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25 Jun 2012

Volume 100, Issue 26, Articles (26xxxx)

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Appl. Phys. Lett. 100, 261104 (2012); http://dx.doi.org/10.1063/1.4711253 (4 pages)

Marcelo Davanço, Jun Rong Ong, Andrea Bahgat Shehata, Alberto Tosi, Imad Agha, Solomon Assefa, Fengnian Xia, William M. J. Green, Shayan Mookherjea, and Kartik Srinivasan
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Ultraviolet emission efficiency enhancement of a-plane AlGaN/GaN multiple-quantum-wells with increasing quantum well thickness

Huei-Min Huang, Chiao-Yun Chang, Yu-Pin Lan, Tien-Chang Lu, Hao-Chung Kuo, and Shing-Chung Wang

Appl. Phys. Lett. 100, 261901 (2012); http://dx.doi.org/10.1063/1.4730438 (4 pages) | Cited 1 time

Online Publication Date: 25 June 2012

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The defect-induced carrier localization in nonpolar a-plane (Al,Ga)N/GaN multiple quantum wells (MQWs) structures with different well thickness have been investigated. A strong variation of temperature-dependent photoluminescence peak energy was observed and attributed to the existence of the localized states. The degree of carrier localization in these defect-induced states was more prominent in the case of MQWs with the wider well width. In addition, the ultraviolet light emission efficiency revealed a 3-fold enhancement with increasing the well width from 1.6 nm to 7.3 nm, due to the strong carrier localization generated from the quantum-wire-like features formed by the intersection between basal stacking faults and quantum wells.
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78.55.Cr III-V semiconductors
78.67.De Quantum wells
61.72.Nn Stacking faults and other planar or extended defects
71.55.Eq III-V semiconductors
73.21.Fg Quantum wells

Quantum Monte Carlo study of high-pressure cubic TiO2

M. Abbasnejad, E. Shojaee, M. R. Mohammadizadeh, M. Alaei, and Ryo Maezono

Appl. Phys. Lett. 100, 261902 (2012); http://dx.doi.org/10.1063/1.4730608 (4 pages) | Cited 3 times

Online Publication Date: 25 June 2012

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We have studied the high-pressure cubic fluorite polymorph of TiO2 (c-TiO2) using the diffusion Monte Carlo (DMC) method. The estimated bulk modulus is within the range reported previously in density functional studies, high, but does not rival that of diamond. The calculated excitation energies within DMC are consistent with the results of GW approximation. The infrared frequency of c-TiO2, obtained via the frozen phonon method within DMC, shows non-negligible anharmonicity. This suggests that c-TiO2 might be stabilized if this anharmonicity is considered. Our DMC results could help to establish more accurate results for c-TiO2 compared with the widely-scattered mean-field results.
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61.66.Fn Inorganic compounds
63.20.Ry Anharmonic lattice modes
78.30.Hv Other nonmetallic inorganics
81.40.Jj Elasticity and anelasticity, stress-strain relations
62.20.de Elastic moduli
62.50.-p High-pressure effects in solids and liquids

Strategy for designing broadband epsilon-near-zero metamaterial with loss compensation by gain media

L. Sun and K. W. Yu

Appl. Phys. Lett. 100, 261903 (2012); http://dx.doi.org/10.1063/1.4730380 (4 pages) | Cited 4 times

Online Publication Date: 26 June 2012

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A strategy is proposed to design the broadband gain-doped epsilon-near-zero (GENZ) metamaterial. Based on the Milton representation of effective permittivity, the strategy starts in a dimensionless spectral space, where the effective permittivity of GENZ metamaterial is simply determined by a pole-zero structure corresponding to the operating frequency range. The physical structure of GENZ metamaterial is retrieved from the pole-zero structure via a tractable inverse problem. The strategy is of great advantage in practical applications and also theoretically reveals the cancellation mechanism of the broadband near-zero permittivity phenomenon in the spectral space.
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42.70.-a Optical materials
77.22.Ch Permittivity (dielectric function)

Tunable coupled surface acoustic cavities

M. M. de Lima, Jr., P. V. Santos, Yu. A. Kosevich, and A. Cantarero

Appl. Phys. Lett. 100, 261904 (2012); http://dx.doi.org/10.1063/1.4730398 (4 pages) | Cited 1 time

Online Publication Date: 26 June 2012

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We demonstrate the electric tuning of the acoustic field in acoustic microcavities (MCs) defined by a periodic arrangement of metal stripes within a surface acoustic delay line on LiNbO3 substrate. Interferometric measurements show the enhancement of the acoustic field distribution within a single MC, the presence of a “bonding” and “anti-bonding” modes for two strongly coupled MCs, as well as the positive dispersion of the “mini-bands” formed by five coupled MCs. The frequency and amplitude of the resonances can be controlled by the potential applied to the metal stripes.
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43.58.Kr Spectrum and frequency analyzers and filters; acoustical and electrical oscillographs; photoacoustic spectrometers; acoustical delay lines and resonators
42.79.Jq Acousto-optical devices

High efficient external resonator Raman laser based on the monoclinic single crystal BaTeMo2O9

Zeliang Gao, Shande Liu, Shaojun Zhang, Weiguo Zhang, Jingliang He, and Xutang Tao

Appl. Phys. Lett. 100, 261905 (2012); http://dx.doi.org/10.1063/1.4730635 (4 pages) | Cited 2 times

Online Publication Date: 26 June 2012

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In this paper spontaneous Raman spectra of monoclinic single crystals of BaTeMo2O9 (BTM) along the Z axis has been characterized, and a highly efficient stimulated Raman scattering laser operating at 1178 nm has been realized based on the BTM crystal. The Raman resonator possesses a threshold of 28 MW/cm2 at 1064 nm and a maximum output pulse energy of 19.2 mJ for the first-order Stokes with an optical-to-optical conversion efficiency of 48% and a slope efficiency of 61.2%. The largest optical-to-optical conversion efficiency can reach 50.4% at a pump energy of 28.8 mJ. Our experimental results demonstrate that BTM can be used as an excellent near and mid-infrared Raman laser material.
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42.55.Ye Raman lasers
42.60.By Design of specific laser systems
42.60.Da Resonators, cavities, amplifiers, arrays, and rings
42.65.Dr Stimulated Raman scattering; CARS
42.65.Es Stimulated Brillouin and Rayleigh scattering

Infrared absorbance of silicene and germanene

Friedhelm Bechstedt, Lars Matthes, Paola Gori, and Olivia Pulci

Appl. Phys. Lett. 100, 261906 (2012); http://dx.doi.org/10.1063/1.4731626 (3 pages) | Cited 6 times

Online Publication Date: 27 June 2012

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Calculating the complex dielectric function for optical interband transitions we show that the two-dimensional crystals silicene and germanene possess the same low-frequency absorbance as graphene. It is determined by the Sommerfeld finestructure constant. Deviations occur for higher frequencies when the first interband transitions outside K or K′ contribute. The low-frequency results are a consequence of the honeycomb geometry but do not depend on the group-IV atom, the sheet buckling, and the orbital hybridization. The two-dimensional crystals may be useful as absorption normals in silicon technology.
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78.30.Na Fullerenes and related materials
61.48.Gh Structure of graphene
73.22.Pr Electronic structure of graphene
77.22.Ch Permittivity (dielectric function)
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
78.67.Wj Optical properties of graphene

Experimental realization of a broadband conformal mapping lens for directional emission

Chendong Gu, Kan Yao, Weixin Lu, Yun Lai, Huanyang Chen, Bo Hou, and Xunya Jiang

Appl. Phys. Lett. 100, 261907 (2012); http://dx.doi.org/10.1063/1.4731877 (3 pages) | Cited 2 times

Online Publication Date: 27 June 2012

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We experimentally realize a transformation lens for directional emission at microwave frequencies. The refractive index of the lens, based on the design of conformal mapping, has variation from 1 to 4 inside a semi-circular geometry. By drilling subwavelength holes in dielectric plates inhomogeneously, we design the lens sample. The scanning measurements of the electric field of the lens demonstrate the high directivity of emission, within a broadband working frequency region of 7-11 GHz.
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84.40.-x Radiowave and microwave (including millimeter wave) technology
41.20.Jb Electromagnetic wave propagation; radiowave propagation

Inter-tube thermal conductance in carbon nanotubes arrays and bundles: Effects of contact area and pressure

William J. Evans, Meng Shen, and Pawel Keblinski

Appl. Phys. Lett. 100, 261908 (2012); http://dx.doi.org/10.1063/1.4732100 (4 pages) | Cited 4 times

Online Publication Date: 28 June 2012

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We use molecular dynamics simulations to compute junction thermal conductance of carbon nanotubes as a function of crossing angle and pressure, and conductivity of arrays and bundles consisting of multiple junctions as a function of pressure. Two types of arrays are investigated: crossbar structures consisting of alternating orthogonal layers of nanotubes and close-packed bundles of parallel oriented tubes. Conductance of 90° junction increases with pressure 4 fold before saturation; cross-plane thermal conductivity of crossbar structures increases by a factor of 2. For parallel junctions pressure doubles the conductance while thermal conductivity of nanotubes bundles is more or less pressure independent.
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66.70.Lm Other systems such as ionic crystals, molecular crystals, nanotubes, etc.
65.80.-g Thermal properties of small particles, nanocrystals, nanotubes, and other related systems
81.07.De Nanotubes
61.48.De Structure of carbon nanotubes, boron nanotubes, and other related systems

Direct evidence for stress-induced (001) anisotropy of rapid-annealed FePt thin films

S. N. Hsiao, S. H. Liu, S. K. Chen, T. S. Chin, and H. Y. Lee

Appl. Phys. Lett. 100, 261909 (2012); http://dx.doi.org/10.1063/1.4730963 (4 pages) | Cited 4 times

Online Publication Date: 28 June 2012

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Roles of rapid thermal annealing (RTA) on the evolution of crystallographic anisotropy of single-layered FePt films have been characterized. We observed a huge biaxial tensile stress of 2.18 GPa induced with increasing heating rate from 0.5 to 40 K/s. The result is a transition of orientation from (111) to perfect (001) texture. The later then degrades at heating rates ≥80 K/s due to morphological variation. The advantages of RTA are to induce tensile stress by densification reaction within a very short time and to simultaneously impede thickness-dependent dynamic stress relaxation.
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75.70.Ak Magnetic properties of monolayers and thin films
81.40.Jj Elasticity and anelasticity, stress-strain relations
68.55.jm Texture
62.40.+i Anelasticity, internal friction, stress relaxation, and mechanical resonances
68.60.Bs Mechanical and acoustical properties
75.30.Gw Magnetic anisotropy

Resonant cavity modes in gallium oxide microwires

Iñaki López, Emilio Nogales, Bianchi Méndez, and Javier Piqueras

Appl. Phys. Lett. 100, 261910 (2012); http://dx.doi.org/10.1063/1.4732153 (3 pages) | Cited 3 times

Online Publication Date: 28 June 2012

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Fabry Perot resonant modes in the optical range 660–770 nm have been detected from single and coupled Cr doped gallium oxide microwires at room temperature. The luminescence is due to chromium ions and dominated by the broad band involving the 4T2–4A2 transition, strongly coupled to phonons, which could be of interest in tunable lasers. The confinement of the emitted photons leads to resonant modes detected at both ends of the wires. The separation wavelength between maxima follows the Fabry-Perot dependence on the wire length and the group refractive index for the Ga2O3 microwires.
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81.05.Hd Other semiconductors
78.67.Uh Nanowires
81.07.Gf Nanowires
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
78.55.Hx Other solid inorganic materials

Coherent phonon dynamics at the martensitic phase transition of Ni2MnGa

S. O. Mariager, A. Caviezel, P. Beaud, C. Quitmann, and G. Ingold

Appl. Phys. Lett. 100, 261911 (2012); http://dx.doi.org/10.1063/1.4730946 (4 pages)

Online Publication Date: 29 June 2012

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We use time-resolved optical reflectivity to study the laser stimulated dynamics in the magnetic shape memory alloy Ni2MnGa. We observe two coherent optical phonons, at 1.2 THz in the martensite phase and at 0.7 THz in the pre-martensite phase, which we interpret as a zone-folded acoustic phonon and a heavily damped amplitudon, respectively. In the martensite phase the martensitic phase transition can be induced by a fs laser pulse on a timescale of a few ps.
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63.20.D- Phonon states and bands, normal modes, and phonon dispersion
64.70.kd Metals and alloys
78.47.jg Time resolved reflection spectroscopy
81.30.Kf Martensitic transformations
81.40.Lm Deformation, plasticity, and creep
62.20.fg Shape-memory effect; yield stress; superelasticity

Orientation-dependent hardness and strain rate sensitivity in nanotwin copper

J. C. Ye, Y. M. Wang, T. W. Barbee, Jr., and A. V. Hamza

Appl. Phys. Lett. 100, 261912 (2012); http://dx.doi.org/10.1063/1.4731242 (5 pages) | Cited 4 times

Online Publication Date: 29 June 2012

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We observe a strong twin-orientation dependent hardness and strain rate sensitivity (m) in nanotwin copper. A highest m value of 0.059 ± 0.004 and an activation volume (V) of ∼10b3 are measured when deformation is predominately vertical to twin boundaries (TBs), whereas a much smaller m and larger V are observed for the direction parallel to TBs. Dislocation density is found to have a stronger impact on m and V in nanotwin materials, compared to that in coarse-grained materials.
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81.40.Np Fatigue, corrosion fatigue, embrittlement, cracking, fracture, and failure
61.72.Hh Indirect evidence of dislocations and other defects (resistivity, slip, creep, strains, internal friction, EPR, NMR, etc.)
61.72.Mm Grain and twin boundaries
62.20.Qp Friction, tribology, and hardness
81.40.Lm Deformation, plasticity, and creep
62.20.F- Deformation and plasticity

A “universal” criterion for metallic glass formation

Li-Min Wang, Yongjun Tian, Riping Liu, and Weihua Wang

Appl. Phys. Lett. 100, 261913 (2012); http://dx.doi.org/10.1063/1.4731881 (4 pages) | Cited 1 time

Online Publication Date: 29 June 2012

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We have established a generally applicable criterion for the critical cooling rates Rc needed for the formation of metallic glasses, based on thermodynamic and kinetic properties of ten categories of metal-based alloys ranging from binary to multicomponent systems. Rc is found to depend on several fundamental properties of materials including the glass transition temperature (normalized with respect to the liquidus temperature), entropy of fusion, and kinetic fragility. Such a relation reproduces the experimental Rc values of 43 metallic glasses remarkably well with a R2 value of 0.94. The explanation of Rc provides guidance in search of metallic glasses.
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64.70.qd Thermodynamics and statistical mechanics
61.43.Fs Glasses
82.60.Fa Heat capacities and heats of phase transitions
82.60.Lf Thermodynamics of solutions
64.70.dj Melting of specific substances
64.70.pe Metallic glasses
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