Spin-polarized spatially indirect excitons in a topological insulator.

The exciton, a bound state of an electron and a hole, is a fundamental quasiparticle induced by coherent light-matter interactions in semiconductors. When the electrons and holes are in distinct spatial locations, spatially indirect excitons are formed with a much longer lifetime and a higher condensation temperature. One of the ultimate frontiers in this field is to create long-lived excitonic topological quasiparticles by driving exciton states with topological properties, to simultaneously leverage both topological effects and correlation1,2. Here we reveal the existence of a transient excitonic topological surface state (TSS) in a topological insulator, Bi2Te3. By using time-, spin- and angle-resolved photoemission spectroscopy, we directly follow the formation of a long-lived exciton state as revealed by an intensity buildup below the bulk-TSS mixing point and an anomalous band renormalization of the continuously connected TSS in the momentum space. Such a state inherits the spin-polarization of the TSS and is spatially indirect along the z axis, as it couples photoinduced surface electrons and bulk holes in the same momentum range, which ultimately leads to an excitonic state of the TSS. These results establish Bi2Te3 as a possible candidate for the excitonic condensation of TSSs3 and, in general, opens up a new paradigm for exploring the momentum space emergence of other spatially indirect excitons, such as moiré and quantum well excitons4-6, and for the study of non-equilibrium many-body topological physics.

Photovoltaic effect by soft phonon excitation.

SignificanceThe quantum-mechanical geometric phase of electrons provides various phenomena such as the dissipationless photocurrent generation through the shift current mechanism. So far, the photocurrent generations are limited to above or near the band-gap photon energy, which contradicts the increasing demand of the low-energy photonic functionality. We demonstrate the photocurrent through the optical phonon excitations in ferroelectric BaTiO3 by using the terahertz light with photon energy far below the band gap. This photocurrent without electron-hole pair generation is never explained by the semiclassical treatment of electrons and only arises from the quantum-mechanical geometric phase. The observed photon-to-current conversion efficiency is as large as that for electronic excitation, which can be well accounted for by newly developed theoretical formulation of shift current.

Photocurrent Induced by a Bicircular Light Drive in Centrosymmetric Systems.

A bicircular light (BCL) consists of left and right circularly polarized lights with different frequencies, and draws a roselike pattern with a rotational symmetry determined by the ratio of the two frequencies. Here we show that an application of a BCL to centrosymmetric systems allows a photocurrent generation through introduction of an effective polarity to the system. We derive formulas for the BCL-induced photocurrent from a standard perturbation theory, which is then applied to a simple 1D model and 3D Dirac and Weyl semimetals. A nonperturbative effect with strong light intensity is also discussed with the Floquet technique.

Giant Magnetochiral Anisotropy in Weyl Semimetal WTe_{2} Induced by Diverging Berry Curvature.

The concept of Berry curvature is essential for various transport phenomena. However, an effect of the Berry curvature on magnetochiral anisotropy, i.e., nonreciprocal magnetotransport, is still elusive. Here, we report that the Berry curvature induces the large magnetochiral anisotropy. In Weyl semimetal WTe_{2}, we observe the strong enhancement of the magnetochiral anisotropy when the Fermi level is located near the Weyl points. Notably, the maximal figure of merit γ[over ¯] reaches 1.2×10^{-6} m^{2} T^{-1} A^{-1}, which is the largest ever reported in bulk materials. Our semiclassical calculation shows that the diverging Berry curvature at the Weyl points strongly enhances the magnetochiral anisotropy.

Defect tolerant zero-bias topological photocurrent in a ferroelectric semiconductor.

Lattice defect is a major cause of energy dissipation in conventional electric current due to the drift and diffusion motions of electrons. Different nature of current emerges when noncentrosymmetric materials are excited by light. This current, called the shift current, originates from the change in the Berry connection of electrons' wave functions during the interband optical transition. Here, we demonstrate the defect tolerance of shift current using single crystals of ferroelectric semiconductor antimony sulfoiodide (SbSI). Although the dark conductance spreads over several orders of magnitude in each crystal due to the difference in the density of defect levels, the observed shift current converges to an identical value. We also reveal that the shift current is scarcely disturbed by the surface defects while they drastically suppress the conventional photocurrent. The defect tolerance is a manifestation of the topological nature of shift current, which will be a crucial advantage in optoelectronic applications.

N2 Gas Adsorption Sites of Single-Walled Carbon Nanotube Bundles: Identifying Interstitial Channels at Very Low Relative Pressure.

Utilizing the nanoscale space created by carbon nanotubes (CNTs) is of importance for applications like energy storage devices, sensors, and functional materials. Gas adsorption is a versatile, quantitative characterization method to analyze nanoscale pore sizes and volumes. Here, we inspected N2 adsorption to the nanospace formed by the bundles of single-walled CNTs with an average nanotube diameter of ca. 2.0 nm and its distributions of 0.7-4.1 nm. Based on comparisons among the as-grown, purified (opened), and heat-treated (closed) CNTs with similar geometric bundle structures, we found that the interstitial channels emerged from a very low relative pressure of approximately 10-8 by removing the impurities from the CNT bundles, which is the first empirical demonstration. These findings can not only be utilized to understand the structures of CNT films, fibers, and bulks but also applied to porous materials science.

[Web Questionnaire Survey on Appetite Loss and Weight Loss Associated with Cancer Cachexia Japanese Evidence for Patients Of Cancer Cachexia(J-EPOCC)-The Understanding of Cancer Cachexia].

Cancer cachexia is multifactor syndrome that occurs in 50-80% of cancer patients and accounts for 20% of cancer deaths. We conducted a web questionnaire survey for healthcare professionals(doctors and medical staff), patients and families to clarify the understanding of cancer cachexia. As a result, it was revealed that the understanding of cancer cachexia among patients and families was low. Cancer cachexia was widely recognized by healthcare professionals, but 3 stages of EPCRC was not. Many of healthcare professionals recalled the image of terminal stage of cancer from the term "cancer cachexia", and they lack awareness that cancer cachexia was a disease complication which developed from the early stage of cancer. Furthermore, there were many doctors who were faced with the problem such as a lack of treatment options for cancer cachexia. From these facts, it is necessary to disseminate scientific concept of cancer cachexia and establish standard of care from the early stage.

[Web Questionnaire Survey on Appetite Loss and Weight Loss Associated with Cancer Cachexia Japanese Evidence for Patients Of Cancer Cachexia(J-EPOCC)-The Problem Awareness of Appetite Loss and Weight Loss].

Appetite loss and weight loss associated with cancer have negative effects on the quality of life and OS of cancer patients. We conducted a web questionnaire survey for healthcare professionals(doctors and medical staff), patients and families to clarify the problem awareness for appetite loss and weight loss associated with cancer. As a result, it turned out that families were more concerned about patients' appetite loss and weight loss, and nearly half of patients haven't consulted their symptoms to healthcare professionals, and it meant that patients missed the opportunity to receive medical intervention due to no consultation. While healthcare professionals have a strong desire to provide dedicated treatment for appetite loss and weight loss in cancer patients, the proportion of patients and families who replied their symptom had improved with the intervention was low. In the near future, it will be necessary to enlighten the importance of symptom management to patients and families and pay attention to their symptom change for healthcare professionals.

Z_{N} Berry Phases in Symmetry Protected Topological Phases.

We show that the Z_{N} Berry phase (Berry phase quantized into 2π/N) provides a useful tool to characterize symmetry protected topological phases with correlation that can be directly computed through numerics of a relatively small system size. The Z_{N} Berry phase is defined in a N-1-dimensional parameter space of local gauge twists, which we call the "synthetic Brillouin zone," and an appropriate choice of an integration path consistent with the symmetry of the system ensures exact quantization of the Berry phase. We demonstrate the usefulness of the Z_{N} Berry phase by studying two 1D models of bosons, SU(3) and SU(4) Affleck-Kennedy-Lieb-Tasaki models, where topological phase transitions are captured by Z_{3} and Z_{4} Berry phases, respectively. We find that the exact quantization of the Z_{N} Berry phase at the topological transitions arises from a gapless band structure (e.g., Dirac cones or nodal lines) in the synthetic Brillouin zone.

Current-Voltage Characteristic and Shot Noise of Shift Current Photovoltaics.

We theoretically study the current-voltage relation, the I-V characteristic, of the photovoltaics due to the shift current, i.e., the photocurrent generated without the external dc electric field in noncentrosymmetric crystals through the Berry connection of the Bloch wave functions. We find that the I-V characteristic and shot noise are controlled by the difference of group velocities between conduction and valence bands, i.e., v_{11}-v_{22}, and the relaxation time τ. Since the shift current itself is independent of these quantities, there are a wide variety of possibilities to design it to maximize the energy conversion rate and also to suppress the noise. We propose that the Landau levels in noncentrosymmetric two-dimensional systems are a promising candidate for energy conversion.

Topological Floquet-Thouless Energy Pump.

We explore adiabatic pumping in the presence of a periodic drive, finding a new phase in which the topologically quantized pumped quantity is energy rather than charge. The topological invariant is given by the winding number of the micromotion with respect to time within each cycle, momentum, and adiabatic tuning parameter. We show numerically that this pump is highly robust against both disorder and interactions, breaking down at large values of either in a manner identical to the Thouless charge pump. Finally, we suggest experimental protocols for measuring this phenomenon.

Large Bulk Photovoltaic Effect and Spontaneous Polarization of Single-Layer Monochalcogenides.

We use a first-principles density functional theory approach to calculate the shift current and linear absorption of uniformly illuminated single-layer Ge and Sn monochalcogenides. We predict strong absorption in the visible spectrum and a large effective three-dimensional shift current (∼100 µA/V^{2}), larger than has been previously observed in other polar systems. Moreover, we show that the integral of the shift-current tensor is correlated to the large spontaneous effective three-dimensional electric polarization (∼1.9 C/m^{2}). Our calculations indicate that the shift current will be largest in the visible spectrum, suggesting that these monochalcogenides may be promising for polar optoelectronic devices. A Rice-Mele tight-binding model is used to rationalize the shift-current response for these systems, and its dependence on polarization, in general terms with implications for other polar materials.

Chiral Anomaly and Giant Magnetochiral Anisotropy in Noncentrosymmetric Weyl Semimetals.

We theoretically propose that giant magnetochiral anisotropy is achieved in Weyl semimetals in noncentrosymmetric crystals as a consequence of the chiral anomaly. The magnetochiral anisotropy is the nonlinearity of the resistivity ρ that depends on the current I and the magnetic field B as ρ=ρ_{0}(1+γI·B), and can be applied to rectifier devices controlled by B. We derive the formula for the coefficient γ in noncentrosymmetric Weyl semimetals. The obtained formula for γ shows that the magnetochiral anisotropy is strongly enhanced when the chemical potential is tuned to Weyl points, and that noncentrosymmetric Weyl semimetals such as TaAs can exhibit much larger magnetochiral anisotropy than that observed in other materials so far.

Charge and spin transport in edge channels of a ν=0 quantum Hall system on the surface of topological insulators.

Three-dimensional topological insulators of finite thickness can show the quantum Hall effect (QHE) at the filling factor ν=0 under an external magnetic field if there is a finite potential difference between the top and bottom surfaces. We calculate energy spectra of surface Weyl fermions in the ν=0 QHE and find that gapped edge states with helical spin structure are formed from Weyl fermions on the side surfaces under certain conditions. These edge channels account for the nonlocal charge transport in the ν=0 QHE which is observed in a recent experiment on (Bi_{1-x}Sb_{x})_{2}Te_{3} films. The edge channels also support spin transport due to the spin-momentum locking. We propose an experimental setup to observe various spintronics functions such as spin transport and spin conversion.

Rapid determination of chromium species in environmental waters using a diol-bonded polymer-stationary column under water-rich conditions coupled with ICPMS.

Chromium speciation analysis in environmental water is of great significance for the monitoring of water pollution and assessing its influences on human health. This study proposes a rapid analytical approach for the simultaneous determination of Cr(VI) and Cr(III) in environmental waters by hydrophilic interaction chromatography (HILIC) coupled with ICPMS under a water-rich condition. 2,6-Pyridinedicarboxylic acid (PDCA) was used to unify Cr(III) species in various chemical forms into a stable Cr(III)-PDCA anion complex and then separated from Cr(VI) oxyanion on a diol-bonded polymer-based HILIC column. An aqueous mobile phase containing 50 mmol L-1 ammonium acetate (pH 7.0), 2 mmol L-1 PDCA, and 4% acetonitrile successfully separates chromium species as well as chloride ions. In addition, our method elutes Cr(VI) preferentially over Cr(III)-PDCA, enabling rapid determination of Cr(VI), and both chromium species were analyzed within 6.2 min. The detection limits of 0.19 µg L-1 for Cr(VI) and 0.35 µg L-1 for Cr(III) at m/z 52 under He collision mode are comparable to or better than the conventional ion exchange chromatography-ICPMS methods, and quantitative recovery was obtained from spike-recovery tests on river water samples containing various levels of matrix. Optimization experiments of the HPLC conditions indicate that the retentions of Cr(VI) and Cr(III)-PDCA are characterized by electrostatic and nonpolar interactions, respectively. The retention behavior of inorganic anions and cations in water-rich conditions observed in this study will provide new insights into the separation mechanism in polymer-based HILIC columns, which has been poorly understood.

Tabular Two-Dimensional Correlation Analysis for Multifaceted Characterization Data.

We propose tabular two-dimensional correlation spectroscopy analysis for extracting features from multifaceted characterization data, essential for understanding material properties. This method visualizes similarities and phase lags in structural parameter changes through heatmaps, combining hierarchical clustering and asynchronous correlations. We applied the proposed method to data sets of carbon nanotube (CNT) films annealed at various temperatures and revealed the complexity of their hierarchical structures, which include elements such as voids, bundles, and amorphous carbon. Our analysis addresses the challenge of attempting to understand the sequence of structural changes, especially in multifaceted characterization data where 11 structural parameters derived from eight characterization methods interact with complex behavior. The results show how phase lags (asynchronous changes from stimuli), and parameter similarities can illuminate the sequence of structural changes in materials, providing insights into phenomena such as the removal of amorphous carbon and graphitization in annealed CNTs. This approach is beneficial even with limited data and holds promise for a wide range of material analyses, demonstrating its potential in elucidating complex material behaviors and properties.

Terahertz photon to dc current conversion via magnetic excitations of multiferroics.

Direct conversion from terahertz photon to charge current is a key phenomenon for terahertz photonics. Quantum geometrical description of optical processes in crystalline solids predicts existence of field-unbiased dc photocurrent arising from terahertz-light generation of magnetic excitations in multiferroics, potentially leading to fast and energy-efficient terahertz devices. Here, we demonstrate the dc charge current generation from terahertz magnetic excitations in multiferroic perovskite manganites with spin-driven ferroelectricity, while keeping an insulating state with no free carrier. It is also revealed that electromagnon, which ranges sub-terahertz to 2 THz, as well as antiferromagnetic resonance shows the giant conversion efficiency. Polar asymmetry induced by the cycloidal spin order gives rise to this terahertz-photon-induced dc photocurrent, and no external magnetic and electric bias field are required for this conversion process. The observed phenomena are beyond the conventional photovoltaics in semi-classical regime and demonstrate the essential role of quantum geometrical aspect in low-energy optical processes. Our finding establishes a paradigm of terahertz photovoltaic phenomena, paving a way for terahertz photonic devices and energy harvesting.

Simultaneous and sensitive analysis of glyphosate, glufosinate, and their metabolites in surface water by HPLC-ICP-MS/MS.

Organophosphorus pesticides such as glyphosate and glufosinate are used worldwide, and environmental regulatory values are being adopted in many countries due to their potential toxicity. In the present work, a pretreatment-free analytical method is established in which these two compounds with their metabolites are isolated from each other by anion-exchange HPLC using ammonium acetate (70 mM, pH 3.7) as eluent, and they are detected by triple quadrupole ICP-MS. Very low detection limits of 0.03-0.17 µg L-1 are acquired through the detection of P+ as PO+ via oxygen reaction mode, and quantitative recovery was obtained from the spike-recovery test on river water samples containing phosphate ion as an isobaric interferent. In addition, a constant sensitivity per molar concentration was achieved regardless of the compounds due to the powerful ion source of ICP-MS. This property suggests that semi-quantitative analysis of unknown P-bearing compounds is possible from one calibration curve.

Visualization of deformation-induced changes in carbon nanotube networks in rubber composites using lock-in thermography.

In this study, we used the lock-in thermography technique (LIT) to successfully visualize the single-walled carbon nanotube (CNT) networks during the tensile deformation of CNT/fluoro-rubber (FKM) composites. The LIT images revealed that the CNT network modes in CNT/FKM during strain-loading and unloading can be classified into four sites: (i) disconnection, (ii) recovery after disconnection, (iii) undestroyable, and (iv) no network. Quantitative analysis of the heat intensity of the LIT also indicated that the change in resistance during strain-loading and unloading plays a role in the balance of disconnection and reconstruction of the conductive network. We demonstrated the ability of LIT to effectively visualize and quantify the network state of the composite under deformation, and the LIT results were found to be strongly correlated with the composite properties. These results highlighted the potential of LIT as a valuable tool for composite characterization and material design.

Size distributions of cellulose nanocrystals in dispersions using the centrifugal sedimentation method.

Nanocellulose is a remarkable biomaterial. It is a plastic alternative with significance from the viewpoint of carbon offset and neutrality. To efficiently develop nanocellulose-based functional materials, it is imperative to evaluate their dispersion states. In this study, the sedimentation equivalent diameter distributions of cellulose nanocrystals (CNC) are analyzed by centrifugal sedimentation. The diameter distribution is well correlated with that estimated from the widths and the lengths of the CNCs obtained by transmission electron microscopy. Hence, centrifugal sedimentation has the potential to assess the dispersion states of nanocellulose on the nanometer scale and should contribute to basic research and applications.