Identification of Nuclear Relaxation Processes in a Gapped Quantum Magnet: Proton NMR in the S=1/2 Heisenberg Ladder Cu2(C5H12N2)2Cl4

Abstract

The proton hyperfine shift K and NMR relaxation rate $1/T_1$ have been measured as a function of temperature in the S=1/2 Heisenberg antiferromagnetic ladder Cu2(C5H12N2)2Cl4. The presence of a spin gap $\Delta \simeq J_{\perp }-J_{\parallel }$ in this strongly coupled ladder ($J_{\parallel }<J_{\perp }$) is supported by the K and $1/T_1$ results. By comparing $1/T_1$ at two different proton sites, we infer the evolution of the spectral functions $S_z(q,{\omega }_n)$ and $S_{\perp }(q,{\omega }_n)$. When the gap is significantly reduced by the magnetic field, two different channels of nuclear relaxation, specific to gapped antiferromagnets, are identified and are in agreement with theoretical predictions.

4 pages, 4 figures, to be published in Phys. Rev. Letters

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