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Full-Text Articles in Physics
Incommensurate Spin Fluctuations In High-Transition Temperature Superconductors, Barrett Wells, Young Lee, Marc Kastner, Rebecca Christianson, Robert Birgeneau, Kazuyoshi Yamada, Yasuo Endoh, Gen Shirane
Incommensurate Spin Fluctuations In High-Transition Temperature Superconductors, Barrett Wells, Young Lee, Marc Kastner, Rebecca Christianson, Robert Birgeneau, Kazuyoshi Yamada, Yasuo Endoh, Gen Shirane
Rebecca J. Christianson
Neutron scattering experiments have revealed a fascinating interplay between the hole doping, the spin fluctuations, and the superconductivity of the cuprate superconductors. Recently, electrochemical techniques have been used to produce large single crystals of La2CuO4+ y, which has mobile oxygen dopants. Staging behavior of the excess oxygen has been demonstrated, and the low-energy spin fluctuations in stage 6 La2CuO4+ y have been measured. The spin fluctuations are incommensurate with the lattice and have spatial, energy, and temperature dependencies very much like those in La2− xSrxCuO4, with similar high transition temperature. This establishes the universality of the incommensurate spin fluctuations among …
Structures And Incommensurate Spin Excitations In Excess Oxygen-Doped La2cuo4+Y, Robert Birgeneau, Rebecca Christianson, Yasuo Endoh, Marc Kastner, Young Lee, Gen Shirane, Barrett Wells, Kazuyoshi Yamada
Structures And Incommensurate Spin Excitations In Excess Oxygen-Doped La2cuo4+Y, Robert Birgeneau, Rebecca Christianson, Yasuo Endoh, Marc Kastner, Young Lee, Gen Shirane, Barrett Wells, Kazuyoshi Yamada
Rebecca J. Christianson
Over the past decade, we have studied in detail the low-energy spin fluctuations in :a2−xSrxCuO4 for xbetween 0 and 0.18. Our experiments, as well as those by others, have revealed a fascinating interplay between the hole doping, the static and dynamic spin fluctuations and superconductivity. Recently, using electrochemical techniques, we have learned how to produce large single crystals of La2CuO4+y which are relatively homogenous. In this latter system, the dopants are characterized by annealed rather than quenched disorder. Furthermore, we have demonstrated staging behavior of the excess oxygen analogous to staging in intercalated graphite. We have now succeeded in carrying …
Viscosity Measurements On Colloidal Dispersions (Nanofluids) For Heat Transfer Applications, Jessica Townsend, Rebecca J. Christianson, D Venerus, J Buongiorno, M A. Kedzierski, Et Al.
Viscosity Measurements On Colloidal Dispersions (Nanofluids) For Heat Transfer Applications, Jessica Townsend, Rebecca J. Christianson, D Venerus, J Buongiorno, M A. Kedzierski, Et Al.
Rebecca J. Christianson
This article reports viscosity data on a series of colloidal dispersions collected as part of the International Nanofluid Property Benchmark Exercise (INPBE). Data are reported for seven different fluids that include dispersions of metal-oxide nanoparticles in water, and in synthetic oil. These fluids, which are also referred to as nanofluids, are currently being researched for their potential to function as heat transfer fluids. In a recently published paper from the INPBE study, thermal conductivity data from more than 30 laboratories around the world were reported and analyzed. Here, we examine the influence of particle shape and concentration on the viscosity …
Nanofluid Properties And Their Effects On Convective Heat Transfer In An Electronics Cooling Application, Jessica Townsend, Rebecca Christianson
Nanofluid Properties And Their Effects On Convective Heat Transfer In An Electronics Cooling Application, Jessica Townsend, Rebecca Christianson
Rebecca J. Christianson
In the search for new, more effective coolant fluids, nanoparticle suspensions have shown promise due to their enhanced thermal conductivity. However, there is a concomitant increase in the viscosity, requiring an increase in pumping power to achieve the same flow rate.Studies of flow cooling in simple geometries indicate that there is a benefit to using nanofluids, but it is difficult to justify extending these results to the far more complicated geometries. Moreover, with the variability of property measurements found in literature, it is possible to show conflicting results from the same set of flow-cooling data. In this work we present …
X-Ray Scattering Study Of The Incommensurate Phase In Mg-Doped Cugeo3, Rebecca J. Christianson, Y. J. Wang, S.C. Lamarra, R. J. Birgeneau, V. Kiryukhin, T. Masuda, I. Tsukada, K. Uchinokura, B. Keimer
X-Ray Scattering Study Of The Incommensurate Phase In Mg-Doped Cugeo3, Rebecca J. Christianson, Y. J. Wang, S.C. Lamarra, R. J. Birgeneau, V. Kiryukhin, T. Masuda, I. Tsukada, K. Uchinokura, B. Keimer
Rebecca J. Christianson
We present results of a systematic x-ray scattering study of the effects of Mg doping on the high-fieldincommensurate phase of CuGeO3. Lorentzian-squared line shapes, the changing of the first-order transition tosecond order, and the destruction of long-range order with infinitesimal doping are observed, consistent withrandom-field effects in a three-dimensional XY system. Values for the soliton width in pure and lightly dopedCuGeO3 are deduced. We find that even a very small doping has a drastic effect on the shape of the latticemodulation.
Two-Magnon Excitations Observed By Neutron Scattering In The Two-Dimensional Spin-5/2 Heisenberg Antiferromagnet Rb2mnf4, T. Huberman, R. Coldea, R. A. Cowley, D. A. Tennant, R. L. Leheny, Rebecca Christianson, C. D. Frost
Two-Magnon Excitations Observed By Neutron Scattering In The Two-Dimensional Spin-5/2 Heisenberg Antiferromagnet Rb2mnf4, T. Huberman, R. Coldea, R. A. Cowley, D. A. Tennant, R. L. Leheny, Rebecca Christianson, C. D. Frost
Rebecca J. Christianson
The low-temperature magnetic excitations of the two-dimensional spin- 52 square-lattice Heisenberg antiferromagnetRb2MnF4 have been probed using pulsed inelastic neutron scattering. In addition to dominant sharppeaks identified with one-magnon excitations, a relatively weak continuum scattering is also observed at higherenergies. This is attributed to neutron scattering by pairs of magnons and the observed intensities are consistentwith predictions of spin wave theory.
Time-Dependent Strength Of Colloidal Gels, S. Manley, Benny Davidovitch, Neil R. Davies, L. Cipelletti, A. E. Bailey, Rebecca J. Christianson, U. Gasser, V. Prasad, P. N. Segre, M. P. Doherty, S. Sankaran, A. L. Jankovsky, B. Shiley, J. Bowen, J. Eggers, C. Kurta, T. Lorik, D. A. Weitz
Time-Dependent Strength Of Colloidal Gels, S. Manley, Benny Davidovitch, Neil R. Davies, L. Cipelletti, A. E. Bailey, Rebecca J. Christianson, U. Gasser, V. Prasad, P. N. Segre, M. P. Doherty, S. Sankaran, A. L. Jankovsky, B. Shiley, J. Bowen, J. Eggers, C. Kurta, T. Lorik, D. A. Weitz
Rebecca J. Christianson
Colloidal silica gels are shown to stiffen with time, as demonstrated by both dynamic light scattering and bulk rheological measurements. Their elastic moduli increase as a power law with time, independent of particle volume fraction; however, static light scattering indicates that there are no large-scale structural changes. We propose that increases in local elasticity arising from bonding between neighboring colloidal particles can account for the strengthening of the network, while preserving network structure.
Spinodal Decomposition In A Model Colloid-Polymer Mixture In Microgravity, A. E. Bailey, W. C. K. Poon, Rebecca J. Christianson, A. B. Schofield, U. Gasser, V. Prasad, S. Manley, P. N. Segre, L. Cipelletti, W. V. Meyer, M. P. Doherty, S. Sankaran, A. L. Jankovsky, W. L. Shiley, J. P. Bowen, J. C. Eggers, C. Kurta, T., Jr. Lorik, P. N. Pusey, D. A. Weitz
Spinodal Decomposition In A Model Colloid-Polymer Mixture In Microgravity, A. E. Bailey, W. C. K. Poon, Rebecca J. Christianson, A. B. Schofield, U. Gasser, V. Prasad, S. Manley, P. N. Segre, L. Cipelletti, W. V. Meyer, M. P. Doherty, S. Sankaran, A. L. Jankovsky, W. L. Shiley, J. P. Bowen, J. C. Eggers, C. Kurta, T., Jr. Lorik, P. N. Pusey, D. A. Weitz
Rebecca J. Christianson
We study phase separation in a deeply quenched colloid-polymer mixture in microgravity on the International Space Station using small-angle light scattering and direct imaging. We observe a clear crossover from early-stage spinodal decomposition to late-stage, interfacial-tension-driven coarsening. Data acquired over 5 orders of magnitude in time show more than 3 orders of magnitude increase in domain size, following nearly the same evolution as that in binary liquid mixtures. The late-stage growth approaches the expected linear growth rate quite slowly.
X-Ray Scattering Studies Of Two Length Scales In The Critical Fluctuations Of Cugeo3, Y. J. Wang, Y. J. Kim, Rebecca J. Christianson, S. C. Lamarra, F. C. Chou, R. J. Birgeneau
X-Ray Scattering Studies Of Two Length Scales In The Critical Fluctuations Of Cugeo3, Y. J. Wang, Y. J. Kim, Rebecca J. Christianson, S. C. Lamarra, F. C. Chou, R. J. Birgeneau
Rebecca J. Christianson
The critical fluctuations of CuGeO3 have been measured by synchrotron x-ray scattering, and two length scales are clearly observed. The ratio between the two length scales is found to be significantly different along the a axis, with the a axis along the surface normal direction. We believe that such a directional preference is a clear sign that random surface strains, especially those caused by dislocations, are the origin of the long length scale fluctuations.
Limits To Gelation In Colloidal Aggregation, S. Manley, L. Cipelletti, V. Trappe, A. E. Bailey, Rebecca J. Christianson, U. Gasser, V. Prasad, P. N. Segre, M. P. Doherty, S. Sankaran, A. L. Jankovsky, B. Shiley, J. Bowen, J. Eggers, C. Kurta, T. Lorik, D. A. Weitz
Limits To Gelation In Colloidal Aggregation, S. Manley, L. Cipelletti, V. Trappe, A. E. Bailey, Rebecca J. Christianson, U. Gasser, V. Prasad, P. N. Segre, M. P. Doherty, S. Sankaran, A. L. Jankovsky, B. Shiley, J. Bowen, J. Eggers, C. Kurta, T. Lorik, D. A. Weitz
Rebecca J. Christianson
We show that the dynamics of large fractal colloid aggregates are well described by a combination of translational and rotational diffusion and internal elastic fluctuations, allowing both the aggregate size and internal elasticity to be determined by dynamic light scattering. The comparison of results obtained in microgravity and on Earth demonstrates that cluster growth is limited by gravity-induced restructuring. In the absence of gravity, thermal fluctuations ultimately inhibit fractal growth and set the fundamental limitation to the lowest volume fraction which will gel.
Critical Dynamics Of A Spin-5/2 Two-Dimensional Isotropic Antiferromagnet, Rebecca J. Christianson, R. L. Leheny, R. J. Birgeneau, R. W. Erwin
Critical Dynamics Of A Spin-5/2 Two-Dimensional Isotropic Antiferromagnet, Rebecca J. Christianson, R. L. Leheny, R. J. Birgeneau, R. W. Erwin
Rebecca J. Christianson
We report a neutron-scattering study of the dynamic spin correlations in Rb2MnF4, a two-dimensional spin-5/2 antiferromagnet. By tuning an external magnetic field to the value for the spin-flop line, we reduce the effective spin anisotropy to essentially zero, thereby obtaining a nearly ideal two-dimensional isotropic antiferromagnet. From the shape of the quasielastic peak as a function of temperature, we demonstrate dynamic scaling for this system and find a value for the dynamical exponent z. We compare these results to theoretical predictions for the dynamic behavior of the two-dimensional Heisenberg model, in which deviations from z=1 provide a measure of the …
Spin Correlations In An Isotropic Spin-5/2 Two-Dimensional Antiferromagnet, R. L. Leheny, Rebecca J. Christianson, R. J. Birgeneau, R. W. Erwin
Spin Correlations In An Isotropic Spin-5/2 Two-Dimensional Antiferromagnet, R. L. Leheny, Rebecca J. Christianson, R. J. Birgeneau, R. W. Erwin
Rebecca J. Christianson
We report a neutron scattering study of the spin correlations for the spin-5/2 two-dimensional antiferromagnet Rb2MnF4 in an external magnetic field. Choosing fields near the system’s bicritical point, we tune the effective anisotropy in the spin interaction to zero, constructing an ideal S = 5/2 Heisenberg system. The correlation length and structure factor amplitude are closely described by the semiclassical theory of Cuccoli 'et al.' over a broad temperature range, but show no indication of approaching the low-temperature renormalized classical regime of the quantum nonlinear sigma model.
Electrochemistry And Staging In La2cuo4+D, P Blakeslee, R J. Birgeneau, F C. Chou, Rebecca J. Christianson, M A. Kastner, Y S. Lee, B O. Wells
Electrochemistry And Staging In La2cuo4+D, P Blakeslee, R J. Birgeneau, F C. Chou, Rebecca J. Christianson, M A. Kastner, Y S. Lee, B O. Wells
Rebecca J. Christianson
Measurements are reported of the time dependence of the current during electrochemical oxidation and reduction at a fixed voltage of single crystals and ceramic samples of La2CuO4+d. Staging peaks in neutron measurements of the single crystals together with the electrochemical measurements and magnetization measurements confirm that stage n=6 corresponds to d=0.055 +/- 0.05, the high-d side of the oxygen-rich–oxygen-poor miscibility gap. Furthermore, stage n=4 occurs at a value of d consistent with d{n^-1. For ceramic samples it is shown that two different superconducting compounds are formed depending on the oxidation voltage used.