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arXiv:1608.08172 · Nature Communications

Field induced spontaneous quasiparticle decay and renormalization of quasiparticle dispersion in a quantum antiferromagnet

Tao Hong, Yiming Qiu, M. Matsumoto, D. M. Tennant, K. Coester, Kai Phillip Schmidt, Firas F. Awwadi, Mark M. Turnbull, Harish K. Agrawal, A. L. Chernyshev

Published May 5, 2017• 32 citations
arXivHTMLPDF

Abstract

Abstract The notion of a quasiparticle, such as a phonon, a roton or a magnon, is used in modern condensed matter physics to describe an elementary collective excitation. The intrinsic zero-temperature magnon damping in quantum spin systems can be driven by the interaction of the one-magnon states and multi-magnon continuum. However, detailed experimental studies on this quantum many-body effect induced by an applied magnetic field are rare. Here we present a high-resolution neutron scattering study in high fields on an S=1/2 antiferromagnet C9H18N2CuBr4. Compared with the non-interacting linear spin–wave theory, our results demonstrate a variety of phenomena including field-induced renormalization of one-magnon dispersion, spontaneous magnon decay observed via intrinsic linewidth broadening, unusual non-Lorentzian two-peak structure in the excitation spectra and a dramatic shift of spectral weight from one-magnon state to the two-magnon continuum.

DOI
10.1038/ncomms15148

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Description of quasiparticle decay by continuous unitary transformations

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In sufficiently dimerized quantum antiferromagnets the elementary excitations are given by gapped spin S = 1 triplon quasiparticles. Although these triplons are protected by a gap at low energies they may decay spontaneously at higher energies where the one-triplon dispersion merges with the two-triplon continuum. First, we illustrate and characterize such a spontaneous decay in low-dimensional quantum antiferromagnets on the basis of a simple one-dimensional bosonic model. The decay implies the breakdown of the quasiparticle picture. No quantitative description by a Lorentzian resonance is possible. In particular, three qualitatively different scenarios are identified depending on the one-triplon dispersion and the two-triplon interaction. To describe unstable quasiparticles in more complex models we introduced an adapted generator for continuous unitary transformations. Its general properties are derived and discussed. Next, we investigate asymmetric antiferromagnetic and asymmetric ferroantiferromagnetic spin S = ½ Heisenberg ladders, which allow for spontaneous triplon decay, to illustrate this approach. Results for the low-energy spectra and the dynamical structure factors for these systems are presented. We show that quasiparticle decay is more pronounced in the case of ferro-antiferromagnetic ladders than in solely antiferromagnetic ladders. Finally, we use continuous unitary transformations and high temperature series expansions to determine a quantitative model for the compound IPA-CuCl3 based on data of inelastic neutron scattering and measurements of the magnetic susceptibility. Our calculations reveal IPA-CuCl3 as system of coupled asymmetric spin S = ½ Heisenberg ladders with the four magnetic couplings J 1 ˜ -2.3 meV, J2 ˜ 1.2 meV, J3 ˜ 2.9 meV and J4 ˜ -0.3 meV. Based on this microscopic model for IPA-CuCl3 the adapted continuous unitary transformation is used to describe the quasiparticle decay in IPA-CuCl3. The results agree very well with the experimental data. In addition the magnetic field dependence of the lowest modes in the condensed phase as well as the temperature dependence of the gap without magnetic field corroborate our microscopic model.

•Jul 16, 2012•3 citations
Abstract The notion of a quasiparticle, such as a phonon, a roton or a magnon, is used in modern condensed matter physics to describe an elementary collective excitation. The intrinsic zero-temperature magnon damping in quantum spin systems can be driven by the interaction of the one-magnon states and multi-magnon continuum. However, detailed experimental studies on this quantum many-body effect induced by an applied magnetic field are rare. Here we present a high-resolution neutron scattering study in high fields on an S=1/2 antiferromagnet C9H18N2CuBr4. Compared with the non-interacting linear spin–wave theory, our results demonstrate a variety of phenomena including field-induced renormalization of one-magnon dispersion, spontaneous magnon decay observed via intrinsic linewidth broadening, unusual non-Lorentzian two-peak structure in the excitation spectra and a dramatic shift of spectral weight from one-magnon state to the two-magnon continuum.
Abstract Quantum magnets in two dimensions display strong quantum interaction effects even when magnetically ordered. Using the metal-organic framework material CuF 2 (D 2 O) 2 (pyz), we investigate the field-dependent spin dynamics of the S = 1/2 square-lattice Heisenberg antiferromagnet by high-resolution inelastic neutron scattering to applied fields beyond one third of saturation. We discover an anomalously sharp, dispersive “shadow mode” residing within the two-magnon continuum, which shadows the dispersion of the transverse one-magnon branches across the Brillouin zone at an offset equal to the Larmor energy. We perform cylinder matrix-product-state (MPS) calculations that reproduce the field-induced spectrum quantitatively and apply a spectrally consistent 1/ S spin-wave theory to deduce that the “Larmor-shadow mode” is a composite two-magnon resonance: a dispersing magnon at wavevector Q couples to the uniform Larmor precession at Γ, its small intrinsic linewidth indicating a non-perturbative effect of attractive magnon-magnon interactions. Another quantum-fluctuation phenomenon, the zero-field (π, 0) anomaly, is lost at increasing fields, which tighten the spectral weight into the one-magnon and Larmor-shadow modes. To our knowledge, these results constitute the first observation of a sharp quasi-bound state embedded in the continuum of a gapless two-dimensional antiferromagnet.
A theoretical overview of the phenomenon of spontaneous magnon decays in quantum antiferromagnets is presented. The intrinsic zero-temperature damping of magnons in quantum spin systems is a fascinating many-body effect, which has recently attracted significant attention in view of its possible observation in neutron-scattering experiments. An introduction to the theory of magnon interactions and a discussion of necessary symmetry and kinematic conditions for spontaneous decays are provided. Various parallels with the decays of anharmonic phonons and excitations in superfluid 4He^{4}\mathrm{He}4He are extensively used. Three principal cases of spontaneous magnon decays are considered: field-induced decays in Heisenberg antiferromagnets, zero-field decays in spiral antiferromagnets, and triplon decays in quantum-disordered magnets. Analytical results are compared with available numerical data and prospective materials for experimental observation of the decay-related effects are briefly discussed.
Quantum magnets in two dimensions display strong quantum interaction effects even when magnetically ordered. Using the metal-organic framework material CuF2_22​(D2_22​O)2_22​(pyz), we investigate the field-dependent spin dynamics of the S=1/2S = 1/2S=1/2 square-lattice Heisenberg antiferromagnet by high-resolution inelastic neutron scattering to applied fields beyond one third of saturation. We discover an anomalously sharp, dispersive ``shadow mode'' residing within the two-magnon continuum, which shadows the dispersion of the transverse one-magnon branches across the Brillouin zone at an offset equal to the Larmor energy. We perform cylinder matrix-product-state (MPS) calculations that reproduce the field-induced spectrum quantitatively and apply a spectrally consistent 1/S1/S1/S spin-wave theory to deduce that the ``Larmor-shadow mode'' is a composite two-magnon resonance: a dispersing magnon at wavevector Q{\bf Q}Q couples to the uniform Larmor precession at ΓΓΓ, its small intrinsic linewidth indicating a non-perturbative effect of attractive magnon-magnon interactions. Another quantum-fluctuation phenomenon, the zero-field (π,0)(π,0)(π,0) anomaly, is lost at increasing fields, which tighten the spectral weight into the one-magnon and Larmor-shadow modes. To our knowledge, these results constitute the first observation of a sharp quasi-bound state embedded in the continuum of a gapless two-dimensional antiferromagnet.
Abstract Dynamics in a quantum material is described by quantized collective motion: a quasiparticle. The single-quasiparticle description is useful for a basic understanding of the system, whereas a phenomenon beyond the simple description such as quasiparticle decay which affects the current carried by the quasiparticle is an intriguing topic. The instability of the quasiparticle is phenomenologically determined by the magnitude of the repulsive interaction between a single quasiparticle and the two-quasiparticle continuum. Although the phenomenon has been studied in several materials, thermodynamic tuning of the quasiparticle decay in a single material has not yet been investigated. Here we show, by using neutron scattering, magnetic field control of the magnon decay in a quantum antiferromagnet RbFeCl3, where the interaction between the magnon and continuum is tuned by the field. At low fields where the interaction is small, the single magnon decay process is observed. In contrast, at high fields where the interaction exceeds a critical magnitude, the magnon is pushed downwards in energy and its lifetime increases. Our study demonstrates that field control of quasiparticle decay is possible in the system where the two-quasiparticle continuum covers wide momentum-energy space, and the phenomenon of the magnon avoiding decay is ubiquitous.
Although the search for quantum many-body phenomena in magnetic materials has a strong focus on highly frustrated systems, even unfrustrated quantum magnets show a multitude of unconventional phenomena in their spin excitation spectra. YbBr_{3} is an excellent realization of the S=1/2 antiferromagnetic Heisenberg model on the honeycomb lattice, and we have performed detailed spectroscopic experiments with both unpolarized and polarized neutrons at all applied magnetic fields up to saturation. We observe extensive excitation continua, which cause strong renormalization and the decay of single magnons at higher fields, while coherent features include field-induced "shadows" of the single magnons and the spectacular emergence of a rotonlike excitation. To guide and interpret our experiments, we performed systematic calculations by the method of cylinder matrix-product states that provide quantitative agreement with the neutron scattering data and a qualitative benchmark for the spectral signatures of strong quantum fluctuations even in the absence of magnetic frustration.
In sufficiently dimerized quantum antiferromagnets the elementary excitations are given by gapped spin S = 1 triplon quasiparticles. Although these triplons are protected by a gap at low energies they may decay spontaneously at higher energies where the one-triplon dispersion merges with the two-triplon continuum. First, we illustrate and characterize such a spontaneous decay in low-dimensional quantum antiferromagnets on the basis of a simple one-dimensional bosonic model. The decay implies the breakdown of the quasiparticle picture. No quantitative description by a Lorentzian resonance is possible. In particular, three qualitatively different scenarios are identified depending on the one-triplon dispersion and the two-triplon interaction. To describe unstable quasiparticles in more complex models we introduced an adapted generator for continuous unitary transformations. Its general properties are derived and discussed. Next, we investigate asymmetric antiferromagnetic and asymmetric ferroantiferromagnetic spin S = ½ Heisenberg ladders, which allow for spontaneous triplon decay, to illustrate this approach. Results for the low-energy spectra and the dynamical structure factors for these systems are presented. We show that quasiparticle decay is more pronounced in the case of ferro-antiferromagnetic ladders than in solely antiferromagnetic ladders. Finally, we use continuous unitary transformations and high temperature series expansions to determine a quantitative model for the compound IPA-CuCl3 based on data of inelastic neutron scattering and measurements of the magnetic susceptibility. Our calculations reveal IPA-CuCl3 as system of coupled asymmetric spin S = ½ Heisenberg ladders with the four magnetic couplings J 1 ˜ -2.3 meV, J2 ˜ 1.2 meV, J3 ˜ 2.9 meV and J4 ˜ -0.3 meV. Based on this microscopic model for IPA-CuCl3 the adapted continuous unitary transformation is used to describe the quasiparticle decay in IPA-CuCl3. The results agree very well with the experimental data. In addition the magnetic field dependence of the lowest modes in the condensed phase as well as the temperature dependence of the gap without magnetic field corroborate our microscopic model.