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The coexistence and coupling between alternatively stacked layers with different functional properties often give rise to exotic physical phenomena, such as high-temperature superconductivity, multiferroic behavior, and giant thermoelectric performance, which are tightly linked with the intrinsic microstructures. Here we unambiguously uncover the structural modulations in multiferroic Bi4.2K0.8Fe2O9+δ (BKFO) nanobelts with a magnetoelectric–dielectric superlattice by scanning transmission electron microscopy (STEM). The octahedrons in the perovskite layers are identified as FeO6 and the ordered arrangements of the Bi and K cations are clearly determined. Quantitative measurements of the positions of the Bi columns indicate that the displacive modulations can be decomposed into a transverse wave and a longitudinal wave, whose amplitudes and phases are layer-dependent. This study may help to understand and optimize the magnetoelectric coupling effect in BKFO.

The remains of trilobites and other species were unexpectedly discovered in a volcanic ash layer beneath the Permian–Triassic Boundary (PTB). Based on a biostratigraphic investigation of the Zhongliangshan section in Chongqing, South China, the quantity of the species gradually decreased with subsequent volcanism. This finding provides an opportunity to further understand the disappearance of trilobites and the evolution of the mass extinction event. The temporal coincidence between the volcanic eruption event and the loss of trilobites and other species supports the idea of a cause-and-effect relationship. The species remains in the ash bed appeared before the disappearance of Clarkina yini and the climax of the negative carbon isotope excursion, which implies that the onset of the mass extinction occurred at the end-Permian. The explosive volcanic events caused massive releases of CO2, toxic gases and volcanic ash and resulted in loss of habitat for certain species in the Tethys domain. This phe...

The design of catalysts that are both highly active and stable is always challenging. Herein, we report that the incorporation of single metal active sites attached to the nitrogen atoms in the basal plane of graphene leads to composite materials with superior activity and stability when used as counter electrodes in dye-sensitized solar cells (DSSCs). A series of composite materials based on different metals (Mn, Fe, Co, Ni, and Cu) were synthesized and characterized. Electrochemical measurements revealed that CoN4 /GN is a highly active and stable counter electrode for the interconversion of the redox couple I(-) /I3 (-) . DFT calculations revealed that the superior properties of CoN4 /GN are due to the appropriate adsorption energy of iodine on the confined Co sites, leading to a good balance between adsorption and desorption processes. Its superior electrochemical performance was further confirmed by fabricating DSSCs with CoN4 /GN electrodes, which displayed a better power conversion efficiency than the Pt counterpart.

The surface electronic structure of transition-metal oxide catalysts plays a decisive role in binding the intermediates of the oxygen evolution reaction (OER) to the oxide surface, in turn influencing the catalytic activity of these materials. However, the approaches to modulating the electronic structure of surface metal ions are rare and far behind the demands. Here, we report a surface single atom decoration for adjusting the surface electronic structure of Co3O4, leading to enhanced electrocatalytic activity for OER, in which the isolated Ru single atoms were uniformly deposited on the surface of Co3O4 by an atomic layer deposition technology. As the OER catalyst, the as-made catalysts have exhibited a significantly enhanced catalytic activity (with increasing to 95.5 times) and a dramatically decreased overpotential. The density functional theory calculations reveal that the single-atom Ru acts as a promotor to adjust the 3d electronic structure of adjacent Co atoms and to tune the binding energy between intermediates and activity sites, finally leading to enhanced catalytic activity.

A theoretical study on the structural dynamics of the temporal behavior of Bragg diffraction is presented and compared with experimental results obtained via ultrafast electron crystallography. In order to describe the time-dependent lattices and calculate the Bragg diffraction intensity, we introduce the basic vector offset matrix, which can be used to quantify the shortening, lengthening and rotation of the three lattice vectors (i.e., lattice deformation). Extensive simulations are performed to evaluate the four-dimensional electron crystallography model. The results elucidate the connection between structural deformations and changes in diffraction peaks, and sheds light on the quantitative analysis and comprehensive understanding of the structural dynamics.

In this work, structural investigation and physical measurements of layered transition-metal dichalcogenides (TMDs) 1 T -MX2materials revealed a series of remarkable phenomena in correlation with structural transitions. Our results show that notable structural transformations, such as charge density wave (CDW) transitions, atomic ordering, and micro-twinning could be introduced via chemical substitution in 1 T -MX2. It was demonstrated that substitution of S by Se in 1 T -TaS2- x Se x resulted in clear changes in the incommensurability of the CDW state; substitution of Se by Te could destruct the CDW state and yield atomic ordering with visible trimerization of the metallic ions. Furthermore, these structural changes yielded a monoclinic stacking along the c -axis direction. Additionally, on substitution of the metal atoms in 1 -MX2, e.g. , 1 -NbTe2, structural distortion in the a – b plane could clearly be observed, and high-density twinning lamella often appeared in the crystals because of enhanced intralayer structural deformation. These phase transitions and their relevant structural features could primarily be corelated with alteration of the superconductivity and other physical properties. FullText for HTML: https://doi.org/10.1209/0295-5075/130/47001

In two-dimensional van der Waals magnets, the presence of magnetic orders, strong spin–orbit coupling, and asymmetry at interfaces is the key ingredient for hosting noncollinear spin textures. Here, we investigate the characteristics and evolution of magnetic domain structures in thin Fe3GeTe2 nanolayers as a function of temperature, applied magnetic field, and specimen thickness using advanced magnetic electron microscopy. Specifically, electron holography analyses reveal the spin configurations of Bloch-type, zero-field-stabilized magnetic bubbles in 20-nm-thick Fe3GeTe2 nanolayers at cryogenic temperature. In situ Lorentz transmission electron microscopy measurements further provide detailed magnetic phase diagrams of noncollinear spin textures, including magnetic spirals and bubbles in Fe3GeTe2 as a function of temperature, applied magnetic field, and specimen thickness. We further estimate the micromagnetic parameters of Fe3GeTe2, such as anisotropy energy density and magnetization at specific specimen temperature using the critical thicknesses measured from Lorentz microscopy measurements. Our experimental results of magnetic domain structures in Fe3GeTe2 nanolayers reveal that due to their intrinsic highly uniaxial magnetocrystalline anisotropy, a very thin film of tens of nanometers of Fe3GeTe2 can support the spontaneous and stable formation of zero-field magnetic bubbles.

We report a comprehensive high-pressure study on the monoclinic TlFeSe2 single crystal, which is an antiferromagnetic insulator with quasi-one-dimensional crystal structure at ambient pressure. It is found that TlFeSe2 undergoes a pressure-induced structural transformation from the monoclinic phase to an orthorhombic structure above P c ≈ 13 GPa, accompanied with a large volume collapse of ΔV/V 0 = 8.3%. In the low-pressure monoclinic phase, the insulating state is easily metallized at pressures above 2 GPa; while possible superconductivity with T c onset ∼ 2 K is found to emerge above 30 GPa in the high-pressure phase. Such a great tunability of TlFeSe2 under pressure indicates that the ternary AFeSe2 system (A = Tl, K, Cs, Rb) should be taken as an important platform for explorations of interesting phenomena such as insulator-metal transition, dimensionality crossover, and superconductivity.

Interfaces between materials with different electronic ground states have become powerful platforms for creating and controlling novel quantum states of matter, in which inversion symmetry breaking and other effects at the interface may introduce additional electronic states. Among the emergent phenomena, superconductivity is of particular interest. Here, by depositing metal films on a newly identified topological semimetal tungsten carbide (WC) single crystal, interfacial superconductivity is obtained, evidenced from soft point-contact spectroscopy. This very robust phenomenon is demonstrated for a wide range of metal/WC interfaces, involving both nonmagnetic and ferromagnetic films, and the superconducting transition temperatures are surprisingly insensitive to the magnetism of thin films. This method offers an opportunity to explore the long-sought topological superconductivity and has potential applications in topological-state-based spin devices.

Cathelicidins are a class of gene-encoded immune effectors in vertebrate innate immune system. Though being extensively studied in mammals, little is known about the roles of cathelicidins in turtles, the water-dwelling vertebrates in the order Testudines. In the present study, six novel cathelicidins (Ps-CATH1-6) with different tissue and inducible expression patterns were characterized from the turtle of economic importance, Chinese soft-shell turtle (Pelodiscus sinensis). Although the structures of Ps-CATH1-6 precursors were identical with most of the other known cathelicidins, the mature peptides of Ps-CATH1-6 showed low sequence similarity with the other cathelicidins. Functional studies indicated that some of them either directly kill pathogens via inducing the permeabilization in bacterial membrane (Ps-CATH4, 6), or boost infection-resolving immunity by selectively inhibiting pro-inflammatory responses (Ps-CATH2, 4, 6) through MAPKs and NF-κB pathways. Ps-CATH2, 4, and 6, which assume the hallmark amphipathic α-helical conformations as most of the other host defense peptides, exhibit evident in vivo protection by significantly reducing the bacterial loads in bacterial infected turtles. Collectively, the discovery of novel Ps-CATHs with pleiotropic structures and functions helps elucidating the roles of cathelicidins in the Chinese softshell turtle innate immunity, and better understanding the survival strategy of Chinese softshell turtle in harsh habitat.

Cutaneous hyperpigmentation from excess melanogenesis causes serious pigmentary disorders and even melasma. Short peptides (SPs) are garnering attention lately owing to their therapeutic potential in dermatological diseases and low systemic side effects. Here, we show an octapeptide, ansin2, designed de novo from antioxidant SPs we previously reported, significantly inhibiting melanogenesis in B16 cells by decreasing tyrosinase production via regulating the MITF pathway. Ansin2 could also inhibit tyrosinase function by covering its catalytic pocket, which was simulated in docking and LIGPLOT studies. Topical application of ansin2 exhibited evident protection in UVB-induced pigmentation in guinea pig models both in terms of prophylaxis and treatment. Interestingly, unlike other hydrophilic and peptidic drugs that need delivery systems, ansin2 can be efficiently delivered topically to the epidermis and dermis per se without an affiliated moiety. Given that ansin2 lacks unwanted toxicities and immunogenicity, it holds great potential in treating hyperpigmentation in the cosmetics and pharmaceutical industries.

Ultrafast laser excitation of electronic crystals reveals hidden quantum states with new polaron cluster ordering., The hidden (H) quantum state in 1T-TaS2 has sparked considerable interest in the field of correlated electron systems. Here, we investigate ultrafast switches to stable H charge density wave (H-CDW) states observed in 1T-TaS2−xSex, with x = 0 and 0.5 crystals, upon excitation with a single femtosecond laser pulse. In situ cooling transmission electron microscopy observations, initiated by a single femtosecond laser pumping with a low fluence, reveal a clear transition from a commensurate CDW phase (qC) to a new CDW order with qH = (1 − δ)qC for the H-CDW state (δ = 1/9) accompanied by an evident phase separation. H-CDW domain relaxation then occurs and yields a stable metallic phase under a high-fluence excitation. Furthermore, electrical resistivity measurements show that the notable drop in x = 0 and 0.5 samples associated with the appearance of H-CDW states depend on laser fluence and temperature. These results potentially provide a new perspective on the photodoping mechanism for the emergence of H-CDW states in the 1T-TaS2−xSex family.

We report $^{75}$As nuclear magnetic resonance (NMR) / nuclear quadrupole resonance (NQR) and transmission electron microscopy (TEM) studies on LaFeAsO$_{1-x}$F$_{x}$. There are two superconducting domes in this material. The first one appears at 0.03 $\leq$ $x$ $\leq$ 0.2 with $T_{\rm c}$$^{max}$ = 27 K, and the second one at 0.25 $\leq$ $x$ $\leq$ 0.75 with $T_{\rm c}$$^{max}$ = 30 K. By NMR and TEM, we demonstrate that a $C4$-to-$C2$ structural phase transition (SPT) takes place above both domes, with the transition temperature $T_{\rm s}$ varying strongly with $x$. In the first dome, the SPT is followed by an antiferromagnetic (AF) transition, but neither AF order nor low-energy spin fluctuations are found in the second dome. In LaFeAsO$_{0.97}$F$_{0.03}$, we find that AF order and superconductivity coexist microscopically via $^{75}$As nuclear spin-lattice relaxation rate (1/$T_1$) measurements. In the coexisting region, 1/$T_1$ decreases at $T_{\rm c}$ but becomes to be proportional to $T$ below 0.6$T_{\rm c}$, indicating gapless excitations. Therefore, in contrast to the early reports, the obtained phase diagram for $x \leq$ 0.2 is quite similar to the doped BaFe$_{2}$As$_{2}$ system. The electrical resistivity in the second dome can be fitted by $\rho = {{\rho }_{0}}+A{{T}^{n}}$ with $n$ = 1 and a maximal coefficient $A$ at around $x_{opt}$ = 0.5$\sim$0.55 where $T_{\rm s}$ extrapolates to zero and $T_{\rm c}$ is the maximal, which suggest the importance of quantum critical fluctuations associated with the SPT. We have constructed a complete phase diagram of LaFeAsO$_{1-x}$F$_{x}$, which provides insight into the relationship between SPT, antiferromagnetism and superconductivity., Comment: 23 pages, 15 figures

Optical tuning and probing ultrafast structural response of nanomaterials driven by electronic excitation constitute a challenging but promising approach for understanding microscopic mechanisms and applications in microelectromechanical systems and optoelectrical devices. Here we use pulsed electron diffraction in a transmission electron microscope to investigate laser-induced tubular lattice dynamics of multi-walled carbon nanotubes (MWCNTs) with varying laser fluence and initial specimen temperature. Our photoexcitation experiments demonstrate cooperative and inverse collective atomic motions in intralayer and interlayer directions, whose strengths and rates depend on pump fluence. The electron-driven and thermally driven structural responses with opposite amplitudes cause a crossover between intralayer and interlayer directions. Our ab initio calculations support these findings and reveal that electrons excited from π to π* orbitals in a carbon tube weaken the intralayer bonds while strengthening the interlayer bonds along the radial direction. Moreover, by probing the structural dynamics of MWCNTs at initial temperatures of 300 and 100 K, we uncover the concomitance of thermal and nonthermal dynamical processes and their mutual influence in MWCNTs. Our results illustrate the nature of electron-driven nonthermal process and electron-phonon thermalization in the MWCNTs, and bear implications for the intricate energy conversion and transfer in materials at the nanoscale.

Multidrug-resistant Staphylococcus aureus, including MRSA (methicillin-resistant) and VRSA (vancomycin-resistant), causes serious healthcare-associated infections, even sepsis and death. Here, we identified six novel cathelicidins (CATHPb1-6) from Python bivittatu, and CATHPb1 displayed the best in vitro pharmacological and toxicological profile. We further show that CATHPb1 exhibited evident protection in mice MRSA/VRSA infection models, given either 24 h before or 4 h after infection. The protection was all effective through different administration routes, but was blocked by in vivo depletion of monocyte/macrophages or neutrophils. CATHPb1 can rapidly and massively modulate macrophages/monocytes and neutrophils trafficking to the infection site, and potentiate their bactericidal functions. Meanwhile, CATHPb1 remarkably augmented neutrophil-mediated bacteria killing by facilitating neutrophil extracellular traps (NETs) formation and preventing its degradation. Acting through MAPKs and NF-κB pathways, CATHPb1 selectively enhanced the levels of chemokines while reducing the production of pro-inflammatory cytokines without undesirable toxicities. The much improved serum half-life and stabilities confer CATHPb1 an excellent prospect to become a novel therapeutic agent against multidrug-resistant staphylococcal infections.

We report the observation of local superconductivity induced at the point contact formed between a normal metal tip and WC -- a triple point topological semimetal with super hardness. Remarkably, the maximum critical temperature is up to near 12 K but insensitive to the tip's magnetism. The lateral dimensions of the superconducting puddles were evaluated and the temperature dependencies of superconducting gap and upper critical field were also obtained. These results put constraints on the explanation of the induced superconductivity and pave a pathway for exploring topological superconductivity., Comment: 7 pages, 6 figures

Bulk polycrystalline samples of LaO 1− x F x BiSe 2 ( x ranges from 0 to 0.5) with nominal composition have been successfully synthesized by solid state reaction. Detailed structural analysis shows that LaOBiSe 2 crystallizes in a tetragonal structure (P4/nmm) with lattice parameters of a =4.1565(1) A and c =14.1074(3) A. Experimental results of electrical transport demonstrate that LaOBiSe 2 is a bad metal with an evident anomaly at about 120 K, while the magnetic susceptibility exhibits an evident anomaly around 120 K, suggesting a possible charge density wave (CDW) transition around this temperature. Furthermore, superconductivity is observed in LaO 0.5 F 0.5 BiSe 2 sample with nominal composition at 3.1 K from magnetic and transport measurements.

Electrocatalytic splitting of water is one of the most efficient technologies for hydrogen production, and two-dimensional (2D) MoS2 has been considered as a potential alternative to Pt-based catalysts in the hydrogen evolution reaction (HER). However, the catalytic activity of 2D MoS2 is always contributed from its edge sites, leaving a large number of in-plane domains useless. Herein, we for the first time demonstrated that the catalytic activity of in-plane S atoms of MoS2 can be triggered via single-atom metal doping in HER. In experiments, single Pt atom-doped, few-layer MoS2 nanosheets (Pt–MoS2) showed a significantly enhanced HER activity compared with pure MoS2, originating from the tuned adsorption behavior of H atoms on the in-plane S sites neighboring the doped Pt atoms, according to the density functional theory (DFT) calculations. Furthermore, the HER activity of MoS2 doped with a number of transition metals was screened by virtue of DFT calculations, resulting in a volcano curve along the adsorption free energy of H atoms , which was further confirmed in experiment by using non-precious metals such as Co and Ni atoms doping 2D MoS2 as the catalysts.

The photoinduced martensitic (MT) transition and reverse transition in a shape memory alloy $\mathrm{M}{\mathrm{n}}_{50}\mathrm{N}{\mathrm{i}}_{40}\mathrm{S}{\mathrm{n}}_{10}$ have been examined by using high spatiotemporal resolution four-dimensional transmission electron microscopy (4D-TEM), and the experimental results clearly demonstrate that the MT transition and reverse transition in this Heusler alloy contain a variety of structural dynamic features at picosecond time scales. The 4D-TEM imaging and diffraction observations clearly show that MT transition and MT domain nucleation, which are related to cooperative atomic motions, occur at between 10 and 20 ps, depending on the thickness of the sample. Moreover, a strong coupling between the MT transition and lattice breathing mode is discovered in this system, which can result in a periodic structural oscillation between the MT phase and austenitic (AUS) phase. This allows us to directly observe the MT nucleation and domain wall motions in transient states using high spatiotemporal imaging. A careful analysis of the ultrafast images demonstrates the presence of remarkable transient states, which exhibit the essential features of MT nucleation, lattice symmetry breaking, and a rapid growth of MT plates. These results not only provide insights into the time-resolved structural dynamics and elementary mechanisms that govern the MT transition but also contribute to the development of a novel technique for future 4D-TEM investigations.

We report the observation of an extremely large magnetoresistance (up to 15 000 000% at 1.8 K in a magnetic field of 9 T) in a simple chemical element, gray arsenic, in which the magnitude of the magnetoresistance increases as approximately the square of the magnetic field strength without any signs of saturation. The Hall-effect study confirms that gray arsenic is a nearly perfect ``compensated semimetal,'' with a small concentration of very mobile carriers, which lead to an extremely large magnetoresistance. The analysis of Shubnikov--de Haas oscillations reveals a nontrivial \ensuremath{\pi} Berry phase, a strong signature of Dirac fermions with three-dimensional dispersion. Furthermore, in the presence of parallel magnetic and electric fields, a weak antilocalization effect and a pronounced negative longitudinal magnetoresistance, which may be linked to novel topological states, are also observed. These findings which uncover the material's basis in gray arsenic not only open avenues in spintronics and magnetic sensor applications but also provide more platforms to study topological materials.

The vortex domains, structural properties and ferroelectric polarization in Y1−x In x MnO3 with 0 ≤ x ≤ 0.6 have been extensively investigated in well-characterized samples. X-ray diffraction measurements demonstrated that the lattice parameters change continuously following the substitution of In for Y. Measurements of magnetic susceptibilities revealed that In substitution could visibly affect the magnetic transition and low-temperature magnetic properties. Transmission electron microscopy study showed that In substitution could result in notable decrease of the size of ferroelectric vortex domains. Cs-corrected scanning transmission electron microscopy observations and our careful analysis on atomic-poling configurations demonstrate that the ferroelectric polarizations of Y1−x In x MnO3 are suppressed with the increase of In content.

Anisotropic lattice movements due to the difference between intralayer and interlayer bonding are observed in the layered transition-metal dichalcogenide 1T-TaSeTe following femtosecond laser pulse excitation. Our ultrafast electron diffraction investigations using 4D-transmission electron microscopy (4D-TEM) clearly reveal that the intensity of Bragg reflection spots often changes remarkably due to the dynamic diffraction effects and anisotropic lattice movement. Importantly, the temporal diffracted intensity from a specific crystallographic plane depends on the deviation parameter s, which is commonly used in the theoretical study of diffraction intensity. Herein, we report on lattice thermalization and structural oscillations in layered 1T-TaSeTe, analyzed by dynamic diffraction theory. Ultrafast alterations of satellite spots arising from the charge density wave in the present system are also briefly discussed.

The structural features and the electron energy loss spectrum of black phosphorus (BP) have been experimentally analyzed and they are discussed based on a theoretical calculation. The low-energy loss spectra of typical samples reveal that the emerging high-mobility two-dimensional material BP often exhibits both bulk and surface plasmon modes. The surface modes of BP are strongly thickness dependent. Electrodynamic analysis indicates that the Fuchs–Kliewer-like surface plasmon modes consist of two branches with different charge symmetry: the upper side and lower side have the same charge polarity as the lower branch and the opposite charge polarity to the upper branch. This study provides fundamental insight into the characteristic nature of BP plasmonics.

High-quality Bi2−x Pb x Sr2CaCu2O8+δ (Bi2212) single crystals have been successfully grown by the traveling solvent floating zone technique with a wide range of Pb substitution (x = 0–0.8). The samples are characterized by transmission electron microscope (TEM) and measured by high resolution laser-based angle-resolved photoemission spectroscopy (ARPES) with different photon energies. A systematic evolution of the electronic structure and superstructure with Pb substitution has been revealed for the first time. The superstructure shows a significant change with Pb substitution and the incommensurate modulation vector ( Q ) decreases with increasing Pb substitution. In the meantime, the superstructure intensity from ARPES measurements also decreases dramatically with increasing Pb concentration. The superstructure in Bi2212 can be effectively suppressed by Pb substitution and it nearly disappears with a Pb substitution of x=0.8. We also find that the superstructure bands in ARPES measurements depend sensitively on the photon energy of lasers used; they can become even stronger than the main band when using a laser photon energy of 10.897 eV. These results provide important information on the origin of the incommensurate superstructure and its control and suppression in bismuth-based high temperature superconductors.

The whisker-like niobium triselenide (NbSe3) nanowires were synthesized using the traditional solid state reaction. X-ray diffraction experiment suggested the monoclinic structure (P2(1)/m), and crystal morphology analysis indicated that the band-like shape is the stable morphology. Two charge density wave (CDW) states were observed at around 140 K and 50 K, respectively, and the nonlinear effect was detected in the CDW states from the R-T and I-V measurements. The doped Fe atoms, as pinning centers, play an important role in the nonlinear properties of the CDW state. Electron diffraction and HRTEM experiments were carried out at different temperatures in order to investigate the structural features and their evolution. The sets of incommensurate modulation spots with modulation vector q(1) similar to (h, k 0.243, I) appeared below 145 K, and other sets of complex superstructure spots with modulation vector q(2) similar to (h, k + 0.3, I + 1.3424), q(3) similar to (h, k-0.3137, 1.5685), = (1/3, k, 1) and q(5) = (0.5, 0.25, 0.5) were observed at [1 0 0] and [3 0 1] zone axis at about 20 K, respectively, suggesting the complex inconnmensurately modulated structures in this material.

The superconducting and lattice properties of {delta}-MoC{sub 0.681} were studied by electromagnetic measurements, synchrotron X-ray diffraction, neutron diffraction, and electron diffraction. The superconducting properties (T{sub c}=12 K) of {delta}-MoC{sub 0.681} were well characterized by a weak coupling model. The carbon vacancies present in the host cubic structure were found to be robust, although the material was synthesized from stoichiometric carbon and Mo powder under a high-pressure of 6 GPa. A thermodynamically-stable structure with ordered vacancies did not account for the robust features of {delta}-MoC{sub 0.681} since the vacancies are unlikely to be ordered in long range in the host structure. A model based on inherent phonon instability theoretically predicted for a stoichiometric MoC phase might be responsible for the robust features of {delta}-MoC{sub 0.681}. - Graphical Abstract: The cubic molybdenum carbide shows an excellent superconductivity with robust carbon vacancies. Inherent phonon instability theoretically predicted for a stoichiometric MoC phase might be responsible for the vacancies rather than a thermodynamically-stable structure with vacancies ordering. Highlights: Black-Right-Pointing-Pointer The 12 K superconductivity is well characterized by a weakly coupling model. Black-Right-Pointing-Pointer Carbon vacancies are robust and disordered in the cubic host structure. Black-Right-Pointing-Pointer Inherent phonon instability might be responsible for the robust carbonmore » vacancies in {delta}-MoC{sub 0.681}.« less

Essential structural properties of the non-trivial “string-wall-bounded” topological defects in hexagonal manganites are studied through homotopy group theory and spherical aberration-corrected scanning transmission electron microscopy. The appearance of a “string-wall-bounded” configuration in RMnO3 is shown to be strongly linked with the transformation of the degeneracy space. The defect core regions (~50 Å) mainly adopt the continuous U(1) symmetry of the high-temperature phase, which is essential for the formation and proliferation of vortices. Direct visualization of vortex strings at atomic scale provides insight into the mechanisms and macro-behavior of topological defects in crystalline materials.

863 Project [2009AA033101]; "973" Projects [2010CB833102]; NSFC [50972164, 51222210]; CAS project [KJCX2-YW-W26]; 100 Talent Project of the Chinese Academy of Sciences, Program for New Century Excellent Talents in University [NCET-09-0628]; SRF for ROCS, SEM

A coordinatively unsaturated single iron site confined in a graphene matrix shows an ultrahigh activity for catalytic oxidation., Coordinatively unsaturated (CUS) iron sites are highly active in catalytic oxidation reactions; however, maintaining the CUS structure of iron during heterogeneous catalytic reactions is a great challenge. Here, we report a strategy to stabilize single-atom CUS iron sites by embedding highly dispersed FeN4 centers in the graphene matrix. The atomic structure of FeN4 centers in graphene was revealed for the first time by combining high-resolution transmission electron microscopy/high-angle annular dark-field scanning transmission electron microscopy with low-temperature scanning tunneling microscopy. These confined single-atom iron sites exhibit high performance in the direct catalytic oxidation of benzene to phenol at room temperature, with a conversion of 23.4% and a yield of 18.7%, and can even proceed efficiently at 0°C with a phenol yield of 8.3% after 24 hours. Both experimental measurements and density functional theory calculations indicate that the formation of the Fe═O intermediate structure is a key step to promoting the conversion of benzene to phenol. These findings could pave the way toward highly efficient nonprecious catalysts for low-temperature oxidation reactions in heterogeneous catalysis and electrocatalysis.

Four-dimensional ultrafast transmission electron microscopy measurements reveal a rich variety of structural dynamic phenomena at a phase transition in the charge density wave (CDW) of $1T\text{\ensuremath{-}}{\mathrm{TaSe}}_{2}$. Under photoexcitation, remarkable changes in both the CDW intensity and orientation are clearly observed, associated with the transformation from a commensurate (C) phase into an incommensurate (IC) phase in a time scale of picoseconds. Moreover, the transient states reveal a notable ``structural isosbestic point'' at a wave vector of ${q}_{\mathrm{iso}}$ where the C and IC phases yield their diffracting efficiencies in equal ratio. This observation reveals that the crystal planes parallel to ${q}_{\mathrm{iso}}$ adopt a common dynamic feature in the CDW phases throughout whole nonequilibrium transition. The second-order characteristics observed in this nonequilibrium phase transition have also been analyzed based on time-resolved structural data.

The electronic structure of multiferroic YMn2O5 material has been studied by use of the generalized gradient approximation (GGA). The results demonstrate that the oxygen 2p and manganese 3d orbitals are strongly hybridized. Considering the on-site Coulomb interaction U, we performed the GGA+U calculations for 0 < U ⩽ 8 eV, and it is found that the increase of U could enlarge the band gap and, on the other hand, weaken the Mn-O hybridization. The experimental measurements of the electron energy-loss spectrometry (EELS) exhibit a rich variety of structural features in both O-K edge and Mn-L edges. A theoretical and experimental analysis on the O-K edge suggests that the on-site Coulomb interaction (U) in YMn2 O5 could be less than 4 eV. Certain electronic structural features of LaMn2O5 have been discussed in comparison with those of YMn2O5.

Martensitic transition and reverse transition in ferromagnetic shape memory alloy MnNiSn contain a variety of structural dynamic features accompanied directly by atomic motions and micro-domain alterations. To investigate the effects of crystalline defects on the dynamical structural phase transitions, we use ultrafast transmission electron microscopy (UTEM) to directly image the rapid structural phase transition in MnNiSn initiated by femtosecond laser pulses. Via high spatiotemporal resolution images, we reveal the pinning effect by the grain boundary on the phonon-driven martensitic transition after fs-laser pulse excitations, and the structural oscillation is also observed as driven by coherent acoustic phonons that start at the sites of the grain boundary and propagate with the speed of sound. These results elucidate the roles of crystallographic defects in the dynamical processes of martensitic transition and highlight the unprecedented capability of UTEM for direct imaging lattice motions with nanometer spatial and picosecond temporal resolutions.

In situ heating transmission electron microscopy observations clearly reveal remarkable interlayer expansion and inner-layer inward contraction in multi-walled boron nitride nanotubes (BNNTs) as the specimen temperature increases. We interpreted the observed inward contraction as being due to the presence of the strong constraints of the outer layers on radial expansion in the tubular structure upon in situ heating. The increase in specimen temperature upon heating can create pressure and stress toward the tubular center, which drive the lattice motion and yield inner diameter contraction for the multi-walled BNNTs. Using a simple model involving a wave-like pattern of layer-wise distortion, we discuss these peculiar structural alterations and the anisotropic thermal expansion properties of the tubular structures. Moreover, our in situ atomic images also reveal Russian-doll-type BN nanotubes, which show anisotropic thermal expansion behaviors.

Dandelion-like micro-crystallites and long belt-like nanostructures of Sr-V-O materials were synthesized under mild hydrothermal reaction conditions in the presence of mineralizer adipic acid at 220C for 60 h and 180C for 60 h, respectively. Scanning electron microscopy measurements show that the Sr-V-O micro-crystallites in general consist of well-arranged microrods with the average length of a few hundred micrometers and diameters ranging from 1 um to 5 m. The long belt-like nanostructures around 100 nm in width exhibit visible twist and waving in shapes, showing high flexibility in nature. Powder X-ray diffraction and electron diffraction measurements both revealed that these micro-/nanometer materials have an orthorhombic structure with the lattice parameter a=9.694 A, b=3.687 A, and c=12.570 A (space group of Pnma) as reported for -SrV2O6 phase. The electron diffraction patterns also indicate that the micro-/nanometer -SrV2O6 crystallites preferentially grow along the 010 crystallographic direction. ...

A 75 K Sr2CuO3+δ superconductor (nominal δ = 0.4), detected to be a K2NiF4-type tetragonal single phase from x-ray powder diffraction (XRD) data, was annealed in a N2 atmosphere at different temperatures, and with raising the annealing temperature, the superconducting transition temperature (Tc) is found to increase from 75 K (as-prepared), in turn, to 89 K (post-annealed at 150 °C), 95 K (post-annealed at 250 °C), and then to disappear when further increasing the annealing temperature above 250 °C. Transmission electron microscopy (TEM) and electron energy-loss spectroscopy (EELS) techniques are utilized to characterize both as-prepared and post-annealed samples in order to further identify the superconducting phases. TEM investigations reveal that almost all grains in the as-prepared sample exhibit modulated structures having the K2NiF4-type tetragonal structure of Sr2CuO3+δ as their basic substructure, and two types of modulated phase, i.e., Fmmm modulated phase ( and c = cp) and C 2/m modulated phase (, b = cp, and β = 101.3°), are found in the sample. With increasing annealing temperature, successive structure phase transitions starting from the C 2/m modulated phase are observed, i.e., C 2/m modulated phase Cmmm modulated phase (a = cp, and ) (annealed at 150 °C) Pmmm modulated phase ( and c = cp) (annealed at 250 °C) unmodulated orthorhombic structure (annealed at 350 °C), while no obvious changes are found for the Fmmm modulated phase up to 250 °C. The experimental results strongly suggest that the C 2/m modulated phase is responsible for Tc at 75 K, the Cmmm modulated phase for Tc at 89 K, and the Pmmm modulated phase for Tc at 95 K, while the Fmmm modulated phase is nonsuperconducting. The absence of superconductivity in the Fmmm modulated phase can be explained by the oxygen vacancies located at the Cu–O planes, based on the previous results. For the superconducting modulated phases, the oxygen vacancies can only be located in the Sr–O layers (i.e., in the apical sites) due to the requirement of oxygen vacancy-free CuO2 planes for superconductivity, and thus their formations are naturally explained by the ordering of apical oxygen vacancies. Besides a minor effect of the hole doping level, the reordering of apical oxygen in those superconducting modulated phases is revealed to be the key factor resulting in the enhancement of Tc.

High quality LaOFeP superconducting materials?have been successfully synthesized by a two-step solid reaction method. We first prepared a La?Fe?P ternary alloy following a pre-alloying treatment; the resultant La?Fe?P ternary alloy was found to be the key precursor for producing the final LaOFeP superconductors in our experiments. The LaOFeP samples in general have a superconducting transition temperature of about Tc?4.1?K as demonstrated by the measurements of resistivity and magnetic susceptibility. The volume ratio of the superconducting phase in the high quality sample is estimated to be as high as 95% at the temperature of 2?K. Structural analysis by means of x-ray diffraction, electron diffraction and high resolution transmission electron microscopy shows that the LaOFeP crystal has a layered tetragonal structure with a P4/nmm space group. Structural defects such as stacking faults in the LaOFeP sample have also been briefly discussed.

Measurements of crystal structures, electrical and magnetic properties for the electron doped Bi x Ca1−x MnO3 (0

We report the structural and superconducting properties of FeSe0.3Te0.7 (FST) thin films with different thicknesses grown on ferroelectric Pb(Mg1/3Nb2/3)0.7Ti0.3O3 substrates. It was shown that the FST films undergo biaxial tensile strains which are fully relaxed for films with thicknesses above 200 nm. Electrical transport measurements reveal that the ultrathin films exhibit an insulating behavior and superconductivity appears for thicker films with Tc saturated above 200 nm. The current-voltage curves around the superconducting transition follow the Berezinskii-Kosterlitz-Thouless (BKT) transition behavior and the resistance-temperature curves can be described by the Halperin–Nelson relation, revealing quasi-two-dimensional phase fluctuation in FST thin films. The Ginzburg number decreases with increasing film thickness indicating the decrease of the strength of thermal fluctuations. Upon applying electric field to the heterostructure, Tc of FST thin film increases due to the reduction of the tensile strain in FST. This work sheds light on the superconductivity, strain effect as well as electric-field modulation of superconductivity in FST films.

Ferroelectricity in multiferroic Pr(Sr0.1 Ca0.9)2 Mn2 O7 is found to originate from the off-centering of Mn ions. This polar displacement is energetically stabilized by the cooperative interplay of lattice deformation induced by orbital ordering and oxygen octahedral tilting. This mechanism implies that magnetism and ferroelectricity arise from the same magnetic ions, providing direct evidence for the magnetic-ion off-centering-driven ferroelectricity.

A single-phase superconductor Sr2CuO3+δ with nominal δ = 0.4 (detected from x-ray powder diffraction (XRD) data) (Tc = 75 K) has been synthesized using a high-pressure technique. The structure and microstructure of this material have been carefully studied by means of transmission electron diffraction (ED), high-resolution transmission electron microscopy (HRTEM), electron energy-loss spectroscopy (EELS) and energy dispersive analysis of x-ray (EDX) techniques. Interestingly enough, ED and HRTEM investigations reveal that almost all grains of the tetragonal form Sr2CuO3+δ exhibit modulated features. This result strongly suggests that the superconductivity at 75 K in the material is associated with the modulated phases. Two types of modulated phases having different primitive cells but equal in volume are found in the sample. One is the face-centred orthorhombic modulated phase with a space group Fmmm and lattice constants of . The other is the base-centred monoclinic modulated phase with a space group C 2/m and unit-cell parameters of , bm = cp, and β = 101.3°. The relative ratio of the former to the latter phase contents is about 4:1. Oxygen K absorption edges from EELS measurements suggest that the two modulated phases have the same hole intensity. Defects such as twin boundaries, small-angle asymmetrical tilt boundaries and dislocations are frequently observed in the C 2/m monoclinic modulated phase, and some of them are carefully studied by means of HRTEM, EELS and EDX.

Multiple-quantum magic angle spinning (MQMAS) and satellite-transition magic angle spinning (STMAS) are two well-known techniques for obtaining high-resolution, or "isotropic", NMR spectra of quadrupolar nuclei. It has recently been shown that dynamics-driven modulation of the quadrupolar interaction on the microsecond timescale results in linewidths in isotropic STMAS spectra that are strongly broadened, while, in contrast, the isotropic MQMAS linewidths remain narrow. Here, we use this novel methodology in an (27)AI (I = 5/2) NMR study of the calcined-dehydrated aluminophosphate AIPO-14 and two forms of as-synthesized AlPO-14, one prepared with isopropylamine (C3H7NH2) as the template molecule and one with piperidine (C5H10NH). For completeness, the P-31 and C-13 (both I=1/2) MAS NMR spectra are also presented. A comparison of the (27)AI MQMAS and STMAS NMR results show that, although calcined AIOPO-14 appears to have a rigid framework structure, the extent of motion in the two as-synthesized forms is significant, with clear evidence for dynamics on the microsecond timescale in the immediate environments of all four AI sites in each material. Variable-temperature (27)AI STMAS NMR studies of the two as-synthesized AIPO forms reveal the dynamics to be complex, with the motions of both the guest water molecules and organic template molecules shown to be contributing. The sensitivity of the STMAS NMR experiment to the presence of microsecond timescale dynamics is such that it seems likely that this methodology will prove useful in NMR studies of host-guest interactions in a wide variety of framework materials.

Previous experimental results have shown important differences between iron selenide and arsenide superconductors, which seem to suggest that the high temperature superconductivity in these two subgroups of iron-based family may arise from different electronic ground states. Here, we report the complete phase diagram of a newly synthesized superconducting (SC) system (Li1-xFex)OHFeSe with a similar structure to FeAs-based superconductors. In the non-SC samples, an antiferromagnetic (AFM) spin-density-wave (SDW) transition occurs at ~127 K. This is the first example to demonstrate such an SDW phase in FeSe-based superconductor system. Transmission electron microscopy (TEM) shows that a well-known sqrr(5) x sqrr(5) iron vacancy ordered state, resulting in an AFM order at ~ 500K in AyFe2-xSe2 (A = metal ions) superconductor system, is absent in both non-SC and SC samples, but a unique superstructure with a modulation wave vector q=1/2(1, 1, 0), identical to that seen in SC phase of KyFe2-xSe2, is dominant in the optimal SC sample (with an SC transition temperature Tc = 40 K). Hence, we conclude that the high-Tc superconductivity in (Li1-xFex)OHFeSe stems from the similarly weak AFM fluctuations as FeAs-based superconductors, suggesting a universal physical picture for both iron selenide and arsenide superconductors., 4 pages, 3 tables, 3 figures, accepted for publication in JACS