Excitation spectra of disordered dimer magnets near quantum criticality

Abstract

For coupled-dimer magnets with quenched disorder, we introduce a generalization of the bond-operator method, appropriate to describe both singlet and magnetically ordered phases. This allows for a numerical calculation of the magnetic excitations at all energies across the phase diagram, including the strongly inhomogeneous Griffiths regime near quantum criticality. We apply the method to the bilayer Heisenberg model with bond randomness and characterize both the broadening of excitations and the transfer of spectral weight induced by disorder. Inside the antiferromagnetic phase this model features the remarkable combination of sharp magnetic Bragg peaks and broad magnons, the latter arising from the tendency to localization of low-energy excitations.

5+9 pages, 5+8 figures; (v2) extended discussion, final version as published

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• Anomalous diffusion and localization in a positionally disordered quantum spin array
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• Dynamic structure factor of disordered quantum spin ladders
• Localized and propagating excitations in gapped phases of spin systems with bond disorder
• Quantum criticality in a three dimensional spin system at zero field and pressure
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• Inhomogeneous mean-field approach to collective excitations in disordered interacting bosons
• Amorphous quantum magnets in a two-dimensional Rydberg atom array
• Chemical composition induced quantum phase transition in Cs${}_{1-x}$Rb${}_x$FeCl${}_3$
• Limits to magnetic quantum criticality from nuclear spins
• Effect of disorder on a pressure-induced $z=1$ magnetic quantum phase transition
• Dynamic structure factor of Heisenberg bilayer dimer phases in the presence of quenched disorder and frustration
• Excitation Spectra and Hard-core Thermodynamics of Bosonic Atoms In Double Well Optical Lattices
• Critical Behavior and Collective Modes at the Superfluid Transition in Amorphous Systems
• Finite-temperature dynamics and the role of disorder in nearly critical Ni(Cl${}_{1-x}$Br${}_x$)${}_2$$\cdot$4SC(NH${}_2$)${}_2$
• Quantum Spin Liquids Stabilized by Disorder in Non-Kramers Pyrochlores