923 research outputs found
Structural differences in amyloid-β fibrils from brains of non-demented elderly individuals and Alzheimer's disease patients
Although amyloid plaques composed of fibrillar amyloid-β (Aβ) assemblies are a diagnostic hallmark of Alzheimer's disease (AD), quantities of amyloid similar to those in AD patients are observed in brain tissue of some nondemented elderly individuals. The relationship between amyloid deposition and neurodegeneration in AD has, therefore, been unclear. Here, we use solid-state NMR to investigate whether molecular structures of Aβ fibrils from brain tissue of nondemented elderly individuals with high amyloid loads differ from structures of Aβ fibrils from AD tissue. Two-dimensional solid-state NMR spectra of isotopically labeled Aβ fibrils, prepared by seeded growth from frontal lobe tissue extracts, are similar in the two cases but with statistically significant differences in intensity distributions of cross-peak signals. Differences in solid-state NMR data are greater for 42-residue amyloid-β (Aβ42) fibrils than for 40-residue amyloid-β (Aβ40) fibrils. These data suggest that similar sets of fibril polymorphs develop in nondemented elderly individuals and AD patients but with different relative populations on average
Statistics of skyrmions in Quantum Hall systems
We analyze statistical interactions of skyrmions in the quantum Hall system
near a critical filling fraction in the framework of the Ginzburg-Landau model.
The phase picked up by the wave-function during an exchange of two skyrmions
close to is , where is the skyrmion's
spin. In the same setting an exchange of two fully polarized vortices gives
rise to the phase . Skyrmions with odd and even numbers of reversed
spins have different quantum statistics. Condensation of skyrmions with an even
number of reversed spins leads to filling fractions with odd denominators,
while condensation of those with an odd number of reversed spins gives rise to
filling fractions with even denominators.Comment: 6 pages in Latex. addendum - skyrmions with odd or even number of
reversed spins have different quantum statistics. They condense to form
respectively even or odd denominator filling fraction state
Critical Behavior of Nuclear-Spin Diffusion in GaAs/AlGaAs Heterostructures near Landau Level Filling \nu=1
Thermal measurements on a GaAs/AlGaAs heterostructure reveal that the state
of the confined two-dimensional electrons dramatically affects the nuclear-spin
diffusion near Landau level filling factor \nu=1. The experiments provide
quantitative evidence that the sharp peak in the temperature dependence of heat
capacity near \nu=1 is due to an enhanced nuclear-spin diffusion from the GaAs
quantum wells into the AlGaAs barriers. We discuss the physical origin of this
enhancement in terms the possible Skyrme solid-liquid phase transition.Comment: 1 LateX file, 3 figures, submitte
Spin Transition in Strongly Correlated Bilayer Two Dimensional Electron Systems
Using a combination of heat pulse and nuclear magnetic resonance techniques
we demonstrate that the phase boundary separating the interlayer phase coherent
quantum Hall effect at in bilayer electron gases from the weakly
coupled compressible phase depends upon the spin polarization of the nuclei in
the host semiconductor crystal. Our results strongly suggest that, contrary to
the usual assumption, the transition is attended by a change in the electronic
spin polarization.Comment: 4 pages, 3 postscript figur
Tackling Systematic Errors in Quantum Logic Gates with Composite Rotations
We describe the use of composite rotations to combat systematic errors in
single qubit quantum logic gates and discuss three families of composite
rotations which can be used to correct off-resonance and pulse length errors.
Although developed and described within the context of NMR quantum computing
these sequences should be applicable to any implementation of quantum
computation.Comment: 6 pages RevTex4 including 4 figures. Will submit to Phys. Rev.
Korringa ratio of ferromagnetically correlated impure metals
The Korringa ratio, , obtained by taking an appropriate combination
of the Knight shift and nuclear spin-lattice relaxation time, is calculated at
finite temperature, , in the three-dimensional electron gas model, including
the electron-electron interaction, , and non-magnetic impurity scatterings.
varies in a simple way with respect to and ; it decreases as
is increased but increases as is raised. However, varies in a
slightly more complicated way with respect to the impurity scatterings; as the
scattering rate is increased, increases for small and low , but
decreases for large or high regime. This calls for a more careful
analysis when one attempts to estimate the Stoner factor from .Comment: 7 pages including 3 figures. To be published in Phys. Rev. B, Dec.
Galilean non-invariance of geometric phase
It is shown that geometric phase in non-relativistic quantum mechanics is not
Galilean invariant.Comment: LaTeX, 6 pages, no figure
Spin Polarizations at and about the Lowest Filled Landau Level
The spin polarization versus temperature at or near a fully filled lowest
Landau level is explored for finite-size systems in a periodic rectangular
geometry. Our results at which also include the finite-thickness
correction are in good agreement with the experimental results. We also find
that the interacting electron system results are in complete agreement with the
results of the sigma model, i.e., skyrmions on a torus have a topological
charge of and the Q=1 solution is like a single spin-flip excitation.
Our results therefore provide direct evidence for the skyrmionic nature of the
excitations at this filling factor.Comment: 4 pages, REVTEX, and 4 .ps files, To be published in Europhysics
Letter
A suggested search for 207Pb nuclear Schiff moment in PbTiO3 ferroelectric
We suggest two types of experiments, NMR and macroscopic magnetometry, with
solid PbTiO3 to search for the nuclear Schiff moment of 207Pb. Both kinds of
experiments promise substantial improvement over the presently achieved
sensitivities. Statistical considerations show that the improvement of the
current sensitivity can be up to 10 orders of magnitude for the magnetometry
experiment and up to 6 orders of magnitude for the NMR experiment. Such
significant enhancement is due to the strong internal electric field of the
ferroelectric, as well as due to the possibility to cool the nuclear-spin
subsystem in the compound down to nanokelvin temperatures.Comment: 4 pages; revised sensitivity estimate for NMR experimen
- …