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Wang M.C.,Peac Institute of Multiscale science | Wang M.C.,Southwest Jiaotong University | Qiao S.,CAS Shanghai Institute of Microsystem and Information Technology | Qiao S.,University of Shanghai for Science and Technology | And 5 more authors.
Physical Review Letters | Year: 2016

We report on a time-resolved ultrafast optical spectroscopy study of the topological insulator Bi2Se3. We unravel that a net spin polarization cannot only be generated using circularly polarized light via interband transitions between topological surface states (SSs), but also via transitions between SSs and bulk states. Our experiment demonstrates that tuning photon energy or temperature can essentially allow for photoexcitation of spin-polarized electrons to unoccupied topological SSs with two distinct spin relaxation times (∼25 and ∼300 fs), depending on the coupling between SSs and bulk states. The intrinsic mechanism leading to such distinctive spin dynamics is the scattering in SSs and bulk states which is dominated by Eg 2 and A1g 1 phonon modes, respectively. These findings are suggestive of novel ways to manipulate the photoinduced coherent spins in topological insulators. © 2016 American Physical Society.


Fan D.,Peac Institute of Multiscale science | Lu L.,Peac Institute of Multiscale science | Lu L.,Anhui University of Science and Technology | Li B.,Peac Institute of Multiscale science | And 8 more authors.
Review of Scientific Instruments | Year: 2014

Real time, in situ, multiframe, diffraction, and imaging measurements on bulk samples under high and ultrahigh strain-rate loading are highly desirable for micro- and mesoscale sciences. We present an experimental demonstration of multiframe transient x-ray diffraction (TXD) along with simultaneous imaging under high strain-rate loading at the Advanced Photon Source beamline 32ID. The feasibility study utilizes high strain-rate Hopkinson bar loading on a Mg alloy. The exposure time in TXD is 2-3 μs, and the frame interval is 26.7-62.5 μs. Various dynamic deformation mechanisms are revealed by TXD, including lattice expansion or compression, crystal plasticity, grain or lattice rotation, and likely grain refinement, as well as considerable anisotropy in deformation. Dynamic strain fields are mapped via x-ray digital image correlation, and are consistent with the diffraction measurements and loading histories. © 2014 AIP Publishing LLC.


An Q.,California Institute of Technology | Goddard W.A.,California Institute of Technology | Zybin S.V.,California Institute of Technology | Luo S.-N.,Peac Institute of Multiscale science
Journal of Physical Chemistry C | Year: 2014

In order to elucidate how shocks in heterogeneous materials affect decomposition and reactive processes, we used the ReaxFF reactive force field in reactive molecules dynamics (RMD) simulations of the effects of strong shocks (2.5 and 3.5 km/s) on a prototype polymer bonded explosive (PBX) consisting of cyclotrimethylene trinitramine (RDX) bonded to hydroxyl-terminated polybutadiene (HTPB). We showed earlier that shock propagation from the high density RDX to the low density polymer (RDX → Poly) across a nonplanar periodic interface (sawtooth) leads to a hotspot at the initial asperity but no additional hotspot at the second asperity. This hotspot arises from shear along the interface induced by relaxation of the stress at the asperity. We now report the case for shock propagation from the low density polymer to the high density RDX (Poly → RDX) where we find a hotspot at the initial asperity and a second more dramatic hotspot at the second asperity. This second hotspot is enhanced due to shock wave convergence from shock wave interaction with nonplanar interfaces. We consider that this second hotspot is likely the source of the detonation in realistic PBX systems. We showed how these hotspots depend on the density mismatch between the RDX and polymer and found that decreasing the density by a factor of 2 dramatically reduces the hotspot. These results suggest that to make PBX less sensitive for propellants and explosives, the binder should be designed to provide low density at the asperity in contact with the RDX. Based on these simulations, we propose a new design for an insensitive PBX in which a low density polymer coating is deposited between the RDX and the usual polymer binder. To test this idea, we simulated shock wave propagation from two opposite directions (RDX → Poly and Poly → RDX) through the interface matched PBX (IM-PBX) material containing a 3 nm coating of low density (0.48 g/cm 3) polymer. These simulations showed that this IM-PBX design dramatically suppresses hotspot formation. © 2014 American Chemical Society.


Huang J.W.,Peac Institute of Multiscale science | Huang J.W.,Southwest Jiaotong University | Huang J.Y.,Peac Institute of Multiscale science | Huang J.Y.,Southwest Jiaotong University | And 4 more authors.
Journal of Synchrotron Radiation | Year: 2016

Dynamic compression experiments are performed on single-crystal Si under split Hopkinson pressure bar loading, together with simultaneous high-speed (250-350?ns resolution) synchrotron X-ray Laue diffraction and phase-contrast imaging. A methodology is presented which determines crystal rotation parameters, i.e. instantaneous rotation axes and angles, from two unindexed Laue diffraction spots. Two-dimensional translation is obtained from dynamic imaging by a single camera. High-speed motion of crystals, including translation and rotation, can be tracked in real time via simultaneous imaging and diffraction.


Cai Y.,Anhui University of Science and Technology | Huang J.Y.,Anhui University of Science and Technology | Wu H.A.,Anhui University of Science and Technology | Zhu M.H.,Southwest Jiaotong University | And 3 more authors.
Journal of Physical Chemistry Letters | Year: 2016

It is well known that strain rate and size effects are both important in material failure, but the relationships between them are poorly understood. To establish this connection, we carry out molecular dynamics (MD) simulations of cavitation in Lennard-Jones and Cu liquids over a very broad range of size and strain rate. These studies confirm that temporal and spatial scales play equivalent roles in the tensile strengths of these two liquids. Predictions based on smallest-scale MD simulations of Cu for larger temporal and spatial scales are consistent with independent simulations, and comparable to experiments on liquid metals. We analyze these results in terms of classical nucleation theory and show that the equivalence arises from the role of both size and strain rate in the nucleation of a daughter phase. Such equivalence is expected to hold for a wide range of materials and processes and to be useful as a predictive bridging tool in multiscale studies. © 2016 American Chemical Society.


Liu H.K.,University of Electronic Science and Technology of China | Liu H.K.,Peac Institute of Multiscale science | Lin Y.,University of Electronic Science and Technology of China | Luo S.N.,Peac Institute of Multiscale science
Journal of Physical Chemistry C | Year: 2014

We investigate with molecular dynamics simulations the dependences of thermal conductivity (κ) of polycrystalline graphene on grain boundary (GB) energy and grain size. Hexagonal grains and grains with random shapes and sizes are explored, and their thermal properties and phonon densities of states are characterized. It is found that κ decreases exponentially with increasing GB energy, and decreasing grain size reduces κ. GB-induced phonon softening and scattering, as well as reduction in the number of heat conducting phonons, contribute to the decrease in thermal conductivity. © 2014 American Chemical Society.


Hudspeth M.,Purdue University | Sun T.,Argonne National Laboratory | Parab N.,Purdue University | Guo Z.,Purdue University | And 3 more authors.
Journal of Synchrotron Radiation | Year: 2015

Using a high-speed camera and an intensified charge-coupled device (ICCD), a simultaneous X-ray imaging and diffraction technique has been developed for studying dynamic material behaviors during high-rate tensile loading. A Kolsky tension bar has been used to pull samples at 1000s-1 and 5000s-1 strain-rates for super-elastic equiatomic NiTi and 1100-O series aluminium, respectively. By altering the ICCD gating time, temporal resolutions of 100ps and 3.37μs have been achieved in capturing the diffraction patterns of interest, thus equating to single-pulse and 22-pulse X-ray exposure. Furthermore, the sample through-thickness deformation process has been simultaneously imaged via phase-contrast imaging. It is also shown that adequate signal-to-noise ratios are achieved for the detected white-beam diffraction patterns, thereby allowing sufficient information to perform quantitative data analysis diffraction via in-house software (WBXRD-GUI). Of current interest is the ability to evaluate crystal d-spacing, texture evolution and material phase transitions, all of which will be established from experiments performed at the aforementioned elevated strain-rates. © 2015 International Union of Crystallography.


Wang L.,Peac Institute of Multiscale science | E J.C.,Peac Institute of Multiscale science | Cai Y.,Peac Institute of Multiscale science | Cai Y.,Anhui University of Science and Technology | And 3 more authors.
Journal of Applied Physics | Year: 2015

We investigate shock-induced deformation of columnar nanocrystalline Al with large-scale molecular dynamics simulations and implement orientation mapping (OM) and selected area electron diffraction (SAED) for microstructural analysis. Deformation mechanisms include stacking fault formation, pronounced twinning, dislocation slip, grain boundary (GB) sliding and migration, and lattice or partial grain rotation. GBs and GB triple junctions serve as the nucleation sites for crystal plasticity including twinning and dislocations, due to GB weakening, and stress concentrations. Grains with different orientations exhibit different densities of twins or stacking faults nucleated from GBs. GB migration occurs as a result of differential deformation between two grains across the GB. High strain rates, appropriate grain orientation and GBs contribute to deformation twinning. Upon shock compression, intra-grain dislocation and twinning nucleated from GBs lead to partial grain rotation and the formation of subgrains, while whole grain rotation is not observed. During tension, stress gradients associated with the tensile pulse give rise to intra-grain plasticity and then partial grain rotation. The simulated OM and SAED are useful to describe lattice/grain rotation, the formation of subgrains, GB migration and other microstructures. © 2015 AIP Publishing LLC.


Li B.,Anhui University of Science and Technology | Li B.,Peac Institute of Multiscale science | Zhao F.P.,Anhui University of Science and Technology | Zhao F.P.,Peac Institute of Multiscale science | And 2 more authors.
Journal of Applied Physics | Year: 2014

We investigate with large-scale molecular dynamics simulations shock-induced surface jetting from grooved Cu as regards microstructure effects, including jetting mass/velocity ratios, directionality, jetting phase diagram, secondary jetting, and underlying mechanisms. The grooves are of wedged, cylindrical, and rectangular shapes. Other microstructure features explored are half angles, crystal structure asymmetry as represented by grain boundaries, geometrical asymmetry, and deformation heterogeneity. The common fundamental mechanism is that jetting is driven by stress gradients due to transverse mass collision. For symmetrical wedged grooves, the velocity ratio (maximum jet head velocity/free surface velocity of flat surface) increases linearly with decreasing half angle, with a slope similar for different materials and at nano- to macroscales, as indicated by our simulations and previous experiments. However, the jetting factor or mass ratio reaches the maximum at certain intermediate half angle. An impact strength vs. half angle phase diagram is established for a typical case of wedged grooves, useful for predicting the critical parameters for jetting (e.g., the critical impact velocity for a given half angle, as well as deducing yield strength). Small asymmetries, including crystal structure and geometrical asymmetries as well as deformation inhomogeneities, may induce considerable deviation of the jetting direction. Wedged, cylindrical, and rectangular grooves form a geometrical hierarchy. Primary jetting can be well described with wedged grooves, and secondary jetting is a result of collision of primary jets. Rectangular grooves may yield pronounced, velocity-enhanced, secondary jetting. © 2014 AIP Publishing LLC.


Cai Y.,Anhui University of Science and Technology | Cai Y.,Peac Institute of Multiscale science | Wu H.A.,Anhui University of Science and Technology | Luo S.N.,Peac Institute of Multiscale science
Journal of Chemical Physics | Year: 2014

Large-scale molecular dynamics (MD) simulations are performed to investigate homogeneous nucleation and growth of nanovoids during cavitation in liquid Cu. We characterize in detail the atomistic cavitation processes by following the temporal evolution of cavities or voids, analyze the nucleation behavior with the mean first-passage time (MFPT) and survival probability (SP) methods, and discuss the results against classical nucleation theory (CNT), the Tolman equation for surface energy, independent calculation of surface tension via integrating the stress profiles, the Johnson-Mehl-Avrami (JMA) growth law, and the power law for nucleus size distributions. Cavitation in this representative metallic liquid is a high energy barrier Poisson processes, and the steady-state nucleation rates obtained from statistical runs with the MFPT and SP methods are in agreement. The MFPT method also yields the critical nucleus size and the Zeldovich factor. Fitting with the Tolman's equation to the MD simulations yields the surface energy of a planar interface (∼ 0.9 J-2) and the Tolman length (0.4-0.5 Å), and those values are in accord with those from integrating the stress profiles of a planar interface. Independent CNT predictions of the nucleation rate (1033 - 34 s -1 m-3) and critical size (3-4 Å in radius) are in agreement with the MFPT and SP results. The JMA law can reasonably describe the nucleation and growth process. The size distribution of subcritical nuclei appears to follow a power law with an exponent decreasing with increasing tension owing to coupled nucleation and growth, and that of the supercritical nuclei becomes flattened during further stress relaxation due to void coalescence. © 2014 AIP Publishing LLC.

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