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22 Feb 2010

Volume 96, Issue 8, Articles (08xxxx)

Issue Cover Spotlight Figure

Appl. Phys. Lett. 96, 081101 (2010); http://dx.doi.org/10.1063/1.3324885 (3 pages)

Wuzhou Song and Demetri Psaltis
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A high-reflectivity atom-focusing mirror stable at room temperature

D. Barredo, G. Laurent, F. Calleja, P. Nieto, J. J. Hinarejos, A. L. Vázquez de Parga, D. Farías, and R. Miranda

Appl. Phys. Lett. 96, 081901 (2010); http://dx.doi.org/10.1063/1.3325033 (3 pages) | Cited 2 times

Online Publication Date: 22 February 2010

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It is shown that the Pb(√3×√3)R30/Si(111) ordered layer is an excellent mirror for neutral He atoms. It focuses more than 15% of the incoming He atoms into the specular peak, and is stable up to 450 K. Moreover, the reflectivity remains almost unchanged in a time scale of several weeks in ultrahigh vacuum. As a consequence, this system is a very good candidate to be used as a mirror in the next generation of the scanning helium atom microscope.
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07.79.Lh Atomic force microscopes
42.79.Bh Lenses, prisms and mirrors

An electronic structure perspective on glass-forming ability in metallic glasses

H. B. Yu (于海滨), W. H. Wang (汪卫华), and H. Y. Bai (白海洋)

Appl. Phys. Lett. 96, 081902 (2010); http://dx.doi.org/10.1063/1.3327337 (3 pages) | Cited 10 times

Online Publication Date: 22 February 2010

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Through systematic study of glass-forming ability (GFA) and the electronic specific heat coefficient in typical ternary (Cu50Zr50)100−xAlx bulk metallic glasses, we provide compelling experimental evidence that the density of electronic energy states at the Fermi level indeed is closely correlated with the GFA of metallic glasses, and the best GFA can be obtained when the Fermi surfaces nearly touch the quasi-Brillouin boundaries, as predicted by the nearly free electron model. Our results highlight the significance of electronic structural effects on the formation of metallic glasses.
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71.23.Cq Amorphous semiconductors, metallic glasses, glasses
65.60.+a Thermal properties of amorphous solids and glasses: heat capacity, thermal expansion, etc.
64.70.pe Metallic glasses

High-excitation effect on photoluminescence of sol-gel ZnO nanopowder

C. H. Chia, Y. J. Lai, T. C. Han, J. W. Chiou, Y. M. Hu, and W. C. Chou

Appl. Phys. Lett. 96, 081903 (2010); http://dx.doi.org/10.1063/1.3327338 (3 pages) | Cited 5 times

Online Publication Date: 22 February 2010

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We studied the power-dependent photoluminescence of ZnO nanopowder grown by sol-gel method at low temperature. At moderate optical pumping intensity, two nonlinear emission bands due to the radiative recombination of free biexciton and the inelastic exciton–exciton scattering were detected. We found that the threshold of the excitation density for the emergence of the nonlinear emission bands is low (<18 W/cm2) in sol-gel ZnO nanopowders.
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78.55.Et II-VI semiconductors
71.35.Gg Exciton-mediated interactions
78.67.Bf Nanocrystals, nanoparticles, and nanoclusters
78.60.Lc Optically stimulated luminescence

Structural transformations in Mn2NiGa due to residual stress

Sanjay Singh, M. Maniraj, S. W. D’Souza, R. Ranjan, and S. R. Barman

Appl. Phys. Lett. 96, 081904 (2010); http://dx.doi.org/10.1063/1.3318461 (3 pages) | Cited 8 times

Online Publication Date: 22 February 2010

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Powder x-ray diffraction study of Mn2NiGa ferromagnetic shape memory alloy shows the existence of a 7M monoclinic modulated structure at room temperature (RT). The structure of Mn2NiGa is found to be highly dependent on residual stress. For higher stress, the structure is tetragonal at RT, and for intermediate stress it is 7M monoclinic. However, only when the stress is considerably relaxed, the structure is cubic, as is expected at RT since the martensitic transition temperature is 230 K.
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75.50.Gg Ferrimagnetics
81.30.Kf Martensitic transformations
75.50.Cc Other ferromagnetic metals and alloys
81.05.Bx Metals, semimetals, and alloys

Scanning tunneling microscope study of striated carbon ridges in few-layer epitaxial graphene formed on 4H-silicon carbide (000math)

S. E. Harrison, M. A. Capano, and R. Reifenberger

Appl. Phys. Lett. 96, 081905 (2010); http://dx.doi.org/10.1063/1.3323092 (3 pages) | Cited 6 times

Online Publication Date: 22 February 2010

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Atomically resolved scanning tunneling microscope images of carbon ridge defects found in few-layer graphene formed on the C-face (000math) of 4H-silicon carbide reveal a striated exterior surface formed from out-of-plane distortions of the hexagonal graphene lattice. While ridge formation is likely explained by compressive in-plane stresses coupled with the small values of the bending modulus for few-layer graphene, the striated structure along the ridges argues for a localized unidirectional stress in the material directed along the ridge length.
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61.48.Gh Structure of graphene
68.55.ag Semiconductors

Mechanically controlled defect mode in cholesteric elastomers

Adriana E. Mota, Laura O. Palomares, and Juan Adrian Reyes

Appl. Phys. Lett. 96, 081906 (2010); http://dx.doi.org/10.1063/1.3323102 (3 pages) | Cited 2 times

Online Publication Date: 22 February 2010

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We consider an axially elongated cholesteric elastomer having a twist defect. We show that its localized mode can be mechanically tuned, and the scaling of the inverse relative line width can be largely enhanced when the values of the deformation and shape anisotropy are near the pseudoisotropic curve. This choice causes a tremendous variation in the behavior of the photon dwell time in the defect mode, which then grows linearly versus the sample thickness. The shift of the defect wavelength, the reflection band width, and the angle between the electric and magnetic fields are also calculated.
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61.30.Jf Defects in liquid crystals
61.30.Vx Polymer liquid crystals
62.10.+s Mechanical properties of liquids
63.50.-x Vibrational states in disordered systems
61.25.he Polymer solutions

Hydrogen in InN: A ubiquitous phenomenon in molecular beam epitaxy grown material

V. Darakchieva, K. Lorenz, N. P. Barradas, E. Alves, B. Monemar, M. Schubert, N. Franco, C. L. Hsiao, L. C. Chen, W. J. Schaff, L. W. Tu, T. Yamaguchi, and Y. Nanishi

Appl. Phys. Lett. 96, 081907 (2010); http://dx.doi.org/10.1063/1.3327333 (3 pages) | Cited 11 times

Online Publication Date: 23 February 2010

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We study the unintentional H impurities in relation to the free electron properties of state-of-the-art InN films grown by molecular beam epitaxy (MBE). Enhanced concentrations of H are revealed in the near surface regions of the films, indicating postgrowth surface contamination by H. The near surface hydrogen could not be removed upon thermal annealing and may have significant implications for the surface and bulk free electron properties of InN. The bulk free electron concentrations were found to scale with the bulk H concentrations while no distinct correlation with dislocation density could be inferred, indicating a major role of hydrogen for the unintentional conductivity in MBE InN.
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81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy
61.72.U- Doping and impurity implantation
61.72.uj III-V and II-VI semiconductors
81.05.Ea III-V semiconductors
61.72.Cc Kinetics of defect formation and annealing
68.55.Ln Defects and impurities: doping, implantation, distribution, concentration, etc.

Alternating photoinduced mass transport triggered by light polarization in azobenzene containing sol-gel films

F. Fabbri, Y. Lassailly, K. Lahlil, J. P. Boilot, and J. Peretti

Appl. Phys. Lett. 96, 081908 (2010); http://dx.doi.org/10.1063/1.3327341 (3 pages) | Cited 3 times

Online Publication Date: 23 February 2010

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Combined shear-force and near-field optical microscopies are used for real-time monitoring of the formation of photoinduced surface relief gratings in photochromic thin films containing azobenzene derivatives. The correlated optical and topographical images provide evidence that the direction of the photoinduced matter migration is defined by the light polarization pattern and that, for a given light intensity pattern, modulating the polarization between two orthogonal states gives rise to alternating mass transport.
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42.79.Wc Optical coatings
42.70.Jk Polymers and organics
42.25.Ja Polarization
81.10.Dn Growth from solutions
81.10.Fq Growth from melts; zone melting and refining
81.15.Lm Liquid phase epitaxy; deposition from liquid phases (melts, solutions, and surface layers on liquids)
68.35.bm Polymers, organics
78.66.Qn Polymers; organic compounds

Focussing bending waves via negative refraction in perforated thin plates

Mohamed Farhat, Sebastien Guenneau, Stefan Enoch, Alexander B. Movchan, and Gunnar G. Petursson

Appl. Phys. Lett. 96, 081909 (2010); http://dx.doi.org/10.1063/1.3327813 (3 pages) | Cited 12 times

Online Publication Date: 23 February 2010

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We propose a design of a periodically perforated thin plate leading to lensing effect of bending waves via negative refraction. To achieve this goal, we first analyze the band spectrum of the bi-harmonic operator for an array of freely vibrating square voids using both numerical (finite elements) and asymptotic methods. We then find some point in the reciprocal space where the acoustic dispersion surface displays a convex isofrequency contour shrinking with frequency. We finally demonstrate that a point force generating a bending wave above a finite array of 221 perforations displays an image underneath according to the Snell–Descartes inverted laws.
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42.25.Gy Edge and boundary effects; reflection and refraction
41.20.Jb Electromagnetic wave propagation; radiowave propagation
43.20.El Reflection, refraction, diffraction of acoustic waves

Photoluminescence from exciton-exciton scattering in a GaAs1−xNx thin film

J. Hashimoto, Y. Maeda, and M. Nakayama

Appl. Phys. Lett. 96, 081910 (2010); http://dx.doi.org/10.1063/1.3309695 (3 pages)

Online Publication Date: 23 February 2010

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We have investigated photoluminescence (PL) properties under high-density-excitation conditions at 10 K in a GaAs1−xNx thin film (x = 0.008) with a narrow band-gap energy of 1.34 eV. A PL band was observed with a threshold-like nature, and its intensity was found to exhibit quadratic dependence on the excitation power. At the threshold excitation power, the PL-peak energy is lower than the energy of the fundamental exciton by the magnitude of the exciton binding energy that is ∼ 4 meV. The results described above indicate that the PL band originates from exciton-exciton scattering, the so-called P emission, which is typically observed in wide-gap semiconductors with large exciton binding energies. Furthermore, we have confirmed the existence of optical gain in the energy region of the P band using a variable-stripe-length method.
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78.55.Cr III-V semiconductors
78.66.Fd III-V semiconductors
73.20.Mf Collective excitations (including excitons, polarons, plasmons and other charge-density excitations)

Visibility and Raman spectroscopy of mono and bilayer graphene on crystalline silicon

Emil B. Song, Bob Lian, Guangyu Xu, Bo Yuan, Caifu Zeng, Amber Chen, Minsheng Wang, Sungmin Kim, Murong Lang, Yi Zhou, and Kang L. Wang

Appl. Phys. Lett. 96, 081911 (2010); http://dx.doi.org/10.1063/1.3323105 (3 pages) | Cited 5 times

Online Publication Date: 23 February 2010

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Experimental studies of pristine graphene devices currently rely on the fact that the graphene crystallites can be visible under optical microscopes when the underlying substrate is engineered to exhibit high contrast. Here, we present that graphene can be visualized not only on a dielectric substrate but also on a crystalline Si surface of a silicon-on-insulator (SOI) wafer (SIMOX and Bonded) with thicknesses of Si ∼ 70 nm and buried oxide ∼ 140 nm, using monochromatic illumination. In addition, we have found that Raman spectroscopy shows similar features to standard graphene on SiO2 substrates independent of the polarity of the Si surface. Finally, the Raman spectrum on SOI exhibits a higher intensity compared to that on bulk Si due to the interference enhancement effect of graphene on SOI. Thus, the usage of optical microscopy and Raman spectroscopy for detecting, locating, and characterizing graphene serves as a high throughput method to further study graphene on semiconductor systems and other substrates beyond SiO2/Si.
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78.67.Wj Optical properties of graphene
78.30.Na Fullerenes and related materials

Phase separation and Si nanocrystal formation in bulk SiO studied by x-ray scattering

O. M. Feroughi, C. Sternemann, Ch. J. Sahle, M. A. Schroer, H. Sternemann, H. Conrad, A. Hohl, G. T. Seidler, J. Bradley, T. T. Fister, M. Balasubramanian, A. Sakko, K. Pirkkalainen, K. Hämäläinen, and M. Tolan

Appl. Phys. Lett. 96, 081912 (2010); http://dx.doi.org/10.1063/1.3323106 (3 pages) | Cited 13 times

Online Publication Date: 23 February 2010

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We present an x-ray scattering study of the temperature-induced phase separation and Si nanocrystal formation in bulk amorphous SiOx with x ≈ 1. X-ray Raman scattering at the Si LII,III-edge reveals a significant contribution of suboxides present in native amorphous SiO. The suboxide contribution decreases with increasing annealing temperature between 800–1200 °C pointing toward a phase separation of SiO into Si and SiO2 domains. In combination with x-ray diffraction and small angle x-ray scattering the SiO microstructure is found to be dominated by internal suboxide interfaces in the native state. For higher annealing temperatures above 900 °C growth of Si nanocrystals with rough surfaces embedded in a silicon oxide matrix can be observed.
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61.46.Hk Nanocrystals
64.75.Jk Phase separation and segregation in nanoscale systems
64.75.Qr Phase separation and segregation in semiconductors
61.72.Cc Kinetics of defect formation and annealing
78.70.Ck X-ray scattering

Ultrasensitive mode-localized mass sensor with electrically tunable parametric sensitivity

P. Thiruvenkatanathan, J. Yan, J. Woodhouse, A. Aziz, and A. A. Seshia

Appl. Phys. Lett. 96, 081913 (2010); http://dx.doi.org/10.1063/1.3315877 (3 pages) | Cited 6 times

Online Publication Date: 25 February 2010

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We use the phenomena of mode localization and vibration confinement in pairs of weakly coupled, nearly identical microelectromechanical (MEMS) resonators as an ultrasensitive technique of detecting added mass on the resonator. The variations in the eigenstates for induced mass additions are studied and compared with corresponding resonant frequency shifts in pairs of MEMS resonators that are coupled electrostatically. We demonstrate that the relative shifts in the eigenstates can be over three orders of magnitude greater than those in resonant frequency for the same addition of mass. We also investigate the effects of voltage controlled electrical spring tuning on the parametric sensitivity of such sensors and demonstrate sensitivities tunable by over 400%.
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85.85.+j Micro- and nano-electromechanical systems (MEMS/NEMS) and devices
07.07.Df Sensors (chemical, optical, electrical, movement, gas, etc.); remote sensing
06.30.Dr Mass and density

Bismuth doping of graphene

Olcay Üzengi Aktürk and Mehmet Tomak

Appl. Phys. Lett. 96, 081914 (2010); http://dx.doi.org/10.1063/1.3334723 (3 pages) | Cited 5 times

Online Publication Date: 25 February 2010

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In this work, we have studied bismuth as a dopant in graphene using density functional theory (DFT). We find that bismuth is weakly physisorbed within DFT. On the other hand, we show that bismuth n-dopes graphene when it is substitutional. We observe that local density approximation results give higher substitutional energy than that of generalized gradient approximation. The electronic structure of graphene is changed when Bi is substitutional.
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61.48.Gh Structure of graphene
73.22.Pr Electronic structure of graphene
71.15.Mb Density functional theory, local density approximation, gradient and other corrections
68.43.-h Chemisorption/physisorption: adsorbates on surfaces

Evidence of localized surface plasmon enhanced magneto-optical effect in nanodisk array

Guan Xiang Du, Tetsuji Mori, Michiaki Suzuki, Shin Saito, Hiroaki Fukuda, and Migaku Takahashi

Appl. Phys. Lett. 96, 081915 (2010); http://dx.doi.org/10.1063/1.3334726 (3 pages) | Cited 12 times

Online Publication Date: 25 February 2010

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Nanodisk array with sandwich structure of Au/[Co/Pt]n/Au was fabricated by electron beam lithography combined with argon ion milling. Excitation of localized surface plasmon resonance (LSPR) was demonstrated for various disk diameters. Magneto-optical (MO) properties were measured by a home-made micro-Faraday system at wavelengths of 633 and 690 nm. Faraday ellipticity at 690 nm showed non-monotonic dependence on disk diameter and reached maximum for disk diameter of 84 nm, which was associated with LSPR at 690 nm. The experimental results show direct evidence for LSPR enhancement effect on MO properties. The optical and MO properties were fitted by average field approximation.
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78.66.Bz Metals and metallic alloys
78.20.Ls Magneto-optical effects
73.20.Mf Collective excitations (including excitons, polarons, plasmons and other charge-density excitations)
78.68.+m Optical properties of surfaces
73.22.Lp Collective excitations

Formulation of phase-field energies for microstructure in complex crystal structures

Lun Yang (杨伦) and Kaushik Dayal

Appl. Phys. Lett. 96, 081916 (2010); http://dx.doi.org/10.1063/1.3319503 (3 pages) | Cited 7 times

Online Publication Date: 26 February 2010

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The unusual properties of many multifunctional materials originate from a structural phase transformation and consequent martensitic microstructure. Phase-field models are typically used to predict the formation of microstructural patterns and subsequent evolution under applied loads. However, formulating a phase-field energy with the correct equilibrium crystal structures and that also respects the crystallographic symmetry is a formidable task in complex materials. This paper presents a simple method to construct such energy density functions for phase-field modeling. The method can handle complex equilibrium structures and crystallographic symmetry with ease. We demonstrate it on a shape memory alloy with 12 monoclinic variants.
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64.70.K- Solid-solid transitions
81.30.Hd Constant-composition solid-solid phase transformations: polymorphic, massive, and order-disorder
62.20.fg Shape-memory effect; yield stress; superelasticity

Ultrafast relaxation dynamics of hot optical phonons in graphene

Haining Wang, Jared H. Strait, Paul A. George, Shriram Shivaraman, Virgil B. Shields, Mvs Chandrashekhar, Jeonghyun Hwang, Farhan Rana, Michael G. Spencer, Carlos S. Ruiz-Vargas, and Jiwoong Park

Appl. Phys. Lett. 96, 081917 (2010); http://dx.doi.org/10.1063/1.3291615 (3 pages) | Cited 46 times

Online Publication Date: 26 February 2010

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Using ultrafast optical pump-probe spectroscopy, we study the relaxation dynamics of hot optical phonons in few-layer and multilayer graphene films grown by epitaxy on silicon carbide substrates and by chemical vapor deposition on nickel substrates. In the first few hundred femtoseconds after photoexcitation, the hot carriers lose most of their energy to the generation of hot optical phonons which then present the main bottleneck to subsequent cooling. Optical phonon cooling on short time scales is found to be independent of the graphene growth technique, the number of layers, and the type of the substrate. We find average phonon lifetimes in the 2.5–2.55 ps range. We model the relaxation dynamics of the coupled carrier-phonon system with rate equations and find a good agreement between the experimental data and the theory. The extracted optical phonon lifetimes agree very well with the theory based on anharmonic phonon interactions.
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78.47.J- Ultrafast spectroscopy (<1 psec)
72.20.Ht High-field and nonlinear effects
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
68.55.aj Insulators
63.20.D- Phonon states and bands, normal modes, and phonon dispersion
73.61.Ng Insulators

Physical mechanism of anisotropic sensitivity in pentaerythritol tetranitrate from compressive-shear reaction dynamics simulations

Sergey V. Zybin, William A. Goddard, Peng Xu, Adri C. T. van Duin, and Aidan P. Thompson

Appl. Phys. Lett. 96, 081918 (2010); http://dx.doi.org/10.1063/1.3323103 (3 pages) | Cited 15 times

Online Publication Date: 26 February 2010

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We propose computational protocol (compressive shear reactive dynamics) utilizing the ReaxFF reactive force field to study chemical initiation under combined shear and compressive load. We apply it to predict the anisotropic initiation sensitivity observed experimentally for shocked pentaerythritol tetranitrate single crystals. For crystal directions known to be sensitive we find large stress overshoots and fast temperature increase that result in early bond-breaking processes whereas insensitive directions exhibit small stress overshoot, lower temperature increase, and little bond dissociation. These simulations confirm the model of steric hindrance to shear and capture the thermochemical processes dominating the phenomena of shear-induced chemical initiation.
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82.40.Fp Shock wave initiated reactions, high-pressure chemistry
62.50.Ef Shock wave effects in solids and liquids
82.30.Lp Decomposition reactions (pyrolysis, dissociation, and fragmentation)
61.50.Lt Crystal binding; cohesive energy
81.40.Gh Other heat and thermomechanical treatments
82.33.Vx Reactions in flames, combustion, and explosions
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