We investigated the thermodynamic properties of the spin-1/2 one-dimensional Heisenberg antiferromagnet KCuGaF
6 by measuring the specific heat in magnetic fields. When this compound is subjected to a uniform magnetic field $\mathbit{H}$, a transverse staggered magnetic field $\mathbit{h}$ is induced in this compound owing to the staggered component of the $\mathbit{g}$ tensor and the Dzyaloshinskii-Moriya interaction with an alternating $\mathbit{D}$ vector. Consequently, the quantum sine-Gordon (SG) model is an effective model of this compound in a uniform magnetic field. In three different field directions, we observed a magnetic-field-induced gap, which increases with
H. We analyzed experimental results using specific heat theory based on quantum SG theory. The thermodynamic property for $H\phantom{\rule{0.16em}{0ex}}\ensuremath{\parallel}\phantom{\rule{0.16em}{0ex}}c$ is very well described in terms of the elementary excitations characteristic of the quantum SG model, while for the other field directions significant contributions from other excitation modes beyond the framework of the quantum SG model were observed. For $H\phantom{\rule{0.16em}{0ex}}\ensuremath{\parallel}\phantom{\rule{0.16em}{0ex}}b$, a quantum phase transition between gapless and gapped ground states was observed.
We study the frustration-induced enhancement of the incommensurate correlation for a bond-alternating quantum spin chain in a magnetic field, which is associated with a quasi-one-dimensional organic compound F5PNN. We investigate the temperature dependence of the staggered susceptibilities by using the density matrix renormalization group, and then find that the incommensurate correlation becomes dominant in a certain range of the magnetic field. We also discuss the mechanism of this enhancement on the basis of the mapping to the effective S=1/2 XXZ chain and a possibility of the field-induced incommensurate long-range order.
We present inelastic neutron-scattering data on the quantum paramagnet
AgVOAsO4 that establish this system as a
S=1/2 alternating spin chain compound and provide a direct measurement of the spin gap $\mathrm{\ensuremath{\Delta}}=1.2\phantom{\rule{0.16em}{0ex}}\mathrm{meV}$. We also present experimental evidence for two different types of field-induced magnetic order between ${\ensuremath{\mu}}_{0}{H}_{c1}=8.4\phantom{\rule{0.16em}{0ex}}\mathrm{T}$ and ${\ensuremath{\mu}}_{0}{H}_{c2}=48.9\phantom{\rule{0.16em}{0ex}}\mathrm{T}$, which may be related to Bose-Einstein condensation (BEC) of triplons. Thermodynamic measurements in magnetic fields up to 60 T and temperatures down to 0.1 K reveal a $H\ensuremath{-}T$ phase diagram consisting of a dome encapsulating two ordered phases with maximum ordering temperatures of 3.8 and 5.3 K, respectively. This complex phase diagram is not expected for a single-$\stackrel{P\vec}{Q}$ BEC system and therefore establishes
AgVOAsO4 as a promising multi-$\stackrel{P\vec}{Q}$ BEC candidate capable of hosting exotic vortex phases.