198 research outputs found
Effects of pressure on spin fluctuations and the exchange interaction in La2CuO4 as determined by twoâmagnon Raman scattering (abstract)
We have measured the twoâmagnon Raman scattering spectrum of magnetic La2CuO4 at pressures of up to 100 kbar. Analysis of the moments of the twoâmagnon line shape indicates that the renormalization parameters resulting from spin fluctuations are essentially pressure independent in this pressure range. Our results provide the first direct determination of the pressure dependence of the inâplane exchange coupling constant J. The pressure dependence of J is compared with that of the NĂ©el temperature and discussed in the context of recent theories for quasiâtwoâdimensional magnetic systems.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/71024/2/JAPIAU-69-8-5392-1.pd
A fixed angle double mirror filter for preparing a pink undulator beam at the Advanced Photon Source
Recent advances in X-ray Photon Correlation Spectroscopy (XPCS) use the full bandwidth of an undulator harmonic in order to maximize the coherent flux for small angle X-ray scattering experiments. X-ray mirrors and filters are typically used to select a given harmonic of the spectrum. At the University of Michigan/Howard University/Lucent Technologies, Bell Labs, Collaborative Access Team (MHATT-CAT) undulator beamline of the Advanced Photon Source, we have designed a fixed-angle Double Mirror Filter which will provide a âpink beamâ (i.e., 2â3% bandwidth) for XPCS experiments. This device uses two small mirrors which vertically reflect a 0.1âmmĂ0.1âmm0.1mmĂ0.1mm white beam in a symmetric geometry. The doubly reflected beam propagates parallel to the incident white beam, but is offset vertically by 35 mm. Using the standard offset of the APS allows one to stop the white beam with a standard APS beam stop. In this report, we will describe our design considerations for this instrument. We also report the results of preliminary tests of the performance. The mirrors preserve the transverse coherence of the source, and filter the undulator spectrum as expected. © 2000 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87429/2/238_1.pd
Probing Internal Stress and Crystallinity in Wet Foam via Raman Spectroscopy
In this article, we correlate the internal stress and the characteristics of
a vibrational mode in wet foam. Using microscope images, we estimate the
average size of the bubbles in wet foam, at specific time intervals, over a
duration of twenty four hours. Raman spectra are also recorded at the same time
intervals, over the same time frame. We show that the internal stress,
originated from the macroscopic structural change of foam with ageing, can be
related to the observed Raman shift of the low frequency methylene rocking mode
of the constituent surfactant molecules in foam. In this report we also show
the capability of the Raman spectroscopy to reveal the crystallinity in foamy
materials, when studied for a longer period of time.Comment: 16 pages, 7 figure
Refractive optics using lithium metal
Thanks to its low x-ray absorption, lithium should be the material of choice for x-ray refractive lenses. This article discusses some of the measurements done to verify lithiumâs relevant properties. Both x-ray transmission and refraction are consistent with expectations. The lens gain suffers from broadening that is related to small-angle scattering. © 2002 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/69944/2/RSINAK-73-3-1492-1.pd
Experience with a fluorescenceâbased beam position monitor at the APS
We have recently redesigned the first crystal mount of our cryogenically cooled monochromator to reduce its sensitivity to pressure fluctuations in the cryogenic lines feeding the Si (111) crystal. With the use of a fluorescenceâbased Xâray beam position monitor (BPM) placed 19 m away from the monochromator, much operational experience has been gained on the sensitivity of the beam position and intensity to small changes in the cooling system. In this presentation, we will describe our Xâray BPM design and performance and will provide examples of changes that have made the beam position more stable on our beamline. One such change for example has been the topâup operation of the Advanced Photon Source (APS), which has reduced the thermal drifts associated with the ring current decay. © 2004 American Institute of PhysicsPeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87659/2/679_1.pd
Homotopic functional connectivity disruptions in glioma patients are associated with tumor malignancy and overall survival
BACKGROUND: Gliomas exhibit widespread bilateral functional connectivity (FC) alterations that may be associated with tumor grade. Limited studies have examined the connection-level mechanisms responsible for these effects. Given the typically strong FC observed between mirroring/homotopic brain regions in healthy subjects, we hypothesized that homotopic connectivity (HC) is altered in low-grade and high-grade glioma patients and the extent of disruption is associated with tumor grade and predictive of overall survival (OS) in a cohort of
METHODS: We used a mirrored FC-derived cortical parcellation to extract blood-oxygen-level-dependent (BOLD) signals and to quantify FC differences between homotopic pairs in normal-appearing brain in a retrospective cohort of glioma patients and healthy controls.
RESULTS: Fifty-nine glioma patients (WHO grade 2,
CONCLUSIONS: These findings demonstrate an association between tumor grade and HC alterations that may underlie global FC changes and provide prognostic information
Speckle from phase ordering systems
The statistical properties of coherent radiation scattered from
phase-ordering materials are studied in detail using large-scale computer
simulations and analytic arguments. Specifically, we consider a two-dimensional
model with a nonconserved, scalar order parameter (Model A), quenched through
an order-disorder transition into the two-phase regime. For such systems it is
well established that the standard scaling hypothesis applies, consequently the
average scattering intensity at wavevector _k and time t' is proportional to a
scaling function which depends only on a rescaled time, t ~ |_k|^2 t'. We find
that the simulated intensities are exponentially distributed, with the
time-dependent average well approximated using a scaling function due to Ohta,
Jasnow, and Kawasaki. Considering fluctuations around the average behavior, we
find that the covariance of the scattering intensity for a single wavevector at
two different times is proportional to a scaling function with natural
variables mt = |t_1 - t_2| and pt = (t_1 + t_2)/2. In the asymptotic large-pt
limit this scaling function depends only on z = mt / pt^(1/2). For small values
of z, the scaling function is quadratic, corresponding to highly persistent
behavior of the intensity fluctuations. We empirically establish a connection
between the intensity covariance and the two-time, two-point correlation
function of the order parameter. This connection allows sensitive testing,
either experimental or numerical, of existing theories for two-time
correlations in systems undergoing order-disorder phase transitions. Comparison
between theory and our numerical results requires no adjustable parameters.Comment: 18 pgs RevTeX, to appear in PR
Asymmetrically cut crystals as optical elements for highly collimated xâray beams
Asymmetrically cut perfect crystals, in both the Laue and Bragg geometries, are examined as single crystal monochromators for xâray beams that are collimated to a small fraction of the Darwin width, as is typical in experiments with coherent x rays. Both the Laue and asymmetric Bragg geometries are plagued by an inherent chromatic aberration that increases the beam divergence much beyond that of the symmetric Bragg geometry. Measurements from a recent experiment at the ESRF are presented to compare Si(220) (symmetric Bragg), diamond(111) (asymmetric Laue), and diamond(111) (symmetric Bragg inclined) geometries. © 1995 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70952/2/RSINAK-66-2-1506-1.pd
Evidence for Two Superconducting Gaps in
We have measured the Raman spectra of polycrystalline MgB from 25 {\cm}
to 1200 {\cm}. When the temperature was decreased below the superconducting
transition temperature , we observed a superconductivity-induced
redistribution in the electronic Raman continuum. Two pair-breaking peaks
appear in the spectra, suggesting the presence of two superconducting gaps.
Furthermore, we have analyzed the measured spectra using a quasi
two-dimensional model in which two s-wave superconducting gaps open on two
sheets of Fermi surface. For the gap values we have obtained (2.7 meV) and (6.2 meV). Our results suggest
that a conventional phonon-mediated pairing mechanism occurs in the planar
boron bands and is responsible for the superconductivity of MgB.Comment: 3 figure
Functional connectivity within glioblastoma impacts overall survival
BACKGROUND: Glioblastoma (GBM; World Health Organization grade IV) assumes a variable appearance on MRI owing to heterogeneous proliferation and infiltration of its cells. As a result, the neurovascular units responsible for functional connectivity (FC) may exist within gross tumor boundaries, albeit with altered magnitude. Therefore, we hypothesize that the strength of FC within GBMs is predictive of overall survival.
METHODS: We used predefined FC regions of interest (ROIs) in de novo GBM patients to characterize the presence of within-tumor FC observable via resting-state functional MRI and its relationship to survival outcomes.
RESULTS: Fifty-seven GBM patients (mean age, 57.8â
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13.9 y) were analyzed. Functionally connected voxels, not identifiable on conventional structural images, can be routinely found within the tumor mass and was not significantly correlated to tumor size. In patients with known survival times (n =â
31), higher intranetwork FC strength within GBM tumors was associated with better overall survival even after accounting for clinical and demographic covariates.
CONCLUSIONS: These findings suggest the possibility that functionally intact regions may persist within GBMs and that the extent to which FC is maintained may carry prognostic value and inform treatment planning
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