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≤0.04x\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, Y2BaNiO5Y_{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