Kenta Kimura

Ultrafast conversion from a high- to low-symmetry structure is important for optical-switching devices. To realize such a conversion, we used nonlinear phononics. Exciting infrared-active modes with an intense terahertz pulse generates symmetry-reducing Raman modes via anharmonic couplings with the infrared-active modes. A terahertz-pump optical-Kerr-ellipticity-probe method was applied to a ferroaxial material, PbWO ₄ , using terahertz pulses with two different bandwidths, revealing that Raman-mode dynamics induced by an anharmonic mode coupling can be distinguished from those due to the stimulated Raman scattering. Our results showed that the anharmonic mode coupling comprises two components. Under terahertz light, forced oscillations of the infrared-active modes drive coherent Raman mode oscillations. After the terahertz light disappears, the square of the infrared-active mode amplitudes induces Raman-mode displacements. By narrowing the bandwidth and broadening the temporal width of the terahertz pulse, the Raman mode displacements are rectified over several picoseconds. This approach is promising for generating a new state with lower symmetry.

We report a comprehensive NMR study of the chiral square-cupola antiferromagnet Pb ( TiO ) Cu 4 ( PO 4 ) 4 and compare its microscopic hyperfine and local-field parameters with the Ba/Sr analogs in the A ( TiO ) Cu 4 ( PO 4 ) 4 family. Above T N ≃ 6.7 K , the P 31 Knight shift tracks the bulk susceptibility and yields nearly isotropic transferred hyperfine couplings H hf [ 010 ] = 6.77 ( 3 ) and H hf [ 001 ] = 6.19 ( 3 ) kOe/ μ B . Below T N , the frequency-swept P 31 spectrum splits into three lines, in contrast to the four-line pattern reported for BaTCPO. The line separation tracks the onset of the static P 31 internal field with a power-law exponent β ≃ 0.23 , consistent with quasi-two-dimensional criticality. Crystal-rotation P 31 NMR in the ordered state resolves all eight symmetry-related P sites and their site-dependent anisotropy. In the ordered state, zero-field Cu 63 , 65 NMR gives a Cu-site internal field B int = 14.50 ( 6 ) T and a quadrupole frequency ν Q = 32.72 ( 5 ) MHz , while point-charge electric-field-gradient calculations including Sternheimer corrections yield an on-site Cu hole occupancy n d = 0.20 ( 4 ) , consistent with a ligand-hole–dominated charge-transfer character. Comparing PbTCPO with BaTCPO and SrTCPO, we find that the transferred hyperfine coupling H hf varies across the series, reflecting changes in local Cu-O-P covalency, whereas the ordered-state P 31 internal field in PbTCPO is 69.5 mT, considerably higher than in BaTCPO (35.6 mT) and SrTCPO (34.6 mT). This enhancement is not captured by dipolar terms alone and points to the combined effects of transferred contributions and stacking-dependent cancellation.

The variation of absorption coefficient with the direction of light propagation in a solid is called nonreciprocal directional dichroism (NDD). In polar magnets with polarization P and magnetization M, NDD depends on the direction of P×M; its magnitude is expected to be controlled via a change in P×M by simultaneously applying electric and magnetic fields. Here, we show that the ultrafast control of NDD can be achieved using a strong terahertz pulse in an antipolar antiferromagnet, Bi_{2}CuO_{4}, in which no macroscopic polarization and magnetization appear. By measuring the dependence of the absorption changes on the incident directions of the terahertz pump and visible probe pulses, as well as on the phase of magnetic domains, the primary component of the NDD changes induced by the terahertz pulse is demonstrated to be proportional to the product of its orthogonal electric field E_{THz} and magnetic field B_{THz}. This is ascribed to the magnetoelectric effect modulation by the electromagnetic fields.

Chirality-induced spin selectivity (CISS) encompasses phenomena such as magnetoresistance and enantiomer separation using ferromagnets. It has recently received notable attention. Despite this growing interest, the microscopic mechanisms driving CISS remain a subject of intense debate. This paper complements and extends the prevailing interpretation that attributes CISS primarily to the electric current in chiral molecules. We propose that molecular vibrations drive spin polarization in chiral molecules and play a critical role in CISS. Our results suggest that magnetic interactions between chiral molecules and ferromagnets, analogous to interlayer exchange coupling, provide a physically consistent mechanism for the observed selectivity. Specifically, we demonstrate that molecular vibrations facilitate the alignment of spin angular momentum dependent on chirality in the presence of an external magnetic field, which is a prerequisite for exchange coupling. These findings necessitate reevaluating spin dynamics, expanding its relevance beyond traditional solid-state physics to fields such as chemical reactions, molecular biology, and drug discovery.