A rigid commensurate host and a defect that switches: vacancies in the frustrated alternating spin-(1, 1/2) Heisenberg chain
Abstract
The alternating spin-(1,1/2) Heisenberg chain with a frustrating next-nearest-neighbor coupling J2 loses its Lieb--Mattis moment through a discontinuous transition to a singlet whose nature was left open. Using an SU(2)-symmetric density-matrix renormalization group, in which the total spin is a quantum-number label, we identify that phase and follow a single vacancy through it. The total spin collapses directly from S=L/4 to S=0, with no partially polarized window. The singlet phase is critical: its singlet--triplet gap closes as 1/L, its entanglement entropy grows logarithmically with a central charge c≃1.8, and its modulation wavevector, extrapolated from L=80 to $200$, is commensurate at 3π/4 and independent of J2 to 5×10−5. Removing the central S=1 site switches the ground state of the doped chain from a triplet to a singlet at J2∗=0.3068(3), where the host shows no feature; the triplet is not bound to the vacancy but spread over one of the two segments it leaves. The transmission of correlations through the vacancy saturates at a finite value below J2∗, T(∞)=0.102(15) and 0.036(9) at J2=0.28 and $0.30$, and the cut-chain law T∝ L−1 is excluded on both sides. We also show that the longitudinal correlator is not an observable in a triplet sector.
Related
- Mutual information across the commensurate--incommensurate crossover and near the Kosterlitz--Thouless point of the spin-1/2 J1--J2 Heisenberg chain
- Emergent Wigner magnon crystals in a fully frustrated Heisenberg four-leg tube
- Microwave spectroscopic evidence of a moiré-commensurate electron crystal in monolayer graphene
- Field-induced incommensurate--commensurate transitions in anisotropic magnets
- Automated Generation of Commensurate Magnetic Structures based on Spin Space Groups and Graph Theory
BibTeX
@misc{silva2026rigid,
title = {{A rigid commensurate host and a defect that switches: vacancies in the frustrated alternating spin-(1, 1/2) Heisenberg chain}},
author = {R. L. Silva and R. C. Silva and A. R. Pereira},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2610.22140},
eprint = {2610.22140},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2610.22140},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - A rigid commensurate host and a defect that switches: vacancies in the frustrated alternating spin-(1, 1/2) Heisenberg chain
AU - R. L. Silva
AU - R. C. Silva
AU - A. R. Pereira
PY - 2026
DA - 2026/10/15
PB - arXiv
DO - 10.48550/arxiv.2610.22140
UR - https://arxiv.org/abs/2610.22140
AB - The alternating spin-(1,1/2) Heisenberg chain with a frustrating next-nearest-neighbor coupling J_2 loses its Lieb--Mattis moment through a discontinuous transition to a singlet whose nature was left open. Using an SU(2)-symmetric density-matrix renormalization group, in which the total spin is a quantum-number label, we identify that phase and follow a single vacancy through it. The total spin collapses directly from S=L/4 to S=0, with no partially polarized window. The singlet phase is critical: its singlet--triplet gap closes as 1/L, its entanglement entropy grows logarithmically with a central charge c≃1.8, and its modulation wavevector, extrapolated from L=80 to $200$, is commensurate at 3π/4 and independent of J_2 to 5×10^−5. Removing the central S=1 site switches the ground state of the doped chain from a triplet to a singlet at J_2^∗=0.3068(3), where the host shows no feature; the triplet is not bound to the vacancy but spread over one of the two segments it leaves. The transmission of correlations through the vacancy saturates at a finite value below J_2^∗, T(∞)=0.102(15) and 0.036(9) at J_2=0.28 and $0.30$, and the cut-chain law T∝ L^−1 is excluded on both sides. We also show that the longitudinal correlator is not an observable in a triplet sector.
ER -