1,672 research outputs found
Spin and Charge Texture around In-Plane Charge Centers in the CuO_2 planes
Recent experiments on La_2Cu_{1-x}Li_xO_4 show that although the doped holes
remain localized near the substitutional Li impurities, magnetic order is
rapidly suppressed. An examination of the spin texture around a bound hole in a
CuO_2 plane shows that the formation of a skyrmion is favored in a wide range
of parameters, as was previously proposed in the context of Sr doping. The spin
texture may be observable by elastic diffuse neutron scattering, and may also
have a considerable effect on NMR lineshapes.Comment: 4 pages, postscript file, hardcopy available upon request, to appear
in PR
Unifying the Phase Diagrams of the Magnetic and Transport Properties of La_(2-x)Sr_xCuO_4, 0 < x < 0.05
An extensive experimental and theoretical effort has led to a largely
complete mapping of the magnetic phase diagram of La_(2-x)Sr_xCuO_4, and a
microscopic model of the spin textures produced in the x < 0.05 regime has been
shown to be in agreement with this phase diagram. Here we use this same model
to derive a theory of the impurity-dominated, low temperature transport. Then,
we present an analysis of previously published data for two samples: x = 0.002
data from Chen et. al., and x = 0.04 data from Keimer et. al. We show that the
transport mechanisms in the two systems are the same, even though they are on
opposite sides of the observed insulator-to-metal transition. Our model of
impurity effects on the impurity band conduction, variable-range hopping
conduction, and coulomb gap conduction, is similar to that used to describe
doped semiconductors. However, for La_(2-x)Sr_xCuO_4 we find that in addition
to impurity-generated disorder effects, strong correlations are important and
must be treated on a equal level with disorder. On the basis of this work we
propose a phase diagram that is consistent with available magnetic and
transport experiments, and which connects the undoped parent compound with the
lowest x value for which La_(2-x)Sr_xCuO_4 is found to be superconducting, x
about 0.06.Comment: 7 pages revtex with one .ps figur
Sr impurity effects on the magnetic correlations of LaSrCuO
We examine the low-temperature magnetic properties of moderately doped
LaSrCuO paying particular attention to the spin-glass (SG) phase and the C-IC
transition as they are affected by Sr impurity disorder. New measurements of
the low-temperature susceptibility in the SG phase show an increase of an
anomalously small Curie constant with doping. This behaviour is explained in
terms of our theoretical work that finds small clusters of AFM correlated
regions separated by disordered domain walls. The domain walls lead to a
percolating sequence of paths connecting the impurities. We predict that for
this spin morphology the Curie constant should scale as , a
result that is quantitatively in agreement with experiment. Also, we find that
the magnetic correlations in the ground states in the SG phase are
commensurate, and that this behaviour should persist at higher temperatures
where the holes should move along the domain walls. However, our results show
that incommensurate correlations develop continuously around 5 % doping,
consistent with recent measurements by Yamada.Comment: 30 pages, revtex, 8 .ps format figures (2 meant to be in colour), to
be published in Physical Review B
Electronic properties of disordered corner-sharing tetrahedral lattices
We have examined the behaviour of noninteracting electrons moving on a
corner-sharing tetrahedral lattice into which we introduce a uniform (box)
distribution, of width W, of random on-site energies. We have used both the
relative localization length and the spectral rigidity to analyze the nature of
the eigenstates, and have determined both the mobility edge trajectories as a
function of W, and the critical disorder, Wc, beyond which all states are
localized. We find (i) that the mobility edge trajectories (energies Ec vs.
disorder W) are qualitatively different from those found for a simple cubic
lattice, and (ii) that the spectral rigidity is scale invariant at Wc and thus
provides a reliable method of estimating this quantity -- we find Wc/t=14.5. We
discuss our results in the context of the metal-to-insulator transition
undergone by LiAlyTi{2-y}O4 in a quantum site percolation model that also
includes the above-mentioned Anderson disorder, and show that the effects
produced by Anderson disorder are far less important than those produced by
quantum site percolation, at least in the determination of the doping
concentration at which the metal-to-insulator transition is predicted to occur
An Exact Diagonalization Demonstration of Incommensurability and Rigid Band Filling for N Holes in the t-J Model
We have calculated S(q) and the single particle distribution function
for N holes in the t - J model on a non--square sqrt{8} X sqrt{32} 16--site
lattice with periodic boundary conditions; we justify the use of this lattice
in compariosn to those of having the full square symmetry of the bulk. This new
cluster has a high density of vec k points along the diagonal of reciprocal
space, viz. along k = (k,k). The results clearly demonstrate that when the
single hole problem has a ground state with a system momentum of vec k =
(pi/2,pi/2), the resulting ground state for N holes involves a shift of the
peak of the system's structure factor away from the antiferromagnetic state.
This shift effectively increases continuously with N. When the single hole
problem has a ground state with a momentum that is not equal to k =
(pi/2,pi/2), then the above--mentioned incommensurability for N holes is not
found. The results for the incommensurate ground states can be understood in
terms of rigid--band filling: the effective occupation of the single hole k =
(pi/2,pi/2) states is demonstrated by the evaluation of the single particle
momentum distribution function . Unlike many previous studies, we show
that for the many hole ground state the occupied momentum states are indeed k =
(+/- pi/2,+/- pi/2) states.Comment: Revtex 3.0; 23 pages, 1 table, and 13 figures, all include
Agronomic characteristics of the spring forms of the wheat landraces (einkorn, emmer, spelt, intermediate bread wheat) grown in organic farming
Organic farmers look to the possibilities of growing neglected crops, such as the spring forms of hulled wheat – einkorn, emmer and spelt – for support in developing the organic farming system. In 2008, 169 landraces from the gene bank at the Crop Research Institute in Prague were tested on certifi ed organic plots. The experiment was aimed at fi nding suitable varieties for the organic farming system. In summary, our fi ndings show that einkorn (Triticum monococcum L.) and emmer wheat [Triticum dicoccum Schrank (Schuebl)] are resistant to powdery mildew and brown rust, spelt wheat (Triticum spelta L.) is less resistant to these two diseases, and the intermediate forms of bread wheat are very sensitive to such infestation. The
varieties evaluated incline to lodging, as they have long and weak stems. Einkorn and emmer wheat have short and dense spikes and a low thousand grains weight, whereas spelt wheat has long and lax spikes. The level of the harvest index is low. Potentially useful varieties were
found during the fi eld experiment and evaluation, and our future efforts will therefore focus on improving resistance to lodging and increasing the productivity of the spike
Comparison of 32-site exact diagonalization results and ARPES spectral functions for the AFM insulator
We explore the success of various versions of the one-band t-J model in
explaining the full spectral functions found in angle-resolved photoemission
spectra for the prototypical, quasi two-dimensional, tetragonal,
antiferromagnetic insulator . After presenting arguments
justifying our extraction of from the experimental data, we rely
on exact-diagonalization results from studies of a square 32-site lattice, the
largest cluster for which such information is presently available, to perform
this comparison. Our work leads us to believe that (i) a one-band model that
includes hopping out to third-nearest neighbours, as well three-site,
spin-dependent hopping, can indeed explain not only the dispersion relation,
but also the quasiparticle lifetimes -- only in the neighbourhood of do we find disagreement; (ii) an energy-dependent broadening
function, , is important in accounting for the
incoherent contributions to the spectral functions.Comment: 8 pages, Revtex
Achieving Cardiovascular Health in Young Adulthood—Which Adolescent Factors Matter?
AbstractPurposeTo examine associations of adolescent body mass index (BMI), tobacco use, and physical activity with optimal physiologic cardiovascular health (CVH) in adulthood.MethodsData were from 12,139 participants in Waves I (1995–1996) and IV (2007–2008) of the National Longitudinal Study of Adolescent Health. We defined optimal CVH as normal blood pressure, glucose, and cholesterol without diabetes or cardiovascular disease. We used logistic regression to estimate odds of having optimal CVH at ages 24–32 years (Wave IV) according to BMI category, smoking status, and physical activity at ages 11–19 years (Wave I).ResultsFew young adults (16%) had optimal CVH. Adolescents with normal BMI were more than twice as likely to have optimal young adult CVH compared to those who were obese (adjusted odds ratio, 2.77; 95% confidence interval, 1.97–3.89). Adolescent smoking and physical activity did not predict young adult CVH.ConclusionsLower adolescent BMI is associated with young adult CVH
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