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arXiv:2409.13446 · Physical review. B./Physical review. B

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

C. J. Gazza, Armando A. Aligia, A. Dobry, G. L. Rossini, D. C. Cabra

Published Jan 30, 2025• 2 citations
arXivHTMLPDF

Abstract

We study a one-dimensional antiferromagnetic-elastic model with magnetic ions having spin $S=3/2$. By extensive 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 be similar to the well-known $S=1/2$ spin-Peierls effect) to a highly distorted phase, driven by the spin-phonon coupling $\ensuremath{\lambda}$. The striking characteristic of the second phase, present at large $\ensuremath{\lambda}$, 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 of the spin gap in both phases is studied by DMRG computation and contrasted with bosonization predictions and perturbation theory around the direct product of dimers. In the FM-AFM phase, robust magnetization plateaus and metamagnetic jumps appear under magnetic fields. The novel phase could be realized in 5d oxides of current interest, with giant spin-phonon coupling. Potential applications of the transition would be associated with the possibility of tuning the transition by external parameters such as striction, magnetic or electric fields, or alloying.

DOI
10.1103/physrevb.111.014443

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We study a one-dimensional antiferromagnetic-elastic model with magnetic ions having spin S=3/2S=3/2S=3/2. By extensive 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 be similar to the well-known S=1/2S=1/2S=1/2 spin-Peierls effect) to a highly distorted phase, driven by the spin-phonon coupling \ensuremathλ\ensuremath{\lambda}\ensuremathλ. The striking characteristic of the second phase, present at large \ensuremathλ\ensuremath{\lambda}\ensuremathλ, 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 of the spin gap in both phases is studied by DMRG computation and contrasted with bosonization predictions and perturbation theory around the direct product of dimers. In the FM-AFM phase, robust magnetization plateaus and metamagnetic jumps appear under magnetic fields. The novel phase could be realized in 5d oxides of current interest, with giant spin-phonon coupling. Potential applications of the transition would be associated with the possibility of tuning the transition by external parameters such as striction, magnetic or electric fields, or alloying.
We report the exact dimer phase, in which the ground states are described by product of singlet dimer, in the extended XYZ model by generalizing the isotropic Majumdar-Ghosh model to the fully anisotropic region. We demonstrate that this phase can be realized even in models when antiferromagnetic interaction along one of the three directions. This model also supports three different ferromagnetic (FM) phases, denoted as x-FM, y-FM and z-FM, polarized along the three directions. The boundaries between the exact dimer phase and FM phases are infinite-fold degenerate. The breaking of this infinite-fold degeneracy by either translational symmetry breaking or [Formula: see text] symmetry breaking leads to exact dimer phase and FM phases, respectively. Moreover, the boundaries between the three FM phases are critical with central charge [Formula: see text] for free fermions. We characterize the properties of these boundaries using entanglement entropy, excitation gap, and long-range spin-spin correlation functions. These results are relevant to a large number of one dimensional magnets, in which anisotropy is necessary to isolate a single chain out from the bulk material. We discuss the possible experimental signatures in realistic materials with magnetic field along different directions and show that the anisotropy may resolve the disagreement between theory and experiments based on isotropic spin-spin interactions.
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Field-induced Peierls phase in S=1S=1S=1 Heisenberg spins coupled to quantum phonons
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Frustrated spin-1/2 model with all-antiferromagnetic exchange on Shastry-Sutherland lattice (SSL) has been extensively explored, and in this work we study the quantum phase diagram of isotropic Heisenberg spin- 1 / 2 model on the SSL with ferromagnetic exchange interactions along leg, and antiferromagnetic couplings along the diagonal bonds ( J ). Using density matrix renormalization group (DMRG) and exact diagonalization calculations, we identify four distinct ground-state (gs) phases for this frustrated spin-1/2 model: stripe ( π , 0 ) , stripe ( 0 , π ) , ferromagnetic, and spiral-dimer phases. In the spiral-dimer phase the gs can exhibit dimer formation along the diagonal bonds with or without spins spiral along the x axis or y axis in different parameter regime and these states are connected through crossovers. There is a continuous transition between stripe with a long-range spin-spin correlations and spiral-dimer phase with a short-range correlations. An effective spin parameter is introduced to accurately determine the phase boundaries. Our DMRG results differ significantly from previous Schwinger boson mean-field theory predictions. The spiral phase occurs in a narrow range of the parameter space and shows short-range correlations in the system.
We investigate the effect of dimensional crossover in the ground state of the antiferromagnetic spin-1 Heisenberg model on the anisotropic triangular lattice that interpolates between the regime of weakly coupled Haldane chains $({J}^{\ensuremath{'}}\ensuremath{\ll}J)$ and the isotropic triangular lattice $({J}^{\ensuremath{'}}=J)$. We use the density-matrix renormalization group (DMRG) and Schwinger boson theory performed at the Gaussian correction level above the saddle-point solution. Our DMRG results show an abrupt transition between decoupled spin chains and the spirally ordered regime at ${({J}^{\ensuremath{'}}/J)}_{c}\ensuremath{\sim}0.42$, signaled by the sudden closing of the spin gap. Coming from the magnetically ordered side, the computation of the spin stiffness within Schwinger boson theory predicts the instability of the spiral magnetic order toward a magnetically disordered phase with one-dimensional features at ${({J}^{\ensuremath{'}}/J)}_{c}\ensuremath{\sim}0.43$. The agreement of these complementary methods, along with the strong difference found between the intra- and the interchain DMRG short spin-spin correlations for sufficiently large values of the interchain coupling, suggests that the interplay between the quantum fluctuations and the dimensional crossover effects gives rise to the one-dimensionalization phenomenon in this frustrated spin-1 Hamiltonian.
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