bluuebunny/crossref_metadata_2025_split huggingface dataset:
Phonon energy renormalization in KDP‐type ferroelectrics
Authors: Wesselinowa J. M. , Apostolov A. T. , Filipova A. 
Published: July 2007

AbstractA Green's function technique is used to study the influence of spin‐phonon interactions in the Ising model with a transverse field including higher‐order anharmonic terms. The renormalized phonon energy has been evaluated for the first time beyond the RPA. At low temperatures both, the anharmonic spin‐phonon and phonon‐phonon interactions including cubic and quadratic terms play an important role, whereas near and above the Curie temperature only the anharmonic phonon‐phonon interaction contributes to the renormalization of the phonon energy.
Determination of the Pressure Dependence of Band‐Structure Parameters by Two‐Photon Spectroscopy
Authors: Reimann K. , Haselhoff M. , Rübenacke St. , Steube M. 
Published: July 2007

AbstractTwo‐photon spectroscopy has been used to determine the pressure dependence of band‐structure parameters such as band gap, exciton binding energy, biexciton binding energy, exchange energy, and Luttinger parameters. Compared to linear spectroscopy it yields not only a higher precision, but is often able to determine a larger number of parameters. Results are presented for ZnTe and CuCl nanocrystals in a LiCl matrix.
Electronic Structure and Pressure Dependence for Some Ternary Calcium Nitrides
Authors: Vansant P. R. , Van Camp P. E. , Van Doren V. E. , Martins J. L. 
Published: July 2007

AbstractIn this paper we present pseudopotential LDA calculations using a plane wave basis set. We investigate the structure of Ca3AsN and Ca3PN and consider its pressure dependence. Between the structures considered no phase transitions were found for positive pressures. Experimentally Ca3AsN and Ca3PN were found to have an orthorhombic structure which corresponds to a distorted cubic structure. Also theoretically this orthorhombic structure was found to be energetically the most favourable. Its corresponding lattice constants are in good agreement with the experimental data. Besides the lattice constants, the bulk moduli and its pressure derivatives we also calculate the electronic bandstructure.
Metal‐Insulator Transition in Degenerately Doped Si and Ge under High Uniaxial Pressure
Authors: Baidakov V. V. , Ermakov V. N. , Gorin A. E. , Kolomoets V. V. , Stuchinska N. V. , Shenderovskii V. A. , Tunstall D. P. 
Published: July 2007

AbstractStrain‐induced metal‐insulator (MI) transitions have been observed in degenerately doped Si:Sb and Ge:Sb crystals at T = 4.2 K and for extremely high uniaxial pressures up to 5 GPa. It is shown that in both Si and Ge the transition from a metallic‐type to an activated conductivity is determined by strain‐induced changes of the conduction band parameters. The data obtained demonstrate that the effective mass donor approach and the scaling theory of localization describe well the MI transition phenomenona observed.
Si in GaN — On the Nature of the Background Donor
Authors: Wetzel C. , Chen A. L. , Suski T. , Ager J. W. , Walukiewicz W. 
Published: July 2007

AbstractA characterization of the Si impurity in GaN is performed by Raman spectroscopy. Applying hydrostatic pressure up to 25 GPa we study the behavior of the LO phonon‐plasmon mode in a series of high mobility Si doped GaN films. In contrast to earlier results on unintentionally doped bulk GaN crystals no freeze out of the free carriers could be observed in Si doped samples. We find that Si is a shallow hydrogenic donor throughout the pressure range studied. This result positively excludes Si incorporation as a dominant source of free electrons in previously studied bulk GaN samples.
Skyrmion Excitations in a Two‐Dimensional Electron Gas under Hydrostatic Pressure
Authors: Maude D. K. , Potemski M. , Henini M. , Portal J. C. , Eaves L. , Hill G. , Pate M. A. 
Published: July 2007

AbstractThe spin excitation gap at Landau filling factor v = 1 has been determined from thermally activated transport measurements in the limit of small Landé g‐factor. The application of hydrostatic pressure allows to probe the region −0.11 < g < 0.055. The observed 50% reduction in the spin excitation gap at g = 0 is fully consistent with the recently proposed spin texture excitations (Skyrmions).
Pressure Dependence of the Electronic Subband Structure of Strained In0.2Ga0.8As/GaAs MQWs
Authors: Li G. H. , Goñi A. R. , Syassen K. , Hou H. Q. , Feng W. , Zhou J. M. 
Published: July 2007

AbstractWe have measured low‐temperature photoluminescence (PL) and optical absorption spectra of an In0.2Ga0.8As/GaAs multiple quantum well (MQW) structure at pressures up to 8 GPa. Below 4.9 GPa, PL shows only the emission of the n = 1 heavy‐hole (HH) exciton. Three new X‐related PL bands appear at higher pressures. They are assigned to spatially indirect (type‐II) and direct (type‐I) transitions from Xz states in GaAs and XXY valleys of InGaAs, respectively, to the HH subband of the wells. From the PL data we obtain a valence band offset of 80 meV for the strained In0.2Ga0.8As/GaAs MQW system. Absorption spectra show three features corresponding to direct exciton transitions in the quantum wells. In the pressure range of 4.5 to 5.5 GPa an additional pronounced feature is apparent in absorption, which is attributed to the pseudo‐direct transition between a HH subband and the folded Xz states of the wells. This gives the first clear evidence for an enhanced strength of indirect optical transitions due to the breakdown of translational invariance at the heterointerfaces in MQWs.
A General Approach to Measurement of Band Offsets of Near‐GaAs Alloys
Authors: Whitaker M. F. , Dunstan D. J. , Missous M. , Gonzalez L. 
Published: July 2007

AbstractBand offsets can be found by high‐pressure spectroscopy if the X‐minima of a heterostructure are type II. However, many heterostructure systems do not have a type II X‐minimum and then this method does not work. We therefore propose a new approach to measuring band offsets. It is capable of obtaining the band offsets of any near‐GaAs alloy against GaAs by comparing the photoluminescence energies of alloy and GaAs quantum wells with AIGaAs barriers below and above crossover. The method is demonstrated for the case of InGaAs. There are other candidate systems which may usefully be used in the same manner as GaAs/AIGaAs. Before they can be used it is necessary to check whether the X‐minimum is type II or not. We present data confirming that the GaInP/GaAs system also has a type II X‐minimum and can therefore also be used.
Zn1−xMgxSe Samples and a ZnSe/Zn0.93Mg0.07Se Quantum Well under Pressure: Optical Absorption and Photoluminescence
Authors: Griebl E. , Haserer B. , Frey T. , Reisinger T. , Gebhardt W. 
Published: July 2007

AbstractWe present optical measurements of freestanding thin Zn1−xMgxSe films and of a ZnSe/Zn0.93Mg0.07Se single quantum well (SQW) under hydrostatic pressure applied by a diamond anvil cell. For the Zn1−xMgxSe compounds the pressure shift of the fundamental gap was determined from absorption measurements at room temperature for X ⩽ 0.37. A hydrostatic deformation potential a(x) = −4.85 + 0.67x eV is derived. The transition pressures Ptr to high pressure phases were determined by visual observation. The pressure dependent absorption and photoluminescence of the ZnSe SQW was investigated at 2 K. The energy shift of two SQW‐related transitions was recorded and is reproduced by a simple theoretical model. LO‐phonon replica were visible in photoluminescence at the sharp 11hh transition.
Nonmonotonic Behavior of the Threshold Current of Compressively Strained 1.3 μm InGaAsP Lasers with Hydrostatic Pressure
Authors: Patel D. , Menoni C. S. , Bernussi A. A. , Temkin H. 
Published: July 2007

AbstractHydrostatic pressure measurements of the threshold current of 1.3 μm compressively strained InGaAsP multiple quantum well lasers show an initial decrease up to 1.2 GPa followed by an increase at higher pressures. This behavior is explained by considering the changes in the differential gain and optical losses determined from measurements of the spontaneous emission and of the external differential efficiency as a function of pressure, respectively.
Bulk Amorphous GaSb Semiconductors Prepared by Thermobaric Treatment: Formation and Properties
Authors: Barkalov O. I. , Kolesnikov A. I. , Antonov V. E. , Ponyatovsky E. G. , Dahlborg U. , Dahlborg M. , Hannon A. 
Published: July 2007

AbstractThe multistage process of solid state amorphization and subsequent crystallization of the quenched “white tin” high pressure phase of Ga‐Sb alloys containing 20 to 80 at% Sb was studied at ambient pressure by differential scanning calorimetry. The heats of both amorphization and crystallization were exothermic and equal to (3.5 ± 0.5) and (8.3 ± 1.0) kJ/mol, respectively. Formation of the bulk amorphous alloys free of any crystalline inclusions was observed for compositions with 47.5 to 52.5 at% Sb. The structure of the amorphous GaSb produced by solid state amorphization was studied by neutron diffraction. It is found to be nearly identical to that of a sample prepared earlier by sputtering.
Pressure Induced Phase Transitions in Alkaline Earth Tungstates
Authors: Christofilos D. , Ves S. , Kourouklis G. A. 
Published: July 2007

AbstractWe present a Raman study of the pressure induced structural phase transitions in the tungstates CaWO4 and SrWO4. Raman spectra up to 24 GPa at room temperature reveal reversible pressure induced phase transitions at 10 and 11.5 GPa for CaW)4 and SrWO4, respectively. The lowest Bg mode, associated with the WO4‐WO4 vibration along the c‐axis, exhibits a softening in the scheelite structure, while in the high‐pressure phase its slope becomes positive. Above the phase transition the modes originating from the antisymmetric v3(Eg) and v3(Bg) stretching modes of the WO4 group exhibit a frequency jump to higher and lower frequencies. In the high pressure phase the modes originating from v3 show an unusual, strongly nonlinear softening in a small pressure region before becoming almost pressure independent.
Optical Properties of C60 Single Crystals at High Pressure
Authors: Meletov K. P. , Christofilos D. , Ves S. , Kourouklis G. A. 
Published: July 2007

AbstractThe Raman scattering, luminescence, and absorption spectra of C60 single crystals at T = 300 K and pressures up to 15 GPa were measured. The pressure dependence of the Raman‐active intramolecular phonons indicates that two phase transitions are induced at pressures of 0.4 and 2.4GPa. The magnitude of the pressure shifts of phonon modes, luminescence and absorption spectra, the values of the Grüneisen parameters, and the deformation potential have been determined.
Influence of Randomly Distributed Surface Steps on Photoemission Lineshape
Authors: Beckmann A. , Meinel K. , Heiler M. , Ammer Ch. , Neddermeyer H. 
Published: July 2007

AbstractThe model of randomly distributed steps is examined with respect to the possibility of non‐lifetime broadening in photoemission spectra from sp‐band surface states. The peak shape is discussed in terms of hole lifetime, terrace width, and angle of emission. The model calculations are based on the parameters of the Cu(III) sp‐surface state.
Electronic Energy Structure of the Ternary Carbide System TiAlC
Authors: Ilyasov V. V. , Ya. Nikiforov I. , Ilyasov Yu. V. 
Published: July 2007

AbstractWith the coherent potential method of the multiple scattering theory the electronic structure of the ternary carbide system Ti1−xAlxC (x = 0.17) is calculated. The cluster approximation with muffintin crystal potential is used. With the same method the electronic structure of the binary TiC system has been calculated for comparison. It was found that Ti1−xAlxC (x = 0.17) can be considered as a semiconductor with a narrow energy gap (2.18 eV). Aluminium doping results in an increase of the valence band width and a decrease of the cohesive energy.
The Influence of the Interaction between Phonons on the Properties of the Exciton in Polar Crystals
Authors: Xiao Wei , Xiao Jing‐Lin 
Published: July 2007

AbstractSome properties of excitons in polar crystals are studied. The effective Hamiltonian of the exciton in polar crystals is derived by a perturbation method. If we consider the interaxtion between phonons of different wave vectors in the recoil process, the influence on the effective potential between electron and hole, the self‐trapping energy, and the self‐trapping condition are discussed.
Magnetic Alloys Containing Crystal Field Split Impurities: Magnetoresistance
Authors: Kumar A. , Ahluwalia P. K. , Sharma K. C. 
Published: July 2007

AbstractThe phenomena of magnetoresistance and thermal conductivity in the presence of a magnetic field are investigated using the Kondo interaction Hamiltonian within the framework of Abrikosov's pseudofermion representation in the presence of a crystalline electric field up to second order for the Ce‐based system CeCu2Si2. The results for both the properties for different values of magnetic field are presented and using the lowest four nondegenerate levels, these are found to be quite consistent with the experimental data.
Spin Wave Spectrum in Magnetic Superlattices with Anisotropic Fields
Authors: Anselmo D. H. A. L. , Albuquerque E. L. 
Published: July 2007

AbstractWe present a microscopic theory, based on the Heisenberg model, to investigate the spin wave spectrum in a magnetic superlattice whose constituents may present the uniaxial and nonuniaxial com‐ ponents of the single‐ion anisotropy. Our calculations are carried out for the exchange‐dominated regime, stressing the contribution of the bulk as well as the surface modes associated with the truncation of the superlattice. We derive general dispersion relations which encompass previous works in the field. Interesting new aspects of the spectrum are obtained due to the presence of the nonuniaxial component of the anisotropic field.
Ps formation in molecular media: low temperature hydrocarbons
Authors: Stepanov S. V. , Byakov V. M. 
Published: July 2007

AbstractA simplified model of low temperature radiolysis of hydrocarbons sufficient for adequate interpretation of radiation chemical and positron annihilation data is suggested. The model describes non‐monotonic accumulation of trapped electrons and other radiolytic products with the absorbed dose in γ ‐radiolysis experiments, as well as variation of the shape of e+ annihilation lifetime spectra with time of the measurements. The developed model is compared with experiments in polyethylene and n‐tridecane. It is shown that intratrack radicals significantly affect Ps formation and time evolution of the e+ states. Some values of intratrack reaction rate constants are estimated. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
ATRAP antihydrogen experiments
Authors: Storry C. H. , Aggarwal M. , Akbari A. , Al‐Rahawi F. , Amole C. , Batrachenko A. , Bebko A. , Carew A. , Chalfin M. , Comeau D. , Gabrielse G. , Garofalo F. , George M. C. , Goldenbaum F. , Grzonka D. , Guise N. , Gura Y. , Hänsch T. , Hessels E. A. , Kolbe D. , Kotlhammer S. , Kuljanishvili I. , Larochelle P. , Lesage D. , Levitt B. , Lewis K. , Lishak B. , Markert F. , Nillius F. , Oelert W. , Patel S. , Popescu P. , Scheid M. , Sefzick T. , Speck A. , Swierad D. , Walz J. , Weel M. , Wrubel J. , Zang Z. 
Published: July 2007

AbstractAntihydrogen (Hbar) was first produced at CERN in 1996. Over the past decade our ATRAP collaboration has made massive progress toward our goal of producing large numbers of cold Hbar atoms that will be captured in a magnetic gradient trap for precise comparison between the atomic spectra of matter and antimatter. The AD at CERN provides bunches of 3 × 107 low energy Pbars every 100 seconds. We capture and cool to 4 K, 0.1% of these in a cryogenic Penning trap. By stacking many bunches we are able to do experiments with 3 × 105 Pbars. ∼100 e+/sec from a 22Na radioactive source are captured and cooled in the trap, with 5 × 106 available experiments.We have developed 2 ways to make Hbar from these cold ingredients, namely 3‐body collisions, and 2‐stage Rydberg charge exchange. In the first case, Pbars are injected into a nested trap containing e+. Hbar is formed when 2 e+ and 1 Pbar collide. In 2‐stage Rydberg charge exchange, laser‐excited caesium (Cs) enters the trap through a small hole. Rydberg positronium is formed when a e+ captures an e‐from a Cs. These atoms exit the trap, some passing through a nearby cloud of cold Pbars. A 2nd charge‐exchange results when a Pbar captures the e+, forming Hbar. We have also developed techniques to measure the excited‐state distribution of the Hbar and measure their velocity. I will present results from these experiments and discuss the next generation of apparatus to be commissioned this year. This new apparatus includes a e+ accumulator built at York University providing many more e+. The new Pbar annihilation detector provides spatial information of annihilations. Windows allow lasers to enter the trap for spectroscopic measurements and for laser cooling of the Hbar. Possibly the most exciting inclusion in this new apparatus is the inclusion of a neutral particle trap which may, for the first time, capture the Hbar and lead to the first atomic spectrum from antimatter. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Study of high‐k gate dielectrics by means of positron annihilation
Authors: Uedono A. , Naito T. , Otsuka T. , Ito K. , Shiraishi K. , Yamabe K. , Miyazaki S. , Watanabe H. , Umezawa N. , Hamid A. , Chikyow T. , Ohdaira T. , Suzuki R. , Ishibashi S. , Inumiya S. , Kamiyama S. , Akasaka Y. , Nara Y. , Yamada K. 
Published: July 2007

AbstractHigh‐dielectric constant (high‐k) gate materials, such as HfSiOx and HfAlOx, fabricated by atomic‐layer‐deposition techniques were characterized using monoenergetic positron beams. Measurements of the Doppler broadening spectra of annihilation radiation and the lifetime spectra of positrons indicated that positrons annihilated from the trapped state by open volumes that exist intrinsically in amorphous structures of the films. The size distributions of the open volumes and the local atomic configurations around such volumes can be discussed using positron annihilation parameters, and they were found to correlate with the electrical properties of the films. We confirmed that the positron annihilation is useful technique to characterize the matrix structure of amorphous high‐k materials, and can be used to determine process parameters for the fabrication of high‐k gate dielectrics. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Digital positron lifetime spectroscopy: present status and outlook
Authors: Bečvář F. 
Published: July 2007

AbstractThis contribution summarizes achievements in instrumentation for positron lifetime measurements with emphasis on digital spectrometric systems. A significant part of the data presented are based on a long‐term exploitation of a conventional positron lifetime spectrometer developed at Charles University in early 90s, on bench‐mark testing measurements with this spectrometer working temporarily in conjunction with a pair of 8‐bit, 4 GS/s digitizers and on analogous measurements with a recently assembled digital positron lifetime spectrometer. In addition, results from testing experiments with microchannel plate photomultipliers are briefly reported. Further development of positron lifetime technique is discussed. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Elevation protective role of Camellia sinensis and Urtica dioica infusion against trichloroacetic acid‐exposed in rats
Authors: Celik Ismail , Tuluce Yasin 
Published: July 2007

AbstractThis study was designed to investigate the protective effects of two traditionally used Turkish medicinal plants, Camellia sinensis (CS) and Urtica dioica L. (UD), beverages used against chemical carcinogen trichloroacetic acid (TCA)‐exposure in rats. The preventive potential of the plant infusions was evaluated by measuring the level of serum marker enzymes, aspartate aminotransferase (AST), alanine aminotransferase (ALT), creatine phosphokinase (CPK), acid phosphatase (ACP), alkaline phosphatase (ALP) and lactate dehydrogenase (LDH); antioxidant defense systems, reduced glutathione (GSH), glutathione reductase (GR), superoxide dismutase (SOD), glutathione‐S‐transferase (GST) and catalase (CAT); and lipid peroxidation (malondialdehyde, MDA) content in various organs of rats. Twenty four healthy rats were randomly allotted into four experimental groups: A (untreated control), B (only TCA‐treated), C (TCA + CS treated) and D (TCA + UD treated). At the end of the 50 days, the plant infusions possessed chemoprotective effects, deduced by the remaining TCA‐induced increased serum damage marker enzyme, lipid peroxidation and antioxidative system in rats compared with those of the control and TCA‐exposed rats. According to the results, while the levels of AST, ALT and CPK increased in group B, no significant changes were observed in groups C and D. The MDA content slightly increased in tissues of all groups, being higher in group B. Antioxidant enzyme activities such as SOD and CAT increased significantly in the brain, liver and kidney of group B while they did not change significantly except for in the kidney in groups C and D. The GSH level and the ancillary enzyme GR activity did not change significantly in organs of all groups. On the other hand, the drug metabolizing enzyme, GST, activity decreased significantly in the brain, liver and kidney of group D while slight changes were observed for the other groups. The results revealed that TCA exposure induced oxidative stress in rat tissues, however, in plant beverage supplemented groups, a significant protective effect of CS and UD against TCA‐induced oxidative injury was recorded. Hence, the study revealed that the constituents present in CS and UD impart protection against carcinogenic chemical induced oxidative injury that may result in the development of cancer. Also the observations, along with changes, suggest that both CS and UD may possess preventive potential during a 50‐day protective exposure. Copyright © 2007 John Wiley & Sons, Ltd.
Anti‐parkinson botanical Mucuna pruriens prevents levodopa induced plasmid and genomic DNA damage
Authors: Tharakan Binu , Dhanasekaran Muralikrishnan , Mize‐Berge Janna , Manyam Bala V. 
Published: July 2007

AbstractLevodopa is considered the ‘gold standard’ for the treatment of Parkinson's disease. However, a serious concern is dyskinesia and motor fluctuation that occurs after several years of use. In vitro experiments have shown that in the presence of divalent copper ions, levodopa may induce intense DNA damage. Mucuna pruriens cotyledon powder (MPCP) has shown anti‐parkinson and neuroprotective effects in animal models of Parkinson's disease that is superior to synthetic levodopa. In the present study two different doses of MPCP protected both plasmid DNA and genomic DNA against levodopa and divalent copper‐induced DNA strand scission and damage. It exhibited chelation of divalent copper ions in a dose‐dependent manner. The copper chelating property may be one of the mechanisms by which MPCP exerts its protective effects on DNA. Copyright © 2007 John Wiley & Sons, Ltd.
Comparison of charge transfer recombination of (3s2 3p) third row S3+ and P2+ ions with H
Authors: Bacchus‐Montabonel M. C. , Łabuda M. , Tergiman Y. S. 
Published: July 2007

AbstractWe present a compared dynamical study of the S3+(3s23p)2P° + H and P2+(3s23p)2P° + H collision systems involving an accurate ab initio calculation of the potential energy curves and couplings of the molecular states involved in each process followed by a semi‐classical collision treatment in the keV laboratory energy range. The results are in good agreement with experiment, when available, and different mechanisms may be pointed out for these reactions in connection with the different molecular interactions involved. © 2007 Wiley Periodicals, Inc. J Quantum Chem, 2007