Zeeman levels with exotic field dependence in the high field phase of a S=1 Heisenberg antiferromagnetic chain

Abstract

We have performed electron spin resonance measurements over a wide frequency and magnetic field range on a single crystal of the S=1 quasi-one-dimensional Heisenberg antiferromagnet $\mathrm{Ni}({\mathrm{C}}_5{\mathrm{H}}_{14}{\mathrm{N}}_2{)}_2{\mathrm{N}}_3(\mathrm{P}{\mathrm{F}}_6)$. We observed gapped excitation branches above the critical field ($H_{\mathrm{c}}$) where the Haldane gap closes. These branches are analyzed by a phenomenological field-theory using complex-field ${\varphi }^4$ model. A satisfactory agreement between experiment and theory is obtained.

4 revtex pages, 3 eps figures included