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Journal of Contemporary Physics (Armenian Academy of Sciences)

Quantum Entanglement in a Mixed Spin Trimer (1/2,1/2,1) with Non-Conserved Magnetization at Finite Temperatures

Zhirayr Adamyan

Published Sep 1, 2024• 2 citations
Publisher

Abstract

Abstract A model of a molecular magnet consisting of three magnetic ions with spins (1/2, 1/2, 1) and non-conserved magnetization arising due to the heterogeneity of Lande multipliers. This study aims to explain the properties of quantum entanglement of the states of the system during magnetization (demagnetization) processes at finite temperatures. We show how the presence of inhomogeneous g-factors results in a significant enhancement of quantum entanglement and sometimes the expansion of the temperature range in which the spins (pseudospins) of magnetic ions of the model are in entangled states.

DOI
10.1134/s1068337224700336

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Abstract A model of a molecular magnet consisting of three magnetic ions with spins (1/2, 1/2, 1) and non-conserved magnetization arising due to the heterogeneity of Lande multipliers. This study aims to explain the properties of quantum entanglement of the states of the system during magnetization (demagnetization) processes at finite temperatures. We show how the presence of inhomogeneous g-factors results in a significant enhancement of quantum entanglement and sometimes the expansion of the temperature range in which the spins (pseudospins) of magnetic ions of the model are in entangled states.
Mixed spin-(1/2,1/2,1) trimer with two different Landé g factors and two different exchange couplings is considered. The main feature of the model is nonconserving magnetization. The Hamiltonian of the system is diagonalized analytically. We presented a detailed analysis of the ground-state properties, revealing several possible ground-state phase diagrams and magnetization profiles. The main focus is on how nonconserving magnetization affects quantum entanglement. We have found that nonconserving magnetization can bring a continuous dependence of the entanglement quantifying parameter (negativity) on the magnetic field within the same eigenstate, while for the case of uniform g factors it is a constant. The main result is an essential enhancement of the entanglement in the case of uniform couplings for one pair of spins caused by an arbitrary small difference in the values of g factors. This enhancement is robust and brings almost a sevenfold increase of the negativity. We have also found a weakening of entanglement for other cases. Thus, nonconserving magnetization offers a broad opportunity to manipulate the entanglement by means of a magnetic field.
Abstract This work considers the spin-1/2 Heisenberg dimer, the mixed spin-(1/2,1) Heisenberg dimer and their quantum entanglement, particularly, the dependence of the entanglement on the system characteristics and the external magnetic field. The quantum entanglement in this work is determined using two different methods, namely ‘Negativity’ and ‘Concurrence’. The dependence of the corresponding quantities on spins Landé g-factors, system anisotropy constants and exchange constant is discussed.
Heisenberg model spin systems offer favourable and manageable physical settings for generating and manipulating entangled quantum states. In this work mixed spin-(1/2,1/2,1) Heisenberg spin trimmer with two different but isotropic Landé g-factors and two different exchange constants is considered. The study undertakes the task of finding the optimal parameters to create entangled states and control them by external magnetic field. The primary objective of this work is to examine the tripartite entanglement of a system and the dependence of the tripartite entanglement on various system parameters. Particularly, the effects of non-conserving magnetization are in the focus of our research. The source of non-commutativity between the magnetic moment operator and the Hamiltonian is non-uniformity of g-factors. To quantify the tripartite entanglement, an entanglement measure refered to as “tripartite negativity” has been used in this work.
Abstract A model of a spin-1/2 trimer coupled to a magnetic ion with spins ranging from 1/2 to 5/2 is introduced. We numerically calculate tripartite negativity of spin trimer. Numerical results show that the tripartite negativity has a nonzero plateau regardless of spin of the magnetic ion. We focus on tripartite entangled state corresponding to this plateau. As spin and exchange coupling of the magnetic ion increase, the tripartite entangled state becomes increasingly stable with respect to changes in external magnetic field and temperature. This phenomenon originates from the fact that internal magnetic field formed by the magnetic ion compensates the external magnetic field applied to spin trimer.
We numerically investigate thermal entanglement of the spins (1/2,1) and (1/2,1/2) in the three-mixed (1/2,1,1/2) anisotropic Heisenberg XXZ spin system on a simple triangular cell under an inhomogeneous magnetic field. We show that the external magnetic field induces strong plateau formation in pairwise thermal entanglement for fixed parameters of Hamiltonian in the case of the ferromagnetic and anti-ferromagnetic interactions. We also observe an unexpected critical point at finite temperature in the therm al entanglement of the spins (1/2,1) for antiferromagnetic case while the entanglement of the spins (1/2,1) for ferromagnetic case and the entanglement of the spins (1/2,1/2) for both ferromagnetic and antiferromagnetic cases almost decays exponentially to zero with increasing temperature. We conclude that this critical point in entanglement of the spi ns (1/2,1) for antiferromagnetic case indicates the presence of the quantum phase transition at finite temperature.
The quantum entanglement of the ground state and the thermal entanglement of the mixed thermal state of a three-qubit system with the Heisenberg interaction are investigated in detail in the presence of an inhomogeneous magnetic field. The concurrence of the model is compared with the magnetization, and the threshold temperature of the entanglement is discussed. The effects of the magnetic field and the coupling coefficient in the thermal entanglement are also examined by considering the concurrence of the three-qubit and two-qubit systems. We found that the thermal entanglement of the system exists at temperatures below the threshold temperature even if the ground state is unentangled.