772 research outputs found
Energy as witness of multipartite entanglement in spin clusters
We derive energy minima for biseparable states in three- and four-spin
systems, with Heisenberg Hamiltonian and s <= 5/2. These provide lower bounds
for tripartite and quadripartite entanglement in chains and rings with larger
spin number N. We demonstrate that the ground state of an -spin Heisenberg
chain is -partite entangled, and compute the energy gap with respect to
biseparable states for N <= 8
Detection of entanglement between collective spins
Entanglement between individual spins can be detected by using thermodynamics
quantities as entanglement witnesses. This applies to collective spins also,
provided that their internal degrees of freedom are frozen, as in the limit of
weakly-coupled nanomagnets. Here, we extend such approach to the detection of
entanglement between subsystems of a spin cluster, beyond such weak-coupling
limit. The resulting inequalities are violated in spin clusters with different
geometries, thus allowing the detection of zero- and finite-temperature
entanglement. Under relevant and experimentally verifiable conditions, all the
required expectation values can be traced back to correlation functions of
individual spins, that are now made selectively available by four-dimensional
inelastic neutron scattering
Entanglement in finite spin rings with noncollinear Ising interaction
We investigate the entanglement properties of finite spin rings, with
noncollinear Ising interaction between nearest neighbours. The orientations of
the Ising axes are determined either by the spin position within the ring
(model A) or by the direction of the bond (model B). In both cases, the
considered spin Hamiltonians have a point group symmetry, rather than a
translation invariance, as in spin rings with collinear Ising interaction. The
ground state of these models exhibit remarkable entanglement properties,
resembling GHZ-like states in the absence of an applied magnetic field (model
B). Besides, the application of an homogeneous magnetic field allows to modify
qualitatively the character of the ground state entanglement, switching from
multipartite to pairwise quantum correlations (both models A and B)
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