Dynamics of a Complex Quantum Magnet

TL;DR

This paper computes low energy states and dynamical susceptibility of large complex quantum spin systems, revealing coherent excitations and unusual spectral properties due to strong interactions.

Contribution

It provides exact results for larger system sizes than previously possible, showing how strong interactions lead to coherent excitations in disordered quantum magnets.

Findings

Ground state is a superposition of many classical states

Dynamical susceptibility shows sharp resonances

Energy spectra depend on system size differently from random graphs

Abstract

We have computed the low energy quantum states and low frequency dynamical susceptibility of complex quantum spin systems in the limit of strong interactions, obtaining exact results for system sizes enormously larger than accessible previously. The ground state is a complex superposition of a substantial fraction of all the classical ground states, and yet the dynamical susceptibility exhibits sharp resonances reminiscent of the behavior of single spins. These results show that strongly interacting quantum systems can organize to generate coherent excitations and shed light on recent experiments demonstrating that coherent excitations are present in a disordered spin liquid. The dependence of the energy spectra on system size differs qualitatively from that of the energy spectra of random undirected bipartite graphs with similar statistics, implying that strong interactions are giving rise to these unusual spectral properties.