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Spinons and triplons in spatially anisotropic frustrated antiferromagnets

Abstract

The search for elementary excitations with fractional quantum numbers is a central challenge in modern condensed-matter physics. It has long been speculated that two-dimensional frustrated magnets might support quantum disordered states with neutral spin-1/2 excitations known as spinons. Despite decades of search, however, no clear experimental examples have been found. We explore the possibility for several materials using a realistic model, the spin-1/2 spatially anisotropic frustrated Heisenberg antiferromagnet in two dimensions. Here, we derive an effective Schrödinger equation valid in the weak interchain coupling regime. The dynamical spin correlations from this approach agree quantitatively without fitting parameters with inelastic neutron measurements of the triangular antiferromagnet Cs2CuCl4. In such antiferromagnets, the spectrum is composed of an incoherent continuum arising from the effects of one-dimensional spinons of individual chains, and a sharp dispersing peak, due to coherently propagating ‘triplon’ bound states of two spinons. We argue that triplons are generic features of spatially anisotropic frustrated antiferromagnets, which arise because the bound spinon pair lowers its kinetic energy by propagating between chains.

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Figure 1: Lattice structure and coupling constants J1, J2, J3 and J.
Figure 2: Dynamical structure factor S(k,ω) for J′(k)<0 and J′(k)>0.
Figure 3: Comparison with the experimental result for dynamical structure factor S(k,ω) at kx=π.
Figure 4: Comparison of S(k,ω) with neutron scattering data on Cs2CuCl4.

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Acknowledgements

We would like to thank J. Alicea, M. P. A. Fisher and R. Shindou for discussions. This work is supported by the Grant-in-aid for Scientific Research (C) No. 10354143 from MEXT, Japan (M.K.), the Petroleum Research Fund ACS PRF 43219-AC10 (O.S.), NSF grant/DMR-0457440 (L.B.) and the Packard Foundation (L.B.). Part of this research was completed at KITP and supported in part by the NSF under Grant No. PHY05-51164.

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Correspondence to Leon Balents.

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Kohno, M., Starykh, O. & Balents, L. Spinons and triplons in spatially anisotropic frustrated antiferromagnets. Nature Phys 3, 790–795 (2007). https://doi.org/10.1038/nphys749

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