Structural transition, spontaneous formation of strong singlet dimers, and metamagnetism in S = 3 / 2 magnetoelastic spin chains

Abstract

We study a one-dimensional antiferromagnetic-elastic model with magnetic ions having spin S = 3/2. Byextensive density matrix renormalization group (DMRG) computations and complementary analytical methods,we uncover a first-order transition from a homogeneous or weakly dimerized phase (a situation that could besimilar to the well-known S = 1/2 spin-Peierls effect) to a highly distorted phase, driven by the spin-phononcoupling λ. The striking characteristic of the second phase, present at large λ, is the appearance of weakly ferromagnetic(FM) couplings alternating with strong antiferromagnetic (AFM) ones (we dub it the FM-AFM phase)with a ground state close to a direct-product state of singlet dimers sitting on the AFM bonds. The behavior ofthe spin gap in both phases is studied by DMRG computation and contrasted with bosonization predictions andperturbation theory around the direct product of dimers. In the FM-AFM phase, robust magnetization plateausand metamagnetic jumps appear under magnetic fields. The novel phase could be realized in 5d oxides ofcurrent interest, with giant spin-phonon coupling. Potential applications of the transition would be associatedwith the possibility of tuning the transition by external parameters such as striction, magnetic or electric fields,or alloying.