In situ TEM revealing the effects of dislocations on lithium-ion migration in transition metal dichalcogenides

The two-dimensional (2D) structure of layered transition metal dichalcogenides (TMDs) provides unusual physical properties [1,2]and chemical reactivity [3,4], which can be influenced by defects such as dislocations [5,6]. For example, dislocations can act as nucleation sites for the onset of deformation when subjected to stress [7].

LnCu_(3)(OH)_(6)Cl_(3) (Ln=Gd, Tb, Dy): Heavy lanthanides on spin-1/2 kagome magnets

The spin-1/2 kagome antiferromagnets are key prototype materials for studying frustrated magnetism.Three isostructural kagome antiferromagnets LnCu_(3)(OH)_(6)Cl_(3)(Ln=Gd,Tb,Dy)have been successfully synthesized by the hydrothermal method.LnCu_(3)(OH)_(6)Cl_(3) adopts space group P3m1 and features the layered Cu-kagome lattice with lanthanide Ln3+cations sitting at the center of the hexagons.Although heavy lanthanides(Ln=Gd,Tb,Dy)in LnCu_(3)(OH)_(6)Cl_(3) provide a large effective magnetic moment and ferromagnetic-like spin correlations compared to light-lanthanides(Nd,Sm,Eu)analogues,Cu-kagome holds an antiferromagnetically ordered state at around 17 K like YCu_(3)(OH)_(6)Cl_(3).

Beating standard quantum limit via two-axis magnetic susceptibility measurement

We report a metrology scheme which measures the magnetic susceptibility of an atomic spin ensemble along the x and z directions and produces parameter estimation with precision beating the standard quantum limit.The atomic ensemble is initialized via one-axis spin squeezing with optimized squeezing time and parameterΦ(to be estimated)assumed as uniformly distributed between 0 and 2πwhile fixed in each estimation.One estimation ofΦcan be produced with every two magnetic susceptibility data measured along the two axes respectively,which has an imprecision scaling(1.43±0.02)/N^(0.687±0.003)with respect to the number N of the atomic spins.The measurement scheme is easy to implement and is robust against the measurement fluctuation caused by environment noise and measurement defects.

Intercalation of van der Waals layered materials: A route towards engineering of electron correlation

Being parent materials of two-dimensional (2D) crystals, van der Waals layered materials have received revived interest. In most 2D materials, the interaction between electrons is negligible. Introducing the interaction can give rise to a variety of exotic properties. Here, via intercalating a van der Waals layered compound VS2, we find evidence for electron correlation by extensive magnetic, thermal, electrical, and thermoelectric characterizations. The low temperature Sommerfeld coefficient is 64 mJ·K-2·mol-1 and the Kadowaki-Woods ratio rKW^0.20a0. Both supports an enhancement of the electron correlation. The temperature dependences of the resistivity and thermopower indicate an important role played by the Kondo effect. The Kondo temperature TK is estimated to be around 8 K. Our results suggest intercalation as a potential means to engineer the electron correlation in van der Waals materials, as well as 2D materials.

Noisy intermediate-scale quantum computers

Quantum computers have made extraordinary progress over the past decade,and significant milestones have been achieved along the path of pursuing universal fault-tolerant quantum computers.Quantum advantage,the tipping point heralding the quantum era,has been accomplished along with several waves of breakthroughs.Quantum hardware has become more integrated and architectural compared to its toddler days.The controlling precision of various physical systems is pushed beyond the fault-tolerant threshold.Meanwhile,quantum computation research has established a new norm by embracing industrialization and commercialization.The joint power of governments,private investors,and tech companies has significantly shaped a new vibrant environment that accelerates the development of this field,now at the beginning of the noisy intermediate-scale quantum era.Here,we first discuss the progress achieved in the field of quantum computation by reviewing the most important algorithms and advances in the most promising technical routes,and then summarizing the next-stage challenges.Furthermore,we illustrate our confidence that solid foundations have been built for the fault-tolerant quantum computer and our optimism that the emergence of quantum killer applications essential for human society shall happen in the future.

Linking a Storm Water Management Model to a Novel Two-Dimensional Model for Urban Pluvial Flood Modeling

This article describes a new method of urban pluvial flood modeling by coupling the 1D storm water management model(SWMM)and the 2D flood inundation model(ECNU Flood-Urban).The SWMM modeling results(the overflow of the manholes)are used as the input boundary condition of the ECNU Flood-Urban model to simulate the rainfall–runoff processes in an urban environment.The analysis is applied to the central business district of East Nanjing Road in downtown Shanghai,considering 5-,10-,20-,50-,and 100-year return period rainfall scenarios.The results show that node overflow,water depth,and inundation area increase proportionately with the growing return periods.Water depths are mostly predicted to be shallow and surface flows generally occur in the urban road network due to its low-lying nature.The simulation result of the coupled model proves to be reliable and suggests that urban surface water flooding could be accurately simulated by using this methodology.Adaptation measures(upgrading of the urban drainage system)can then be targeted at specific locations with significant overflow and flooding.

Linear Strain-Gradient Effect on the Energy Bandgap in Bent CdS Nanowires

Although possible non-homogeneous strain effects in semiconductors have been investigated for over a half century and the strain-gradient can be over 1% per micrometer in flexible nanostructures, we still lack an understanding of their influence on energy bands. Here we conduct a systematic cathodoluminescence spectroscopy study of the strain-gradient induced exciton energy shift in elastically curved CdS nanowires at low temperature, and find that the red-shift of the exciton energy in the curved nanowires is proportional to the strain-gradient, an index of lattice distortion. Density functional calculations show the same trend of band gap reduction in curved nanostructures and reveal the underlying mechanism. The significant linear straingradient effect on the band gap of semiconductors should shed new light on ways to tune optical-electronic properties in nanoelectronics.

Ultrafast epitaxial growth of metre-sized single-crystal graphene on industrial Cu foil

A foundation of the modern technology that uses single-crystal silicon has been the growth of highquality single-crystal Si ingots with diameters up to 12 inches or larger. For many applications of graphene, large-area high-quality(ideally of single-crystal) material will be enabling. Since the first growth on copper foil a decade ago, inch-sized single-crystal graphene has been achieved. We present here the growth, in 20 min, of a graphene film of(5 ×50) cm^2 dimension with >99% ultra-highly oriented grains.This growth was achieved by:(1) synthesis of metre-sized single-crystal Cu(1 1 1) foil as substrate;(2)epitaxial growth of graphene islands on the Cu(1 1 1) surface;(3) seamless merging of such graphene islands into a graphene film with high single crystallinity and(4) the ultrafast growth of graphene film.These achievements were realized by a temperature-gradient-driven annealing technique to produce single-crystal Cu(1 1 1) from industrial polycrystalline Cu foil and the marvellous effects of a continuous oxygen supply from an adjacent oxide. The as-synthesized graphene film, with very few misoriented grains(if any), has a mobility up to ~23,000 cm^2 V^(-1)s^(-1)at 4 K and room temperature sheet resistance of ~230 Ω/□. It is very likely that this approach can be scaled up to achieve exceptionally large and high-quality graphene films with single crystallinity, and thus realize various industrial-level applications at a low cost.

Simulation of higher-order topological phases and related topological phase transitions in a superconducting qubit

Higher-order topological phases give rise to new bulk and boundary physics,as well as new classes of topological phase transitions.While the realization of higher-order topological phases has been confirmed in many platforms by detecting the existence of gapless boundary modes,a direct determination of the higher-order topology and related topological phase transitions through the bulk in experiments has still been lacking.To bridge the gap,in this work we carry out the simulation of a twodimensional second-order topological phase in a superconducting qubit.Owing to the great flexibility and controllability of the quantum simulator,we observe the realization of higher-order topology directly through the measurement of the pseudo-spin texture in momentum space of the bulk for the first time,in sharp contrast to previous experiments based on the detection of gapless boundary modes in real space.Also through the measurement of the evolution of pseudo-spin texture with parameters,we further observe novel topological phase transitions from the second-order topological phase to the trivial phase,as well as to the first-order topological phase with nonzero Chern number.Our work sheds new light on the study of higher-order topological phases and topological phase transitions.

Towards intrinsically pure graphene grown on copper

The state-of-the-art semiconductor industry is built on the successful production of silicon ingot with extreme purity as high as 99.999999999%,or the so-called"eleven nines".The coming high-end applications of graphene in electronics and optoelectronics will inevitably need defect-free pure graphene as well.Due to its two-dimensional(2D)characteristics,graphene restricts all the defects on its surface and has the opportunity to eliminate all kinds of defects,i.e.,line defects at grain boundaries and point or dot defects in grains,and produce intrinsically pure graphene.In the past decade,epitaxy growth has been adopted to grow graphene by seamlessly stitching of aligned grains and the line defects at grain boundaries were eliminated finally.However,as for the equally common dot and point defects in graphene grain,there are rare ways to detect or reduce them with high throughput and efficiency.Here,we report a methodology to realize the production of ultrapure graphene grown on copper by eliminating both the dot and point defects in graphene grains.The dot defects,proved to be caused by the silica particles shedding from quartz tube during the high-temperature growth,were excluded by a designed heat-resisting box to prevent the deposition of particles on the copper surface.The point defects were optically visualized by a mild-oxidation-assisted method and further reduced by etching-regrowth process to an ultralow level of less than 1/1,000 μm^(2).Our work points out an avenue for the production of intrinsically pure graphene and thus lays the foundation for the large-scale graphene applications at the integrated-circuit level.

Nonreciprocal coherent coupling of nanomagnets by exchange spin waves

Nanomagnets are widely used to store information in non-volatile spintronic devices.Spin waves can transfer information with low-power consumption as their propagations are independent of charge transport.However,to dynamically couple two distant nanomagnets via spin waves remains a major challenge for magnonics.Here we experimentally demonstrate coherent coupling of two distant Co nanowires by fast propagating spin waves in an yttrium iron garnet thin film with sub-50 nm wavelengths.Magnons in two nanomagnets are unidirectionally phase-locked with phase shifts controlled by magnon spin torque and spin-wave propagation.The coupled system is finally formulated by an analytical theory in terms of an effective non-Hermitian Hamiltonian.Our results are attractive for analog neuromorphic computing that requires unidirectional information transmission.

Surface Exciton-Plasmon Polariton Enhanced Light Emission via Integration of Single Semiconductor Nanowires with Metal Nanostructures

The light emission enhancement behavior from single ZnO nanowires integrated with metallic contacts is investigated by micro-photoluminescence measurements.Apart from surface plasmon polaritons at the air/metal interface,the emission of a single ZnO nanowire can be coupled into guided modes of surface exciton-plasmon polaritons(SEPPs).The out-coupling avenues of SEPP guided modes are modeled in the presence of nanostructures,such as corrugation and gratings,on the metal surface.The guided modes of SEPPs in metal-contacted ZnO nanowires are calculated using the effective index method.The enhanced light emission from single semiconductor nanowires shows promise for use in highly efficient nano-emitters and nano-lasers,as well as macroscopic solid state light sources with very high efficiency.

Electrical and mechanical performance of graphene sheets exposed to oxidative environments

Graphene coatings have been shown to protect the underlying material from oxidation when exposed to different media. However, the passivating properties of graphene in air at room temperature, which corresponds to the operating conditions of many electronic devices, still remain undear. In this work, we analyze the oxidation kinetics of graphene/Cu samples in air at room temperature for long periods of time （from I day to 113 days） using scanning electron microscopy, conductive atomic force microscopy and Auger electron microscop3~ and we compare the results with those obtained for similar samples treated in H202. We observe that unlike the graphene sheets exposed to H202, in which the accumulation of oxygen at the graphene domain boundaries evolves in a very controlled and progressive way, the local oxidation of graphene in air happens in a disordered manner. In both cases the oxide hillocks formed at the graphene domain boundaries can propagate to the domains until reaching a limiting width and height. Our results demonstrate that the local oxidation of the underlying material along the domain boundaries can dramatically decrease the roughness, conductivity, mechanical resistance and frictional characteristics of the graphene sheet, which reduces the performance of the whole device.

Bending-Induced Conductance Increase in Individual Semiconductor Nanowires and Nanobelts

Reliable ohmic contacts were established in order to study the strain sensitivity of nanowires and nanobelts.Significant conductance increases of up to 113%were observed on bending individual ZnO nanowires or CdS nanobelts.This bending strain-induced conductance enhancement was confirmed by a variety of bending measurements,such as using different manipulating tips(silicon,glass or tungsten)to bend the nanowires or nanobelts,and is explained by bending-induced effective tensile strain based on the principle of the piezoresistance effect.

Observation of two-times self-focusing of femtosecond laser beam in ZnO crystal by two-photon luminescence

By ‘‘seeing" the green two-photon luminescence, two separate focusing points are observed on the propagation axis of a converging femtosecond laser beam in a ZnO single crystal rod. It is found that the selffocusing effect makes a significant contribution to the formation of the first focusing point, while the second focusing point is caused by self-refocusing. The position of the first focusing point is in good agreement with the value predicted by a model developed by Chin and his co-workers. These experimental findings could be the unprecedented evidence for the self-focusing and refocusing effect of the femtosecond laser filament propagation in nonlinear media.

Visualizing grain boundaries in monolayer MoSe2 using mild H2O vapor etching

Beyond graphene, two-dimensional （2D） transition metal dichalcogenides （TMDs） have attracted significant attention owing to their potential in next-generation nanoelectronics and optoelectronics. Nevertheless, grain boundaries are ubiquitous in large-area as-grown TMD materials and would significantly affect their band structure, electrical transport, and optical properties. Therefore, the characterization of grain boundaries is essential for engineering the properties and optimizing the growth in TMD materials. Although the existence of boundaries can be measured using scanning tunneling microscopy, transmission electron microscopy, or nonlinear optical microscop~ a universal, convenient, and accurate method to detect boundaries with a twist angle over a large scale is still lacking. Herein, we report a high-throughput method using mild hot H20 etching to visualize grain boundaries of TMDs under an optical microscope, while ensuring that the method is nearly noninvasive to grain domains. This technique utilizes the reactivity difference between stable grain domains and defective grain boundaries and the mild etching capacity of hot water vapor. As grain boundaries of two domains with twist angles have defective lines, this method enables to visualize all types of grain boundaries unambiguously. Moreover, the characterization is based on an optical microscope and therefore naturally of a large scale. We further demonstrate the successful application of this method to other TMD materials such as MoS2 and WSe2. Our technique facilitates the large-area characterization of grain boundaries and will accelerate the controllable growth of large single-crystal TMDs.

Ordered space-time structures: Quantum carpets from Gaussian sum theory

The term "quantum carpet" can be observed in many closed quantum systems, where the evolution of a wave function exhibits a carpet-like pattern. Quantum carpet mechanisms are also akin to the classical interference patterns of light. Although the origins of quantum carpets have previously been studied by various researchers, many interesting details are still worth exploring. In this study, we present a unified framework for simultaneously analyzing three different features of quantum carpets: full revival,fractional revival, and diagonal canal. For the fractional revival feature, a complete formula is presented to explain its formation through Gaussian sum theory, in which all essential features, including phases and amplitudes, are captured analytically. We also reveal important relationships between the interference terms of diagonal canals and their geometric interpretations such that a better understanding of the development of diagonal canals can be supported.

Spontaneous ferromagnetism and magnetoresistance hysteresis in Ge_(1–x)Sn_(x)alloys

Introducing ferromagnetism into non-magnetic systems without the participation of magnetic elements is promising for all-electric spintronic devices[1,2].Many approaches have been pursued,such as non-magnetic defects induced magnetization in layered materials[3–5]or the inversion symmetry breaking induced magnetization in magic-angle bilayer graphene[6–8],etc.However,these approaches have to tackle with the localization effects or the inevitable precise control of twist angle,which hinders the future application into large-scale spintronic information devices.Theorists also predicted that the spontaneous ferromagnetism could emerge in the quasi-2D crystals[9]like GaSe,but no experimental results have been reported.Here,we report the spontaneous ferromagnetism induced by van Hove singularity[9–13]in non-magnetic groupⅣGe_(1–x)Sn_(x)alloys grown by the molecular beam epitaxy(MBE)technique.Our findings experimentally open up an opportunity to realize spintronics in groupⅣsemiconductors.

Bending strain effects on the optical and optoelectric properties of GaN nanowires

Elastic strain has been an important method to regulate the electronic structures and physical properties of nanoscale semiconductors due to the promising potentials in improving the performance of their optoelectronic devices.Here,we report the investigation of bending strain effects on the optical and optoelectric properties of individual gallium nitride(GaN)nanowires(NWs).By charactering the near-band emission spectrum of individual GaN NWs at different bending strains with low temperature cathodoluminescence(CL),we reveal that the near-band emission splits into two peaks,where the low energy peak displays a linear redshift with increasing the bending strain while the high energy one shows a slight blueshift.Further localized ultraviolet(UV)photoresponse measurements illustrate that the photoresponse of the GaN NWs shows a linear increase with the bending train,and the maximum enhancement is more than two orders of magnitude.The experimental observations are well interpreted by theoretical calculations on the strain modulation on the electronic band structure of GaN combined with analysis of carrier dynamics and optical waveguide effect in the bending strain field.Our results not only shed light on the bending strain effects on the optical and optoelectric properties of semiconductors,but also hold potential to help the future design of high performance nano-optoelectric devices.

Selection of Single-Walled Carbon Nanotubes According to Both Their Diameter and Chirality via Nanotweezers

Diameter- and chirality-dependent interactions between aromatic molecule-based nanotweezers and single-walled carbon nanotubes (SWNTs) are revealed by density functional theory calculations. We found that the threshold diameter of selected SWNTs is determined by the end-to-end distance of the nanotweezer. Large-diameter SWNTs are preferred by a nanotweezer with an obtuse folding angle, whereas small-diameter SWNTs are favored by a nanotweezer with an acute folding angle. The adsorption can be further stabilized by the orientational alignment of the hexagonal rings of the nanotweezer and the SWNT sidewall. Therefore, by taking advantage of the supramolecular recognition ability of the aromatic molecule-based nanotweezer, SWNTs can be enriched with both controllable diameter and chirality.