Quantum phase transitions in integer spin chains

High field inelastic neutron scattering experiments on the $S$=1 bond-alternating 1D antiferromagnet NTENP, the anisotropic $S=1$-chain Haldane-gap compound NDMAP and the isotropic ``composite'' Haldane spin chain IPA-CuCl$_3$ [T. Masuda {\it et al.}, cond-mat/0506382] reveal key differences in the spin dynamics of these distinct types of quantum spin liquids. In modest applied fields the spectra of NDMAP [A. Zheludev {\it et al.}, Phys. Rev. Lett. {\bf 88}, 077206 (2002)] and IPA-CuCl$_3$ feature three sharp stable gap excitations. In contrast, in NTENP the highest mode is anomalously weak at $H=0$ and rapidly broadens and vanishes when the field is turned on. Above the critical field of 1D Bose condensation of magnons and long-range ordering NDMAP retains a triplet of massive long-lived excitations [A. Zheludev {\it et al.}, Phys. Rev. B $\bf{68}$, 134438 (2003)]. In IPA-CuCl$_3$ only two sharp gap excitations persist, with possibly an additional gapless mode. In NTENP only one sharp excitation branch is observed in this regime [Hagiwara {\it et al.}, Phys. Rev. Lett. {\bf 94}, 177202 (2005)], but there is new evidence of low-lying excitation continua. Work at ORNL was carried out under DOE Contract No. DE-AC05-00OR22725.