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We have investigated the temperature- and field-variation of electronic state for the Dirac semimetal of ${\mathrm{EuMnBi}}_{2}$ by means of optical spectroscopy and theoretical calculation. The optical conductivity spectra show a clear Drude peak in the paramagnetic phase, which gradually diminishes in the Mn-$3d$ antiferromagnetic phase with decreasing temperature. Meanwhile, the absorption peaks due to the interband transition grow at low temperatures, resulting in a pseudogap feature with an energy scale of 0.07 eV. The analysis of Drude weight shows that the Drude response is nearly governed by the Dirac electrons at low temperatures. On the contrary, both the antiferromagnetic transition and spin reorientation of Eu-$4f$ moment do not significantly change the spectra except the moderate variation of Drude weight. As a comparison, we have also investigated the charge dynamics for ${\mathrm{EuZnBi}}_{2}$, which is an analog without the Mn-$3d$ antiferromagnetic ordering. In ${\mathrm{EuZnBi}}_{2}$, the optical conductivity spectra do not show the pseudogap structure, but show an intense Drude peak at all temperatures. Combined with the results of ab initio calculation, in ${\mathrm{EuMnBi}}_{2}$, it is likely that the reconstruction of electronic state driven by the Mn-$3d$ antiferromagnetic ordering causes the Dirac semimetallic state with tiny hole pockets, wherein electronic states other than the Dirac band are nearly gapped-out from the Fermi level.

${\mathrm{Cd}}_{3}{\mathrm{As}}_{2}$ is one of the prototypical topological Dirac semimetals. Here, we manipulate the band inversion responsible for the emergence of Dirac nodes by alloying ${\mathrm{Cd}}_{3}{\mathrm{As}}_{2}$ with topologically trivial ${\mathrm{Zn}}_{3}{\mathrm{As}}_{2}$. The carrier density monotonically decreases and the resistivity is enhanced as $x$ is increased. For larger $x$, the thermoelectric figure of merit exhibits comparably large values exceeding 0.3 at room temperature, due to the combined effects of a strong enhancement of the thermopower, an only moderate increase in the resistivity, and a suppression of the thermal conductivity. Complementary quantum-oscillation data and optical-conductivity measurements allow us to infer that the Dirac nodes are gapped out, and a band structure with a partly flat dispersion likely emerges in the higher-$x$ region in ${\mathrm{Cd}}_{3\ensuremath{-}x}{\mathrm{Zn}}_{x}{\mathrm{As}}_{2}$.

Spin-charge conversion via spin-orbit interaction is one of the core concepts in the current spintronics research. The efficiency of the interconversion between charge and spin current is estimated based on Berry curvature of Bloch wave function in the linear-response regime. Beyond the linear regime, nonlinear spin-charge conversion in the higher-order electric field terms has recently been demonstrated in noncentrosymmetric materials with nontrivial spin texture in the momentum space. Here, we report the observation of the nonlinear charge-spin conversion in a nominally centrosymmetric oxide material ${\mathrm{SrIrO}}_{3}$ by breaking inversion symmetry at the interface. A large second-order magnetoelectric coefficient is observed at room temperature because of the antisymmetric spin-orbit interaction at the interface of Dirac semimetallic bands, which is subject to the symmetry constraint of the substrates. Our study suggests that nonlinear spin-charge conversion can be induced in many materials with strong spin-orbit interaction at the interface by breaking the local inversion symmetry to give rise to spin splitting in otherwise spin degenerate systems.

We have investigated the charge dynamics of the correlated Dirac semimetallic perovskite ${\mathrm{CaIrO}}_{3}$ by infrared spectroscopy. The optical conductivity spectra show the $\ensuremath{\omega}$-linear-type interband transition and a tiny Drude response at low temperatures, suggesting that the Dirac node is pinned near the Fermi energy due to the Mott criticality. Moreover, a large polaron absorption appears at around 0.04 eV, when thermally excited carriers diminish and the plasma frequency decreases below the optical phonon energy. The electron correlation and resultant reduced plasma frequency are likely to enhance the electron-phonon interaction via the insufficient electronic screening, realizing the Dirac polaron.

In this chapter, we review the recent research on the spin-state ordering in epitaxial thin films of perovskite LaCoO3 from the viewpoint of optical spectroscopy and X-ray scattering. A remarkable feature is that a variety of spin/orbital orderings, which have not been identified in bulk samples, can be induced by tuning the epitaxial strain in thin film samples. In the moderately tensile-strained films, a spin-state/orbital ordering accompanying the ordering of high-spin-state site and low-spin-state site occurs around 130 K, and subsequently, a ferromagnetic/ferrimagnetic ordering emerges around 90 K. In the weakly strained films, another spin-state/orbital ordered phase with longer modulation period emerges around 40 K, followed by a ferromagnetic ordering transition around 20 K. By analyzing the Co-3d orbital state by the resonant soft X-ray scattering technique, we identified that two kinds of high-spin state with different site symmetries emerges in this phase. Moreover, by means of grazing-incidence resonant soft X-ray scattering technique, it is demonstrated that the transition temperature of spin-state ordering is higher in the surface-layer of film than inside the film due to the strain relaxation. These results demonstrate that the strain engineering of epitaxial thin films offers a fertile playground to study the spin-state/orbital ordering in the correlated spin-crossover material.

We study possible Weyl semimetals of strongly correlated electrons by investigating magnetotransport properties in pyrochlore ${R}_{2}{\mathrm{Ir}}_{2}{\mathrm{O}}_{7}$ ($R$ denotes rare-earth ions), choosing three types of $R$ ions to design the exchange coupling scheme between $R\phantom{\rule{4pt}{0ex}}4f$ and Ir $5d$ moments: nonmagnetic Eu $(4{f}^{6})$, isotropic Gd $(4{f}^{7})$, and anisotropic Tb $(4{f}^{8})$. In the doping-induced semimetallic state, distinctive features of magnetoresistance and the Hall effect are observed in $R=\mathrm{Gd}$ and Tb compounds due to the effects of the exchange-enhanced isotropic and anisotropic Zeeman fields, respectively, exemplifying the double-Weyl semimetal and the two-in two-out line-node semimetal as predicted by theories. In particular, the Hall angle of an $R=\mathrm{Gd}$ compound is strongly enhanced to 1.5% just above the critical doping for the Mott transition. Furthermore, an unconventional Hall contribution is discerned for a lower doping regime of the $R=\mathrm{Gd}$ compound, which can be ascribed to the emergence of Weyl points with the field-distorted all-in all-out order state. These findings indicate that the hole-doping-induced Mott transition as well as the characteristic $f\text{\ensuremath{-}}d$ exchange interaction stabilizes versatile topological semimetal states in a wide range of material parameter space.

In the mixed-valence manganites, a near-infrared laser typically melts the orbital and spin order simultaneously, corresponding to the photoinduced d1d0→d0d1 excitations in the Mott-Hubbard bands of manganese. Here, we use ultrafast methods-both femtosecond resonant X-ray diffraction and optical reflectivity-to demonstrate that the orbital response in the layered manganite Nd1-xSr1+xMnO4(x=2/3) does not follow this scheme. At the photoexcitation saturation fluence, the orbital order is only diminished by a few percent in the transient state. Instead of the typical d1d0→d0d1 transition, a near-infrared pump in this compound promotes a fundamentally distinct mechanism of charge transfer, the d0→d1L, where L denotes a hole in the oxygen band. This finding may pave a different avenue for selectively manipulating specific types of order in complex materials of this class.

Epitaxial strain provides important pathways to control the magnetic and electronic states in transition-metal oxides. However, the large strain is usually accompanied by a strong reduction of the oxygen-vacancy formation energy, which hinders the direct manipulation of their intrinsic properties. Here, using a postdeposition ozone annealing method, we obtain a series of oxygen stoichiometric SrCoO3 thin films with the tensile strain up to 3.0%. We observe a robust ferromagnetic ground state in all strained thin films, while interestingly the tensile strain triggers a distinct metal-to-insulator transition along with the increase of the tensile strain. The persistent ferromagnetic state across the electrical transition therefore suggests that the magnetic state is directly correlated with the localized electrons, rather than the itinerant ones, which then calls for further investigation of the intrinsic mechanism of this magnetic compound beyond the double-exchange mechanism.

Significance Shift current is one of the bulk photovoltaic phenomena in the materials without inversion symmetry, originating from the geometric Berry phase of the constituting electron bands. This concept of photocurrent generation based on the real-space shift of the electron cloud on the short timescale of optical transition is distinct from that of conventional p–n junction photovoltaics, where the carriers are driven by the built-in Coulomb potential. We experimentally demonstrate for a ferroelectric polar semiconductor how the subpicosecond charge swing on the relevant chemical bond changes its dynamics while scanning the excitation photon energy across the bandgap. On the interband photoexcitation above the bandgap, a finite net charge flow is produced along the electrically polar direction., Photoexcitation in solids brings about transitions of electrons/holes between different electronic bands. If the solid lacks an inversion symmetry, these electronic transitions support spontaneous photocurrent due to the geometric phase of the constituting electronic bands: the Berry connection. This photocurrent, termed shift current, is expected to emerge on the timescale of primary photoexcitation process. We observe ultrafast evolution of the shift current in a prototypical ferroelectric semiconductor antimony sulfur iodide (SbSI) by detecting emitted terahertz electromagnetic waves. By sweeping the excitation photon energy across the bandgap, ultrafast electron dynamics as a source of terahertz emission abruptly changes its nature, reflecting a contribution of Berry connection on interband optical transition. The shift excitation carries a net charge flow and is followed by a swing over of the electron cloud on a subpicosecond timescale. Understanding these substantive characters of the shift current with the help of first-principles calculation will pave the way for its application to ultrafast sensors and solar cells.

Electrons in conventional metals become less mobile under the influence of electron correlation. Contrary to this empirical knowledge, we report here that electrons with the highest mobility ever found in known bulk oxide semiconductors emerge in the strong-correlation regime of the Dirac semimetal of perovskite CaIrO3. The transport measurements reveal that the high mobility exceeding 60,000 cm2V−1s−1 originates from the proximity of the Fermi energy to the Dirac node (ΔE, Electron correlation normally makes electrons less mobile, but it is still not clear when correlation becomes very strong in Dirac semimetals. Here, Fujioka et al. report a very high electron mobility exceeding 60,000 cm2V−1s−1 in correlated Dirac semimetal of perovskite CaIrO3, due to the enhanced electron correlation nearby the Mott transition.

We review recent advances in strongly correlated oxides as thermoelectric materials in pursuit of energy harvesting. We discuss two topics: one is the enhancement of the ordinary thermoelectric properties by controlling orbital degrees of freedom and orbital fluctuation not only in bulk but also at the interface of correlated oxides. The other topic is the use of new phenomena driven by spin-orbit coupling (SOC) of materials. In 5d electron oxides, we show some SOC-related transport phenomena, which potentially contribute to energy harvesting. We outline the current status and a future perspective of oxides as thermoelectric materials., Graphical Abstract

The impact of electron correlation on the Dirac semimetal state is investigated for perovskite ${\text{CaIrO}}_{3}$ in terms of the magnetotransport properties under varying pressures. The reduction of electron correlation with a pressure of 1 GPa enhances the Fermi velocity as much as 40%, but it reduces the mobility by an order of magnitude by detuning the Dirac node from the Fermi energy. Moreover, the giant magnetoresistance at the quantum limit due to the one-dimensional confinement of Dirac electrons is critically suppressed under pressure. These results indicate that the electron correlation is a crucial knob for controlling the transport of a correlated Dirac semimetal.

We report polarization-resolved Raman scattering measurements on single crystals of the pyrochlore compound ${\mathrm{Eu}}_{2}{\mathrm{Ir}}_{2}{\mathrm{O}}_{7}$ which exhibits a ground state with noncollinear magnetic order, as well as its paramagnetic counterpart ${\mathrm{Pr}}_{2}{\mathrm{Ir}}_{2}{\mathrm{O}}_{7}$. The spectra reveal the six Raman-active optical phonons expected for the pyrochlore lattice symmetry. Combined with density functional calculations, polarization analysis of the Raman intensity allows us to assign all observed peaks to specific vibration patterns. Whereas most phonon profiles are weakly temperature dependent, an Ir-O-Ir bond-bending vibration in ${\mathrm{Eu}}_{2}{\mathrm{Ir}}_{2}{\mathrm{O}}_{7}$ exhibits a pronounced Fano asymmetry in the paramagnetic state, and marked softening and line-shape anomalies at the magnetic phase transition. These observations indicate strong electron-phonon interactions, which must be considered in models of the electronic properties and phase behavior of the pyrochlore iridates. In addition, a prominent feature appears at 210 ${\mathrm{cm}}^{\ensuremath{-}1}$ in the Raman spectrum of ${\mathrm{Eu}}_{2}{\mathrm{Ir}}_{2}{\mathrm{O}}_{7}$ below the magnetic transition temperature, possibly due to a magnetic excitation.

Topological quantum states of matter, characterized by geometrical features of electronic band structures, have been extensively studied. Among them, the topological electronic state with magnetic order remains elusive because of a scarce number of examples. Here we present experimental observations proving that the pyrochlore iridate, when electronically tuned, can be a topological Weyl semimetal as predicted by recent theories. We observe a sizable spontaneous Hall conductivity with minimal magnetization only within a few Kelvin below the all-in all-out magnetic ordering temperature. Our theoretical calculation, which is quantitatively consistent with the observation, suggests that the presence of linearly-dispersing crossing points (Weyl points), acting as a source/sink of a quantized magnetic flux, potentially gives rise to such an enormous effect. The manifestation of the salient Hall response provides one important example of topological states, which promotes a better understanding of Weyl semimetal and indicates the new research direction for the topological-materials design., Material with both topological state and magnetic order remains rare. Here, Ueda et al. observe a sizable spontaneous Hall conductivity with minimal magnetization at a few Kelvin below the all-in-all-out magnetic ordering temperature in the predicted magnetic Weyl semimetal R2Ir2O7.

The spin states of ${\mathrm{Co}}^{3+}$ ions in perovskite-type ${\mathrm{LaCoO}}_{3}$, governed by the complex interplay between the electron-lattice interactions and the strong electron correlations, still remain controversial due to the lack of experimental techniques which can directly detect them. In this Letter, we revealed the tensile-strain dependence of spin states, i.e., the ratio of the high- and low-spin states, in epitaxial thin films and a bulk crystal of ${\mathrm{LaCoO}}_{3}$ via resonant inelastic soft x-ray scattering. A tensile strain as small as 1.0% was found to realize different spin states from that in the bulk.

Visualizing conducting domain walls When a metal undergoes a phase transition and becomes insulating, it sometimes also becomes magnetically ordered. It is possible that some metallicity survives along the boundaries of magnetic domains, the so-called domain walls, but the question is difficult to address directly in experiments. Ma et al. did just that by mapping out the conductance of the material Nd 2 Ir 2 O 7 in its low-temperature magnetic insulating phase, using microwave impedance microscopy. The magnetic domain walls showed up clearly in the images as regions of high conductance. Science , this issue p. 538

We report the polarization rotation of terahertz light resonant with the magnetoelectric (ME) spin excitation in the multiferroic ${(\mathrm{Fe},\mathrm{Zn})}_{2}{\mathrm{Mo}}_{3}{\mathrm{O}}_{8}$. 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.

The development of antibiotics cannot keep up with the speed of resistance acquired by microorganisms. Recently, the development of antimicrobial photodynamic therapy (aPDT) has been a necessary antimicrobial strategy against antibiotic resistance. Among the wide variety of bacteria found in the oral flora, Porphyromonas gingivalis (P. gingivalis) is one of the etiological agents of periodontal disease. aPDT has been studied for periodontal disease, but has risks of cytotoxicity to normal stained tissue. In this study, we performed aPDT using protoporphyrin IX (PpIX), an intracellular pigment of P. gingivalis, without an external photosensitizer. We confirmed singlet oxygen generation by PpIX in a blue-light irradiation intensity-dependent manner. We discovered that blue-light irradiation on P. gingivalis is potentially bactericidal. The sterilization mechanism seems to be oxidative DNA damage in bacterial cells. Although it is said that no resistant bacteria will emerge using aPDT, the conventional method relies on an added photosensitizer dye. PpIX in P. gingivalis is used in energy production, so aPDT applied to PpIX of P. gingivalis should limit the appearance of resistant bacteria. This approach not only has potential as an effective treatment for new periodontal diseases, but also offers potential antibacterial treatment for multiple drug resistant bacteria.

The interplay between electron correlation and spin–orbit coupling in solids has been proven to be an abundant gold mine for emergent topological phases. Here we report the results of systematic magnetotransport study on bandwidth-controlled pyrochlore iridates R2Ir2O7 near quantum metal-insulator transition (MIT). The application of a magnetic field along [001] crystallographic direction (H//[001]) significantly decreases resistivity while producing a unique Hall response, which indicates the emergence of the novel semi-metallic state in the course of the magnetic transformation from all-in all-out (AIAO, 4/0) to 2-in 2-out (2/2) spin configuration. For H//[111] that favours 3-in 1-out (3/1) configuration, by contrast, the resistivity exhibits saturation at a relatively high value typical of a semimetal. The observed properties can be identified to reflect the emergence of multiple Weyl semimetal states with varying numbers of Weyl points and line nodes in respective spin configurations. With tuning effective bandwidth, all these states appear to concentrate around the quantum MIT region, which may open a promising venue for topological phenomena and functions., The interplay between multiple electronic interactions in solid promotes the emergence of exotic phases. Here, Ueda et al. report magnetotransport study on pyrochlore iridates R 2Ir2O7 near quantum metal-insulator transition reflecting the emergence of multiple Weyl semimetal states.

We investigated magnetotransport properties and charge dynamics of strain-free perovskite ${\mathrm{SrIrO}}_{3}$. Both the longitudinal and transverse magnetoresistivity (MR) are significantly enhanced with decreasing temperature, in accord with the evolution of the Dirac semimetallic state. The electron correlation effect in the Dirac state shows up as a dramatic change in charge dynamics with temperature and as an enhanced paramagnetic susceptibility. We propose that the field-induced topological transition of the Dirac node coupled to the enhanced paramagnetism causes the unique MR of correlated Dirac electrons.

A mid-infrared free-electron laser (FEL) is operated as a pulsed and linearly polarized laser with tunable wavelengths within infrared region. Although the FEL can ablate soft tissues with minimum collateral damage in surgery, the potential of FEL for dissecting protein aggregates is not fully understood. Protein aggregates such as amyloid fibrils are in some cases involved in serious diseases. In our previous study, we showed that amyloid-like lysozyme fibrils could be disaggregated into the native form with FEL irradiation specifically tuned to the amide I band (1,620 cm(-1)). Here, we show further evidence for the FEL-mediated disaggregation of amyloid-like fibrils using insulin fibrils. Insulin fibrils were prepared in acidic solution and irradiated by the FEL, which was tuned to either 1,620 or 2,000 cm(-1) prior to the experiment. The Fourier transform infrared spectroscopy (FT-IR) spectrum after irradiation with the FEL at 1,620 cm(-1) indicated that the broad peak (1,630-1,660 cm(-1)) became almost a single peak (1,652 cm(-1)), and the β-sheet content was reduced to 25 from 40% in the fibrils, while that following the irradiation at 2,000 cm(-1) remained at 38%. The Congo Red assay as well as transmission electron microscopy observation confirmed that the number of fibrils was reduced by FEL irradiation at the amide I band. Size-exclusion chromatography analysis indicated that the disaggregated form of fibrils was the monomeric form. These results confirm that FEL irradiation at the amide I band can dissect amyloid-like protein fibrils into the monomeric form in vitro.

We have investigated the charge transport in thin films of correlated Dirac semimetal of perovskite CaIrO3 by measurements of resistivity and optical spectra. The semimetallic transport of either electron-type or hole-type carrier is observed in the strain-relaxed thin films. By controlling the strain relaxation via thermal annealing, the carrier density decreases in both n-type and p-type samples, while enhancing the carrier mobility up to 160 cm2 V−1 s−1 at an electron density of 2.5 × 1018 cm−3 at 2 K. We propose that the energy of Dirac line node, which locates in proximity to the Fermi level, varies with the lattice distortion or strain-sensitive defect character, causing the sign change in the charge carrier as well as the mobility enhancement.

We report sub-picosecond photocarrier dynamics observed via emitted terahertz waves in a ferroelectric semiconductor Sn2P2S6. Excitation photon energy, intensity and polarization dependences of the photocarrier dynamics testify that the ultrafast photocurrent originates from the shift-current under interband excitation. The photocurrent excitation spectrum shows a quantitative agreement with that derived from first-principles calculations with the Berry connection integrated, showing the shift-current to be a sensitive feature in ferroelectrics. The terahertz emission spectroscopy is a powerful tool to identify the intricate carrier dynamics under pulsed photoexcitation.

Magnetic excitations are investigated for a hexagonal polar magnet $\mathrm{F}{\mathrm{e}}_{2}\mathrm{M}{\mathrm{o}}_{3}{\mathrm{O}}_{8}$ by terahertz spectroscopy. We observed magnon modes including an electric-field active magnon, electromagnon, in the collinear antiferromagnetic phase with spins parallel to the $c$ axis. We unravel the nature of these excitations by investigating the correlation between the evolution of the mode profile and the magnetic transition from antiferromagnetic to ferrimagnetic order induced by a magnetic field or Zn doping. We propose that the observed electromagnon mode involves the electric polarization oscillating within the $c$ plane induced by the collective precession of the spins through the same mechanism as producing the linear magnetoelectric effect.