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  • 1
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    Quasi-transverse optical phonon mode in self-generated semipolar AlN grains embedded in c-oriented AlN matrix grown on sapphire using hydride vapor phase epitaxy (2016)
    American Institute of Physics (AIP)
    Details
    Publication Date: 2016-05-26
    Description: In this study, we present a microspectroscopic investigation on the quasi-transverse optical phonon modes Q(TO) in some self-generated aluminum nitride (AlN) grains grown on sapphire using hydride vapor phase epitaxy. Using X-ray diffraction and transmission electron microscope, these grains were confirmed to be embedded in (0001)-AlN ( c -AlN) epitaxial matrix with an appearance plane of ( 10 1 ¯ 1 ) ( s -plane). Two beam bright field images further showed that the AlN grains were free of dislocation. In-plane phonon anisotropy of the AlN grains was discussed in detail using angular-dependent polarized Raman spectroscopy. The dependence of pure Raman phonons intensity on rotation angle agrees well with the calculation. The Q(TO) phonon intensity exhibited similar behavior to that of A 1 (TO) phonon, which can be explained by Loudon's formula. However, the observed frequency fluctuation for the Q(TO) phonon differs from that of the pure phonon modes, which cannot be directly understood from the classic Loudon's formula. A modified Loudon's formula appropriate to non-normal incidence was presented to explain the observed Q(TO) phonon frequency fluctuation. Combining with the angular-dependent Raman spectra, we proposed that a small inclination of s -plane along with the various in-plane orientations in c -AlN matrix lead to the frequency fluctuation of Q(TO) in these embedded semipolar AlN grains.
    Print ISSN: 0021-8979
    Electronic ISSN: 1089-7550
    Topics: Physics
    Published by American Institute of Physics (AIP)
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  • 2
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    Ion-acoustic shocks with self-regulated ion reflection and acceleration (2016)
    American Institute of Physics (AIP)
    Details
    Publication Date: 2016-04-13
    Description: An analytic solution describing an ion-acoustic collisionless shock, self-consistently with the evolution of shock-reflected ions, is obtained. The solution extends the classic soliton solution beyond a critical Mach number, where the soliton ceases to exist because of the upstream ion reflection. The reflection transforms the soliton into a shock with a trailing wave and a foot populated by the reflected ions. The solution relates parameters of the entire shock structure, such as the maximum and minimum of the potential in the trailing wave, the height of the foot, as well as the shock Mach number, to the number of reflected ions. This relation is resolvable for any given distribution of the upstream ions. In this paper, we have resolved it for a simple “box” distribution. Two separate models of electron interaction with the shock are considered. The first model corresponds to the standard Boltzmannian electron distribution in which case the critical shock Mach number only insignificantly increases from M ≈ 1.6 (no ion reflection) to M ≈ 1.8 (substantial reflection). The second model corresponds to adiabatically trapped electrons. They produce a stronger increase, from M ≈ 3.1 to M ≈ 4.5 . The shock foot that is supported by the reflected ions also accelerates them somewhat further. A self-similar foot expansion into the upstream medium is described analytically.
    Print ISSN: 1070-664X
    Electronic ISSN: 1089-7674
    Topics: Physics
    Published by American Institute of Physics (AIP)
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  • 3
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    Water adsorption induced in-plane domain switching on BaTiO3 surface (2015)
    American Institute of Physics (AIP)
    Details
    Publication Date: 2015-09-05
    Description: In this study, the influences of the adsorption of water molecules on the changes in the atomic and electric structures of BaTiO 3 surface were investigated using ab initio calculation. Water molecules are molecularly and dissociatively adsorbed on the BaTiO 3 surface, which makes electrons transfer from water molecules to the BaTiO 3 surface. The redistribution of electrons in the BaTiO 3 surface layers weakens the Ba-O interactions and strengthens the Ti-O interactions, so that the Ti atom shifts in TiO 2 plane, i.e., an in-plane domain switching. The adsorption of water molecules on BaTiO 3 surfaces also results in a reduction in the surface rumpling.
    Print ISSN: 0021-8979
    Electronic ISSN: 1089-7550
    Topics: Physics
    Published by American Institute of Physics (AIP)
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  • 4
    Electronic Resource
    Electronic Resource
    Omega head—an experimental 120-turn inductive head : 38th Annual Conference on Magnetism and Magnetic Materials (1994)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 75 (1994), S. 6397-6399 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Experimental 120-turn thin-film inductive heads have been built. The key features of this head are the 6-μm pitch helical coils and an omega-shaped, planar yoke structure having dual easy axes. Hardbaked photoresist insulator layers are used to encapsulate the yoke and to smooth out the wafer surface topography. Micro-Kerr studies show that the easy axis remains in the transverse direction in the yokes after multiple anneals. The P1/G/P2 is 3.8/0.3/3.8 μm, and the yoke length is close to 1 mm. The helical coils were built with a novel process that combines yoke/stud coplating and a photoresist planarization process. The coil resistance is 68Ω and the inductance is 5.5 μH. The yoke saturates at 5 mA. The heads were tested over disks having Mrt of 2.5 memu/cm2 and Hc of 1500 Oe. The write threshold current is 5.3 mA (peak-to-peak) and the overwrite is 30 dB. The isolated pulse amplitude Vpp is 10.3 μV/(TwV), where track width Tw is in μm and the linear velocity V in m/s.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.355362
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  • 5
    Electronic Resource
    Electronic Resource
    An experimental investigation of electromechanical responses in a polyurethane elastomer (1997)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 81 (1997), S. 2770-2776 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The electromechanical response of a polyurethane elastomer was investigated at room temperature and in the temperature range near its glass transition. It was found that the Maxwell stress contribution to the strain response can be significant at temperatures higher than the glass transition temperature. In addition, the material exhibits a very high electrostrictive coefficient Q, about two orders of magnitude higher than that of polyvinylidene fluoride. It was also found that in a polymeric material, the chain segment motions can be divided into those related to the polarization response and those related to the mechanical response and the overlap between the two yields the electromechanical response of the material. In general, the activation energies for different types of motion can be different, resulting in different relaxation times in the dielectric, the elastic compliance, and the electrostrictive data, as observed in the polyurethane elastomer investigated. The experimental results indicate that at the temperatures investigated, the activation energy for the mechanical related segment motions is higher than that of nonmechanical related segment motions. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.363981
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  • 6
    Electronic Resource
    Electronic Resource
    A bimorph based dilatometer for field induced strain measurement in soft and thin free standing polymer films (1998)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 69 (1998), S. 2480-2483 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: A bimorph based dilatometer has been developed which enables one to characterize the electric field induced strain response in the out-of-plane direction in thin and soft free standing polymer film samples conveniently over a relatively wide frequency range (1 Hz to 1 kHz). The test results demonstrate that the newly developed dilatometer is capable of detecting displacement down to sub-angstrom range. The agreement between the test results from the bimorph dilatometer, the results from a laser dilatometer, and the model analysis indicates that the device can indeed be used reliably for these measurement with high sensitivity. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1148947
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  • 7
    Electronic Resource
    Electronic Resource
    Dielectric studies of Zn1−xMnxSe epilayers (1999)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 86 (1999), S. 1630-1633 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We report on the dielectric properties of Zn1−xMnxSe(0≤x≤0.78) epilayers, by capacitance and dissipation factor measurements at temperature 5 K〈T〈475 K and frequency 20 Hz〈f〈100 kHz. A Debye-like relaxational contribution to the dielectric response is observed, which requires the presence of charge redistribution. The relaxation is found to be a thermally activated process, and the activation energies obtained from the dissipation factor and capacitance are in good agreement. The capacitance is found to increase with a decrease in test frequency. From our results it is established that the dielectric response is caused by carrier hopping among structural defects. A monotonic variation is found in the relationship between the activation energy and the Mn concentration. This monotonic variation is interpreted in terms of the four-center model, in which the number of Mn atoms appearing in the nearest-neighbor sites of the defect can have four possible configurations. The measured defect behavior reflects the overall average of all energy levels involved in the center, and the result is weighted by the relative concentrations. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.370938
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  • 8
    Electronic Resource
    Electronic Resource
    Laser wake-field acceleration and optical guiding in a hollow plasma channel (1995)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 2 (1995), S. 310-318 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The accelerating and focusing wake fields that can be excited by a short laser pulse in a hollow underdense plasma are examined. The evacuated channel in the plasma serves as an optical fiber to guide the laser pulse over many Rayleigh lengths. Wake fields excited by plasma current at the edge of the channel extend to the center where they may be used for ultrahigh gradient acceleration of particles over long distances. The wake field and equilibrium laser profiles are found analytically and compared to two-dimensional (2-D) particle-in-cell (PIC) simulations. Laser propagation is simulated over more than ten Rayleigh lengths. The accelerating gradients on the axis of a channel of radius c/ωp are of order of one-half of the gradients in a uniform plasma. For present high-power lasers, multi-GeV/m gradients are predicted. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.871107
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  • 9
    Electronic Resource
    Electronic Resource
    Wave collapse at the lower-hybrid resonance (1993)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 5 (1993), S. 3148-3162 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The modulational instability and collapse of waves in the vicinity of the lower-hybrid resonance including both magnetosonic and lower-hybrid waves are investigated by analytical and numerical methods. The mechanism leading to the modulational instability is the nonlinear coupling of lower-hybrid waves with the much lower-frequency quasineutral density perturbations via the ponderomotive force. The result is a filamentation of the high-frequency field producing elongated, cigar-shaped nonlinear wave packets aligned along the magnetic field with the plasma expelled outside (cavities). The analytical self-similar solutions describing cavity collapse are obtained and compared with the results of numerical simulation for both two- and three-dimensional cavity geometries. It is shown that in three-dimensional solutions the transverse, with respect to the magnetic field, contraction remains prevailing. The possibility of ion acceleration as the result of the lower-hybrid collapse is discussed and detailed comparison is made with the observations of the phenomena in the auroral ionosphere.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.860652
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  • 10
    Electronic Resource
    Electronic Resource
    Plasma physics at the final focus of high-energy colliders (1990)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 2 (1990), S. 1384-1389 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The plasma physics associated with intense electron and positron beams in high-energy colliders is discussed. The focusing and plasma oscillation of these beams are studied in the presence of an externally supplied plasma (a plasma lens) and in the self-consistent fields of the interpenetrating beams. A plasma lens can provide strong focusing of a beam by shielding the beam's radial space-charge electric field and allowing it to self-pinch in its self-generated azimuthal magnetic field. The focusing force of a plasma lens can be equivalent to quadrupole magnet strengths of the order of hundreds of MG/cm. Theoretical and computational work characterizing the aberrations and final spot sizes possible with plasma lenses is presented. Furthermore, when the e+ and e− beams overlap at the interaction point (i.e., collide), they behave (temporarily) as a two-component plasma. The self-pinching or disruption of one beam by another in the e+e− plasma is modeled analytically and with two-dimensional particle-in-cell simulations.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.859560
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