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Register a new userNonlinear phononics in 2D SnTe: a ferroelectric material with phonon dynamical amplification of electric polarization
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Ultrafast optical control of ferroelectricity using intense terahertz fields has attracted significant interest. Here we show that the nonlinear interactions between two optical phonons in SnTe, a two-dimensional in-plane ferroelectric material, enables a dynamical amplification of the electric polarization within subpicoseconds time domain. Our first principles time dependent simulations show that the infrared-active out-of-plane phonon mode, pumped to nonlinear regimes, spontaneously generates in-plane motions, leading to rectified oscillations in the in-plane electric polarization. We suggest that this dynamical control of ferroelectric material, by nonlinear phonon excitation, can be utilized to achieve ultrafast control of the photovoltaic or other non-linear optical responses.
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Nonlinear interactions between phonon modes govern the behavior of vibrationally highly excited solids and molecules. Here, we demonstrate theoretically that optical cavities can be used to control the redistribution of energy from a highly excited coherent infrared-active phonon state into the other vibrational degrees of freedom of the system. The hybridization of the infrared-active phonon mode with the fundamental mode of the cavity induces a polaritonic splitting that we use to tune the nonlinear interactions with other vibrational modes in and out of resonance. We show that not only can the efficiency of the redistribution of energy be enhanced or decreased, but also the underlying scattering mechanisms may be changed. This work introduces the concept of cavity control to the field of nonlinear phononics, enabling nonequilibrium quantum optical engineering of new states of matter.
Ultrashort light pulses can selectively excite charges, spins and phonons in materials, providing a powerful approach for manipulating their properties. Here we use femtosecond laser pulses to coherently manipulate the electron and phonon distributions, and their couplings, in the charge density wave (CDW) material 1T-TaSe$_2$. After exciting the material with a short light pulse, spatial smearing of the electrons launches a coherent lattice breathing mode, which in turn modulates the electron temperature. This indicates a bi-directional energy exchange between the electrons and the strongly-coupled phonons. By tuning the laser excitation fluence, we can control the magnitude of the electron temperature modulation, from ~ 200 K in the case of weak excitation, to ~ 1000 K for strong laser excitation. This is accompanied by a switching of the dominant mechanism from anharmonic phonon-phonon coupling to coherent electron-phonon coupling, as manifested by a phase change of $pi$ in the electron temperature modulation. Our approach thus opens up possibilities for coherently manipulating the interactions and properties of quasi-2D and other quantum materials using light.
We report phonon renormalization induced by an external electric field E in ferroelectric poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)] nanofibers through measuring the E-dependent thermal conductivity. Our experimental results are in excellent agreement with the theoretical ones derived from the lattice dynamics. The renormalization is attributed to the anharmonicity that modifies the phonon spectrum when the atoms are pulled away from their equilibrium positions by the electric field. Our finding provides an efficient way to manipulate the thermal conductivity by tuning external fields in ferroelectric materials.
We report that the lowest energy transverse-optic phonon in metallic SnTe softens to near zero energy at the structural transition at $T_C=75 text{~K}$ and importantly show that the energy of this mode below $T_C$ increases as the temperature decreases. Since the mode is a polar displacement this proves unambiguously that SnTe undergoes a ferroelectric displacement below $T_C$. Concentration gradients and imperfect stoichiometry in large crystals may explain why this was not seen in previous inelastic neutron scattering studies. Despite SnTe being metallic we find that the ferroelectric transition is similar to that in ferroelectric insulators, unmodified by the presence of conduction electrons: we find that (i) the damping of the polar mode is dominated by coupling to acoustic phonons rather than electron-phonon coupling (ii) the transition is almost an ideal continuous transition (iii) comparison with density functional calculations identifies the importance of dipolar-dipolar screening for understanding this behaviour.
We propose a new 2D semiconductor material (TTA-2D) based on the molecular structure of Thiophene-Tetrathia-Annulene (TTA). The TTA-2D structural, electronic, and optical properties were investigated using textit{ab initio} methods. Our results show that TTA-2D is a small indirect bandgap semiconductor ($0.6$ eV). A semiconductor-metal transition can be induced by applying a uniaxial strain. Our results also show that TTA-2D is thermally stable up to $T=1000$ K. TTA-2D absorbs in a large spectral range, from infrared to ultraviolet regions. Values of refractive index and reflectivity show that TTA-2D reflects only $10%$ of the incident light in the visible region. These results suggest that TTA-2D is a promising material for solar cell applications.
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