Search

Studies of the proteome would benefit greatly from methods to directly sequence and digitally quantify proteins and detect posttranslational modifications with single-molecule sensitivity. Here, we demonstrate single-molecule protein sequencing using a dynamic approach in which single peptides are probed in real time by a mixture of dye-labeled N-terminal amino acid recognizers and simultaneously cleaved by aminopeptidases. We annotate amino acids and identify the peptide sequence by measuring fluorescence intensity, lifetime, and binding kinetics on an integrated semiconductor chip. Our results demonstrate the kinetic principles that allow recognizers to identify multiple amino acids in an information-rich manner that enables discrimination of single amino acid substitutions and posttranslational modifications. With further development, we anticipate that this approach will offer a sensitive, scalable, and accessible platform for single-molecule proteomic studies and applications.

The effect of competing crystal-field anisotropies on magnetic order has been investigated in a series of Ho/Er superlattices using neutron and resonant X-ray magnetic diffraction techniques. The neutron diffraction reveals that for temperatures in the interval T-N(Er) less than or equal to T less than or equal to T-N(Ho) the Ho basal-plane order propagates coherently through the paramagnetic Er, and that below T-N(Er) the longitudinal component of the Er moments fail to order across the Ho block. The magnetic superlattice peaks observed in the X-ray scattering display an anomalous energy dependence: a sharp resonance is found at L(III)(Ho), with no resonance visible at L(III)(Er). These results are discussed with reference to models of exchange in metallic superlattices.

The effect of large hydrogen concentrations (\ensuremath{\approx}30% atomic), on the superconducting transition temperature ${T}_{c}$ of Nb films is discussed. The hydrogen is incorporated in solutionlike phases which occur at these concentrations because the (\ensuremath{\alpha} to \ensuremath{\beta}) hydride transition in these films is suppressed. X-ray-diffraction data show an asymmetric expansion of the [110] Nb interplanar spacing. This can be as high as a 6--8 % expansion perpendicular to the plane of the film for the addition of \ensuremath{\sim}70% hydrogen, with only a 1--2 % change in the plane of the film. ${T}_{c}$ is depressed to a value near 50% of that of the undoped film with about 30% atomic hydrogen, and returns to its initial value when the hydrogen is removed. A discussion is given of how both disorder and changes in the electronic structure can affect ${T}_{c}.$

The authors describe synchrotron based X-ray diffraction techniques and issues related to in situ studies of intercalation processes in battery electrodes. They then demonstrate the utility of this technique, through a study of two batches of Li{sub x}Mn{sub 2}O{sub 4} cathode materials. The structural evolution of these spinel materials was monitored in situ during the initial charge of these electrodes in actual battery cells. Significant differences were observed in the two batches, particularly in the intercalation range of x = 0.45 to 0.20. The first-order structural transitions in this region indicated coexistence of two cubic phases in the batch 2 material, whereas the batch 1 material showed suppressed two-phase coexistence. Batch 2 cells also indicated structural evolution in the low-potential region below 3.0 V in contrast to the batch 1 material. Differences in structural evolution between batches of Li{sub x}Mn{sub 2}O{sub 4} could have important ramifications in their cycle life and stability characteristics.

The alignment and the formation of (Bi, Pb)2Sr2Ca2Cu3O10 inside an Ag sheath were investigated for tapes from four different precursor powders. Microstructural characterization along with in situ transmission x-ray diffraction experiments revealed that the kinetics depended strongly on the processing technique and the phase assemblage of the precursors. The alignment process was governed by the preferential grain growth of the Bi-cuprates along the a-b plane and the constraint applied by the sheath. The formation mechanism of (Bi, Pb)2Sr2Ca2Cu3O10 was either the intercalation or the nucleation and growth, depending on whether excessive liquid is adjacent to the platelets.

Hydrogen uptake and discharge by thin metallic films under potentiostatic control was studied using x-ray diffraction at the National Synchrotron Light Source (NSLS). The formation of metal-hydrogen phases in Pd, Pd-capped Nb and Pd/Nb multilayer electrode structures was deduced from x-ray diffraction data and correlated with the cyclic voltammetry (CV) peaks. The x-ray data was also used to construct a plot of the hydrogen concentration as a function of cell potential for a multilayered thin film. {copyright} {ital 1997 Materials Research Society.}

Hard x-rays from a synchrotron source were utilized in diffraction experiments performed at elevated temperatures (up to ˜ 870 °C) on (Bi, Pb)2Sr2Ca2Cu3O10 (Bi-2223) tapes completely encased in silver. The general behavior of the phase and texture development under typical processing conditions was determined, and the effects that several variations in processing conditions had on the phase and texture development were examined. These results and their implications for improving processing conditions are discussed.

We describe the construction and performance of a small‐angle x‐ray scattering (SAXS) instrument which we have used on several beam lines at the National Synchrotron Light Source. The analyzer crystal was a channel cut Si(1,1,1) designed for use at λ=1.54 A with a measured efficiency of 60% and an angular resolution full width at half maximum of 0.001°. In the case of strongly scattering samples (i.e., powders), momentum transfer q between 1×10−4 A

The structural properties of superconducting (Bi,Pb)2Sr2Ca2Cu3O10–Ag (2223) tapes have been measured using synchrotron x‐ray scattering techniques. The x‐ray photon energy was tuned just below the silver K absorption edge so the penetration depth was large, which allowed the measurements to be performed in a transmission geometry without removing the silver cladding. Analysis of the peaks in 2θ scans indicates that residual (Bi,Pb)2Sr2CaCu2O8 (2212) superconductor starting material is present in all samples studied. The amount of 2212 varied widely among the tapes, and was not homogeneous along the length of each individual tape. Residual 2212 content increased near the ends of most samples, suggesting that 2223 phase development is sensitive to whether the superconducting material is encased in silver or not. The bulk c‐axis alignment was measured in ∼100 mono‐ and multifilament samples, and correlations between c‐axis alignment and current carrying capacity at 77 K were found. Multifilament samples generally had better alignment than monofilament samples. The c‐axis alignment along the length of the tapes was uniform, and the superconducting material within ∼1 μm of the Ag was better textured than the bulk of the sample. Intermediate pressings were directly shown to have an adverse affect on c‐axis alignment. Finally, the evolution of texture and phase development was examined in a series of samples annealed for varying times. The 2212 starting material acquired the final c‐axis alignment state after brief heating times, and only after much longer heating times did the 2212 transform into the 2223 phase. These results and their implications for improving processing procedures are discussed.The structural properties of superconducting (Bi,Pb)2Sr2Ca2Cu3O10–Ag (2223) tapes have been measured using synchrotron x‐ray scattering techniques. The x‐ray photon energy was tuned just below the silver K absorption edge so the penetration depth was large, which allowed the measurements to be performed in a transmission geometry without removing the silver cladding. Analysis of the peaks in 2θ scans indicates that residual (Bi,Pb)2Sr2CaCu2O8 (2212) superconductor starting material is present in all samples studied. The amount of 2212 varied widely among the tapes, and was not homogeneous along the length of each individual tape. Residual 2212 content increased near the ends of most samples, suggesting that 2223 phase development is sensitive to whether the superconducting material is encased in silver or not. The bulk c‐axis alignment was measured in ∼100 mono‐ and multifilament samples, and correlations between c‐axis alignment and current carrying capacity at 77 K were found. Multifilament samples gene...

The effect of competing crystal-field anisotropies on magnetic order has been investigated in a series of Ho/Er superlattices using a combination of x-ray and neutron scattering techniques. For temperatures in the interval TN(Er)≤T≤TN(Ho) the Ho basal-plane order propagates coherently through the paramagnetic Er over a typical length scale of 1000 A. At low temperatures the Ho moments retain their bulk-like helical configuration, whereas the magnetic structure in the Er blocks has both basal-plane and c-axis components. Below TN(Er), the coherence length of the basal-plane order decreases on cooling, while the longitudinal component of the Er moments fails to order across the Ho block. It is argued that these results require an extension of current models of indirect exchange in superlattices to explicitly include the superlattice band structure.

We present the results of x-ray- and neutron-scattering studies of the temperature dependence of the magnetic scattering exhibited by the type-I, triple-{ital Q} antiferromagnet UO{sub 2}. Our neutron-scattering results are consistent with those of earlier studies, including the observation of short-ranged magnetic correlations at temperatures near and above {ital T}{sub {ital N}}. However, it is found by x-ray diffraction that a second, longer length scale is induced near {ital T}{sub {ital N}} when the near-surface volume of the sample is mechanically roughened. The longitudinal and transverse widths of the additional scattering increase continuously with increasing temperature above {ital T}{sub {ital N}}, similar to that which has been observed near the magnetic ordering transitions of Ho, Tb, and NpAs and near the tetragonal-to-cubic transitions of various perovskites. Another unusual feature of the present results for UO{sub 2} involves the apparent shift with temperature of the magnetic scattering along the surface normal direction at the (1,1,0) reflection, but not at the (2,1,0) reflection. To our knowledge, this is the first observation of a second length scale near a first-order transition. {copyright} {ital 1996 The American Physical Society.}

Bi-2223 tapes were manufactured from a fine “two-powder” product produced by using an aerosol spray pyrolysis technique. Critical current density of 22000 A/ cm2 at 77K and 0 T was achieved. Nondestructive transmission x-ray diffraction study indicated good alignment of the superconducting grains. The texturing process of the superconducting phase was found to be nearly complete after the first 24 h of heat treatment for the samples studied. Pressing was found to play little role in the texturing process. The texturing can be enhanced by Ag-doping. Jc, however, was not found to be improved significantly, presumably due to the reduced effective cross-sectional area. A new phase, Bi-4435, was identified which may play a significant role in the formation of 2223.

We present resonant x-ray magnetic scattering studies of the temperature dependence of the magnetic order parameters of Dy, Ho, Er, and Tm single crystals near their antiferromagnetic phase transitions. The experimentally determined values of the critical exponent \ensuremath{\beta} of Er and Tm, which have c-axis modulated structures, are nearly equal and consistent with the mean-field value (\ensuremath{\beta}=0.47\ifmmode\pm\else\textpm\fi{}0.05 and \ensuremath{\beta}=0.49\ifmmode\pm\else\textpm\fi{}0.06, respectively). The measured values of Dy and Ho, which have spiral magnetic structures, are lower (\ensuremath{\beta}=0.36\ifmmode\pm\else\textpm\fi{}0.04 and \ensuremath{\beta}=0.41\ifmmode\pm\else\textpm\fi{}0.04, respectively). In addition to the primary magnetic order parameters, we have measured the temperature dependence of the intensities of up to four higher harmonics. The exponents of the higher harmonic satellites of Er and Tm exhibit mean-field-like scaling, while those of Ho do not. We discuss these results within the context of simple corrections to mean-field scaling, based on the three-dimensional XY model. We also report measurements of the temperature dependence of the c-axis lattice constants and magnetic wave vectors of all four metals. It is found that the magnetic correlation lengths are reduced near transitions to ferrimagnetic and ferromagnetic phases.

In pyrite-structure antiferromagnetic NiS{sub 2}, the magnetic susceptibility {chi}({ital T}) shows downward curvature above the Neel transition ({ital T}{sub {ital N}1}=39.2 K), suggesting that magnetic correlations persist to {ital T}=400 K. However, neutron-scattering measurements indicate that the bulk magnetic behavior above {ital T}{sub {ital N}1} is conventional. We present evidence that the unusual temperature dependence of {chi}({ital T}) arises from a surface contribution. At {ital T}{sub {ital N}2}=29.75 K a transition occurs to a weakly ferromagnetic state, accompanied by a small structural distortion. We show that this distortion is not tetragonal as suggested previously, but is most likely rhombohedral. Below {ital T}{sub {ital N}2}, {chi}({ital T}) is anisotropic; our data suggest that in this range {chi}({ital T}) is dominated by next-nearest-neighbor exchange interactions.

We report the results of resonant x-ray magnetic scattering experiments on bulk and thin-film single crystals of holmium. The scattering at the principal magnetic reflection has been characterized as a function of the temperature in the spiral phases near and below their respective Neel temperatures. The integrated intensity of the principal magnetic peak in both samples shows power-law behavior versus reduced temperature with nearly equal exponents. The exponents for the scattering at the resonant second and third harmonics in the bulk sample are not simple integer multiples of the first, and motivate the consideration of simple scaling corrections to mean-field theory. We also present and compare the results of high-resolution measurements of the temperature dependence of the magnetic wave vectors, [ital c]-axis lattice constants, and correlation lengths of the magnetic scattering of the two samples in their spiral phases. Although the qualitative behavior is similar, systematic differences are found, including uniformly larger magnetic wave vectors and the suppression of the 1/6 phase in the film. The spiral magnetic structure of the film forms a domain state at all temperatures in the ordered phase. The magnetic correlation lengths of both samples are greatest near the Neel temperature, where that of themore » film appears to exceed the translational correlation lengths of the lattice. As the temperature decreases, the magnetic correlation lengths also decrease. These results are discussed in terms of the strain present in the samples.« less

Neutron-scattering experiments have been performed to study the dynamic spin properties of La{sub 1.85}Sr{sub 0.15}CuO{sub 4} ({ital T}{sub {ital c}}=33 K) in the temperature range 1.9 K{le}{ital T}{le}150 K and energy range 1.5 meV{le}{omega}{le}20 meV; these span, respectively, {ital T}{sub {ital c}} and the weak-coupling BCS gap energy 2{Delta}{similar_to}10 meV. The width of {ital S}({bold q},{omega}) in momentum space decreases with decreasing temperature at fixed energy as well as with decreasing energy at fixed {ital T}, becoming very sharp at both low {omega} and low temperature. It is found that both in the normal state at {ital T}=35 K and in the superconducting state at {ital T}=10 K the integrated generalized susceptibility {chi}{prime}{prime}({omega})={integral}{sub ({pi},{pi})} d{ital q}{sub 2D}{chi}{prime}{prime}({bold q},{omega}) is constant above {omega}{similar_to}10 meV with an amplitude about three times that in La{sub 1.98}Sr{sub 0.02}CuO{sub 4} and begins to decrease with decreasing {omega} at {omega}{similar_to}10 meV, which corresponds to the BCS superconducting gap energy 2{Delta}. However, we observe nonzero magnetic scattering well below the pseudogap energy at temperatures as low as 2 K. These results are compared with those in La{sub 1.98}Sr{sub 0.02}CuO{sub 4} and YBa{sub 2}Cu{sub 3}O{sub 6.6}.

We report a neutron scattering study of the spin correlations in the model 2D, [ital S]=1/2, square-lattice Heisenberg antiferromagnetic Sr[sub 2]CuO[sub 2]Cl[sub 2]. The spin correlation lengths obtained agree quantitatively with values deduced from Monte Carlo simulations over a wide range of temperature. The combined data, which cover the length scale from 1 to 200 lattice constants, are predicted accurately with no adjustable parameters by renormalized classical theory for the quantum nonlinear sigma model.

We describe measurements of the magnetic critical fluctuations of holmium by x-ray scattering techniques. The x-ray results are compared to those obtained in neutron scattering experiments performed on the same sample.

We report a detailed synchrotron X-ray scattering study of the magnetic correlations in two samples of Mn0.75Zn0.25F2 as a function of temperature and applied field. The critical behavior of this system is believed to be isomorphic with that of the three-dimensional random field Ising model (RFIM). On cooling in an external magnetic field (FC), the first sample exhibits a transition to long range order (LRO) in the near-surface region, at a field dependent temperature,T N (H). In contrast, bulk neutron scattering studies show a long lived metastable domain state forming below a metastability boundary,T M (H). The transition temperatureT M (H), lies below the metastability boundary,T M (H). The temperature difference,T M (H)−T M (H), increases with increasing field and agrees closely with the value deduced from an extrapolation from aboveT M (H) of earlier, equilibrium neutron scattering results on the spin-spin correlation length. On cooling, the order parameter exponent is found to be large, β=0.30±0.05. We speculate that there is an imbalance in the random fields in the neighborhood of linear surface defects (scratches) in this sample, and that the consequent net staggered field initiates a regular random Ising transition. The second sample was cut from the first and underwent a more extensive polishing process, resulting in a smoother surface with a small density of visible defects. Interestingly, it does not attain a LRO state on cooling, but rather it forms a domain state consistent with that observed by neutron diffraction. Both samples may be prepared in a LRO state, either by cooling in zero field and subsequently applying a field (ZFC) or, at high fields, by heating from the XY phase (FHXY). We have studied the evolution of the metastable LRO state in each sample on warming. We find universal behavior in both samples at all fields studied. Specifically there is a powerlaw-like decay of the order parameter with exponent β=0.20±0.05, and a rounded transition region which may be described by a Gaussian distribution of transition temperatures. The width of this distribution scales asH2. A scaling plot of all the warming data as a function of the scaling variable (T−T C (H)/H2 is constructed. We label this non-equilibrium pseudo-critical behavior, “trompe l'oeil critical behaviour.” Phenomenologically, these results enable us to explain many previous, apparently contradictory, results in the literature.

We report a synchrotron magnetic x-ray scattering study of the temperature evolution of the magnetic long range order (LRO) of Mn[sub 0.75]Zn[sub 0.25]F[sub 2] in a magnetic field with emphasis on the behavior after zero field cooling (ZFC). We show that with increasing temperature the metastable ZFC LRO vanishes continuously at a temperature well above the equilibrium [ital T][sub [ital N]]; the LRO follows a rounded power law with exponent [beta][sub ZFC]=0.20[plus minus]0.02 and a transition width which scales as [similar to][ital H][sup 2] but with no divergent critical fluctuations. We argue that this behavior is generic to the random field Ising model.

Neutron scattering experiments have been performed to study the dynamic spin properties of La 1.98 Sr 0.02 CuO 4 , a crystal with just enough Sr to destroy three-dimensional long range magnetic order and induce spin-glass behavior at low temperatures. The q -integrated susceptibility \(\int \,{\rm d}^{2}q_{\rm 2D}\,{\rm Im}\,\chi _{\rm 2D}(q_{\rm 2D},\,\omega )\) follows the scaling function of ω/ T suggested by Keimer et al. Below 3 meV, however, clear deviation from the scaling function is observed at low temperatures. This probably originates from a gap in the spin-wave-like excitations due to the out-of-plane anisotropy.

Neutron scattering measurements have been made of the phase diagrams of the nearly two-demensional antiferromagnets Rb2MnF4 and Rb2Mn0.7Mg0.3F4 in a magnetic field applied along thec-axis. In Rb2MnF4 there is at low temperatures a spin-flop phase at fields above 5.5 T which has long range order. The observation of true long range order rather than the algebraic decay of the order characteristic of the two-dimensional XY model is presumably due to subtle anisotropy effects in the plane as well as weak three-dimensional coupling. The phase boundaries of the uniaxial and transverse phases are shown to be consistent with renormalization group predictions for two-dimensional systems. The two lines become exponentially close to each other at low temperatures. The weak three-dimensional coupling moves the bicritical point fromT=0 to a non-zero temperature. The situation is more complex in Rb2Mn0.7Mg0.3F4 because of Ising random field effects. At low fields we observe typical random field metastable behavior with a sharp metastability boundary and a gange of length scales which are time independent below that boundary. At higher fields there are substantial uniaxial fluctuations. The transverse phase boundary and the metastability line appear to intercept atT=0 showing that the random field fluctuations do have a large effect on the phase diagram. The theory of the phase diagrams has been extended to include the random field fluctuations and good agreement is obtained with the observed transverse phase boundary. Unfortunately, there is as yet no theory of the metastable uniaxial phase with which to compare our results.

Neutron-scattering experiments with markedly improved signal-to-noise ratios have been performed on single-crystal superconducting La[sub 1.85]Sr[sub 0.15]CuO[sub 4] ([ital T][sub [ital c]]=33 K) at energies down to 1.5 meV. The measurements provide quantitative information on the temperature dependence of [chi][prime][prime]([bold q],[omega]). Surprisingly, [chi][prime][prime]([bold q][sub peak],[omega]) is approximately constant below [ital T][sub [ital c]] for the measured energies 1.5 meV[le][omega][le]6 meV, that is, for energies much less than the mean-field BCS gap energy of [similar to]10 meV, hence implying other than conventional [ital s]-wave superconductivity.

Stimulated by the pioneering work of Michael Fisher and collaborators on bicritical phase diagrams in pure systems, we consider the corresponding behavior in systems with uniaxial random fields. We discuss experiments in the two- and three-dimensional n = 3 systems Rb2Mn0.7Mg0.3F4 and Mn0.75Zn0.25F2, respectively. We also report a new theory for the 2D n = 3 system, which predicts a novel phase boundary geometry. In both two and three dimensions the Ising component is dominated by metastability effects. However, the XY component shows a reversible transition to long range order. Experiments in the bicritical region in Mn0.75Zn0.25F2 are inconclusive. However, the theory describes the measured XY phase boundary in Rb2Mn0.7Mg0.3F4 quite well.

Neutron-scattering measurements have been performed on (Nd,Pr){sub 2{minus}{ital x}}Ce{sub {ital x}}CuO{sub 4} (0.0{lt}{ital x}{lt}0.125) single crystals. All samples exhibit three-dimensional (3D) long-range magnetic order; the temperature dependence of the 2D magnetic correlation lengths in all is similar in character to that observed in pure and lightly doped La{sub 2}CuO{sub 4} albeit with renormalized spin stiffness constants {rho}{sub {ital s}}. Thus, in the nonsuperconducting doping regime the hole- and electron-doped systems exhibit analogous magnetic behavior except for the drastically different dopant-concentration scales for the destruction of magnetic order.

Combined muon-spin-relaxation (${\mathrm{\ensuremath{\mu}}}^{+}$SR) and elastic magnetic neutron-scattering experiments on a large single crystal of ${\mathrm{La}}_{1.94}$${\mathrm{Sr}}_{0.06}$${\mathrm{CuO}}_{4}$ are reported. ${\mathrm{\ensuremath{\mu}}}^{+}$SR data taken without an external field indicate that the Cu spins are fluctuating slowly or frozen below ${\mathit{T}}_{\mathit{f}}^{\mathrm{\ensuremath{\mu}}}$\ensuremath{\approxeq}6.0 K. At 3.9 K, measurements done with an applied longitudinal field show that the muon depolarization is almost completely due to static moments. Neutron measurements of the magnetic peaks Q=(1,0,0) and (0,1,1) show that no three-dimensional long-range order is present down to 1.8 K. However, the quasielastic scattering intensity from the two-dimensional ridge Q=(1,k,0) at k=-0.12 increases sharply between 40 and 20 K. Below ${\mathit{T}}_{\mathit{f}}^{\mathit{N}}$=20 K the scattering intensity is roughly constant. The difference between these two freezing temperatures is explained in terms of the different range of fluctuation frequencies to which each of these probes is sensitive. The dependence of the freezing temperature on the frequency windows of different probes is similar to that seen in traditional spin-glass materials like CuMn.

A two‐step process is proposed for the formation of c‐axis aligned (Bi,Pb)2Sr2Ca2Cu3O10+δ [Bi(2:2:2:3)] platelets in a silver sheath. The process involves: (1) the formation of c‐axis aligned (Bi,Pb)2Sr2CaCu2O8+δ at early stage of heat treatment and (2) the subsequent intercalation of Ca–Cu–O layers to form Bi(2:2:2:3). This is based on the measurements of (1) the rocking curves for c‐axis alignment and the two theta scans for the Bi(2:2:2:3) conversion ratio, both by a transmission x‐ray technique, and (2) a quantitative study of the phase conversion due to intercalation of Ca–Cu–O layers into existing Bi(2:2:1:2) by transmission electron microscopy.

Hard x rays from a synchrotron source were utilized in diffraction experiments which probed the bulk of electrode materials while they were operating in situ in battery cells. Two technologically relevant electrode materials were examined; an AB2‐type anode in a nickel–metal–hydride cell and a LiMn2O4 cathode in a Li‐ion ‘‘rocking chair’’ cell. Structural features such as lattice expansions and contractions, phase transitions, and the formation of multiple phases were easily observed as either hydrogen or lithium was electrochemically intercalated in and out of the electrode materials. The relevance of this technique for future studies of battery electrode materials is discussed.

Improved strain sensitivity of critical currents Ic of the multicored over the monocored (Bi,Pb)2Sr2Ca2Cu3O10–Ag tapes is shown for bending strains, but not for axial tensile strains. The improved mechanical tolerance is due, in part, to a better alignment of the Bi‐cuprate platelets achieved in the multicored tapes. Furthermore, the difference in the response to these tapes to axial tensile and bending strains is due to the difference in the precise manner in which the strains are applied to the tapes.

The purpose of this CRADA was to characterize the structural properties of the superconductor material within the wires in order to determine which processing procedures produce the best superconductor texture and phase development, and hence the best ultimate current carrying capacity.

In this paper, we discuss changes in the phase diagram of hydrogen in both bilayer (i.e. 200–2000 Å Nb/ 100 Å Pd on glass) and multilayer configurations. In fact, it is the changes in this phase diagram, in addition to the Pd coating, that makes theses bilayer systems useful for hydrogen gas sensors. Comparison of x-ray diffraction, electrical resistivity and volumetric measurements of the films before and after hydrogen charging indicate that the phase equilibria between a correlated (high concentration) and a dilute phase of hydrogen in Nb is not sensitive to the number of layers in the films. On the other hand, the experimental methods show different behavior for 200 Å thick Nb films and thicker (>400 Å) Nb layers. Diffraction results showing unisotropic lattice expansion will also be discussed..

The short-ranged correlations associated with magnetic ordering in the rare-earth antiferromagnet holmium have been characterized in high-resolution x-ray- and neutron-scattering studies. For temperatures within about 2 K above ${\mathit{T}}_{\mathit{N}}$, the short-ranged magnetic fluctuations exhibit two length scales, instead of the single one expected in an ideal system. Well above ${\mathit{T}}_{\mathit{N}}$, the shorter of the two length scales exhibits power-law behavior consistent with normal critical fluctuations; the line shape in momentum space is well described by a Lorentzian, and the measured critical exponents are \ensuremath{\nu}=0.54\ifmmode\pm\else\textpm\fi{}0.04 and \ensuremath{\gamma}=1.24\ifmmode\pm\else\textpm\fi{}0.15. The longer of the two length scales is well described by a squared-Lorentzian line shape, and exhibits a power-law temperature dependence. Both the shorter and the longer length-scale fluctuations are approximately spatially isotropic. We propose that the longer length-scale fluctuations are related to random strain fields which are localized at or near the sample surfae. These results are reminiscent of behavior observed at the cubic-to-tetragonal structural phase transitions of some perovskite materials.

The short-ranged correlations associated with magneitc ordering in the rare earth antiferromagnet holmium have been characterized in high-resolution x-ray and neutron scattering studies. We find that within about 2 K of ${\mathit{T}}_{\mathit{c}}$, the magnetic fluctuations exhibit two length scales, instead of one as expected in an ideal system. This result is reminiscent of behavior observed at the cubic-to-tegragonal structural phase transitions of the perovskites.

We report both synchrotron-x-ray and neutron-diffraction studies of the magnetic ordering in a crystal of Mn{sub 0.75}Zn{sub 0.25}F{sub 2} in a magnetic field. The neutron experiments reproduce previous results showing the establishment of a short-range domain state on cooling in a field. Remarkably, the magnetic x-ray scattering shows that within a few microns of the surface there is a second-order transition to true long-range order on field cooling. Implications of these results are discussed.

Resonant magnetic X-ray scattering from UAs using the Miv uranium absorption edge allows a detailed examination of the first order antiferromagnetic phase transition occuring at 122 K which appears to proceed via the formation of antiferromagnetically coupled surface layers.

Only recently has it become clear that quantum fluctuations do not destroy long-range oreder in the ground state of the 2D S = 1 2 square-lattice Heisenberg antiferromagnet. We reporta neutron scattering study of the static structure factor of Su2CuO2Cl2, which is an optimal experimental realization of this model. Excellent agreement for the magnetic correlation length is found to exist between theory. Monte Carlo results and our experiment.

Synchrotron magnetic x-ray scattering studies have been performed on the site-diluted antiferromagnet ${\mathrm{Mn}}_{0.5}$${\mathrm{Zn}}_{0.5}$${\mathrm{F}}_{2}$ in zero field and in a field of 0.36 T. No hysteresis was observed in either experiment nor was any broadening due to random fields observed at the ${10}^{\mathrm{\ensuremath{-}}4}$ A${\r{}}^{\mathrm{\ensuremath{-}}1}$ level in the 0.36 T external field. The sublattice magnetization was measured and the effective exponents \ensuremath{\beta}=0.33 \ifmmode\pm\else\textpm\fi{}0.02 and 0.21\ifmmode\pm\else\textpm\fi{}0.07 were extracted for 0 and 0.36 T external fields, respectively. Order-parameter curves for both fields exhibit mean-field behavior for an extended region below ${T}_{N}$ consistent with expectations based on the Ginzburg criterion. An average over published experiments gives \ensuremath{\beta}=0.34\ifmmode\pm\else\textpm\fi{}0.01 in good agreement with theory, for the random-exchange Ising model.

Among Cu/sup 2+/ compounds, cupric oxide (CuO) has unusual magnetic properties, some of which are shared by the superconducting copper oxides. In order to study these properties further, neutron scattering experiments on a CuO single crystal have been performed. Two magnetic phase transitions at T/sub N//sub 1/ = 231 K and T/sub N//sub 2/ = 212.5 K have been observed, with the magnetic structure below T/sub N//sub 2/ antiferromagnetic and the structure between T/sub N//sub 1/ and T/sub N//sub 2/ helical in nature. The dynamics of the magnetism was also studied. As in La/sub 2/CuO/sub 4/, the dispersion of the low-temperature spin-wave excitations is unusually steep (dE/dk = 250 +- 75 meV A/sup -1/), indicating strong magnetic interactions. Inelastic magnetic scattering has also been measured above T/sub N//sub 1/. At temperatures slightly above the 231-K phase transition, dynamic critical slowing has been observed.

Using neutron-scattering techniques, we have studied the temperature dependence of the magnetic excitations at varied energies in a high-quality single crystal of ${\mathrm{La}}_{1.85}$${\mathrm{Sr}}_{0.15}$${\mathrm{CuO}}_{4}$. This crystal exhibits a sharp superconducting transition at 33 K with nearly 100% flux exclusion. For magnetic excitation energies of 9 meV and lower, the intensity drops dramatically with decreasing temperature for temperatures below \ensuremath{\sim}150 K. However, at 12 MeV the intensity is independent of temperature between 300 and 5 K.

We review the results of recent neutron-scattering studies of the spin fluctuations in samples of La 1.89 Sr 0.11 CuO 4 which are ∼80% superconducting with T c = 10 K. The structure factor, S(Q), reflects three-dimensional modulated spin correlations with an in-plane correlation length of order 18 ± 6 A. The fluctuations evolve with temperature from being predominantly dynamic at high temperatures to mainly quasi-elastic (|ΔE| < 0.5 meV) at low temperatures. No significant differences are observed in the normal and superconducting states.

The structural phase transition from the tetragonal to the orthorhombic phase of doped and undoped samples of La2−xSrxCuO4 has been investigated by using inelastic neutron scattering techniques. The rotational nature of the soft mode leads to moderate electron-phonon coupling and the mode is unlikely to enhance significantly conventional phonon mediated superconductivity.

Using large single crystals ({similar to}2 cm{sup 3}), we have extended previous elastic and inelastic neutron scattering studies of the spin dynamics in the two-dimensional (2D) quantum antiferromagnet La{sub 2}CuO{sub 4}. We observe well-defined magnetic peaks for constant energy scans across the 2D magnetic rod for a wide range of temperatures and energies. The spectra are compared quantitatively with theoretical models. Simple spin-wave theory satisfactorily describes the experimental results well below {ital T}{sub {ital N}}. The thermal excitations of the low-energy ({h bar}{omega}{le}3 meV) spectra, however, appear to deviate from the theory as {ital T}{sub {ital N}} is approached. The overall temperature and energy dependence of the spectra above {ital T}{sub {ital N}} (245--520 K) can be explained by the formula of Chakravarty, Halperin, Nelson, and Tyc for the dynamical structure factor {ital S}({bold q},{omega}) with no adjustable parameters. We do, however, observe a 2D elastic component which grows rapidly as {ital T}{sub {ital N}} is approached; it is hypothesized that this arises primarily from magnetic defects and is analogous to the central peak'' observed for many structural phase transitions.

We report the results of a high-resolution synchrotron x-ray scattering study of the magnetic scattering from Mn${\mathrm{F}}_{2}$. The temperature dependence of the magnetic order parameter was obtained from the intensity of the direct magnetic scattering of x rays from the ${\mathrm{Mn}}^{2+}$ spins. The intense, highly collimated synchrotron beam allowed for a clear, extinction-free separation of the Bragg component from the diffuse scattering even very close to the N\'eel temperature ${T}_{N}$. The excellent magnetic scattering signal-to-noise ratio should allow for the observation of critical scattering for temperatures within 1 mK of ${T}_{N}$.

Using a flux technique we have grown sizable single crystals of ${\mathrm{La}}_{2\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Sr}}_{\mathrm{x}}$${\mathrm{CuO}}_{4}$. With x rays and neutrons we have studied both the static and dynamic aspects of the tetragonal to orthorhombic structural phase transition; classic soft-phonon behavior is observed at the ((1/2, 1) / 2 , 0) zone boundary involving rotations of ${\mathrm{CuO}}_{6}$ octahedra. The pure and lightly-doped single crystals show hopping conductivity, ln\ensuremath{\rho}\ensuremath{\sim}(${T}_{0}$/T${)}^{1/4}$, indicating that the electronic states at the Fermi energy are localized.

We report a neutron scattering study of the spin fluctuations in samples of ${\mathrm{La}}_{1.89}$${\mathrm{Sr}}_{0.11}$Cu${\mathrm{O}}_{4}$ which are \ensuremath{\sim}80% superconducting with ${T}_{c}=10$ K. The structure factor $S(\mathrm{Q})$ reflects three-dimensional modulated spin correlations with an in-plane correlation length of order 18\ifmmode\pm\else\textpm\fi{}6 \AA{}. The fluctuations evolve with temperature from being predominantly dynamic at high temperatures to mainly quasielastic ($|\ensuremath{\Delta}E|l0.5$ meV) at low temperatures. No significant differences are observed in the normal and superconducting states.

We report elastic, quasielastic (F dE), and inelastic neutron-scattering studies of the instantaneous and dynamic spin fluctuations in as-grown and doped ${\mathrm{La}}_{2}$${\mathrm{CuO}}_{4}$. Four samples have been studied: (A) as-grown ${\mathrm{La}}_{2}$${\mathrm{CuO}}_{4}$ with ${T}_{N}$=195 K, (B) oxygenated ${\mathrm{La}}_{2}$${\mathrm{CuO}}_{4}$ with ${\mathit{T}}_{\mathit{N}}$\ensuremath{\simeq}100 K, (C) ${\mathrm{La}}_{2}$${\mathrm{Cu}}_{0.95}$${\mathrm{Li}}_{0.05}$${\mathrm{O}}_{4}$, and (D) ${\mathrm{La}}_{1.97}$${\mathrm{Sr}}_{0.03}$${\mathrm{Cu}}_{0.95}$${\mathrm{Li}}_{0.05}$${\mathrm{O}}_{4}$. All crystals exhibit variable-range-hopping conductivity behavior. At room temperature each sample exhibits two-dimensional (2D) antiferromagnetic instantaneous correlations in the ${\mathrm{CuO}}_{2}$ sheets with correlation length varying from \ensuremath{\sim}200 A\r{} in crystal A to \ensuremath{\sim}14 A\r{} in crystal D. The integrated intensity and therefore the effective moment is, however, constant to within the experimental error. In samples A and B the 2D correlation length becomes sufficiently large with decreasing temperature that the interplanar coupling is able to drive a transition to 3D long-range order. The spin dynamics have been studied in detail in crystals A and B and quite unusual behavior is observed. In contrast to previously studied planar antiferromagnets, there is no significant E\ensuremath{\simeq}0 component for temperatures \ensuremath{\ge}${T}_{N}$ and instead the 2D response function is highly inelastic. The effective dispersion of the spin excitations is \ensuremath{\ge}0.4 eV A\r{}. This large energy scale for the spin fluctuations gives credence to models of the superconductivity in doped ${\mathrm{La}}_{2}$${\mathrm{CuO}}_{4}$ in which the pairing is magnetic in origin.

We have carried out elastic, quasielastic (..integral.. dE), and inelastic neutron-scattering studies of the antiferromagnetic spin correlations in La/sub 2-//sub x/Sr/sub x/CuO/sub 4/ with x varying between 0.02 and 0.18. The crystals, which were grown in three different laboratories, exhibit behavior that varies smoothly with x. In all cases, antiferromagnetic correlations with a scattering amplitude corresponding to a fully occupied Cu/sup 2+/ square lattice are observed. However, the Neel state is destroyed by the doping and spin-spin correlation lenght xi is quite short, varying from approx.35 to approx.8 A as x varies between 0.02 and 0.18; the local order is, however, the same as in pure La/sub 2/CuO/sub 4/. The fluctuations are dynamic in character as in La/sub 2/CuO/sub 4/ above the Neel temperature T/sub N/. To a first approximation, xi = 3.8/ ..sqrt..x A, the average separation between the holes introduced by the Sr/sup 2+/ doping. The x = 0.08 sample exhibits superconductivity with T/sub c/ = 10 K and with a Meissner fraction exceeding 15% at 5 K; no important differences in the magnetic scattering are observed in the normal and superconducting states. In an appendix we present additional data on the spin dynamics in pure La/sub 2/CuO/submore » 4/ at T = 300 K in a sample with T/sub N/ = 235 K.« less