Order from Disorder: Non Magnetic Impurities in the Spin-gap Phase of the Cuprates

Abstract

We solve the problem of $N$ non magnetic impurities in the staggered flux phase of the Heisenberg model which we assume to be a good mean-field approximation for the spin-gap phase of the cuprates. The density of states is evaluated exactly in the unitary limit and is porportional to $1/\left (\omega \ln^2(|\omega|/D))$, in analogy with the 1D case of doped spin-Peierls and two-leg ladders compounds. We argue that the system exhibits a quasi long-range order at T=0 with instantaneous spin-spin correlations decreasing as $n_i/ \ln^2\left (n_i R_{ij})$ for large distances $R_{ij}$ and we predict enhanced low energy fluctuations in Neutron Scattering.