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6 Feb 2012

Volume 100, Issue 6, Articles (06xxxx)

Issue Cover Spotlight Figure

Appl. Phys. Lett. 100, 061101 (2012); http://dx.doi.org/10.1063/1.3665180 (3 pages)

I. E. Khodasevych, C. M. Shah, S. Sriram, M. Bhaskaran, W. Withayachumnankul, B. S. Y. Ung, H. Lin, W. S. T. Rowe, D. Abbott, and A. Mitchell
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Twin-Foucault imaging method

Ken Harada (原田研)

Appl. Phys. Lett. 100, 061901 (2012); http://dx.doi.org/10.1063/1.3681780 (3 pages) | Cited 2 times

Online Publication Date: 6 February 2012

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A method of Lorentz electron microscopy, which enables observation two Foucault images simultaneously by using an electron biprism instead of an objective aperture, was developed. The electron biprism is installed between two electron beams deflected by 180° magnetic domains. Potential applied to the biprism deflects the two electron beams further, and two Foucault images with reversed contrast are then obtained in one visual field. The twin Foucault images are able to extract the magnetic domain structures and to reconstruct an ordinary electron micrograph. The developed Foucault method was demonstrated with a 180° domain structure of manganite La0.825Sr0.175MnO3.
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75.60.Ch Domain walls and domain structure
75.25.-j Spin arrangements in magnetically ordered materials (including neutron and spin-polarized electron studies, synchrotron-source x-ray scattering, etc.)

Suppression of photoconductivity by magnetic field in epitaxial manganite thin films

E. J. Guo, L. Wang, H. B. Lu, H. Z. Guo, M. He, K. J. Jin, G. Z. Yang, and J. Gao

Appl. Phys. Lett. 100, 061902 (2012); http://dx.doi.org/10.1063/1.3683551 (5 pages)

Online Publication Date: 6 February 2012

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The erasure of photoinduced resistance (PR) by the magnetic field was investigated in manganite films. The PR was significantly suppressed when a magnetic field was introduced at low temperature. The decrease (or increase) of PR with increment of magnetic field was observed in ferromagnetic (or paramagnetic) phases of films, respectively. Our results are suggested to be the coaction of two effects under magnetic fields: (i) the reorientation of domains and spin directions of photoexcited carriers and (ii) electrons trapped around oxygen vacancies released and recombined with majority carriers in films. The interplay of the external fields is a good demonstration of the strong coupling between spins and charges in colossal magnetoresistance materials.
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72.40.+w Photoconduction and photovoltaic effects
75.70.Ak Magnetic properties of monolayers and thin films
75.47.Gk Colossal magnetoresistance
73.61.Ng Insulators
73.50.Gr Charge carriers: generation, recombination, lifetime, trapping, mean free paths
73.50.Pz Photoconduction and photovoltaic effects

Oxygen vacancy segregation and space-charge effects in grain boundaries of dry and hydrated BaZrO3

B. Joakim Nyman, Edit E. Helgee, and Göran Wahnström

Appl. Phys. Lett. 100, 061903 (2012); http://dx.doi.org/10.1063/1.3681169 (3 pages) | Cited 6 times

Online Publication Date: 6 February 2012

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A space-charge model is applied to describe the equilibrium effects of segregation of double-donor oxygen vacancies to grain boundaries in dry and wet acceptor-doped samples of the perovskite oxide BaZrO3. The grain boundary (GB) core vacancy concentrations and electrostatic potential barriers resulting from different vacancy segregation energies were evaluated. Density-functional calculations on vacancy segregation to the mirror-symmetric Σ3 (112) [math10] tilt grain boundary are also presented. Our results indicate that oxygen vacancy segregation can be responsible for the low grain boundary proton conductivity in BaZrO3 reported in the literature.
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61.72.jd Vacancies
64.75.Gh Phase separation and segregation in model systems (hard spheres, Lennard-Jones, etc.)
66.30.H- Self-diffusion and ionic conduction in nonmetals
61.72.Mm Grain and twin boundaries
77.22.Jp Dielectric breakdown and space-charge effects

Interplay of point defects, biaxial strain, and thermal conductivity in homoepitaxial SrTiO3 thin films

S. Wiedigen, T. Kramer, M. Feuchter, I. Knorr, N. Nee, J. Hoffmann, M. Kamlah, C. A. Volkert, and Ch. Jooss

Appl. Phys. Lett. 100, 061904 (2012); http://dx.doi.org/10.1063/1.3682300 (4 pages)

Online Publication Date: 7 February 2012

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Separating out effects of point defects and lattice strain on thermal conductivity is essential for improvement of thermoelectric properties of SrTiO3. We study relations between defects generated during deposition, induced lattice strain, and their impact on thermal conductivity κ in homoepitaxial SrTiO3 films prepared by ion-beam sputtering. Lowering the deposition temperature gives rise to lattice expansion by enhancement of point defect density which increases the hardness of the films. Due to a fully coherent substrate-film interface, the lattice misfit induces a large biaxial strain. However, we can show that the temperature dependence of κ is mainly sensitive on the defect concentration.
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72.20.Pa Thermoelectric and thermomagnetic effects
73.50.Lw Thermoelectric effects
81.15.Cd Deposition by sputtering
66.70.Lm Other systems such as ionic crystals, molecular crystals, nanotubes, etc.
68.55.aj Insulators
68.60.Bs Mechanical and acoustical properties

High‐pressure phases of NaAlH4 from first principles

Xiang‐Feng Zhou (周向锋), Xiao Dong (董校), Zhisheng Zhao (赵智胜), Artem R. Oganov, Yongjun Tian (田永君), and Hui‐Tian Wang (王慧田)

Appl. Phys. Lett. 100, 061905 (2012); http://dx.doi.org/10.1063/1.3682317 (4 pages) | Cited 1 time

Online Publication Date: 8 February 2012

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High‐pressure phases of NaAlH4 are predicted using the ab initio evolutionary algorithm. our first‐principles calculations reveal that the low‐pressure I41/a phase containing AlH4 tetrahedra transforms into the monoclinic P21/c phase at 3.8 GPa and then into the orthorhombic Ima2 phase at 20.5 GPa. The predicted two structures contain AlH6 octahedra and AlH7 decahedra, respectively. Coexistence of P21/c and Ima2 phases can explain better the measured x‐ray diffraction patterns, transition pressure, and volume contraction. Our results provide a comprehensive understanding for the experimental findings.
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62.50.-p High-pressure effects in solids and liquids
81.30.Hd Constant-composition solid-solid phase transformations: polymorphic, massive, and order-disorder
61.66.Fn Inorganic compounds
64.70.K- Solid-solid transitions

Crystal and electronic structures of CuxS solar cell absorbers

Qiang Xu, Bing Huang, Yufeng Zhao, Yanfa Yan, Rommel Noufi, and Su-Huai Wei

Appl. Phys. Lett. 100, 061906 (2012); http://dx.doi.org/10.1063/1.3682503 (4 pages)

Online Publication Date: 8 February 2012

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The crystal and electronic band structures of CuxS(1.25 < x ≤ 2) are systematically studied using the density-functional theory method. For Cu2S, all the three chalcocite phases, i.e., the low-chalcocite, the high-chalcocite, and the cubic-chalcocite phases have direct band gaps around 1.3–1.5 eV, with the low-chalcocite being the most stable one. However, Cu vacancies can form spontaneously in these compounds, causing instability of Cu2S. We find that under Cu-rich condition, the anilite Cu1.75S is the most stable structure. It has a predicted band gap of 1.4 eV and could a promising solar cell absorber.
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88.40.jn Thin film Cu-based I-III-VI2 solar cells
71.15.Mb Density functional theory, local density approximation, gradient and other corrections
71.20.Nr Semiconductor compounds

Symmetry-breaking diffraction and dynamic self-trapping in optically induced hexagonal photonic lattices

Sheng Liu, Yi Hu, Peng Zhang, Xuetao Gan, Cibo Lou, Daohong Song, Jianlin Zhao, Jingjun Xu, and Zhigang Chen

Appl. Phys. Lett. 100, 061907 (2012); http://dx.doi.org/10.1063/1.3682510 (4 pages) | Cited 2 times

Online Publication Date: 8 February 2012

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We demonstrate both experimentally and numerically linear symmetry-breaking diffraction and nonlinear dynamic self-trapping of an optical beam in hexagonal photonic lattices. We show that a stripe multivortex beam undergoes asymmetric linear diffraction, but evolves into a moving self-trapped beam under a self-defocusing nonlinearity. Fine features of symmetry-breaking in diffraction of elliptical multivortex beams are also observed and discussed.
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42.65.Jx Beam trapping, self-focusing and defocusing; self-phase modulation
42.50.Wk Mechanical effects of light on material media, microstructures and particles

Enhancement of photoluminescence signal from ultrathin layers with silicon nanocrystals

S. A. Dyakov, D. M. Zhigunov, A. Hartel, M. Zacharias, T. S. Perova, and V. Yu. Timoshenko

Appl. Phys. Lett. 100, 061908 (2012); http://dx.doi.org/10.1063/1.3682537 (4 pages) | Cited 1 time

Online Publication Date: 8 February 2012

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Using the model of oscillating dipoles, we simulated the photoluminescence intensity of a triple-layered structure where the silicon nanocrystals layer was enclosed by buffer and capping silicon dioxide layers. It was found that a structure with an optimized buffer layer thickness exhibited photoluminescence which was approximately 20 times more intense than that from the structure without a buffer layer. Theoretical simulations were verified by photoluminescence measurements for the corresponding structures with silicon nanocrystals fabricated by plasma enhanced chemical vapour deposition.
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81.07.Bc Nanocrystalline materials
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
61.46.Df Structure of nanocrystals and nanoparticles ("colloidal" quantum dots but not gate-isolated embedded quantum dots)
78.55.Ap Elemental semiconductors
81.05.Cy Elemental semiconductors

Development of tilted hexagonal platelet ZnO using atmospheric pressure chemical vapor deposition and investigation of its growth mechanism

Y. Z. Yoo, S. H. Kim, G. S. Yoon, E. H. Choi, J.-W. Park, J. H. Park, B.-G. Kim, S. C. Jung, and B. M. Park

Appl. Phys. Lett. 100, 061909 (2012); http://dx.doi.org/10.1063/1.3681163 (3 pages) | Cited 1 time

Online Publication Date: 8 February 2012

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Large area ZnO films (25 × 25 cm2) grown via atmospheric pressure chemical vapor deposition (APCVD) showed unique surface morphologies of tilted hexagonal platelets. In response to the tilt angle change from 50° to 75°, haze values increased from 7% to 25%, indicating that tilt angles directly affected the light-trapping capabilities of films. These unique surfaces were created when H2O oxidizers were used, while powder-like ZnO formed when O2 or O3 oxidizers were used. Based on experimental results, corresponding density function theory, and thermodynamic calculations, the initial growth and property of APCVD for ZnO on glass were elucidated.
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81.05.Dz II-VI semiconductors
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)

Local structure of nitrogen in N-doped amorphous and crystalline GeTe

Alexander V. Kolobov, Paul Fons, Bérangère Hyot, Bernard André, Junji Tominaga, Yusuke Tamenori, Hideki Yoshikawa, and Keisuke Kobayashi

Appl. Phys. Lett. 100, 061910 (2012); http://dx.doi.org/10.1063/1.3683522 (4 pages) | Cited 2 times

Online Publication Date: 9 February 2012

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The local structure around nitrogen species in N-doped GeTe has been investigated using (soft) x-ray absorption spectroscopy and high-resolution x-ray photoemission spectroscopy. The obtained results demonstrate that while in as-deposited amorphous GeTe nitrogen is predominantly bonded to Ge atoms, upon crystallization the majority of nitrogen forms N2 molecules that are likely to be located in the grain boundaries, with only a small fraction of nitrogen species remaining bonded to Ge.
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61.43.Dq Amorphous semiconductors, metals, and alloys
61.66.Fn Inorganic compounds
64.70.kg Semiconductors
61.72.Mm Grain and twin boundaries
78.70.Dm X-ray absorption spectra
79.60.Bm Clean metal, semiconductor, and insulator surfaces

Nanostructure thermal conductivity prediction by Monte Carlo sampling of phonon free paths

Alan J. H. McGaughey and Ankit Jain

Appl. Phys. Lett. 100, 061911 (2012); http://dx.doi.org/10.1063/1.3683539 (3 pages) | Cited 5 times

Online Publication Date: 9 February 2012

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We propose a method by which the thermal conductivity of a nanostructure with arbitrary geometry can be predicted through Monte Carlo sampling of the free paths associated with phonon-phonon and phonon-boundary scattering. The required inputs are the nanostructure geometry and the bulk phonon frequencies, group velocities, and mean free paths. The method is applied to a thin film in the in-plane and cross-plane directions and to a polycrystalline bulk material. For the film, a faster approach to the bulk thermal conductivity is found compared to predictions made using the Matthiessen rule with the bulk mean free path and an average phonon-boundary scattering length.
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66.70.Df Metals, alloys, and semiconductors
63.22.-m Phonons or vibrational states in low-dimensional structures and nanoscale materials

Non-radiative complete surface acoustic wave bandgap for finite-depth holey phononic crystal in lithium niobate

Didit Yudistira, Yan Pennec, Bahram Djafari Rouhani, Samuel Dupont, and Vincent Laude

Appl. Phys. Lett. 100, 061912 (2012); http://dx.doi.org/10.1063/1.3684839 (4 pages) | Cited 1 time

Online Publication Date: 10 February 2012

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We demonstrate the existence of non-radiative complete surface acoustic wave (SAW) bandgaps for two-dimensional piezoelectric phononic crystals of holes. Holes of finite depth in a semi-infinite LiNbO3 substrate are specifically considered. SAW bandgaps are determined from the band structure calculated with a three-dimensional finite element method taking into account material anisotropy and piezoelectricity. The effect of hole geometry on the bandgaps has been investigated. It is further found that the complete band gap does not close for moderately conical holes.
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77.65.Dq Acoustoelectric effects and surface acoustic waves (SAW) in piezoelectrics
77.84.Ek Niobates and tantalates
68.35.Iv Acoustical properties
63.20.D- Phonon states and bands, normal modes, and phonon dispersion
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