arXiv:1612.07609 · Physical review. B./Physical review. B
Crossover from an incommensurate singlet spiral state with a vanishingly small spin gap to a valence-bond solid state in dimerized frustrated ferromagnetic spin chains
Cliò Efthimia Agrapidis, Stefan‐Ludwig Drechsler, Jeroen van den Brink, Satoshi Nishimoto
Abstract
Motivated by the magnetic properties of the spin-chain compounds ${\mathrm{LiCuSbO}}_{4}\ensuremath{\equiv}{\mathrm{LiSbCuO}}_{4}$ and ${\mathrm{Rb}}_{2}{\mathrm{Cu}}_{2}{\mathrm{Mo}}_{3}{\mathrm{O}}_{12}$, we study the ground state of the Heisenberg chain with dimerized nearest-neighbor ferromagnetic (FM) (${J}_{1},{J}_{1}^{\ensuremath{'}}<0$) and next-nearest-neighbor antiferromagnetic (${J}_{2}>0$) couplings. Using the density-matrix renormalization group technique and spin-wave theory, we find a first-order transition between a fully polarized FM and an incommensurate spiral state at $2\ensuremath{\alpha}=\ensuremath{\beta}/(1+\ensuremath{\beta})$, where $\ensuremath{\alpha}$ is the frustration ratio ${J}_{2}/|{J}_{1}|$ and $\ensuremath{\beta}$ the degree of dimerization ${J}_{1}^{\ensuremath{'}}/{J}_{1}$. In the singlet spiral state the spin-gap is vanishingly small in the vicinity of the FM transition, corresponding to a situation of ${\mathrm{LiCuSbO}}_{4}$. For larger $\ensuremath{\alpha}$, corresponding to ${\mathrm{Rb}}_{2}{\mathrm{Cu}}_{2}{\mathrm{Mo}}_{3}{\mathrm{O}}_{12}$, and smaller $\ensuremath{\beta}$ there is a crossover from this frustration induced incommensurate state to an Affleck-Lieb-Kennedy-Tasaki-type valence-bond solid state with substantial spin gaps.