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Journal of Applied Physics

Issue 3 • Date Feb 2010

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Displaying Results 1 - 25 of 129
  • Terahertz dielectric response and optical conductivity of n-type single-crystal ZnO epilayers grown by metalorganic chemical vapor deposition

    Page(s): 033101 - 033101-4
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    Using terahertz time-domain spectroscopy, we measured the frequency dependent complex dielectric response and conductivity of n-type single-crystal ZnO epilayers with different carrier concentrations over the frequency range from 0.1 to 3.0 THz. The measured complex dielectric response and conductivity are analyzed using Drude model. View full abstract»

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  • Optical properties of ZnO/Zn0.9Mg0.1O multiple quantum wells grown on (111) Si using buffer assisted pulsed-laser deposition

    Page(s): 033102 - 033102-4
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    This work investigates the photoluminescence properties of ZnO/Zn0.9Mg0.1O multiple quantum wells (MQWs), which have been fabricated by pulsed-laser deposition on (111) Si substrates using intervening epitaxial Lu2O3 buffer layers. In ZnO/Zn0.9Mg0.1O MQWs, the luminescence is dominated by localized exciton (LE) emission throughout the whole temperature range studied. With increasing temperature from 10 to 300 K, the LE emission redshifts by 38 meV. This redshift is believed to be due to the thermalized excitons occupying higher-lying localized states where they emit higher energy radiation and temperature-induced band gap shrinkage. Moreover, the LE emission from the MQWs decays more slowly than exciton emission from ZnO. In addition, the LE emission in the MQWs shows a systematic blueshift with decreasing well width, which is consistent with a quantum size effect. View full abstract»

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  • Ultralow threshold green lasing and optical bistability in ZBNA (ZrF4BaF2NaFAlF3) microspheres

    Page(s): 033103 - 033103-6
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    Upconversion lasing and fluorescence from active microspheres fabricated from a novel fluorozirconate, Er3+ doped glass, ZBNA (ZrF4BaF2NaFAlF3), when pumped at 978 nm via a tapered optical fiber is demonstrated. An ultralow, green lasing threshold of ∼3 μW for 550 nm emissions is measured. This is one order of magnitude lower than that previously reported for ZBLAN (ZrF4BaF2LaF3AlF3NaF) microspheres. Optical bistability effects in ZBNA microspheres are reported and the bistable mechanism is discussed and attributed to shifts of the whispering gallery modes due to thermal expansion of the sphere, where heating is achieved by optical pumping around 978 nm. The effect of the bistability on the upconversion lasing is examined and we report multiple bistability loops within the microspheres. View full abstract»

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  • Femtosecond carrier dynamics in native and high resistivity iron-doped GaxIn1-xAs

    Page(s): 033104 - 033104-4
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    We report femtosecond transient reflectivity measurements of as-grown and iron-doped GaxIn1-xAs. A hybrid vertical Bridgman and gradient freezing directional solidification process was employed for the growth of high quality Ga0.69In0.31As:Fe crystals with the uniform impurity doping concentration necessary for high resistivity (1.6×107 Ω cm) and high mobility [(2–3)×103 cm2/Vs] material. Carrier lifetimes range from ∼62 fs for as-grown Ga0.09In0.91As to ∼306 fs for Ga0.69In0.31As:Fe. The high carrier mobility along with high resistivity and subpicosecond carrier lifetimes make Ga0.69In0.31As:Fe an excellent candidate for photoconducting antenna based terahertz emitters. View full abstract»

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  • The challenge of unity wall plug efficiency: The effects of internal heating on the efficiency of light emitting diodes

    Page(s): 033105 - 033105-6
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    We develop a self-consistent model to describe the internal heating of high power light emitting diodes (LEDs) and use this model to simulate the operation of GaAs–AlGaAs double heterostructure LEDs. We account for the heating by nonradiative recombination processes in the simulations and solve self-consistently the steady state junction temperature. Based on the simulation results, we discuss the plausibility of unity conversion efficiency in LEDs and also the mechanisms underlying the efficiency droop. We show that the rise in the junction temperature limits the light output available from LEDs and further degrades the efficiency of operation at high operating currents. In addition to high power applications we study the optimal operating point and discuss the methods to increase the efficiency of LEDs toward the thermodynamical limits. View full abstract»

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  • Neodymium doping in UV-IR transparent ferroelectric BaMgF4

    Page(s): 033106 - 033106-5
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    The incorporation of Nd3+ in the ferroelectric fluoride BaMgF4 crystal is investigated for its potential application as self-frequency-converter solid state laser. Low temperature high-resolution optical spectroscopy shows that Nd3+ is located at two well distinguished sites referred as A and B. The whole set of energy levels in the excited 4F3/2, 4I11/2, and ground 4I9/2 states are identified for both Nd3+ sites and their crystallographic location in BaMgF4 lattice is discussed. The radiative lifetime of the 4F3/2 metastable laser level is obtained to be 1430 and 800 μs for site A and B, respectively, and the luminescence quantum efficiency of the 4F3/2 metastable state for the dominant A site is 87%. The possibility of annihilating the contribution of the B site in the optical spectrum is demonstrated by means of the incorporation Na+ into the crystal. View full abstract»

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  • Interaction of vacuum ultraviolet excimer laser radiation with fused silica. I. Positive ion emission

    Page(s): 033107 - 033107-13
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    We report time- and mass-resolved measurements of Si+ and O+ emission from ultraviolet-grade fused silica during exposure to pulsed 157 nm excimer laser radiation at fluences below the threshold for optical breakdown. The emission intensities are increased by treatments that increase the density of surface defects, such as abrasion, and are reduced by treatments that reduce the density of surface defects, such as annealing. Ion emission is a sensitive probe of mechanical damage on silica surfaces. The mean ion kinetic energies are typically several eV: 8–9 eV for Si+ and about 4 eV for O+. Hartree–Fock studies of candidate defect sites suggest that antibonding states excited by 157 nm photons play a critical role in the release of these ions. We propose that positive ion emission from fused silica under these conditions is best explained by a hybrid mechanism involving (a) the excitation of an antibonding chemical state (Menzel–Gomer–Redhead mechanism) and (b) the acceleration of the positive ion by repulsive electrostatic forces due to the photoionization of nearby electron traps. View full abstract»

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  • Interaction of vacuum ultraviolet excimer laser radiation with fused silica: II. Neutral atom and molecule emission

    Page(s): 033108 - 033108-11
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    We report mass-resolved time-of-flight measurements of neutral Si, O, and SiO from ultraviolet-grade fused silica during pulsed 157-nm irradiation at fluences well below the threshold for optical breakdown. Although the emission intensities are strongly affected by thermal treatments that affect the density of strained bonds in the lattice, they are not consistently affected by mechanical treatments that alter the density of point defects, such as polishing and abrasion. We propose that the absorption of single 157 nm photons cleave strained bonds to produce defects that subsequently diffuse to the surface. There they react with dangling bonds to release neutral atoms and molecules. Hartree–Fock calculations on clusters containing these defects support the contention that defect interactions can yield emission. More direct emission by the photoelectronic excitation of antibonding chemical states is also supported. View full abstract»

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  • Interaction of vacuum ultraviolet excimer laser radiation with fused silica. III. Negative ion formation

    Page(s): 033109 - 033109-9
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    We report mass- and time-resolved measurements of negative ions produced by exposing fused silica to 157 nm radiation at fluences below the threshold for optical breakdown. The principal observed negative ions are O-, Si-, and SiO-, in order of decreasing intensity. The peak in the negative ion time-of-flight signals occurs after the peak in the positive ion signal and before the peak in the corresponding neutral atom or molecule signal. The negative ion intensities are strong functions of the degree of overlap between the positive ion and neutral atom densities. We propose that O-, Si-, and SiO- are created after the laser pulse, by electron attachment to these neutral particles and that the electrons participating in attachment events are trapped in the electrostatic potential of the positive ions. View full abstract»

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  • Intense 1.54 μm fluorescence of Er3+/Yb3+ codoped phosphate glass and the three-photon phenomenon of near infrared upconversion luminescence

    Page(s): 033110 - 033110-5
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    Transparent P2O5CaONa2O glasses codoped with Er3+ and Yb3+ ions were fabricated by high-temperature melting technique. Strong near infrared and visible upconversion luminescence were observed under 975 nm laser diode excitation, and the luminescence processes were explained in details. For 1.54 μm band, the calculated peak emission cross sections is 1.04×10-20 cm2, and it is better than that in germinate, tellurite, silicate, and other phosphate glasses. The near infrared upconversion emission around 828 nm contains two-photon and three-photon processes, and it is attributed to the transition 4S3/24I13/2, which enhances the population of the upper level for 1.54 μm laser emission and reduces the energy loss due to upconversion. Efficient near infrared fluorescence indicates Er3+/Yb3+ codoped phosphate glass is a promising laser and amplifier material. View full abstract»

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  • Continuously tunable, phase-locked, continuous-wave terahertz generator based on photomixing of two continuous-wave lasers locked to two independent optical combs

    Page(s): 033111 - 033111-7
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    A continuously tunable, phase-locked, single-frequency, continuous-wave (cw) terahertz generator has been demonstrated around 120 GHz, corresponding to the spectral bandwidth of the F-band unitraveling-carrier photodiode (UTC-PD) used as a photomixer in this study. This cw-terahertz generator is based on photomixing of an accurately tunable cw laser and a tightly fixed cw laser in the optical frequency region, phase locked to two independent optical combs. The continuous tuning range of the presented method was three orders of magnitude around 0.1 THz and two orders around 1 THz broader than that of a previous photomixing method in which two cw lasers are phase locked to a single optical comb, and fully covered the available spectral bandwidth of the F-band UTC-PD. The spectral behavior of the tight locking and continuous tuning of 120 GHz cw-terahertz radiation was confirmed in real time by use of a terahertz-comb-referenced spectrum analyzer. This cw-terahertz generator shows promise as a terahertz clock and synthesizer for terahertz frequency metrology. View full abstract»

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  • Effect of liquid environment on laser-induced backside wet etching of fused silica

    Page(s): 033112 - 033112-8
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    In laser-induced backside wet etching (LIBWE), the liquid absorbent indirectly heats the transparent material, causing explosive phase change and cavitation. Accordingly, the hydrodynamics of the absorbing liquid, including the size of the liquid chamber, is strongly related to the ablation process. Because the hydrodynamics is dependent on the elastic deformation of the sample, the sample thickness also affects the performance of LIBWE. In this work, experimental analyses were performed to elucidate the hydrodynamics in LIBWE and the effect on the etch rate by varying the liquid chamber size and sample thickness. A KrF excimer laser was used to ablate fused silica samples in toluene and the etch rate was quantified using a scanning profilometer. Laser flash shadowgraphy and photodeflection probing techniques were employed for in situ measurement of the laser-induced hydrodynamics and displacement of the sample, respectively, with a time resolution of approximately nanoseconds. To directly observe the effect of increased liquid pressure on LIBWE, the liquid pressure on the etching spot was locally increased by an external shock wave and the etch results are examined. This study confirms that the photomechanical effects from the laser-induced bubble plays a key role in the LIBWE process, revealing a linear relationship between the etch rate the applied recoil momentum. However, the dependence of the etch rate on the chamber size and sample thickness could not be explained by the change in recoil momentum, i.e., by the bubble pressure. Instead, transient deformation of the sample by the pressure impact was estimated to be mainly responsible for the dependence. View full abstract»

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  • Ultraviolet photorefraction at 325 nm in doped lithium niobate crystals

    Page(s): 033113 - 033113-7
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    We studied the photorefractive effect of lithium niobate (LiNbO3) doped with Mg, Zn, In, Hf, or codoped with Mg and Fe at an ultraviolet (UV) wavelength down to 325 nm. It is found that the UV photorefraction of LiNbO3 doped with Mg, Zn, In, or Hf was enhanced significantly as compared to that of the nominally pure LiNbO3. Our results show that the property of resistance against photorefraction in highly Mg, Zn, In, or Hf doped LiNbO3 is true only in the visible and near-infrared wavelength range. By contrast, these crystals exhibit excellent photorefractive characteristics at UV wavelength of 325 nm, even better than those at 351 nm. For example, the photorefractive two-wave coupling gain coefficient Γ and the photorefractive recording sensitivity at 325 nm were measured to be ∼38 cm-1 and 37.7 cm/J, respectively, in a LiNbO3 crystal doped with 9 mol% Zn. The photorefractive response time of a Mg:LiNbO3 with a 9 mol% Mg was measured to be 73 ms with a total recording intensity of 614 mW/cm2 at 325 nm. In highly Mg, Zn, In, or Hf doped LiNbO3 crystals, diffusion dominates over photovoltaic effect and electrons are the dominant charge carriers in UV photorefraction at 325 nm. The results are also of interest to the study on the defect structure of LiNbO3 near to the absorption edge. View full abstract»

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  • Influence of an electric field on the operation of terahertz intracenter silicon lasers

    Page(s): 033114 - 033114-5
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    We have investigated the influence of a pulsed electric field on the intracenter population inversion between phosphorus donor states in silicon. Terahertz-range electroluminescence resulted from the populated excited donor states. It grows linearly above the impurity breakdown field but saturates at excitation powers exceeding 100 W at 5 μs pulse length. An electric field applied to the optically pumped silicon laser reduces the efficiency of the 2p0→1s(T2) phosphorus transition already at voltages below the impurity breakdown. The appearance of a current through the laser sample results in a fast quenching of the laser emission that indicates a reduction in the inversed population between the laser states. View full abstract»

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  • Nonlinear and saturable absorption characteristics of amorphous InSe thin films

    Page(s): 033115 - 033115-6
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    We prepared very thin amorphous InSe films and investigated the thickness dependence of the nonlinear absorption by pump-probe and open aperture Z-scan techniques. While thinner films (20 and 52 nm) exhibit saturable absorption, thicker films (70 and 104 nm) exhibit nonlinear absorption for 4 ns, 65 ps, and 44 fs pulse durations. This behavior is attributed to increasing localized defect states in the energy band gap as the film thickness increases. We developed a theoretical model incorporating one photon, two photon, and free carrier absorptions and their saturations to derive the transmission in the open aperture Z-scan experiment. The theory of open aperture Gaussian beam Z-scan based on the Adomian decomposition method was used to fit the experimental curves. Nonlinear absorption coefficients along with saturation intensity thresholds were extracted from fitting the experimental results for all pulse durations. The lowest saturation threshold was found about 3×10-3 GW/cm2 for 20 nm film thickness with nanosecond pulse duration and increased about four orders of magnitude for 104 nm film thickness. View full abstract»

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  • Atom lithography with metastable helium

    Page(s): 033116 - 033116-6
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    A bright metastable helium (He*) beam is collimated sequentially with the bichromatic force and three optical molasses velocity compression stages. Each He* atom in the beam has 20 eV of internal energy that can destroy a molecular resist assembled on a gold coated silicon wafer. Patterns in the resist are imprinted onto the gold layer with a standard selective etch. Patterning of the wafer with the He* was demonstrated with two methods. First, a mesh was used to protect parts of the wafer making an array of grid lines. Second, a standing wave of λ=1083 nm light was used to channel and focus the He* atoms into lines separated by λ/2. The patterns were measured with an atomic force microscope establishing an edge resolution of 80 nm. Our results are reliable and repeatable. View full abstract»

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  • Self-masking controlled by metallic seed layer during glass dry-etching for optically scattering surfaces

    Page(s): 033301 - 033301-6
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    During reactive ion etching in a high density Ar/CF4 plasma, nonvolatile halogen compounds are generated on the glass surface and act as statistically distributed micromasks. As a consequence surface roughness occurs, which can be used as additional surface functionality in certain classes of applications, where defined optical scattering is desired. The glass etch process described in this contribution enables user-defined scattering characteristics. The deposition of a thin metallic layer of Cu on top of the samples before dry-etching provides an additional seed for the production of micromasks, resulting in higher reproducibility. By varying the etch parameters a multitude of different surface morphologies can be realized both on borosilicate glasses and on fused silica. View full abstract»

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  • Effect of surface roughness on thermal conductivity of silicon nanowires

    Page(s): 033501 - 033501-5
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    Reducing the thermal conductivity of nanowires may enhance their already exciting efficiency of thermoelectric energy conversion. Using molecular dynamics simulations, we demonstrate that the thermal conductivity of silicon nanowires could be significantly decreased by patterning (or etching) induced roughness of the nanowire surfaces. The type, amplitude, and wavelength of the surface roughness all have profound effects, and the thermal conductivity could be reduced more when the wavelength is smaller or the amplitude is larger. Such an effect of roughness on the thermal conductivity is furthermore found to be coupled with the effects of nanowire cross-sectional size and length. Typically, the roughness effect is more prominent in longer and larger nanowires. View full abstract»

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  • Optical constants of Cu(In1-xGax)5Se8 crystals

    Page(s): 033502 - 033502-6
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    Spectroscopic ellipsometry has been used to characterize the dielectric functions of bulk Cu(In1-xGax)5Se8 crystals. Spectra were measured at room temperature over the energy range 0.74–5.2 eV. The dielectric functions as well as the complex refractive index, the absorption coefficient, and the normal-incidence reflectivity have been modeled using a modification of the Adachi model. The results are in a good agreement with the experimental data over the entire range of photon energies. The model parameters (strength, threshold energy, and broadening) have been determined using the simulated annealing algorithm. The transition energies E0 and E1A are found to shift linearly to higher energies as the gallium content increases. View full abstract»

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  • The role of rare earth elements in the structures of FeB-based glass forming liquid alloys

    Page(s): 033503 - 033503-4
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    The structures of liquid Fe72RE6B22 (RE=Sc, Er, Ho, Dy, Y, Sm, Gd, Nd, and Ce) alloys were investigated by ab initio molecular dynamics simulation. The results show that the chemical affinity of Fe-RE and RE-B may influence the glass forming ability more than the atomic size of RE atom in these alloys. As expected, the <0,3,6,0> polyhedron dominates around B atoms and is significantly enriched by adding RE elements to the liquid Fe78B22 alloy. The good glass formers do not correspond to the larger percentages but to more RE atoms in the shell of <0,3,6,0> polyhedron. These features suggest that the effect of the chemical composition of <0,3,6,0> polyhedron on the glass forming ability may be larger than that of its quantity in these alloys. View full abstract»

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  • Slit waveguide based terahertz near-field microscopy: Prospects and limitations

    Page(s): 033504 - 033504-7
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    We experimentally and numerically investigate the transmission of single cycle terahertz pulses through subwavelength slit apertures featuring zero cutoff frequency and very low attenuation. Employing a polaritonic approach we demonstrate that wave forms transmitted through slit samples with slit widths as small as λ/1000 can be visualized and analyze the applicability of this approach to terahertz near-field microscopy. Finite element simulations are used to quantitatively investigate resolution limitations due to imperfect experimental configurations. Our results show that resolutions on the scale of the slit width are possible; however, they demand an accurate control of the distance between the imaging aperture and the sample. This is because the presence of small gaps leads to around-the-bend waveguiding effects resulting in a significant reduction of the attainable resolution. View full abstract»

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  • Atomic structure of a Ni diffused Si (001) surface layer: Precursor to formation of NiSi2 at low temperature

    Page(s): 033505 - 033505-4
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    The atomic structure in a Ni-diffused Si (001) surface layer was determined by using aberration-corrected scanning-transmission electron microscopy (STEM). The STEM investigation revealed three key findings. First, Ni atoms occupy the tetrahedral interstitial voids of a Si crystal at an occupancy factor of about 0.5. Second, the Si atoms in the surface layer can dissociate from their lattice site at a low substrate temperature (300 °C). Third, CaF2-type NiSi2 is formed at that substrate temperature and is lattice matched to the surface layer. View full abstract»

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  • On eigenmodes, stiffness, and sensitivity of atomic force microscope cantilevers in air versus liquids

    Page(s): 033506 - 033506-9
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    The effect of hydrodynamic loading on the eigenmode shapes, modal stiffnesses, and optical lever sensitivities of atomic force microscope (AFM) microcantilevers is investigated by measuring the vibrations of such microcantilevers in air and water using a scanning laser Doppler vibrometer. It is found that for rectangular tipless microcantilevers, the measured fundamental and higher eigenmodes and their equivalent stiffnesses are nearly identical in air and in water. However, for microcantilevers with a tip mass or for picket shaped cantilevers, there is a marked difference in the second (and higher) eigenmode shapes between air and water that leads to a large decrease in their modal stiffness in water as compared to air as well as a decrease in their optical lever sensitivity. These results are explained in terms of hydrodynamic interactions of microcantilevers with nonuniform mass distribution. The results clearly demonstrate that tip mass and hydrodynamic loading must be taken into account in stiffness calibration and optical lever sensitivity calibration while using higher-order eigenmodes in dynamic AFM. View full abstract»

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  • Electronic and ionic contributions to the constant-volume specific heat of carbon tetrachloride shocked at pressures up to 23 GPa

    Page(s): 033507 - 033507-5
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    For carbon tetrachloride, a temperature Hugoniot at 7–23 GPa and a distribution of the constant-volume specific heat on the Hugoniot CV(T) at 1057–3275 K are simultaneously estimated from the Walsh–Christian (WC) equation such that the Hugoniot fits well to the existing measured data. The estimated CV(T) distribution reveals the significance of the contribution of electrons and ions to the specific heat. That is, in contrast to the almost uniform distribution of the specific heat predicted from the Debye equation, the CV(T) distribution increases significantly with an increase in the Hugoniot temperature due to thermal excitation of electrons at 1057–1500 K (7–10.1 GPa), additional activation of the dimerization reaction at 1500–2350 K (10.1–16 GPa), and also additional activation of the polymerization reaction at 2350–3275 K (16–23 GPa). As an example, evidence is presented that carbon tetrachloride is a semiconductor at 1500 K. The CV(T) distribution in each temperature range is formulated and a temperature Hugoniot is reevaluated from the WC equation using the specific heat equations formulated. It is confirmed by a good fit of the reevaluated Hugoniot to the existing measured data that the specific heat equations express the CV(T) distribution appropriately. View full abstract»

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  • Phase compatibility and thermoelectric properties of compounds in the Sr–Ca–Co–O system

    Page(s): 033508 - 033508-6
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    Two low-dimensional cobaltite series in the Sr–Ca–Co–O system have been investigated for their solid solution limit, structure, and compatibility phase relationships (850 °C in air). Thermoelectric properties have been measured for selected members of these solid solutions. In (Ca,Sr)3Co4O9, which has a misfit layered structure, Sr was found to substitute in the Ca site to a limit of (Ca0.8Sr0.2)3Co4O9. Compounds in the homologous series, An+2ConCoO3n+3 [where A=Sr, Ca, (Ca,Sr), or (Sr,Ca)], consist of one-dimensional parallel Co2O66- chains that are built from successive alternating face-sharing CoO6 trigonal prisms and “n” units of CoO6 octahedra along the hexagonal c-axis. In the Can+2ConCoO3n+3 series, only the n=1 phase (Ca3Co2O6) could be prepared under the present synthesis conditions. Sr substitutes in the Ca site of Ca3Co2O6 to a limit of (Ca0.9Sr0.1)3Co2O6. In the Srn+2ConCoO3n+3 series, Ca substitutes in the Sr site of the n=2, 3, and 4 members to a limit of (Sr0.7Ca0.3)4Co3O9, (Sr0.67Ca0.33)5Co4O12, and (Sr0.725Ca0.275)6Co5O15, respectively. While the members of the Can+2ConCoO3n+3 and Srn+2ConCoO3n+3 series have reasonably high Seebeck coefficients and relatively low thermal conductivity, the electrical conductivity needs to be increased in order to achieve high ZT values. View full abstract»

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Aims & Scope

Journal of Applied Physics is the American Institute of Physics' (AIP) archival journal for significant new results in applied physics

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Editor
P. James Viccaro
Argonne National Laboratory