Three-dimensional flat bands in pyrochlore metal CaNi2.

Electronic flat-band materials host quantum states characterized by a quenched kinetic energy. These flat bands are often conducive to enhanced electron correlation effects and emergent quantum phases of matter1. Long studied in theoretical models2-4, these systems have received renewed interest after their experimental realization in van der Waals heterostructures5,6 and quasi-two-dimensional (2D) crystalline materials7,8. An outstanding experimental question is if such flat bands can be realized in three-dimensional (3D) networks, potentially enabling new materials platforms9,10 and phenomena11-13. Here we investigate the C15 Laves phase metal CaNi2, which contains a nickel pyrochlore lattice predicted at a model network level to host a doubly-degenerate, topological flat band arising from 3D destructive interference of electronic hopping14,15. Using angle-resolved photoemission spectroscopy, we observe a band with vanishing dispersion across the full 3D Brillouin zone that we identify with the pyrochlore flat band as well as two additional flat bands that we show arise from multi-orbital interference of Ni d-electrons. Furthermore, we demonstrate chemical tuning of the flat-band manifold to the Fermi level that coincides with enhanced electronic correlations and the appearance of superconductivity. Extending the notion of intrinsic band flatness from 2D to 3D, this provides a potential pathway to correlated behaviour predicted for higher-dimensional flat-band systems ranging from tunable topological15 to fractionalized phases16.

Palladium-Catalyzed Oxidative Amination of Unactivated Olefins with Primary Aliphatic Amines.

Direct coupling of unactivated olefins with primary alkylamines is considered to be an efficient but unknown method for the construction of complex amines. Herein we report a catalytic intermolecular oxidative amination of unactivated olefins with primary aliphatic amines based on the combination of a palladium catalyst, a bidentate phosphine ligand, and duroquinone. A range of secondary allylic amines were obtained in good yields with excellent regio- and stereoselectivity. Mechanistic control experiments revealed that the reaction proceeds by allylic C(sp3)-H activation and nucleophilic amination. The utility of the protocol is further demonstrated with the late-stage modification and streamlined synthesis of drug molecules.

Multi-level semantic fusion network for Chinese medical named entity recognition.

Medical named entity recognition (MNER) is a fundamental component of understanding the unstructured medical texts in electronic health records, and it has received widespread attention in both academia and industry. However, the previous approaches of MNER do not make full use of hierarchical semantics from morphology to syntactic relationships like word dependency. Furthermore, extracting entities from Chinese medical texts is a more complex task because it usually contains for example homophones or pictophonetic characters. In this paper, we propose a multi-level semantic fusion network for Chinese medical named entity recognition, which fuses semantic information on morphology, character, word and syntactic level. We take radical as morphology semantic, pinyin and character dictionary as character semantic, word dictionary as word semantic, and these semantic features are fused by BiLSTM to get the contextualized representation. Then we use a graph neural network to model word dependency as syntactic semantic to enhance the contextualized representation. The experimental results show the effectiveness of the proposed model on two public datasets and robustness in real-world scenarios.

Identifying Influences in Patient Decision-making Processes in Online Health Communities: Data Science Approach.

BACKGROUND: In recent years, an increasing number of users have joined online health communities (OHCs) to obtain information and seek support. Patients often look for information and suggestions to support their health care decision-making. It is important to understand patient decision-making processes and identify the influences that patients receive from OHCs. OBJECTIVE: We aimed to identify the posts in discussion threads that have influence on users who seek help in their decision-making. METHODS: We proposed a definition of influence relationship of posts in discussion threads. We then developed a framework and a deep learning model for identifying influence relationships. We leveraged the state-of-the-art text relevance measurement methods to generate sparse feature vectors to present text relevance. We modeled the probability of question and action presence in a post as dense features. We then used deep learning techniques to combine the sparse and dense features to learn the influence relationships. RESULTS: We evaluated the proposed techniques on discussion threads from a popular cancer survivor OHC. The empirical evaluation demonstrated the effectiveness of our approach. CONCLUSIONS: It is feasible to identify influence relationships in OHCs. Using the proposed techniques, a significant number of discussions on an OHC were identified to have had influence. Such discussions are more likely to affect user decision-making processes and engage users' participation in OHCs. Studies on those discussions can help improve information quality, user engagement, and user experience.

Signature of Many-Body Localization of Phonons in Strongly Disordered Superlattices.

Many-body localization (MBL) has attracted significant attention because of its immunity to thermalization, role in logarithmic entanglement entropy growth, and opportunities to reach exotic quantum orders. However, experimental realization of MBL in solid-state systems has remained challenging. Here, we report evidence of a possible phonon MBL phase in disordered GaAs/AlAs superlattices. Through grazing-incidence inelastic X-ray scattering, we observe a strong deviation of the phonon population from equilibrium in samples doped with ErAs nanodots at low temperature, signaling a departure from thermalization. This behavior occurs within finite phonon energy and wavevector windows, suggesting a localization-thermalization crossover. We support our observation by proposing a theoretical model for the effective phonon Hamiltonian in disordered superlattices, and showing that it can be mapped exactly to a disordered 1D Bose-Hubbard model with a known MBL phase. Our work provides momentum-resolved experimental evidence of phonon localization, extending the scope of MBL to disordered solid-state systems.

Exosomes derived from tendon stem cells promote cell proliferation and migration through the TGF ß signal pathway.

Tendon stem cells (TSCs) are a kind of progenitor cells found in tendon niches, which play a key role in the repair of tendon injuries. Exosomes that mediate cell communication are involved in physiological processes and various diseases, while the effect of exosomes derived from TSCs (TSC-exo) on TSCs is still unclear. The purpose of this study is to explore the effect of TSC-exo on TSCs. Analyzing the characteristics of TSC-exo, we found that the TSC-exo were enriched in a large amount of transforming growth factor ß (TGF ß) by western blotting. We also found that the TGF ß carried by TSC-exo can effectively accelerate the proliferation and migration of TSCs. We further found that TGF ß carried by TSC-exo can activate the TGF ß-Smad2/3 and the ERK1/2 signaling pathway in TSCs. Furthermore, matrix metalloenzyme 2 (MMP2), a downstream molecule of Smad2, is regulated by TGF ß carried by TSC-exo. Collectively, our findings provide molecular insights into TSC-exo and indicate that TSC-exo are a potential strategy for treating tendon injuries.

miR-142 downregulation alleviates rat PTSD-like behaviors, reduces the level of inflammatory cytokine expression and apoptosis in hippocampus, and upregulates the expression of fragile X mental retardation protein.

BACKGROUND: FMRP is a selective mRNA-binding protein that regulates protein synthesis at synapses, and its loss may lead to the impairment of trace fear memory. Previously, we found that FMRP levels in the hippocampus of rats with post-traumatic stress disorder (PTSD) were decreased. However, the mechanism underlying these changes remains unclear. METHODS: Forty-eight male Sprague-Dawley rats were randomly divided into four groups. The experimental groups were treated with the single-prolonged stress (SPS) procedure and injected with a lentivirus-mediated inhibitor of miR-142-5p. Behavior test as well as morphology and molecular biology experiments were performed to detect the effect of miR-142 downregulation on PTSD, which was further verified by in vitro experiments. RESULTS: We found that silence of miRNA-142 (miR-142), an upstream regulator of FMRP, could alleviate PTSD-like behaviors of rats exposed to the SPS paradigm. MiR-142 silence not only decreased the levels of proinflammatory mediators, such as interleukin-1ß, interleukin-6, and tumor necrosis factor-α, but also increased the expressive levels of synaptic proteins including PSD95 and synapsin I in the hippocampus, which was one of the key brain regions associated with PTSD. We further detected that miR-142 silence also downregulated the transportation of nuclear factor kappa-B (NF-κB) into the nuclei of neurons and might further affect the morphology of neurons. CONCLUSIONS: The results revealed miR-142 downregulation could alleviate PTSD-like behaviors through attenuating neuroinflammation in the hippocampus of SPS rats by binding to FMRP.

Electron mean-free-path filtering in Dirac material for improved thermoelectric performance.

Recent advancements in thermoelectric materials have largely benefited from various approaches, including band engineering and defect optimization, among which the nanostructuring technique presents a promising way to improve the thermoelectric figure of merit (zT) by means of reducing the characteristic length of the nanostructure, which relies on the belief that phonons' mean free paths (MFPs) are typically much longer than electrons'. Pushing the nanostructure sizes down to the length scale dictated by electron MFPs, however, has hitherto been overlooked as it inevitably sacrifices electrical conduction. Here we report through ab initio simulations that Dirac material can overcome this limitation. The monotonically decreasing trend of the electron MFP allows filtering of long-MFP electrons that are detrimental to the Seebeck coefficient, leading to a dramatically enhanced power factor. Using SnTe as a material platform, we uncover this MFP filtering effect as arising from its unique nonparabolic Dirac band dispersion. Room-temperature zT can be enhanced by nearly a factor of 3 if one designs nanostructures with grain sizes of ∼10 nm. Our work broadens the scope of the nanostructuring approach for improving the thermoelectric performance, especially for materials with topologically nontrivial electronic dynamics.

Understanding Disorder in 2D Materials: The Case of Carbon Doping of Silicene.

We investigate the effect of lattice disorder and local correlation effects in finite and periodic silicene structures caused by carbon doping using first-principles calculations. For both finite and periodic silicene structures, the electronic properties of carbon-doped monolayers are dramatically changed by controlling the doping sites in the structures, which is related to the amount of disorder introduced in the lattice and electron-electron correlation effects. By changing the position of the carbon dopants, we found that a Mott-Anderson transition is achieved. Moreover, the band gap is determined by the level of lattice disorder and electronic correlation effects. Finally, these structures are ferromagnetic even under disorder which has potential applications in Si-based nanoelectronics, such as field-effect transistors (FETs).

Topological Singularity Induced Chiral Kohn Anomaly in a Weyl Semimetal.

The electron-phonon interaction (EPI) is instrumental in a wide variety of phenomena in solid-state physics, such as electrical resistivity in metals, carrier mobility, optical transition, and polaron effects in semiconductors, lifetime of hot carriers, transition temperature in BCS superconductors, and even spin relaxation in diamond nitrogen-vacancy centers for quantum information processing. However, due to the weak EPI strength, most phenomena have focused on electronic properties rather than on phonon properties. One prominent exception is the Kohn anomaly, where phonon softening can emerge when the phonon wave vector nests the Fermi surface of metals. Here we report a new class of Kohn anomaly in a topological Weyl semimetal (WSM), predicted by field-theoretical calculations, and experimentally observed through inelastic x-ray and neutron scattering on WSM tantalum phosphide. Compared to the conventional Kohn anomaly, the Fermi surface in a WSM exhibits multiple topological singularities of Weyl nodes, leading to a distinct nesting condition with chiral selection, a power-law divergence, and non-negligible dynamical effects. Our work brings the concept of the Kohn anomaly into WSMs and sheds light on elucidating the EPI mechanism in emergent topological materials.

Phonon Hydrodynamic Heat Conduction and Knudsen Minimum in Graphite.

In the hydrodynamic regime, phonons drift with a nonzero collective velocity under a temperature gradient, reminiscent of viscous gas and fluid flow. The study of hydrodynamic phonon transport has spanned over half a century but has been mostly limited to cryogenic temperatures (∼1 K) and more recently to low-dimensional materials. Here, we identify graphite as a three-dimensional material that supports phonon hydrodynamics at significantly higher temperatures (∼100 K) based on first-principles calculations. In particular, by solving the Boltzmann equation for phonon transport in graphite ribbons, we predict that phonon Poiseuille flow and Knudsen minimum can be experimentally observed above liquid nitrogen temperature. Further, we reveal the microscopic origin of these intriguing phenomena in terms of the dependence of the effective boundary scattering rate on momentum-conserving phonon-phonon scattering processes and the collective motion of phonons. The significant hydrodynamic nature of phonon transport in graphite is attributed to its strong intralayer sp2 hybrid bonding and weak van der Waals interlayer interactions. More intriguingly, the reflection symmetry associated with a single graphene layer is broken in graphite, which opens up more momentum-conserving phonon-phonon scattering channels and results in stronger hydrodynamic features in graphite than graphene. As a boundary-sensitive transport regime, phonon hydrodynamics opens up new possibilities for thermal management and energy conversion.

Nonperturbative Quantum Nature of the Dislocation-Phonon Interaction.

Despite the long history of dislocation-phonon interaction studies, there are many problems that have not been fully resolved during this development. These include an incompatibility between a perturbative approach and the long-range nature of a dislocation, the relation between static and dynamic scattering, and their capability of dealing with thermal transport phenomena for bulk material only. Here by utilizing a fully quantized dislocation field, which we called a "dislon", a phonon interacting with a dislocation is renormalized as a quasi-phonon, with shifted quasi-phonon energy, and accompanied by a finite quasi-phonon lifetime, which are reducible to classical results. A series of outstanding legacy issues including those above can be directly explained within this unified phonon renormalization approach. For instance, a renormalized phonon naturally resolves the decade-long debate between dynamic and static dislocation-phonon scattering approaches, as two limiting cases. In particular, at nanoscale, both the dynamic and static approaches break down, while the present renormalization approach remains valid by capturing the size effect, showing good agreement with lattice dynamics simulations.

Tailoring Superconductivity with Quantum Dislocations.

Despite the established knowledge that crystal dislocations can affect a material's superconducting properties, the exact mechanism of the electron-dislocation interaction in a dislocated superconductor has long been missing. Being a type of defect, dislocations are expected to decrease a material's superconducting transition temperature (Tc) by breaking the coherence. Yet experimentally, even in isotropic type I superconductors, dislocations can either decrease, increase, or have little influence on Tc. These experimental findings have yet to be understood. Although the anisotropic pairing in dirty superconductors has explained impurity-induced Tc reduction, no quantitative agreement has been reached in the case a dislocation given its complexity. In this study, by generalizing the one-dimensional quantized dislocation field to three dimensions, we reveal that there are indeed two distinct types of electron-dislocation interactions. Besides the usual electron-dislocation potential scattering, there is another interaction driving an effective attraction between electrons that is caused by dislons, which are quantized modes of a dislocation. The role of dislocations to superconductivity is thus clarified as the competition between the classical and quantum effects, showing excellent agreement with existing experimental data. In particular, the existence of both classical and quantum effects provides a plausible explanation for the illusive origin of dislocation-induced superconductivity in semiconducting PbS/PbTe superlattice nanostructures. A quantitative criterion has been derived, in which a dislocated superconductor with low elastic moduli and small electron effective mass and in a confined environment is inclined to enhance Tc. This provides a new pathway for engineering a material's superconducting properties by using dislocations as an additional degree of freedom.

Esaki Diodes in van der Waals Heterojunctions with Broken-Gap Energy Band Alignment.

van der Waals (vdW) heterojunctions composed of two-dimensional (2D) layered materials are emerging as a solid-state materials family that exhibits novel physics phenomena that can power a range of electronic and photonic applications. Here, we present the first demonstration of an important building block in vdW solids: room temperature Esaki tunnel diodes. The Esaki diodes were realized in vdW heterostructures made of black phosphorus (BP) and tin diselenide (SnSe2), two layered semiconductors that possess a broken-gap energy band offset. The presence of a thin insulating barrier between BP and SnSe2 enabled the observation of a prominent negative differential resistance (NDR) region in the forward-bias current-voltage characteristics, with a peak to valley ratio of 1.8 at 300 K and 2.8 at 80 K. A weak temperature dependence of the NDR indicates electron tunneling being the dominant transport mechanism, and a theoretical model shows excellent agreement with the experimental results. Furthermore, the broken-gap band alignment is confirmed by the junction photoresponse, and the phosphorus double planes in a single layer of BP are resolved in transmission electron microscopy (TEM) for the first time. Our results represent a significant advance in the fundamental understanding of vdW heterojunctions and broaden the potential applications of 2D layered materials.

Proximity-Driven Enhanced Magnetic Order at Ferromagnetic-Insulator-Magnetic-Topological-Insulator Interface.

Magnetic exchange driven proximity effect at a magnetic-insulator-topological-insulator (MI-TI) interface provides a rich playground for novel phenomena as well as a way to realize low energy dissipation quantum devices. Here we report a dramatic enhancement of proximity exchange coupling in the MI/magnetic-TI EuS/Sb(2-x)V(x)Te3 hybrid heterostructure, where V doping is used to drive the TI (Sb2Te3) magnetic. We observe an artificial antiferromagneticlike structure near the MI-TI interface, which may account for the enhanced proximity coupling. The interplay between the proximity effect and doping in a hybrid heterostructure provides insights into the engineering of magnetic ordering.

Experimental verification of the van Vleck nature of long-range ferromagnetic order in the vanadium-doped three-dimensional topological insulator Sb(2)Te(3).

We demonstrate by high resolution low temperature electron energy loss spectroscopy (EELS) measurements that the long range ferromagnetic (FM) order in the vanadium- (V-)doped topological insulator Sb_{2}Te_{3} has the nature of van Vleck-type ferromagnetism. The positions and the relative amplitudes of two core-level peaks (L_{3} and L_{2}) of the V EELS spectrum show unambiguous change when the sample is cooled from room temperature to T=10 K. Magnetotransport and comparison of the measured and simulated EELS spectra confirm that these changes originate from the onset of FM order. Crystal field analysis indicates that in V-doped Sb_{2}Te_{3}, partially filled core states contribute to the FM order. Since van Vleck magnetism is a result of summing over all states, this magnetization of core level verifies the van Vleck-type ferromagnetism in a direct manner.

Boson peak in deeply cooled confined water: a possible way to explore the existence of the liquid-to-liquid transition in water.

The boson peak in deeply cooled water confined in nanopores is studied with inelastic neutron scattering. We show that in the (P, T) plane, the locus of the emergence of the boson peak is nearly parallel to the Widom line below ∼ 1600 bar. Above 1600 bar, the situation is different and from this difference the end pressure of the Widom line is estimated. The frequency and width of the boson peak correlate with the density of water, which suggests a method to distinguish the hypothetical "low-density liquid" and "high-density liquid" phases in deeply cooled water.

One role of hydration water in proteins: key to the "softening" of short time intraprotein collective vibrations of a specific length scale.

High resolution inelastic X-ray scattering (IXS) experiments show that the "phonon energy softening" and "phonon population enhancement" observed in a hydrated native protein when increasing the temperature from 200 K to physiological temperature are not directly related to the protein structure. Such phenomena were also observed in a denatured sample without a defined tertiary structure and with a limited residual secondary structure. However, in a dry sample, such "softening" is strongly suppressed. These facts suggest that the above-mentioned protein "softening" phenomenon is water-induced. In addition, increasing the hydration level can also induce "phonon energy softening" at room temperature, but not at 200 K. This change may be due to a qualitative difference in the dynamics of hydration water at 200 K and at room temperature.

Near-field optical effect of a core-shell nanostructure in proximity to a flat surface.

We provide an analytical solution for studying the near-field optical effect of a core-shell nanostructure in proximity to a flat surface, within quasi-static approximation. The distribution of electrostatic potential and the field enhancement in this complex geometry are obtained by solving a set of linear equations. This analytical result can be applied to a wide range of systems associated with near-field optics and surface plasmon polaritons. To illustrate the power of this technique, we study the field-attenuation effect of an oxidized shell in a silver tip in a near-field scanning microscope. The thickness of oxidized layer can be monitored by measuring the intensity of light. We also find a linear relation between resonant frequency and temperature in an Ag-Au core-shell structure, which provides insight for local temperature detection with nm scale resolution. Our results also show good agreement with recent finite element method results.