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arXiv:2505.22420

Robustness of topological edge states in alternating spin chains against environment

Alexander Sattler, Maria Daghofer

Published May 28, 2025• 0 citations
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Abstract

Both the Haldane spin-$1$ chain and dimerized chains of spin-$1/2$ exhibit topologically protected edge states that are robust against specific perturbations. Recently, such spin chains have been specifically assembled on surfaces and we investigate here the robustness of these edge states against coupling to the surface. Since no physical system can be considered perfectly isolated, it is crucial to examine whether topological robustness is maintained in the presence of environmental coupling. We apply exact diagonalization to a Lindblad master equation that couples an alternating Heisenberg spin chain based on spins $1/2$ to a surface via various jump operators. The robustness of topological states is assessed via the time evolution of quantities such as the ground-state degeneracy, correlation function, entropy, and magnetization of edge states. We investigate chains built from dimers with antiferromagnetic and ferromagnetic intra-dimer coupling, which resemble Su-Schrieffer-Heeger and the Haldane models, resp., and assess the impact of $z$-axis anisotropy and longer-ranged couplings. Generally, we find that signatures of topological properties are more robust in Su-Schrieffer-Heeger-like chains than in Haldane-like chains.

DOI
10.48550/arxiv.2505.22420

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Both the Haldane spin-111 chain and dimerized chains of spin-1/21/21/2 exhibit topologically protected edge states that are robust against specific perturbations. Recently, such spin chains have been specifically assembled on surfaces and we investigate here the robustness of these edge states against coupling to the surface. Since no physical system can be considered perfectly isolated, it is crucial to examine whether topological robustness is maintained in the presence of environmental coupling. We apply exact diagonalization to a Lindblad master equation that couples an alternating Heisenberg spin chain based on spins 1/21/21/2 to a surface via various jump operators. The robustness of topological states is assessed via the time evolution of quantities such as the ground-state degeneracy, correlation function, entropy, and magnetization of edge states. We investigate chains built from dimers with antiferromagnetic and ferromagnetic intra-dimer coupling, which resemble Su-Schrieffer-Heeger and the Haldane models, resp., and assess the impact of zzz-axis anisotropy and longer-ranged couplings. Generally, we find that signatures of topological properties are more robust in Su-Schrieffer-Heeger-like chains than in Haldane-like chains.
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Antiferromagnetic spin-1 chains host the celebrated symmetry-protected topological Haldane phase, whose spin-1/21/21/2 edge states were evidenced in bulk by, e.g., electron spin resonance (ESR). To fully reveal and manipulate the spin-1/21/21/2 degree of freedom, one may build on the recent success in assembling effective spin-1 antiferromagnetic chains from nanographene and porphyrin molecules, and on the nascent technique of combined ESR and scanning tunneling microscopy, which is able to measure the spin dynamics site by site with atomic real-space resolution. In this Letter, we combine exact diagonalization and density matrix renormalization group to investigate the local dynamic spin structure factor of the different phases of the bilinear-biquadratic Hamiltonian with single-ion anisotropy in the presence of an external magnetic field. We find that the signature of the Haldane phase is a low-energy peak created by singlet-triplet transitions in the edge-state manifold. We predict that the signature peak is experimentally observable, although for chains of length above N=30N=30N=30 its energy should be first tuned by application of an external magnetic field. We fully characterize the peak in real space and energy, and further show its robustness to weak anisotropy and a relevant range of temperatures.
We investigate interacting Su-Schrieffer-Heeger (SSH) chains with two- and three-site unit cells using density matrix renormalization group (DMRG) simulations. By selecting appropriate filling fractions and sweeping across interaction strength Jz{J}_{z}Jz​ and dimerization $\ensuremath{\delta}$, we map out their phase diagrams and identify transition lines via entanglement entropy and magnetization measurements. In the two-site model, we observe the emergence of an interaction-induced antiferromagnetic intermediate phase between the topologically trivial and nontrivial regimes, as well as a critical region at negative Jz{J}_{z}Jz​ with suppressed magnetization and finite-size scaling of entanglement entropy. In contrast, the three-site model lacks an intermediate phase and exhibits asymmetric edge localization and antiferromagnetic ordering in both positive and negative Jz{J}_{z}Jz​ regimes. We further examine the response of edge states to Ising perturbations. In the two-site model, zero-energy edge modes are topologically protected and remain robust up to a finite interaction strength. However, in the three-site model, where the edge states reside at finite energy, this protection breaks down. Despite this, the edge-localized nature of these states survives in the form of polarized modes whose spatial profiles reflect the noninteracting limit.
The antiferromagnetic Heisenberg spin chain with integer spin has a short-range magnetic order and an excitation energy gap above the ground state. This so-called Haldane gap is proportional to the exchange coupling constant J of the Heisenberg chain. In this study, we discuss recent results about the spin dependence of the Haldane gap and conjecture an analytical formula valid asymptotically for large spin values. We then discuss the robustness of the edge states of the spin-1 chain by studying a spin-1 ladder using the density matrix renormalization group (DMRG) algorithm. We show that the peculiar hidden topological order of the spin-1 chain disappears smoothly by increasing the ladder rung coupling without any intervening phase transition. This is evidence for the fragile character of the topological order of the spin-1 chain.
Quantum quenches drive edge states far from equilibrium, yet whether the memory of a topological initial state survives in a non-integrable, interacting system has remained largely unexplored. We study this question in the bond-alternating XXZ chain---an interacting Su--Schrieffer--Heeger model hosting symmetry-protected topological edge modes with markedly enhanced boundary magnetization---and analyze quenches across all combinations of single-particle and many-body initial and final Hamiltonians. The results organize by a single distinction as we rigorously establish in this work: whether the post-quench Hamiltonian is free or genuinely interacting. For a free post-quench Hamiltonian, the dynamics is solved exactly by a correlation-matrix approach; the boundary-mode return amplitude decays as t−3/2t^{-3/2}t−3/2, and initial interactions enter only through a dressed one-body density matrix. For a genuinely interacting post-quench Hamiltonian, finite-time stability bounds prove that away from local resonances the first-dimer magnetization remains stable on time windows growing as arbitrarily large powers of the inverse inter-dimer coupling. Matrix product state simulations across all four protocols show that interactions in the final Hamiltonian markedly extend finite-time boundary memory---with local suppression near the isotropic SU(2)SU(2)SU(2) point---revealing a many-body protection mechanism in a non-integrable system where scrambling would otherwise wash out initial-state memory fast.
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