96 research outputs found

### Spin dynamics in hole-doped two-dimensional S=1/2 Heisenberg antiferromagnets: ^{63}Cu NQR relaxation in La_{2-x}Sr_xCuO_4 for $x\leq 0.04$

The effects on the correlated Cu^{2+} S = 1/2 spin dynamics in the
paramagnetic phase of La_{2-x}Sr_xCuO_4 (for $x \lesssim 0.04$) due to the
injection of holes are studied by means of ^{63}Cu NQR spin-lattice relaxation
time T_1 measurements. The results are discussed in the framework of the
connection between T_1 and the in-plane magnetic correlation length
$\xi_{2D}(x,T)$. It is found that at high temperatures the system remains in
the renormalized classical regime, with a spin stiffness constant $\rho_s(x)$
reduced by small doping to an extent larger than the one due to Zn doping. For
$x\gtrsim 0.02$ the effect of doping on $\rho_s(x)$ appears to level off. The
values for $\rho_s(x)$ derived from T_1 for $T\gtrsim 500$ K are much larger
than the ones estimated from the temperature behavior of sublattice
magnetization in the ordered phase ($T\leq T_N$). It is argued that these
features are consistent with the hypothesis of formation of stripes of
microsegregated holes.Comment: 10 pages, 3 figure

### 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

### Comparison of S=0 and S=1/2 Impurities in Haldane Chain Compound, $Y_{2}BaNiO_{5}$

We present the effect of Zn (S=0) and Cu (S=1/2) substitution at the Ni site
of S=1 Haldane chain compound $Y_{2}BaNiO_{5}$. $^{89}$Y NMR allows us to
measure the local magnetic susceptibility at different distances from the
defects. The $^{89}$Y NMR spectrum consists of one central peak and several
less intense satellite peaks. The shift of the central peak measures the
uniform susceptibility, which displays a Haldane gap $Delta$$equiv$100 K and it
corresponds to an AF coupling J$equiv$260 K between the near-neighbor Ni spins.
Zn or Cu substitution does not affect the Haldane gap. The satellites, which
are evenly distributed on the two sides of the central peak, probe the
antiferromagnetic staggered magnetization near the substituted site, which
decays exponentially. Its extension is found identical for both impurities and
corresponds accurately to the correlation length $xi$(T) determined by Monte
Carlo (QMC) simulations for the pure compound. In the case of non-magnetic Zn,
the temperature dependence of the induced magnetization is consistent with a
Curie law with an "effective" spin S=0.4 on each side of Zn, which is well
accounted by Quantum Monte Carlo computations of the spinless-defect-induced
magnetism. In the case of magnetic Cu, the similarity of the induced magnetism
to the Zn case implies a weak coupling of the Cu spin to the nearest- neighbor
Ni spins. The slight reductionin the induced polarization with respect to Zn is
reproduced by QMC computations by considering an antiferromagnetic coupling of
strength J'=0.1-0.2 J between the S=1/2 Cu-spin and nearest-neighbor Ni-spin.Comment: 15 pages, 18 figures, submitted to Physical Review

### Possible Localized Modes in the Uniform Quantum Heisenberg Chains of Sr2CuO3

A model of mobile-bond defects is tentatively proposed to analyze the
"anomalies" observed on the NMR spectrum of the quantum Heisenberg chains of
Sr2CuO3. A bond-defect is a local change in the exchange coupling. It results
in a local alternating magnetization (LAM), which when the defect moves,
creates a flipping process of the local field seen by each nuclear spin. At low
temperature, when the overlap of the LAM becomes large, the defects form a
periodic structure, which extends over almost all the chains. In that regime,
the density of bond-defects decreases linearly with T.Comment: 4 pages + 3 figures. To appear in Physical Review

### (89)Y nuclear magnetic resonance study of Ca-doped Y(1-x)Ca(x)Ba(2)Cu(3)O(y) from the underdoped to the overdoped superconducting regime

(89)Y NMR linewidth, Knight shift, spin-echo dephasing, and spin-lattice relaxation measurements have been carried out in Y(1-x)Ca(x)Ba(2)Cu(3)O(y). Underdoped and overdoped samples have been obtained by means of Y(3+) for Ca(2+) substitutions in the parent chain-empty antiferromagnetic (AF) YBa(2)Cu(3)O(6.1) and in the chain-full YBa(2)Cu(3)O(7), respectively. Unexpected effects, as the divergence of relaxation rate with the concurrent broadening of the NMR line in the underdoped superconducting phase and the inadequacy of the Korringa relation between the relaxation rate 1/T(1) and Knight shift, even in the overdoped regime, suggest that a revision of the commonly accepted view of YBa(2)Cu(3)O(6+y) compounds is required. In particular the linear temperature dependence of T(1)(-1) and the temperature behavior of Knight shift cannot be accounted for over all the temperature range. In the underdoped superconducting phase the divergence of 1/T(1) on cooling is associated with the slowing down of excitations possibly related to sliding motions of orbital currents, or with the concurrent freezing of AF correlated spins. Echo-dephasing measurements evidence an extreme slowing down of longitudinal spin fluctuations which appear to be driven by a different dynamic, related either to flux line motions or to (63,65)Cu spin-lattice relaxation

### 89Y nuclear magnetic resonance study of Ca-doped Y 1-xCaxBa2Cu3Oy from the underdoped to the overdoped superconducting regime

89Y NMR linewidth, Knight shift, spin-echo dephasing, and spin-lattice relaxation measurements have been carried out in Y 1-xCaxBa2Cu3Oy. Underdoped and overdoped samples have been obtained by means of Y3+ for Ca2+ substitutions in the parent chain-empty antiferromagnetic (AF) YBa2Cu3O6.1 and in the chain-full YBa 2Cu3O7, respectively. Unexpected effects, as the divergence of relaxation rate with the concurrent broadening of the NMR line in the underdoped superconducting phase and the inadequacy of the Korringa relation between the relaxation rate 1/T1 and Knight shift, even in the overdoped regime, suggest that a revision of the commonly accepted view of YBa2Cu3O6+y compounds is required. In particular the linear temperature dependence of T1 -1 and the temperature behavior of Knight shift cannot be accounted for over all the temperature range. In the underdoped superconducting phase the divergence of 1/T1 on cooling is associated with the slowing down of excitations possibly related to sliding motions of orbital currents, or with the concurrent freezing of AF correlated spins. Echo-dephasing measurements evidence an extreme slowing down of longitudinal spin fluctuations which appear to be driven by a different dynamic, related either to flux line motions or to 63,65Cu spin-lattice relaxation

### 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 YBa$_2$Cu$_3$O$_{6+x}$. The
fluctuations of the coupled Cu magnetic moments in the vicinity of Li are
probed by near-neighbour $^{89}$Y {\it and} $^7$Li NMR spin lattice relaxation.
The data indicates that the magnetic perturbation fluctuates as a single entity
with a correlation time $\tau$ which scales with the local static
susceptibility. This behaviour is reminiscent of the low $T$ 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

### Magnetization profiles and NMR spectra of doped Haldane chains at finite temperatures

Open segments of S=1 antiferromagnetic spin chains are studied at finite
temperatures and fields using continuous time Quantum Monte Carlo techniques.
By calculating the resulting magnetization profiles for a large range of chain
lengths with fixed field and temperature we reconstruct the experimentally
measured NMR spectrum of impurity doped Y$_2$BaNi$_{1-x}$Mg$_x$O$_5$. For
temperatures above the gap the calculated NMR spectra are in excellent
agreement with the experimental results, confirming the existence of $S=1/2$
excitations at the end of open S=1 chain segments. At temperatures below the
gap, neglecting inter chain couplings, we still find well defined peaks in the
calculated NMR spectra corresponding to the $S=1/2$ chain end excitations. At
low temperatures, inter chain couplings could be important, resulting in a more
complicated phase.Comment: 7 pages, 5 figures, minor correction

### Susceptibility and dilution effects of the kagome bi-layer geometrically frustrated network. A Ga-NMR study of SrCr_(9p)Ga_(12-9p)O_(19)

We present an extensive gallium NMR study of the geometrically frustrated
kagome bi-layer compound SrCr_(9p)Ga_(12-9p)O_(19) (Cr^3+, S=3/2) over a broad
Cr-concentration range (.72<p<.95). This allows us to probe locally the kagome
bi-layer susceptibility and separate the intrinsic properties due to the
geometric frustration from those related to the site dilution. Our major
findings are: 1) The intrinsic kagome bi-layer susceptibility exhibits a
maximum in temperature at 40-50 K and is robust to a dilution as high as ~20%.
The maximum reveals the development of short range antiferromagnetic
correlations; 2) At low-T, a highly dynamical state induces a strong wipe-out
of the NMR intensity, regardless of dilution; 3) The low-T upturn observed in
the macroscopic susceptibility is associated to paramagnetic defects which stem
from the dilution of the kagome bi-layer. The low-T analysis of the NMR
lineshape suggests that the defect can be associated with a staggered
spin-response to the vacancies on the kagome bi-layer. This, altogether with
the maximum in the kagome bi-layer susceptibility, is very similar to what is
observed in most low-dimensional antiferromagnetic correlated systems; 4) The
spin glass-like freezing observed at T_g=2-4 K is not driven by the
dilution-induced defects.Comment: 19 pages, 19 figures, revised version resubmitted to PRB Minor
modifications: Fig.11 and discussion in Sec.V on the NMR shif

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