Spin-gap behaviour in the 2-leg spin-ladder BiCu2PO6

We present magnetic suscceptibility and heat capacity data on a new S=1/2 two-leg spin ladder compound BiCu2PO6. From our susceptibility analysis, we find that the leg coupling J1/k_B is ~ 80 K and the ratio of the rung to leg coupling J2/J1 ~ 0.9. We present the magnetic contribution to the heat capacity of a two-leg ladder. The spin-gap Delta/k_B =3 4 K obtained from the heat capacity agrees very well with that obtained from the magnetic susceptibility. Significant inter-ladder coupling is suggested from the susceptibility analysis. The hopping integrals determined using Nth order muffin tin orbital (NMTO) based downfolding method lead to ratios of various exchange couplings in agreement with our experimental data. Based on our band structure analysis, we find the inter-ladder coupling in the bc-plane J2 to be about 0.75J1 placing the compound presumably close to the quantum critical limit.Comment: 8 pages, 5 figure

Mn local moments prevent superconductivity in iron-pnictides Ba(Fe 1-x Mn x)2As2

75As nuclear magnetic resonance (NMR) experiments were performed on Ba(Fe1-xMnx)2As2 (xMn = 2.5%, 5% and 12%) single crystals. The Fe layer magnetic susceptibility far from Mn atoms is probed by the75As NMR line shift and is found similar to that of BaFe2As2, implying that Mn does not induce charge doping. A satellite line associated with the Mn nearest neighbours (n.n.) of 75As displays a Curie-Weiss shift which demonstrates that Mn carries a local magnetic moment. This is confirmed by the main line broadening typical of a RKKY-like Mn-induced staggered spin polarization. The Mn moment is due to the localization of the additional Mn hole. These findings explain why Mn does not induce superconductivity in the pnictides contrary to other dopants such as Co, Ni, Ru or K.Comment: 6 pages, 7 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

Comment on "Localized behavior near the Zn impurity in YBa2Cu4O8 as measured by nuclear quadrupole resonance"

Williams and Kramer [Phys. Rev. B {\bf 64}, 104506 (2001)] have recently argued against the existence of staggered magnetic moments residing on several lattice sites around Zn impurities in YBCO superconductors. This claim, which is in line with an earlier publication by Williams, Tallon and Dupree [Phys. Rev. B {\bf 61}, 4319 (2000)], is however in contradiction with a large body of experimental data from different NMR groups. On the contrary, the authors argue in favor of a very localized spin and charge density on Cu sites first neighbors to Zn. We show that the conclusions of Williams and Kramer arise from erroneous interpretations of NMR and NQR data.Comment: 4 page

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

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

Absence of static phase separation in the high-Tc cuprate YBa2Cu3O6+yYBa_{2}Cu_{3}O_{6+y}

We use 89Y NMR in $YBa_{2}Cu_{3}O_{6+y}$ 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

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