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arXiv:0904.0123 · Physical Review B

Quantum magnets with weakly confined spinons: Multiple length scales and quantum impurities

R. L. Doretto, Matthias Vojta C

Published Jul 10, 2009• 14 citations
arXivHTMLPDF

Abstract

In magnets with strong quantum fluctuations, paramagnetic ground states with or without confinement of spinon excitations can be realized. Here we discuss the physics of the confined phase in the vicinity of a confinement-deconfinement transition: this regime, likely relevant to a multitude of frustrated spin systems, is characterized by multiple length scales. In addition to the magnetic correlation length, a confinement length can be defined, which can be probed, e.g., by local static measurements near nonmagnetic impurities. We illustrate the ideas by explicit calculations for dimerized spin chains, but our qualitative results remain valid in higher dimensions as well. In particular, we study crossover from weak to strong confinement visible in the antiferromagnetic polarization cloud around a nonmagnetic impurity. We also discuss the effective magnetic interaction between impurities, relevant for impurity-induced magnetic order, and consequences for nuclear magnetic resonance and neutron-scattering experiments.

DOI
10.1103/physrevb.80.024411

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In magnets with strong quantum fluctuations, paramagnetic ground states with or without confinement of spinon excitations can be realized. Here we discuss the physics of the confined phase in the vicinity of a confinement-deconfinement transition: this regime, likely relevant to a multitude of frustrated spin systems, is characterized by multiple length scales. In addition to the magnetic correlation length, a confinement length can be defined, which can be probed, e.g., by local static measurements near nonmagnetic impurities. We illustrate the ideas by explicit calculations for dimerized spin chains, but our qualitative results remain valid in higher dimensions as well. In particular, we study crossover from weak to strong confinement visible in the antiferromagnetic polarization cloud around a nonmagnetic impurity. We also discuss the effective magnetic interaction between impurities, relevant for impurity-induced magnetic order, and consequences for nuclear magnetic resonance and neutron-scattering experiments.
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We investigate the effective interaction between two localized spin impurities embedded in a frustrated spin-12\frac {1}{2}21​ J1J_1J1​–J2J_2J2​ Heisenberg chain. Treating the impurity spins as classical moments coupled locally to the host, we combine second-order perturbation theory with large-scale density matrix renormalization group (DMRG) calculations to determine the impurity–impurity interaction as a function of separation, coupling strength, and magnetic frustration. In the weak-coupling regime, we show that the interaction is governed by the static spin susceptibility of the host and exhibits oscillatory power-law decay in the gapless phase, modified by universal logarithmic corrections at the SU(2)-symmetric critical point. In the gapped dimerized phase, the interaction decays exponentially with distance. For intermediate and strong impurity–host coupling, we observe a crossover to a boundary-dominated regime characterized by pronounced parity effects associated with the length of the chain segment between impurities, signaling a breakdown of the simple RKKY-like description. Our results establish impurity–impurity interactions as a sensitive probe of frustrated quantum spin liquids and provide a controlled framework for distinguishing gapless and gapped phases through local perturbations. Abstract Published by the Jagiellonian University 2026 authors
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Finite-size effects are ubiquitous in quantum matter, yet typically invisible in the thermodynamic limit; when system size is restricted, boundaries and confinement reveal modified spectra, correlations, and emergent phenomena. With growing access to well-controlled few-spin systems, these effects are now directly observable and offer a route to probing universal physics and enabling quantum information applications. Here, we study finite-size physics in the quasi-one-dimensional Ising-like quantum antiferromagnet Ba(Co1−x_{1-x}1−x​Mgx_xx​)2_22​V2_22​O8_88​ with x=2%x = 2\%x=2% and x=5%x = 5\%x=5%, where non-magnetic Mg2+^{2+}2+ ions fragment the Co2+^{2+}2+ spin chains into finite segments. Combining inelastic neutron scattering and numerical calculations, we identify distinct regimes with fundamentally different low-energy quantum excitations: spinons confined to chain boundaries, and a cluster mode in shorter chains corresponding to a collective reversal of the Néel-ordered segment without classical analogue. Our results demonstrate how chemical substitution acts as a quantum microscope providing direct access to boundary-shaped excitations in strongly correlated spin systems.
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The infinite Heisenberg XXZ spin-12\frac{1}{2}21​ chain in the gapped antiferromagnetic regime has two degenerate vacua and kink topological excitations (which are also called spinons) interpolating between these vacua as elementary excitations. Application of an arbitrary weak staggered longitudinal magnetic field hhh induces a long-range attractive potential between two adjacent spinons leading to their confinement into ``meson'' bound states. Exploiting the integrability of the XXZ model in the deconfined phase h=0h=0h=0, we perform perturbative calculations of the energy spectra of the two-spinon bound states in the weak confinement regime at $h\ensuremath{\rightarrow}+0$, using the strength of the staggered magnetic field hhh as a small parameter. Both transverse and longitudinal dynamical structure factors of the local spin operators are calculated as well in the two-spinon approximation in the weak confinement regime to the leading order in hhh.