Tunable correlated Chern insulator and ferromagnetism in a moiré superlattice.

Studies of two-dimensional electron systems in a strong magnetic field revealed the quantum Hall effect1, a topological state of matter featuring a finite Chern number C and chiral edge states2,3. Haldane4 later theorized that Chern insulators with integer quantum Hall effects could appear in lattice models with complex hopping parameters even at zero magnetic field. The ABC-trilayer graphene/hexagonal boron nitride (ABC-TLG/hBN) moiré superlattice provides an attractive platform with which to explore Chern insulators because it features nearly flat moiré minibands with a valley-dependent, electrically tunable Chern number5,6. Here we report the experimental observation of a correlated Chern insulator in an ABC-TLG/hBN moiré superlattice. We show that reversing the direction of the applied vertical electric field switches the moiré minibands of ABC-TLG/hBN between zero and finite Chern numbers, as revealed by large changes in magneto-transport behaviour. For topological hole minibands tuned to have a finite Chern number, we focus on quarter filling, corresponding to one hole per moiré unit cell. The Hall resistance is well quantized at h/2e2 (where h is Planck's constant and e is the charge on the electron), which implies C = 2, for a magnetic field exceeding 0.4 tesla. The correlated Chern insulator is ferromagnetic, exhibiting substantial magnetic hysteresis and a large anomalous Hall signal at zero magnetic field. Our discovery of a C = 2 Chern insulator at zero magnetic field should open up opportunities for discovering correlated topological states, possibly with topological excitations7, in nearly flat and topologically nontrivial moiré minibands.

Publisher Correction: Tunable correlated Chern insulator and ferromagnetism in a moiré superlattice.

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

Non-Fermi Liquids from Kinetic Constraints in Tilted Optical Lattices.

We study Fermi-Hubbard models with kinetically constrained dynamics that conserves both total particle number and total center of mass, a situation that arises when interacting fermions are placed in strongly tilted optical lattices. Through a combination of analytics and numerics, we show how the kinetic constraints stabilize an exotic non-Fermi liquid phase described by fermions coupled to a gapless bosonic field, which in several respects mimics a dynamical gauge field. This offers a novel route towards the study of non-Fermi liquid phases in the precision environments afforded by ultracold atom platforms.

Continuous Mott Transition in Moiré Semiconductors: Role of Long-Wavelength Inhomogeneities.

Recent experiments in moiré transition metal dichalcogenide materials have reported the observation of a continuous bandwidth-tuned transition from a metal to a paramagnetic Mott insulator at a fixed filling of one electron per moiré unit cell. The electrical transport measurements reveal a number of puzzling features that are seemingly at odds with the theoretical expectations of an interaction-induced, but disorder-free, bandwidth-tuned metal-insulator transition. In this Letter, we include the effects of long-wavelength inhomogeneities, building on the results for a continuous metal-insulator transition at fixed filling in the clean limit. We examine the effects of mesoscale inhomogeneities near the critical point on transport using the framework of random resistor networks, highlighting the salient differences from a simple percolation-based picture. We place our results in the context of recent and ongoing experiments.

Gamma irradiation effect on photocatalytic properties of Cu and Sr ions codoped PbS.

Gamma-irradiation effects on photocatalytic action of PbS nanocrystallites codoped with Cu and Sr ions were performed for organic dye degradation. The physical and chemical characterizations of these nanocrystallites were examined employing X-ray diffraction, Raman, and field emission electron microscopic analysis. The optical bandgaps of gamma-irradiated PbS with co-dopants have shifted from 1.95 eV (pristine PbS) to 2.45 eV in the visible spectrum. Under direct sunlight, the photocatalytic action of these compounds against methylene blue (MB) was investigated. Observations indicated that gamma-irradiated Pb(0.98)Cu0.01Sr0.01S nanocrystallite sample exhibits a higher photocatalytic degradation activity of 74.02% in 160 min and stability of 69.4% after three cycles, suggesting that gamma irradiation could potentially influence organic MB degradation. This is due to combined action of high-energy gamma irradiation (at an optimzed dose), which causes sulphur vacancies, and defects created by dopant ions, which alter the crystal structure by inducing strain in the crystal lattice, hence altering the crystallinity of PbS.

Unraveling the complexity of amyloid polymorphism using gold nanoparticles and cryo-EM.

Increasing evidence suggests that amyloid polymorphism gives rise to different strains of amyloids with distinct toxicities and pathology-spreading properties. Validating this hypothesis is challenging due to a lack of tools and methods that allow for the direct characterization of amyloid polymorphism in hydrated and complex biological samples. Here, we report on the development of 11-mercapto-1-undecanesulfonate-coated gold nanoparticles (NPs) that efficiently label the edges of synthetic, recombinant, and native amyloid fibrils derived from different amyloidogenic proteins. We demonstrate that these NPs represent powerful tools for assessing amyloid morphological polymorphism, using cryogenic transmission electron microscopy (cryo-EM). The NPs allowed for the visualization of morphological features that are not directly observed using standard imaging techniques, including transmission electron microscopy with use of the negative stain or cryo-EM imaging. The use of these NPs to label native paired helical filaments (PHFs) from the postmortem brain of a patient with Alzheimer's disease, as well as amyloid fibrils extracted from the heart tissue of a patient suffering from systemic amyloid light-chain amyloidosis, revealed a high degree of homogeneity across the fibrils derived from human tissue in comparison with fibrils aggregated in vitro. These findings are consistent with, and strongly support, the emerging view that the physiologic milieu is a key determinant of amyloid fibril strains. Together, these advances should not only facilitate the profiling and characterization of amyloids for structural studies by cryo-EM, but also pave the way to elucidate the structural basis of amyloid strains and toxicity, and possibly the correlation between the pathological and clinical heterogeneity of amyloid diseases.

Strange Metals as Ersatz Fermi Liquids.

A long-standing mystery of fundamental importance in correlated electron physics is to understand strange non-Fermi liquid metals that are seen in diverse quantum materials. A striking experimental feature of these metals is a resistivity that is linear in temperature (T). In this Letter we ask what it takes to obtain such non-Fermi liquid physics down to zero temperature in a translation invariant metal. If in addition the full frequency (ω) dependent conductivity satisfies ω/T scaling, we argue that the T-linear resistivity must come from the intrinsic physics of the low energy fixed point. Combining with earlier arguments that compressible translation invariant metals are "ersatz Fermi liquids" with an infinite number of emergent conserved quantities, we obtain powerful and practical conclusions. We show that there is necessarily a diverging susceptibility for an operator that is odd under inversion and time reversal symmetries, and has zero crystal momentum. We discuss a few other experimental consequences of our arguments, as well as potential loopholes, which necessarily imply other exotic phenomena.

A simple, versatile and robust centrifugation-based filtration protocol for the isolation and quantification of α-synuclein monomers, oligomers and fibrils: Towards improving experimental reproducibility in α-synuclein research.

Increasing evidence suggests that the process of alpha-synuclein (α-syn) aggregation from monomers into amyloid fibrils and Lewy bodies, via oligomeric intermediates plays an essential role in the pathogenesis of different synucleinopathies, including Parkinson's disease (PD), multiple system atrophy and dementia with Lewy bodies (DLB). However, the nature of the toxic species and the mechanisms by which they contribute to neurotoxicity and disease progression remain elusive. Over the past two decades, significant efforts and resources have been invested in studies aimed at identifying and targeting toxic species along the pathway of α-syn fibrillization. Although this approach has helped to advance the field and provide insights into the biological properties and toxicity of different α-syn species, many of the fundamental questions regarding the role of α-syn aggregation in PD remain unanswered, and no therapeutic compounds targeting α-syn aggregates have passed clinical trials. Several factors have contributed to this slow progress, including the complexity of the aggregation pathways and the heterogeneity and dynamic nature of α-syn aggregates. In the majority of experiment, the α-syn samples used contain mixtures of α-syn species that exist in equilibrium and their ratio changes upon modifying experimental conditions. The failure to quantitatively account for the distribution of different α-syn species in different studies has contributed not only to experimental irreproducibility but also to misinterpretation of results and misdirection of valuable resources. Towards addressing these challenges and improving experimental reproducibility in Parkinson's research, we describe here a simple centrifugation-based filtration protocol for the isolation, quantification and assessment of the distribution of α-syn monomers, oligomers and fibrils, in heterogeneous α-syn samples of increasing complexity. The protocol is simple, does not require any special instrumentation and can be performed rapidly on multiple samples using small volumes. Here, we present and discuss several examples that illustrate the applications of this protocol and how it could contribute to improving the reproducibility of experiments aimed at elucidating the structural basis of α-syn aggregation, seeding activity, toxicity and pathology spreading. This protocol is applicable, with slight modifications, to other amyloid-forming proteins.

How specific are the conformation-specific α-synuclein antibodies? Characterization and validation of 16 α-synuclein conformation-specific antibodies using well-characterized preparations of α-synuclein monomers, fibrils and oligomers with distinct structures and morphology.

Increasing evidence suggests that alpha-synuclein (α-syn) oligomers are obligate intermediates in the pathway involved in α-syn fibrillization and Lewy body (LB) formation, and may also accumulate within LBs in Parkinson's disease (PD) and other synucleinopathies. Therefore, the development of tools and methods to detect and quantify α-syn oligomers has become increasingly crucial for mechanistic studies to understand their role in PD, and to develop new diagnostic methods and therapies for PD and other synucleinopathies. The majority of these tools and methods rely primarily on the use of aggregation state-specific or conformation-specific antibodies. Given the impact of the data and knowledge generated using these antibodies on shaping the foundation and directions of α-syn and PD research, it is crucial that these antibodies are thoroughly characterized, and their specificity or ability to capture diverse α-syn species is tested and validated. Herein, we describe an antibody characterization and validation pipeline that allows a systematic investigation of the specificity of α-syn antibodies using well-defined and well-characterized preparations of various α-syn species, including monomers, fibrils, and different oligomer preparations that are characterized by distinct morphological, chemical and secondary structure properties. This pipeline was used to characterize 18 α-syn antibodies, 16 of which have been reported as conformation- or oligomer-specific antibodies, using an array of techniques, including immunoblot analysis (slot blot and Western blot), a digital ELISA assay using single molecule array technology and surface plasmon resonance. Our results show that i) none of the antibodies tested are specific for one particular type of α-syn species, including monomers, oligomers or fibrils; ii) all antibodies that were reported to be oligomer-specific also recognized fibrillar α-syn; and iii) a few antibodies showed high specificity for oligomers and fibrils but did not bind to monomers. These findings suggest that the great majority of α-syn aggregate-specific antibodies do not differentiate between oligomers and fibrils, thus highlighting the importance of exercising caution when interpreting results obtained using these antibodies. Our results also underscore the critical importance of the characterization and validation of antibodies before their use in mechanistic studies and as diagnostic tools or therapeutic agents. This will not only improve the quality and reproducibility of research and reduce costs but will also reduce the number of therapeutic antibody failures in the clinic.

Ferromagnetism in Narrow Bands of Moiré Superlattices.

Many graphene moiré superlattices host narrow bands with nonzero valley Chern numbers. We provide analytical and numerical evidence for a robust spin and/or valley polarized insulator at total integer band filling in nearly flat bands of several different moiré materials. In the limit of a perfectly flat band, we present analytical arguments in favor of the ferromagnetic state substantiated by numerical calculations. Further, we numerically evaluate its stability for a finite bandwidth. We provide exact diagonalization results for models appropriate for ABC trilayer graphene aligned with hBN, twisted double bilayer graphene, and twisted bilayer graphene aligned with hBN. We also provide DMRG results for a honeycomb lattice with a quasiflat band and nonzero Chern number, which extend our results to larger system sizes. We find a maximally spin and valley polarized insulator at all integer fillings when the band is sufficiently flat. We also show that interactions may induce effective dispersive terms strong enough to destabilize this state. These results still hold in the case of zero valley Chern number (for example, trivial side of TLG/hBN). We give an intuitive picture based on extended Wannier orbitals, and emphasize the role of the quantum geometry of the band, whose microscopic details may enhance or weaken ferromagnetism in moiré materials.

Strange Metal in Magic-Angle Graphene with near Planckian Dissipation.

Recent experiments on magic-angle twisted bilayer graphene have discovered correlated insulating behavior and superconductivity at a fractional filling of an isolated narrow band. Here we show that magic-angle bilayer graphene exhibits another hallmark of strongly correlated systems-a broad regime of T-linear resistivity above a small density-dependent crossover temperature-for a range of fillings near the correlated insulator. This behavior is reminiscent of similar behavior in other strongly correlated systems, often denoted "strange metals," such as cuprates, iron pnictides, ruthenates, and cobaltates, where the observations are at odds with expectations in a weakly interacting Fermi liquid. We also extract a transport "scattering rate," which satisfies a near Planckian form that is universally related to the ratio of (k_{B}T/ℏ). Our results establish magic-angle bilayer graphene as a highly tunable platform to investigate strange metal behavior, which could shed light on this mysterious ubiquitous phase of correlated matter.

Possible Quantum Paramagnetism in Compressed Sr_{2}IrO_{4}.

The effect of compression on the magnetic ground state of Sr_{2}IrO_{4} is studied with x-ray resonant techniques in the diamond anvil cell. The weak interlayer exchange coupling between square-planar 2D IrO_{2} layers is readily modified upon compression, with a crossover between magnetic structures around 7 GPa mimicking the effect of an applied magnetic field at ambient pressure. Higher pressures drive an order-disorder magnetic phase transition with no magnetic order detected above 17-20 GPa. The persistence of strong exchange interactions between J_{eff}=1/2 magnetic moments within the insulating IrO_{2} layers up to at least 35 GPa points to a highly frustrated magnetic state in compressed Sr_{2}IrO_{4}, opening the door for realization of novel quantum paramagnetic phases driven by extended 5d orbitals with entangled spin and orbital degrees of freedom.

Electrical behavior and enhanced photocatalytic activity of (Ag, Ni) co-doped ZnO nanoparticles synthesized from co-precipitation technique.

Methylene blue (MB) dye is the most common harmful, toxic, and non-biodegradable effluent produced by the textile industries. The present study investigates the effect of zinc oxide (ZnO) nanoparticles (NPs) and Ag-Ni doped ZnO NPs on the performance of photocatalytic degradation of MB dye. Pure ZnO and Ag-Ni doped ZnO NPs are synthesized using the co-precipitation method. The crystalline nature and surface morphology of the synthesized pure ZnO and Ag-Ni doped ZnO NPs was characterized by powder X-ray diffraction, scanning electron microscopy (SEM), and high resolution transmission electron microscopy (HRTEM) analysis. The presence of spherical-like morphologies was confirmed from SEM and HRTEM analysis. The presence of Ni-O and Zn-O bands in the synthesized materials was found by Fourier transform infrared (FTIR) spectroscopy analysis. The MB dye was degraded under UV-light exposure in various pH conditions. The Ag (0.02%)-Ni doped ZnO NPs exhibits highest photocatalytic activity of 77% under pH 4.

Structural characterization of amyloid fibrils from the human parathyroid hormone.

Amyloid deposits are common in various tissues as a consequence of misfolded proteins. However, secretory protein and peptides are often stored in membrane coated granules as functional amyloids. In this article, we present a detailed characterization of in vitro generated amyloid fibrils from human parathyroid hormone (hPTH(1-84)). Fully mature fibrils could be obtained after a short lag phase within less than one hour at 65°C. These fibrils showed all characteristic of a cross-ß structure. Protease cleavage combined with mass spectrometry identified the central region of the peptide hormone involved in the fibril core formation. EGCG, an inhibitor of amyloid fibril formation, showed binding to residues in the peptide monomers corresponding to the later fibril core and thus explaining the inhibition of the fibril growth. Conformational and dynamic studies by solid-state NMR further corroborated the cross-ß core of the fibrils, but also identified highly mobile segments with a random coil structure not belonging to the rigid fibril core.

Liquefaction of semen generates and later degrades a conserved semenogelin peptide that enhances HIV infection.

UNLABELLED: Semen enhances HIV infection in vitro, but how long it retains this activity has not been carefully examined. Immediately postejaculation, semen exists as a semisolid coagulum, which then converts to a more liquid form in a process termed liquefaction. We demonstrate that early during liquefaction, semen exhibits maximal HIV-enhancing activity that gradually declines upon further incubation. The decline in HIV-enhancing activity parallels the degradation of peptide fragments derived from the semenogelins (SEMs), the major components of the coagulum that are cleaved in a site-specific and progressive manner upon initiation of liquefaction. Because amyloid fibrils generated from SEM fragments were recently demonstrated to enhance HIV infection, we set out to determine whether any of the liquefaction-generated SEM fragments associate with the presence of HIV-enhancing activity. We identify SEM1 from amino acids 86 to 107 [SEM1(86-107)] to be a short, cationic, amyloidogenic SEM peptide that is generated early in the process of liquefaction but that, conversely, is lost during prolonged liquefaction due to the activity of serine proteases. Synthetic SEM1(86-107) amyloids directly bind HIV-1 virions and are sufficient to enhance HIV infection of permissive cells. Furthermore, endogenous seminal levels of SEM1(86-107) correlate with donor-dependent variations in viral enhancement activity, and antibodies generated against SEM1(86-107) recognize endogenous amyloids in human semen. The amyloidogenic potential of SEM1(86-107) and its virus-enhancing properties are conserved among great apes, suggesting an evolutionarily conserved function. These studies identify SEM1(86-107) to be a key, HIV-enhancing amyloid species in human semen and underscore the dynamic nature of semen's HIV-enhancing activity. IMPORTANCE: Semen, the most common vehicle for HIV transmission, enhances HIV infection in vitro, but how long it retains this activity has not been investigated. Semen naturally undergoes physiological changes over time, whereby it converts from a gel-like consistency to a more liquid form. This process, termed liquefaction, is characterized at the molecular level by site-specific and progressive cleavage of SEMs, the major components of the coagulum, by seminal proteases. We demonstrate that the HIV-enhancing activity of semen gradually decreases over the course of extended liquefaction and identify a naturally occurring semenogelin-derived fragment, SEM1(86-107), whose levels correlate with virus-enhancing activity over the course of liquefaction. SEM1(86-107) amyloids are naturally present in semen, and synthetic SEM1(86-107) fibrils bind virions and are sufficient to enhance HIV infection. Therefore, by characterizing dynamic changes in the HIV-enhancing activity of semen during extended liquefaction, we identified SEM1(86-107) to be a key virus-enhancing component of human semen.

Ball/dumbbell-like structured micrometer-sized Sb2S3 particles as a scattering layer in dye-sensitized solar cells.

To improve the light harvesting efficiency in dye-sensitized solar cells (DSSC) the light scattering layer is important. In this Letter, we present ball/dumbbell-like structured micrometer-sized Sb2S3 particles for photon propagation in DSSCs and demonstrate their effective usage in photoelectrodes. The analysis of the photoelectrode by a UV-vis spectrophotometer indicates that the absorption wavelength of an electrode with scattering layer can be obviously promoted from ultraviolet to visible light. The synthesized Sb2S3 particle structures were also characterized by x-ray diffraction, scanning electron microscopy, and transmission electron microscopy. The photovoltaic performance of the ball/dumbbell-like structured Sb2S3 based cell exhibits excellent power conversion efficiency.

Elastic nailing for pediatric subtrochanteric and supracondylar femur fractures.

BACKGROUND: Subtrochanteric and supracondylar femur fractures are difficult injuries to treat in children. Although elastic stable intramedullary nails are commonly used for pediatric femur shaft fractures, there is little information on their effectiveness for managing pediatric subtrochanteric and supracondylar femur fractures. QUESTIONS/PURPOSES: We (1) evaluated radiographic union rates and fracture alignment after elastic nailing of pediatric subtrochanteric and supracondylar femur fractures, (2) identified complications, and (3) determined risk factors for complications. METHODS: Between 2005 and 2011, 36 subtrochanteric fractures and eight supracondylar femur fractures were treated with elastic stable intramedullary nails and had complete followup until clinical and radiographic union. Elastic nailing was used for subtrochanteric fractures in children 5 to 12 years of age or after failed spica cast treatment in younger children and for displaced supracondylar fractures in children older than 5 years. Fracture alignment and union were measured on radiographs, and complications were identified from review of patient charts. Patients with and without complications were compared using nonparametric tests to identify risk factors. RESULTS: All fractures healed; 23 of 33 (70%) subtrochanteric femur fractures and five of seven (71%) supracondylar femur fractures healed with anterior angulation of about 5°. For subtrochanteric fractures, complications included repositioning/removal of nails before radiographic union (n = 4), malunion (n = 2), fracture (n = 1), irritation (n = 1) at nail insertion site, and limb length discrepancy (n = 1); despite these complications, there were 22 (61%) excellent, 12 (33%) satisfactory, and only two (6%) poor outcomes. For supracondylar fractures, complications included infection after nail removal (n = 1) and nail site irritation (n = 2); there were three (38%) excellent, five (62%) satisfactory, and no poor outcomes. Complications were more likely after subtrochanteric fracture during motor vehicle accident (p = 0.045). CONCLUSIONS: Although complication rates are high with elastic nailing for pediatric subtrochanteric (22%) and supracondylar (38%) femur fractures, elastic nailing represents an important option for difficult-to-manage femur fractures. LEVEL OF EVIDENCE: Level IV, therapeutic study. See Instructions for Authors for a complete description of levels of evidence.

Integer quantum Hall effect for bosons.

A simple physical realization of an integer quantum Hall state of interacting two dimensional bosons is provided. This is an example of a symmetry-protected topological (SPT) phase which is a generalization of the concept of topological insulators to systems of interacting bosons or fermions. Universal physical properties of the boson integer quantum Hall state are described and shown to correspond with those expected from general classifications of SPT phases.

Mechanical, microstructural and fracture studies on inconel 825-SS316L functionally graded wall fabricated by wire arc additive manufacturing.

This paper presents a novel method that uses the cold metal transfer based wire arc additive manufacturing process to fabricate functionally graded Inconel 825-SS316L walls. The optical micrograph of Inconel 825 exhibits continuous and discontinuous dendritic structures. The SS316L region comprises 5% of δ-ferrite in primary austenitic (γ) dendrites which was confirmed by the Creq/Nieq ratio of 1.305. The functionally graded interface reveals a partially mixed zone with a transition from the elongated dendrites to fine equiaxed dendrites. The tensile properties of the fabricated wall were determined at room temperature using specimens extracted from Inconel 825, SS316L, and the interface regions. The morphology of the tensile tested specimens revealed significant plastic deformation, indicating ductile failure. The fracture toughness of the wall was experimentally investigated by employing the crack tip opening displacement (CTOD) test. The fracture morphology exhibited a ductile mode of fracture with striations perpendicular to the direction of crack development. Elemental mapping revealed that there was no evidence of elemental segregation on the fractured surfaces, and the elements were uniformly dispersed. The CTOD measures 0.853 mm, 0.873 mm on the Inconel 825 side and the SS316L side respectively. The test results confirm that both the Inconel 825 and SS316L sides have good fracture toughness.

γ-Ray-Induced Photocatalytic Activity of Bi-Doped PbS toward Organic Dye Removal under Sunlight.

Photocatalysts based on semiconducting chalcogenides due to their adaptable physio-chemical characteristics are attracting attention. In this work, Bi-doped PbS (henceforth PbS:Bi) was prepared using a straightforward chemical precipitation approach, and the influence of γ-irradiation on PbS's photocatalytic ability was investigated. Synthesized samples were confirmed structurally and chemically. Pb(1-x)BixS (x = 0, 0.005, 0.01, 0.02) samples that were exposed to gamma rays showed fine-tuning of the optical bandgap for better photocatalytic action beneath visible light. The photocatalytic degradation rate of the irradiated Pb0.995Bi0.005S sample was found to be 1.16 times above that of pure PbS. This is due to the occupancy of Bi3+ ions at surface lattice sites as a result of their lower concentration in PbS, which effectively increases interface electron transport and the annealing impact of gamma irradiation. Scavenger tests show that holes are active species responsible for deterioration of the methylene blue. The irradiated PbS:Bi demonstrated high stability after being used repeatedly for photocatalytic degradation.