Two-dimensional ferroelectricity in a single-element bismuth monolayer.

Ferroelectric materials are fascinating for their non-volatile switchable electric polarizations induced by the spontaneous inversion-symmetry breaking. However, in all of the conventional ferroelectric compounds, at least two constituent ions are required to support the polarization switching1,2. Here, we report the observation of a single-element ferroelectric state in a black phosphorus-like bismuth layer3, in which the ordered charge transfer and the regular atom distortion between sublattices happen simultaneously. Instead of a homogenous orbital configuration that ordinarily occurs in elementary substances, we found the Bi atoms in a black phosphorous-like Bi monolayer maintain a weak and anisotropic sp orbital hybridization, giving rise to the inversion-symmetry-broken buckled structure accompanied with charge redistribution in the unit cell. As a result, the in-plane electric polarization emerges in the Bi monolayer. Using the in-plane electric field produced by scanning probe microscopy, ferroelectric switching is further visualized experimentally. Owing to the conjugative locking between the charge transfer and atom displacement, we also observe the anomalous electric potential profile at the 180° tail-to-tail domain wall induced by competition between the electronic structure and electric polarization. This emergent single-element ferroelectricity broadens the mechanism of ferroelectrics and may enrich the applications of ferroelectronics in the future.

Ultrathin quantum light source with van der Waals NbOCl2 crystal.

Interlayer electronic coupling in two-dimensional materials enables tunable and emergent properties by stacking engineering. However, it also results in significant evolution of electronic structures and attenuation of excitonic effects in two-dimensional semiconductors as exemplified by quickly degrading excitonic photoluminescence and optical nonlinearities in transition metal dichalcogenides when monolayers are stacked into van der Waals structures. Here we report a van der Waals crystal, niobium oxide dichloride (NbOCl2), featuring vanishing interlayer electronic coupling and monolayer-like excitonic behaviour in the bulk form, along with a scalable second-harmonic generation intensity of up to three orders higher than that in monolayer WS2. Notably, the strong second-order nonlinearity enables correlated parametric photon pair generation, through a spontaneous parametric down-conversion (SPDC) process, in flakes as thin as about 46 nm. To our knowledge, this is the first SPDC source unambiguously demonstrated in two-dimensional layered materials, and the thinnest SPDC source ever reported. Our work opens an avenue towards developing van der Waals material-based ultracompact on-chip SPDC sources as well as high-performance photon modulators in both classical and quantum optical technologies1-4.

Topological Chern vectors in three-dimensional photonic crystals.

The paradigmatic example of a topological phase of matter, the two-dimensional Chern insulator1-5, is characterized by a topological invariant consisting of a single integer, the scalar Chern number. Extending the Chern insulator phase from two to three dimensions requires generalization of the Chern number to a three-vector6,7, similar to the three-dimensional (3D) quantum Hall effect8-13. Such Chern vectors for 3D Chern insulators have never been explored experimentally. Here we use magnetically tunable 3D photonic crystals to achieve the experimental demonstration of Chern vectors and their topological surface states. We demonstrate Chern vector magnitudes of up to six, higher than all scalar Chern numbers previously realized in topological materials. The isofrequency contours formed by the topological surface states in the surface Brillouin zone form torus knots or links, whose characteristic integers are determined by the Chern vectors. We demonstrate a sample with surface states forming a (2, 2) torus link or Hopf link in the surface Brillouin zone, which is topologically distinct from the surface states of other 3D topological phases. These results establish the Chern vector as an intrinsic bulk topological invariant in 3D topological materials, with surface states possessing unique topological characteristics.

Three-dimensional quantum Hall effect and metal-insulator transition in ZrTe5.

The discovery of the quantum Hall effect (QHE)1,2 in two-dimensional electronic systems has given topology a central role in condensed matter physics. Although the possibility of generalizing the QHE to three-dimensional (3D) electronic systems3,4 was proposed decades ago, it has not been demonstrated experimentally. Here we report the experimental realization of the 3D QHE in bulk zirconium pentatelluride (ZrTe5) crystals. We perform low-temperature electric-transport measurements on bulk ZrTe5 crystals under a magnetic field and achieve the extreme quantum limit, where only the lowest Landau level is occupied, at relatively low magnetic fields. In this regime, we observe a dissipationless longitudinal resistivity close to zero, accompanied by a well-developed Hall resistivity plateau proportional to half of the Fermi wavelength along the field direction. This response is the signature of the 3D QHE and strongly suggests a Fermi surface instability driven by enhanced interaction effects in the extreme quantum limit. By further increasing the magnetic field, both the longitudinal and Hall resistivity increase considerably and display a metal-insulator transition, which represents another magnetic-field-driven quantum phase transition. Our findings provide experimental evidence of the 3D QHE and a promising platform for further exploration of exotic quantum phases and transitions in 3D systems.

The Coexistence of Superconductivity and Topological Order in Van der Waals InNbS2.

The research on systems with coexistence of superconductivity and nontrivial band topology has attracted widespread attention. However, the limited availability of material platforms severely hinders the research progress. Here, it reports the first experimental synthesis and measurement of high-quality single crystal van der Waals transition-metal dichalcogenide InNbS2 , revealing it as a topological nodal line semimetal with coexisting superconductivity. The temperature-dependent measurements of magnetization susceptibility and electrical transport show that InNbS2 is a type-II superconductor with a transition temperature Tc of 6 K. First-principles calculations predict multiple topological nodal ring states close to the Fermi level in the presence of spin-orbit coupling. Similar features are also observed in the as-synthesized BiNbS2 and PbNbS2 samples. This work provides new material platforms ANbS2 (A = In, Bi, and Pb) and uncovers their intriguing potential for exploring the interplay between superconductivity and band topology.

Orbital Origin of the Intrinsic Planar Hall Effect.

Recent experiments reported an antisymmetric planar Hall effect, where the Hall current is odd in the in plane magnetic field and scales linearly with both electric and magnetic fields applied. Existing theories rely exclusively on a spin origin, which requires spin-orbit coupling to take effect. Here, we develop a general theory for the intrinsic planar Hall effect (IPHE), highlighting a previously unknown orbital mechanism and connecting it to a band geometric quantity-the anomalous orbital polarizability (AOP). Importantly, the orbital mechanism does not request spin-orbit coupling, so sizable IPHE can occur and is dominated by an orbital contribution in systems with weak spin-orbit coupling. Combined with first-principles calculations, we demonstrate our theory with quantitative evaluation for bulk materials TaSb_{2}, NbAs_{2}, and SrAs_{3}. We further show that AOP and its associated orbital IPHE can be greatly enhanced at topological band crossings, offering a new way to probe topological materials.

Orbital Magneto-Nonlinear Anomalous Hall Effect in Kagome Magnet Fe_{3}Sn_{2}.

It has been theoretically predicted that perturbation of the Berry curvature by electromagnetic fields gives rise to intrinsic nonlinear anomalous Hall effects that are independent of scattering. Two types of nonlinear anomalous Hall effects are expected. The electric nonlinear Hall effect has recently begun to receive attention, while very few studies are concerned with the magneto-nonlinear Hall effect. Here, we combine experiment and first-principles calculations to show that the kagome ferromagnet Fe_{3}Sn_{2} displays such a magneto-nonlinear Hall effect. By systematic field angular and temperature-dependent transport measurements, we unambiguously identify a large anomalous Hall current that is linear in both applied in-plane electric and magnetic fields, utilizing a unique in-plane configuration. We clarify its dominant orbital origin and connect it to the magneto-nonlinear Hall effect. The effect is governed by the intrinsic quantum geometric properties of Bloch electrons. Our results demonstrate the significance of the quantum geometry of electron wave functions from the orbital degree of freedom and open up a new direction in Hall transport effects.

Symmetry-selective quasiparticle scattering and electric field tunability of the ZrSiS surface electronic structure.

Three-dimensional Dirac semimetals with square-net non-symmorphic symmetry, such as ternary ZrXY (X = Si, Ge; Y = S, Se, Te) compounds, have attracted significant attention owing to the presence of topological nodal lines, loops, or networks in their bulk. Orbital symmetry plays a profound role in such materials as the different branches of the nodal dispersion can be distinguished by their distinct orbital symmetry eigenvalues. The presence of different eigenvalues suggests that scattering between states of different orbital symmetry may be strongly suppressed. Indeed, in ZrSiS, there has been no clear experimental evidence of quasiparticle scattering reported between states of different symmetry eigenvalues at small wave vectorq⃗.Here we show, using quasiparticle interference, that atomic step-edges in the ZrSiS surface facilitate quasiparticle scattering between states of different symmetry eigenvalues. This symmetry eigenvalue mixing quasiparticle scattering is the first to be reported for ZrSiS and contrasts quasiparticle scattering with no mixing of symmetry eigenvalues, where the latter occurs with scatterers preserving the glide mirror symmetry of the crystal lattice, e.g. native point defects in ZrSiS. Finally, we show that the electronic structure of the ZrSiS surface, including its unique floating band surface state, can be tuned by a vertical electric field locally applied by the tip of a scanning tunneling microscope (STM), enabling control of a spin-orbit induced avoided crossing near the Fermi level by as much as 300%.

A Dual-Signal Sensing for the Visual and Luminescent Detection of p-Phenylenediamine Based on Cerium-Nitrogen-Co-Doped Carbon Dots.

Herein, a visual and luminescent dual-mode (colorimetric and fluorometric) method for the detection of P-phenylenediamine (PPD) in hair dye was successfully established based on cerium-nitrogen co-doped carbon dots (Ce, N-CDs) that displayed remarkable luminescence and peroxidase activity. Ce, N-CDs catalyzed H2O2 to produce superoxide anion, which then oxidized the colorless 3,3,5,5-tetramethylbenzidine (TMB) into blue oxidized TMB (oxTMB), capable of quenching the fluorescence through fluorescence resonance energy transfer (FRET) between Ce, N-CDs and oxTMB. The reducing properties of PPD could reduce oxTMB back to TMB, leading to a decrease in the absorption intensity of oxTMB and a fluorescence recovery of Ce, N-CDs. As a result, the quantitative detection of PPD could be achieved by measuring the absorption values of oxTMB and the fluorescence signal of Ce, N-CDs. The detection limits for PPD were calculated as 0.36 µM and 0.10 µM for colorimetry and fluorimetry, respectively. Furthermore, smartphone application (ColorPicker) capable of measuring the RGB value of the color was utilized in the detection system, facilitating on-site quantitative detection. This approach effectively shortens the detection time and simplifies the operation, offering a powerful and convenient tool for real-time monitoring of PPD.

Electric-field-tuned topological phase transition in ultrathin Na3Bi.

The electric-field-induced quantum phase transition from topological to conventional insulator has been proposed as the basis of a topological field effect transistor1-4. In this scheme, 'on' is the ballistic flow of charge and spin along dissipationless edges of a two-dimensional quantum spin Hall insulator5-9, and 'off' is produced by applying an electric field that converts the exotic insulator to a conventional insulator with no conductive channels. Such a topological transistor is promising for low-energy logic circuits4, which would necessitate electric-field-switched materials with conventional and topological bandgaps much greater than the thermal energy at room temperature, substantially greater than proposed so far6-8. Topological Dirac semimetals are promising systems in which to look for topological field-effect switching, as they lie at the boundary between conventional and topological phases3,10-16. Here we use scanning tunnelling microscopy and spectroscopy and angle-resolved photoelectron spectroscopy to show that mono- and bilayer films of the topological Dirac semimetal3,17 Na3Bi are two-dimensional topological insulators with bulk bandgaps greater than 300 millielectronvolts owing to quantum confinement in the absence of electric field. On application of electric field by doping with potassium or by close approach of the scanning tunnelling microscope tip, the Stark effect completely closes the bandgap and re-opens it as a conventional gap of 90 millielectronvolts. The large bandgaps in both the conventional and quantum spin Hall phases, much greater than the thermal energy at room temperature (25 millielectronvolts), suggest that ultrathin Na3Bi is suitable for room-temperature topological transistor operation.

Microwave synthesized novel biomass carbon dots applied in the fluorescent detection of crystal violet.

To establish a new method for detecting crystal violet (CV), a harmful dye, herein, a genre of novel biomass carbon dots (CDs) was synthesized via a microwave method and employed as a fluorescent probe, in which water spinach and polyethylene glycol (PEG) performed as raw materials. Based on the inner filter effect (IFE) between the luminescent CDs and CV, the blue emission of this probe at 430 nm could be quenched by CV. Hence, a new strategy was proposed to selectively determine CV in aquaculture ambient. Moreover, under the optimal experiment conditions, this method showed a good linearity between the concentration of CV (c) and fluorescence quenching rate (ΔF/F0) in the concentration range of 4-200 µmol/L with the corresponding correlation coefficient (r) and the detection limit of 0.997 and 710 nmol/L, respectively. With advantages of environmental protectivity, sensitivity, affordability, and user-friendliness, the facilely fabricated CDs could be successfully applied in detecting CV in aquaculture samples, providing a technical foundation for monitoring the pollution of CV and ensuring the quality and safety of aquatic products.

Magnetic Real Chern Insulator in 2D Metal-Organic Frameworks.

Real Chern insulators have attracted great interest, but so far, their material realization is limited to nonmagnetic crystals and systems without spin-orbit coupling. Here, we reveal the magnetic real Chern insulator (MRCI) state in a recently synthesized metal-organic framework material Co3(HITP)2. Its ground state with in-plane ferromagnetic ordering hosts a nontrivial real Chern number, enabled by the C2zT symmetry and robustness against spin-orbit coupling. Distinct from previous nonmagnetic examples, the topological corner zero modes of MRCIs are spin-polarized. Furthermore, under small tensile strains, the material undergoes a topological phase transition from the MRCI to a magnetic double-Weyl semimetal phase, via a pseudospin-1 critical state. Similar physics can also be found in closely related materials Mn3(HITP)2 and Fe3(HITP)2, which also exist. Possible experimental detections and implications of an emerging magnetic flat band in the system are discussed.

Universal Molecular Control Strategy for Scalable Fabrication of Perovskite Light-Emitting Diodes.

Despite the rapid progress in perovskite light-emitting diodes (PeLEDs), the electroluminescence performance of large-area perovskite devices lags far behind that of laboratory-size ones. Here, we report a 3.5 cm × 3.5 cm large-area PeLED with a record-high external quantum efficiency of 12.1% by creating an amphipathic molecular interface modifier of betaine citrate (BC) between the perovskite layer and the underlying hole transport layer (HTL). It is found that the surface wettability for various HTLs can be efficiently improved as a result of the coexistence of methyl and carboxyl groups in the BC molecules that makes favorable groups to selectively contact with the HTL surface and increases the surface free energy, which greatly facilitates the scalable process of solution-processed perovskite films. Moreover, the luminous performance of perovskite emitters is simultaneously enhanced through the coordination between C═O in the carboxyl groups and Pb dangling bonds.

Competition-Based Two-Dimensional Photonic Crystal Dually Cross-Linked Supramolecular Hydrogel for Colorimetric and Fluorescent Dual-Mode Sensing of Bisphenol A.

Bisphenol A (BPA), one of the most abundantly produced endocrine disrupting chemicals, is widely used in everyday plastic products and thus must be monitored. Multimode sensing platforms are able to combine the advantages of different strategies while solving the issues of inaccurate test results of single signal sensing. However, the exploration in this field is limited due to the compromise of sensing conditions and inevitable mutual interferences of different systems. Herein, we constructed a two-dimensional photonic crystal dually cross-linked supramolecular hydrogel (2DPCDCSH) by utilizing a host-guest pair of ß-cyclodextrin (ß-CD) and tert-butyl (t-Bu) as the second cross-linking for colorimetric and fluorescent dual-mode sensing of BPA. Based on the fact that BPA can act as a competitive guest to break the host-guest interaction between ß-CD and t-Bu, the cross-linking density decreased and an expansion-induced structural color change occurred. Sensitive and selective BPA detection can be easily achieved by measuring the Debye diffraction ring diameter or observing the color change of 2DPC with a detection limit of 1 µg mL-1. Moreover, the formation of the ß-CD/BPA complex gave a significant enhancement of the intrinsic fluorescence of BPA, obtaining a detection limit of 0.001 µg mL-1. The two sensing systems can share the same reaction condition and yield a wider dynamic response range than the single signal strategy. Overall, the proposed method presented an efficient, rapid, cost-effective, and regenerative dual-mode method for BPA analysis and shed new insights for the design of diversified sensing platforms.

Intrinsic Nonlinear Planar Hall Effect.

We propose an intrinsic nonlinear planar Hall effect, which is of band geometric origin, independent of scattering, and scales with the second order of electric field and first order of magnetic field. We show that this effect is less symmetry constrained compared with other nonlinear transport effects and is supported in a large class of nonmagnetic polar and chiral crystals. Its characteristic angular dependence provides an effective way to control the nonlinear output. Combined with first-principles calculations, we evaluate this effect in the Janus monolayer MoSSe and report experimentally measurable results. Our work reveals an intrinsic transport effect, which offers a new tool for material characterization and a new mechanism for nonlinear device application.

Giant and Nonanalytic Negative Piezoelectric Response in Elemental Group-Va Ferroelectric Monolayers.

Materials with negative longitudinal piezoelectric response have been a focus of recent research. So far, reported examples are mostly three-dimensional bulk materials, either compounds with strong ionic bonds or layered materials with van der Waals interlayer gaps. Here, we report the first example in two-dimensional elemental materials-the class of group-Va monolayers. From first-principles calculations, we show that these materials possess giant negative longitudinal piezoelectric coefficient e_{11}. Importantly, its physical mechanism is also distinct from all previous proposals, connected with the special buckling driven polarization in these elemental systems. As a result, the usually positive internal strain contribution to piezoelectricity becomes negative and even dominates over the clamped ion contribution in Bi monolayers. Based on this new mechanism, we also find several 2D crystal structures that may support negative longitudinal piezoelectricity. As another consequence, piezoelectric response in Bi monolayers exhibits a significant nonanalytic behavior, namely, the e_{11} coefficient takes sizably different values (differed by ∼18%) under tensile and compressive strains, a phenomenon not known before and helpful for the development of novel electromechanical devices.

Control over Berry Curvature Dipole with Electric Field in WTe_{2}.

Berry curvature dipole plays an important role in various nonlinear quantum phenomena. However, the maximum symmetry allowed for nonzero Berry curvature dipole in the transport plane is a single mirror line, which strongly limits its effects in materials. Here, via probing the nonlinear Hall effect, we demonstrate the generation of Berry curvature dipole by applied dc electric field in WTe_{2}, which is used to break the symmetry constraint. A linear dependence between the dipole moment of Berry curvature and the dc electric field is observed. The polarization direction of the Berry curvature is controlled by the relative orientation of the electric field and crystal axis, which can be further reversed by changing the polarity of the dc field. Our Letter provides a route to generate and control Berry curvature dipole in broad material systems and to facilitate the development of nonlinear quantum devices.

Time-Reversal-Even Nonlinear Current Induced Spin Polarization.

We propose a time-reversal-even spin generation in second order of electric fields, which dominates the current induced spin polarization in a wide class of centrosymmetric nonmagnetic materials, and leads to a novel nonlinear spin-orbit torque in magnets. We reveal a quantum origin of this effect from the momentum space dipole of the anomalous spin polarizability. First-principles calculations predict sizable spin generations in several nonmagnetic hcp metals, in monolayer TiTe_{2}, and in ferromagnetic monolayer MnSe_{2}, which can be detected in experiment. Our work opens up the broad vista of nonlinear spintronics in both nonmagnetic and magnetic systems.

Two-dimensional Janus Si dichalcogenides: a first-principles study.

Strong structural asymmetry is actively explored in two-dimensional (2D) materials, because it can give rise to many interesting physical properties. Motivated by the recent synthesis of monolayer Si2Te2, we explored a family of 2D materials, named Janus Si dichalcogenides (JSD), which parallel the Janus transition metal dichalcogenides and exhibit even stronger inversion asymmetry. Using first-principles calculations, we show that their strong structural asymmetry leads to a pronounced intrinsic polar field, sizable spin splitting, and large piezoelectric response. The spin splitting involves an out-of-plane spin component, which is beyond the linear Rashba model. The piezoelectric tensor has a large value in both in-plane d11 coefficient and out-of-plane d31 coefficient, making monolayer JSDs distinct among existing 2D piezoelectric materials. In addition, we find interesting strain-induced phase transitions in these materials. Particularly, there are multiple valleys that compete for the conduction band minimum, which will lead to notable changes in the optical and transport properties under strain. Our work reveals a new family of Si based 2D materials, which could find promising applications in spintronic and piezoelectric devices.

First Report of Alternaria alternata Causing White Leaf Spot on Allium tuberosum in China.

Puding County is the major Allium tuberosum growing area in Guizhou Province of China. In 2019, white leaf spots were observed on Allium tuberosum in Puding County (26.31°N, 105.64°E). The white spots, ranging from elliptic to irregular in shape, first appeared on leaf tips. With disease aggravation, spots gradually coalesced, forming necrotic patches with yellow margins causing leaf necrosis; sometimes there was gray mold on dead leaves. The incidence of the diseased leaf rate was estimated to be 27-48%. To identify the pathogenic agent, 150 leaf tissues (5 mm × 5 mm) were obtained from disease-healthy junctions of 50 diseased leaves. Leaf tissues were disinfected in 75% ethanol for 30 s, soaked in 0.5% sodium hypochlorite for 5 min, and flushed three times with sterile water, before being placed on potato dextrose agar (PDA) in the dark at 25 °C. When colonies appeared, the mycelial tips were picked and placed on new PDA. Purified fungus was obtained after repeating this last step several times. The colonies were grayish-green with white round margins. Conidiophores (2.7-4.5 µm × 27-81 µm) were brown, straight, or flexuous with branches and septa. Conidia (8-34 µm × 5-16 µm) were brown, with 0-5 transverse septa and 0-4 longitudinal septa. The 18S nuclear ribosomal DNA (nrDNA; SSU), 28S nrDNA (LSU), RNA polymerase II second largest subunit (RPB2), internal transcribed spacer (ITS), glyceraldehyde-3-phosphate dehydrogenase (GAPDH), and translation elongation factor 1-alpha (TEF-α) (Woudenberg et al. 2013) were amplified and sequenced. The sequences were deposited in GenBank (ITS: OP703616, LSU: OP860684, SSU: OP860685, GAPDH: OP902372, RPB2: OP902373, TEF1-α: OP902374). According to BLAST analysis, the ITS, LSU, GAPDH, RPB2, SSU, and TEF1-α of the straishowed 100% (689 of 731 base pairs; bp), 100% (916 of 938 bp), 100% (579 of 600 bp), 100% (946 of 985 bp), 100% (1093 of 1134 bp), and 100% (240 of 240 bp) sequence identity to those of Alternaria alternata (ITS: LC440581.1, LSU: KX609781.1, GAPDH: MT109295.1, RPB2: MK605900.1, SSU: ON055699.1 and TEF1-α: OM220081.1). A phylogenetic tree was constructed using PAUP4 and the maximum parsimony method with 1000 replicas of bootstrapping for all datasets. According to morphological characteristics and phylogenetic analysis, FJ-1 was identified as Alternaria alternata (Simmons 2007, Woudenberg et al. 2015). The strain was preserved in the Agricultural Culture Collection of China (preservation number: ACC39969). To determine the pathogenicity of Alternaria alternata against Allium tuberosum, wounded healthy leaves were inoculated with a conidial suspension (106 conidial/mL) and round mycelial plugs (4mm). Sterile agar PDA plugs with no mycelium or sterile water were inoculated as negative controls. Three days later, white spots appeared on the wounded leaves inoculated with mycelial plugs or conidial suspension. However, the symptoms caused by conidial suspensions were weaker than those caused by mycelial plugs. No symptoms were observed in the control group. The experimental symptoms were consistent with the phenomena observed in the field. The same fungus was reisolated from necrotic lesions and identified as Alternaria alternata using the method described above. To our knowledge, this is the first report of Alternaria alternata causing white leaf spots on Allium tuberosum in China, a disease seriously affected the yield and quality of Allium tuberosum and caused economic losses to farmers. Reference: Simmons EG (2007) Alternaria: an identification manual. CBS Fungal Biodiversity Centre, Utrecht, the Netherlands. Woudenberg JHC, Groenewald JZ, Binder M, Crous PW ( 2013) Alternaria redefined. Stud Mycol, 75: 171-212. https://doi.org/10.3114/sim0015. Woudenberg JHC, Seidl MF, Groenewald JZ, Vries M de, Stielow JB, Thomma BPHJ, Crous PW (2015) Alternaria section Alternaria: Species, formae speciales or pathotypes? Stud Mycol, 82:1-21. https://doi.org/10.1016/j.simyco.2015.07.001.