Ping Sheng

The Hong Kong University of Science and Technology, Chiu-lung, Kowloon City, Hong Kong

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Publications (360)1470.06 Total impact

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    ABSTRACT: By employing a metal-coated central platelet and a rigid mesh electrode which is transparent to acoustic wave, we show that the membrane-type acoustic metamaterials (MAMs) can be easily tuned by applying an external voltage. With static voltage, the MAM's eigenfrequencies and therefore the phase of the transmitted wave are tunable up to 70 Hz. The MAM's vibration can be significantly suppressed or enhanced by using phase-matched AC voltage. Functionalities such as phase modulation and acoustic switch with on/off ratio up to 21.3 dB are demonstrated.
    Applied Physics Letters 03/2015; 106:091904. DOI:10.1063/1.4913999] · 3.52 Impact Factor
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    ABSTRACT: Bilayer graphene, unlike monolayer graphene, provides a tunable electronic energy gap when an electrical field is applied across the layers, therefore holds great potential in semiconductor industry. Here, we demonstrate a facile technique to obtain bilayer graphene structures on the growth substrate by controlling oxidation to remove monolayers, while retaining the bilayer electronic properties. We found that this precise oxidation process selectively destructs monolayers while preserves the qualities of bilayers, evidenced by the expected quantum hall effect and exceptional room temperature carrier mobilities of ~3500 cm2 v−1 s−1 obtained from electrical transport measurement. In addition, visualization of bilayers, which serves as nuclei for graphene growth, opens the door for the understanding the actual mechanism of graphene growth process which eventually can lead to the optimized synthesis.
    RSC Advances 03/2015; 5(32). DOI:10.1039/C5RA00865D · 3.71 Impact Factor
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    ABSTRACT: Decorated membrane, comprising a thin layer of elastic film with small rigid platelets fixed on top, has been found to be an efficient absorber of low frequency sound. In this work we consider the problem of sound absorption from a perspective aimed at deriving upper bounds under different scenarios, i.e., whether the sound is incident from one side only or from both sides, and whether there is a reflecting surface on the back side of the membrane. By considering the negligible thickness of the membrane, usually on the order of a fraction of one millimeter, we derive a relation showing that the sum of the incoming sound waves' (complex) pressure amplitudes, averaged over the area of the membrane, must be equal to that of the outgoing waves. By using this relation, and without going to any details of the wave solutions, it is shown that the maximum absorption achievable from one-side incident is 50%, while the maximum absorption with a back reflecting surface can reach 100%. The latter was attained by the hybridized resonances. All the results are shown to be in excellent agreement with the experiments. This generalized perspective, when used together with the Green function formalism, can be useful in gaining insights and delineating the constraints on what are achievable in scatterings and absorption by thin film structures.
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    ABSTRACT: The geometric phase that specifically characterizes the topological property of bulk bands in one-dimensional periodic systems is known as the Zak phase. Recently, it has been found that topological notions can also characterize the topological phase of mechanical isostatic lattices. Here, we present a theoretical framework and two experimental methods to determine the Zak phase in a periodic acoustic system. We constructed a phononic crystal with a topological transition point in the acoustic band structure where the band inverts and the Zak phase in the bulk band changes following a shift in system parameters. As a consequence, the topological characteristics of the bandgap change and interface states form at the boundary separating two phononic crystals having diierent bandgap topological characteristics. Such acoustic interface states with large sound intensity enhancement are observed at the phononic crystal interfaces. We use a simple photonic crystal system to demonstrate geometric phase effects and the existence of topological transition points in acoustic systems.
    Nature Physics 02/2015; 11(3):240. DOI:10.1038/NPHYS3228 · 20.60 Impact Factor
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    ABSTRACT: The metal-insulator transition is one of the remarkable electrical properties of atomically thin molybdenum disulphide. Although the theory of electron-electron interactions has been used in modelling the metal-insulator transition in molybdenum disulphide, the underlying mechanism and detailed transition process still remain largely unexplored. Here we demonstrate that the vertical metal-insulator-semiconductor heterostructures built from atomically thin molybdenum disulphide are ideal capacitor structures for probing the electron states. The vertical configuration offers the added advantage of eliminating the influence of large impedance at the band tails and allows the observation of fully excited electron states near the surface of molybdenum disulphide over a wide excitation frequency and temperature range. By combining capacitance and transport measurements, we have observed a percolation-type metal-insulator transition, driven by density inhomogeneities of electron states, in monolayer and multilayer molybdenum disulphide. In addition, the valence band of thin molybdenum disulphide layers and their intrinsic properties are accessed.
    Nature Communications 01/2015; 6:6088. DOI:10.1038/ncomms7088 · 10.74 Impact Factor
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    ABSTRACT: Bilayer graphene (BLG) can provide a tunable band gap when exposed to a vertical electric field. We report here an approach to the synthesis of large single-crystal BLG structures with diameters up to 0.54 mm. We found that both absorption-diffusion and gas-phase penetration mechanisms contributed to the growth of the lower second layer and that the absorption-diffusion mechanism favors faster BLG growth. Our strategy was to suppress nucleation in the growth of the first layer using an established surface oxidation method to maintain a low coverage of graphene on Cu foil. We subsequently maximized the growth of the second layer by increasing the duration of absorption-diffusion. The chemical treatment used to polish the Cu surface to reduce the nucleation of growth in the monolayer increased the nucleation density during the growth of the second layer. Electron transport measurements on dual-gated field-effect transistors showed that the BLG fabricated was of high quality with a sizeable tunable band gap. Our approach may have broad applications for the controlled synthesis of bilayers in materials chemistry.
    Nanoscale 01/2015; 7(6). DOI:10.1039/c4nr06607c · 6.74 Impact Factor
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    ABSTRACT: We report a direct measurement of the friction coefficient ξ_{c} of two fluctuating contact lines formed on a fiber surface when a long glass fiber intersects the two water-air interfaces of a thin soap film. The glass fiber of diameter d in the range of 0.4-4 μm and length 100-300 μm is glued onto the front end of a rectangular cantilever used for atomic force microscopy. As a sensitive mechanical resonator, the hanging fiber probe can accurately measure a minute change of its viscous damping caused by the soap film. By measuring the broadening of the resonant peak of the hanging fiber probe with varying viscosity η of the soap film and different surface treatments of the glass fiber, we confirm that the contact line dissipation obeys a universal scaling law, ξ_{c}=απdη, where the coefficient α=1.1±0.3 is insensitive to the change of liquid-solid contact angle. The experimental result is in good agreement with the numerical result based on the phase field model under the generalized Navier boundary conditions.
    Physical Review E 01/2015; 91(1-1):012404. DOI:10.1103/PhysRevE.91.012404 · 2.33 Impact Factor
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    ABSTRACT: We report new developments on the chemical vapor deposition growth of 0.4 nm single-walled carbon nanotubes (SWCNTs) inside the linear channels of the aluminophosphate zeolite, AlPO4-5 (AFI), single crystals (0.4 nm-SWCNT@AFI). Ethylene (C2H4) and carbon monoxide (CO) were used as the feedstock. Polarized Raman spectroscopy was used to analyze the structure and quality of SWCNTs, both the radial breathing mode and G-band are much clearer and stronger than the samples grown by the old process which used template tripropylamine molecules for growing SWCNT@AFI. From the Raman spectra, it is clearly seen that the RBM is composed of two peaks at 535 and 551 cm−1. By using the pseudopotential module in Material Studio to calculate the Raman lines, the 535 cm−1 peak is attributed to the (5,0) SWCNTs and the 551 cm−1 peak to the (3,3) SWCNTs. The abundance of (4,2) is relatively small. Thermal gravity analysis showed that while the samples grown by CO display less than 1 wt% of carbon, for the samples heated in C2H4 atmosphere the weight percentage of SWCNTs is around 10%, which implies ∼30% of the AFI channels are occupied with SWCNTs, a significant increase compared with the previous samples.
    Carbon 09/2014; 76:401–409. DOI:10.1016/j.carbon.2014.04.094 · 6.16 Impact Factor
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    ABSTRACT: By carrying out simultaneous longitudinal and Hall measurements in graphene, we find that the 1/f noise for the charge carrier density is negatively correlated to that of mobility, with a governing behavior that differs significantly from the relation between their mean values. The correlation in the noise data can be quantitatively explained by a single-parameter theory whose underlying physics is the trapping and detrapping of the fluctuating charge carriers by the oppositely charged Coulomb scattering centers. This can alter the effective density of long-range scattering centers in a transient manner, with the consequent fluctuating effect on the mobility.
    Physical Review B 08/2014; 90(8). DOI:10.1103/PhysRevB.90.085434 · 3.66 Impact Factor
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    ABSTRACT: An impedance-matched surface has the property that an incident wave generates no reflection. Here we demonstrate that by using a simple construction, an acoustically reflecting surface can acquire hybrid resonances and becomes impedance-matched to airborne sound at tunable frequencies, such that no reflection is generated. Each resonant cell of the metasurface is deep-subwavelength in all its spatial dimensions, with its thickness less than the peak absorption wavelength by two orders of magnitude. As there can be no transmission, the impedance-matched acoustic wave is hence either completely absorbed at one or multiple frequencies, or converted into other form(s) of energy, such as an electrical current. A high acoustic-electrical energy conversion efficiency of 23% is achieved.
    Nature Material 06/2014; DOI:10.1038/nmat3994 · 36.43 Impact Factor
  • The European Physical Journal Applied Physics 06/2014; 66(3):31301. DOI:10.1051/epjap/2014130388 · 0.79 Impact Factor
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    Dataset: 1D goes 2D
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    ABSTRACT: In this paper, we report a first-principles study of the lattice dynamics of small graphene nanoribbon with zigzag edges. Our investigation is based on spin polarized density functional calculations (DFT). Nesting properties in the electronic band structure are very different for nanoribbons with unpolarized, ferromagnetic, and antiferromagnetic configurations. As a result, the phonon spectrum and nesting related softening in phonon frequencies differ in these cases. The unpolarized and ferromagnetic structures show nesting related phonon softening and considerable electron phonon linewidth, while for the antiferromagnetic structure, a band gap at the Fermi energy eliminates these effects. Saturating the nanoribbon edge with hydrogen has negligible effect on the phonon spectra for the magnetic structures while for the unpolarized configuration all structures without hydrogen are unstable due to soft phonon modes. The electron-phonon coupling coefficients have also been calculated and implications for Peierls transition and superconductivity are discussed.
    Physical Review B 05/2014; 89(20). DOI:10.1103/PhysRevB.89.205404 · 3.66 Impact Factor
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    ABSTRACT: In this work we treat the Poisson-Nernst-Planck (PNP) equations as the basis for a consistent framework of the electrokinetic (EK) effects. The static limit of the PNP equations is shown to be the charge-conserving Poisson-Boltzmann (CCPB) equation, with guaranteed charge neutrality within the computational domain. We propose a surface potential trap model that attributes an energy cost to the interfacial charge dissociation. In conjunction with the CCPB, the surface potential trap can effect a surface-specific adsorbed charge layer . By defining a chemical potential that arises from the charge neutrality constraint, a re-formulated CCPB can be reduced to the form of the Poisson-Boltzmann (PB) equation, whose prediction of the Debye screening layer profile is in excellent agreement with that of the PB equation when the channel width is much larger than the Debye length. However, important differences emerge when the channel width is small so that the Debye screening layers from the opposite sides of the channel overlap with each other. In particular, the theory automatically yields a variation of that is generally known as the “charge regulation” behavior, attendant with predictions of force variation as a function of nanoscale separation between two charged surfaces that is in good agreement with the experiments, with no adjustable or additional parameters. We give a generalized definition of the potential that reflects the strength of the EK effect; its variations with the concentration of surface specific and non-specific salt ions are shown to be in good agreement with the experiments. To delineate the behavior of the electro-osmotic (EO) effect, the coupled PNP and Navier-Stokes (NS) equations are solved numerically under an applied electric field tangential to the fluid-solid interface. The EO effect is shown to exhibit intrinsic time dependence that is non-inertial in its origin. Under a step function applied electric field, a pulse of fluid flow is followed by relaxation to a new ion distribution, owing to the diffusive counter current. We have numerically evaluated the Onsager coefficients associated with the EO effect, L21, and its reverse SP effect, L12, and show L12=L21 in accordance with the Onsager relation. We conclude by noting some of the challenges ahead.
    Physical Review X 03/2014; 4(1):011042. DOI:10.1103/PhysRevX.4.011042 · 8.39 Impact Factor
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    ABSTRACT: We present a homogenization scheme for acoustic metamaterials that is based on reproducing the lowest orders of scattering amplitudes from a finite volume of metamaterials. This approach is noted to differ significantly from that of coherent potential approximation, which is based on adjusting the effective-medium parameters to minimize scatterings in the long-wavelength limit. With the aid of metamaterials' eigenstates, the effective parameters, such as mass density and elastic modulus can be obtained by matching the surface responses of a metamaterial's structural unit cell with a piece of homogenized material. From the Green's theorem applied to the exterior domain problem, matching the surface responses is noted to be the same as reproducing the scattering amplitudes. We verify our scheme by applying it to three different examples: a layered lattice, a two-dimensional hexagonal lattice, and a decorated-membrane system. It is shown that the predicted characteristics and wave fields agree almost exactly with numerical simulations and experiments and the scheme's validity is constrained by the number of dominant surface multipoles instead of the usual long-wavelength assumption. In particular, the validity extends to the full band in one dimension and to regimes near the boundaries of the Brillouin zone in two dimensions.
    Physical Review B 02/2014; 89(6):064309. DOI:10.1103/PhysRevB.89.064309 · 3.66 Impact Factor
  • Xiaobin Xiao, Ping Sheng
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    ABSTRACT: The Onsager theory is known to be inaccurate in its prediction of the critical transition density for small aspect ratio hard rods. In this paper we generalize the Onsager theory in two dimensions by taking into account the short-range order as well as the higher-order virial coefficients, up to the fourth order. By carrying out molecular dynamics (MD) simulations on "molecules" comprising linked hard disks with an aspect ratio ℓ ranging from 5 to 13, we show that the generalized theory is much improved as compared to the traditional theory, with its predictions of the transition density agreeing well with the simulation results. This indicates the importance of short-range order considerations (in conjunction with steric repulsion) for molecules with ℓ≤10, a group which includes the most commonly encountered thermotropic liquid crystals. MD simulations further yield evidence for hexagonal order for molecules with ℓ≤8, indicating an intermediate hexagonal phase before solidifying at higher densities.
    Physical Review E 12/2013; 88(6-1):062501. DOI:10.1103/PhysRevE.88.062501 · 2.33 Impact Factor
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    ABSTRACT: An acoustic/vibrational energy absorption metamaterial includes at least one enclosed planar frame with an elastic membrane attached having one or more rigid plates are attached. The rigid plates have asymmetric shapes, with a substantially straight edge at the attachment to said elastic membrane, so that the rigid plate establishes a cell having a predetermined mass. Vibrational motions of the structure contain a number of resonant modes with tunable resonant frequencies.
    Ref. No: US20140060962 A1, Year: 11/2013
  • Ping Sheng
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    ABSTRACT: We present a structurally and conceptually simple acoustic double negative metamaterial comprising two coupled membranes [Phys. Rev. Lett. 110, 134301 (2013)]. Owing to its symmetry, the system can generate both monopolar and dipolar resonances that are separately tunable, thereby making broadband double negativity possible. A homogenization scheme is implemented that enables the exact characterization of our metamaterial by the effective mass density and bulk modulus even beyond the usual long-wavelength regime, with the measured displacement fields on the sample's surfaces as inputs. Double negativity is achieved in the frequency range of 520-830 Hz. Transmission and reflection predictions using effective parameters are shown to agree remarkably well with the experiment. Work done in collaboration with M. Yang, G. C. Ma, and Z. Y. Yang.
    The Journal of the Acoustical Society of America 11/2013; 134(5):4026. DOI:10.1121/1.4830696 · 1.56 Impact Factor
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    ABSTRACT: We study theoretically and experimentally the nature of formation and properties of acoustic surface-guided modes (SGMs) at the interface of two phononic crystals (PCs). It is demonstrated that in the shared region of the frequency gaps, the SGMs can exist at the interface as a result of total reflections from both PCs, which causes the formation of nodal planes by the interference of the forward- and backward-propagating waves. The experiments show excellent agreement with the theory predictions.
    EPL (Europhysics Letters) 11/2013; 104(3):34005-. DOI:10.1209/0295-5075/104/34005 · 2.27 Impact Factor
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    ABSTRACT: A device with simultaneous negative effective mass density and bulk modulus has at least one tubular section and front and back membranes sealing the tubular section. The front and back membranes sealing the tubular sections seal the tubular section sufficiently to establish a sealed or restricted enclosed fluid space defined by the tubular section and the membranes, and restrict escape or intake of fluid resulting from acoustic vibrations. A pair of platelets are mounted to the membranes, with the individual platelets substantially centered on respective ones of the front and back membranes.
    Ref. No: US20140116802 A1, Year: 10/2013

Publication Stats

9k Citations
1,470.06 Total Impact Points


  • 1970–2015
    • The Hong Kong University of Science and Technology
      • Department of Physics
      Chiu-lung, Kowloon City, Hong Kong
  • 2014
    • Pennsylvania State University
      • Department of Mathematics
      University Park, Maryland, United States
  • 2001–2013
    • Nano Science and Technology Institute
      Austin, Texas, United States
  • 2012
    • South China University of Technology
      • Department of Applied Physics
      Shengcheng, Guangdong, China
  • 2010
    • Soochow University (PRC)
      • School of Physical Science and Technology School of Energy
      Suzhou, Jiangsu Sheng, China
  • 2002
    • Wuhan University
      • Department of Physics
      Wu-han-shih, Hubei, China
  • 2000
    • Fudan University
      • Department of Physics
      Shanghai, Shanghai Shi, China
  • 1997
    • The Chinese University of Hong Kong
      • Department of Physics
      Hong Kong, Hong Kong
  • 1987
    • Boston University
      • Department of Physics
      Boston, Massachusetts, United States