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Journal of the Physical Society of Japan

Spin-wave Dynamics of Percolating Heisenberg Antiferromagnets

Kousuke Yakubo, Takamichi Terao, Tsuneyoshi Nakayama

Published Sep 15, 1994• 11 citations
Publisher

Abstract

The fracton dynamics of percolating Heisenberg antiferromagnets are studied in terms of a dynamic scaling argument and large-scale numerical analysis. It is shown that the spectral (or fracton) dimension for antiferromagnetic fractons, \(\tilde{d}_{\rm AF}\), is bounded by the relation \(\tilde{d}_{\rm AF}\le 1\). The densities of states (DOS) are calculated for very large percolating antiferromagnets at the critical concentration p c formed on d =2, 3, and 4 simple cubic lattices. It is concluded, from the calculated DOS's, that the spectral dimensions \(\tilde{d}_{\rm AF}\) are very close to unity for any Euclidean dimension d (≥2). We have also calculated the DOS for percolating antiferromagnets above p c in order to clarify the crossover behavior from extended magnons to fractons. In addition, the dynamical structure factor S ( q ,ω) is investigated both analytically and numerically. It is found, postulating a single-length scaling, that the asymptotic behaviors can be characterized by two exponents, the dynamical exponent z AF and a new exponent y . The numerical results for S ( q ,ω) support the validity of the single-length-scale postulate.

DOI
10.1143/jpsj.63.3431

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The fracton dynamics of percolating Heisenberg antiferromagnets are studied in terms of a dynamic scaling argument and large-scale numerical analysis. It is shown that the spectral (or fracton) dimension for antiferromagnetic fractons, (\tilde{d}{\rm AF}), is bounded by the relation (\tilde{d}{\rm AF}\le 1). The densities of states (DOS) are calculated for very large percolating antiferromagnets at the critical concentration p c formed on d =2, 3, and 4 simple cubic lattices. It is concluded, from the calculated DOS's, that the spectral dimensions (\tilde{d}_{\rm AF}) are very close to unity for any Euclidean dimension d (≥2). We have also calculated the DOS for percolating antiferromagnets above p c in order to clarify the crossover behavior from extended magnons to fractons. In addition, the dynamical structure factor S ( q ,ω) is investigated both analytically and numerically. It is found, postulating a single-length scaling, that the asymptotic behaviors can be characterized by two exponents, the dynamical exponent z AF and a new exponent y . The numerical results for S ( q ,ω) support the validity of the single-length-scale postulate.
We report the observations of the dynamical structure factors S(q,ω) for antifferomagnetic fractons in diluted three-dimensional (3d) and two-dimensional (2d) Heisenberg systems, RbMn0.4Mg0.6F3 and Rb2Mn0.598Mg0.402F4, with a magnetic concentration close to the percolation concentration, which were obtained by means of high-resolution (ΔE = 17.5 μeV) inelastic neutron scattering experiments. The peak intensity A(q) and the dispersion relation E(q) show the clear scaling laws following to A(q) ∝ q−y with y = 2.9 ± 0.1 and E(q) ∝ qz with z = 2.5 ± 0.1 for the 3d system, and y = 2.9 ± 0.2 and z = 1.8 ± 0.2 for the 2d system. The validity of the single-length-scaling postulate (SLSP) for S(q,ω) are demonstrated, for the first time. In addition, we show that the spectral dimension of antiferromagnetic fractons is unity independent of the embedding Euclidean dimension of the systems. These values are consistent with the theoretical predictions.
The scaling behavior of the dynamical structure factor S(q,\ensuremath{\omega}) of percolating Heisenberg antiferromagnets is investigated in terms of a dynamic scaling argument and numerical calculations. It is found, based on the single-length-scaling postulate, that the asymptotic behavior of S(q,\ensuremath{\omega}) can be characterized by only two exponents: the dynamic exponent za{\mathit{z}}_{\mathit{a}}za​ and a new exponent y. This theoretical prediction is confirmed by direct numerical calculations. The values of the exponents za{\mathit{z}}_{\mathit{a}}za​ and y are determined for the case of d=2 percolating Heisenberg antiferromagnets.
The dynamic structure factors S ( q ,ω) of an ideal percolating network, the three-dimensional (3 d ) dilute Heisenberg antiferromagnet RbMn 0.4 Mg 0.6 F 3 , obtained from high resoluton (Δ E = 17.5 µeV) inelastic neutron scattering (INS) experiments are analyzed for the first time within the framework of the single-length-scaling postulate (SLSP). The analysis confirms the validity of the SLSP and is also used to extract the values of the key exponents governing the spin dynamics, the dynamic exponent (zAF=Df/d~AFz_{\text{AF}} = D_{\text{f}}/\tilde{d}_{\text{AF}}zAF​=Df​/d~AF​) being 2.5 ±0.1 and the spectral dimension d~AF\tilde{d}_{\text{AF}}d~AF​ for antiferromagnetic (AFM) fractons taking a value of unity.
It is found, through simulation of the density of states on a large scale, that the spectral dimensions d~AF\tilde{d}_{\rm AF}d~AF​ of percolating Heisenberg antiferromagnets take a value very close to unity independent of Euclidean dimension d . This finding is interpreted in terms of a dynamic scaling argument.
The dynamical structure factors, S ( q ,ω), for antiferromagnetic fractons in diluted three-dimensional (3D) and two-dimensional (2D) Heisenberg systems, RbMn 0.4 Mg 0.6 F 3 and Rb 2 Mn 0.598 Mg 0.402 F 4 , respectively, with a magnetic concentration close to the percolation concentration, were observed through high-resolution (Δ E = 17.5 µeV) inelastic neutron scattering experiments well below T N . The exponents describing the peak intensity ( A ( q ) ∝ q - y ) and the dispersion relation ( E ( q ) ∝ q z ) were determined to be y = 2.9 ±0.1 and z = 2.5 ±0.1 for the 3D system and y = 2.9 ±0.2 and z = 1.8 ±0.2 for the 2D system, respectively. In addition, the validity of the single-length-scaling postulate (SLSP) was confirmed for S ( q ,ω) by using the above values of y and z . We experimentally demonstrated, for the first time, that the spectral dimension, d~AF=1\tilde{d}_{\text{AF}} = 1d~AF​=1, is universal and independent of the embedding Euclidean dimensions of the systems.
The dynamical structure factor S(q, ω) of Heisenberg antiferromagnets with random fractal structures is investigated in terms of a dynamical scaling argument. It is found that the asymptotic behavior of S(q, ω) can be characterized by the dynamic exponent za and a new exponent y. We have performed numerical calculations of S(q, ω) in order to confirm its predicted scaling behaviors for d=2 percolating antiferromagnets. The calculated values for the exponents y and za are consistent with our theoretical prediction.