Direct visualization of ultrafast coupling between charge carriers and lattice degrees of freedom in photoexcited semiconductors has remained a long-standing challenge and is critical for understanding the light-induced physical behaviour of materials under extreme non-equilibrium conditions. Here we obtain a direct visualization of the structural dynamics in monocrystalline 2D perovskites. We achieve this by monitoring the evolution of wavevector-resolved ultrafast electron diffraction intensity following above-bandgap high-density photoexcitation. Our analysis reveals a light-induced ultrafast reduction in antiferro-distortion resulting from a strong interaction between the electron–hole plasma and perovskite lattice, which induces an in-plane octahedra rotation towards a more symmetric phase. Correlated ultrafast spectroscopy performed at the same carrier density as ultrafast electron diffraction reveals that the creation of a dense electron–hole plasma triggers the relaxation of lattice distortion at shorter timescales by modulating the crystal cohesive energy. Finally, we show that the interaction between carrier gas and lattice can be altered by tailoring the rigidity of the 2D perovskite by choosing an appropriate organic spacer layer. The ultrafast structural dynamics in 2D perovskites are an important part of their non-equilibrium properties. Now, their visualization reveals a light-induced reduction in the antiferro-distortion initiated by the electron–hole plasma. 

/pdf/ultrafast-relaxation-of-lattice-distortion-in-two-3eu5n1cq.pdf

Ultrafast relaxation of lattice distortion in two-dimensional perovskites

We investigate the influence of three- and four-phonon scattering, perturbative anharmonic phonon renormalization, and off-diagonal terms of coherent phonons on the thermal conductivity of ${\mathrm{CsPbBr}}_{3}$ phase change perovskite, by using advanced implementations and first-principles simulations. Our study spans a wide temperature range covering the entire structural spectrum. Notably, we demonstrate that the interactions between acoustic and optical phonons result in contrasting trends of phonon frequency shifts for the high-lying optical phonons in orthorhombic and cubic ${\mathrm{CsPbBr}}_{3}$ as temperature varies. Our findings highlight the significance of wavelike tunneling of coherent phonons in ultralow and glasslike thermal conductivity in halide perovskites. 

Role of high-order anharmonicity and off-diagonal terms in thermal conductivity: A case study of multiphase <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi>CsPbBr</mml:mi><mml:mn>3</mml:mn></mml:msub></mml:math>

We present a joint experimental and computational study of second harmonic generation and Kerr effect induced by intense terahertz electric fields in the quantum paraelectric material ${\mathrm{KTaO}}_{3}$. The single-cycle terahertz (THz) electric pulses induce the free induction decay of the polar soft phonon mode and a ferroelectric polarization response that lasts for several picoseconds in both experiment and computation. We observe a much longer-lasting (43-ps) transient Kerr effect that follows the THz excitation. We explain the disparity in the relaxation times between the THz-induced second harmonic generation and the Kerr effect by the presence of the polar nanoregions intrinsic to ${\mathrm{KTaO}}_{3}$. The origin of the slow THz Kerr effect dynamics is not presently understood. We speculate that it may be associated with the rotational motion of polar nanoregions due to the material's proximity to the ferroelectric phase transition. 

/pdf/terahertz-pulse-induced-second-harmonic-generation-and-kerr-1dcry8hm.pdf

Terahertz pulse induced second harmonic generation and Kerr effect in the quantum paraelectric 
KTaO3

We reveal a photoinduced control of ferroelectricity in hybrid-improper ferroelectric superlattices, using ab initio theory. Along with a notable photostriction effect, thermalized carriers from photoexcitation affect octahedral tiltings along different directions. A trilinear coupling between antipolar and tilting modes drives corresponding cationic displacements, and this results in a decrease in polarization under light. Together with recent experimental evidence, our study therefore demonstrates that light is a viable route to engineer functionalities in materials. 

Photoinduced control of ferroelectricity in hybrid-improper ferroelectric superlattices

Direct visualization of ultrafast lattice ordering triggered by an electron-hole plasma in 2D perovskites Hao Zhang, Wenbin Li, Joseph Essman, Claudio Quarti, Isaac Metcalf, Wei-Yi Chiang, Siraj Sidhik, Jin Hou, Austin Fehr, Andrew Attar, Ming-Fu Lin, Alexander Britz, Xiaozhe Shen, Stephan Link, Xijie Wang, Uwe Bergmann, Mercouri G. Kanatzidis, Claudine Katan, Jacky Even, Jean-Christophe Blancon* and Aditya D. Mohite* Department of Chemical and Biomolecular Engineering, Rice University, Houston, Texas 77005, USA. Applied Physics Program, Smalley‐Curl Institute, Rice University, Houston, TX, 77005, USA.

Direct visualization of ultrafast lattice ordering triggered by an electron-hole plasma in 2D perovskites

We use an x-ray free-electron laser to study the lattice dynamics following photoexcitation with ultrafast near-UV light (wavelength 266 nm, 50 fs pulse duration) of the incipient ferroelectric potassium tantalate, ${\mathrm{KTaO}}_{3}$. By probing the lattice dynamics corresponding to multiple Brillouin zones through the x-ray diffuse scattering with pulses from the Linac Coherent Light Source (LCLS) (wavelength 1.3 \AA{} and $&lt;10\text{ }\text{ }\mathrm{fs}$ pulse duration), we observe changes in the diffuse intensity associated with a hardening of the transverse acoustic phonon branches along $\mathrm{\ensuremath{\Gamma}}$ to $X$ and $\mathrm{\ensuremath{\Gamma}}$ to $M$. Using force constants from density functional theory, we fit the quasiequilibrium intensity and obtain the instantaneous lattice temperature and density of photoexcited charge carriers. The density functional theory calculations demonstrate that photoexcitation transfers charge from oxygen $2p$ derived $\ensuremath{\pi}$-bonding orbitals to Ta $5d$ derived antibonding orbitals, further suppressing the ferroelectric instability and increasing the stability of the cubic, paraelectric structure. 

/pdf/ultrafast-suppression-of-the-ferroelectric-instability-in-2hk6zr4f.pdf

Ultrafast Suppression of the Ferroelectric Instability in 
<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi>KTaO</mml:mi></mml:mrow><mml:mrow><mml:mn>3</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>

We use an x-ray free-electron laser to study the lattice dynamics following photoexcitation with ultrafast near-UV light (wavelength 266 nm, 50 fs pulse duration) of the incipient ferroelectric potassium tantalate, KTaO_{3}. By probing the lattice dynamics corresponding to multiple Brillouin zones through the x-ray diffuse scattering with pulses from the Linac Coherent Light Source (LCLS) (wavelength 1.3 Å and <10  fs pulse duration), we observe changes in the diffuse intensity associated with a hardening of the transverse acoustic phonon branches along Γ to X and Γ to M. Using force constants from density functional theory, we fit the quasiequilibrium intensity and obtain the instantaneous lattice temperature and density of photoexcited charge carriers. The density functional theory calculations demonstrate that photoexcitation transfers charge from oxygen 2p derived π-bonding orbitals to Ta 5d derived antibonding orbitals, further suppressing the ferroelectric instability and increasing the stability of the cubic, paraelectric structure.

Ultrafast Suppression of the Ferroelectric Instability in KTaO_{3}.

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Ultrafast Suppression of the Ferroelectric Instability in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi>KTaO</mml:mi></mml:mrow><mml:mrow><mml:mn>3</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>

Ultrafast Suppression of the Ferroelectric Instability in KTaO_{3}.