Tsuyoshi Kimura

Hematite ( α − F e 2 O 3 ) is a prototypical room-temperature antiferromagnet whose time-reversal-odd magnetic structure has recently attracted renewed attention. While such magnetic symmetry can be characterized in terms of higher-order multipoles beyond the magnetic dipole, their manifestation in measurable physical phenomena has remained largely elusive. In this work, we investigate x-ray absorption near the Fe K -edge of hematite under an applied electric field, which explicitly breaks space-inversion symmetry. We observe an electric-field-induced x-ray nonreciprocal linear dichroism that reflects the time-reversal-odd nature of the magnetic order. A symmetry-resolved multipole analysis attributes this response to the electric-field-induced magnetic quadrupole and the magnetic toroidal octupole induced by the applied electric field. Furthermore, numerical simulations based on density functional theory reproduce the observed spectra, including their dependence on the antiferromagnetic domain and x-ray polarization. These results indicate that electric-field-modulated x-ray absorption measurements, combined with simulations, provide a framework for identifying the higher-order multipoles contributing to measurable physical responses in time-reversal-odd antiferromagnets, thereby facilitating the investigation of hidden symmetry properties in magnetic materials.

Magnetoelectric multiferroics promise direct cross control between coexisting ferroelectric and ferromagnetic orders, which is of interest for applications in magnetism and spintronics. A particularly interesting type of cross control is found in spin-spiral multiferroic Mn 2 GeO 4 , in which a ferroelectric multidomain distribution can be globally inverted by a single magnetic field sweep. In this work we show that this repeatable response is not obtained immediately after entering the multiferroic phase via zero-field coolling. Instead, an initial metastable multidomain state forms in which polarization, magnetization, and antiferromagnetic order do not yet minimize the trilinear magnetoelectric coupling. Using optical second-harmonic generation for ferroelectric domain imaging, we demonstrate that a single field cycle deterministically drives the system from the initial metastable state into an equilibrium domain configuration that shows repeatable domain interconversion. This initialization behavior originates from a deterministic pathway from metastable to equilibrium domain patterns, in contrast to the more common—and less reliable—stochastic domain “training” procedures that require repeated field cycles. Understanding the initial domain evolution thus enables reliable cross control in magnetoelectric devices with highly interlinked order parameters.

Quantum phase transitions are driven at absolute zero by quantum‐mechanical fluctuations, offering unique opportunities for the emergence of exotic phenomena ranging from unconventional superconductivity to quantum magnetism. In multiferroics, where magnetic and ferroelectric orders are cross‐correlated, this raises an intriguing possibility: a state of quantum criticality in which both fluctuations are intertwined. Here we show that the quasi‐one‐dimensional Ising antiferromagnet Sr _{1− x} Ba _x Co ₂ V ₂ O ₈ hosts multiferroic quantum criticality through the coexistence of ferroelectric and magnetic quantum critical points (QCPs) tuned by independent non‐thermal parameters. Structural and dielectric analyses establish this system as a proper displacive‐type ferroelectric—an exceptionally rare feature among magnetic materials. Isovalent Ba substitution continuously suppresses the ferroelectric transition to zero temperature, producing a quantum paraelectric state with a signature T ² scaling of the inverse permittivity. In this regime, a transverse magnetic field drives the system to a magnetic QCP. Their resulting coexistence gives rise to a pronounced magnetocapacitance, revealing a strong interplay between fluctuations in the lattice and spin systems. Our findings establish Sr _{1− x} Ba _x Co ₂ V ₂ O ₈ as a unique platform for opening new avenues at the intersection of quantum criticality and multiferroicity.

We investigated the optical absorption spectra of the time-reversal-odd antiferromagnet ErCrO_{3} under an applied electric field (E). This material, with its two distinct magnetic sites (Er^{3+} and Cr^{3+}), exhibits successive antiferromagnetic (AFM) transitions at T_{N}≈133 K and T_{SR}≈10 K. We observed nonreciprocal directional dichroism induced by E (ENDD) in the AFM phases, spanning the near-infrared to visible light regions. The phenomenon is explained by the electrotoroidic effect, where a magnetic toroidal moment is induced by the applied electric field. We also found that the spatial distributions of ENDD-reflecting magnetic domain structures-obtained at photon energies corresponding to Er^{3+} f-f and Cr^{3+} d-d transitions, were in good agreement. The result indicates a strong interplay between Er^{3+} 4f and Cr^{3+} 3d moments in the AFM phases of this rare-earth orthochromite. The combination of ENDD spectroscopy and microscopy is a powerful tool for elucidating site-specific magnetic information in time-reversal-odd antiferromagnets such as altermagnets.

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.

Raman optical activity (ROA), the dependence of Raman intensity on the circular polarization of incident and scattered light, has traditionally been observed in chiral molecules and magnetic materials, where inversion or time-reversal symmetry is broken. Here we demonstrate that ROA can also arise in a centrosymmetric and nonmagnetic ferroaxial crystal. Using circularly polarized Raman spectroscopy on single-crystalline NiTiO_{3}, we observed a pronounced ROA signal in the cross-circular polarization configurations, which correlates with the ferroaxial domain structure. Our symmetry analysis, first-principles calculations of phonons, and tight-binding model calculations reveal that the natural ROA originates from the ferroaxial order and persists even within the electric dipole approximation. These results establish ROA as a powerful probe of ferroaxial order in centrosymmetric systems.

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.