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arXiv:cond-mat/9904074 · Kluwer Academic Publishers eBooks

On the Estimate of the Spin-Gap in Quasi-1D Heisenberg Antiferromagnets from Nuclear Spin-Lattice Relaxation

Roberto Melzi, P. Carretta

Published Mar 29, 2006• 0 citations
arXivHTMLPDF

Abstract

We present a careful analysis of the temperature dependence of the nuclear spin-lattice relaxation rate l/ T 1 in gapped quasi-1D Heisenberg antiferromagnets. It is found that in order to estimate the value of the gap correctly from l/ T 1 , the peculiar features of the dispersion curve for the triplet excitations must be taken into account. The temperature dependence of l/ T 1 due to two-magnon processes is reported for different values of the ratio r = J ⌊ / J ‖ between the superexchange constants in a 2-leg ladder. As an illustrative example, we compare our results to the experimental findings for 63 Cu l/ T 1 in the dimerized chains and 2-leg ladders contained in Sr 14 Cu 24 O 41

DOI
10.1007/0-306-47100-0_37

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We present a careful analysis of the temperature dependence of the nuclear spin-lattice relaxation rate l/ T 1 in gapped quasi-1D Heisenberg antiferromagnets. It is found that in order to estimate the value of the gap correctly from l/ T 1 , the peculiar features of the dispersion curve for the triplet excitations must be taken into account. The temperature dependence of l/ T 1 due to two-magnon processes is reported for different values of the ratio r = J ⌊ / J ‖ between the superexchange constants in a 2-leg ladder. As an illustrative example, we compare our results to the experimental findings for 63 Cu l/ T 1 in the dimerized chains and 2-leg ladders contained in Sr 14 Cu 24 O 41
We study the nuclear spin-lattice relaxation rate 1/T1{1/T}_{1}1/T1​ in the two-leg antiferromagnetic spin-1/2 Heisenberg ladder. More specifically, we consider the contribution to 1/T1{1/T}_{1}1/T1​ from the processes with momentum transfer $(\ensuremath{\pi},\ensuremath{\pi}).$ In the limit of weak coupling between the two chains, this contribution is of activation type with gap $2\ensuremath{\Delta}$ at low temperatures $(\ensuremath{\Delta}$ is the spin gap), but crosses over to a slowly decaying temperature dependence at the crossover temperature $T\ensuremath{\approx}\ensuremath{\Delta}.$ This crossover possibly explains the recent high-temperature NMR results on ladder-containing cuprates by Imai et al.
Magnetic properties of SrCu2{\mathrm{Cu}}_{2}Cu2​O3{\mathrm{O}}_{3}O3​ and Sr2{\mathrm{Sr}}_{2}Sr2​Cu3{\mathrm{Cu}}_{3}Cu3​O5{\mathrm{O}}_{5}O5​, containing two-leg and three-leg S=12S=\frac{1}{2}S=21​ ladders made of antiferromagnetic Cu-O-Cu linear bonds, were investigated. The susceptibility of the two-leg ladder material, SrCu2{\mathrm{Cu}}_{2}Cu2​O3{\mathrm{O}}_{3}O3​, is characteristic of thermal excitation from a nonmagnetic ground state with a spin gap of about 420 K, while the susceptibility of Sr2{\mathrm{Sr}}_{2}Sr2​Cu3{\mathrm{Cu}}_{3}Cu3​O5{\mathrm{O}}_{5}O5​, containing three-leg ladders, reflects a gapless spin excitation spectrum. The temperature dependence of the nuclear-spin lattice relaxation rate, 1T1\frac{1}{{T}_{1}}T1​1​, of 63Cu^{63}\mathrm{Cu}63Cu NMR also indicates the existence of a spin gap only for SrCu2{\mathrm{Cu}}_{2}Cu2​O3{\mathrm{O}}_{3}O3​. The spin gap we observed for SrCu2{\mathrm{Cu}}_{2}Cu2​O3{\mathrm{O}}_{3}O3​ confirms a recent theoretical prediction for the magnetic behavior of spin = \textonehalf{} antiferromagnetically coupled Heisenberg chains.
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