Abstract
The electronic instabilities in CsV
3
Sb
5
are believed to originate from the V 3
d
-electrons on the kagome plane, however the role of Sb 5
p
-electrons for 3-dimensional orders is largely ...unexplored. Here, using resonant tender X-ray scattering and high-pressure X-ray scattering, we report a rare realization of conjoined charge density waves (CDWs) in CsV
3
Sb
5
, where a 2 × 2 × 1 CDW in the kagome sublattice and a Sb 5
p
-electron assisted 2 × 2 × 2 CDW coexist. At ambient pressure, we discover a resonant enhancement on Sb
L
1
-edge (2
s
→5
p
) at the 2 × 2 × 2 CDW wavevectors. The resonance, however, is absent at the 2 × 2 × 1 CDW wavevectors. Applying hydrostatic pressure, CDW transition temperatures are separated, where the 2 × 2 × 2 CDW emerges 4 K above the 2 × 2 × 1 CDW at 1 GPa. These observations demonstrate that symmetry-breaking phases in CsV
3
Sb
5
go beyond the minimal framework of kagome electronic bands near van Hove filling.
The plastic deformation and fracture behavior of two different types of Cu/X (X=Nb, Zr) nanostructured multilayered films (NMFs) were systematically investigated over wide ranges of modulation period ...(λ) and modulation ratio (η, the ratio of X layer thickness to Cu layer thickness). It was found that both the ductility and fracture mode of the NMFs were predominantly related to the constraining effect of ductile Cu layers on microcrack-initiating X layers, which showed a significant length-scale dependence on λ and η. Experimental observations and theoretical analyses also revealed a transition in strengthening mechanism, from single dislocation slip in confined layers to a load-bearing effect, when the Cu layer thickness was reduced to below ∼15nm by either decreasing λ or increasing η. This is due to the intense suppression of dislocation activities in the thin Cu layers, which causes a remarkable reduction in the deformability of the Cu layers. Concomitantly, the constraining effect of Cu layers on microcrack propagation is weakened, which can be used to explain the experimentally observed λ and η-dependent fracture mode transition from shear mode to an opening mode. Furthermore, the fracture toughness of the NMFs is also found to be sensitive to both λ and η. A fracture mechanism-based micromechanical model is developed to quantitatively assess the length-scale-dependent fracture toughness, and these calculations are in good agreement with experimental findings.
CsV3 Sb5 is a newly discovered Z2 topological kagome metal showing the coexistence of a charge-density-wave (CDW)-like order at T* = 94 K and superconductivity (SC) at Tc = 2.5 K at ambient pressure. ...Here, we study the interplay between CDW and SC in CsV3 Sb5 via measurements of resistivity, dc and ac magnetic susceptibility under various pressures up to 6.6 GPa. We find that the CDW transition decreases with pressure and experience a subtle modification at Pc1 ≈ 0.6 – 0.9 GPa before it vanishes completely at Pc2 ≈ 2 GPa . Correspondingly, Tc(P) displays an unusual M -shaped double dome with two maxima around Pc1 and Pc2 , respectively, leading to a tripled enhancement of Tc to about 8 K at 2 GPa. The obtained temperature-pressure phase diagram resembles those of unconventional superconductors, illustrating an intimated competition between CDW-like order and SC. The competition is found to be particularly strong for the intermediate pressure range Pc1 ≤ P ≤ Pc2 as evidenced by the broad superconducting transition and reduced superconducting volume fraction. The modification of CDW order around Pc1 has been discussed based on the band structure calculations. This work not only demonstrates the potential to raise Tc of the V-based kagome superconductors, but also offers more insights into the rich physics related to the electron correlations in this novel family of topological kagome metals.
Understanding the Fundamentals of Empty-Car Routing in Ridesharing Systems
How to efficiently route empty-cars in ridesharing systems? In this paper “Empty-car Routing in Ridesharing Systems,” A. ...Braverman, J.G. Dai, X. Liu, and L. Ying introduce a novel model based on closed queueing networks and propose an optimization framework to optimize empty-car routing for maximizing system-wide utility functions. We propose a fluid-based optimal routing policy by solving the optimization problem in a large market regime. We establish both process-level and steady-state convergence of the closed queueing network to the fluid-limit and prove the optimal network utility obtained from the fluid-based optimization is an upper bound on the utility in the finite car system for any routing policy under which the closed queueing network has a stationary distribution. This upper bound is achieved asymptotically under the fluid-based optimal routing policy.
This paper considers a closed queueing network model of ridesharing systems, such as Didi Chuxing, Lyft, and Uber. We focus on empty-car routing, a mechanism by which we control car flow in the network to optimize system-wide utility functions, for example, the availability of empty cars when a passenger arrives. We establish both process-level and steady-state convergence of the queueing network to a fluid limit in a large market regime where demand for rides and supply of cars tend to infinity and use this limit to study a fluid-based optimization problem. We prove that the optimal network utility obtained from the fluid-based optimization is an upper bound on the utility in the finite car system for any routing policy, both static and dynamic, under which the closed queueing network has a stationary distribution. This upper bound is achieved asymptotically under the fluid-based optimal routing policy. Simulation results with real-world data released by Didi Chuxing demonstrate the benefit of using the fluid-based optimal routing policy compared with various other policies.
Hardness, activation volume and strain-rate sensitivity of Cu/Cr (face-centered cubic (fcc)/body-centered cubic) and Cu/Zr (fcc/hexagonal close-packed) nanostructured multilayer films have been ...systematically measured as a function of modulation period (L) and modulation ratio (η), respectively. Significant size effects were found for all the three plastic deformation characteristics, i.e. enhanced hardness and activation volume but reduced strain-rate sensitivity with decreasing the dimension length L. Microstructure evolution was statistically examined to rationalize these size dependences. It was crucially observed that abundant nanotwins existed in the Cu grains, though nanotwin formation was depressed with smaller L. This inverse size-dependent nanotwin formation is responsible for the reduction in strain-rate sensitivity, because the negative effect induced by the decreased nanotwins predominates over the positive effect coming from the raised interfaces/boundaries. A mechanistic model is modified to account for the interface effect as well as the nanotwin effect, which yields calculations of strain-rate sensitivity in broad agreement with the experimental results when L is larger than about 20nm. Below this critical length size, there are discrepancies between the calculations and the experimental results, due to the change in deformation mechanism from dislocation nucleation/slip in confined layers to dislocation crossing interfaces. A confined layer slip model is also modified by considering the nanotwin strengthening to quantitatively describe the L-dependent hardness. In addition, the effects of constituent phases and their relative content on the activation volume and strain-rate sensitivity of NMFs are discussed with regard to variation in η.
In this work, we synthesize large-area thin films of a conjugated, imine-based, two-dimensional covalent organic framework at the solution/air interface. Thicknesses between ∼2-200 nm are achieved. ...Films can be transferred to any desired substrate by lifting from underneath, enabling their use as the semiconducting active layer in field-effect transistors.
Li‐ion batteries are promising candidates for electrical energy storage in applications ranging from portable electronics to hybrid and electric vehicles. In this context, layered compounds in the ...Li1+δ(TMxMn1‐x)1‐δO2 family (TM = transition metal) have received much attention due to their high capacity and stability. In this Research News article we describe recent advances on structural characterization of Li‐ion electrode materials using state‐of‐the‐art electron microscopy. Direct evidence of the monoclinic nature of Li2MnO3 has been provided. It has been demonstrated that differences in Z‐contrast imaging between Li2MnO3 and LiTMO2 may be used to screen samples for phase separation in the 10–100 nm scale.
Structure of (rhombohedral) layered lithium oxides consisting of a NaCl‐like stack of hexagonal close‐packed planes (O‐red, Li‐yellow, Metal‐blue). LiMn substitutionals (yellow dots) in Li2MnO3 arrange in regular hexagonal sublattices at Mn planes, resulting in a monoclinic structure which unit cell is indicated in the figure by a blue box.
This paper presents the results of an experimental study on the behavior of concrete confined by fiber reinforced polymer (FRP) jackets with a large rupture strain (LRS). The FRP composites ...considered herein are formed by embedding polyethylene naphthalate (PEN) and polyethylene terephthalate (PET) fibers in a suitable epoxy resin matrix. The PEN and PET fibers are usually made from recycled materials (e.g., PET bottles) and have a strain capacity greater than 5%. They are ideal for use in seismic retrofit applications where increases in ductility and energy absorption capacity are of prime concern. The present study has two specific objectives: (1) to develop a good understanding of the compressive stress-strain behavior of concrete confined with LRS FRP; and (2) to examine whether existing confinement models developed for conventional FRPs are applicable to LRS FRPs. As the existing models have been developed and verified mainly based on test data for CFRP and GFRP, which have a jacket hoop rupture strain of less than 2%, their accuracy in the hoop/lateral strain range beyond 2% is unclear. Results presented in this paper indicate that the two LRS FRPs made from PEN and PET fibers possess a bilinear tensile stress-strain relationship, which has a significant effect on the axial compressive stress-strain behavior of FRP-confined concrete. A recent confinement model for conventional FRPs is compared with the present test results, indicating that the model significantly overestimates the ultimate axial strain. A modified version of the model is then presented to provide more accurate predictions of the test results.
The layered oxides being considered as intercalation compounds for lithium batteries display significant differences between the long-range crystal structure and local arrangements around individual ...atoms. These differences are important, because the local atomic environments affect Li-ion transport and, hence, the oxide’s rate capability, by determining activation barrier energies, by blocking or opening Li-diffusion pathways, etc. Traditional diffraction methods provide key information on the average crystal structure. However, no single experimental technique can unequivocally determine the average long-range crystal structure and the distribution of local environments over crystallographic distances while retaining atomic-scale resolution. Therefore, in this study, we have employed a combination of diffraction, microscopy, and spectroscopy techniques to investigate the long-range (∼1 μm) and local structure (≤1 nm) of Li1.2Co0.4Mn0.4O2, which is a model compound for layered oxides being considered for transportation applications. We find that Li1.2Co0.4Mn0.4O2 contains mostly Mn4+ in Li2MnO3-like atomic environments and Co3+ in LiCoO2-like atomic environments, which are intimately mixed over length scales of ≥2−3 nm, resulting in a Li1.2Co0.4Mn0.4O2 crystallite composition that appears homogeneous over the long-range. In addition, we observed a quasi-random distribution of locally monoclinic structures, topotaxially integrated within a rhombohedral-NaFeO2 framework. Based on these observations, we propose a dendritic microstructure model for Li1.2Co0.4Mn0.4O2 consisting of well integrated LiCoO2- and Li2MnO3-like structures.
PDF
