Showing 1–33 of 33 results for author: Jing, L

Observation of electromagnons in a monolayer multiferroic

Authors: Mohammad Amini, Tiago V. C. Antão, Liwei Jing, Ziying Wang, Antti Karjasilta, Robert Drost, Shawulienu Kezilebieke, Jose L. Lado, Adolfo O. Fumega, Peter Liljeroth

Abstract: Van der Waals multiferroics have emerged as a promising platform to explore novel magnetoelectric phenomena. Recently, it has been shown that monolayer NiI$_2$ hosts robust type-II multiferroicity down to the two-dimensional limit, a giant dynamical magnetoelectric coupling at terahertz frequencies, and an electrically switchable spin polarization. These developments present the possibility of eng… ▽ More Van der Waals multiferroics have emerged as a promising platform to explore novel magnetoelectric phenomena. Recently, it has been shown that monolayer NiI$_2$ hosts robust type-II multiferroicity down to the two-dimensional limit, a giant dynamical magnetoelectric coupling at terahertz frequencies, and an electrically switchable spin polarization. These developments present the possibility of engineering ultrafast, low-energy-consumption, and electrically-tunable spintronic devices based on the collective excitations of the multiferroic order, electromagnons. However, the direct visualization of these bosonic modes in real space and within the monolayer limit remains elusive. Here, we report the atomic-scale observation of electromagnons in monolayer NiI$_2$ using low-temperature scanning tunneling microscopy. By tracking the thermal evolution of the multiferroic phase, we establish the energy scale and resolve coherent in-gap excitations of the symmetry-broken multiferroic state. Comparison with first-principles and spin-model calculations reveals that the low-energy modes originate from electromagnon excitations. Spatially resolved inelastic tunneling spectroscopy maps show a stripe-like modulation of the local spectral function at electromagnon energies, matching theoretical predictions. These results provide direct evidence of the internal structure of electromagnons and establish a methodology to probe these modes at the atomic scale, opening avenues for electrically tunable spintronics. △ Less

Submitted 31 July, 2026; v1 submitted 9 October, 2025; originally announced October 2025.

White-box machine learning for uncovering physically interpretable dimensionless governing equations for granular materials

Authors: Xu Han, Lu Jing, Chung-Yee Kwok, Gengchao Yang, Yuri Dumaresq Sobral

Abstract: Granular material has significant implications for industrial and geophysical processes. A long-lasting challenge, however, is seeking a unified rheology for its solid- and liquid-like behaviors under quasi-static, inertial, and even unsteady shear conditions. Here, we present a data-driven framework to discover the hidden governing equation of sheared granular materials. The framework, PINNSR-DA,… ▽ More Granular material has significant implications for industrial and geophysical processes. A long-lasting challenge, however, is seeking a unified rheology for its solid- and liquid-like behaviors under quasi-static, inertial, and even unsteady shear conditions. Here, we present a data-driven framework to discover the hidden governing equation of sheared granular materials. The framework, PINNSR-DA, addresses noisy discrete particle data via physics-informed neural networks with sparse regression (PINNSR) and ensures dimensional consistency via machine learning-based dimensional analysis (DA). Applying PINNSR-DA to our discrete element method simulations of oscillatory shear flow, a general differential equation is found to govern the effective friction across steady and transient states. The equation consists of three interpretable terms, accounting respectively for linear response, nonlinear response and energy dissipation of the granular system, and the coefficients depends primarily on a dimensionless relaxation time, which is shorter for stiffer particles and thicker flow layers. This work pioneers a pathway for discovering physically interpretable governing laws in granular systems and can be readily extended to more complex scenarios involving jamming, segregation, and fluid-particle interactions. △ Less

Submitted 28 September, 2025; v1 submitted 17 September, 2025; originally announced September 2025.

Comments: 24 pages, 6 figures, 1 table

Basal layer of granular flow down smooth and rough inclines: kinematics, slip laws and rheology

Authors: Teng Wang, Lu Jing, Fiona C. Y. Kwok, Yuri D. Sobral, Thomas Weinhart, Anthony R. Thornton

Abstract: Granular flow down an inclined plane is ubiquitous in geophysical and industrial applications. On rough inclines, the flow exhibits Bagnold's velocity profile and follows the so-called $μ(I)$ local rheology. On insufficiently rough or smooth inclines, however, velocity slip occurs at the bottom and a basal layer with strong agitation emerges below the bulk, which is not predicted by the local rheo… ▽ More Granular flow down an inclined plane is ubiquitous in geophysical and industrial applications. On rough inclines, the flow exhibits Bagnold's velocity profile and follows the so-called $μ(I)$ local rheology. On insufficiently rough or smooth inclines, however, velocity slip occurs at the bottom and a basal layer with strong agitation emerges below the bulk, which is not predicted by the local rheology. Here, we use discrete element method simulations to study detailed dynamics of the basal layer in granular flows down both smooth and rough inclines. We control the roughness via a dimensionless parameter, $R_a$, varied systematically from 0 (flat, frictional plane) to near 1 (very rough plane). Three flow regimes are identified: a slip regime ($R_a \lesssim 0.45$) where a dilated basal layer appears, a no-slip regime ($R_a \gtrsim 0.6$) and an intermediate transition regime. In the slip regime, the kinematics profiles (velocity, shear rate and granular temperature) of the basal layer strongly deviate from Bagnold's profiles. General basal slip laws are developed which express the slip velocity as a function of the local shear rate (or granular temperature), base roughness and slope angle. Moreover, the basal layer thickness is insensitive to flow conditions but depends somewhat on the inter-particle coefficient of restitution. Finally, we show that the rheological properties of the basal layer do not follow the $μ(I)$ rheology, but are captured by Bagnold's stress scaling and an extended kinetic theory for granular flows. Our findings can help develop more predictive granular flow models in the future. △ Less

Submitted 22 June, 2025; originally announced June 2025.

Journal ref: J. Fluid Mech. 1025 (2025) A27

Unified Flow Rule of Undeveloped and Fully Developed Dense Granular Flows Down Rough Inclines

Authors: Yanbin Wu, Thomas Pähtz, Zixiao Guo, Lu Jing, Zhao Duan, Zhiguo He

Abstract: We report on chute measurements of the free-surface velocity $v$ in dense flows of spheres and diverse sands and spheres-sand mixtures down rough inclines. These and previous measurements are inconsistent with standard flow rules, in which the Froude number $v/\sqrt{gh}$ scales linearly with $h/h_s$ or $(\tanθ/μ_r)^2h/h_s$, where $μ_r$ is the dynamic friction coefficient, $h$ the flow thickness, a… ▽ More We report on chute measurements of the free-surface velocity $v$ in dense flows of spheres and diverse sands and spheres-sand mixtures down rough inclines. These and previous measurements are inconsistent with standard flow rules, in which the Froude number $v/\sqrt{gh}$ scales linearly with $h/h_s$ or $(\tanθ/μ_r)^2h/h_s$, where $μ_r$ is the dynamic friction coefficient, $h$ the flow thickness, and $h_s(θ)$ its smallest value that permits a steady, uniform dense flow state at a given inclination angle $θ$. This is because the characteristic length $L$ a flow needs to fully develop can exceed the chute or travel length $l$ and because neither rule is universal for fully developed flows across granular materials. We use a dimensional analysis motivated by a recent unification of sediment transport to derive a flow rule that solves both problems in accordance with our and previous measurements: $v=v_\infty[1-\exp(-l/L)]^{1/2}$, with $v_\infty\proptoμ_r^{3/2}\left[(\tanθ-μ_r)h\right]^{4/3}$ and $L\proptoμ_r^3\left[(\tanθ-μ_r)h\right]^{5/3}h$. △ Less

Submitted 17 January, 2025; originally announced January 2025.

Journal ref: Physical Review Letters 134 (2), 028201 (2025)

Effects of particle elongation on dense granular flows down a rough inclined plane

Authors: Jixiong Liu, Lu Jing, Thomas Pähtz, Yifei Cui, Gordon G. D. Zhou, Xudong Fu

Abstract: Granular materials in nature are nearly always non-spherical, but particle shape effects in granular flow remain largely elusive. This study uses discrete element method simulations to investigate how elongated particle shapes affect the mobility of dense granular flows down a rough incline. For a range of systematically varied particle length-to-diameter aspect ratios (AR), we run simulations wit… ▽ More Granular materials in nature are nearly always non-spherical, but particle shape effects in granular flow remain largely elusive. This study uses discrete element method simulations to investigate how elongated particle shapes affect the mobility of dense granular flows down a rough incline. For a range of systematically varied particle length-to-diameter aspect ratios (AR), we run simulations with various flow thicknesses $h$ and slope angles $θ$ to extract the well-known $h_\textrm{stop}(θ)$ curves (below which the flow ceases) and the $Fr$-$h/h_\textrm{stop}$ relations following Pouliquen's approach, where $Fr=u/\sqrt{gh}$ is the Froude number, $u$ is the mean flow velocity, and $g$ is the gravitational acceleration. The slope $β$ of the $Fr$-$h/h_\textrm{stop}$ relations shows an intriguing S-shaped dependence on AR, with two plateaus at small and large AR, respectively, transitioning with a sharp increase. We understand this S-shaped dependence by examining statistics of particle orientation, alignment, and hindered rotation. We find that the rotation ability of weakly elongated particles ($\textrm{AR}\lesssim1.3$) remains similar to spheres, leading to the first plateau in the $β$-AR relation, whereas the effects of particle orientation saturates beyond $\textrm{AR}\approx2.0$, explaining the second plateau. An empirical sigmoidal function is proposed to capture this non-linear dependence. The findings are expected to enhance our understanding of how particle shape affects the flow of granular materials from both the flow- and particle-scale perspectives. △ Less

Submitted 17 January, 2025; originally announced January 2025.

Journal ref: Physical Review E 110 (4), 044902 (2024)

Granular segregation across flow geometries: a closure model for the particle segregation velocity

Authors: Yifei Duan, Lu Jing, Paul B. Umbanhowar, Julio M. Ottino, Richard M. Lueptow

Abstract: Predicting particle segregation has remained challenging due to the lack of a general model for the segregation velocity that is applicable across a range of granular flow geometries. Here, a segregation velocity model for dense granular flows is developed by exploiting momentum balance and recent advances in particle-scale modelling of the segregation driving and drag forces over a wide range of… ▽ More Predicting particle segregation has remained challenging due to the lack of a general model for the segregation velocity that is applicable across a range of granular flow geometries. Here, a segregation velocity model for dense granular flows is developed by exploiting momentum balance and recent advances in particle-scale modelling of the segregation driving and drag forces over a wide range of particle concentrations, size and density ratios, and flow conditions. This model is shown to correctly predict particle segregation velocity in a diverse set of idealized and natural granular flow geometries simulated using the discrete element method. When incorporated in the well-established advection-diffusion-segregation formulation, the model has the potential to accurately capture segregation phenomena in many relevant industrial application and geophysical settings. △ Less

Submitted 15 July, 2025; v1 submitted 10 October, 2024; originally announced October 2024.

Journal ref: J. Fluid Mech. 1016 (2025) A2

Strain-induced two-dimensional topological crystalline insulator

Authors: Liwei Jing, Mohammad Amini, Adolfo O. Fumega, Orlando J. Silveira, Jose L. Lado, Peter Liljeroth, Shawulienu Kezilebieke

Abstract: Topological crystalline insulators (TCIs) host topological phases of matter protected by crystal symmetries. Topological surface states in three-dimensional TCIs have been predicted and observed in IV-VI SnTe-class semiconductors. Despite the prediction of a two-dimensional (2D) TCI characterized by two pairs of edge states inside the bulk gap, materials challenges have thus far prevented its expe… ▽ More Topological crystalline insulators (TCIs) host topological phases of matter protected by crystal symmetries. Topological surface states in three-dimensional TCIs have been predicted and observed in IV-VI SnTe-class semiconductors. Despite the prediction of a two-dimensional (2D) TCI characterized by two pairs of edge states inside the bulk gap, materials challenges have thus far prevented its experimental realization. Here we report the growth and characterization of bilayer SnTe on the 2$H$-NbSe$_2$ substrate by molecular beam epitaxy and scanning tunneling microscopy. We experimentally observe two anticorrelated, periodically modulated pairs of conducting edge states along the perimeters of the sample with a large band gap exceeding $0.2$ eV. We identify these states with a 2D TCI through first principles calculations. Finally, we probe the coupling of adjacent topological edge states and demonstrate the resulting energy shift driven by a combination of electrostatic interactions and tunneling coupling. Our work opens the door to investigations of tunable topological states in 2D TCIs, of potential impact for spintronics and nanoelectronics applications at room temperature. △ Less

Submitted 9 October, 2024; originally announced October 2024.

Journal ref: Nat Commun 17, 817 (2026)

General model for segregation forces in flowing granular mixtures

Authors: Yifei Duan, Lu Jing, Paul B. Umbanhowar, Julio M. Ottino, Richard M. Lueptow

Abstract: Particle segregation in dense flowing size-disperse granular mixtures is driven by gravity and shear, but predicting the associated segregation force due to both effects has remained an unresolved challenge. Here, a model of the combined gravity- and kinematics-induced segregation force on a single intruder particle is integrated with a model of the concentration dependence of the gravity-induced… ▽ More Particle segregation in dense flowing size-disperse granular mixtures is driven by gravity and shear, but predicting the associated segregation force due to both effects has remained an unresolved challenge. Here, a model of the combined gravity- and kinematics-induced segregation force on a single intruder particle is integrated with a model of the concentration dependence of the gravity-induced segregation force. The result is a general model of the net particle segregation force in flowing size-bidisperse granular mixtures. Using discrete element method simulations for comparison, the model correctly predicts the segregation force for a variety of mixture concentrations and flow conditions in both idealized and natural shear flows. △ Less

Submitted 23 September, 2023; originally announced September 2023.

Journal ref: J. Fluid Mech. 989 (2024) A17

Noise-resistant quantum memory enabled by Hamiltonian engineering

Authors: Lei Jing, Peng Du, Hui Tang, Wenxian Zhang

Abstract: Nuclear spins in quantum dots are promising candidates for fast and scalable quantum memory. By utilizing the hyperfine interaction between the central electron and its surrounding nuclei, quantum information can be transferred to the collective state of the nuclei and be stored for a long time. However, nuclear spin fluctuations in a partially polarized nuclear bath deteriorate the quantum memory… ▽ More Nuclear spins in quantum dots are promising candidates for fast and scalable quantum memory. By utilizing the hyperfine interaction between the central electron and its surrounding nuclei, quantum information can be transferred to the collective state of the nuclei and be stored for a long time. However, nuclear spin fluctuations in a partially polarized nuclear bath deteriorate the quantum memory fidelity. Here we introduce a noise-resistant protocol to realize fast and high-fidelity quantum memory through Hamiltonian engineering. With analytics and numerics, we show that high-fidelity quantum state transfer between the electron and the nuclear spins is achievable at relatively low nuclear polarizations, due to the strong suppression of nuclear spin noises. For a realistic quantum dot with $10^4$ nuclear spins, a fidelity surpassing 80% is possible at a polarization as low as 30%. Our approach reduces the demand for high nuclear polarization, making experimentally realizing quantum memory in quantum dots more feasible. △ Less

Submitted 2 January, 2023; originally announced January 2023.

Comments: 6 pages, 3 figures

Optical-plug-assisted spin vortex in a $^{87}$Rb dipolar spinor Bose-Einstein condensate

Authors: Hui Tang, Peng Du, Lei Jing, Su Yi, Wenxian Zhang

Abstract: Generating a spin vortex in a $^{87}$Rb dipolar spinor Bose-Einstein condensate in a controllable way is still experimentally challenging. We propose an experimentally easy and tunable way to produce spin vortex by varying the potential barrier height and the width of an additionally applied optical plug. A topological phase transition occurs from the trivial single mode approximation phase to the… ▽ More Generating a spin vortex in a $^{87}$Rb dipolar spinor Bose-Einstein condensate in a controllable way is still experimentally challenging. We propose an experimentally easy and tunable way to produce spin vortex by varying the potential barrier height and the width of an additionally applied optical plug. A topological phase transition occurs from the trivial single mode approximation phase to the optical-plug-assisted-vortex one, as the barrier height increases and the width lies in an appropriate range. The optical plug causes radial density variation thus the spin vortex is favored by significantly lowering the intrinsic magnetic dipolar energy. A type of coreless spin vortex, different from the conventional polar core vortex, is predicted by our numerical results. Our proposal removes a major obstacle to investigate the topological phase transition in a $^{87}$Rb dipolar spinor BEC. △ Less

Submitted 3 June, 2022; originally announced June 2022.

Topogivity: A Machine-Learned Chemical Rule for Discovering Topological Materials

Authors: Andrew Ma, Yang Zhang, Thomas Christensen, Hoi Chun Po, Li Jing, Liang Fu, Marin Soljačić

Abstract: Topological materials present unconventional electronic properties that make them attractive for both basic science and next-generation technological applications. The majority of currently known topological materials have been discovered using methods that involve symmetry-based analysis of the quantum wavefunction. Here we use machine learning to develop a simple-to-use heuristic chemical rule t… ▽ More Topological materials present unconventional electronic properties that make them attractive for both basic science and next-generation technological applications. The majority of currently known topological materials have been discovered using methods that involve symmetry-based analysis of the quantum wavefunction. Here we use machine learning to develop a simple-to-use heuristic chemical rule that diagnoses with a high accuracy whether a material is topological using only its chemical formula. This heuristic rule is based on a notion that we term topogivity, a machine-learned numerical value for each element that loosely captures its tendency to form topological materials. We next implement a high-throughput procedure for discovering topological materials based on the heuristic topogivity-rule prediction followed by ab initio validation. This way, we discover new topological materials that are not diagnosable using symmetry indicators, including several that may be promising for experimental observation. △ Less

Submitted 23 January, 2023; v1 submitted 10 February, 2022; originally announced February 2022.

Comments: Main text: 6 pages, 3 figures; supplementary materials: 43 pages, 62 figures, 5 tables

Journal ref: Nano Lett. 2023, 23, 3, 772-778

Segregation forces in dense granular flows: Closing the gap from single intruders to mixtures

Authors: Yifei Duan, Lu Jing, Paul B. Umbanhowar, Julio M. Ottino, Richard M. Lueptow

Abstract: Using simulations and a virtual-spring-based approach, we measure the segregation force, Fseg, over a range of size-bidisperse mixture concentrations, particle size ratios, and shear rates to develop a model for Fseg that extends its applicability from the well-studied non-interacting intruders regime to finite-concentration mixtures where cooperative phenomena occur. The model predicts the concen… ▽ More Using simulations and a virtual-spring-based approach, we measure the segregation force, Fseg, over a range of size-bidisperse mixture concentrations, particle size ratios, and shear rates to develop a model for Fseg that extends its applicability from the well-studied non-interacting intruders regime to finite-concentration mixtures where cooperative phenomena occur. The model predicts the concentration below which the single intruder assumption applies and provides an accurate description of the pressure partitioning between species. △ Less

Submitted 25 August, 2021; originally announced August 2021.

Journal ref: J. Fluid Mech. (2022), vol. 935, R1

A unified description of gravity- and kinematics-induced segregation forces in dense granular flows

Authors: Lu Jing, Julio M. Ottino, Richard M. Lueptow, Paul B. Umbanhowar

Abstract: Particle segregation is common in natural and industrial processes involving flowing granular materials. Complex, and seemingly contradictory, segregation phenomena have been observed for different boundary conditions and forcing. Using discrete element method simulations, we show that segregation of a single particle intruder can be described in a unified manner across different flow configuratio… ▽ More Particle segregation is common in natural and industrial processes involving flowing granular materials. Complex, and seemingly contradictory, segregation phenomena have been observed for different boundary conditions and forcing. Using discrete element method simulations, we show that segregation of a single particle intruder can be described in a unified manner across different flow configurations. A scaling relation for the net segregation force is obtained by measuring forces on an intruder particle in controlled-velocity flows where gravity and flow kinematics are varied independently. The scaling law consists of two additive terms: a buoyancy-like gravity-induced pressure gradient term and a shear rate gradient term, both of which depend on the particle size ratio. The shear rate gradient term reflects a kinematics-driven mechanism whereby larger (smaller) intruders are pushed toward higher (lower) shear rate regions. The scaling is validated, without refitting, in wall-driven flows, inclined wall-driven flows, vertical silo flows, and free surface flows down inclines. Comparing the segregation force to the intruder weight results in predictions of the segregation direction that match experimental and computational results for various flow configurations. △ Less

Submitted 16 March, 2021; originally announced March 2021.

Modified Archimedes' principle predicts rising and sinking of intruders in sheared granular flows

Authors: Lu Jing, Julio M. Ottino, Richard M. Lueptow, Paul B. Umbanhowar

Abstract: We computationally determine the force on single spherical intruder particles in sheared granular flows as a function of particle size, particle density, shear rate, overburden pressure, and gravitational acceleration. The force scales similarly to, but deviates from, the buoyancy force predicted by Archimedes' principle. The deviation depends only on the intruder to bed particle size ratio, but n… ▽ More We computationally determine the force on single spherical intruder particles in sheared granular flows as a function of particle size, particle density, shear rate, overburden pressure, and gravitational acceleration. The force scales similarly to, but deviates from, the buoyancy force predicted by Archimedes' principle. The deviation depends only on the intruder to bed particle size ratio, but not the density ratio or flow conditions. We propose a simple force model that successfully predicts whether intruders rise or sink, knowing only the size and density ratios, for a variety of flow configurations in physical experiments. △ Less

Submitted 7 June, 2019; originally announced June 2019.

Journal ref: Phys. Rev. Research 2, 022069 (2020)

Characterization of base roughness for granular chute flows

Authors: Lu Jing, C. Y. Kwok, Y. F. Leung, Y. D. Sobral

Abstract: Base roughness plays an important role to the dynamics of granular flows but is yet poorly understood due to the difficulty of its quantification. For a bumpy base made by spheres, at least two factors should be considered to characterize its geometric roughness, namely the size ratio of base- to flow-particles and the packing of base particles. In this paper, we propose a definition of base rough… ▽ More Base roughness plays an important role to the dynamics of granular flows but is yet poorly understood due to the difficulty of its quantification. For a bumpy base made by spheres, at least two factors should be considered to characterize its geometric roughness, namely the size ratio of base- to flow-particles and the packing of base particles. In this paper, we propose a definition of base roughness, Ra, which is a function of both the size ratio and the packing arrangement of base particles. The function is generalized for random and regular packing of multi-layered spheres, where the range of possible values of Ra is studied, along with the optimal values to create maximum base roughness. The new definition is applied to granular flows down chute in both two- and three-dimensional configurations. It is proven to be a good indicator of slip condi- tion, and a transition occurs from slip to non-slip condition as Ra increases. Critical values of Ra are identified for the construction of a non-slip base. The effects of contact parameters on base velocity are studied, and it is shown that while the coefficient of friction is less influential, normal damping has more profound effect on base velocity at lower values of Ra. The application of present definition to other base geometries is also discussed. △ Less

Submitted 11 June, 2016; originally announced June 2016.

Comments: 18 figures

Squeeze expulsion in grain size segregation

Authors: Lu Jing, C. Y. Kwok, Y. F. Leung

Abstract: Grain segregation occurs under various conditions, such as vibration, shear and mixing. In the gravity-driven shear flow, size segregation is triggered by the percolation of small particles through the opened voids (kinetic sieving), and large particles are accumulated in the upper layer. One hypothesis for the upward migration of large particles is the squeeze expulsion mechanism, which remains y… ▽ More Grain segregation occurs under various conditions, such as vibration, shear and mixing. In the gravity-driven shear flow, size segregation is triggered by the percolation of small particles through the opened voids (kinetic sieving), and large particles are accumulated in the upper layer. One hypothesis for the upward migration of large particles is the squeeze expulsion mechanism, which remains yet ambiguous due to limited evidences from either physical or numerical experiments. Here we show statistically how the percolation of small particles facilitates the upward movement of large particles. We found that in large particles, the mechanical anisotropy (strong force network) coincides with the geometric anisotropy (contact network), which indicates squeeze, and the connectivity of large particles is much higher when they are squeezed through small particles. The presence of small particles filling the voids hinders the downward movements and provides dynamic steps for the climbing large particles. Furthermore, increasing the coefficient of friction promotes rotation and empowers more large particles to reach the top layer, implying that the climbing particles tend to rotate relative to their neighbors. Our findings of the microstructure and movement pattern of individual particles add new evidences to the mechanism of squeeze expulsion and provide new perspective for the study of segregation. △ Less

Submitted 25 May, 2016; originally announced May 2016.

Comments: 5 figures

Topologically protected edge gap solitons of interacting Bosons in one-dimensional superlattices

Authors: Z. Z. Wang, C. S. Liu, T. F. Xu, L. H. Wen, X. L. Jing, Y. Guo

Abstract: We comprehensively investigate the nontrivial states of interacting Bose system in one-dimensional optical superlattices under the open boundary condition. Our results show that there exists a kind of stable localized states: edge gap solitons. We argue that the states originate from the eigenstates of independent edge parabolas. In particular, the edge gap solitons exhibit a nonzero topological i… ▽ More We comprehensively investigate the nontrivial states of interacting Bose system in one-dimensional optical superlattices under the open boundary condition. Our results show that there exists a kind of stable localized states: edge gap solitons. We argue that the states originate from the eigenstates of independent edge parabolas. In particular, the edge gap solitons exhibit a nonzero topological invariant. The topological nature is due to the connection of the present model to the quantized adiabatic particle transport problem. In addition, the composition relations between the gap solitons and the extend states under the open boundary condition are discussed. △ Less

Submitted 16 March, 2016; v1 submitted 15 March, 2016; originally announced March 2016.

Comments: This paper has been withdrawn by the author since some paragraphs have to be rewriteen

Pressure-induced phase transitions of multiferroic BiFeO3

Authors: Zhang XiaoLi, Wu Ye, Zhang Qian, Dong JunCai, Wu Xiang, Liu Jing, Wu ZiYu, Chen DongLiang

Abstract: Pressure-induced phase transitions of multiferroic BiFeO3 have been investigated using synchrotron radiation X-ray diffraction with diamond anvil cell technique at room temperature. Present experimental data clearly show that rhombohedral (R3c) phase of BiFeO3 first transforms to monoclinic (C2/m) phase at 7 GPa, then to orthorhombic (Pnma) phase at 11 GPa, which is consistent with recent theoreti… ▽ More Pressure-induced phase transitions of multiferroic BiFeO3 have been investigated using synchrotron radiation X-ray diffraction with diamond anvil cell technique at room temperature. Present experimental data clearly show that rhombohedral (R3c) phase of BiFeO3 first transforms to monoclinic (C2/m) phase at 7 GPa, then to orthorhombic (Pnma) phase at 11 GPa, which is consistent with recent theoretical ab initio calculation. However, we observe another peak at 2θ=7° in the pressure range of 5-7 GPa that has not been reported previously. Further analysis reveals that this reflection peak is attributed to the orthorhombic (Pbam) phase, indicating the coexisting of monoclinic phase with orthorhombic phase in low pressure range. △ Less

Submitted 28 August, 2013; originally announced August 2013.

Comments: 10pages,7figures

Transport Measurement of Landau level Gaps in Bilayer Graphene

Authors: J. Velasco Jr., Y. Lee, Z. Zhao, Lei Jing, P. Kratz, Marc Bockrath, C. N. Lau

Abstract: Landau level gaps are important parameters for understanding electronic interactions and symmetry-broken processes in bilayer graphene (BLG). Here we present transport spectroscopy measurements of LL gaps in double-gated suspended BLG with high mobilities in the quantum Hall regime. By using bias as a spectroscopic tool, we measure the gap Δ for the quantum Hall (QH) state at filling factor ν={\pm… ▽ More Landau level gaps are important parameters for understanding electronic interactions and symmetry-broken processes in bilayer graphene (BLG). Here we present transport spectroscopy measurements of LL gaps in double-gated suspended BLG with high mobilities in the quantum Hall regime. By using bias as a spectroscopic tool, we measure the gap Δ for the quantum Hall (QH) state at filling factor ν={\pm}4 and -2. The single-particle gap for ν=4 scales linearly with magnetic field B and is independent of the out-of-plane electric field E. For the symmetry-broken ν=-2 state, the measured values of gap are 1.1 meV/T and 0.17 meV/T for singly-gated geometry and dual-gated geometry at E=0, respectively. The difference between the two values arises from the E-dependence of the gap, suggesting that the ν=-2 state is layer polarized. Our studies provide the first measurements of the gaps of the broken symmetry QH states in BLG with well-controlled E, and establish a robust method that can be implemented for studying similar states in other layered materials. △ Less

Submitted 14 January, 2014; v1 submitted 14 March, 2013; originally announced March 2013.

Comments: revised figures and discussion

Journal ref: Nano Letters, 14, 1324 (2014)

Broken Symmetry Quantum Hall States in Dual Gated ABA Trilayer Graphene

Authors: Yongjin Lee, Jairo Velasco Jr, David Tran, Fan Zhang, Wenzhong Bao, Lei Jing, Kevin Myhro, Dmitry Smirnov, Chun Ning Lau

Abstract: We present low temperature transport measurements on dual-gated suspended trilayer graphene in the quantum Hall (QH) regime. We observe QH plateaus at filling factors ν=-8, -2, 2, 6, and 10, in agreement with the full-parameter tight binding calculations. In high magnetic fields, odd-integer plateaus are also resolved, indicating almost complete lifting of the 12-fold degeneracy of the lowest Land… ▽ More We present low temperature transport measurements on dual-gated suspended trilayer graphene in the quantum Hall (QH) regime. We observe QH plateaus at filling factors ν=-8, -2, 2, 6, and 10, in agreement with the full-parameter tight binding calculations. In high magnetic fields, odd-integer plateaus are also resolved, indicating almost complete lifting of the 12-fold degeneracy of the lowest Landau levels (LL). Under an out-of-plane electric field E, we observe degeneracy breaking and transitions between QH plateaus. Interestingly, depending on its direction, E selectively breaks the LL degeneracies in the electron-doped or hole-doped regimes. Our results underscore the rich interaction-induced phenomena in trilayer graphene. △ Less

Submitted 29 October, 2012; v1 submitted 24 October, 2012; originally announced October 2012.

Quantum transport in double-gated graphene devices

Authors: J. Velasco Jr., Y. Lee, L. Jing, G. Liu, W. Bao, C. N. Lau

Abstract: Double-gated graphene devices provide an important platform for understanding electrical and optical properties of graphene. Here we present transport measurements of single layer, bilayer and trilayer graphene devices with suspended top gates. In zero magnetic fields, we observe formation of pnp junctions with tunable polarity and charge densities, as well as a tunable band gap in bilayer graphen… ▽ More Double-gated graphene devices provide an important platform for understanding electrical and optical properties of graphene. Here we present transport measurements of single layer, bilayer and trilayer graphene devices with suspended top gates. In zero magnetic fields, we observe formation of pnp junctions with tunable polarity and charge densities, as well as a tunable band gap in bilayer graphene and a tunable band overlap in trilayer graphene. In high magnetic fields, the devices' conductance are quantized at integer and fractional values of conductance quantum, and the data are in good agreement with a model based on edge state equilibration at pn interfaces. △ Less

Submitted 24 July, 2012; originally announced July 2012.

Journal ref: Solid State Communications, 152, 1301 (2012)

Visualizing Electrical Breakdown and ON/OFF States in Electrically Switchable Suspended Graphene Break Junctions

Authors: Hang Zhang, Wenzhong Bao, Zeng Zhao, Jhao-Wun Huang, Brian Standley, Gang Liu, Fenglin Wang, Philip Kratz, Lei Jing, Marc Bockrath, Chun Ning Lau

Abstract: Narrow gaps are formed in suspended single to few layer graphene devices using a pulsed electrical breakdown technique. The conductance of the resulting devices can be programmed by the application of voltage pulses, with a voltage of 2.5V~4.5V corresponding to an ON pulse and voltages ~8V corresponding to OFF pulses. Electron microscope imaging of the devices shows that the graphene sheets typica… ▽ More Narrow gaps are formed in suspended single to few layer graphene devices using a pulsed electrical breakdown technique. The conductance of the resulting devices can be programmed by the application of voltage pulses, with a voltage of 2.5V~4.5V corresponding to an ON pulse and voltages ~8V corresponding to OFF pulses. Electron microscope imaging of the devices shows that the graphene sheets typically remain suspended and that the device conductance tends to zero when the observed gap is large. The switching rate is strongly temperature dependent, which rules out a purely electromechanical switching mechanism. This observed switching in suspended graphene devices strongly suggests a switching mechanism via atomic movement and/or chemical rearrangement, and underscores the potential of all-carbon devices for integration with graphene electronics. △ Less

Submitted 1 May, 2012; originally announced May 2012.

Comments: in situ SEM videos are available on Nano letters website

Journal ref: Nano Lett., 2012, 12 (4), pp 1772-1775

Interplay between periodicity and nonlinearity of indirect excitons in coupled quantum wells

Authors: T. F. Xu, X. L. Jing, H. G. Luo, W. C. Wu, C. S. Liu

Abstract: Inspired by a recent experiment of localization-delocalization transition (LDT) of indirect excitons in lateral electrostatic lattices [M. Remeika \textit{et al.}, Phys. Rev. Lett. \textbf{102}, 186803 (2009)], we investigate the interplay between periodic potential and nonlinear interactions of indirect excitons in coupled quantum wells. It is shown that the model involving both attractive two-bo… ▽ More Inspired by a recent experiment of localization-delocalization transition (LDT) of indirect excitons in lateral electrostatic lattices [M. Remeika \textit{et al.}, Phys. Rev. Lett. \textbf{102}, 186803 (2009)], we investigate the interplay between periodic potential and nonlinear interactions of indirect excitons in coupled quantum wells. It is shown that the model involving both attractive two-body and repulsive three-body interactions can lead to a natural account for the LDT of excitons across the lattice when reducing lattice amplitude or increasing particle density. In addition, the observations that the smooth component of the photoluminescent energy increases with increasing exciton density and exciton interaction energy is close to the lattice amplitude at the transition are also qualitatively explained. Our model provides an alternative way for understanding the underlying physics of the exciton dynamics in lattice potential wells. △ Less

Submitted 1 March, 2012; originally announced March 2012.

Comments: 5 pages, 3 figures

Minimum Conductivity and Evidence for Phase Transitions in Ultra-clean Bilayer Graphene

Authors: Wenzhong Bao, Jairo Velasco Jr, Fan Zhang, Lei Jing, Brian Standley, Dmitry Smirnov, Marc Bockrath, Allan MacDonald, Chun Ning Lau

Abstract: Bilayer graphene (BLG) at the charge neutrality point (CNP) is strongly susceptible to electronic interactions, and expected to undergo a phase transition into a state with spontaneous broken symmetries. By systematically investigating a large number of singly- and doubly-gated bilayer graphene (BLG) devices, we show that an insulating state appears only in devices with high mobility and low extri… ▽ More Bilayer graphene (BLG) at the charge neutrality point (CNP) is strongly susceptible to electronic interactions, and expected to undergo a phase transition into a state with spontaneous broken symmetries. By systematically investigating a large number of singly- and doubly-gated bilayer graphene (BLG) devices, we show that an insulating state appears only in devices with high mobility and low extrinsic doping. This insulating state has an associated transition temperature Tc~5K and an energy gap of ~3 meV, thus strongly suggesting a gapped broken symmetry state that is destroyed by very weak disorder. The transition to the intrinsic broken symmetry state can be tuned by disorder, out-of-plane electric field, or carrier density. △ Less

Submitted 15 February, 2012; originally announced February 2012.

Journal ref: Proc. Nat. Acad. Sci., 109, 10802 (2012)

Gap solitons of a super-Tonks-Girardeau gas in a one-dimensional periodic potential

Authors: T. F. Xu, X. L. Jing, H. G. Luo, C. S. Liu

Abstract: We study the stability of gap solitons of the super-Tonks-Girardeau bosonic gas in one-dimensional periodic potential. The linear stability analysis indicates that increasing the amplitude of periodic potential or decreasing the nonlinear interactions, the unstable gap solitons can become stable. In particular, the theoretical analysis and numerical calculations show that, comparing to the lower-f… ▽ More We study the stability of gap solitons of the super-Tonks-Girardeau bosonic gas in one-dimensional periodic potential. The linear stability analysis indicates that increasing the amplitude of periodic potential or decreasing the nonlinear interactions, the unstable gap solitons can become stable. In particular, the theoretical analysis and numerical calculations show that, comparing to the lower-family of gap solitons, the higher-family of gap solitons are easy to form near the bottoms of the linear Bloch band gaps. The numerical results also verify that the composition relations between various gap solitons and nonlinear Bloch waves are general and can exist in the super-Tonks-Girardeau phase. △ Less

Submitted 1 February, 2012; originally announced February 2012.

Comments: 7 pages,6 figures

MSC Class: J.2 ACM Class: F.2.1; G.1.8

Transport Spectroscopy of Symmetry-Broken Insulating States in Bilayer Graphene

Authors: J. Velasco Jr., L. Jing, W. Bao, Y. Lee, P. Kratz, V. Aji, M. Bockrath, C. N. Lau, C. Varma, R. Stillwell, D. Smirnov, Fan Zhang, J. Jung, A. H. MacDonald

Abstract: The flat bands in bilayer graphene(BLG) are sensitive to electric fields E\bot directed between the layers, and magnify the electron-electron interaction effects, thus making BLG an attractive platform for new two-dimensional (2D) electron physics[1-5]. Theories[6-16] have suggested the possibility of a variety of interesting broken symmetry states, some characterized by spontaneous mass gaps, whe… ▽ More The flat bands in bilayer graphene(BLG) are sensitive to electric fields E\bot directed between the layers, and magnify the electron-electron interaction effects, thus making BLG an attractive platform for new two-dimensional (2D) electron physics[1-5]. Theories[6-16] have suggested the possibility of a variety of interesting broken symmetry states, some characterized by spontaneous mass gaps, when the electron-density is at the carrier neutrality point (CNP). The theoretically proposed gaps[6,7,10] in bilayer graphene are analogous[17,18] to the masses generated by broken symmetries in particle physics and give rise to large momentum-space Berry curvatures[8,19] accompanied by spontaneous quantum Hall effects[7-9]. Though recent experiments[20-23] have provided convincing evidence of strong electronic correlations near the CNP in BLG, the presence of gaps is difficult to establish because of the lack of direct spectroscopic measurements. Here we present transport measurements in ultra-clean double-gated BLG, using source-drain bias as a spectroscopic tool to resolve a gap of ~2 meV at the CNP. The gap can be closed by an electric field E\bot \sim13 mV/nm but increases monotonically with a magnetic field B, with an apparent particle-hole asymmetry above the gap, thus providing the first mapping of the ground states in BLG. △ Less

Submitted 8 August, 2011; originally announced August 2011.

Comments: 4 figures

Journal ref: Nature Nanotechnology, 7, 156 (2012)

Stacking-Dependent Band Gap and Quantum Transport in Trilayer Graphene

Authors: W. Bao, L. Jing, Y. Lee, J. Velasco Jr., P. Kratz, D. Tran, B. Standley, M. Aykol, S. B. Cronin, D. Smirnov, M. Koshino, E. McCann, M. Bockrath, C. N. Lau

Abstract: In a multi-layer electronic system, stacking order provides a rarely-explored degree of freedom for tuning its electronic properties. Here we demonstrate the dramatically different transport properties in trilayer graphene (TLG) with different stacking orders. At the Dirac point, ABA-stacked TLG remains metallic while the ABC counterpart becomes insulating. The latter exhibits a gap-like dI/dV cha… ▽ More In a multi-layer electronic system, stacking order provides a rarely-explored degree of freedom for tuning its electronic properties. Here we demonstrate the dramatically different transport properties in trilayer graphene (TLG) with different stacking orders. At the Dirac point, ABA-stacked TLG remains metallic while the ABC counterpart becomes insulating. The latter exhibits a gap-like dI/dV characteristics at low temperature and thermally activated conduction at higher temperatures, indicating an intrinsic gap ~6 meV. In magnetic fields, in addition to an insulating state at filling factor ν=0, ABC TLG exhibits quantum Hall plateaus at ν=-30, \pm 18, \pm 9, each of which splits into 3 branches at higher fields. Such splittings are signatures of the Lifshitz transition induced by trigonal warping, found only in ABC TLG, and in semi-quantitative agreement with theory. Our results underscore the rich interaction-induced phenomena in trilayer graphene with different stacking orders, and its potential towards electronic applications. △ Less

Submitted 20 January, 2012; v1 submitted 31 March, 2011; originally announced March 2011.

Comments: minor revision; published version

Journal ref: Nature Physics 7, 948--952 (2011)

Gap solitons and Bloch waves of interacting bosons in one-dimensional optical lattices: From the weak to the strong interaction limits

Authors: T. F. Xu, X. M. Guo, X. L. Jing, W. C. Wu, C. S. Liu

Abstract: We study the gap solitons and nonlinear Bloch waves of interacting bosons in one-dimensional optical lattices, taking into account the interaction from the weak to the strong limits. It is shown that composition relation between the gap solitons and nonlinear Bloch waves exists for the whole span of the interaction strength. The linear stability analysis indicates that the gap solitons are stable… ▽ More We study the gap solitons and nonlinear Bloch waves of interacting bosons in one-dimensional optical lattices, taking into account the interaction from the weak to the strong limits. It is shown that composition relation between the gap solitons and nonlinear Bloch waves exists for the whole span of the interaction strength. The linear stability analysis indicates that the gap solitons are stable when their energies are near the bottom of the linear Bloch band gap. By increasing the interaction strength, the stable gap solitons can turn into unstable. It is argued that the stable gap solitons can easily be formed in a weakly interacting system with energies near the bottoms of the lower-level linear Bloch band gaps. △ Less

Submitted 6 March, 2011; originally announced March 2011.

Comments: 8 papges, 7 figures

Suspension and Measurement of Graphene and Bi2Se3 Atomic Membranes

Authors: Jairo Velasco Jr., Zeng Zhao, Hang Zhang, Fenglin Wang, Zhiyong Wang, Philip Kratz, Lei Jing, Wenzhong Bao, Jing Shi, Chun Ning Lau

Abstract: Coupling high quality, suspended atomic membranes to specialized electrodes enables investigation of many novel phenomena, such as spin or Cooper pair transport in these two dimensional systems. However, many electrode materials are not stable in acids that are used to dissolve underlying substrates. Here we present a versatile and powerful multi-level lithographical technique to suspend atomic me… ▽ More Coupling high quality, suspended atomic membranes to specialized electrodes enables investigation of many novel phenomena, such as spin or Cooper pair transport in these two dimensional systems. However, many electrode materials are not stable in acids that are used to dissolve underlying substrates. Here we present a versatile and powerful multi-level lithographical technique to suspend atomic membranes, which can be applied to the vast majority of substrate, membrane and electrode materials. Using this technique, we fabricated suspended graphene devices with Al electrodes and mobility of 5500 cm^2/Vs. We also demonstrate, for the first time, fabrication and measurement of a free-standing thin Bi2Se3 membrane, which has low contact resistance to electrodes and a mobility of >~500 cm^2/Vs. △ Less

Submitted 12 November, 2010; originally announced November 2010.

Journal ref: Nanotechnology 22, 285305 (2011)

Quantum Transport and Field Induced Insulating States in Bilayer Graphene pnp Junctions

Authors: Lei Jing, Jairo Velasco Jr., Philip Kratz, Gang Liu, Wenzhong Bao, Marc Bockrath, Chun Ning Lau

Abstract: We perform transport measurements in high quality bilayer graphene pnp junctions with suspended top gates. At a magnetic field B=0, we demonstrate band gap opening by an applied perpendicular electric field, with an On/Off ratio up to 20,000 at 260mK. Within the band gap, the conductance decreases exponentially by 3 orders of magnitude with increasing electric field, and can be accounted for by va… ▽ More We perform transport measurements in high quality bilayer graphene pnp junctions with suspended top gates. At a magnetic field B=0, we demonstrate band gap opening by an applied perpendicular electric field, with an On/Off ratio up to 20,000 at 260mK. Within the band gap, the conductance decreases exponentially by 3 orders of magnitude with increasing electric field, and can be accounted for by variable range hopping with a gate-tunable density of states, effective mass, and localization length. At large B, we observe quantum Hall conductance with fractional values, which arise from equilibration of edge states between differentially-doped regions, and the presence of an insulating state at filling factor ν=0. Our work underscores the importance of bilayer graphene for both fundamental interest and technological applications. △ Less

Submitted 23 September, 2010; v1 submitted 23 September, 2010; originally announced September 2010.

Comments: 4 figures, to appear in Nano Lett. Minor typos corrected

Magnetoconductance Oscillations in High-Mobility Suspended Bilayer and Trilayer Graphene

Authors: Wenzhong Bao, Zeng Zhao, Hang Zhang, Gang Liu, Philip Kratz, Lei Jing, Jairo Velasco Jr, Dmitry Smirnov, Chun Ning Lau

Abstract: We report pronounced magnetoconductance oscillations observed on suspended bilayer and trilayer graphene devices with mobilities up to 270,000 cm2/Vs. For bilayer devices, we observe conductance minima at all integer filling factors nu between 0 and -8, as well as a small plateau at ν=1/3. For trilayer devices, we observe features at nu=-1, -2, -3 and -4, and at ν~0.5 that persist to 4.5K at B=8T.… ▽ More We report pronounced magnetoconductance oscillations observed on suspended bilayer and trilayer graphene devices with mobilities up to 270,000 cm2/Vs. For bilayer devices, we observe conductance minima at all integer filling factors nu between 0 and -8, as well as a small plateau at ν=1/3. For trilayer devices, we observe features at nu=-1, -2, -3 and -4, and at ν~0.5 that persist to 4.5K at B=8T. All of these features persist for all accessible values of Vg and B, and could suggest the onset of symmetry breaking of the first few Landau (LL) levels and fractional quantum Hall states. △ Less

Submitted 30 November, 2010; v1 submitted 30 April, 2010; originally announced May 2010.

Comments: to appear in Phys. Rev. Lett

Journal ref: PRL 105, 246601 (2010)

Probing Charging and Localization in the Quantum Hall Regime by Graphene pnp Junctions

Authors: Jairo Velasco Jr., Gang Liu, Lei Jing, Philip Kratz, Wenzhong Bao, Marc Bockrath, Chun Ning Lau

Abstract: Using high quality graphene pnp junctions, we observe prominent conductance fluctuations on transitions between quantum Hall (QH) plateaus as the top gate voltage Vtg is varied. In the Vtg-B plane, the fluctuations form crisscrossing lines that are parallel to those of the adjacent plateaus, with different temperature dependences for the conductance peaks and valleys. These fluctuations arise fr… ▽ More Using high quality graphene pnp junctions, we observe prominent conductance fluctuations on transitions between quantum Hall (QH) plateaus as the top gate voltage Vtg is varied. In the Vtg-B plane, the fluctuations form crisscrossing lines that are parallel to those of the adjacent plateaus, with different temperature dependences for the conductance peaks and valleys. These fluctuations arise from Coulomb-induced charging of electron- or hole-doped localized states when the device bulk is delocalized, underscoring the importance of electronic interactions in graphene in the QH regime. △ Less

Submitted 16 December, 2009; originally announced December 2009.

Average Density of States in Disordered Graphene systems

Authors: Shangduan Wu, Lei Jing, Qunxiang Li, Q. W. Shi, Jie Chen, Xiaoping Wang, Jinlong Yang

Abstract: In this paper, the average density of states (ADOS) with a binary alloy disorder in disordered graphene systems are calculated based on the recursion method. We observe an obvious resonant peak caused by interactions with surrounding impurities and an anti-resonance dip in ADOS curves near the Dirac point. We also find that the resonance energy (Er) and the dip position are sensitive to the conc… ▽ More In this paper, the average density of states (ADOS) with a binary alloy disorder in disordered graphene systems are calculated based on the recursion method. We observe an obvious resonant peak caused by interactions with surrounding impurities and an anti-resonance dip in ADOS curves near the Dirac point. We also find that the resonance energy (Er) and the dip position are sensitive to the concentration of disorders (x) and their on-site potentials (v). An linear relation, not only holds when the impurity concentration is low but this relation can be further extended to high impurity concentration regime with certain constraints. We also calculate the ADOS with a finite density of vacancies and compare our results with the previous theoretical results. △ Less

Submitted 7 November, 2007; originally announced November 2007.

Comments: 10 pages, 8 figures