Direct Observation of Field-Induced Incommensurate Fluctuations in a One-Dimensional S=1/2 Antiferromagnet

TL;DR

This study provides direct experimental evidence of field-dependent incommensurate fluctuations and a novel field-induced energy gap in a one-dimensional S=1/2 antiferromagnet, revealing complex magnetic behavior under external magnetic fields.

Contribution

First direct neutron scattering observation of field-dependent incommensurate modes and a novel energy gap in a 1D S=1/2 antiferromagnet, confirming theoretical predictions.

Findings

Incommensurate low energy modes occur at wavevectors q=π ± dq(H).

Energy gap Δ(H) scales as H^α with α ≈ 0.65.

Gap magnitude varies with field orientation, from 0.06 to 0.3 J at 7 T.

Abstract

Neutron scattering from copper benzoate, Cu(C6D5COO)2 3D2O, provides the first direct experimental evidence for field-dependent incommensurate low energy modes in a one-dimensional spin S = 1/2 antiferromagnet. Soft modes occur for wavevectors q=\pi +- dq(H) where dq(H) ~ 2 \pi M(H)/g\mu_B as predicted by Bethe ansatz and spinon descriptions of the S = 1/2 chain. Unexpected was a field-induced energy gap $\Delta(H) \propto H^\alpha$, where $\alpha = 0.65(3)$ as determined from specific heat measurements. At H = 7 T (g\mu_B H/J = 0.52), the magnitude of the gap varies from 0.06 - 0.3 J depending on the orientation of the applied field.