Mingqiang Gu

Observation of Quasi-Two-Dimensional Polar Domains and Ferroelastic Switching in a Metal, Ca3Ru2O7

Polar domains arise in insulating ferroelectrics when free carriers are unable to fully screen surface-bound charges. Recently discovered binary and ternary polar metals exhibit broken inversion symmetry coexisting with free electrons that might be expected to suppress the electrostatic driving force for domain formation. Contrary to this expectation, we report the first direct observation of polar domains in single crystals of the polar metal Ca3Ru2O7. By a combination of mesoscale optical second-harmonic imaging and atomic-resolution scanning transmission electron microscopy, the polar domains are found to possess a quasi-two-dimensional slab geometry with a lateral size of ∼100 μm and thickness of ∼10 nm. Electronic structure calculations show that the coexistence of electronic and parity-lifting orders arise from anharmonic lattice interactions, which support 90° and 180° polar domains in a metal. Using in situ transmission electron microscopy, we also demonstrate a strain-tuning route to achieve ferroelastic switching of polar metal domains.

Coupled Raman-Raman modes in the ionic Raman scattering process

We employ a lattice Hamiltonian to examine the dynamics of coupled normal modes excited through a nonlinear phononic process, extending the model beyond infrared and first-order Raman active mode couplings to include interactions between symmetry-allowed Raman active modes. We examine the strength of the interactions between two Raman active modes on the targeted driven-mode dynamics to confirm that all symmetry allowed Raman modes interact with each other. We apply the model to the correlated insulating ferromagnet YTiO3 and present the resulting renormalization effects on the driven-mode dynamics from the anharmonic interactions. Owing to the dependence of the displacive amplitude of the Raman active mode on such interactions, especially for those modes with small amplitude, we suggest that models of anharmonic phononic coupling in materials with electronic, ferroic, or superconducting properties derived from competing Raman-like distortions should include these low-order terms in the equations of motion describing the excited phonons to obtain accurate physical models.We employ a lattice Hamiltonian to examine the dynamics of coupled normal modes excited through a nonlinear phononic process, extending the model beyond infrared and first-order Raman active mode couplings to include interactions between symmetry-allowed Raman active modes. We examine the strength of the interactions between two Raman active modes on the targeted driven-mode dynamics to confirm that all symmetry allowed Raman modes interact with each other. We apply the model to the correlated insulating ferromagnet YTiO3 and present the resulting renormalization effects on the driven-mode dynamics from the anharmonic interactions. Owing to the dependence of the displacive amplitude of the Raman active mode on such interactions, especially for those modes with small amplitude, we suggest that models of anharmonic phononic coupling in materials with electronic, ferroic, or superconducting properties derived from competing Raman-like distortions should include these low-order terms in the equations of motio...

Ultrafast Band Engineering and Transient Spin Currents in Antiferromagnetic Oxides

We report a dynamic structure and band engineering strategy with experimental protocols to induce indirect-to-direct band gap transitions and coherently oscillating pure spin-currents in three-dimensional antiferromagnets (AFM) using selective phononic excitations. In the Mott insulator LaTiO3, we show that a photo-induced nonequilibrium phonon mode amplitude destroys the spin and orbitally degenerate ground state, reduces the band gap by 160 meV and renormalizes the carrier masses. The time scale of this process is a few hundreds of femtoseconds. Then in the hole-doped correlated metallic titanate, we show how pure spin-currents can be achieved to yield spin-polarizations exceeding those observed in classic semiconductors. Last, we demonstrate the generality of the approach by applying it to the non-orbitally degenerate AFM CaMnO3. These results advance our understanding of electron-lattice interactions in structures out-of-equilibrium and establish a rational framework for designing dynamic phases that may be exploited in ultrafast optoelectronic and optospintronic devices.