In the present work, we investigate the quantum thermal entanglement in molecular magnets composed of dimers of spin S , using an Entanglement Witness built from measurements of magnetic susceptibility. An entanglement temperature, T e , is then obtained for some values of spin S . From this, it is shown that T e is proportional to the intradimer exchange interaction J and that entanglement appears only for antiferromagnetic coupling. The results are compared to experiments carried on three isostructural materials: KNaMSi 4 O 10 (M=Mn, Fe or Cu).
Entanglement has been extensively studied from the perspective of Quantum Information Processing for the last decade and various measures of entanglement, to quantify it are now available. Experimentally also it has been possible to measure entanglement in pure state, mostly in optics. However, in recent years there has been significant development in Quantum Condensed Matter systems, especially spin chains, where extraction of entanglement has been made possible through macroscopic witness operators like magnetic susceptibility. In this work, we have studied the magnetic properties of some spin chains, where the exchange interaction can be tuned by preparation technique and we have extracted a magnetic field and temperature dependence of entanglement from susceptibility and magnetization data taken at various temperatures and fields. The spin chains considered here fall in two categories, first one is an alternating dimer model and the second is an infinite spin chain. We have given the experimental plots of entanglement as a function of temperature and magnetic fields and compared them with the theoretical plots.
The entanglement thermal (equilibrium) properties in spin-1/2 Ising-Heisenberg model on an ideal diamond chain is analyzed. Due to the classical character of Ising-type exchange interactions between two neighboring antiferromagnetic Heisenberg dimers, the calculation of quantum entanglement (by tracing out Ising spins) can be performed for each of the dimers separately. The concurrence, as a measure of entanglement is obtained and different regimes depending on the values of exchange interactions are revealed. The effects of the magnetic field are incorporated and critical temperatures corresponding to the vanishing or arising of entanglement are considered.
Detection of entangled states from experimental magnetic susceptibility is reported for an organic radical. The present system exemplifies an ideal spin ½ chain with full isotropy. Experimental susceptibility data are modelled by the formulation described by Bonner and Fisher. An explicit correlation is established between the molecular structure and the spin chain interaction. Using experimental magnetic susceptibility as entanglement witness, entanglement is observed to exist up to 28 K in the present system. Evolution of entanglement with magnetic field is also studied and a surface plot is generated, which clearly exhibits the variation of entanglement with magnetic field and temperature.
Exchange interactions in spin systems can give rise to quantum entanglement in the ground and thermal states of the systems. We consider a spin tetramer, with spins of magnitude
21, in which the spins interact via nearest-neighbor, diagonal, and four-spin interactions of strength
J1,
J2, and
K, respectively. The ground- and thermal-state entanglement properties of the tetramer are calculated analytically in the various limiting cases. Both bipartite and multipartite entanglements are considered and a signature of the quantum phase transition (QPT), in terms of the entanglement ratio, is identified. The first-order QPT is accompanied by discontinuities in the nearest-neighbor and diagonal concurrences. The magnetic properties of a
S=21 antiferromagnetic polyoxovanadate compound V12 are well explained by tetramers, with
J2=0,
K=0, in which the spins interact via the isotropic Heisenberg exchange interaction Hamiltonian. Treating the magnetic susceptibility $\ensuremath{\chi}$ as an entanglement witness (EW), an estimate of the lower bound of the critical entanglement temperature
Tc below which entanglement is present in the experimental compound, is determined. Two other cases considered include the symmetric tetramer---i.e., tetrahedron
(J1=J2,K=0)---and the symmetric trimer. In both the cases, there is no entanglement between a pair of spins in the thermal state but multipartite entanglement is present. A second EW based on energy provides an estimate of the entanglement temperature
TE below which the thermal state is definitely entangled. This EW detects bipartite entanglement in the case of the tetramer describing a square of spins (the case of V12 ) and multipartite entanglement in the cases of the tetrahedron and symmetric trimer.
This paper investigates the entanglement in the supermolecular dimer [Mn 4 ] 2 consisting of a pair of single molecular magnets with antiferromagnetic exchange-coupling J . The conventional von Neumann entropy as a function of the exchange-coupling is calculated explicitly for all eigenstates with the quantum number range from M = M 1 + M 2 = −9 to 0. It is shown that the von Neumann entropy is not a monotonic function of the coupling strength. However, it is significant that the entropy of entanglement has the maximum values and the minimum values for most eigenstates, which is extremely useful in the quantum computing. It also presents the time-evolution of entanglement from various initial states. The results are useful in the design of devices based on the entanglement of two molecular magnets.
We study the entanglement in Heisenberg dimer single molecular magnets V 4+ with a strong magnetic field by means of the measure of entanglement called ``concurrence'' and find that thermal entanglement exists for both the ferromagnetic (FM) and antiferromagnetic (AFM) cases. In the FM case, entanglement vanishes for anisotropic parameter λ = 0 while in the AFM case, entanglement exits in the whole region of anisotropic parameter 0⩽λ⩽1. Entanglement variations with anisotropic parameter λ, magnetic field B and temperature T are evaluated. We find a critical magnetic field at which the concurrence changes sharply from zero to maximum entanglement.