Coherent Fraction Research Articles

The noble gas geochemistry of natural CO 2 gas reservoirs from the Colorado Plateau and Rocky Mountain provinces, USA

Identification of the source of CO 2 in natural reservoirs and development of physical models to account for the migration and interaction of this CO 2 with the groundwater is essential for developing a quantitative understanding of the long term storage potential of CO 2 in the subsurface. We present the results of 57 noble gas determinations in CO 2 rich fields (>82%) from three natural reservoirs to the east of the Colorado Plateau uplift province, USA (Bravo Dome, NM., Sheep Mountain, CO. and McCallum Dome, CO.), and from two reservoirs from within the uplift area (St. John’s Dome, AZ., and McElmo Dome, CO.). We demonstrate that all fields have CO 2/ 3He ratios consistent with a dominantly magmatic source. The most recent volcanics in the province date from 8 to 10 ka and are associated with the Bravo Dome field. The oldest magmatic activity dates from 42 to 70 Ma and is associated with the McElmo Dome field, located in the tectonically stable centre of the Colorado Plateau: CO 2 can be stored within the subsurface on a millennia timescale. The manner and extent of contact of the CO 2 phase with the groundwater system is a critical parameter in using these systems as natural analogues for geological storage of anthropogenic CO 2. We show that coherent fractionation of groundwater 20Ne/ 36Ar with crustal radiogenic noble gases ( 4He, 21Ne, 40Ar) is explained by a two stage re-dissolution model: Stage 1: Magmatic CO 2 injection into the groundwater system strips dissolved air-derived noble gases (ASW) and accumulated crustal/radiogenic noble gas by CO 2/water phase partitioning. The CO 2 containing the groundwater stripped gases provides the first reservoir fluid charge. Subsequent charges of CO 2 provide no more ASW or crustal noble gases, and serve only to dilute the original ASW and crustal noble gas rich CO 2. Reservoir scale preservation of concentration gradients in ASW-derived noble gases thus provide CO 2 filling direction. This is seen in the Bravo Dome and St. John’s Dome fields. Stage 2: The noble gases re-dissolve into any available gas stripped groundwater. This is modeled as a Rayleigh distillation process and enables us to quantify for each sample: (1) the volume of groundwater originally ‘stripped’ on reservoir filling; and (2) the volume of groundwater involved in subsequent interaction. The original water volume that is gas stripped varies from as low as 0.0005 cm 3 groundwater/cm 3 gas (STP) in one Bravo Dome sample, to 2.56 cm 3 groundwater/cm 3 gas (STP) in a St. John’s Dome sample. Subsequent gas/groundwater equilibration varies within all fields, each showing a similar range, from zero to ∼100 cm 3 water/cm 3 gas (at reservoir pressure and temperature).

Lithospheric modification during crustal extension in the Main Ethiopian Rift

Quaternary lavas erupted in zones of tectonomagmatic extension within the Main Ethiopian Rift (MER) preserve details of lithospheric structure in the East African Rift System. Despite observed source heterogeneity, basalts, trachybasalts, and basaltic trachyandesites erupted in the Wonjii Fault Belt (WFB) and the Silti‐Debre Zeyit Fault Zone (SDFZ) form coherent fractionation paths dominated by variable removal of observed phenocryst phases. Crustal assimilation is not widespread, though it is observed at the southern end of the WFB where both fault belts merge; farther north, assimilation of cumulate phases related to fractional crystallization of previous magmas is identified. Shallow fractionation conditions (∼1 kbar) within the WFB do not change from north to south. In contrast, lavas erupted within the contemporaneous SDFZ fractionate at various crustal depths. These results indicate a better developed magmatic system beneath the WFB where magmas rose quickly before undergoing more significant fractionation at near surface levels and a less developed system beneath the SDFZ. The distribution of magmatism and extant geophysical data indicate thinned crust and a single rift‐centered zone of magmatic activity northeast of 8°30′N, consistent with a transitional lithosphere between continental and oceanic settings. Southwest of 8°30′N, thicker crust and rift‐marginal axes of extension suggest lithosphere with continental affinities. The WFB is propagating southward in response to extension within the Red Sea Rift; the northward propagating SDFZ is related to rifting within the East African Rift System. This region records the unification of two rift systems, requiring care in interpreting the MER as simply transitional between continental and oceanic environments.

Chaoticity parameter λ in Hanbury-Brown–Twiss interferometry

In Hanbury-Brown--Twiss interferometry measurements using identical bosons, the chaoticity parameter \ensuremath{\lambda} has been introduced phenomenologically to represent the momentum correlation function at zero relative momentum. It is useful to study an exactly solvable problem in which the \ensuremath{\lambda} parameter and its dependence on the coherence properties of the boson system can be worked out in great detail. We are therefore motivated to study the state of a gas of noninteracting identical bosons at various temperatures held together in a harmonic oscillator potential that arises either externally or from bosons' own mean fields. We determine the degree of Bose-Einstein condensation and its momentum correlation function as a function of the attributes of the boson environment. The parameter \ensuremath{\lambda} can then be evaluated from the momentum correlation function. We find that the $\ensuremath{\lambda}(p,T)$ parameter is a sensitive function of both the average pair momentum $p$ and the temperature $T$, and the occurrence of $\ensuremath{\lambda}=1$ is not a consistent measure of the absence of a coherent condensate fraction. In particular, for large values of $p$, the \ensuremath{\lambda} parameter attains the value of unity even for significantly coherent systems with large condensate fractions. We find that if a pion system maintains a static equilibrium within its mean field, and if it contains a root-mean-squared radius, a pion number, and a temperature typical of those in high-energy heavy-ion collisions, then it will contain a large fraction of the Bose-Einstein pion condensate.

X-ray standing wave analysis of overlayer-induced substrate relaxation: The clean and Bi-covered (110) GaP surface

The relaxation of the surface P atoms, for both the clean and Bi-covered GaP(110) surface, was studied with x-ray standing wave (XSW) spectroscopy using surface-sensitive x-ray photoelectron as the XSW modulated signal. The photoemission signal of the outermost surface layer is mixed with the signal from the remaining near surface of the underlying substrate, so further analysis is required to calculate the geometry of the relaxation of the surface atoms. We present a general analysis method for extracting the geometry of the surface reconstruction that minimizes the propagation of the uncertainties associated with fitting XSW data. It takes advantage of the fact that the coherent distance may be more accurately determined than the coherent fraction in XSW data analysis. This method makes use of the electron attenuation length, and shows that the relaxation is only weakly dependent on the uncertainties of this parameter. Results indicate that, for the clean GaP surface, P relaxes with a small outward rotational displacement, with the axis of the rotation located at the second-layer Ga site, whereas, for the Bi-covered case, relaxation consists of a rotation in the opposite direction. The magnitude of the contraction is not negligible, and might be important in the interpretation of low-energy electron diffraction data and in ab initio calculations.

Sequential mechanism of electron transport in the resonant tunneling diode with thick barriers

A frequency-dependent impedance analysis (0.1–50 GHz) of an InGaAs/InAlAs-based resonant tunneling diode with a 5-nm-wide well and 5-nm-thick barriers showed that the transport mechanism in such a diode is mostly sequential, rather than coherent, which is consistent with estimates. The possibility of determining the coherent and sequential mechanism fractions in the electron transport through the resonant tunneling diode by its frequency dependence on the impedance is discussed.

Adsorbed molecular shuttlecocks: An NIXSW study of Sn phthalocyanine on Ag(1 1 1) using Auger electron detection

Normal incidence X-ray standing wave (NIXSW) spectroscopy has been used to determine the orientation of Sn phthalocyanine (SnPc) molecules in a highly ordered, but incommensurate, monolayer on the Ag(1 1 1) surface. Our sample preparation procedure differs from that used in previous work on this system [C. Stadler, S. Hansen, F. Pollinger, C. Kumpf, E. Umbach, T.-L. Lee, J. Zegenhagen, Phys. Rev. B 74 (2006) 035404] and leads to a different unit cell with basis vector lengths of ∼15.0 Å and 15.3 Å ( γ = 98°) which is oriented at an angle of ∼5° to the underlying Ag(1 1 1) lattice. Structural parameters extracted from Sn MNN NIXSW spectra indicate that SnPc, a buckled, ‘shuttlecock’ phthalocyanine, adsorbs in a Sn-down geometry with the Sn atom approximately 2.3 Å above the Ag(1 1 1) surface plane. Despite the incommensurate nature of the overlayer, we find a surprisingly high coherent fraction for standing wave data taken for the ( 1 ¯ 1 1 ) reflection and argue that this arises from the small domain size of the superstructure.

Resonant inelastic soft x-ray scattering of Be chalcogenides

The bulk electronic band structures of BeTe and BeSe have been studied by resonant inelastic soft x-ray scattering (RIXS) at the Be $1s$ edge. We derive direct and indirect bulk band gaps and observe the coexistence of core excitons and momentum conservation. A quantitative analysis of the coherent spectral fraction gives insight into the femtosecond time scale of the relevant dephasing processes. Experimental results agree very well with calculations based on the Kramers-Heisenberg and density-functional theories.

Noble gas tracing of groundwater/coalbed methane interaction in the San Juan Basin, USA

The San Juan Basin natural gas field, located in northwestern New Mexico and southwestern Colorado in the USA, is a case-type coalbed methane system. Groundwater is thought to play a key role in both biogenic methane generation and the CO 2 sequestration potential of coalbed systems. We show here how noble gases can be used to construct a physical model that describes the interaction between the groundwater system and the produced gas. We collected 28 gas samples from producing wells in the artesian overpressured high production region of the basin together with 8 gas samples from the underpressured low production zone as a control. Stable isotope and major species determination clearly characterize the gas in the high production region as dominantly biogenic in origin, and the underpressured low producing region as having a significant admix of thermogenic coal gas. 3He/ 4He ratios increase from 0.0836R a at the basin margin to 0.318R a towards the center, indicating a clear but small mantle He signature in all gases. Coherent fractionation of water-derived 20Ne/ 36Ar and crustal 4He/ 40Ar* are explained by a simple Rayleigh fractionation model of open system groundwater degassing. Low 20Ne concentrations compared to the model predicted values are accounted for by dilution of the groundwater-associated gas by desorbed coalbed methane. This Rayleigh fractionation and dilution model together with the gas production history allows us to quantify the amount of water involved in gas production at each well. The quantified water volumes in both underpressured and overpressured zones range from 1.7 × 10 3 m 3 to 4.2 × 10 5 m 3, with no clear distinction between over- and underpressured production zones. These results conclusively show that the volume of groundwater seen by coal does not play a role in determining the volume of methane produced by secondary biodegradation of these coalbeds. There is no requirement of continuous groundwater flow for renewing the microbes or nutrient components. We furthermore observe strong mass related isotopic fractionation of 20Ne/ 22Ne and 38Ar/ 36Ar isotopic ratios. This can be explained by a noble gas concentration gradient in the groundwater during gas production, which causes diffusive partial re-equilibration of the noble gas isotopes. It is important for the study of other systems in which extensive groundwater degassing may have occurred to recognize that severe isotopic fractionation of air-derived noble gases can occur when such concentration gradients are established during gas production. Excess air-derived Xe and Kr in our samples are shown to be related to the diluting coalbed methane and can only be accounted for if Xe and Kr are preferentially and volumetrically trapped within the coal matrix and released during biodegradation to form CH 4.

The effect of ramping casting speed and casting temperature on temperature distribution and melt flow patterns in the sump of a DC cast billet

The interpretation of experimental results obtained upon direct-chill (DC) casting frequently requires knowledge of the shape and position of liquidus and solidus isotherms and of the flow patterns in the billet sump. Computer simulations for the steady-state casting conditions are usually used to obtain such information, although experiments are frequently performed under transient casting conditions, e.g. at a ramping casting speed in order to promote hot tearing, which obviously results in a deviation from the steady-state condition. The first goal of this work is to examine by modelling the differences in temperature distribution and flow patterns in the billet sump occurring in the steady-state and under transient casting conditions (caused by casting speed ramping) and to assess the implications of such differences for the structure and defect formation. The second goal is to evaluate the effect of the other important casting parameter, casting temperature, on the temperature and flow patterns during DC casting. To achieve these two goals, the concept of a hybrid model proposed in [C.M. Oldenburg, F.J. Spera, Numer. Heat Transfer, Part B 21 (1992) 217–229] is employed, in which the so-called coherent solid fraction contour divides the two-phase region into the slurry region and the mushy zone. Good spatial resolution of the two-phase region in the calculations is achieved by unstructured meshing, which improve the accuracy of the calculation. The calculation results are discussed and compared with the results of previously reported experiments.

XSW feasibility study of C [formula omitted]Ph [formula omitted]–OH on Ag(1 1 1): combining hard X-rays, low Z and, low coverage

We have undertaken a ”proof of principle” Normal Incidence X-ray standing wave (NIXSW) experiment on a monolayer of C 60 Ph 5 –OH, a functionalised fullerene, on Ag(111) and show that it is indeed possible to use XSW in case of extremely low coverage of low Z elements such as oxygen. We find a tentative coherent fraction f co of 30% and a coherent position D co of 2.35Å.

Excitations of a Superfluid in a Three-Dimensional Optical Lattice

We prepare a Bose-Einstein condensed gas in a three-dimensional optical lattice and study the excitation spectrum of the superfluid phase for different interaction strengths. We probe the response of the system by modulating the depth of the optical lattice along one axis. The interactions can be controlled independently by varying the tunnel coupling along the other two lattice axes. In the weakly interacting regime we observe a small susceptibility of the superfluid to excitations, while for stronger interactions an unexpected resonance appears in the excitation spectrum. In addition we measure the coherent fraction of the atomic gas, which determines the depletion of the condensate.

Interaction of Uranyl with Calcite in the Presence of EDTA

Adsorption of uranyl at the surface of calcite was investigated by using batch sorption experiments and synchrotron X-ray standing wave (XSW) measurements. Aqueous solutions containing 236U(VI) (4.5 x 10(-7) to 1.0 x 10(-4) M) and EDTA (5.0 x 10(-7) to 1.1 x 10(-4) M) were reacted for 90 s to 60 min with freshly cleaved calcite (104) surfaces and calcite powders. Surface exchange coefficients, sorption kinetics, and influence of powder surface area/solution volume (SA/V) ratio were investigated by alpha-counting of 236U. Powder sorption results at SA/V = 870 cm2/mL fit a Freundlich isotherm [log [U]surface (in monolayers) = log K + n log [U]aq (in moles/L)], where K = 1.9+/-0.5 and n = 0.9+/-0.1, consistent with uptake of U(VI) by a specific surface reaction where the availability of sorption sites is nonlimiting in the U concentration range measured. Measured U(VI) coverages along this isotherm, based on the calcite (104) surface Ca site density, ranged from 0.04% to 5.4% of a monolayer. Steady state surface coverages were obtained within 90 s. Sorption of U(VI) on calcite (104) single-crystal cleavage surfaces using identical solutions yielded higher coverages, because of increased step density induced by dissolution at the relatively low SA/V ratio (approximately 1) of these measurements. The crystallographic location of the sorbed U(VI) was examined with the synchrotron XSW technique. Measurements were performed at the Advanced Photon Source on fresh calcite (104) cleavage surfaces reacted for 90 s with U(VI) solutions. Coherent fractions for sorbed U ranged from 0.14 to 0.62, and the mean value of the U coherent position was 0.84+/-0.02. This position was independent of dissolved U(VI) concentration and corresponds to a distance between the U atom and the calcite (104) plane of 2.55+/-0.06 A. These results are consistent with U(VI) adsorption atthe calcite surface as an inner-sphere uranyl-carbonate surface complex bonded with the outer oxygen atom(s) of a single surface carbonate group. Steric considerations allow this observed U(VI) surface complex to occur both at step sites ((441)_ and (481)_) and on terrace areas adjacent to Ca vacancies.

X-ray standing waves in a heterostructure: application to a Zn(1-x)CoxO epilayer on ZnO(00.1)-O substrate.

X-ray standing waves (XSW) in a thin epitaxic film are treated in the framework of the dynamical theory. It is demonstrated that the fluorescence yield around the main peak of the rocking curve has essentially the same characteristics as that of the usual XSW on a bulk crystal surface. Thus, XSW provide a direct method to probe the atom position in a thin film. The method was applied to an epilayer of the diluted magnetic semiconductor Zn0.94Co0.06O, in order to determine the Co-atom position. The XSW established that Co atoms occupy the substitutional Zn site in the ZnO matrix, although their coherent fraction, which measures the degree of order, is rather low. Moreover, the measurement of the Zn fluorescence in the film gives approximately the same value for the coherent fraction of the Zn atoms. Besides, by using the substrate rocking curve, it is shown that the XSW signal of the Zn atoms in the substrate can be detected through the film. This interesting approach allows the coherent fraction of an element of a substrate below an interface to be probed in situ. For the Zn fluorescence, the coherent fraction is lower near the interface than in the bulk. These results should relate to strains and defects on both sides of the interface.

Transition from a strongly interacting 1d superfluid to a Mott insulator.

We study 1D trapped Bose gases in the strongly interacting regime. The systems are created in an optical lattice and are subject to a longitudinal periodic potential. Bragg spectroscopy enables us to investigate the excitation spectrum in different regimes. In the superfluid phase a broad continuum of excitations is observed which calls for an interpretation beyond the Bogoliubov spectrum taking into account the effect of strong interactions. In the Mott insulating phase a discrete spectrum is measured. Both phases are compared to the 3D situation and to the crossover regime from 1D to 3D. The coherence length and coherent fraction of the gas are measured in all configurations. We observe signatures for increased fluctuations characteristic for 1D systems. Moreover, the collective oscillations cease near the transition to the Mott insulator phase.

Does an Encapsulated Atom ‘feel' the Effects of Adsorption?: X-ray Standing Wave Spectroscopy of Ce@C82 on Ag(111)

A comparison of normal incidence X-ray standing wave (NIXSW) spectra from bulk films and monolayers of Ce@C82 on Ag(111) clearly shows that the Ce atom does not adopt a preferred intramolecular bonding site as a result of adsorption. NIXSW profiles measured at room temperature and 100 K are each associated with a coherent fraction of zero, indicating a high level of static (rather than vibrational) disorder in the intramolecular Ce positions.

X-ray standing wave studies of strained InxGa1−xAs/InP short-period superlattices

We report an x-ray standing wave (XSW) study on a set of structurally well-characterized InxGa1−xAs/InP short-period superlattices grown by metal–organic chemical vapor deposition and chemical-beam epitaxy techniques. It was possible to model the x-ray standing wave profiles only once the superlattice period has been assumed to be constituted by four layers of well-defined chemical composition [barrier (InP), first interface (InAs0.7P0.3), well (In0.53Ga0.47As), and second interface (In0.53Ga0.47As0.7P0.3)], and of variable thickness. The thickness of the four layers have been obtained by fitting the high resolution x-ray diffraction profiles of the heterostructures. The presence of partially disordered interface layers, as evidenced by a transmission electron microscopy study, causes a significant reduction of the coherent fraction, F, of both Ga and As atoms. The difference in F values among measured samples illustrates how the XSW can provide important information on the quality of semiconductor superlattices. Comparison with a “long period (160 Å)” In0.53Ga0.47As/InP superlattice, where the role played by InAs0.7P0.3 and In0.53Ga0.47As0.7P0.3 interface layers is negligible, confirms this picture. The coherent fraction of both As and Ga correlates well with the average perpendicular lattice misfit 〈Δa⊥/a〉 determined by x-ray diffraction.

X-ray standing wave study of Si/Ge/Si( [formula omitted]) heterostructures grown with Bi as a surfactant

X-ray standing wave (XSW) analysis was used for an atomic-scale structural study of ultra-thin Si/Ge heterostructures grown on Si(0 0 1) by surfactant mediated epitaxy with Bi as the surfactant. XSW measurements were performed for the Si(0 0 4) and Si(0 2 2) Bragg reflections for Ge coverages from 1 to 10 monolayers. The measured Ge coherent positions agree with the calculated Ge positions for a tetragonally distorted Ge lattice formed on Si(0 0 1) using continuum elasticity theory. However, the measured Ge coherent fractions are smaller than expected. The quality of the Si cap layer and its registry with the underlying Si(0 0 1) substrate lattice was also determined by combining the XSW technique with evanescent-wave emission.

Structure of ultrathin films of Mn on Cu(111) investigated by the normal-incidence x-ray standing-wave method

The growth of Mn films on Cu(111) has been investigated in the range of 0.5--4.5 monolayer equivalents using the normal-incidence x-ray standing-wave method. The films have been found to be incommensurate at all coverages as determined by the coherent fraction of the Mn in the substrate $(1\ifmmode\bar\else\textasciimacron\fi{}11)$ reflection. The (111) reflection gives a coherent position that increases with coverage, but a coherent fraction that decreases. At submonolayer exposure, the low coherent position reveals that the majority of the Mn atoms are at a z position near to, but slightly expanded from, the Cu(111) d spacing, but that $15%\ifmmode\pm\else\textpm\fi{}5%$ of the Mn are at a z position of $0.57\ifmmode\pm\else\textpm\fi{}0.18$ of the substrate d spacing. Modeling of the trends in coherent position and fraction with Mn exposure eliminates the possibility of the overlayer being \ensuremath{\alpha}-Mn, \ensuremath{\gamma}-Mn (fcc), or \ensuremath{\delta}-Mn (bcc). However, the trends are entirely consistent with the Mn overlayer being a Laves phase similar in structure to ${\mathrm{Zn}}_{2}\mathrm{Mg}.$

Normal incidence X-ray standing wave study of Fe on Cu( [formula omitted

The structure of annealed ultra-thin Fe films on Cu(1 1 1) has been investigated using normal incidence X-ray standing wave (NIXSW). Films of between 0.5 and 5.3 monolayer equivalence (MLE) have been deposited in situ and briefly annealed to 513 K. NIXSW measurements of the (1 1 1) and ( 1 ̄ 1 1) reflections using 2p 3/2 photoemission show that the film is pseudomorphic up to 2 MLE, with near complete occupation of the fcc hollow sites. For this regime, the value of the (1 1 1) coherent position is equivalent to a film spacing relative to the lattice of 2.00±0.03 Å. For the 5.3 MLE film, there is a drastic reduction in the ( 1 ̄ 1 1) coherent fraction and shift in the ( 1 ̄ 1 1) coherent position that is consistent with the transformation of the entire film to a Kurjumov-Sachs oriented bcc(1 1 0) structure.

Solution to the initial value problem for a high-gain FEL via Van Kampen's method

Using Van Kampen's normal mode expansion, we solve the initial value problem for a high-gain free-electron laser described by the three-dimensional Maxwell–Klimontovich equations. An expression of the radiation spectrum is given for the process of coherent amplification and self-amplified spontaneous emission. It is noted that the input coupling coefficient for either process increases with the initial beam energy spread. The effective start-up noise is identified as the coherent fraction of the spontaneous undulator radiation in one field gain length, and is larger with increasing energy spread and emittance mainly because of the increase in gain length.