arXiv subjects

Chun Zhang

Publications and source records attributed to Chun Zhang.

At least 37 records · Page 2Linked to original sources

arXiv2021

Sub-angstrom Non-invasive Imaging of Atomic Arrangement in 2D Hybrid Perovskites

Non-invasive imaging of the atomic arrangement in two-dimensional (2D) Ruddlesden-Popper hybrid Perovskites (RPPs), as well as understanding the related effects is challenging, due to the insulating nature and softness of the organic layers which also obscure the underlying inorganic lattice. Here, we demonstrate a sub-angstrom resolution imaging of both soft organic layers and inorganic framework in a prototypical 2D lead-halide RPP crystal via combined tip-functionalized Scanning Tunneling Microscopy (STM) and non-contact Atomic Force Microscope (ncAFM) corroborated by theoretical simulations. STM measurements unveil the atomic reconstruction of the inorganic lead-halide lattice and overall twin-domain composition of the RPP crystal, while ncAFM measurements with a CO-tip enable non-perturbative visualization of the cooperative reordering of surface organic cations driven by their hydrogen bonding interactions with the inorganic lattice. Moreover, such a joint technique also allows for the atomic-scale imaging of the electrostatic potential variation across the twin-domain walls, revealing alternating quasi-one-dimensional (1D) electron and hole-channels at neighboring twin-boundaries, which may influence in-plane exciton transport and dissociation.

cond-mat.mtrl-sci↗

arXiv2020

Variational study of the ground state and spin dynamics of the spin-$\frac{1}{2}$ Kagome antiferromagnetic Heisenberg model and its implication on Hebertsmithite ZnCu$_{3}$(OH)$_{6}$Cl$_{2}$

We find that the best RVB state of the spin-$\frac{1}{2}$ Kagome antiferromagnetic Heisenberg model(spin-$\frac{1}{2}$ KAFH) is described by a $Z_{2}$ gapped mean field ansatz, which hosts a mean field spinon dispersion very different from that of the widely studied $U(1)$ Dirac spin liquid state. However, we find that the physical spin fluctuation spectrum calculated from the Gutzwiller projected RPA(GRPA) theory above such an RVB state is actually gapless and is almost identical to that above the $U(1)$ Dirac spin liquid state. We find that such a peculiar behavior can be attributed to the unique flat band physics on the Kagome lattice, which makes the mapping between the mean field ansatz and the RVB state non-injective. We find that the spin fluctuation spectrum of the spin-$\frac{1}{2}$ KAFH is not at all featureless, but is characterized by a prominent spectral peak at about $0.25J$ around the $\mathbf{M}$ point, which is immersed in a broad continuum extending to $2.7J$. Based on these results, we argue that the spectral peak below 2 meV in the inelastic neutron scattering(INS) spectrum of Hebertsmithite ZnCu$_{3}$(OH)$_{6}$Cl$_{2}$, which has been attributed to the contribution of Cu$^{2+}$ impurity spins occupying the Zn$^{2+}$ site, should rather be understood as the intrinsic contribution from the Kagome layer. We propose to verify such a picture by measuring the Knight shift on the Cu site, rather than the O site, which is almost blind to the spin fluctuation at the $\mathbf{M}$ point as a result of the strong antiferromagnetic correlation between nearest neighboring spins on the Kagome lattice.

cond-mat.str-el↗

arXiv2020

Theory of the anomalous spin dynamics of spin-$\frac{1}{2}$ triangular lattice Heisenberg antiferromagnet and its application to Ba$_3$CoSb$_2$O$_9$

Although it is well accepted that the spin-$\frac{1}{2}$ triangular lattice Heisenberg antiferromagnet(TLHAF) has a long range ordered ground state, a thorough understanding of its spin dynamics is still missing. While the linear spin wave theory(LSWT) predicts three branches of magnon mode in the magnetic Brillouin zone(MBZ), the 1/S expansion at the next order is found to break down in a large portion of the MBZ centered around the M point, leaving the fate of the magnon modes there undecided. Recent neutron scattering measurement on Ba$_3$CoSb$_2$O$_9$, an ideal realization of the spin-$\frac{1}{2}$ TLHAF, provides a surprising answer to this issue. Two, rather than three branches of magnon mode are observed around the M point, whose dispersion are strongly renormalized with respect to the LSWT prediction and exhibit pronounced roton-like minimum at the M point. This is accompanied by a strong spin fluctuation continuum at higher energy, inside which two strong and broad spectral peaks of unknown origin are observed. In this work, we propose a simple picture for these spectral anomalies by invoking the resonating valence bond(RVB) physics in the description of the ground state of the system. We find that the roton-like minimum in the magnon dispersion can be explained by the coupling between the collective spin fluctuation and the continuum of Dirac spinon excitation moving in a $π$-flux background. We also propose that the two broad peaks in the continuum can be understood respectively as the Landau damped third magnon mode and the Landau damped longitudinal mode. Such a picture can be verified by studying the polarization character of the various spectral features.

cond-mat.str-el↗

arXiv2020

Long-term prediction of chaotic systems with recurrent neural networks

Reservoir computing systems, a class of recurrent neural networks, have recently been exploited for model-free, data-based prediction of the state evolution of a variety of chaotic dynamical systems. The prediction horizon demonstrated has been about half dozen Lyapunov time. Is it possible to significantly extend the prediction time beyond what has been achieved so far? We articulate a scheme incorporating time-dependent but sparse data inputs into reservoir computing and demonstrate that such rare "updates" of the actual state practically enable an arbitrarily long prediction horizon for a variety of chaotic systems. A physical understanding based on the theory of temporal synchronization is developed.

cs.LG↗

arXiv2019

Essence Knowledge Distillation for Speech Recognition

It is well known that a speech recognition system that combines multiple acoustic models trained on the same data significantly outperforms a single-model system. Unfortunately, real time speech recognition using a whole ensemble of models is too computationally expensive. In this paper, we propose to distill the knowledge of essence in an ensemble of models (i.e. the teacher model) to a single model (i.e. the student model) that needs much less computation to deploy. Previously, all the soften outputs of the teacher model are used to optimize the student model. We argue that not all the outputs of the ensemble are necessary to be distilled. Some of the outputs may even contain noisy information that is useless or even harmful to the training of the student model. In addition, we propose to train the student model with a multitask learning approach by utilizing both the soften outputs of the teacher model and the correct hard labels. The proposed method achieves some surprising results on the Switchboard data set. When the student model is trained together with the correct labels and the essence knowledge from the teacher model, it not only significantly outperforms another single model with the same architecture that is trained only with the correct labels, but also consistently outperforms the teacher model that is used to generate the soft labels.

cs.CL↗

arXiv2018

3D characterization of ultrasonic melt processing on the microstructural refinement of Al-Cu alloys by synchrotron X-ray tomography

The effect of ultrasonic melting processing on three-dimensional architecture of intermetallic phases and pores in two multicomponent cast Al-5.0Cu-0.6Mn-0.5 Fe alloys is characterized using conventional microscopy and synchrotron X-ray microtomography. The two alloys are found to contain intermetallic phases such as Al15(FeMn)3Cu2, Al7Cu2Fe, Al3(FeMn), Al6(FeMn), and Al2Cu that have complex networked morphology in 3D. The application of USP in alloys can obtained refined and equiaxed microstructures. The grain size of 0.5Fe and 1.0 Fe alloys is greatly decreased from 16.9 m, 15.8 m without USP to 13.3 m, 12.2 m with USP, respectively. The results show that USP significantly reduce the volume fraction, grain size, interconnectivity, and equivalent diameter of the intermetallic phases in both alloys. The volume fraction of pores in both alloys is reduced due to the USP degassing effect. The refinement mechanism of USP induced fragmentation of primary and secondary dendrites via acoustic bubbles and acoustic streaming flow were discussed.

cond-mat.mtrl-sci↗

arXiv2018

Synchrotron X-ray tomography investigation of 3D morphology of intermetallic phases and pores and their effect on the mechanical properties of cast Al-Cu alloys

The influence of Fe content on the three-dimensional (3D) morphology of Fe-rich intermetallic phases (Fe phases), Al2Cu, and pores and mechanical properties of cast Al-5.0Cu-0.6Mn alloys with 0.5 and 1.0 wt. % Fe are characterized using synchrotron X-ray tomography and a tensile test. The results show that both Fe phases and Al2Cu exhibit a complex 3D network structure, and the pores are irregular with complex interconnected and near-globular shape. As the Fe content increases from 0.5 % to 1.0 %, the volume fraction and equivalent diameter of Fe phases decrease, whereas both their interconnectivities decrease. Skeletonization analysis shows that the Chinese-script-shaped Fe phase is compacted than the plate-like Fe phases. The equivalent diameter and sphericity of pores vary with Fe content, and their relationships follow exponential functions, Y = 7.14*X-1.29 and Y = 7.06*X-1.20, respectively. The addition of Fe results in a decrease in the ultimate tensile strength and elongation from 223.7 MPa to 199.8 MPa and from 5.51 % to 3.64 %, respectively, owing to increasing volume fraction of sharp-edged Fe phases and pores, resulting in stress concentration during tensile test.

cond-mat.mtrl-sci↗

arXiv2018

A Bio-inspired Collision Detecotr for Small Quadcopter

Sense and avoid capability enables insects to fly versatilely and robustly in dynamic complex environment. Their biological principles are so practical and efficient that inspired we human imitating them in our flying machines. In this paper, we studied a novel bio-inspired collision detector and its application on a quadcopter. The detector is inspired from LGMD neurons in the locusts, and modeled into an STM32F407 MCU. Compared to other collision detecting methods applied on quadcopters, we focused on enhancing the collision selectivity in a bio-inspired way that can considerably increase the computing efficiency during an obstacle detecting task even in complex dynamic environment. We designed the quadcopter's responding operation imminent collisions and tested this bio-inspired system in an indoor arena. The observed results from the experiments demonstrated that the LGMD collision detector is feasible to work as a vision module for the quadcopter's collision avoidance task.

cs.NE↗

arXiv2015

Phosphorene and Transition Metal Dichalcogenide 2D Heterojunctions: Application in Excitonic Solar Cells

Using the first-principles GW-Bethe-Salpeter equation method, here we study the excited-state properties, including quasi-particle band structures and optical spectra, of phosphorene, a two-dimensional (2D) atomic layer of black phosphorus. The quasi-particle band gap of monolayer phosphorene is 2.15 eV and its optical gap is 1.6 eV, which is suitable for excitonic thin film solar cell applications. Next, this potential application is analysed by considering type-II heterostructures with single layered phosphorene and transition metal dichalcogenides (TMDs). These heterojunctions have a potential maximum power conversion efficiency of up to 12\%, which can be further enhanced to 20\% by strain engineering. Our results show that phosphorene is not only a promising new material for use in nanoscale electronics, but also in optoelectronics.

cond-mat.mtrl-sci↗

arXiv2013

Uniform Electron Gas under An External Bias: The Generalized Thomas-Fermi-Dirac Model and the Dual-Mean-Field Theory

The uniform electron gas placed between two reservoirs is used as a model system for molecular junctions under an external bias. The energetics of the electron gas are calculated by generalizing the Thomas-Fermi-Dirac (TFD) model to nonequilibrium cases. We show that when the bias voltage is not zero, the first Hohenberg-Kohn (HK) theorem breaks down, and energies of the electron gas can be determined by the total electron density together with the density of nonequilibrium electrons, supporting the dual-mean-field (DMF) theory recently proposed by us [J. Chem. Phys. 139, (2013) 191103]. The generalization of TFD functionals to DMF ones is also discussed.

cond-mat.mes-hall↗

arXiv2013

Energy-gap Opening and Quenching in Graphene under Periodic External Potentials

In this paper, we investigated the effects of periodic external potentials on properties of charge carriers in graphene using both the first-principles method based on density functional theory (DFT) and a theoretical approach based on a generalized effective spinor Hamiltonian. DFT calculations were done in a modified Kohn-Sham procedure that includes effects of the periodic external potential. Unexpected energy band gap opening and quenching were predicted for the graphene superlattice with two symmetrical sublattices and those with two unsymmetrical sublattices, respectively. Theoretical analysis based on the spinor Hamiltonian showed that the correlations between pseudospins of Dirac fermions in graphene and the applied external potential, and the potential-induced intervalley scattering, play important roles in energy-gap opening and quenching.

cond-mat.mes-hall↗

arXiv2013

Band gap engineering in graphene and hexagonal BN antidot lattices: A first principles study

Effects of antidot lattices on electronic structures of graphene and hexagonal BN (h-BN) are investigated using the first principles method based on density functional theory. For graphene, we find that when the antidot lattice is along the zigzag direction, the band gap opening can be related to the inter-valley scattering, and does not follow the simple scaling rule previously proposed in literature for the antidot lattice along the armchair direction. For h-BN, our calculations show that the antidot lattice results in reducing of band gaps. Coupled with doping of carbon atoms, the band gap of an h-BN antidot lattice can be reduced to below 2 eV, which might have implications in light-emitting devices or photoelectrochemistry.

cond-mat.mtrl-sci↗

arXiv2013

Mn-doped Thiolated Au$_{25}$ Nanoclusters: Atomic Configuration, Magnetic Properties, and A Possible High-performance Spin Filter

We report an \emph{ab inito} investigation on the ground-state atomic configuration, electronic structures, magnetic and spin-dependent transport properties of Mn-doped Au$_{25}$ nanoclusters protected by thiolate. It is found that the most stable dopant sites are near surfaces, rather than the center positions of the nanoparticles. Transport calculations show that high-performance spin filters can be achieved by sandwiching these doped clusters between two nonmagnetic Au electrodes. The nearly perfect spin filtering originates from localized magnetic moments of these clusters that are well protected by ligands from the presence of electrodes.

cond-mat.mes-hall↗

arXiv2013

Reversible magnetism switching in graphene-based systems via the decoration of photochromic molecules

By first principles calculations, we demonstrate that when decorated with photochromic molecules, it is possible to use light to reversibly control the magnetic properties of a nanoscale magnetic system. The combination of a graphene-based magnetic system and a photochromic azobenzene molecule is chosen as a model system. The {\it trans} and {\it cis} isomers of the azobenzene molecule that can be converted between each other by means of photoexcitations are found to have drastically different effects on the magnetic properties of the system. The results may pave the way for the future design of light controllable molecular-scale spintronic devices.

cond-mat.mes-hall↗

arXiv2013

Electronic and Transport Properties of Molecular Junctions under a Finite Bias: A Dual Mean Field Approach

We show that when a molecular junction is under an external bias, its properties can not be uniquely determined by the total electron density in the same manner as the density functional theory (DFT) for ground state (GS) properties. In order to correctly incorporate bias-induced nonequilibrium effects, we present a dual mean field (DMF) approach. The key idea is that the total electron density together with the density of current-carrying electrons are sufficient to determine the properties of the system. Two mean fields, one for current-carrying electrons and the other one for equilibrium electrons can then be derived.By generalizing the Thomas-Fermi-Dirac (TFD) model to non-equilibrium cases, we analytically derived the DMF exchange energy density functional. We implemented the DMF approach into the computational package SIESTA to study non-equilibrium electron transport through molecular junctions. Calculations for a graphene nanoribbon (GNR) junction show that compared with the commonly used \textit{ab initio} transport theory, the DMF approach could significantly reduce the electric current at low biases due to the non-equilibrium corrections to the mean field potential in the scattering region.

cond-mat.mes-hall↗

arXiv2013

Massless Dirac Fermions in Graphene under an External Periodic Magnetic Field

By solving two-component spinor equation for massless Dirac Fermions, we show that graphene under a periodic external magnetic field exhibits a unique energy spectrum: At low energies, Dirac Fermions are localized inside the magnetic region with discrete Landau energy levels, while at higher energies, Dirac Fermions are mainly found in non-magnetic regions with continuous energy bands originating from wavefunctions analogous to particle-in-box states of electrons. These findings offer a new methodology for the control and tuning of massless Dirac Fermions in graphene.

cond-mat.mes-hall↗

arXiv2013

Bonding, Conductance and Magnetization of Oxygenated Au Nanowires

Spin-density-functional calculations of tip-suspended gold chains, with molecular oxygen, or dissociated oxygen atoms, incorporated in them, reveal structural transitions for varying lengths. The nanowires exhibit enhanced strength for both oxygen incorporation modes, and upon stretching tip atoms join the wire. With incorporated molecular oxygen the wire conductance is about 1(2e2/h), transforming to an insulating state beyond a critical length. The nanowire conductance with embedded oxygen atoms is low, 0.2 (2e2/h), and it develops magnetic moments localized on the oxygens and the neighboring Au atoms.

cond-mat.mes-hall↗

arXiv2013

Coherent electron transport through an azobenzene molecule: A light-driven molecular switch

We apply a first-principles computational approach to study a light-sensitive molecular switch. The molecule that comprises the switch can convert between a trans and a cis configuration upon photo-excitation. We find that the conductance of the two isomers vary dramatically, which suggests that this system has potential application as a molecular device. A detailed analysis of the band structure of the metal leads and the local density of states of the system reveals the mechanism of the switch.

cond-mat.mes-hall↗