Self-flux-grown Ba4Fe4ClO9.5-x crystals exhibiting structures with tunable modulation.

The synthesis and X-ray structural study of the new family of compounds Ba4Fe4ClO9.5-x with tunable structural modulation are reported. The framework of the structure has the Ba2Fe4O9.5-x composition, with open hexagonal channels extending along the c-axis. The channels are filled with linear [Ba-Cl-Ba] triplets. The oxygen stoichiometry and the oxidation state of iron both are controlled by the redox conditions during crystal preparation. The modulation of the crystal structure arises from the distribution of the oxygen atoms in the framework and iron coordination polyhedra are a combination of FeO4-tetrahedra, FeO5-bipyramids, and FeO6-octahedra. The structure modulation also originates from the ordered or disordered distribution of the [Ba-Cl-Ba] triplets filling the channels which is also affected by the conditions of the thermal treatment of the crystals. The structure investigation reveals a composition variation from Ba4Fe4ClO9.5 (x = 0), in which Fe exhibits a 3+ oxidation state, to Ba4Fe4ClO8 (x = 1.5) with the framework built exclusively of FeO4 tetrahedra.

Magnetic Field Induced Quantum Spin Liquid in the Two Coupled Trillium Lattices of K_{2}Ni_{2}(SO_{4})_{3}.

Quantum spin liquids are exotic states of matter that form when strongly frustrated magnetic interactions induce a highly entangled quantum paramagnet far below the energy scale of the magnetic interactions. Three-dimensional cases are especially challenging due to the significant reduction of the influence of quantum fluctuations. Here, we report the magnetic characterization of K_{2}Ni_{2}(SO_{4})_{3} forming a three-dimensional network of Ni^{2+} spins. Using density functional theory calculations, we show that this network consists of two interconnected spin-1 trillium lattices. In the absence of a magnetic field, magnetization, specific heat, neutron scattering, and muon spin relaxation experiments demonstrate a highly correlated and dynamic state, coexisting with a peculiar, very small static component exhibiting a strongly renormalized moment. A magnetic field B≳4 T diminishes the ordered component and drives the system into a pure quantum spin liquid state. This shows that a system of interconnected S=1 trillium lattices exhibits a significantly elevated level of geometrical frustration.

Chemical exchange at the ferroelectric phase transition of lead germanate revealed by solid state 207Pb nuclear magnetic resonance.

The nature of the dynamics and structural changes that take place at the ferroelectric phase transition in lead oxides is a rich field of study. Solid-state nuclear magnetic resonance (NMR) of 207Pb is well suited to study the local structure and disorder in lead oxide ferroelectric transitions at the atomic level. However, very large 207Pb shielding anisotropy results in poor resolution in 1D static and magic angle spinning (MAS) NMR spectra. We address this problem by using short high-power adiabatic pulses (SHAPs) with magic-angle-turning sequences to correlate the isotropic and anisotropic parts of the 207Pb chemical shift tensor in a 2D NMR experiment, yielding resolved 207Pb NMR spectra of the nine distinct lead sites in uniaxial ferroelectric lead germanate (Pb5Ge3O11). Using this technique we detect the magnetic environments of displaced Pb2+ ions and unambiguously identify the nature of the phase transition as mixed displacive and order-disorder. We also observe that the atomic-level process responsible for the phase transition in ferroelectric lead germanate is chemical exchange on the kilohertz timescale. We derive an activation energy of 103.4 ± 1.7 kJ mol-1 and compare it to dielectric spectroscopy studies on similar materials. These results show that this method can be used to characterize ferroelectric phase transitions of complex materials with high resolution using nuclei that are typically inaccessible due to their large shielding anisotropy.

In Situ Electric Field Skyrmion Creation in Magnetoelectric Cu2OSeO3.

Exploiting additional degrees of freedom in solid-state materials may be the most-promising solution when approaching the quantum limit of Moore's law for the conventional electronic industry. Recently discovered topologically nontrivial spin textures, skyrmions, are outstanding among such possibilities. However, the controlled creation of skyrmions, especially by electric means, remains a pivotal challenge in technological applications. Here, we report that skyrmions can be created locally via electric field in the magnetoelectric helimagnet Cu2OSeO3. Using Lorentz transmission electron microscopy, we successfully write skyrmions in situ from a helical-spin background. Our discovery is highly coveted because it implies that skyrmionics can be integrated into modern field effect transistor based electronic technology, in which very low energy dissipation can be achieved and, hence, realize a large step forward toward its practical applications.

Spin-Resolved Electronic Response to the Phase Transition in MoTe_{2}.

The semimetal MoTe_{2} is studied by spin- and angle-resolved photoemission spectroscopy across the centrosymmetry-breaking structural transition temperature of the bulk. A three-dimensional spin-texture is observed in the bulk Fermi surface in the low temperature, noncentrosymmetric phase that is consistent with first-principles calculations. The spin texture and two types of surface Fermi arc are not completely suppressed above the bulk transition temperature. The lifetimes of quasiparticles forming the Fermi arcs depend on thermal history and lengthen considerably upon cooling toward the bulk structural transition. The results indicate that a new form of polar instability exists near the surface when the bulk is largely in a centrosymmetric phase.

Filming the formation and fluctuation of skyrmion domains by cryo-Lorentz transmission electron microscopy.

Magnetic skyrmions are promising candidates as information carriers in logic or storage devices thanks to their robustness, guaranteed by the topological protection, and their nanometric size. Currently, little is known about the influence of parameters such as disorder, defects, or external stimuli on the long-range spatial distribution and temporal evolution of the skyrmion lattice. Here, using a large (7.3 × 7.3 µm(2)) single-crystal nanoslice (150 nm thick) of Cu2OSeO3, we image up to 70,000 skyrmions by means of cryo-Lorentz transmission electron microscopy as a function of the applied magnetic field. The emergence of the skyrmion lattice from the helimagnetic phase is monitored, revealing the existence of a glassy skyrmion phase at the phase transition field, where patches of an octagonally distorted skyrmion lattice are also discovered. In the skyrmion phase, dislocations are shown to cause the emergence and switching between domains with different lattice orientations, and the temporal fluctuation of these domains is filmed. These results demonstrate the importance of direct-space and real-time imaging of skyrmion domains for addressing both their long-range topology and stability.

Time-resolved ARPES at LACUS: Band Structure and Ultrafast Electron Dynamics of Solids.

The manipulation of the electronic properties of solids by light is an exciting goal, which requires knowledge of the electronic structure with energy, momentum and temporal resolution. Time- and angle-resolved photoemission spectroscopy (tr-ARPES) is the most direct probe of the effects of an optical excitation on the band structure of a material. In particular, tr-ARPES in the extreme ultraviolet (VUV) range gives access to the ultrafast dynamics over the entire Brillouin zone. VUV tr-ARPES experiments can now be performed at the ASTRA (ARPES Spectrometer for Time-Resolved Applications) end station of Harmonium, at LACUS. Its capabilities are illustrated by measurements of the ultrafast electronic response of ZrSiTe, a novel topological semimetal characterized by linearly dispersing states located at the Brillouin zone boundary.

Crystal Structure, Transport, and Magnetic Properties of an Ir(6+) Compound Ba8Al2IrO14.

The novel iridate Ba8Al2IrO14 was prepared as single crystals by self-flux method, thereby providing a rare example of an all-Ir(VI) compound that can be synthesized under ambient pressure conditions. The preparation of all-Ir(6+) iridate without using traditional high-pressure techniques has to our knowledge previously only been reported in Nd2K2IrO7 and Sm2K2IrO7. The monoclinic crystal structure (space group C2/m, No.12) is stable down to 90 K and contains layers of IrO6 octahedra separated by Ba and AlO4 tetrahedra. The material exhibits insulating behavior with a narrow band gap of ∼0.6 eV. The positive Seebeck coefficient indicates hole-like dominant charge carriers. Susceptibility measurement shows antiferromagnetic coupling with no order down to 2 K.

Nanopore integrated nanogaps for DNA detection.

A high-throughput fabrication of sub-10 nm nanogap electrodes combined with solid-state nanopores is described. These devices should allow concomitant tunneling and ionic current detection of translocating DNA molecules. We report the optimal fabrication parameters in terms of dose, resist thickness, and gap shape that allow easy reproduction of the fabrication process at wafer scale. The device noise and current voltage characterizations performed and the influence of the nanoelectrodes on the ionic current noise is identified. In some cases, ionic current rectification for connected or biased nanogap electrodes is also observed. In order to increase the extremely low translocation rates, several experimental strategies were tested and modeled using finite element analysis. Our findings are useful for future device designs of nanopore integrated electrodes for DNA sequencing.

High-performance multipanel biosensors based on a selective integration of nanographite petals.

We report the first selective growth of nanographite petals and various carbon nanomaterials onto a multipanel electrochemical platform. Different types of nanomaterials can be obtained by fine-tuning the growth parameters of the chemical vapor deposition (CVD) process. First, absolute novelty is the catalytic CVD selective growth of different carbon nanomaterials only on the working electrodes of the platform. A second novelty is the growth obtained at complementary metal-oxide-semiconductor compatible temperatures. These novel electrodes have been incorporated in sensors in which performance characteristics improve with the content of nanostructures. Unprecedented sensing parameters with respect to both direct and enzyme-mediated electrochemical biodetection have been obtained.

High-resolution crystal structure of the double nitrate hydrate [La(NO3)6]2[Ni(H2O)6]3·6H2O.

This study introduces bis-[hexa-kis-(nitrato-κ2 O,O')lanthanum(III)] tris-[hexa-aqua-nickel(II)] hexa-hydrate, [La(NO3)6]2[Ni(H2O)6]3·6H2O, with a structure refined in the hexa-gonal space group R . The salt com-prises [La(NO3)6]3- icosa-hedra and [Ni(H2O)6]2+ octa-hedra, thus forming an intricate network of inter-penetrating honeycomb lattices arranged in layers. This arrangement is stabilized through strong hydrogen bonds. Two successive layers are connected via the second [Ni(H2O)6]2+ octa-hedra, forming sheets which are stacked perpendicular to the c axis and held in the crystal by van der Waals forces. The synthesis of [La(NO3)6]2[Ni(H2O)6]3·6H2O involves dissolving lanthanum(III) and nickel(II) oxides in nitric acid, followed by slow evaporation, yielding green hexa-gonal plate-like crystals.

Diuron sorbed to carbon nanotubes exhibits enhanced toxicity to Chlorella vulgaris.

Carbon nanotubes (CNT) are more and more likely to be present in the environment, where they will associate with organic micropollutants due to strong sorption. The toxic effects of these CNT-micropollutant mixtures on aquatic organisms are poorly characterized. Here, we systematically quantified the effects of the herbicide diuron on the photosynthetic activity of the green alga Chlorella vulgaris in presence of different multiwalled CNT (industrial, purified, pristine, and oxidized) or soot. The presence of carbonaceous nanoparticles reduced the adverse effect of diuron maximally by <78% (industrial CNT) and <34% (soot) at 10.0 mg CNT/L, 5.0 mg soot/L, and diuron concentrations in the range 0.73-2990 µg/L. However, taking into account the measured dissolved instead of the nominal diuron concentration, the toxic effect of diuron was equal to or stronger in the presence of CNT by a factor of up to 5. Sorbed diuron consequently remained partially bioavailable. The most pronounced increase in toxicity occurred after a 24 h exposure of algae and CNT. All results point to locally elevated exposure concentration (LEEC) in the proximity of algal cells associated with CNT as the cause for the increase in diuron toxicity.

Light-Induced Metastable Hidden Skyrmion Phase in the Mott Insulator Cu2 OSeO3.

The discovery of a novel long-lived metastable skyrmion phase in the multiferroic insulator Cu2 OSeO3 visualized with Lorentz transmission electron microscopy for magnetic fields below the equilibrium skyrmion pocket is reported. This phase can be accessed by exciting the sample non-adiabatically with near-infrared femtosecond laser pulses and cannot be reached by any conventional field-cooling protocol, referred as a hidden phase. From the strong wavelength dependence of the photocreation process and via spin-dynamics simulations, the magnetoelastic effect is identified as the most likely photocreation mechanism. This effect results in a transient modification of the magnetic free energy landscape extending the equilibrium skyrmion pocket to lower magnetic fields. The evolution of the photoinduced phase is monitored for over 15 min and no decay is found. Because such a time is much longer than the duration of any transient effect induced by a laser pulse in a material, it is assumed that the newly discovered skyrmion state is stable for practical purposes, thus breaking ground for a novel approach to control magnetic state on demand at ultrafast timescales and drastically reducing heat dissipation relevant for next-generation spintronic devices.

Electric field writing and erasing of skyrmions in magnetoelectric Cu2OSeO3 with an ultralow energy barrier.

Skyrmions are chiral magnetic textures with non-trivial topology, and due to their unique properties they are widely considered as promising information carriers in novel magnetic storage applications. While electric field writing/erasing and manipulation of skyrmions have been recently achieved, quantitative insights into the energetics of those phenomena remain scarce. Here, we report our in situ electric field writing/erasing of skyrmions in magnetoelectric helimagnet Cu2OSeO3 utilizing real-space and real-time Lorentz transmission electron macroscopy. Through the quantitavie analysis on our massive video data, we obtained a linear dependence of the number of skyrmions on the amplitude of the applied electric field, from which a local energy barried to write/erase skyrmions is estimated to be per skyrmion. Such an ultralow energy barrier implies the potential of precise control of skyrmions in future spintronics applications.

Direct Visualisation of Skyrmion Lattice Defect Alignment at Grain Boundaries.

We present a method to directly visualise a statistical analysis of skyrmion defect alignment at grain boundaries in the skyrmion host [Formula: see text]OSeO3. Using Lorentz transmission electron microscopy, we collected large data sets with several hundreds of frames containing skyrmion lattices with grain boundaries in them. To address the behaviour of strings of dislocations in these grain boundaries, we developed an algorithm to automatically extract and classify strings of dislocations separating the grains. This way we circumvent the problem of having to create configurations with well-defined relative grain orientations by performing a statistical analysis on a dynamically rearranging image sequence. With this statistical method, we are able to experimentally extract the relationship between grain boundary alignment and defect spacing and find an agreement with geometric expectations. The algorithms used can be extended to other types of lattices such as Abrikosov lattices or colloidal systems in optical microscopy.

Are carbon nanotube effects on green algae caused by shading and agglomeration?

Due to growing production, carbon nanotubes (CNT) may soon be found in a broad range of products and thus in the environment. In this work, an algal growth test was developed to determine effects of pristine and oxidized CNT on the green algae Chlorella vulgaris and Pseudokirchneriella subcapitata. CNT suspensions were prepared in algal test medium and characterized taking into account the suspension age, the reduced light transmittance of nanoparticle suspensions defined as shading of CNT and quantified by UV/vis spectroscopy, and the agglomeration of the CNT and of the algal cells. Growth inhibition and photosynthetic activity were investigated as end points. Growth of C. vulgaris was inhibited with effect concentrations of 50% (EC(50)) values of 1.8 mg CNT/L and of 24 mg CNT/L in well dispersed and in agglomerated suspensions, respectively, and 20 mg CNT/L and 36 mg CNT/L for P. subcapitata, respectively. However, the photosynthetic activity was not affected. Growth inhibition was highly correlated with the shading of CNT and the agglomeration of algal cells. This suggests that the reduced algal growth might be caused mainly by indirect effects, i.e. by reduced availability of light and different growth conditions caused by the locally elevated algal concentration inside of CNT agglomerates.

Ba5(IO6)2: crystal structure evolution from room temperature to 80†K.

The crystal structure of Ba5(IO6)2, penta-barium bis-(orthoperiodate), has been re-investigated at room temperature based on single-crystal X-ray diffraction data. In comparison with a previous crystal structure determination by the Rietveld method, an improved precision of the structural parameters was achieved. Additionally, low-temperature measurements allowed the crystal structure evolution to be studied down to 80 K. No evidence of structural transition was found even at the lowest temperature. Upon cooling, the lattice contraction is more pronounced along the b axis. This contraction is found to be inhomogeneous along different crystallographic axes. The inter-atomic distances between different Ba atoms reduce drastically with lowering temperature, resulting in a closer packing around the IO6 octa-hedra, which remain largely unaffected.

Management of nanomaterials safety in research environment.

Despite numerous discussions, workshops, reviews and reports about responsible development of nanotechnology, information describing health and environmental risk of engineered nanoparticles or nanomaterials is severely lacking and thus insufficient for completing rigorous risk assessment on their use. However, since preliminary scientific evaluations indicate that there are reasonable suspicions that activities involving nanomaterials might have damaging effects on human health; the precautionary principle must be applied. Public and private institutions as well as industries have the duty to adopt preventive and protective measures proportionate to the risk intensity and the desired level of protection. In this work, we present a practical, 'user-friendly' procedure for a university-wide safety and health management of nanomaterials, developed as a multi-stakeholder effort (government, accident insurance, researchers and experts for occupational safety and health). The process starts using a schematic decision tree that allows classifying the nano laboratory into three hazard classes similar to a control banding approach (from Nano 3--highest hazard to Nano1--lowest hazard). Classifying laboratories into risk classes would require considering actual or potential exposure to the nanomaterial as well as statistical data on health effects of exposure. Due to the fact that these data (as well as exposure limits for each individual material) are not available, risk classes could not be determined. For each hazard level we then provide a list of required risk mitigation measures (technical, organizational and personal). The target 'users' of this safety and health methodology are researchers and safety officers. They can rapidly access the precautionary hazard class of their activities and the corresponding adequate safety and health measures. We succeed in convincing scientist dealing with nano-activities that adequate safety measures and management are promoting innovation and discoveries by ensuring them a safe environment even in the case of very novel products. The proposed measures are not considered as constraints but as a support to their research. This methodology is being implemented at the Ecole Polytechnique de Lausanne in over 100 research labs dealing with nanomaterials. It is our opinion that it would be useful to other research and academia institutions as well.

Publisher Correction: Melting of a skyrmion lattice to a skyrmion liquid via a hexatic phase.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.