87 research outputs found
89Y NMR Probe of Zn Induced Local Magnetism in YBa2(Cu(1-y)Zn(y))3O(6+x)
We present detailed data and analysis of the effects of Zn substitution on
the planar Cu site in YBaCuO (YBCO) as evidenced from
our Y NMR measurements on oriented powders. For we find
additional NMR lines which are associated with the Zn substitution. From our
data on the intensities and temperature dependence of the shift, width, and
spin-lattice relaxation rate of these resonances, we conclude that the spinless
Zn 3 state induces local moments on the near-neighbour () Cu
atoms. Additionally, we conjecture that the local moments actually extend to
the farther Cu atoms with the magnetization alternating in sign at subsequent
sites. We show that this analysis is compatible with ESR data taken on
dilute Gd doped (on the Y site) and on neutron scattering data reported
recently on Zn substituted YBCO. For optimally doped compounds
Y resonances are not detected, but a large % -dependent
contribution to the Y NMR linewidth is evidenced and is also attributed
to the occurence of a weak induced local moment near the Zn. These results are
compatible with macroscopic magnetic measurements performed on YBCO
samples prepared specifically in order to minimize the content of impurity
phases. We find significant differences between the present results on the
underdoped YBCO samples and Al NMR data taken on Al
substituted on the Cu site in optimally doped LaCuO. Further
experimental work is needed to clarify the detailed evolution of the impurity
induced magnetism with hole content in the cuprates.Comment: To be published in EPJB 15 pages of text and figures in eps forma
Normal State Magnetic Properties of Ni and Zn Substituted in YBa_{2}Cu_{3} O_{6+x}: Hole-Doping Dependence
We present SQUID susceptibility data on Zn and Ni substituted
YBa_{2}Cu_{3}O_{6+x}. Cross-checks with NMR yield an unprecedented accuracy in
the estimate of the magnetic susceptibility associated with the substituants,
from the underdoped to the lightly overdoped case. This allows us to determine
the Weiss temperature \theta for YBCO: its value is very small for all hole
dopings n_h. Since in conventional metals, the Kondo temperature,
, magnetic screening effects would not be expected for ; in contrast, increasing n_h produces a reduction of the small moment
induced by Zn^{2+} and a nearly constant effective moment for Ni^{2+}
corresponding to a spin 1/2 rather than to a spin 1.Comment: 4 pages, 5 figures, to be published in Europhysics Letter
Cu NMR evidence for enhanced antiferromagnetic correlations around Zn impurities in YBa2Cu3O6.7
Doping the high-Tc superconductor YBa2Cu3O6.7 with 1.5 % of non-magnetic Zn
impurities in CuO2 planes is shown to produce a considerable broadening of 63Cu
NMR spectra, as well as an increase of low-energy magnetic fluctuations
detected in 63Cu spin-lattice relaxation measurements. A model-independent
analysis demonstrates that these effects are due to the development of
staggered magnetic moments on many Cu sites around each Zn and that the
Zn-induced moment in the bulk susceptibility might be explained by this
staggered magnetization. Several implications of these enhanced
antiferromagnetic correlations are discussed.Comment: 4 pages including 2 figure
Room temperature coexistence of large electric polarization and magnetic order in BiFeO3 single crystals
From an experimental point of view, room temperature ferroelectricity in
BiFeO3 is raising many questions. Electric measurements made a long time ago on
solid-solutions of BiFeO3 with Pb(Ti,Zr)O3 indicate that a spontaneous electric
polarization exists in BiFeO3 below the Curie temperature TC=1143K. Yet in most
reported works, the synthesised samples are too conductive at room temperature
to get a clear polarization loop in the bulk without any effects of extrinsic
physical or chemical parameters. Surprisingly, up to now there has been no
report of a P(E) (polarization versus electric field) loop at room temperature
on single crystals of BiFeO3. We describe here our procedure to synthesize
ceramics and to grow good quality sizeable single crystals by a flux method. We
demonstrate that BiFeO3 is indeed ferroelectric at room-temperature through
evidence by Piezoresponse Force Microscopy and P(E) loops. The polarization is
found to be large, around 60 microC/cm2, a value that has only been reached in
thin films. Magnetic measurements using a SQUID magnetometer and Mossbauer
spectroscopy are also presented. The latter confirms the results of NMR
measurements concerning the anisotropy of the hyperfine field attributed to the
magnetic cycloidal structure.Comment: 27 pages, 12 figure
Absence of static phase separation in the high-Tc cuprate
We use 89Y NMR in in order to evaluate with high
sensitivity the distribution of hole content p in the CuO2 planes. For y=1 and
y=0.6, this hole doping distribution is found narrow with a full width at half
maximum smaller than Delta p=0.025. This rules out any large static phase
separation between underdoped and optimally doped regions in contrast with the
one observed by STM in Bi2212 and by NQR in LaSrCuO. This establishes that
static electronic phase separation is not a generic feature of the cuprates.Comment: published in Phys.Rev.Lett. 89, 157002 (2002) (only minor changes as
compared to previous version) Article of 4 pages + 3 figure
Spinless impurities in high Tc cuprates: Kondo-like behavior
We compare the effects of in-plane non magnetic Li and Zn
impurities on the normal state of high-T cuprates. Y NMR shows that the
extra hole introduced by Li is not localized in its vicinity. The Tc depression
and induced moments on near neighbour Cu sites of Zn or Li are found identical.
These universal effects of spinless impurities establish the major influence of
the spin perturbation with respect to the charge defect. The susceptibility of
the induced moment measured by Li NMR displays a 1/(T+Theta) behavior. Theta
increases with doping up to about 200 K in the overdoped regime. We attribute
this to a "Kondo like" effect.Comment: To appear in Phys.Rev.Lett. (22 nov. 99) Minor modifications compared
to previous version. 8 pages (4 pages for text + 4 figures
Planar 17O NMR study of Pr_yY_{1-y}Ba_2Cu_3O_{6+x}
We report the planar ^{17}O NMR shift in Pr substituted YBa_{2}Cu_{3}O_{6+x},
which at x=1 exhibits a characteristic pseudogap temperature dependence,
confirming that Pr reduces the concentration of mobile holes in the CuO_{2}
planes. Our estimate of the rate of this counterdoping effect, obtained by
comparison with the shift in pure samples with reduced oxygen content, is found
insufficient to explain the observed reduction of T_c. From the temperature
dependent magnetic broadening of the ^{17}O NMR we conclude that the Pr moment
and the local magnetic defect induced in the CuO_2 planes produce a long range
spin polarization in the planes, which is likely associated with the extra
reduction of T_c. We find a qualitatively different behaviour in the oxygen
depleted Pr_yY_{1-y}Ba_2Cu_3O_{6.6}, i.e. the suppression of T is nearly
the same, but the magnetic broadening of the ^{17}O NMR appears weaker. This
difference may signal a weaker coupling of the Pr to the planes in the
underdoped compound, which might be linked with the larger Pr to CuO_2 plane
distance, and correspondingly weaker hybridization.Comment: 8 pages, 9 figures, accepted in Phys Rev
Dynamics of the Local Moment Induced by Nonmagnetic Defects in Cuprates
We present a study of the spin dynamics of magnetic defects induced by Li
substitution of the plane Cu in the normal state of YBaCuO. The
fluctuations of the coupled Cu magnetic moments in the vicinity of Li are
probed by near-neighbour Y {\it and} Li NMR spin lattice relaxation.
The data indicates that the magnetic perturbation fluctuates as a single entity
with a correlation time which scales with the local static
susceptibility. This behaviour is reminiscent of the low Kondo state of
magnetic impurities in conventional metals. Surprisingly it extends well above
the ``Kondo'' temperature for the underdoped pseudogapped case.Comment: 4 pages, 5 figures (same), major modifications to text, accepted in
PR
Persistence of Li Induced Kondo Moments in the Superconducting State of Cuprates
We measure the magnetic susceptibility nearby Li spinless impurities in the
superconducting phase of the high Tc cuprate YBaCuO. The induced moment which
was found to exist above Tc persists below Tc. In the underdoped regime, it
retains its Curie law below Tc. In contrast, near optimal doping, the large
Kondo screening observed above Tc (T_K=135 K) is strongly reduced below Tc as
expected theoretically when the superconducting gap develops. This moment still
extends essentially on its 4 near neighbour Cu, showing the persistence of AF
correlations in the superconducting state. A direct comparison with recent STM
results of Pan et al. is proposed.Comment: accepted for publication in Phys. Rev. Lett. (issue of 30 april 2001)
Revised version : 8 pages including 4 pages of text and 4 figure
Genetic and genomic monitoring with minimally invasive sampling methods
Funding: Marie Slodowska Curie Fellowship, (Behaviour-Connect) funded by the EU Horizon2020 program (ELC).The decreasing cost and increasing scope and power of emerging genomic technologies are reshaping the field of molecular ecology. However, many modern genomic approaches (e.g., RAD-seq) require large amounts of high quality template DNA. This poses a problem for an active branch of conservation biology: genetic monitoring using minimally invasive sampling (MIS) methods. Without handling or even observing an animal, MIS methods (e.g. collection of hair, skin, faeces) can provide genetic information on individuals or populations. Such samples typically yield low quality and/or quantities of DNA, restricting the type of molecular methods that can be used. Despite this limitation, genetic monitoring using MIS is an effective tool for estimating population demographic parameters and monitoring genetic diversity in natural populations. Genetic monitoring is likely to become more important in the future as many natural populations are undergoing anthropogenically-driven declines, which are unlikely to abate without intensive adaptive management efforts that often include MIS approaches. Here we profile the expanding suite of genomic methods and platforms compatible with producing genotypes from MIS, considering factors such as development costs and error rates. We evaluate how powerful new approaches will enhance our ability to investigate questions typically answered using genetic monitoring, such as estimating abundance, genetic structure and relatedness. As the field is in a period of unusually rapid transition, we also highlight the importance of legacy datasets and recommend how to address the challenges of moving between traditional and next generation genetic monitoring platforms. Finally, we consider how genetic monitoring could move beyond genotypes in the future. For example, assessing microbiomes or epigenetic markers could provide a greater understanding of the relationship between individuals and their environment.Publisher PDFPeer reviewe
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