Local spin polarization in underdoped cuprates with impurities

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Abstract

We present a theory of magnetic (Ni) and nonmagnetic (Zn) impurities substituted into planar Cu sites in the normal state of underdoped cuprates exhibiting a spin gap. Both types of impurities induce magnetic moments on neighboring Cu sites. In the case of Ni these moments partially screen the inherent impurity spin, resulting in an effective S=1/2 moment. The characteristic Kondo scale is found to have a power-law dependence on the coupling constant. We investigate the spatial shape of the impurity-induced spin density, taking into account the presence of short-ranged AF correlations, and calculate the ^{17}O NMR line broadening induced by impurity doping.

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Magnetic scaling in cuprate superconductors

V. Barzykin, D Pines (1995)

We determine the magnetic phase diagram for the YBa\(_2\)Cu\(_3\)O\(_{6+x}\) and La\(_{2-x}\)Sr\(_x\)CuO\(_4\) systems from various NMR experiments. We discuss the possible interpretation of NMR and neutron scattering experiments in these systems in terms of both the non-linear \(\sigma\)-model of nearly localized spins and a nearly antiferromagnetic Fermi liquid description of magnetically coupled quasiparticles. We show for both the 2:1:4 and 1:2:3 systems that bulk properties, such as the spin susceptibiltiy, and probes at the antiferromagnetic wavevector \((\pi, \pi)\), such as \(^{63}T_1\), the \( ^{63}Cu\) spin relaxation time, both display a crossover at a temperature \(T_{cr}\), which increases linearly with decreasing hole concentration, from a non-universal regime to a \(z=1\) scaling regime characterized by spin pseudogap behavior. We pursue the consequences of the ansatz that \(T_{cr}\) corresponds to a fixed value of the antiferromagnetic correlation length, \(\xi\), and show how this enables one to extract the magnitude and temperature dependence of \(\xi\) from measurements of \(T_1\) alone. We show that like \(T_{cr}\), the temperature \(T_*\) which marks a crossover at low temperatures from the \(z=1\) scaling regime to a quantum disordered regime, exhibits the same dependence on doping for the 2:1:4 and 1:2:3 systems, and so arrive at a unified description of magnetic behavior in the cuprates, in which the determining factor is the planar hole concentration. We apply our quantitative results for YBa\(_2\)Cu\(_3\)O\(_7\) to the recent neutron scattering experiments of Fong {\em et al}, and show that the spin excitation near \(40 meV\) measured by them corresponds to a spin gap excitation, which is overdamped in the normal state, but becomes visible in the superconducting state.