Machine learning of theΓ-point gap and flat bands of twisted bilayer graphene at arbitrary angles

The novel electronic properties of bilayer graphene can be fine-tuned via twisting,which may induce flat bands around the Fermi level with nontrivial topology.In general,the band structure of such twisted bilayer graphene(TBG)can be theoretically obtained by using first-principles calculations,tight-binding method,or continuum model,which are either computationally demanding or parameters dependent.In this work,by using the sure independence screening sparsifying operator method,we propose a physically interpretable three-dimensional(3D)descriptor which can be utilized to readily obtain theΓ-point gap of TBG at arbitrary twist angles and different interlayer spacings.The strong predictive power of the descriptor is demonstrated by a high Pearson coefficient of 99%for both the training and testing data.To go further,we adopt the neural network algorithm to accurately probe the flat bands of TBG at various twist angles,which can accelerate the study of strong correlation physics associated with such a fundamental characteristic,especially for those systems with a larger number of atoms in the unit cell.

A Time-Dependent Random State Approach for Large-Scale Density Functional Calculations

We develop a self-consistent first-principle method based on the density functional theory.Physical quantities such as the density of states,Fermi energy and electron density are obtained using a time-dependent random state method without diagonalization.The numerical error for calculating either global or local variables always scales as1/√SN_(e)where N_(e)is the number of electrons and S is the number of random states,leading to a sublinear computational cost with the system size.In the limit of large systems,one random state could be enough to achieve reasonable accuracy.The accuracy and scaling properties of using the method are derived analytically and verified numerically in different condensed matter systems.Our time-dependent random state approach provides a powerful strategy for large-scale density functional calculations.

Preparation and Photocatalytic Performance of Double-Shelled Hollow W_(18)O_(49)@C_(3)N_(4)@Ti_(3)C_(2)Microspheres

C_(3)N_(4),C_(3)N_(4)@Ti_(3)C_(2)and W_(18)O_(49)@C_(3)N_(4)@Ti_(3)C_(2)hollow spheres were successfully prepared by using SiO_(2)template followed by gradual deposition method.The degradation of phenol solution and photolysis ability were tested to characterize its photocatalytic activity.Compared with the single-shelled C_(3)N_(4)and C_(3)N_(4)@Ti_(3)C_(2)hollow spheres,double-shelled W_(18)O_(49)@C_(3)N_(4)@Ti_(3)C_(2)hollow spheres possessed larger surface area and fast charge separation efficiency,exhibiting about 8.9 times and 4.0 times higher H_(2)evolution than those of C_(3)N_(4),C_(3)N_(4)@Ti_(3)C_(2)hollow spheres,respectively.The photocatalytic mechanism of the W_(18)O_(49)@C_(3)N_(4)@Ti_(3)C_(2)hollow spheres were carefully investigated according to the results of morphology design and photoelectric performance.A Z scheme mechanism based on the construction of heterojunctions was proposed to explain the improvement of photocatalytic performance.This new charge transfer mechanism appears to greatly inhibit the recombination of electrons/holes during the charge transfer process,while maintaining its strong hydrogen reduction ability,resulting in a higher photocatalytic performance.

Broadband tunable external cavity laser using a bent-waveguide quantum-dot superluminescent diode as gain device

A broadband tunable grating-coupled external cavity laser is realized by employing a self-assembled InAs/GaAs quantum-dot (QD) superluminescent diode (SLD) as the gain device. The SLD device is processed with a bent-waveguide structure and facet antireflection (AR) coating. Tuning bandwidths of 106 nm and 117 nm are achieved under 3-A and 3.5-A injection currents, respectively. The large tuning range originates essentially from the broad gain spectrum of self-assembled QDs. The bent waveguide structure combined with the facet AR coating plays a role in suppressing the inner-cavity lasing under a large injection current.

Phase transition and critical behavior of spin–orbital coupled spinel ZnV_2O_4

We present the temperature-dependent susceptibility and specific heat measurement of spinel ZnV_2O_4.The structural transition with orbital ordering and the antiferromagnetic transition with spin ordering were observed at 50 K and 37 K,respectively.By analysis of the hysteresis behavior between the specific heat curves obtained in warming and cooling processes,the structural transition was confirmed to be the first-order transition,while the antiferromagnetic transition was found to be of the second-order type.At the structural transition,the latent heat and entropy change were calculated from the excess specific heat,and the derivative of pressure with respect to temperature was obtained using the Clausius-Clapayron equation.At the magnetic transition,the width of the critical fluctuation region was obtained to be about 0.5 K by comparing with Gaussian fluctuations.In the critical region,the critical behavior was analyzed by using renormalization-group theory.The critical amplitude ratio A^+/A^- = 1.46,which deviates from the 3D Heisenburg model;while the critical exponent α is-0.011,which is close to the 3D XY model.We proposed that these abnormal critical behaviors can be attributed to strong spin-orbital coupling accompanied with the antiferromagnetic transition.Moreover,in the low temperature range(2-5 K),the Fermi energy,the density of states near the Fermi surface,and the low limit of Debye temperature were estimated to be2.42 eV,2.48 eV^(-1),and 240 K,respectively.

Electrical Conductivity of a Single Electro-deposited CoZn Nanowire

CoZn nanowires are fabricated by the electrodeposition method at constant voltage mode with porous anodic aluminum oxide as templates. Scanning electron microscope and transmission electron microscope images show that the CoZn nanowires have a rather smooth surface. The nanowires have an average diameter of 50 nm,which coincides with the diameter of the used templates. The x-ray diffraction pattern reveals the polycrystalline structure of the CoZn nanowires. The electrical conductivity of a single CoZn nanowire is studied. The metallic behavior is observed at temperatures from 230 K to 30 K. Moreover, an abnormal behavior appears around 30 K.The resistance shows the slight upturn phenomenon below 30 K down to 2 K, which is due to the major conduction role of the oxidation layer on the surface of the CoZn nanowire.

A low noise, high fidelity cross phase modulation in multi-level atomic medium

We develop a hybrid scheme of cross phase modulation based on electromagnetically induced transparency(EIT)and active Raman gain(ARG)in a multi-level atomic medium.The cross phase modulation,with low loss and without noise,is demonstrated in a room-temperature ^(85)Rb vapor.We show that a p radian nonlinear Kerr phase shift of the signal light relative to a reference light is observed when the signal light is modulated by the phase control field with the low light intensity.We also show that the linear and the third-order absorption can be eliminated via the Raman gain,and the phase noise of the signal light can be ignored when the phase control light is applied in this hybrid scheme.

Dual Topological Features of Weyl Semimetallic Phases in Tetradymite BiSbTe_(3)

Based on first-principles calculations and symmetry arguments,we reveal that the non-centrosymmetric ternary tetradymite BiSbTe_(3) possesses exotic dual topological features of Weyl semimetallic phases with Z_(2) index(1:000).The results show that the helical Dirac-type surface states protected by the time-reversal symmetry are present in the vicinity of the Brillouin zone center,which is consistent with the experimental report.Furthermore,we show that four pairs of Weyl points reside exactly at the Fermi level,which are guaranteed to be located on high-symmetry planes due to mirror symmetries.The helical surface states and the projected Weyl nodes are well separated in the momentum space,facilitating their observations in experiments.This work not only uncovers a unique quantum phenomenon with dual topological features in the tetradymite family but also paves a fascinating avenue for exploring the coexistence of multi-topological states with wide applications.

Random Green's Function Method for Large-Scale Electronic Structure Calculation

We report a linear-scaling random Green's function(rGF) method for large-scale electronic structure calculation. In this method, the rGF is defined on a set of random states and is efficiently calculated by projecting onto Krylov subspace. With the rGF method, the Fermi–Dirac operator can be obtained directly, avoiding the polynomial expansion to Fermi–Dirac function. To demonstrate the applicability, we implement the rGF method with the density-functional tight-binding method. It is shown that the Krylov subspace can maintain at small size for materials with different gaps at zero temperature, including H_(2)O and Si clusters. We find with a simple deflation technique that the rGF self-consistent calculation of H_(2)O clusters at T = 0 K can reach an error of~ 1 me V per H_(2)O molecule in total energy, compared to deterministic calculations. The rGF method provides an effective stochastic method for large-scale electronic structure simulation.

Recent developments in CVD growth and applications of 2D transition metal dichalcogenides

Two-dimensional(2D)transition metal dichalcogenides(TMDs)with fascinating electronic energy band structures,rich valley physical properties and strong spin–orbit coupling have attracted tremendous interest,and show great potential in electronic,optoelectronic,spintronic and valleytronic fields.Stacking 2D TMDs have provided unprecedented opportunities for constructing artificial functional structures.Due to the low cost,high yield and industrial compatibility,chemical vapor deposition(CVD)is regarded as one of the most promising growth strategies to obtain high-quality and large-area 2D TMDs and heterostructures.Here,state-of-the-art strategies for preparing TMDs details of growth control and related heterostructures construction via CVD method are reviewed and discussed,including wafer-scale synthesis,phase transition,doping,alloy and stacking engineering.Meanwhile,recent progress on the application of multi-functional devices is highlighted based on 2D TMDs.Finally,challenges and prospects are proposed for the practical device applications of 2D TMDs.

Effects of electron-phonon coupling on the phonon transport properties of the Weyl semimetals NbAs and TaAs:A comparative study

It has now become recognized that the electron-phonon coupling(EPC)may play an important role in governing the phonon transport,especially for metallic and semiconducting systems at high carrier concentration.Here we focus on the Weyl semimetals TaAs and NbAs and give a comparative study on their phonon transport properties by explicitly including the EPC in first-principles calculations.It is found that the lattice thermal conductivities of both systems are significantly reduced by the EPC,which is more pronounced for the TaAs compared with the NbAs at the same carrier concentration.Detailed analysis indicates that the TaAs exhibits smaller EPC phonon relaxation time,as characterized by stronger EPC strength which is associated with larger deformation potential constant and Born effective charge.Moreover,we see that the TaAs exhibits obviously larger overlap between the EPC relaxation time and that from intrinsic phonon-phonon scattering,which could further reduce the lattice thermal conduc-tivity.Our work not only highlights the vital importance of EPC in accurately predicting the phonon transport behaviors,but also offers a simple alternative to evaluate the EPC strength of various material systems.

Predicting lattice thermal conductivity via machine learning: a mini review

Over the past few decades,molecular dynamics simulations and first-principles calculations have become two major approaches to predict the lattice thermal conductivity(κ_(L)),which are however limited by insufficient accuracy and high computational cost,respectively.To overcome such inherent disadvantages,machine learning(ML)has been successfully used to accurately predictκL in a high-throughput style.In this review,we give some introductions of recent ML works on the direct and indirect prediction ofκL,where the derivations and applications of data-driven models are discussed in details.A brief summary of current works and future perspectives are given in the end.

When the orbital degree of freedom meets higher-order topology

The orbital degree of freedom(ODoF),which has a significant impact on exotic quantum states of matter and solidstate materials,has now been combined with higher-order topology.The experimental realization of a photonic p-orbital higher-order topological insulator can lead to exploring a wide range of novel topological phases involving the ODoF.

尖晶石:二维磁性材料家族新成员

Ultrathin two-dimensional(2D)materials have attracted considerable interest in recent years due to their fascinating properties and enormous potential in various fields.Among the vast family of 2D materials,2D magnetic materials are especially notable.In light of the established Mermin-Wagner theorem,it has long been believed that the long-range magnetic order can hardly survive in a strictly 2D isotropic Heisenberg system at non-zero temperatures.

SnSe+Ag_2Se composite engineering with ball milling for enhanced thermoelectric performance

Earth-abundant IV-VI semiconductor SnSe is regarded as a promising thermoelectric material due to its intrinsic low thermal conductivity. In this report, the highly textured SnSe/Ag2 Se composites were first designed by solid solution method followed by spark plasma sintering(SPS) and their thermoelectric properties in two directions were investigated, and then, the performance of composites was further optimized with an additional ball milling. The coexistence of SnSe and Ag2 Se phases is clearly confirmed by energy-dispersive X-ray spectroscopy(EDX) in transmission electron microscopy(TEM). After ball milling, the size of SnSe grains as well as the incorporated Ag2 Se particles reduces effectively, which synergistically optimizes the electrical and thermal transport properties at high temperature range. As a result, a maximum ZT of ～0.74 at 773 K for SnSe+1.0%Ag_2 Se in the direction vertical to the pressing direction is achieved. Composite engineering with additional ball milling is thus proved to be an efficient way to improve the thermoelectric properties of SnSe, and this strategy could be applicable to other thermoelectric systems.

Gas sensing capabilities of TiO_(2) porous nanoceramics prepared through premature sintering

Pure and noble metal(Pt,Pd,and Au)doped TiO_(2)nanoceramics have been prepared from TiO_(2)nanoparticles through traditional pressing and sintering.For those samples sintered at 550℃,a typical premature sintering occurred,which led to the formation of a highly porous microstructure with a Brunauer-Emmett-Teller(BET)specific surface area of 23 m^(2)/g.At room temperature,only Pt-doped samples showed obvious response to hydrogen,with sensitivities as high as~500 for 1000 ppm H_(2)in N_(2);at 300℃,all samples showed obvious responses to CO,while the responses of noble metal doped samples were much higher than that of the undoped ones.The mechanism for the observed sensing capabilities has been discussed,in which the catalytic effect of Pt for hydrogen is believed responsible for the room-temperature hydrogen sensing capabilities,and the absence of glass frit as commonly used in commercial thick-film metal oxide gas sensors is related to the high sensitivities.It is proposed that much attention should be paid to metal oxide porous nanoceramics in developing gas sensors with high sensitivities and low working temperatures.

Controlling disorder in host lattice by hetero-valence ion doping to manipulate luminescence in spinel solid solution phosphors

Phosphor materials have been rapidly developed in the past decades. Developing phosphors with desired properties including strong luminescence intensity and long lifetime has attracted widespread attention. Herein, we show that hetero-valence ion doping can serve as a potent strategy to manipulate luminescence in persistent phosphors by controlling disorder in the host lattice. Specifically, spinel phosphor Zn(Ga_(1-x)Zn_x)(Ga_(1-x)Ge_x)O_4:Cr is developed by doping ZnGa_2O_4:Cr with tetravalent Ge^(^(4+)).Compared to the original ZnGa_2O_4:Cr, the doped Zn(Ga_(1-x)Zn_x)(Ga_(1-x)Ge_x)O_4:Cr possesses significantly enhanced persistent luminescence intensity and prolonged decay time. Rietveld refinements show that Ge^(4+)enters into octahedral sites to substitute Ga^(3+), which leads to the co-substitution of Ga^(3+) by Zn^(2+) for charge compensation. The hetero-valence substitution of Ga^(3+) by Ge^(4+)and Zn^(2+) enriches the charged defects in Zn(Ga_(1-x)Zn_x)(Ga_(1-x)Ge_x)O_4:Cr, making it possible to trap large amounts of charge carriers within the defects during excitation. Electron paramagnetic resonance measurement further confirms that the amount of Cr^(3+) neighboring charged defects increases with Ge^(4+)doping. Thus charge carriers released from defects can readily combine with the neighboring Cr^(3+) to produce bright persistent luminescence after excitation ceases. The hetero-valence ion doping strategy can further be employed to develop many other phosphors and contributes to lighting, photocatalysis and bioimaging.

Machine learning method for tight-binding Hamiltonian parameterization from ab-initio band structure

The tight-binding(TB)method is an ideal candidate for determining electronic and transport properties for a large-scale system.It describes the system as real-space Hamiltonian matrices expressed on a manageable number of parameters,leading to substantially lower computational costs than the ab-initio methods.Since the whole system is defined by the parameterization scheme,the choice of the TB parameters decides the reliability of the TB calculations.The typical empirical TB method uses the TB parameters directly from the existing parameter sets,which hardly reproduces the desired electronic structures quantitatively without specific optimizations.It is thus not suitable for quantitative studies like the transport property calculations.The ab-initio TB method derives the TB parameters from the ab-initio results through the transformation of basis functions,which achieves much higher numerical accuracy.However,it assumes prior knowledge of the basis and may encompass truncation error.Here,a machine learning method for TB Hamiltonian parameterization is proposed,within which a neural network(NN)is introduced with its neurons acting as the TB matrix elements.This method can construct the empirical TB model that reproduces the given ab-initio energy bands with predefined accuracy,which provides a fast and convenient way for TB model construction and gives insights into machine learning applications in physical problems.

能带调控助力基于MnS空穴传输层的全无机高效稳定Sb_(2)(S,Se)_(3)太阳能电池

近年,锑硫化合物[Sb_(2)S_(x)Se_(3-0)<x<3)已迅速成为太阳能电池中的新型吸光材料研究热点.然而,高效(超过9%)的锑基太阳能电池通常是使用昂贵且稳定性差的有机空穴传输层(HTL,Spiro-OMeTAD).因此,开发成本低且稳定性好的新型无机HTL迫在眉睫.本文利用热蒸法制备MnS薄膜,并利用它作为HTL构建Sb_(2)(S,Se)_(3)太阳能电池。研究表明,在空气中低温退火有利于MnS与Sb_(2)(S,Se)_(3)形成更好的能带匹配从而实现空穴有效提取.基于此,作者制备得到迄今为止最高效率(9.24%)的全无机Sb_(2)(S,Se)_(3)太阳能电池.

Preliminary study of light yield dependence on LAB liquid scintillator composition

Liquid scintillator(LS) will be adopted as the detector material in JUNO(Jiangmen Underground Neutrino Observatory). The energy resolution requirement of JUNO is 3%, which has never previously been reached.To achieve this energy resolution, the light yield of liquid scintillator is an important factor. PPO(the fluor) and bis-MSB(the wavelength shifter) are the two main materials dissolved in LAB. To study the influence of these two materials on the transmission of scintillation photons in LS, 25 and 12 cm-long quartz vessels were used in a light yield experiment. LS samples with different concentration of PPO and bis-MSB were tested. At these lengths, the light yield growth is not obvious when the concentration of PPO is higher than 4 g/L. The influence from bis-MSB becomes insignificant when its concentration is higher than 8 mg/L. This result could provide some useful suggestions for the JUNO LS.