Signatures of Ultrafast Reversal of Excitonic Order in Ta_{2}NiSe_{5}.

In the presence of electron-phonon coupling, an excitonic insulator harbors two degenerate ground states described by an Ising-type order parameter. Starting from a microscopic Hamiltonian, we derive the equations of motion for the Ising order parameter in the phonon coupled excitonic insulator Ta_{2}NiSe_{5} and show that it can be controllably reversed on ultrashort timescales using appropriate laser pulse sequences. Using a combination of theory and time-resolved optical reflectivity measurements, we report evidence of such order parameter reversal in Ta_{2}NiSe_{5} based on the anomalous behavior of its coherently excited order-parameter-coupled phonons. Our Letter expands the field of ultrafast order parameter control beyond spin and charge ordered materials.

Optical Magnetoelectric Resonance in a Polar Magnet (Fe,Zn)_{2}Mo_{3}O_{8} with Axion-Type Coupling.

We report the polarization rotation of terahertz light resonant with the magnetoelectric (ME) spin excitation in the multiferroic (Fe,Zn)_{2}Mo_{3}O_{8}. This resonance reflects the frequency dispersion of the diagonal ME susceptibility (axion term), with which we quantitatively reproduce the thermal and magnetic-field evolution of the observed polarization rotation spectra. The application of the sum rule on the extrapolated dc value of the spectral weight of the ME oscillator provides insight into the dc linear ME effect. The present finding highlights a novel optical functionality of spin excitations in multiferroics that originates from diagonal ME coupling.

Shift current photovoltaic effect in a ferroelectric charge-transfer complex.

Shift current is a steady-state photocurrent generated in non-centrosymmetric single crystals and has been considered to be one of the major origins of the bulk photovoltaic effect. The mechanism of this effect is the transfer of photogenerated charges by the shift of the wave functions, and its amplitude is closely related to the polarization of the electronic origin. Here, we report the photovoltaic effect in an organic molecular crystal tetrathiafulvalene-p-chloranil with a large ferroelectric polarization mostly induced by the intermolecular charge transfer. We observe a fairly large zero-bias photocurrent with visible-light irradiation and switching of the current direction by the reversal of the polarization. Furthermore, we reveal that the travel distance of photocarriers exceeds 200 µm. These results unveil distinct features of the shift current and the potential application of ferroelectric organic molecular compounds for novel optoelectric devices.The bulk photovoltaics refers to an effect whereby electrons move directionally in non-centrosymmetric crystals upon light radiation. Here, Nakamura et al. observe this effect in a ferroelectric organic charge-transfer complex, which shows large diffusion distance of photogenerated electrons over 200 µm.

Giant thermal Hall effect in multiferroics.

Multiferroics, in which dielectric and magnetic orders coexist and couple with each other, attract renewed interest for their cross-correlated phenomena, offering a fundamental platform for novel functionalities. Elementary excitations in such systems are strongly affected by the lattice-spin interaction, as exemplified by the electromagnons and the magneto-thermal transport. Here we report an unprecedented coupling between magnetism and phonons in multiferroics, namely, the giant thermal Hall effect. The thermal transport of insulating polar magnets (ZnxFe1-x)2Mo3O8 is dominated by phonons, yet extremely sensitive to the magnetic structure. In particular, large thermal Hall conductivities are observed in the ferrimagnetic phase, indicating unconventional lattice-spin interactions and a new mechanism for the Hall effect in insulators. Our results show that the thermal Hall effect in multiferroic materials can be an effective probe for strong lattice-spin interactions and provide a new tool for magnetic control of thermal currents.

Magnetic-field induced competition of two multiferroic orders in a triangular-lattice helimagnet MnI2.

Magnetic and dielectric properties with varying magnitude and direction of magnetic-field H have been investigated for a triangular-lattice helimagnet MnI_{2}. The in-plane electric polarization P emerges in the proper screw magnetic ground state below 3.5 K, showing the rearrangement of six possible multiferroic domains as controlled by the in-plane H. With every 60° rotation of H around the [001] axis, discontinuous 120° flop of the P vector is observed as a result of the flop of magnetic modulation vector q. With increasing the in-plane H above 3 T, however, the stable q direction changes from q‖(110[ ¯over 0]) to q‖(110), leading to a change of P-flop patterns under rotating H. At the critical field region (∼3 T), due to the phase competition and resultant enhanced q flexibility, the P vector smoothly rotates clockwise twice while the H vector rotates counterclockwise once.