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Communications in Theoretical Physics

Macroscopic Quantum Coherence in an Antiferromagnetic Spin Chain with Biaxial Anisotropy

Zhao Qun-Chao, Gong-Ping Zheng, Zheng Wang

Published Aug 1, 2009• 0 citations
Publisher

Abstract

Instanton configurations of (1+1)-dimensions in an antiferromagnetic biaxial-anisotropy-spin-chain are obtained explicitly in the strong anisotropy limit, which interpolate between degenerate equilibrium orientations of the Néel vector along easy axis and are seen to be responsible for quantum tunneling. Macroscopic quantum coherence of the domain walls is demonstrated in terms of the instantons.

DOI
10.1088/0253-6102/52/2/34

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Instanton configurations of (1+1)-dimensions in an antiferromagnetic biaxial-anisotropy-spin-chain are obtained explicitly in the strong anisotropy limit, which interpolate between degenerate equilibrium orientations of the Néel vector along easy axis and are seen to be responsible for quantum tunneling. Macroscopic quantum coherence of the domain walls is demonstrated in terms of the instantons.
We investigate the macroscopic quantum spin coherence of a spinor condensate confined in an anisotropic potential. Under the single-mode approximation, we show that a spin-1 Rb condensate can be modeled as a biaxial quantum magnet. A direct consequence of the biaxial anisotropy is that the tunneling splitting oscillates as a function of the external magnetic field applied along the hard axis. We also propose an experimental scheme to detect the oscillatory behavior of the tunneling splitting by employing the Landau-Zener tunneling.
Macroscopic quantum coherence oscillations of the N\'eel vector are proved to appear in the antiferromagnetic nuclear spin chain of eight 129Xe{}^{129}\mathrm{Xe}129Xe atoms placed on a magnetic surface. The oscillation period at zero temperature is calculated numerically as a function of the chain constant. The environmental decoherence effects at finite temperature are accounted assuming a dipole coupling between the spin chain and the fluctuating magnetic field of the surface. With this coupling the N\'eel vector oscillations are damped by a rate $(N\ensuremath{-}1)/\ensuremath{\tau},$ where N is the number of spins and $\ensuremath{\tau}$ is the relaxation time of a single spin.
Antiferromagnetic Heisenberg integer-spin chains are characterized by a spin-liquid ground state with no long-range order, due to the relevance of quantum fluctuations. Spin anisotropy, however, freezes quantum fluctuations, and the system is magnetized in the presence of a sufficiently large easy-axis anisotropy. We numerically investigate the case S=1, by means of the density-matrix renormalization group, and find that the freezing of the spin liquid into a Néel spin solid, with increasing easy-axis anisotropy, is a continuous quantum phase transition. Numerical evidence indicates that the transition is not in the two-dimensional Ising universality class.
We investigate one-dimensional S=1/2S=1/2S=1/2 and S=1S=1S=1 antiferromagnetic quantum-spin chains with easy-axis anisotropies in a transverse field. We calculate both the uniform magnetization and the staggered magnetization, using a variant of the density-matrix renormalization-group method. We find that the staggered magnetizations exhibit reentrant behavior as functions of the transverse field, where the competition between the classical N\'eel order and the quantum fluctuations plays an important role. We also discuss the critical behavior associated with the staggered magnetizations.
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A distinct integrable quantum-spin model is proposed. The model has a biaxial magnetic anisotropy of alternating coupling between spins together with multiple spin interactions. Our model gives the possibility to exactly find thermodynamic characteristics of the considered spin chain. The ground state of the model can reveal spontaneous values of the total magnetic and antiferromagnetic moments caused by multiple spin couplings. Also, in the ground state, depending on the strength of multiple spin couplings, our model manifests several quantum critical points, some of which are governed by the external magnetic field.