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Physical review. B./Physical review. B

Unconventional criticality and phase diagram of a tetramer Heisenberg bond-alternating chain with single-ion anisotropy

Luan M. Veríssimo, Jordana Torrico, S. M. de Souza, Onofre Rojas, Maria S. S. Pereira, M. L. Lyra

Published Nov 8, 2025• 1 citation
Publisher

Abstract

We investigate the phase diagram of a quantum mixed-spin $(1/2,1/2,1,1)$ Heisenberg tetramer chain featuring bond-alternating antiferromagnetic nearest-neighbor exchange interactions and single-ion anisotropy. This model exhibits a rich phase structure comprising three gapped phases: the antiferromagnetic, topological valence-bond-solid (TVBS), and the trivial nonmagnetic phases (TnM). We identify a multicritical Wess-Zumino-Witten $SU{(2)}_{1}$ point where two Ising critical lines merge, forming a higher-symmetry point that realizes conformal embedding on the lattice. Beyond this multicritical point, a Gaussian critical line emerges, determining the boundary between the TVBS and the TnM phases. Utilizing the tangential finite-size scaling method, we demonstrate the precision with which critical points and correlation length critical exponents can be extracted along this Gaussian line. Our results reveal rich critical phenomena induced by the interplay of bond-alternating exchange and anisotropic single-ion interactions in quantum many-body systems.

DOI
10.1103/r5dq-sgm6

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We investigate the phase diagram of a quantum mixed-spin (1/2,1/2,1,1)(1/2,1/2,1,1)(1/2,1/2,1,1) Heisenberg tetramer chain featuring bond-alternating antiferromagnetic nearest-neighbor exchange interactions and single-ion anisotropy. This model exhibits a rich phase structure comprising three gapped phases: the antiferromagnetic, topological valence-bond-solid (TVBS), and the trivial nonmagnetic phases (TnM). We identify a multicritical Wess-Zumino-Witten SU(2)1SU{(2)}_{1}SU(2)1​ point where two Ising critical lines merge, forming a higher-symmetry point that realizes conformal embedding on the lattice. Beyond this multicritical point, a Gaussian critical line emerges, determining the boundary between the TVBS and the TnM phases. Utilizing the tangential finite-size scaling method, we demonstrate the precision with which critical points and correlation length critical exponents can be extracted along this Gaussian line. Our results reveal rich critical phenomena induced by the interplay of bond-alternating exchange and anisotropic single-ion interactions in quantum many-body systems.
The interplay between competing interactions in low-dimensional quantum many-body systems gives rise to a wide variety of quantum phases and critical phenomena. Motivated by this scenario, we investigate the quantum phase diagram of the mixed-spin 1 2 , 1 antiferromagnetic Heisenberg chain with modulated single-ion anisotropy by means of the density matrix renormalization group (DMRG) method. The resulting phase diagram comprises four distinct quantum phases: three gapped phases, namely the 1 / 3 magnetization plateau, the trivial nonmagnetic (TnM) phase, and the topological valence-bond-solid (TVBS) phase, together with a gapless Luttinger liquid (LL) phase. The different phases are characterized through the analysis of magnetization, spin correlation functions, nonlocal order parameters, and entanglement-based observables. The TVBS phase is identified by the presence of exponentially localized edge states, finite values of the nonlocal string order parameter and twist operator, and the characteristic degeneracy pattern of the entanglement spectrum. The boundaries of the LL phase are determined from the transverse spin correlation function and the corresponding Luttinger parameter, revealing two Kosterlitz–Thouless transition lines characterized by the critical value K c = 2.0 , in agreement with finite-size scaling analyses of the spin gap. In addition, we identify a Gaussian critical line associated with the topological quantum phase transition separating the TVBS and TnM phases. By employing the tangential finite-size scaling approach, we accurately determine this critical line by extracting the correlation-length critical exponent ν and the conformal field theory central charge c . Our results demonstrate that the competition between exchange interactions and modulated single-ion anisotropy gives rise to a rich interplay between topology, quantum criticality, and magnetic ordering, highlighting the fundamental role of these interactions in determining the ground-state properties of low-dimensional quantum spin systems.
We report a phase diagram of the antiferromagnetic spin-1 chain with nearest-neighbor Heisenberg and three-site interactions in the presence of single-ion anisotropy. We show that the Gaussian and Ising transitions that separate the topological Haldane phase from the two anisotropic phases eventually fuse into a higher symmetry point characterized by the Wess-Zumino-Witten (WZW) SU(2)2_22​ critical theory providing a lattice realization of the conformal embedding. On the other side of the WZW multi-critical point, the Ising critical line reappears together with the eight-vertex transition. This transition is a one-dimensional realization of a deconfined quantum criticality separating the dimerized and Ising antiferromagnetic phases - two ordered phases with incompatible order parameters.
= 1 andS=3/2dimers is studied in detail as a function of two considered exchange interactions ascribed to similar and dissimilar spin pairs. At zero magnetic field, the ferrimagnetic mixed spin-(1, 1, 3/2, 3/2) Heisenberg tetramer chain displays, depending on a mutual interplay between two considered exchange interactions, three distinct gapped valence-bond-solid phases separated by gap-closing quantum critical points. Using density-matrix renormalization group calculations we construct the full ground-state phase diagram as a function of the interaction ratio and magnetic field, which exhibits besides three gapped valence-bond-solid phases special Kosterlitz-Thouless and topological quantum critical points. A tangential finite-size scaling analysis is employed to obtain precise estimates of the zero-field valence-bond-solid transitions and unveil their common logarithmic correction to a power-law scaling of the correlation length.
Scaling aspects of Gaussian topological phase transitions in quantum spin chains are investigated using the prototypical one-dimensional spin-1 XXZ Heisenberg model with uniaxial single-ion anisotropy DDD. This model presents a critical line separating the gapped Haldane and large-DDD phases, with the relevant energy gap closing at the transition point. We show that a proper tangential finite-size scaling analysis is able to accurately locate the Gaussian critical line and to probe the continuously varying set of correlation length critical exponents. The specific features of the tangential scaling are highlighted, in contrast to the standard scaling holding in the Ising-like transition between the gapless antiferromagnetic N\'eel and gapped Haldane phases. Our results are compared with field-theoretic predictions and available high-accuracy data for specific points along the Gaussian line.
Long-range interactions can fundamentally reshape the low-energy properties of low-dimensional quantum matter, altering both continuous symmetry breaking and topological phenomena. However, their impact on the quantum criticality separating these regimes remains poorly understood. We determine the ground-state phase diagram and critical properties of the spin-one Heisenberg chain with single-ion anisotropy and staggered antiferromagnetic power-law interactions, using matrix-product state (MPS) calculations complemented by high-order series expansions (pCUT+MC). Such long-range, non-frustrated interactions circumvent the Hohenberg-Mermin-Wagner theorem, thereby stabilizing continuous symmetry breaking (CSB) phases in direct competition with the Haldane phase. We map out the resulting phase diagram and analyze the entanglement entropy scaling behavior in the U(1) and SU(2) CSB phases, finding logarithmic corrections beyond the short-range, universal contributions expected from linearly dispersed Goldstone modes. We further characterize all critical boundaries through finite-size scaling of either the entanglement entropy or the staggered magnetization. In particular, the large-D-to-U(1)-CSB transition exhibits unconventional, continuously varying critical exponents as a function of the long-range decay exponent with a strong dependence on the imposed boundary conditions leading to distinct finite-size scalings for sufficiently long-range potentials. Remarkably, the Haldane-to-U(1)-CSB transition likewise displays unconventional quantum criticality with distinct continuously varying critical exponents. Our work positions this model as a target for near-term atomic platforms with tunable long-range couplings and exhibiting natural single-ion anisotropy, offering a minimal playground for exploring the interplay between long-range interactions, continuous symmetry breaking, and topology.
One dimensional S =1 antiferromagnetic Heisenberg chains with bond alternation and uniaxial single-ion-type anisotropy are studied by numerical exact diagonalization of finite size systems. We calculate the ground state phase diagram using the twisted boundary condition method for D ≥0. The ground states of this model contain the Haldane state, large- D state and the dimer state. Using conformal field theory and the level spectroscopy method, we calculate the D and δ-dependence of the Luttinger liquid paraments K and the critical exponent ν of the energy gap.