bluuebunny/crossref_metadata_2025 huggingface dataset:
Small polarons
Authors: Emin David
Published: September 2008

We usually think of electrons in solids and liquids as behaving like free particles whose motion is impeded by occasional collisions. Evidence is now accumulating for charge carriers with qualitatively different behavior. These new charge carriers, in the simplest case, are extra electrons in condensed matter that become trapped in potential wells of their own creation, forming units known as small polarons.
Modeling the electrical conductivity in BaTiO3 on the basis of first-principles calculations
Authors: Erhart Paul , Albe Karsten
Published: September 2008

The dependence of the electrical conductivity on the oxygen partial pressure is calculated for the prototypical perovskite BaTiO3 based on data obtained from first-principles calculations within density functional theory. The equilibrium point defect concentrations are obtained via a self-consistent determination of the electron chemical potential. This allows one to derive charge carrier concentrations for a given temperature and chemical environment and eventually the electrical conductivity. The calculations are in excellent agreement with experimental data if an accidental acceptor dopant level of 1017 cm3 is assumed. It is shown that doubly charged oxygen vacancies are accountable for the high-temperature n-type conduction under oxygen-poor conditions. The high-temperature p-type conduction observed at large oxygen pressures is due to barium vacancies and titanium-oxygen divacancies under Ti- and Ba-rich conditions, respectively. Finally, the connection between the present approach and the mass-action law approach to point defect thermodynamics is discussed.
Sub-30-nm patterning on quartz for imprint lithography templates
Authors: Srinivasan Charan , Hohman J. Nathan , Anderson Mary E. , Weiss Paul S. , Horn Mark W.
Published: September 2008

A parallel and economical method for obtaining nanoscale features on large-area quartz substrates has been developed for use in nanoimprint lithography template fabrication. Self-assembled multilayer films (molecular rulers) are used in conjunction with photolithographically defined metallic features to generate precise nanogaps with sub-30-nm resolution on quartz substrates. These nanopatterns are then transferred to the quartz substrates using the metallic thin films as etch masks.
Quantum Monte Carlo study of porphyrin transition metal complexes
Authors: Koseki Jun , Maezono Ryo , Tachikawa Masanori , Towler M. D. , Needs R. J.
Published: September 2008

Diffusion quantum Monte Carlo (DMC) calculations for transition metal (M) porphyrin complexes (MPo, M=Ni,Cu,Zn) are reported. We calculate the binding energies of the transition metal atoms to the porphin molecule. Our DMC results are in reasonable agreement with those obtained from density functional theory calculations using the B3LYP hybrid exchange-correlation functional. Our study shows that such calculations are feasible with the DMC method.
A comparison of CuO and Cu2O hole-injection layers for low voltage organic devices
Authors: Murdoch G. B. , Greiner M. , Helander M. G. , Wang Z. B. , Lu Z. H.
Published: September 2008

Cu 2 O and CuO have been grown with an aim to reduce junction electrical resistance when interfaced with N,N′-bis(1-naphthyl)-N,N′-diphenyl-1,1′ biphenyl 4,4′-diamine (NPB). Organic light-emitting diodes employing Cu/CuO anodes have equivalent driving voltages as devices made with indium tin oxide. Hole-injection barriers are calculated from current-voltage characteristics of CuO/NPB/Cu and Cu2O/NPB/Cu devices via theoretical simulation. Photoelectron spectroscopies are used to measure oxide valence band spectra, interfacial dipole formation, and band bending during in situ sequential deposition of NPB on each oxide. Calculated hole-injection barriers and those derived from photoemission results accord well, explaining the superior hole injection at the CuO-NPB interface.
A benchmark theoretical study of the electron affinities of benzene and linear acenes
Authors: Hajgató B. , Deleuze M. S. , Tozer D. J. , De Proft F.
Published: September 2008

A benchmark theoretical determination of the electron affinities of benzene and linear oligoacenes ranging from naphthalene to hexacene is presented, using the principles of a focal point analysis. These energy differences have been obtained from a series of single-point calculations at the Hartree–Fock, second-, third-, and partial fourth-order Møller–Plesset (MP2, MP3, and MP4SDQ) levels and from coupled cluster calculations including single and double excitations (CCSD) as well as perturbative estimates of connected triple excitations [CCSD(T)], using basis sets of improving quality, containing up to 1386, 1350, 1824, 1992, 1630, and 1910 basis functions in the computations, respectively. Studies of the convergence properties of these energy differences as a function of the size of the basis set and order attained in electronic correlation enable a determination of the vertical electron affinities of the four larger terms of the oligoacene (C2+4nH2+2n) series within chemical accuracy (0.04eV). According to our best estimates, these amount to +0.28, +0.82, +1.21, and +1.47eV when n=3, 4, 5, and 6. Adiabatic electron affinities have been further calculated by incorporating corrections for zero-point vibrational energies and for geometrical relaxations. The same procedure was applied to determine the vertical electron affinities of benzene and naphthalene, which are found to be markedly negative (∼−1.53 and ∼−0.48eV, respectively). Highly quantitative insights into experiments employing electron transmission spectroscopy on these compounds were also amenable from such an approach, provided diffuse atomic functions are deliberately removed from the basis set, in order to enforce confinement in the molecular region and enable a determination of pseudoadiabatic electron affinities (with respect to the timescale of nuclear motions). Comparison was made with calculations employing density functional theory and especially designed models that exploit the integer discontinuity in the potential or incorporate a potential wall in the unrestricted Kohn–Sham orbital equation for the anion.
Reliability study of La2O3 capped HfSiON high-permittivity n-type metal-oxide-semiconductor field-effect transistor devices with tantalum-rich electrodes
Authors: O’Sullivan B. J. , Mitsuhashi R. , Pourtois G. , Aoulaiche M. , Houssa M. , Van der Heyden N. , Schram T. , Harada Y. , Groeseneken G. , Absil P. , Biesemans S. , Nakabayashi T. , Ikeda A. , Niwa M.
Published: September 2008

A significant difference in the magnitude of the threshold voltage (Vt) reduction achieved by lanthanum oxide (La2O3) incorporation in ultra-thin dielectrics is observed for TaCx and TaCxNy electrodes. This is explained by dielectric consumption resulting from the thermodynamically unstable TaCx electrode, which gives rise to electrically active oxygen vacancies in the high-permittivity dielectric. We report extensively on the presence and origins of an aberrant negative Vt shift during positive bias temperature instability stressing and, conversely, a positive shift during negative bias temperature instability stressing for La2O3 capped HfSiON dielectrics observed for TaCx electroded films, not seen for the TaCxNy case. Although, oxygen vacancies are inherent due to the differences in electronic configuration between lanthanum and hafnium, this effect is exacerbated by the electrode instability, which is reflected in the Vt dependence.
Influence of metal capping layer on the work function of Mo gated metal-oxide semiconductor stacks
Authors: Li Z. , Schram T. , Stesmans A. , Franquet A. , Witters T. , Pantisano L. , Yamada N. , Tsunoda T. , Hooker J. , De Gendt S. , De Meyer K.
Published: September 2008

It was found that applying a capping layer has an important impact on the work function (WF) of Mo gated metal-oxide semiconductor stacks. Before any postmanufacturing thermal treatment, uncapped Mo has a similar WF as one capped with a TaC layer. However, after forming gas anneal (FGA), the uncapped Mo gate exhibits a significantly higher WF than the TaC capped one does. This is understood as O incorporation during deposition, storage or FGA, and its subsequent piling up at the Mo/dielectric interface during FGA in the former case, which is an effect prevented by TaC capping.
Enhancement of multiple-phonon resonant Raman scattering in Co-doped ZnO nanorods
Authors: Phan The-Long , Vincent Roger , Cherns David , Dan Nguyen Huy , Yu Seong-Cho
Published: September 2008

We have studied Raman scattering in Co-doped ZnO nanorods prepared by thermal diffusion. Experimental results show that the features of their non-resonant spectra are similar to Raman spectra from Co-doped ZnO materials investigated previously. Under resonant conditions, however, there is a strong enhancement of multiple-phonon Raman scattering processes. Longitudinal optical (LO)-phonon overtones up to eleventh order are observed. The modes become more obvious when the Co concentration diffused into ZnO nanorods goes to an appropriate value. This phenomenon is explained due to the shift of the band-gap energy and also due to the decrease in the intensity of near-band-edge luminescence. Our observation is in agreement with the prediction [J. F. Scott, Phys. Rev. B 2, 1209 (1970)] that the number of LO-phonon lines in ZnO is higher than that observed for CdS.
Boron activation and diffusion in silicon and strained silicon-on-insulator by rapid thermal and flash lamp annealings
Authors: Lanzerath F. , Buca D. , Trinkaus H. , Goryll M. , Mantl S. , Knoch J. , Breuer U. , Skorupa W. , Ghyselen B.
Published: September 2008

We present experimental results on the activation and diffusion behaviors of boron in silicon-on-insulator and strained silicon-on-insulator using standard rapid thermal processing treatments as well as flash lamp annealing. After boron implantation at different doses and at a low energy of 1 keV, samples were annealed to activate the dopants, and secondary ion mass spectrometry and Hall measurements were carried out to determine boron diffusion and the amount of activated dopants, respectively. In contrast to rapid thermal annealing, flash lamp annealing enables the activation without significant diffusion of dopants. In addition, we investigated the effect of coating the samples with antireflection layers to increase the absorbed energy during flash annealing. As a result, the activation was increased significantly to values comparable with the activation obtained with standard annealing. Furthermore, the relation between the observed boron diffusion and activation as a function of the implantation and annealing parameters is discussed in terms of the kinetics of the defects involved in these processes.
Dynamic force spectroscopy: Analysis of reversible bond-breaking dynamics
Authors: Diezemann Gregor , Janshoff Andreas
Published: September 2008

The problem of diffusive bond dissociation in a double well potential under application of an external force is scrutinized. We compute the probability distribution of rupture forces and present a detailed discussion of the influence of finite rebinding probabilities on the dynamic force spectrum. In particular, we focus on barrier crossing upon extension, i.e., under linearly increased load, and upon relaxation starting from completely separated bonds. For large loading rates the rupture force and the rejoining force depend on the loading rate in the expected manner determined by the shape of the potential. For small loading rates the mean forces obtained from pull and relax modes approach each other as the system reaches equilibrium. We investigate the dependence of the rupture force distributions and mean rupture forces on external parameters such as cantilever stiffness and influence of a soft linker. We find that depending on the implementation of a soft linker the equilibrium rupture force is either unaffected by the presence of the linker or changes in a predictable way with the linker compliance. Additionally, we show that it is possible to extract the equilibrium constant of the on and off rates from the determination of the equilibrium rupture forces.
Carbon atom and cluster sputtering under low-energy noble gas plasma bombardment
Authors: Oyarzabal E. , Doerner R. P. , Shimada M. , Tynan G. R.
Published: September 2008

Exit-angle resolved carbon atom and cluster (C2 and C3) sputtering yields are measured during different noble gas (Xe, Kr, Ar, Ne, and He) ion bombardments from a plasma, for low incident energies (75–225 eV). A quadrupole mass spectrometer (QMS) is used to detect the fraction of sputtered neutrals that is ionized in the plasma and to obtain the angular distribution by changing the angle between the target normal and the QMS aperture. A one-dimensional Monte Carlo code is used to simulate the interaction of the plasma and the sputtered particles in the region between the sample and the QMS. The effective elastic scattering cross sections of C, C2, and C3 with the different bombarding gas neutrals are obtained by varying the distance between the sample and the QMS and by performing a best fit of the simulation results to the experimental results. The total sputtering yield (C+C2+C3) for each bombarding gas is obtained from weight-loss measurements and the sputtering yield for C, C2, and C3 is then calculated from the integration of the measured angular distribution, taking into account the scattering and ionization of the sputtered particles between the sample and the QMS. We observe undercosine angular distributions of the sputtered atoms and clusters for all the studied bombarding gases and a clear decrease of the atom to cluster (C2 and C3) sputtering ratio as the incident ion mass increases, changing from a carbon atom preferential erosion for the lower incident ion masses (He, Ne, and Ar) to a cluster preferential erosion for the higher incident ion masses (Kr and Xe).
Electrical behavior of memory devices based on fluorene-containing organic thin films
Authors: Dimitrakis Panagiotis , Normand Pascal , Tsoukalas Dimitris , Pearson Christopher , Ahn Jin H. , Mabrook Mohammed F. , Zeze Dagou A. , Petty Michael C. , Kamtekar Kiran T. , Wang Changsheng , Bryce Martin R. , Green Mark
Published: September 2008

We report on switching and negative differential resistance (NDR) behaviors of crossed bar electrode structures based on Al/organic layer/Al devices in which the organic layer was a spin-coated layer of 7-{4-[5-(4-tert-butylphenyl)-1,3,4-oxadiazol-2-yl]phenyl}-9,9-dihexyl-N,N-diphenyl-fluoren-2-amine. The addition of gold nanoparticles (0.5wt%) did not change the switching behavior of thicker film structures; however, devices incorporating the nanoparticles showed more reproducible characteristics. In most cases, a “forming” process, in which a large positive voltage was applied to the top Al electrode, was required before the NDR and conductivity switching were observed. Three different electrical conductivity mechanisms have been identified: Poole–Frenkel conductivity in unformed structures, linear current versus voltage characteristics for the ON state in formed devices, and superlinear current versus voltage behavior for the OFF state in formed devices. Models based on metallic filaments or on the injection and storage of charge do not explain all our experimental observations satisfactorily. Instead, an explanation based on the formation of nanocrystalline regions within the thin film is suggested. The devices can be used as two-terminal memory cells operating with unipolar voltage pulses.
Regimes of near-wall vortex dynamics in potential flow through gaps
Authors: Makarov V. G. , Bulgakov S. N.
Published: September 2008

A two-dimensional problem of the motion of a single vortex near an infinite straight wall with singular gaps is solved both analytically, using a point-vortex approach, and numerically based on the method of contour surgery for a vortex patch. The background irrotational flow was generated by a balanced point source-sink system located at the gaps. Three different regimes of vortex evolution were detected and studied in detail: (i) Complete or partial transit, i.e., continuation of the motion along the wall; (ii) complete destruction, i.e., the “penetration” through the sink gap; and (iii) capture in a certain area against the wall between the gaps. These regimes are controlled by three parameters: the ratio of the vortex size and the distance between the gaps, the remoteness of the vortex trajectory from the wall, and the ratio of the intensities of the background flow and the vortex. A bifurcational character of the transition between the regimes was observed. Steady-state solutions were found numerically, including the orbital O state, where the vortex's centroid moves along a constant orbit, while the shape of the vortex changes periodically. Capturing the vortex was usually carried out in a form close to this state.
Equation of state for diamond in wide ranges of pressure and temperature
Authors: Tse John S. , Holzapfel Wilfried B.
Published: September 2008

Theoretical total energy and phonon calculations for diamond under strong compressions up to 3 TPa are analyzed with different analytical forms for the representation of p-V isotherms. A simple model with only two characteristic (volume dependent) phonon parameters—an effect acoustic Debye temperature and an additional Einstein temperature of the optical phonons—provides a complete modeling of all the thermophysical properties of diamond in this extended range of pressure up to the melting curve. The comparison with thermophysical data at ambient pressure provides additional information on weak intrinsic anharmonic contributions and gives the frame for the complete modeling up to 3 TPa in pressure and temperatures up to the melting curve.
rf resistance and inductance of massively parallel single walled carbon nanotubes: Direct, broadband measurements and near perfect 50Ω impedance matching
Authors: Rutherglen Chris , Jain Dheeraj , Burke Peter
Published: September 2008

We report using dielectrophoresis to accumulate hundred to thousands of solubilized single walled carbon nanotubes in parallel to achieve impedance values very close to 50Ω. This allows us to clearly measure the real (resistive) and imaginary (inductive) impedance over a broad frequency range. We find a negligible to mild frequency dependent resistance for the devices and an imaginary impedance that is significantly smaller then the resistance over the range of dc to 20GHz. This clearly and unambiguously demonstrates that kinetic inductance is not the major issue facing nanotube array interconnects, when compared to the real impedance (the resistance).
Helium implanted gallium nitride evidence of gas-filled rod-shaped cavity formation along the c-axis
Authors: Barbot Jean-François , Pailloux Frédéric , David Marie-Laure , Pizzagalli Laurent , Oliviero Erwan , Lucas Guillaume
Published: September 2008

The structural defects induced by He implantation in GaN epilayer at high fluence (1×1017He/cm2) and elevated temperature (750 °C) have been studied by conventional and high resolution transmission electron microscopy. In addition to the planar interstitial-type defects lying in the basal plane usually observed after high fluence implantation into GaN, a continuous layer of bubbles arranged in rows parallel to the implanted surface is observed in the region of maximum He concentration. This arrangement of bubbles is ascribed to interactions with dislocations. Beyond, one dimensional rod-shaped defects appear perpendicular to the implanted surface. Contrast analysis of high resolution images and atomistic simulations gives converging results in the determination of the nature and structure of these defects, i.e., gas-filled rod-shaped cavities in an overpressurized state.
Angle-independent microwave absorption by ultrathin microcavity arrays
Authors: Brown J. R. , Hibbins A. P. , Lockyear M. J. , Lawrence C. R. , Sambles J. R.
Published: September 2008

The resonant absorption of microwave radiation by thin, two-dimensional microcavity arrays has been studied. Resonant modes associated with these structures, formed from copper-clad FR4 laminates, exhibit both an azimuthal and polar angle independent electromagnetic response. The experimental data agree well with the predictions of a finite element method computer model, which has been utilized to explore the electromagnetic character of the resonant modes supported.
Vacancy-mediated intermixing in InAs/InP(001) quantum dots subjected to ion implantation
Authors: Dion C. , Desjardins P. , Schiettekatte F. , Chicoine M. , Robertson M. D. , Shtinkov N. , Poole P. J. , Wu X. , Raymond S.
Published: September 2008

We have investigated the influence of defects emanating from phosphorus implantation damage in the InP capping layer on postgrowth thermally induced intermixing in self-assembled InAs/InP(001) quantum dots (QDs). Photoluminescence (PL) spectra from as-grown samples could be described as the superposition of separate PL peaks where each peak corresponded to emission from an ensemble of QDs with a particular height ranging from 4 to 13 ML. Blueshift of up to 270 meV and significant bandwidth broadening were observed in the PL spectra after ion implantation with a fluence of 5×1011−1014 cm−2 and subsequent annealing at temperatures ranging from 450 to 600 °C. From the analysis of the evolution of the QD peaks upon intermixing, which revealed the coexistence of intact QD PL and a broad PL feature related to heavily intermixed QDs, it was suggested that the bandwidth broadening resulted from spatial inhomogeneity in the compositional intermixing. In order to better understand the mechanism responsible for the ion-implantation-induced intermixing, samples capped with a stack of compressively strained In0.75Ga0.25As/InP quantum wells (QWs) were prepared to trap vacancies released by the implantation damage while not inhibiting the effect of the interstitials. Both blueshift and bandwidth broadening were suppressed in samples containing the strained InGaAs QWs, whereas the evolution of the PL spectra from the QDs behaves as expected for interstitial-mediated intermixing. The vacancies were thus believed to be trapped in the QWs and indicated that intermixing in ion-implanted InP capped samples is mediated by vacancies. The shape of the QDs changed from a truncated pyramid in the as-grown state to a double convex lens structure after intermixing as confirmed by cross-sectional scanning transmission electron microscopy imaging. Furthermore, the change in shape and compositional intermixing of the QDs were attributed to vacancy trapping in the vicinity of the QDs as based on atomistic strain calculations.
Subgap biasing of superconducting tunnel junctions without a magnetic field
Authors: Segall K. , Moyer J. , Mazo Juan J.
Published: September 2008

Superconducting tunnel junctions (STJs) have been successfully used as single-photon detectors but require the use of a magnetic field to operate. A recent paper has proposed the idea to use a circuit of three junctions in place of a single junction in order to achieve the necessary biasing without applying a magnetic field. The nonlinear interaction between the different junctions in the circuit causes the existence of a stable subgap state for one of the junctions, which acts as the detector junction. In this paper, we present the first measurements demonstrating the existence of such a biasing state feasible for STJ detectors. Single junction measurements with an applied magnetic field help determine the functional form of the subgap current versus voltage; then the operating point of a three-junction circuit is measured and fit to theory. The excellent match between theory and experiment demonstrates the existence of the subgap biasing state. The outlook for possible use in detector applications is discussed.
Magnetic properties of electron doped Sm0.1Ca0.9−yBayMnO3 (y=0.02, 0.06) manganites: Pressure effects on competitive ferromagnetic and antiferromagnetic interactions
Authors: Markovich V. , Fita I. , Puzniak R. , Martin C. , Wisniewski A. , Hébert S. , Maignan A. , Mogilyansky D. , Gorodetsky G.
Published: September 2008

Magnetic properties of polycrystalline Sm0.1Ca0.9−yBayMnO3 (y=0.02, 0.06) samples have been investigated in the temperature range of 2–320 K, magnetic fields of up to 90 kOe, and under hydrostatic pressures of up to 12 kbar. It was found that the volume fraction of ferromagnetic (FM) phase at 5 K is about 27% for Sm0.1Ca0.88Ba0.02MnO3 and only ∼1% for Sm0.1Ca0.84Ba0.06MnO3. The compound Sm0.1Ca0.88Ba0.02MnO3 exhibits magnetic phase separation below TN≈TC≈99 K, which is consistent with a model of FM clusters embedded in antiferromagnetic (AFM) G-type matrix. For this sample, magnetization and ac susceptibility exhibit a cluster glasslike behavior below TC, while the resistivity exhibits pronounced thermal hysteresis around TC, indicating upon a plausible first order magnetic phase transition. For Sm0.1Ca0.84Ba0.06MnO3 two peaks observed in the temperature dependence of the ac susceptibility at low temperatures may be related to magnetic transitions, associated with the C- and G-AFM structures. Both, a significant hysteresis of magnetization observed at T<100 K and an increase in magnetoresistance at low temperatures are apparently an indicative of magnetic field induced structural transformation from monoclinic to orthorhombic phase. This transformation occurs concurrently with magnetic transition from C-type AFM to G-type AFM structure. It was found that an applied pressure enhances TC of Sm0.1Ca0.88Ba0.02MnO3 with a pressure coefficient dTC/dP≈0.6 K/kbar. The spontaneous FM moment of Sm0.1Ca0.88Ba0.02MnO3 decreases by 15% under pressure of about 12 kbar, while under the same pressure it only slightly increases for Sm0.1Ca0.84Ba0.06MnO3. For both samples, the paramagnetic Curie temperature decreases under pressure indicating upon an increase in AFM interactions. Stronger suppression of the FM phase for the sample with y=0.06, in comparison with that one for 0.02 sample, is attributed to the effect of the chemical disorder, playing a more significant role than the effect related to the changes in ⟨rA⟩.
Dispersionful analog of the Whitham hierarchy
Authors: Szablikowski Błażej M. , Błaszak Maciej
Published: September 2008

The transition from the zero-genus universal Whitham hierarchy to its dispersionful counterpart, making use only of the Lax representations, is presented. This is an alternative approach to that of Takasaki who has recently shown that the dispersionless limit of the charged multicomponent Kadomtsev-Petviashvili (KP) hierarchy is the Whitham hierarchy. The advantage of the presented approach is the construction of finite-field reductions, which are the main concern in this paper. The theory is illustrated by several significant examples.
Input impedance and gain of a gigahertz amplifier using a dc superconducting quantum interference device in a quarter wave resonator
Authors: Spietz Lafe , Irwin Kent , Aumentado José
Published: September 2008

Due to their superior noise performance, superconducting quantum interference devices (SQUIDs) are an attractive alternative to high electron mobility transistors for constructing ultra-low-noise microwave amplifiers for cryogenic use. We describe the use of a lumped element SQUID inductively coupled to a quarter wave resonator. The resonator acts as an impedance transformer and also makes it possible to accurately measure the input impedance and intrinsic microwave characteristics of the SQUID. We present a model for input impedance and gain, compare it to the measured scattering parameters, and describe how to use the model for the systematic design of low-noise microwave amplifiers with a wide range of performance characteristics.
Very high channel conductivity in low-defect AlN/GaN high electron mobility transistor structures
Authors: Dabiran A. M. , Wowchak A. M. , Osinsky A. , Xie J. , Hertog B. , Cui B. , Look D. C. , Chow P. P.
Published: September 2008

Low defect AlN/GaN high electron mobility transistor (HEMT) structures, with very high values of electron mobility (>1800 cm2/V s) and sheet charge density (>3×1013 cm−2), were grown by rf plasma-assisted molecular beam epitaxy (MBE) on sapphire and SiC, resulting in sheet resistivity values down to ∼100 Ω/◻ at room temperature. Fabricated 1.2 μm gate devices showed excellent current-voltage characteristics, including a zero gate saturation current density of ∼1.3 A/mm and a peak transconductance of ∼260 mS/mm. Here, an all MBE growth of optimized AlN/GaN HEMT structures plus the results of thin-film characterizations and device measurements are presented.
Half-metallicity in hybrid BCN nanoribbons
Authors: Kan Er-Jun , Wu Xiaojun , Li Zhenyu , Zeng X. C. , Yang Jinlong , Hou J. G.
Published: September 2008

The established chemical synthetic strategy toward graphene nanoribbons has greatly prompted and justified the research of theoretical designs of novel materials based on graphene. In this paper, we report the novel half-metallicity in C and BN hybrid zigzag nanoribbons even though stand-alone C or BN nanoribbon possesses a finite band gap. By performing first-principles electronic-structure calculations, we find this unexpected half-metallicity in the hybrid nanostructures stems from a competition between the charge and spin polarizations, as well as from the π orbital hybridization between C and BN. Molecular dynamics simulations indicate that the hybrid nanoribbons are stable. Our results point out a possibility of making spintronic devices solely based on nanoribbons and a new way of fabricating metal-free half metals.
Comparison of the OKα x-ray emission bands in micro- and mesoporous silica materials and in α-quartz
Authors: Khyzhun O. Yu. , Strunskus T. , Wöll Ch. , Gies H. , Staemmler V.
Published: September 2008

X-ray emission spectroscopy (XES) at the OKα threshold has been used to investigate the electronic structure of a microporous pure calcined zeolite with the crystal structure of the MFI-type framework (silicalite), a deboronated MFI zeolite (DB-MFI), a pure mesoporous cubic MCM-48 material, a MCM-48 loaded with copper and zinc oxide nanoparticles (Cu∕ZnO-MCM-48), and a crystalline layered silicic acid H-RUB-18. For comparison, the XES OKα spectrum of pure α-quartz has also been recorded. In the nonresonant energy regime the XES OKα spectra for all these compounds look very similar indicating that the electronic structure of the micro- and mesoporous silica materials is very similar to that of quartz. In the resonant regime, however, the spectra exhibit significant differences. In all the materials under study, the resonant XES OKα spectra recorded at photon energies close to the positions of the OK edges show Raman-type inelastic peaks with an energy loss of 11eV, originating from electronic excitations within these insulating materials. The prominent features in the XES OKα spectra of α-quartz and H-RUB-18 are analyzed by means of quantum chemical ab initio cluster calculations.
Salting out of methane by sodium chloride: A scaled particle theory study
Authors: Graziano Giuseppe
Published: September 2008

The salting out of methane by adding NaCl to water at 25°C and 1atm is investigated by calculating the work of cavity creation by means of scaled particle theory and the methane-solvent energy of attraction. The latter quantity changes to little extent on passing from pure water to an aqueous 4M NaCl solution, whereas the magnitude of the work of cavity creation increases significantly, accounting for the salting out effect. There is quantitative agreement between the experimental values of the hydration Gibbs energy and the calculated ones. The behavior of the work of cavity creation is due to the increase in the volume packing density of NaCl solutions, since the average effective molecular diameter does not change, being always 2.80Å. The same approach allows the rationalization of the difference in methane salting out along the alkali chloride series. These results indicate that, fixed the aqueous solution density, the solubility of nonpolar species is mainly determined by the effective diameter of solvent molecules and the corresponding volume packing density. There is no need to take into account the H-bond rearrangement because it is characterized by an almost complete enthalpy-entropy compensation.
Blends of poly[2-(2′,5′-bis(2′-ethyl-hexyloxy)phenyl)-1,4-phenylenevinylene] and poly(3-hexylthiophene) as base materials for broad band light emission devices
Authors: Valadares M. , Silvestre I. , Calado H. D. R. , Neves B. R. A. , Guimarães P. S. S. , Cury L. A.
Published: September 2008

Photoluminescence (PL) and electroluminescence (EL) emission from blended films composed by poly[2-(2′,5′-bis(2′-ethyl-hexyloxy)phenyl)-1,4-phenylenevinylene] (BEHP-PPV) and poly(3- hexylthiophene) (P3HT) conjugated polymers with different relative concentrations are investigated. The main changes observed on the PL spectra of the blended films with temperature and excitation power are associated to an increase of the emission efficiency of the P3HT constituent. The EL spectrum of a light emission device (LED) fabricated with the BEHP-PPV:P3HT blend presents a similar broad emission as in the PL measurement but shifted to higher wavelengths. The blended LEDs show a significantly higher emission efficiency than the LEDs made with the pure constituents. The apparent color of the blended LED shifts to a purer red as the applied voltage is increased. This tuning capacity was interpreted as due to a change in the effective gap of the blend caused by the redistribution of injected carriers in the BEHP-PPV:P3HT interfaces under bias. These characteristics make the BEHP-PPV:P3HT blend a very attractive material for fabrication of efficient red LEDs.
Quaternionic and Poisson–Lie structures in three-dimensional gravity: The cosmological constant as deformation parameter
Authors: Meusburger C. , Schroers B. J.
Published: September 2008

Each of the local isometry groups arising in three-dimensional (3d) gravity can be viewed as a group of unit (split) quaternions over a ring which depends on the cosmological constant. In this paper we explain and prove this statement and use it as a unifying framework for studying Poisson structures associated with the local isometry groups. We show that, in all cases except for the case of Euclidean signature with positive cosmological constant, the local isometry groups are equipped with the Poisson–Lie structure of a classical double. We calculate the dressing action of the factor groups on each other and find, among others, a simple and unified description of the symplectic leaves of SU(2) and SL(2,R). We also compute the Poisson structure on the dual Poisson–Lie groups of the local isometry groups and on their Heisenberg doubles; together, they determine the Poisson structure of the phase space of 3d gravity in the so-called combinatorial description.
Characteristics of leakage current in the dielectric layer due to Cu migration during bias temperature stress
Authors: Hwang Sang-Soo , Jung Sung-Yup , Joo Young-Chang
Published: September 2008

The dielectric breakdown by Cu ion migration under applied bias is important for the reliability of damascene Cu interconnects as well as Cu through-hole bias in three-dimensional interconnects. The mechanism responsible for the leakage current through the dielectric during bias temperature stress is investigated in Cu/SiO2 and Al/SiO2 metal-insulator-semiconductor samples. When a constant electric field is applied, a gradual increase in leakage currents was observed in the case of Cu/SiO2, but no increase in leakage current before failure was observed for Al/SiO2. The voltage ramping tests conducted on the samples before and during the constant electric field application showed that the increase in leakage current in the Cu/SiO2 sample is by the Poole–Frenkel (PF) conduction mechanism. Cu ions, which migrate into the dielectric under the applied field, act as trap sites for PF conduction. This was confirmed by negative bias test as well as simulation of the leakage current by one-dimensional finite differential methods.
Exchange bias and its training effect in the martensitic state of bulk polycrystalline Ni49.5Mn34.5In16
Authors: Wang B. M. , Liu Y. , Wang L. , Huang S. L. , Zhao Y. , Yang Y. , Zhang H.
Published: September 2008

Exchange bias phenomena are observed in the bulk polycrystalline Ni49.5Mn34.5In16 alloy in which ferromagnetic and antiferromagnetic phases coexist in the martensitic state. Both the exchange bias field and coercivity are strongly dependent on temperature. The training effect of the exchange bias is found to be very small in the present alloy and can be explained by the depinning of uncompensated antiferromagnet spins. These results suggest that the ferromagnetic and antiferromagnetic domains couple at the interfaces and as a result induce the exchange bias. Such behavior is an addition to the multifunctional properties of the Ni49.5Mn34.5In16 ferromagnetic shape memory alloy.
Luminescence evolution of ZnO single crystal under low-energy electron beam irradiation
Authors: Dierre B. , Yuan X. L. , Sekiguchi T.
Published: September 2008

The effects of electron beam irradiation on the luminescence of ZnO single crystals were investigated by cathodoluminescence. We have found that the evolution of the intensity during the e-beam irradiation depends on the surface polarity. For O-face, the ultraviolet (UV) emission decreases exponentially and approaches an asymptotic value. For Zn-face, it first increases and then decreases. The decrease components are similar in both faces. If we halt the e-beam irradiation, the UV intensity recovers partially. These results suggest that the decrease in the UV evolution is related to metastable bulk defect reactions at the subsurface region while the increase is related to surface reaction such as electron-stimulated desorption.
Spin accumulation and pure spin current in a three-terminal quantum dot ring with Rashba spin-orbit effect
Authors: Chi Feng , Zheng Jun , Sun Lian-Liang
Published: September 2008

Based on the Keldysh nonequilibrium Green’s function technique, we study the spin accumulation and the pure spin current in a three-terminal quantum ring device, in which a quantum dot (QD) is inserted in one arm of the ring and the Rashba spin-orbit interaction (RSOI) exists in the other. We find that under certain circumstances, the pure spin current in the middle lead and the spin accumulation in the QD emerge simultaneously. The sign and the magnitude of them can be modulated by adjusting the bias voltages, the RSOI-induced phase factor and the magnetic flux penetrating the ring. The origin of the spin accumulation and the pure spin current is interpreted in terms of the spin-dependent total effective coupling strengths between the leads and the QD.
Dependence of spectral behavior in an InGaN/GaN quantum-well light-emitting diode on the prestrained barrier thickness
Authors: Lu Chih-Feng , Huang Chi-Feng , Chen Yung-Sheng , Yang C. C.
Published: September 2008

The dependencies of output spectral overall redshift and current-density-induced spectral blueshift on the prestrained barrier thickness in the InGaN/GaN quantum-well light-emitting diodes (LEDs) of prestrained growth are demonstrated. Due to the stronger prestrain effect in a sample of a thinner prestrained barrier, the overall spectral redshift range increases and the current-density-induced blueshift range decreases with decreasing prestrained barrier thickness. Also, in terms of device resistance and saturation current, the LED performances of prestrained samples are superior to that of a conventional LED. With a thinner prestrained barrier, the device performance becomes better. The results are attributed to the higher average-indium content and stronger indium-rich clustering behavior in a sample of stronger prestrain. Such attributions are supported by the observations of strain state analysis in transmission electron microscopy measurements.
Second-order, two-electron Dyson propagator theory: Comparisons for vertical double ionization potentials
Authors: Ida T. , Ortiz J. V.
Published: September 2008

The second-order, two-electron Dyson propagator is derived using superoperator theory with a spin-adapted formulation. To include certain ladder diagrams to all orders, the shifted-denominator (SD2) approximation is made. Formal and computational comparisons with other approximations illustrate the advantages of the SD2 procedure. Vertical double ionization potentials (DIPs) for a set of closed-shell molecules are evaluated with the second-order propagator and the SD2 method. The results of the SD2 approximation are in good agreement with experiment. To systematically examine the quality of the results, we compared SD2 and equation-of-motion, coupled-cluster predictions. The average absolute discrepancy is 0.26eV for 36 doubly ionized states.
Quasiclassical trajectory study of the SiH4+H→SiH3+H2 reaction on a global ab initio potential energy surface
Authors: Wang Manhui , Sun Xiaomin , Bian Wensheng
Published: September 2008

The SiH4+H→SiH3+H2 reaction has been investigated by the quasiclassical trajectory (QCT) method on a recent global ab initio potential energy surface [M. Wang et al., J. Chem. Phys. 124, 234311 (2006)]. The integral cross section as a function of collision energy and thermal rate coefficient for the temperature range of 300–1600K have been obtained. At the collision energy of 9.41kcal∕mol, product energy distributions and rovibrational populations are explored in detail, and H2 rotational state distributions show a clear evidence of two reaction mechanisms. One is the conventional rebound mechanism and the other is the stripping mechanism similar to what has recently been found in the reaction of CD4+H [J. P. Camden et al., J. Am. Chem. Soc. 127, 11898 (2005)]. The computed rate coefficients with the zero-point energy correction are in good agreement with the available experimental data.
The vibronic level structure of the cyclopentadienyl radical
Authors: Ichino Takatoshi , Wren Scott W. , Vogelhuber Kristen M. , Gianola Adam J. , Lineberger W. Carl , Stanton John F.
Published: September 2008

The 351.1 nm photoelectron spectrum of the cyclopentadienide ion has been measured, which reveals the vibronic structure of the X̃ E21″ state of the cyclopentadienyl radical. Equation-of-motion ionization potential coupled-cluster (EOMIP-CCSD) calculations have been performed to construct a diabatic model potential of the X̃ E21″ state, which takes into account linear Jahn–Teller effects along the e2′ normal coordinates as well as bilinear Jahn–Teller effects along the e2′ and ring-breathing a1′ coordinates. A simulation based on this ab initio model potential reproduces the spectrum very well, identifying the vibronic levels with linear Jahn–Teller angular momentum quantum numbers of ±1/2. The angular distributions of the photoelectrons for these vibronic levels are highly anisotropic with the photon energies used in the measurements. A few additional weak photoelectron peaks are observed when photoelectrons ejected parallel to the laser polarization are examined. These peaks correspond to the vibronic levels for out-of-plane modes in the ground X̃ E21″ state, which arise due to several pseudo-Jahn–Teller interactions with excited states of the radical and quadratic Jahn–Teller interaction in the X̃ E21″ state. A variant of the first derivative of the energy for the EOMIP-CCSD method has been utilized to evaluate the strength of these nonadiabatic couplings, which have subsequently been employed to construct the model potential of the X̃ E21″ state with respect to the out-of-plane normal coordinates. Simulations based on the model potential successfully reproduce the weak features that become conspicuous in the 0° spectrum. The present study of the photoelectron spectrum complements a previous dispersed fluorescence spectroscopic study Miller and co-workers [J. Chem. Phys. 114, 4855 (2001); 4869 (2001) Miller and co-workers [J. Chem. Phys.114, 4869 (2001)] to provide a detailed account of the vibronic structure of X̃ E21″ cyclopentadienyl. The electron affinity of the cyclopentadienyl radical is determined to be 1.808±0.006 eV. This electron affinity and the gas-phase acidity of cyclopentadiene have been combined in a negative ion thermochemical cycle to determine the C–H bond dissociation energy of cyclopentadiene; D0(C5H6,C–H)=81.5±1.3 kcal mol−1. The standard enthalpy of formation of the cyclopentadienyl radical has been determined to be ΔfH298(C5H5)=63.2±1.4 kcal mol−1.
On the influence of bandstructure on transport properties of magnetic tunnel junctions with Co2Mn1−xFexSi single and multilayer electrode
Authors: Schmalhorst J. , Ebke D. , Weddemann A. , Hütten A. , Thomas A. , Reiss G. , Turchanin A. , Gölzhäuser A. , Balke B. , Felser C.
Published: September 2008

The transport properties of magnetic tunnel junctions with different (110)-textured Heusler alloy electrodes such as Co2MnSi, Co2FeSi or Co2Mn0.5Fe0.5Si, AlOx barrier, and Co–Fe counterelectrode are investigated. The bandstructure of Co2Mn1−xFexSi is predicted to show a systematic shift in the position of the Fermi energy EF through the gap in the minority density of states while the composition changes from Co2MnSi toward Co2FeSi. Although this shift is indirectly observed by x-ray photoemission spectroscopy, all junctions show a large spin polarization of around 70% at the Heusler alloy/Al–O interface and are characterized by a very similar temperature and bias voltage dependence of the tunnel magnetoresistance. This suggests that these transport properties of these junctions are dominated by inelastic excitations and not by the electronic bandstructure.
Feedback suppression of rotating external kink instabilities in the presence of noise
Authors: Hanson Jeremy M. , De Bono Bryan , James Royce W. , Levesque Jeffrey P. , Mauel Michael E. , Maurer David A. , Navratil Gerald A. , Pedersen Thomas Sunn , Shiraki Daisuke
Published: September 2008

The authors report on the first experimental demonstration of active feedback suppression of rotating external kink modes near the ideal wall limit in a tokamak using Kalman filtering to discriminate the n=1 kink mode from background noise. The Kalman filter contains an internal model that captures the dynamics of a rotating, growing n=1 mode. Suppression of the external kink mode is demonstrated over a broad range of phase angles between the sensed mode and applied control field, and performance is robust at noise levels that render proportional gain feedback ineffective. Suppression of the kink mode is accomplished without excitation of higher frequencies as was observed in previous experiments using lead-lag loop compensation [A. J. Klein et al., Phys Plasmas 12, 040703 (2005)].
Fabrication of zinc oxide nanorods based heterojunction devices using simple and economic chemical solution method
Authors: Reddy N. Koteeswara , Ahsanulhaq Q. , Hahn Y. B.
Published: September 2008

In this letter we reported the behavior of zinc oxide nanorods/gallium nitride (ZnO/GaN) heterojunctions at different temperatures. The well-aligned ZnO nanorods were synthesized on GaN coated alumina (Al2O3) substrate using a solution method at lower temperatures. The as-grown p-n junction diode exhibited a low turn-on voltage of ∼0.65 V with an excellent rectifying behavior. While increasing temperature, the series resistance of the device slightly increased due to the formation of metallic bonds between metal and semiconductor. These results, therefore, emphasize that the as-grown heterostructures are quite stable even at higher temperatures.