Observation of a linked-loop quantum state in a topological magnet.

Quantum phases can be classified by topological invariants, which take on discrete values capturing global information about the quantum state1-13. Over the past decades, these invariants have come to play a central role in describing matter, providing the foundation for understanding superfluids5, magnets6,7, the quantum Hall effect3,8, topological insulators9,10, Weyl semimetals11-13 and other phenomena. Here we report an unusual linking-number (knot theory) invariant associated with loops of electronic band crossings in a mirror-symmetric ferromagnet14-20. Using state-of-the-art spectroscopic methods, we directly observe three intertwined degeneracy loops in the material's three-torus, T3, bulk Brillouin zone. We find that each loop links each other loop twice. Through systematic spectroscopic investigation of this linked-loop quantum state, we explicitly draw its link diagram and conclude, in analogy with knot theory, that it exhibits the linking number (2, 2, 2), providing a direct determination of the invariant structure from the experimental data. We further predict and observe, on the surface of our samples, Seifert boundary states protected by the bulk linked loops, suggestive of a remarkable Seifert bulk-boundary correspondence. Our observation of a quantum loop link motivates the application of knot theory to the exploration of magnetic and superconducting quantum matter.

Quantum-limit Chern topological magnetism in TbMn6Sn6.

The quantum-level interplay between geometry, topology and correlation is at the forefront of fundamental physics1-15. Kagome magnets are predicted to support intrinsic Chern quantum phases owing to their unusual lattice geometry and breaking of time-reversal symmetry14,15. However, quantum materials hosting ideal spin-orbit-coupled kagome lattices with strong out-of-plane magnetization are lacking16-21. Here, using scanning tunnelling microscopy, we identify a new topological kagome magnet, TbMn6Sn6, that is close to satisfying these criteria. We visualize its effectively defect-free, purely manganese-based ferromagnetic kagome lattice with atomic resolution. Remarkably, its electronic state shows distinct Landau quantization on application of a magnetic field, and the quantized Landau fan structure features spin-polarized Dirac dispersion with a large Chern gap. We further demonstrate the bulk-boundary correspondence between the Chern gap and the topological edge state, as well as the Berry curvature field correspondence of Chern gapped Dirac fermions. Our results point to the realization of a quantum-limit Chern phase in TbMn6Sn6, and may enable the observation of topological quantum phenomena in the RMn6Sn6 (where R is a rare earth element) family with a variety of magnetic structures. Our visualization of the magnetic bulk-boundary-Berry correspondence covering real space and momentum space demonstrates a proof-of-principle method for revealing topological magnets.

Electrical discharge triggers quasicrystal formation in an eolian dune.

We report the discovery of a dodecagonal quasicrystal Mn72.3Si15.6Cr9.7Al1.8Ni0.6-composed of a periodic stacking of atomic planes with quasiperiodic translational order and 12-fold symmetry along the two directions perpendicular to the planes-accidentally formed by an electrical discharge event in an eolian dune in the Sand Hills near Hyannis, Nebraska, United States. The quasicrystal, coexisting with a cubic crystalline phase with composition Mn68.9Si19.9Ni7.6Cr2.2Al1.4, was found in a fulgurite consisting predominantly of fused and melted sand along with traces of melted conductor metal from a nearby downed power line. The fulgurite may have been created by a lightning strike that combined sand with material from downed power line or from electrical discharges from the downed power line alone. Extreme temperatures of at least 1,710 °C were reached, as indicated by the presence of SiO2 glass in the sample. The dodecagonal quasicrystal is an example of a quasicrystal of any kind formed by electrical discharge, suggesting other places to search for quasicrystals on Earth or in space and for synthesizing them in the laboratory.

A de novo protein catalyzes the synthesis of semiconductor quantum dots.

De novo proteins constructed from novel amino acid sequences are distinct from proteins that evolved in nature. Construct K (ConK) is a binary-patterned de novo designed protein that rescues Escherichia coli from otherwise toxic concentrations of copper. ConK was recently found to bind the cofactor PLP (pyridoxal phosphate, the active form of vitamin B6). Here, we show that ConK catalyzes the desulfurization of cysteine to H2S, which can be used to synthesize CdS nanocrystals in solution. The CdS nanocrystals are approximately 3 nm, as measured by transmission electron microscope, with optical properties similar to those seen in chemically synthesized quantum dots. The CdS nanocrystals synthesized using ConK have slower growth rates and a different growth mechanism than those synthesized using natural biomineralization pathways. The slower growth rate yields CdS nanocrystals with two desirable properties not observed during biomineralization using natural proteins. First, CdS nanocrystals are predominantly of the zinc blende crystal phase; this is in stark contrast to natural biomineralization routes that produce a mixture of zinc blende and wurtzite phase CdS. Second, in contrast to the growth and eventual precipitation observed in natural biomineralization systems, the CdS nanocrystals produced by ConK stabilize at a final size. Future optimization of CdS nanocrystal growth using ConK-or other de novo proteins-may help to overcome the limits on nanocrystal quality typically observed from natural biomineralization by enabling the synthesis of more stable, high-quality quantum dots at room temperature.

Surface Adatom Diffusion-Assisted Dislocation Nucleation in Metal Nanowires.

We employ a hybrid diffusion- and nucleation-based kinetic Monte Carlo model to elucidate the significant impact of adatom diffusion on incipient surface dislocation nucleation in metal nanowires. We reveal a stress-regulated diffusion mechanism that promotes preferential accumulation of diffusing adatoms near nucleation sites, which explains the experimental observations of strong temperature but weak strain-rate dependence as well as temperature-dependent scatter of the nucleation strength. Furthermore, the model demonstrates that a decreasing rate of adatom diffusion with an increasing strain rate will lead to stress-controlled nucleation being the dominant nucleation mechanism at higher strain rates. Overall, our model offers new mechanistic insights into how surface adatom diffusion directly impacts the incipient defect nucleation process and resulting mechanical properties of metal nanowires.

Atomic Resolution Imaging of Highly Air-Sensitive Monolayer and Twisted-Bilayer WTe2.

Bulk Td-WTe2 is a semimetal, while its monolayer counterpart is a two-dimensional (2D) topological insulator. Recently, electronic transport resembling a Luttinger liquid state was found in twisted-bilayer WTe2 (tWTe2) with a twist angle of ∼5°. Despite the strong interest in 2D WTe2 systems, little experimental information is available about their intrinsic microstructure, leaving obstacles in modeling their physical properties. The monolayer, and consequently tWTe2, are highly air-sensitive, and therefore, probing their atomic structures is difficult. In this study, we develop a robust method for atomic-resolution visualization of monolayers and tWTe2 obtained through mechanical exfoliation and fabrication. We confirm the high crystalline quality of mechanically exfoliated WTe2 samples and observe that tWTe2 with twist angles of ∼5 and ∼2° retains its pristine moiré structure without substantial deformations or reconstructions. The results provide a structural foundation for future electronic modeling of monolayer and tWTe2 moiré lattices.

Atomically Sharp Internal Interface in a Chiral Weyl Semimetal Nanowire.

Internal interfaces in Weyl semimetals (WSMs) are predicted to host distinct topological features that are different from the commonly studied external interfaces (crystal-to-vacuum boundaries). However, the lack of atomically sharp and crystallographically oriented internal interfaces in WSMs makes it difficult to experimentally investigate topological states buried inside the material. Here, we study a unique internal interface known as merohedral twin boundary in chemically synthesized single-crystal nanowires (NWs) of CoSi, a chiral WSM of space group P213 (No. 198). Scanning transmission electron microscopy reveals that this internal interface is a (001) twin plane which connects two enantiomeric counterparts at an atomically sharp interface with inversion twinning. Ab initio calculations show localized internal Fermi arcs at the (001) twin plane that can be clearly distinguished from both external Fermi arcs and bulk states. These merohedrally twinned CoSi NWs provide an ideal platform to explore topological properties associated with internal interfaces in WSMs.

Evidence of a room-temperature quantum spin Hall edge state in a higher-order topological insulator.

Room-temperature realization of macroscopic quantum phases is one of the major pursuits in fundamental physics1,2. The quantum spin Hall phase3-6 is a topological quantum phase that features a two-dimensional insulating bulk and a helical edge state. Here we use vector magnetic field and variable temperature based scanning tunnelling microscopy to provide micro-spectroscopic evidence for a room-temperature quantum spin Hall edge state on the surface of the higher-order topological insulator Bi4Br4. We find that the atomically resolved lattice exhibits a large insulating gap of over 200 meV, and an atomically sharp monolayer step edge hosts an in-gap gapless state, suggesting topological bulk-boundary correspondence. An external magnetic field can gap the edge state, consistent with the time-reversal symmetry protection inherent in the underlying band topology. We further identify the geometrical hybridization of such edge states, which not only supports the Z2 topology of the quantum spin Hall state but also visualizes the building blocks of the higher-order topological insulator phase. Our results further encourage the exploration of high-temperature transport quantization of the putative topological phase reported here.

Coexistence of Bulk-Nodal and Surface-Nodeless Cooper Pairings in a Superconducting Dirac Semimetal.

The interplay of nontrivial topology and superconductivity in condensed matter physics gives rise to exotic phenomena. However, materials are extremely rare where it is possible to explore the full details of the superconducting pairing. Here, we investigate the momentum dependence of the superconducting gap distribution in a novel Dirac material PdTe. Using high resolution, low temperature photoemission spectroscopy, we establish it as a spin-orbit coupled Dirac semimetal with the topological Fermi arc crossing the Fermi level on the (010) surface. This spin-textured surface state exhibits a fully gapped superconducting Cooper pairing structure below T_{c}∼4.5 K. Moreover, we find a node in the bulk near the Brillouin zone boundary, away from the topological Fermi arc. These observations not only demonstrate the band resolved electronic correlation between topological Fermi arc states and the way it induces Cooper pairing in PdTe, but also provide a rare case where surface and bulk states host a coexistence of nodeless and nodal gap structures enforced by spin-orbit coupling.

Signatures of Weyl Fermion Annihilation in a Correlated Kagome Magnet.

The manipulation of topological states in quantum matter is an essential pursuit of fundamental physics and next-generation quantum technology. Here we report the magnetic manipulation of Weyl fermions in the kagome spin-orbit semimetal Co_{3}Sn_{2}S_{2}, observed by high-resolution photoemission spectroscopy. We demonstrate the exchange collapse of spin-orbit-gapped ferromagnetic Weyl loops into paramagnetic Dirac loops under suppression of the magnetic order. We further observe that topological Fermi arcs disappear in the paramagnetic phase, suggesting the annihilation of exchange-split Weyl points. Our findings indicate that magnetic exchange collapse naturally drives Weyl fermion annihilation, opening new opportunities for engineering topology under correlated order parameters.

Transplantation of Human Umbilical Cord Blood-Derived Cellular Fraction Improves Left Ventricular Function and Remodeling After Myocardial Ischemia/Reperfusion.

Rationale: Human umbilical cord blood (hUCB) contains diverse populations of stem/progenitor cells. Whether hUCB-derived nonhematopoietic cells would induce cardiac repair remains unknown. Objective: To examine whether intramyocardial transplantation of hUCB-derived CD45-Lin- nonhematopoietic cellular fraction after a reperfused myocardial infarction in nonimmunosuppressed rats would improve cardiac function and ameliorate ventricular remodeling. Methods and Results: Nonhematopoietic CD45-Lin- cells were isolated from hUCB. Flow cytometry and quantitative polymerase chain reaction were used to characterize this subpopulation. Age-matched male Fischer 344 rats underwent a 30-minute coronary occlusion followed by reperfusion and 48 hours later received intramyocardial injection of vehicle or hUCB CD45-Lin- cells. After 35 days, compared with vehicle-treated rats, CD45-Lin- cell-treated rats exhibited improved left ventricular function, blunted left ventricular hypertrophy, greater preservation of viable myocardium in the infarct zone, and superior left ventricular remodeling. Mechanistically, hUCB CD45-Lin- cell injection favorably modulated molecular pathways regulating myocardial fibrosis, cardiomyocyte apoptosis, angiogenesis, and inflammation in postinfarct ventricular myocardium. Rare persistent transplanted human cells could be detected at both 4 and 35 days after myocardial infarction. Conclusions: Transplantation of hUCB-derived CD45-Lin- nonhematopoietic cellular subfraction after a reperfused myocardial infarction in nonimmunosuppressed rats ameliorates left ventricular dysfunction and improves remodeling via favorable paracrine modulation of molecular pathways. These findings with human cells in a clinically relevant model of myocardial ischemia/reperfusion in immunocompetent animals may have significant translational implications.Visual Overview: An online visual overview is available for this article.

Induced Pluripotent Stem Cell (iPSC)-Derived Extracellular Vesicles Are Safer and More Effective for Cardiac Repair Than iPSCs.

RATIONALE: Extracellular vesicles (EVs) are tiny membrane-enclosed droplets released by cells through membrane budding or exocytosis. The myocardial reparative abilities of EVs derived from induced pluripotent stem cells (iPSCs) have not been directly compared with the source iPSCs. OBJECTIVE: To examine whether iPSC-derived EVs can influence the biological functions of cardiac cells in vitro and to compare the safety and efficacy of iPSC-derived EVs (iPSC-EVs) and iPSCs for cardiac repair in vivo. METHODS AND RESULTS: Murine iPSCs were generated, and EVs isolated from culture supernatants by sequential centrifugation. Atomic force microscopy, high-resolution flow cytometry, real-time quantitative RT-PCR, and mass spectrometry were used to characterize EV morphology and contents. iPSC-EVs were enriched in miRNAs and proteins with proangiogenic and cytoprotective properties. iPSC-EVs enhanced angiogenic, migratory, and antiapoptotic properties of murine cardiac endothelial cells in vitro. To compare the cardiac reparative capacities in vivo, vehicle, iPSCs, and iPSC-EVs were injected intramyocardially at 48 hours after a reperfused myocardial infarction in mice. Compared with vehicle-injected mice, both iPSC- and iPSC-EV-treated mice exhibited improved left ventricular function at 35 d after myocardial infarction, albeit iPSC-EVs rendered greater improvement. iPSC-EV injection also resulted in reduction in left ventricular mass and superior perfusion in the infarct zone. Both iPSCs and iPSC-EVs preserved viable myocardium in the infarct zone, whereas reduction in apoptosis was significant with iPSC-EVs. iPSC injection resulted in teratoma formation, whereas iPSC-EV injection was safe. CONCLUSIONS: iPSC-derived EVs impart cytoprotective properties to cardiac cells in vitro and induce superior cardiac repair in vivo with regard to left ventricular function, vascularization, and amelioration of apoptosis and hypertrophy. Because of their acellular nature, iPSC-EVs represent a safer alternative for potential therapeutic applications in patients with ischemic myocardial damage.

In Situ Nano-thermomechanical Experiment Reveals Brittle to Ductile Transition in Silicon Nanowires.

Silicon (Si) nanostructures are widely used in microelectronics and nanotechnology. Brittle to ductile transition in nanoscale Si is of great scientific and technological interest but this phenomenon and its underlying mechanism remain elusive. By conducting in situ temperature-controlled nanomechanical testing inside a transmission electron microscope (TEM), here we show that the crystalline Si nanowires under tension are brittle at room temperature but exhibit ductile behavior with dislocation-mediated plasticity at elevated temperatures. We find that reducing the nanowire diameter promotes the dislocation-mediated responses, as shown by 78 Si nanowires tested between room temperature and 600 K. In situ high-resolution TEM imaging and atomistic reaction pathway modeling reveal that the unconventional 1/2⟨110⟩{001} dislocations become highly active with increasing temperature and thus play a critical role in the formation of deformation bands, leading to transition from brittle fracture to dislocation-mediated failure in Si nanowires at elevated temperatures. This study provides quantitative characterization and mechanistic insight for the brittle to ductile transition in Si nanostructures.

Soft Chemical Synthesis of HxCrS2: An Antiferromagnetic Material with Alternating Amorphous and Crystalline Layers.

We report a new HxCrS2-based crystalline/amorphous layered material synthesized by soft chemical methods. We study the structural nature and composition of this material with atomic resolution scanning transmission electron microscopy (STEM), revealing a complex structure consisting of alternating layers of amorphous and crystalline lamellae. Furthermore, the magnetic properties show evidence for increased magnetic frustration compared to the parent compound NaCrS2. Finally, we show that this material can be exfoliated, thus providing a facile synthesis method for chromium-sulfide-based ultrathin layers. The material reported herein can not only be a source of new thin TMD-related sheets for potential application in catalysis but also be of interest for realizing new 2D magnetic materials.

Deletion of Interleukin-6 Attenuates Pressure Overload-Induced Left Ventricular Hypertrophy and Dysfunction.

RATIONALE: The role of interleukin (IL)-6 in the pathogenesis of cardiac myocyte hypertrophy remains controversial. OBJECTIVE: To conclusively determine whether IL-6 signaling is essential for the development of pressure overload-induced left ventricular (LV) hypertrophy and to elucidate the underlying molecular pathways. METHODS AND RESULTS: Wild-type and IL-6 knockout (IL-6(-/-)) mice underwent sham surgery or transverse aortic constriction (TAC) to induce pressure overload. Serial echocardiograms and terminal hemodynamic studies revealed attenuated LV hypertrophy and superior preservation of LV function in IL-6(-/-) mice after TAC. The extents of LV remodeling, fibrosis, and apoptosis were reduced in IL-6(-/-) hearts after TAC. Transcriptional and protein assays of myocardial tissue identified Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) and signal transducer and activator of transcription 3 (STAT3) activation as important underlying mechanisms during cardiac hypertrophy induced by TAC. The involvement of these pathways in myocyte hypertrophy was verified in isolated cardiac myocytes from wild-type and IL-6(-/-) mice exposed to prohypertrophy agents. Furthermore, overexpression of CaMKII in H9c2 cells increased STAT3 phosphorylation, and exposure of H9c2 cells to IL-6 resulted in STAT3 activation that was attenuated by CaMKII inhibition. Together, these results identify the importance of CaMKII-dependent activation of STAT3 during cardiac myocyte hypertrophy via IL-6 signaling. CONCLUSIONS: Genetic deletion of IL-6 attenuates TAC-induced LV hypertrophy and dysfunction, indicating a critical role played by IL-6 in the pathogenesis of LV hypertrophy in response to pressure overload. CaMKII plays an important role in IL-6-induced STAT3 activation and consequent cardiac myocyte hypertrophy. These findings may have significant therapeutic implications for LV hypertrophy and failure in patients with hypertension.

Anomalous Tensile Detwinning in Twinned Nanowires.

In spite of numerous studies on mechanical behaviors of nanowires (NWs) focusing on the surface effect, there is still a general lack of understanding on how the internal microstructure of NWs influences their deformation mechanisms. Here, using quantitative in situ transmission electron microscopy based nanomechanical testing and molecular dynamics simulations, we report a transition of the deformation mechanism from localized dislocation slip to delocalized plasticity via an anomalous tensile detwinning mechanism in bitwinned metallic NWs with a single twin boundary (TB) running parallel to the NW length. The anomalous tensile detwinning starts with the detwinning of a segment of the preexisting TB under no resolved shear stress, followed by the propagation of a pair of newly formed TB and grain boundary leading to a large plastic deformation. An energy-based criterion is proposed to describe this transition of the deformation mechanism, which depends on the volume ratio between the two twin variants and the cross-sectional aspect ratio.

Evolution of Irradiation-Induced Vacancy Defects in Boron Nitride Nanotubes.

Irradiation-induced vacancy defects in multiwalled (MW) boron nitride nanotubes (BNNTs) are investigated via in situ high-resolution transmission electron microscope operated at 80 kV, with a homogeneous distribution of electron beam intensity. During the irradiation triangle-shaped vacancy defects are gradually generated in MW BNNTs under a mediate electron current density (30 A cm(-2)), by knocking the B atoms out. The vacancy defects grow along a well-defined direction within a wall at the early stage as a result of the curvature induced lattice strain, and then develop wall by wall. The orientation or the growth direction of the vacancy defects can be used to identify the chirality of an individual wall. With increasing electron current density, the shape of the irradiation-induced vacancy defects changes from regular triangle to irregular polygon.