160 research outputs found
Study of one-dimensional nature of (Sr,Ba)_2Cu(PO_4)_2 and BaCuP_2O_7 via 31P NMR
The magnetic behavior of the low-dimensional phosphates (Sr,Ba)_2 Cu(PO_4)_2
and BaCuP_2O_7 was investigated by means of magnetic susceptibility and ^{31}P
nuclear magnetic resonance (NMR) measurements. We present here the NMR shift
K(T), the spin-lattice 1/T_1 and spin-spin 1/T_2 relaxation-rate data over a
wide temperature range 0.02 K < T < 300 K. The T-dependence of the NMR K(T) is
well described by the S=1/2 Heisenberg antiferromagnetic chain model with an
intrachain exchange of J/k_B = 165 K, 151 K, and 108 K in Sr_2Cu(PO_4)_2,
Ba_2Cu(PO_4)_2, and BaCuP_2O_7, respectively. Our measurements suggest the
presence of magnetic ordering at 0.8 K in BaCuP_2O_7 (J/k_B = 108 K). For all
the samples, we find that 1/T_1 is nearly T-independent at low-temperatures (1
K < T < 10 K), which is theoretically expected for 1D chains when relaxation is
dominated by fluctuations of the staggered susceptibility. At high
temperatures, 1/T_1 varies nearly linearly with temperature
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
NMR study of the magnetic and metal-insulator transitions in Na0.5CoO2: a nesting scenario
Co and Na NMR are used to probe the local susceptibility and charge state of
the two Co sites of the Na-ordered orthorhombic Na0.5CoO2. Above T_N=86K, both
sites display a similar T-dependence of the spin shift, suggesting that there
is no charge segregation into Co3+ and Co4+ sites. Below T_N, the magnetic long
range commensurate order found is only slightly affected by the metal-insulator
transition (MIT) at T_MIT=51K. Furthermore, the electric field gradient at the
Co site does not change at these transitions, indicating the absence of charge
ordering. All these observations can be explained by successive SDW induced by
two nestings of the Fermi Surface specific to the x=0.5 Na-ordering.Comment: 4 pages, 4 figure
Correlation length in cuprates deduced from the impurity-induced magnetization
We report a new multi-nuclei based NMR method which allows us to image the
staggered polarization induced by nonmagnetic Li impurities in underdoped O6.6
and slightly overdoped O7 YBa2Cu3O6+y above T_C. The spatial extension of the
polarization xi_imp approximately follows a Curie law, increasing up to six
lattice constants at T=80K at O6.6 in the pseudogap regime. Near optimal
doping, the staggered magnetization has the same shape, with xi_imp reduced by
a factor 2. xi_imp is argued to reveal the intrinsic magnetic correlation
length of the pure system. It is found to display a smooth evolution through
the pseudogap regime.Comment: 8 latex pages + 8 figures, to appear in Physical Review B, this
resubmitted version is twice longer than the previous one : we detail here
our method to determine the impurity-induced magnetizatio
Impurity-induced spin polarization and NMR line broadening in underdoped cuprates
We present a theory of magnetic (S=1) Ni and nonmagnetic Zn impurities in
underdoped cuprates. Both types of impurities are shown to induce S=1/2 moments
on Cu sites in the proximity of the impurity, a process which is intimately
related to the spin gap phenomenon in cuprates. Below a characteristic Kondo
temperature, the Ni spin is partially screened by the Cu moments, resulting in
an effective impurity spin S=1/2. We further analyze the
Ruderman-Kittel-Kasiya-Yosida-type response of planar Cu spins to a
polarization of the effective impurity moments and derive expressions for the
corresponding ^{17}O NMR line broadening. The peculiar aspects of recent
experimental NMR data can be traced back to different spatial characteristics
of Ni and Zn moments as well as to an inherent temperature dependence of local
antiferromagnetic correlations.Comment: PRB B1 01June9
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
On-site magnetization in open antiferromagnetic chains: a classical analysis versus NMR experiments in a spin-1 compound
The response of an open spin chain with isotropic antiferromagnetic
interactions to a uniform magnetic field is studied by classical Monte Carlo
simulations. It is observed how the induced on-site magnetization is non
uniform, due to the occurrence of edge staggered terms which decay
exponentially over a distance equal to the zero field correlation length of the
infinite chain. The total magnetic moment associated to each staggered term is
found to be about half of the original single-spin magnitude and to decrease as
the inverse of temperature (i.e. to behave as a Curie-like moment). The
numerical results are compared to recent NMR findings in spinless-doped
Y(2)BaNiO(5); the remarkable agreement found shows that, for temperatures above
the Haldane gap, the classical approach gives a correct picture of the boundary
effects observed in the Heisenberg S=1 chain.Comment: 4 pages, 4 eps figures; minor changes in the text; added reference
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
Impurity effects in coupled-ladder BiCu2PO6 studied by NMR and quantum Monte Carlo simulations
We present a 31P NMR study of the coupled spin 1/2 ladder compound BiCu2PO6.
In the pure material, intrinsic susceptibility and dynamics show a spin gap of
about . Substitution of non magnetic Zn or magnetic Ni impurity at Cu site
induces a staggered magnetization which results in a broadening of the 31P NMR
line, while susceptibility far from the defects is unaffected. The effect of Ni
on the NMR line broadening is twice that of Zn, which is consistent with
Quantum Monte Carlo (QMC) calculations assuming that Ni couples
ferromagnetically to its adjacent Cu. The induced moment follows a 1/T
temperature dependence due to the Curie-like development of the moment
amplitude while its extension saturates and does not depend on impurity content
or nature. This allow us to verify the generically expected scenario for
impurity doping and to extend it to magnetic impurity case: in an
antiferromagnetically correlated low dimensional spin system with
antiferromagnetic correlations, any type of impurity induces a staggered moment
at low temperature, whose extension is not linked to the impurity nature but to
the intrinsic physics at play in the undoped pure system, from 1D to 2D
systems.Comment: 11 pages, 16 figure
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