Nature of spin excitations in fluctuating quantum magnets

Dynamical spin correlations as imaged by inelastic neutron scattering  
are an important source of information about the nature of elementary  
excitations in quantum magnets. We review numerical results based on  
exact diagonalization and Quantum Monte Carlo exploring the diversity  
of response features. We present three prototypical cases (S=1/2): i)  
The square lattice Heisenberg antiferromagnet, where there are well  
defined spin waves at low magnetic fields, which become unstable and  
decay into continua at high magnetic fields [1]. ii) Spin ladders in a  
magnetic field, where the coherent triplon fractionalizes into spinon  
continua as the spin gap is closed. Here we compare theoretical  
results with INS experiments performed on the Dpip ladder material  
[2]. And iii) the Heisenberg antiferromagnet on the kagome lattice,  
which is a highly frustrated quantum magnet with a magnetically  
disordered spin liquid ground state. The dynamical response is  
surprisingly broad with a characteristic increase towards low energies  
[3].

[1] A. Lüscher and A. Läuchli, The antiferromagnetic spin-1/2  
Heisenberg model on the square lattice in a magnetic field,
Phys. Rev. B 79, 195102 (2009).

[2] B. Thielemann et al., Direct Observation of Magnon  
Fractionalization in the Quantum Spin Ladder,
Phys. Rev. Lett. 102, 107204 (2009).

[3] A. Läuchli and C. Lhuillier, Dynamical Correlations of the Kagome  
S=1/2 Heisenberg Quantum Antiferromagnet,
arxiv:0901.1065