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At least 1,531 records · Page 85
All sourcesNASA NTRSDOE OSTI
DOE OSTI2010

Charged-Particle Multiplicity Density at Midrapidity in Central Pb-Pb Collisions at {radical}(s{sub NN})=2.76 TeV

The first measurement of the charged-particle multiplicity density at midrapidity in Pb-Pb collisions at a center-of-mass energy per nucleon pair {radical}(s{sub NN})=2.76 TeV is presented. For an event sample corresponding to the most central 5% of the hadronic cross section, the pseudorapidity density of primary charged particles at midrapidity is 1584{+-}4(stat){+-}76(syst), which corresponds to 8.3{+-}0.4(syst) per participating nucleon pair. This represents an increase of about a factor 1.9 relative to pp collisions at similar collision energies, and about a factor 2.2 to central Au-Au collisions at {radical}(s{sub NN})=0.2 TeV. This measurement provides the first experimental constraint for models of nucleus-nucleus collisions at LHC energies.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2010

Frustrated exchange interactions formation at low temperatures and high hydrostatic pressures in La{sub 0.70}Sr{sub 0.30}Mn{sub O2.85}

The magnetic and thermal properties of the anion-deficient La{sub 0.70}Sr{sub 0.30}MnO{sub 2.85} manganite are investigated in wide temperature (4-350 K) range, including under hydrostatic pressure (0-1.1 GPa). Throughout the pressure range investigated, the sample is spin glass with diffused phase transition into paramagnetic state. It is established, that spin glass state is a consequence of exchange interaction frustration of the ferromagnetic clusters embeded into antiferromagnetic clusters. The magnetic moment freezing temperature T{sub f} of ferromagnetic clusters increases under pressure, freezing temperature dependence on pressure is characterized by derivative value {approx}4.5 K/GPa, while the magnetic ordering T{sub MO} temperature dependence is characterized by derivative value {approx}13 K/GPa. The volume fraction of sample having ferromagnetic state is V{sub fer} {approx} 13% and it increases under a pressure of 1.1 GPa by {Delta}V{sub fer} {approx} 6%. Intensification of ferromagnetic properties of the anion-deficient La{sub 0.70}Sr{sub 0.30}MnO{sub 2.85} manganite under hydrostatic pressure is a consequence of oxygen vacancies redistribution and unit cell parameters decrease. The most likely mechanism of frustrated exchange interactions formation is discussed.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Disorder effects on the band structure of ZnGeN 2 : Role of exchange defects

The role of exchange defects on the band structure of ZnGeN 2 is investigated. Exchange defects are defined through the exchange of cations Zn and Ge starting from the ideal Pna2 1 crystal structure, which obeys the local octet rule. Each such exchange creates several nitrogen-centered tetrahedra which violate the local octet rule, although overall charge neutrality is preserved. We study several distributions of exchange defects, some with all antisites making up the exchange defect close to each other and with increasing numbers of exchange defects, and others where the two types of antisites Zn Ge and Ge Zn are kept separated from each other. We also compare the results for these models with a fully random distribution of Zn and Ge on the cation sites. We show that for a single-nearest-neighbor exchange defect, the band gap is narrowed by about 0.5 eV due to two effects: (1) the Zn Ge antisites form filled acceptor states just above and merging with the valence-band maximum (VBM) of perfect crystal ZnGeN 2 and (2) the Ge Zn antisites form a resonance in the conduction band which lowers the conduction-band minimum (CBM). When more exchange defects are created, these acceptor states broaden into bands which can lower the gap further. When tetrahedra occur surrounded completely by four Zn atoms, states even deeper in the gap are found localized all near these tetrahedra, forming a separate intermediate band. Finally, for phase-segregated Zn Ge and Ge Zn , the gap is significantly more reduced, but no separate band is found to occur. The Zn Ge acceptorlike states now form a percolating defect band which is significantly wider and hence reaches deeper into the gap. In all cases, the wave functions near the top of the new VBM remain, to some extent, localized near the Zn Ge sites. For a fully random case, the gap is even more severely reduced by almost 3 eV. The total energy of the system increases with the number of octet-rule-violating tetrahedra and the energy cost per exchange defect of order 2 eV is quite high.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Nature of native atomic defects in ZrTe 5 and their impact on the low-energy electronic structure

Over the past decades, investigations of the anomalous low-energy electronic properties of ZrTe 5 have reached a wide array of conclusions. An open question is the growth method's impact on the stoichiometry of ZrTe 5 samples, especially given the very small density of states near its chemical potential. In this work, we report on high-resolution scanning tunneling microscopy and spectroscopy measurements performed on samples grown via different methods. Using density functional theory calculations, we identify the most prevalent types of atomic defects on the surface of ZrTe 5 , namely, Te vacancies and intercalated Zr atoms. Finally, we precisely quantify their density and outline their role as ionized defects in the anomalous resistivity of this material.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2010

Luminescence and creation of electron centers in UV-irradiated YAlO{sub 3} single crystals

Luminescence and defect creation processes were studied by the photoluminescence, thermally stimulated luminescence, and electron paramagnetic resonance methods in the UV-irradiated single crystals of undoped YAlO{sub 3}, containing small amounts of Ce, Mo, and Ti ions as accidental impurities. The luminescence of the electron antisite Y{sub Al}{sup 2+}-type centers of different structures was found around 2.45 eV and studied at 4.2-500 K. The luminescence of the Ti{sup 3+}-related centers (2.03 and 1.73 eV) and Ti{sup 4+} centers (2.78 eV) was observed as well. Dependences of the number of the Y{sub Al}{sup 2+}-type and Ti{sup 3+}-related centers on the UV irradiation energy, temperature, and duration, as well as on various crystal heat-treatment procedures were examined. As a result of the photostimulated electron transfer from the O{sup 2-} ligand ions to Mo{sup 4+} and Ti{sup 4+} ions, the paramagnetic hole O{sup -}-type centers and electron Ti{sup 3+} and Mo{sup 3+} centers are created. The antisite Y{sub Al}{sup 2+}-type centers are created due to the photostimulated release of electrons mainly from the Mo{sup 3+} centers to the conduction band and their subsequent trapping at the Y{sub Al}{sup 3+} ions located near an oxygen vacancy or a defect at the neighboring Y{sup 3+} site.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Direct observation of N-(group V) bonding defects in dilute nitride semiconductors using hard x-ray photoelectron spectroscopy

Using bulk sensitive hard x-ray photoelectron spectroscopy, we directly observe a spectrum related to N-As bonding defects in (Ga,In)(N,As)/Ga(N,As) heterostructure. The defects are most likely attributed to split interstitials. Their concentration is in the order of 10{sup 19} cm{sup -3}, close to the detection limit of the measurement. Rapid thermal annealing eliminates the defects, leading to those undetectable. Similar phenomenon is observed for N-P bonding defects in In(N,P). The results indicate common features in dilute nitride semiconductor system: existence of N-(group V) bonding defects and their behavior on postgrowth annealing.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2011

Optical modulation of persistent photoconductivity in ZnO nanowires

In this study, ZnO nanowires (ZNWs)-based optoelectric devices are found to exhibit strong persistent photoconductivity (PPC) effect. An optical modulation on the PPC effect of the ZNWs with 980 nm infrared (IR) laser has been investigated. It was found that the decay time for the PPC can be significantly shortened by IR irradiation. The modulation mechanism related with the oxygen vacancies and the subband gap excitation is proposed. Based on this mechanism, the modulation behavior of the IR can be well explained. The present optical modulation on the PPC is suggested to have potential applications in enhancing the performance of ZnO-based photodetectors.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2011

Diffusion of Mn interstitials in (Ga,Mn)As epitaxial layers

The magnetic properties of thin (Ga,Mn)As layers improve during annealing by out-diffusion of interstitial Mn ions to a free surface. Out-diffused Mn atoms participate in the growth of a Mn-rich surface layer and a saturation of this layer causes an inhibition of the out-diffusion. We combine high-resolution x-ray diffraction with x-ray absorption spectroscopy and a numerical solution of the diffusion problem for the study of the out-diffusion of Mn interstitials during a sequence of annealing steps. Our data demonstrate that the out-diffusion of the interstitials is substantially affected by the internal electric field caused by an inhomogeneous distribution of charges in the (Ga,Mn)As layer.

36 MATERIALS SCIENCE↗
DOE OSTI2011

First-principles study of noble gas impurities and defects in UO{sub 2}

We performed a series of density functional theory + U (DFT + U) calculations to explore the energetics of various defects in UO{sub 2}, i.e., noble gases (He, Ne, Ar, Kr, Xe), Schottky defects, and the interaction between these defects. We found the following: (1) collinear antiferromagnetic UO{sub 2} has an energy-lowering distortion of the oxygen sublattice from ideal fluorite positions; (2) DFT + U qualitatively affects the formation volume of Schottky defect clusters in UO{sub 2} (without U the formation volume is negative, but including U the formation volume is positive); (3) the configuration of the Schottky defect cluster is dictated by a competition between electrostatic and surface energy effects; (4) the incorporation energy of inserting noble gas atoms into an interstitial site has a strong dependence on the volume of the noble gas atom, corresponding to the strain it causes in the interstitial site, from He (0.98 eV) to Xe (9.73 eV); (5) the energetics of each of the noble gas atoms incorporated in Schottky defects show strong favorable binding, due to strain relief associated with moving the noble gas atom from the highly strained interstitial position into the vacant space of the Schottky defect; and (6) for argon, krypton, and xenon, the binding energy of a noble gas impurity with the Schottky defect is larger than the formation energy of a Schottky defect, thereby making the formation of Schottky defects thermodynamically favorable in the presence of these large impurities.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Hydrogen adsorption and anomalous electronic properties of nitrogen-doped graphene

We investigate hydrogen adsorption effects on stabilities and electronic properties of nitrogen defects in graphene using first-principles electronic-structure calculations within the density-functional theory. We find that the adsorption of hydrogen atoms on the pyridine-type nitrogen defects in graphene becomes energetically favorable, whereas in the case of the substitutional nitrogen defect the hydrogen adsorption becomes unfavorable. We also find that a transition from p-type to n-type doping properties occurs by hydrogen adsorption on the pyridine-type defects, suggesting that even the carrier type is controllable in nitrogen-doped graphene.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2010

Structural and magnetic properties of self assembled Fe-doped Cu{sub 2}O nanorods

Cuprous oxide (Cu{sub 2}O) nanorods doped with iron impurities have been synthesized by the polyol method using sodium dodecyl sulfate as the surfactant. The X-ray diffraction measurement reveals the pure phase of simple cubic Cu{sub 2}O and the electron microscopy displays its one dimensional morphology. Ferromagnetism was observed at room temperature in the magnetic measurements of the doped samples while undoped sample exhibits only diamagnetism. Room temperature Moessbauer spectra for the samples exhibited only doublets but no sextet, which corresponds to the presence of paramagnetic iron sites. As magnetic moment contribution of the doped ions was insignificant for the observed magnetism, ferromagnetic property in the doped samples could have been originated from the defects as cation vacancies. Existence of the defects was supported by the room temperature photoluminescence spectra of the doped samples in reference to the undoped sample. - Ferromagnetism at room temperature was observed in the Fe doped Cu{sub 2}O nanorods. The origin seems to be the defects of cation vacancies created by the dopant ions.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2011

Formation mechanisms of precursors of radiation-induced color centers during fabrication of silica optical fiber preform

Samples in the form of transverse slices of rods and optical fiber preforms made from the high-hydroxyl KU-1 and low-hydroxyl KS-4V silica by the plasma outside deposition (POD) method are {gamma}-irradiated to a dose of {approx}1 MGy (SiO{sub 2}). Next, the radial dependences of the radiation-induced nonbridging oxygen hole center (NBOHC) and E'-center (three-coordinated silicon) in the samples are constructed by measuring the amplitudes of their 4.8 and 5.8 eV absorption bands, respectively. Based on the analysis of these radial dependences and considering the temperature and duration of the preirradiation heat treatment of the rods and preforms at the POD-installation, we determine the ratio of the oscillator strengths of the above bands and the microscopic thermoinduced processes occurring during preform fabrication and producing precursors of the radiation-induced NBOHC and E'-center. These processes are found to be associated with the escape of either H{sub 2} or H{sub 2}O from neighboring hydroxyl groups, and, therefore, can occur in high-hydroxyl silica only. It is concluded that enhancement of the radiation resistance of high-hydroxyl silica optical fibers requires decreasing the temperature and duration of the preform fabrication process, in particular, changing from the POD-technology to the low-temperature plasmachemical vapor deposition (PCVD) or surface PCVD (SPCVD)-technology.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Preparation, crystal structure, and superconductive characteristics of new oxynitrides (Nb{sub 1-x}M{sub x})(N{sub 1-y}O{sub y}) where M=Mg, Si, and x{approx}y

New niobium oxynitrides containing either magnesium or silicon were prepared at 1000 deg. C by ammonia nitridation of oxide precursors obtained via the citrate route. The products had rock-salt type crystal structures. Crystallinity was improved by annealing in 0.5 MPa N{sub 2} and the final compositions were (Nb{sub 0.95}Mg{sub 0.05})(N{sub 0.92}O{sub 0.08}) at 1500 deg. C and (Nb{sub 0.87}Si{sub 0.09{open_square}0.04})(N{sub 0.87}O{sub 0.13}) at 1200 deg. C. The magnesium and oxide ions partially co-substitute the niobium and nitride ions in the octahedral sites of the {delta}-NbN lattice, respectively. Silicon ions were also successfully doped together with oxide ions into the rock-salt type NbN lattice. The Si doped product exhibited relatively large displacement at the octahedral sites and was accompanied by a small amount of cation vacancies. Superconductivity was improved by annealing to obtain critical temperatures/volume fractions of T{sub c}=17.6 K/100% for Mg- and T{sub c}=16.2 K/95% for the Si-doped niobium oxynitrides. - Graphical Abstract: Doped silicon ions substituting Nb in (Nb{sub 0.87}Si{sub 0.09{open_square}0.04})(N{sub 0.87}O{sub 0.13}) introduced a large displacement at the 4a site mainly because of the accompanied vacancy. The silicon ions may also be slightly shifted from the regular octahedral 4a site (green) towards the interstitial tetrahedral site (red) statistically in the rock-salt type lattice. Highlights: > New superconducting niobium oxynitrides containing either magnesium or silicon were prepared. > Their annealed products had chemical compositions of (Nb{sub 0.95}Mg{sub 0.05})(N{sub 0.92}O{sub 0.08}) and (Nb{sub 0.87}Si{sub 0.09{open_square}0.04})(N{sub 0.87}O{sub 0.13}) in the rock-salt type structure. > Their superconductivities were T{sub c}/volume fraction=17.6 K/100% and 16.2 K/95%, respectively.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2010

Ultraviolet radiation effects on paramagnetic defects in low-{kappa} dielectrics for ultralarge scale integrated circuit interconnects

The electronic properties of low-{kappa} interlayer dielectric and etch stop layers are important issues in ultralarge scale integrated circuits development. Leakage currents are critical problems that are not well understood. A topic of current interest is ultraviolet curing of these films. We report on electron spin resonance and electrical measurements of low-{kappa} films with and without ultraviolet exposure. This work provides fundamental understanding of the deep level defects likely involved in leakage currents.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Spin correlations and electron transport in MnBi:Au films

The structural, magnetic, and electron transport properties of Mn{sub 55-x}Au{sub x}Bi{sub 45} (x = 0, 4.5) thin films prepared by magnetron sputtering have been investigated. The magnetization of the MnBi films decreases and the coercivity increases due to Au doping. The temperature dependence of resistivity between 2 to 300 K shows that the films are metallic but the 4.5% Au-doped film shows a Kondo behavior with resistance minimum at 10.2 K. The magnetoresistance is anisotropic and the positive transverse magnetoresistance is significantly enhanced (16.3% at 70 kOe) by Au doping. We interpret these data in terms of a model in which Au atoms preferentially substitute for Mn atoms on the Mn lattice, and some Mn atoms are displaced to interstitial sites in the NiAs structure. These interstitial Mn atoms are coupled antiferromagnetically to the Mn atoms on the original Mn lattice leading to the large decrease in magnetization, Kondo effect, and the positive magnetoresistance.

36 MATERIALS SCIENCE↗
DOE OSTI2011

IR studies of the impact of Ge doping on the successive conversion of VO{sub n} defects in Czochralski-Si containing carbon

We report infrared absorption studies of oxygen-related defects in electron-irradiated Ge-doped Czochralski-Si. Our investigation was mainly focused on the reaction channel leading to the formation of VO{sub n} (1{<=}n{<=}6) defects. The VO{sub n} defects form mainly upon annealing, as a result of the successive aggregation of oxygen atoms in the initial VO defect produced by the irradiation: (VO+O{sub i}{yields}VO{sub 2}+O{sub i}{yields}VO{sub 3}+O{sub i}{yields}VO{sub 4},...). It was found that the ratio of the conversion of VO{sub n} to VO{sub n+1} defects is sensitive to the Ge content of the material. In particular, the ratio of the conversion of the VO to the VO{sub 2} defects was found to decrease with the increase in Ge concentration of the samples, although the opposite trend was observed for the VO{sub 3} to VO{sub 4} conversion. However, the VO{sub 2} to VO{sub 3} conversion changes only slightly with Ge content, being practically unaffected for Ge concentrations up to 2x10{sup 20} cm{sup -3}. In the case of VO{sub 2} formation, the phenomenon was attributed to the elastic strains induced in the lattice due to the Ge presence which affects the balance between the reactions VO+O{sub i}{yields}VO{sub 2}, VO+Si{sub I}{yields}O{sub i}, mainly involved in the decay of the VO and the growth of the VO{sub 2} defects. In the case of VO{sub 4} formation, the phenomenon was discussed by taking into account the enhancement of the diffusivity of the O{sub i} atoms in the Ge-doped Si, which could lead to an enhancement of the rate of the reaction VO{sub 3}+O{sub i}{yields}VO{sub 4}. For the VO{sub 3} formation this effect is practically negligible due to the fact that at the temperatures of VO{sub 2} to VO{sub 3} conversion oxygen diffusivity is quite small. The exhibited behavior in the conversion of the VO{sub n} to VO{sub n+1} defects (n=1,2,3) was similar in Ge-doped samples with low carbon content ([C{sub s}]<2x10{sup 16} cm{sup -3}) and in Ge-doped samples with high carbon content ([C{sub s}]{>=}1x10{sup 17} cm{sup -3}). The impact of C as well as its role in the conversion efficiency of VO to VO{sub 2} was studied by comparing the spectra in low carbon and high carbon Ge free Si material. Furthermore, a pair of bands at (1037,1051 cm{sup -1}) was attributed to the VO{sub 5} defect. The origin of another pair of bands (967,1005 cm{sup -1}) was discussed and tentatively correlated with a VO{sub n}C{sub s} structure. The role of Ge and C in the formation of the latter two pairs of bands was discussed.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Bandgap tailoring of rutile TiO{sub 2}(110) via surface patterning with electron cyclotron resonance sputtering

Crystalline TiO{sub 2} nanodots have been formed on single crystal rutile TiO{sub 2}(110) surfaces via ion beam sputtering method by utilizing Ar ion beams from electron cyclotron resonance source. Nearly five times enhancement in absorbance of visible light, {approx}5 times increase in luminescence, and {approx}0.1 eV narrowing of bandgap are observed for nanodot-patterned surfaces, in the absence of any dopant material. Formation of crystalline rutile TiO{sub 2} nanodots and development of Ti interstitials on the TiO{sub 2}(110) surface, after ion beam sputtering, are responsible for these observations. Results suggest that these nanodot-patterned rutile TiO{sub 2} surfaces can become effective photocatalysts.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Electronic structure studies of Fe doped CeO{sub 2} thin films by resonance photoemission spectroscopy

We have studied the modification in the electronic properties of pulsed laser deposited CeO{sub 2} thin films due to Fe doping (2 and 6 at %), with the help of x-ray photoemission spectroscopy (XPS) and resonance photoemission spectroscopy (RPES) measurements. XPS results indicate the ionic state of Fe in the Fe doped films, ruling out the possibility of Fe metallic clusters. Valence band spectra of CeO{sub 2} show an additional feature after Fe doping, suggesting its incorporation in the CeO{sub 2} matrix. RPES studies on these films reveal the hybridization between oxygen vacancy induced Ce localized states and Fe derived states.

36 MATERIALS SCIENCE↗