Speaker
Description
The present work deals with the exactly solvable spin-1/2 Ising-Heisenberg model on an
infinite but regular two-dimensional lattice composed of identical inter-connected bipyramidal
plaquettes with the aim to clarify a bipartite entanglement between the Heisenberg spins at
zero as well as finite temperatures. The quantity called concurrence is used as an indicator for
determining a strength of this quantum-mechanical correlation.
It is demonstrated that the Heisenberg spins of each bipyramidal plaquette can be mutually
entangled at zero temperature only if the two-fold degenerate spontaneously ordered quantum
phase characterized by a symmetric quantum superposition of three possible up-up-down (or
down-down-up) states of these spins is stable. Otherwise, the bipartitite quantum
entanglement of the Heisenberg spins is totally absent. Interestingly, the entanglement
between the Heisenberg spin pairs persists also at finite temperatures, even far above the
critical temperature of the model if the exchange anisotropy between these spins is
sufficiently strong. The entangled Heisenberg spin states can also be thermally activated
above non-entangled ground state if the values of the exchange anisotropy parameter are
taken sufficiently close to the boundary with the quantum ground-state phase.
