changes in the c-axis tetragonality. The ultrafast structural characterization described here enables direct comparison with first-principles-based molecular dynamics simulations, with good agreement obtained.
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Intense, few-cycle pulses in the terahertz frequency range have strong potential for schemes of control over vibrational modes in solid-state materials in the electronic ground-state. Here we report an experiment using single cycle terahertz pulses to directly excite lattice vibrations in the ferroelectric material $mathrm{Sn_2P_2S_6}$ and ultrafast x-ray diffraction to quantify the resulting structural dynamics. A model of a damped harmonic oscillator driven by the transient electric field of the terahertz pulses describes well the movement of the Sn$^{2+}$ ion along the ferroelectric soft mode. Finally, we describe an anharmonic extension of this model which predicts coherent switching of domains at peak THz-frequency fields of 790 kV/cm.
We use ultrafast x-ray pulses to characterize the lattice response of SrTiO3 when driven by strong terahertz (THz) fields. We observe transient changes in the diffraction intensity with a delayed onset with respect to the driving field. Fourier analysis reveals two frequency components corresponding to the two lowest energy zone-center optical modes in SrTiO3. The lower frequency mode exhibits clear softening as the temperature is decreased while the higher frequency mode shows slight temperature dependence.
We find that in BaTiO$_3$ the phonon angular momentum is dominantly pointing in directions perpendicular to the electrical polarization. Therefore, external electric field in ferroelectric BaTiO$_3$ does not control only the direction of electrical polarization, but also the direction of phonon angular momentum. This finding opens up the possibility for electric-field control of physical phenomena that rely on phonon angular momentum. We construct an intuitive model, based on our first-principles calculations, that captures the origin of the relationship between phonon angular momentum and electric polarization.
The recent discovery of topology-protected charge transport of ultimate thinness on surfaces of three-dimensional topological insulators (TIs) are breaking new ground in fundamental quantum science and transformative technology. Yet a challenge remains on how to isolate and disentangle helical spin transport on the surface from bulk conduction. Here we show that selective midinfrared femtosecond photoexcitation of exclusive intraband electronic transitions at low temperature underpins topological enhancement of terahertz (THz) surface transport in doped Bi2Se3, with no complication from interband excitations or need for controlled doping. The unique, hot electron state is characterized by conserved populations of surface/bulk bands and by frequency-dependent hot carrier cooling times that directly distinguish the faster surface channel than the bulk. We determine the topological enhancement ratio between bulk and surface scattering rates, i.e., $gamma_text{BS}/gamma_text{SS}sim$3.80 in equilibrium. These behaviors are absent at elevated lattice temperatures and for high pumpphoton frequencies and uences. The selective, mid-infrared-induced THz conductivity provides a new paradigm to characterize TIs and may apply to emerging topological semimetals in order to separate the transport connected with the Weyl nodes from other bulk bands.
The three dimensional distribution of the X-ray diffuse scattering intensity of BaTiO$_3$ has been recorded in a synchrotron experiment and simultaneously computed using molecular dynamics simulations of a shell-model. Together these have allowed the details of the disorder in paraelectric BaTiO$_3$ to be clarified. The narrow sheets of diffuse scattering, related to the famous anisotropic longitudinal correlations of Ti ions, are shown to be caused entirely by the overdamped anharmonic soft phonon branch. This finding demonstrates that the occurrence of narrow sheets of diffuse scattering agrees with a displacive picture of the cubic phase of this textbook ferroelectric material.
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