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19 Mar 2012

Volume 100, Issue 12, Articles (12xxxx)

Issue Cover Spotlight Figure

Appl. Phys. Lett. 100, 121101 (2012); http://dx.doi.org/10.1063/1.3693413 (4 pages)

Jolly Xavier, Raktim Dasgupta, Sunita Ahlawat, Joby Joseph, and Pradeep Kumar Gupta
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Thermal rectification in a fluid reservoir

Sohail Murad and Ishwar K. Puri

Appl. Phys. Lett. 100, 121901 (2012); http://dx.doi.org/10.1063/1.3696022 (5 pages) | Cited 2 times

Online Publication Date: 19 March 2012

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An organized nonuniform mass distribution in solids leads to a monotonically varying thermal conductivity in a nanomaterial so that the heat flux is directionally dependent. We investigate through molecular dynamics simulations if the influence of an organized mass distribution in a fluid also leads to thermal rectification. Heat transfer is monitored in a water reservoir placed between two (hot and cold) silicon walls. The distribution of the fluid in the reservoirs is organized by applying an external force to each water molecule in a specified direction, creating a density gradient. This external force is smaller than the intermolecular forces in water, in most cases by much more than an order of magnitude. The simulations reveal that mass graded fluid-containing nanosystems can be engineered to possess an asymmetric axial thermal conductance that leads to greater heat flow in the direction of decreasing mass density. The rectification improves as the thermal conductivity is enhanced by increasing the fluid density adjacent to a hot wall, since doing so decreases the interfacial resistance and increases the heat flux.
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61.20.Ja Computer simulation of liquid structure
66.25.+g Thermal conduction in nonmetallic liquids
73.40.-c Electronic transport in interface structures

Tensile softening of metallic-glass-matrix composites in the supercooled liquid region

J. W. Qiao, Y. Zhang, H. L. Jia, H. J. Yang, P. K. Liaw, and B. S. Xu

Appl. Phys. Lett. 100, 121902 (2012); http://dx.doi.org/10.1063/1.3696026 (4 pages) | Cited 3 times

Online Publication Date: 19 March 2012

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A Ti-based metallic-glass-matrix composite exhibits tensile softening (necking) in the supercooled liquid region, accompanied by a large tensile ductility and a fragmentation of dendrites. Subjected to high temperatures, concurrent crystallization does not occur, suggesting a good thermal stability of the glass matrix. The presence of high-volume-fractioned dendrites lowers the rheology of the viscous glass matrix at high temperatures, which results in an absence of super elongation as monolithic bulk metallic glasses (BMGs). A tensile strength of 970 MPa is higher than those of most BMGs under varying strain rates, ascribing to the retardation of softening by the dendrites.
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81.40.Lm Deformation, plasticity, and creep
81.30.Fb Solidification
62.20.fk Ductility, malleability
62.20.fq Plasticity and superplasticity
68.70.+w Whiskers and dendrites (growth, structure, and nonelectronic properties)

Oxygen-vacancy and depth-dependent violet double-peak photoluminescence from ultrathin cuboid SnO2 nanocrystals

L. Z. Liu, X. L. Wu, J. Q. Xu, T. H. Li, J. C. Shen, and Paul K. Chu

Appl. Phys. Lett. 100, 121903 (2012); http://dx.doi.org/10.1063/1.3696044 (4 pages) | Cited 4 times

Online Publication Date: 20 March 2012

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A double peak in the violet region between 360 and 400 nm is observed from the photoluminescence spectra acquired from cuboid SnO2 nanocrystals and the energy separation between the two subpeaks increases with nanocrystal size. The phenomenon arises from band edge recombination caused by different in-depth distributions of oxygen vacancies (OVs). Density functional theory calculations disclose that variations in the oxygen vacancies with depth introduce valence-band peak splitting leading to the observed splitting and shift of the double peak.
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61.72.jd Vacancies
78.55.Hx Other solid inorganic materials
71.15.Mb Density functional theory, local density approximation, gradient and other corrections
73.20.At Surface states, band structure, electron density of states

Electronic states and photoluminescence of TiO2 nanotubes with adsorbed surface oxygen

L. Z. Liu, W. Xu, X. L. Wu, Y. Y. Zhang, T. H. Chen, and Paul K. Chu

Appl. Phys. Lett. 100, 121904 (2012); http://dx.doi.org/10.1063/1.3695167 (4 pages) | Cited 1 time

Online Publication Date: 20 March 2012

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The electronic states associated with enhanced photocatalytic activity of anodic anatase TiO2 nanotubes (NTs) annealed in N2 and O2 are investigated by photoluminescence (PL). The NTs annealed in N2 show a green peak related to oxygen vacancies and its position blueshifts with deceasing temperature, whereas those annealed in O2 show a double peak at 475–600 nm and the energy separation increases with decreasing temperature. Spectral analysis and density function theory calculation disclose that the double peak results from residual oxygen vacancies and oxygen atoms on the NT wall and the increased energy separation arises from the larger difference between the inner and outer NT stress at low temperature.
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78.67.Ch Nanotubes
78.55.Hx Other solid inorganic materials
82.65.+r Surface and interface chemistry; heterogeneous catalysis at surfaces
82.50.-m Photochemistry
61.72.jd Vacancies
61.72.Cc Kinetics of defect formation and annealing

High performance magnetocaloric perovskites for magnetic refrigeration

Christian R. H. Bahl, David Velázquez, Kaspar K. Nielsen, Kurt Engelbrecht, Kjeld B. Andersen, Regina Bulatova, and Nini Pryds

Appl. Phys. Lett. 100, 121905 (2012); http://dx.doi.org/10.1063/1.3695338 (3 pages) | Cited 11 times

Online Publication Date: 20 March 2012

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We have applied mixed valance manganite perovskites as magnetocaloric materials in a magnetic refrigeration device. Relying on exact control of the composition and a technique to process the materials into single adjoined pieces, we have observed temperature spans above 9 K with two materials. Reasonable correspondence is found between experiments and a 2D numerical model, using the measured magnetocaloric properties of the two materials as input.
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75.30.Sg Magnetocaloric effect, magnetic cooling

Switching nonlinearity in a superconductor-enhanced metamaterial

Cihan Kurter, Philippe Tassin, Alexander P. Zhuravel, Lei Zhang, Thomas Koschny, Alexey V. Ustinov, Costas M. Soukoulis, and Steven M. Anlage

Appl. Phys. Lett. 100, 121906 (2012); http://dx.doi.org/10.1063/1.3696297 (3 pages) | Cited 3 times

Online Publication Date: 21 March 2012

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We demonstrate a nonlinear metamaterial that can be switched between low and high transmission by controlling the power level of the incident beam. The origin of this nonlinear response is the superconducting Nb thin film employed in the metamaterial structure. We show that with moderate RF power of about 22 dBm it is possible to quench the superconducting state as a result of extremely strong current densities at the corners of the metamaterial’s split-ring resonators. We measure a transmission contrast of 10 dB and a change in group delay of 70 ns between the low and high power states.
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81.05.Xj Metamaterials for chiral, bianisotropic and other complex media
74.78.-w Superconducting films and low-dimensional structures
74.25.Sv Critical currents

Ratcheting of silicon island electrodes on substrate due to cyclic intercalation

Hamed Haftbaradaran and Huajian Gao

Appl. Phys. Lett. 100, 121907 (2012); http://dx.doi.org/10.1063/1.3696298 (4 pages) | Cited 6 times

Online Publication Date: 22 March 2012

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Analytical and numerical calculations are conducted to demonstrate a ratcheting mechanism of irreversible and accumulative deformation in patterned Si islands on substrates during lithiation/delithiation cycling. It is shown that ratcheting occurs as soon as one allows the yield stress of Si and/or the friction strength of the interface to vary from lithiation to delithiation half-cycles, and that this important failure mode can be avoided by simply reducing the lateral size of the islands below a critical length scale. The present study provides a feasible explanation for the experimentally observed ratcheting in fractured Si films on substrates.
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81.40.Lm Deformation, plasticity, and creep
62.20.fg Shape-memory effect; yield stress; superelasticity
62.20.Qp Friction, tribology, and hardness
81.40.Pq Friction, lubrication, and wear
68.35.Gy Mechanical properties; surface strains

CdSe/CdTe type-II superlattices grown on GaSb (001) substrates by molecular beam epitaxy

Jing-Jing Li, Xinyu Liu, Shi Liu, Shumin Wang, David J. Smith, Ding Ding, Shane R. Johnson, Jacek K. Furdyna, and Yong-Hang Zhang

Appl. Phys. Lett. 100, 121908 (2012); http://dx.doi.org/10.1063/1.3697676 (3 pages) | Cited 1 time

Online Publication Date: 22 March 2012

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CdSe/CdTe superlattices are grown on GaSb substrates using molecular beam epitaxy. X-ray diffraction measurements and cross-sectional transmission electron microscopy images indicate high crystalline quality. Photoluminescence (PL) measurements show the effective bandgap varies with the superlattice layer thicknesses and confirm the CdSe/CdTe heterostructure has a type-II band edge alignment. The valence band offset between unstrained CdTe and CdSe is determined as 0.63 ± 0.06 eV by fitting the measured PL peak positions using the envelope function approximation and the Kronig-Penney model. These results suggest that CdSe/CdTe superlattices are promising candidates for multi-junction solar cells and other optoelectronic devices based on GaSb substrates.
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81.05.Dz II-VI semiconductors
81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy
78.55.Et II-VI semiconductors
73.21.Cd Superlattices
68.65.Cd Superlattices
78.67.Pt Multilayers; superlattices; photonic structures; metamaterials

InGaN channel high electron mobility transistor structures grown by metal organic chemical vapor deposition

O. Laboutin, Y. Cao, W. Johnson, R. Wang, G. Li, D. Jena, and H. Xing

Appl. Phys. Lett. 100, 121909 (2012); http://dx.doi.org/10.1063/1.3697415 (3 pages) | Cited 2 times

Online Publication Date: 23 March 2012

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High electron mobility transistor (HEMT) structures of AlInGaN/AlN/InGaN/GaN were grown by metal-organic chemical vapor deposition. A combination of low growth rate and high growth temperature during synthesis of the InGaN channel layer led to significant improvement in HEMT electron transport properties. The improvement was correlated with an evolution of both surface roughness and photoluminescence intensity of InGaN. Record electron mobilities from 1070 to 1290 cm2/V·s with associated sheet charge density of ∼2 × 1013 cm−2 were obtained across the InxGa1-xN channel composition range x = 0.05 to 0.10.
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85.30.Tv Field effect devices
68.47.Fg Semiconductor surfaces
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)

A microstructurally motivated description of the deformation of vertically aligned carbon nanotube structures

Shelby B. Hutchens, Alan Needleman, and Julia R. Greer

Appl. Phys. Lett. 100, 121910 (2012); http://dx.doi.org/10.1063/1.3697686 (4 pages) | Cited 6 times

Online Publication Date: 23 March 2012

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Show Abstract
Vertically aligned carbon nanotube’s extreme compliance and mechanical energy absorption/dissipation capabilities are potentially promising aspects of their multi-functionality. Mathematical models have revealed that a hardening-softening-hardening material relation can capture the unique sequential, periodic buckling behavior displayed by vertically aligned carbon nanotubes under uniaxial compression. Yet the physical origins of these models remain unknown. We provide a microstructure-based motivation for such a phenomenological constitutive relation and use it to explore changes in structural response with nanotube volume fraction.
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61.48.De Structure of carbon nanotubes, boron nanotubes, and other related systems
81.07.De Nanotubes
62.25.-g Mechanical properties of nanoscale systems
81.40.Lm Deformation, plasticity, and creep
62.20.mq Buckling
62.20.F- Deformation and plasticity
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