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21 Jul 2008

Volume 93, Issue 3, Articles (03xxxx)

Issue Cover Spotlight Figure

Appl. Phys. Lett. 93, 031101 (2008); http://dx.doi.org/10.1063/1.2959092 (3 pages)

Di Xu, Kevin P. Chen, Kris Ohlinger, and Yuankun Lin
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Phase transformation accommodated plasticity in nanocrystalline nickel

X. Y. Zhang, X. L. Wu, Q. Liu, R. L. Zuo, A. W. Zhu, P. Jiang, and Q. M. Wei

Appl. Phys. Lett. 93, 031901 (2008); http://dx.doi.org/10.1063/1.2953545 (3 pages) | Cited 3 times

Online Publication Date: 21 July 2008

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Based on detailed x-ray diffraction and transmission electron microscopy we have found body-centered-cubic (bcc) Ni upon room-temperature rolling of nanocrystalline (nc) face-centered-cubic (fcc) Ni. The bcc phase forms via the Kurdjumov–Sachs (KS) martensitic transformation mechanism when the von Mises equivalent strain exceeds ∼ 0.3, much higher than accessible in tensile testing. The fcc and bcc phases keep either the KS or the Nishiyama–Wasserman orientation relationship. Our results provide insights into the deformation physics in nc Ni, namely, the fcc-to-bcc phase transformation can also accommodate plasticity at large plastic strains.
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81.30.Kf Martensitic transformations
64.70.kd Metals and alloys
81.40.Lm Deformation, plasticity, and creep
62.20.fq Plasticity and superplasticity
62.25.-g Mechanical properties of nanoscale systems
61.46.Df Structure of nanocrystals and nanoparticles ("colloidal" quantum dots but not gate-isolated embedded quantum dots)

In situ metal-organic chemical vapor deposition atomic-layer deposition of aluminum oxide on GaAs using trimethyaluminum and isopropanol precursors

Cheng-Wei Cheng and Eugene A. Fitzgerald

Appl. Phys. Lett. 93, 031902 (2008); http://dx.doi.org/10.1063/1.2960574 (3 pages) | Cited 25 times

Online Publication Date: 21 July 2008

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In situ atomic-layer deposition (ALD) of Al2O3 on p-GaAs in metal-organic chemical vapor deposition system is demonstrated in this article. Isopropanol was chosen as the oxygen source for Al2O3 ALD, instead of common H2O. The ALD mechanism is discussed and it is proposed that water does not form in the process. The saturation growth rate of Al2O3 is about 0.8 Å/cycle. X-ray photoetectron spectroscopy depth profiles were performed and no arsenic oxide is observed at the interface. The capacitance-voltage measurements show a small accumulation capacitance dispersion and voltage shift in the depletion region. The interfacial defect density near the midgap of the GaAs bandgap has been determined with the conductance-frequency method. The interfacial defect density is determined as 2.5×1011 eV−1 cm−2 at the midgap of the GaAs.
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81.15.-z Methods of deposition of films and coatings; film growth and epitaxy
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
68.55.at Other materials
81.15.Kk Vapor phase epitaxy; growth from vapor phase
79.60.Bm Clean metal, semiconductor, and insulator surfaces

Giant effective g-factor in PbxEu1−xTe epitaxial films

E. Heredia, P. H. de Oliveira Rappl, P. Motisuke, A. L. Gazoto, F. Iikawa, and M. J. S. P. Brasil

Appl. Phys. Lett. 93, 031903 (2008); http://dx.doi.org/10.1063/1.2961018 (3 pages) | Cited 6 times

Online Publication Date: 21 July 2008

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We investigated PbxEu1−xTe films with x ⩽ 0.2 by magneto-optical measurements. For x ∼ 0.01, the optical emission is similar to high quality EuTe films with two narrow lines attributed to excitonic recombinations associated with magnetic polarons. For increasing x, the emission becomes dominated by a broader lower energy band, which is very efficient as compared to the binary emission. The magneto-optical properties of the ternary films show various similarities with EuTe results, such as quenchings at similar temperatures and magnetic fields. Most remarkably, they also present a giant effective g-factor that makes this material a strong candidate for spintronic applications.
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78.66.Li Other semiconductors
78.20.Ls Magneto-optical effects
75.70.Ak Magnetic properties of monolayers and thin films
78.55.Hx Other solid inorganic materials
71.38.-k Polarons and electron-phonon interactions
71.35.-y Excitons and related phenomena

Evolution of valence-band alignment with nitrogen content in GaNAs/GaAs single quantum wells

Jun Shao, Wei Lu, M. Sadeghi, Xiang Lü, S. M. Wang, Lili Ma, and A. Larsson

Appl. Phys. Lett. 93, 031904 (2008); http://dx.doi.org/10.1063/1.2958232 (3 pages) | Cited 7 times

Online Publication Date: 21 July 2008

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We report on experimental evidence for the transition of valence-band alignment from type I to type II in GaNxAs1−x/GaAs single quantum wells by photoreflectance measurements. The substitutional nitrogen content covers a range of 1.4%–5.9%. The turning point of the type I–type II transition occurs at x≳4.7%. The experimental observations can be well interpreted by a combination of band anticrossing model and model-solid theory when nonlinear behavior of either the shear deformation potential or the average valence-band energy is taken into account. The effect of dilute nitrogen on the valence-band offset of GaNAs/GaAs quantum well structure is hence clarified.
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73.21.Fg Quantum wells
78.67.De Quantum wells

Modulus mapping of nanoscale closure variants in Ni–Mn–Ga

Yaniv Ganor and Doron Shilo

Appl. Phys. Lett. 93, 031905 (2008); http://dx.doi.org/10.1063/1.2961023 (3 pages) | Cited 5 times

Online Publication Date: 21 July 2008

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The twinned magnetic microstructure of Ni2MnGa ferromagnetic shape-memory alloy is investigated by high resolution nanoscale modulus mapping. A surprisingly fine near-surface nanoscale substructure of closure magnetic twin variants was observed. The lateral distance between adjacent closure variants was found to be 100 nm. The small size of twin variant prisms provides a unique opportunity for evaluating the twin boundary energy by considering the competition between the magnetic field and interface energies. Our estimate shows a relatively small twin boundary energy of 3 ergs/cm2, which suggests the ability of Ni2MnGa to form nanoscale structures of magnetic twin variants.
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61.72.Mm Grain and twin boundaries
75.50.Cc Other ferromagnetic metals and alloys
81.40.Lm Deformation, plasticity, and creep
68.35.Md Surface thermodynamics, surface energies
62.20.F- Deformation and plasticity

Improved GaN film overgrown with a molybdenum nanoisland mask

Chaotong Lin, Guanghui Yu, Xinzhong Wang, Mingxia Cao, Hang Gong, Ming Qi, and Aizhen Li

Appl. Phys. Lett. 93, 031906 (2008); http://dx.doi.org/10.1063/1.2962983 (3 pages) | Cited 3 times

Online Publication Date: 21 July 2008

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We report the improved crystalline and optical quality of GaN film overgrown by hydride vapor phase epitaxy adopting a molybdenum (Mo) nanoisland mask (MNM). The MNM is fabricated following thermal annealing of the nanometer-thick Mo film deposited by electron-beam evaporation. The full width at half maximum values of high-resolution x-ray diffraction (HRXRD) rocking curves for the GaN film with MNM are 188 arc sec (002 reflection) and 219 arc sec (102 reflection), while those for the GaN film without MNM are 256 and 364 arc sec, respectively. This result indicates a significant reduction of dislocation density in the overgrown GaN film with MNM. Photoluminescence spectra measurements reveal the compressive strain relaxation and improvement in the quality of the overgrown GaN film with MNM as compared to the regrown GaN film without MNM, which is consistent with the trend observed by HRXRD.
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81.05.Ea III-V semiconductors
81.15.Kk Vapor phase epitaxy; growth from vapor phase
68.55.ag Semiconductors
42.70.Nq Other nonlinear optical materials; photorefractive and semiconductor materials
78.66.Fd III-V semiconductors
78.55.Cr III-V semiconductors

Shear band melting and serrated flow in metallic glasses

K. Georgarakis, M. Aljerf, Y. Li, A. LeMoulec, F. Charlot, A. R. Yavari, K. Chornokhvostenko, E. Tabachnikova, G. A. Evangelakis, D. B. Miracle, A. L. Greer, and T. Zhang

Appl. Phys. Lett. 93, 031907 (2008); http://dx.doi.org/10.1063/1.2956666 (3 pages) | Cited 39 times

Online Publication Date: 23 July 2008

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Scanning electron microscopy observations of shear steps on Zr-based bulk metallic glasses show direct evidence of shear band melting due to heat generated by elastic energy release. The estimated range of attained temperatures and the observed morphologies are consistent with shear steps forming at a subsonic speed limited by a required redistribution of local microscopic stresses. The calculations indicate that a 0.2 μm layer melts in the vicinity of a shear band forming a 1 μm shear step. The plastic part of the stress strain curve is serrated but a majority of shear events are not associated to serrations.
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81.40.Lm Deformation, plasticity, and creep
62.20.fq Plasticity and superplasticity
62.20.de Elastic moduli
61.43.Fs Glasses
81.40.Jj Elasticity and anelasticity, stress-strain relations
64.70.dj Melting of specific substances

Mechanical twinning and omega transition by ⟨111⟩ {112} shear in a metastable β titanium alloy

H. Xing and J. Sun

Appl. Phys. Lett. 93, 031908 (2008); http://dx.doi.org/10.1063/1.2959183 (3 pages) | Cited 11 times

Online Publication Date: 23 July 2008

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{112} ⟨111⟩ mechanical twinning and stress-induced omega transition were observed by high-resolution transmission electron microscope in a metastable β titanium alloy with chemical composition of Ti-23Nb-0.7Ta-2Zr-1.2O at. % after deformation. The orientation relationships between the ω phase and β parent matrix are (math010)ω‖(211)β, [1math10]ω‖[0math1]β and [0001]ω‖[math11]β, and the habit plane of (math010)ω‖(211)β for the stress-induced ω transition is different from that of (0001)ω‖(111)β often observed for the thermal ω transition. Both mechanical twinning and ω transition arise from the shear along ⟨111⟩ {112}. A dislocation mechanism for mechanical twinning and stress-induced ω transition was discussed additionally.
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61.72.Mm Grain and twin boundaries
61.72.Ff Direct observation of dislocations and other defects (etch pits, decoration, electron microscopy, x-ray topography, etc.)
81.30.Hd Constant-composition solid-solid phase transformations: polymorphic, massive, and order-disorder
64.70.kd Metals and alloys
81.40.Lm Deformation, plasticity, and creep
62.20.F- Deformation and plasticity

Voltage dependent director of a homeotropic negative liquid crystal cell

L. Z. Ruan, Fuzi Yang, and J. R. Sambles

Appl. Phys. Lett. 93, 031909 (2008); http://dx.doi.org/10.1063/1.2963977 (2 pages) | Cited 1 time

Online Publication Date: 23 July 2008

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Thin layers of obliquely (60° to normal) thermally evaporated SiOx lead to homeotropic alignment of a nematic liquid crystal (LC) having negative dielectric anisotropy. Under application of an ac voltage the director, as characterized by the fully leaky waveguide technique, is found to realign with a voltage controlled tilt along the evaporation direction. This behavior is in complete contrast with that of a LC having positive dielectric anisotropy and may have important implications for modern LC display technology.
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42.79.Kr Display devices, liquid-crystal devices
61.30.Eb Experimental determinations of smectic, nematic, cholesteric, and other structures

Deformation twin formed by self-thickening, cross-slip mechanism in nanocrystalline Ni

X. L. Wu, J. Narayan, and Y. T. Zhu

Appl. Phys. Lett. 93, 031910 (2008); http://dx.doi.org/10.1063/1.2949685 (3 pages) | Cited 2 times

Online Publication Date: 23 July 2008

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We report the observation of a deformation twin formed by a recently proposed self-thickening, cross-slip twinning mechanism. This observation verifies one more twinning mechanism, in addition to those reported before, in nanocrystalline face-centered-cubic metals. In this mechanism, once the first Shockley partial is emitted from a grain boundary, and cross slips onto another slip plane, a deformation twin could nucleate and grow in both the primary and cross-slip planes without requiring the nucleation of additional Shockley partials from the grain boundary.
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81.40.Lm Deformation, plasticity, and creep
62.20.F- Deformation and plasticity
61.72.Mm Grain and twin boundaries
61.46.Hk Nanocrystals

Spherical indentation of a finite poroelastic coating

M. Galli and M. L. Oyen

Appl. Phys. Lett. 93, 031911 (2008); http://dx.doi.org/10.1063/1.2957993 (3 pages) | Cited 5 times

Online Publication Date: 23 July 2008

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Indentation testing of a finite poroelastic layer is considered. Finite element modeling was used to investigate spherical contact creep tests, with emphasis on the influence of layer thickness and of finite rise time on the time-dependent deformation. Thin layers are stiffened by the substrate constraint even at very small relative indenter penetrations and reach steady state more quickly than thick layers. The degree of consolidation following loading is affected by the interaction of layer thickness and rise time and cannot be predicted from either alone. These results provide guidance for micro- and nanoindentation testings of hydrogel coatings for biomedical applications.
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81.40.Np Fatigue, corrosion fatigue, embrittlement, cracking, fracture, and failure
62.20.M- Structural failure of materials
81.40.Jj Elasticity and anelasticity, stress-strain relations
62.20.D- Elasticity
81.70.Bt Mechanical testing, impact tests, static and dynamic loads
81.40.Lm Deformation, plasticity, and creep
62.20.F- Deformation and plasticity

Unusual surface reliefs from photoinduced creeping and aggregation behavior of azopolymer

Seungwoo Lee, Yong-Cheol Jeong, and Jung-Ki Park

Appl. Phys. Lett. 93, 031912 (2008); http://dx.doi.org/10.1063/1.2959062 (3 pages) | Cited 6 times

Online Publication Date: 24 July 2008

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We report the spontaneous formation of unusual surface reliefs, in which two sets of sinusoidal gratings were hierarchically structured, merely by single-step holographic inscription on amorphous azopolymer films. By monitoring of growth behavior of surface reliefs during holographic inscription, we found that the formation of additional grating is caused by the creeping and resulting aggregation of dome structures. Our direct observation of creeping and aggregation behavior is expected to contribute to enhancing the understanding of unusual surface reliefs, and also in fabricating complex surface reliefs.
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42.70.Jk Polymers and organics
42.40.Eq Holographic optical elements; holographic gratings
61.41.+e Polymers, elastomers, and plastics

High strength Ni–Zr binary ultrafine eutectic-dendrite composite with large plastic deformability

Jin Man Park, Tae Eung Kim, Sung Woo Sohn, Do Hyang Kim, Ki Buem Kim, Won Tae Kim, and Jürgen Eckert

Appl. Phys. Lett. 93, 031913 (2008); http://dx.doi.org/10.1063/1.2952755 (3 pages) | Cited 11 times

Online Publication Date: 24 July 2008

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A Ni–8Zr high strength ultrafine eutectic-dendrite composite with large plasticity has been developed in the Ni–Zr binary eutectic system. The excellent mechanical properties are attributed to the specific heterogeneous microstructure with distinctly different length scales, i.e., micrometer-size ductile dendrites combined with an ultrafine eutectic matrix. The plastic deformation mainly proceeds through a shear banding mechanism. However, there is no significant shear localization due to the constraint effect of ductile solid solution Ni phases including dendrites and/or alternating lamellar layers. Furthermore, excessive shear stress and accumulated shear strain can be effectively released and accommodated by delocalization and multiplication of shear bands.
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81.40.Lm Deformation, plasticity, and creep
62.20.fk Ductility, malleability

Thin film NiTi coatings on optical fiber Bragg sensors

K. P. Mohanchandra, S. Karnani, M. C. Emmons, W. L. Richards, and G. P. Carman

Appl. Phys. Lett. 93, 031914 (2008); http://dx.doi.org/10.1063/1.2961002 (3 pages)

Online Publication Date: 24 July 2008

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This paper describes the sputter deposition and characterization of nickel titanium (NiTi) thin film shape memory alloy onto the surface of an optical fiber Bragg grating. The NiTi coating uniformity, crystallinity, and transformation temperatures are measured using scanning electron microscope, x-ray diffraction, and differential scanning calorimeter, respectively. The strain in the optical fiber is measured using centroid calculation of wavelength shifts. Results show distinct and abrupt changes in the optical fiber signal with the four related transformation temperatures represented by the austenite-martensite forward and reverse phase transformations. These tests demonstrate a coupling present between optical energy and thermal energy, i.e., a modified multiferroic material.
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42.81.Pa Sensors, gyros
81.15.Cd Deposition by sputtering
42.79.Wc Optical coatings
42.79.Dj Gratings
42.81.Bm Fabrication, cladding, and splicing
42.81.Cn Fiber testing and measurement of fiber parameters

Bilinear responses and rippling morphologies of multiwalled carbon nanotubes under torsion

Xu Huang, Jian Zou, and Sulin Zhang

Appl. Phys. Lett. 93, 031915 (2008); http://dx.doi.org/10.1063/1.2965800 (3 pages) | Cited 10 times

Online Publication Date: 25 July 2008

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We present coarse-grained simulations of torsion induced rippling deformation morphology of multiwalled carbon nanotubes (MWCNTs). Our simulations reveal that beyond the torsional bifurcation, the rippling pattern propagates from outer to inner layers, with the two innermost layers acting as a hard core that hardly ripples. Despite the highly nonlinear rippling deformation, the mechanical response of MWCNTs follows a simple bilinear law, with a nearly constant ratio ( ∼ 60%) of post- to prerippling torsional rigidities. The bifurcation torsion scales inversely with the square of tube radius. This bilinear constitutive relation may be exploited by large-scale simulations of MWCNT-based materials and devices.
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81.40.Lm Deformation, plasticity, and creep
81.07.De Nanotubes
61.46.Fg Nanotubes

Microstructural evolution in H ion induced splitting of freestanding GaN

O. Moutanabbir, R. Scholz, S. Senz, U. Gösele, M. Chicoine, F. Schiettekatte, F. Süßkraut, and R. Krause-Rehberg

Appl. Phys. Lett. 93, 031916 (2008); http://dx.doi.org/10.1063/1.2955832 (3 pages) | Cited 11 times

Online Publication Date: 25 July 2008

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We investigated the microstructural transformations during hydrogen ion-induced splitting of GaN thin layers. Cross-sectional transmission electron microscopy and positron annihilation spectroscopy data show that the implanted region is decorated with a high density of 1–2 nm bubbles resulting from vacancy clustering during implantation. These nanobubbles persist up to 450 °C. Ion channeling data show a strong dechanneling enhancement in this temperature range tentatively attributed to strain-induced lattice distortion. The dechanneling level decreases following the formation of plateletlike structures at 475 °C. Extended internal surfaces develop around 550 °C leading to the exfoliation of GaN thin layer.
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61.72.U- Doping and impurity implantation
61.72.jd Vacancies
61.80.Jh Ion radiation effects
78.70.Bj Positron annihilation
47.55.D- Drops and bubbles
61.72.Qq Microscopic defects (voids, inclusions, etc.)

A study of the pair distribution function of self-organized Ge quantum dots

Marco Bernardi, Anna Sgarlata, Massimo Fanfoni, Adalberto Balzarotti, and Nunzio Motta

Appl. Phys. Lett. 93, 031917 (2008); http://dx.doi.org/10.1063/1.2965122 (3 pages) | Cited 3 times

Online Publication Date: 25 July 2008

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We explore the use of the pair distribution function to study the self-organization process of Ge quantum dots on both nanopatterned and nonpatterned oxidized Si(001) surfaces. Dots formation and ordering upon annealing of a Ge thin film are analyzed. The method we use is not limited to this case study. We show how it can be applied to determine short and long range self-ordering of nanostructures. We support our results by applying a software routine to simulate patterns of dots to finally spot the relevant physical aspects of Ge islands self-assembly.
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81.16.Dn Self-assembly
68.65.Hb Quantum dots (patterned in quantum wells)
61.72.Cc Kinetics of defect formation and annealing
68.55.ag Semiconductors

Temperature independence of pressure-induced amorphization of the phase-change memory alloy Ge2Sb2Te5

M. Krbal, A. V. Kolobov, J. Haines, A. Pradel, M. Ribes, P. Fons, J. Tominaga, C. Levelut, R. Le Parc, and M. Hanfland

Appl. Phys. Lett. 93, 031918 (2008); http://dx.doi.org/10.1063/1.2956409 (3 pages) | Cited 12 times

Online Publication Date: 25 July 2008

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In the temperature range from room temperature to about 150 °C, the prototypic phase-change material Ge2Sb2Te5 becomes amorphous upon hydrostatic compression. In the studied temperature range, the onset of amorphization is at about 15 GPa and the material completely amorphizes at 25 GPa; these values do not depend on temperature. Upon decompression, the amorphous phase is stable at lower temperatures, yet at higher temperatures (145 °C), the initial fcc phase is recovered upon decompression. A possible mechanism of pressure-induced amorphization and its implications for phase-change memories are discussed.
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64.70.K- Solid-solid transitions
81.30.Hd Constant-composition solid-solid phase transformations: polymorphic, massive, and order-disorder
61.43.-j Disordered solids

Elasticity of ideal single-walled carbon nanotubes via symmetry-adapted tight-binding objective modeling

D.-B. Zhang and T. Dumitrică

Appl. Phys. Lett. 93, 031919 (2008); http://dx.doi.org/10.1063/1.2965465 (3 pages) | Cited 14 times

Online Publication Date: 25 July 2008

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The elastic response for a large catalog of carbon nanotubes subjected to axial and torsional strain is derived from atomistic calculations that rely on an accurate tight-binding description of the covalent binding. The application of the computationally expensive quantum treatment is possible due to the simplification in the number of atoms introduced by accounting for the helical and angular symmetries exhibited by the elastically deformed nanotubes. The elasticity of nanotubes larger than ∼ 1.25 nm in diameter can be represented with an isotropic elastic continuum.
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81.40.Jj Elasticity and anelasticity, stress-strain relations
81.40.Lm Deformation, plasticity, and creep
62.25.-g Mechanical properties of nanoscale systems
62.23.-c Structural classes of nanoscale systems
62.20.D- Elasticity
62.20.F- Deformation and plasticity
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