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Study of Macroscopic Quantum Effect in Ni4 Molecular Magnets

Quantum tunneling is one of the basic physical phenomena of particles, yet the phenomenon is rarely observed in the macroscopic system because of the environment-induced decoherence and the statistic average of different individuals. For the molecular magnet materials, the negligibly weak interactions between molecules make the molecules are highly identical and magnetically independent of each other, the factors which destroy the quantum tunneling effect in a macroscopic scale no longer exist, and therefore, the quantum tunneling effect is seen in the magnetization measurements. Study of this kind of macroscopic quantum tunneling effect may help us understand the difference and relationship between quantum and classical Rare earth magnets behaviors in the macroscopic scale. It was thought that the macroscopic quantum tunneling effect would be more complex or even disappear when the interactions between molecules cannot be ignored.Ni4 molecular magnet material is unique among the molecular magnet family because of the relatively strong exchange interactions between molecules, which lead to an antiferromagnetic phase transition at 0.91K as observed in our measurements of heat capacity and magnetic ac susceptibility. Beyond our expectation,this material shows a clear, simple macroscopic quantum tunneling pattern that the resonant quantum tunneling occurs at certain fields of equal intervals.We have proposed a simple picture to explain the unique quantum tunneling behavior of Ni4 molecular magnets.

The essence of this picture is that whether the quantum tunneling takes place depends not only on the spin state of the molecule under consideration, but also on the spin states of its nearest neighbors antiferromagnetically coupled to it. The Hamiltonian of the molecule is hereby presented as where Sz,i represents the z components of spin operator for their nearest neighbors,J is the exchange energy constant. In this context, quantum tunneling occurs only when the energies of molecules before and after spin-flipping. calculated from the above Hamiltonian, do not have differences. we have calculated the exchange interaction constant J=0.019K from our the resonant magnetic fields data. Besides, based on the above picture, we have reasonably explained the following two experimental phenomena: the disappearing of quantum tunneling at zero field, and the dependence of the resonant quantum tunneling upon the initial state of the material.Our picture gives the following predictions. Firstly, the resonant quantum tunneling at 0.11T or-0.11T may well disappear if the initial state is antiferromagnetic phase at zero field. Secondly, if we scan the field from a saturation field to the resonant field of 0.11T with a canning rate high enough to avoid the quantum tunneling at the resonant field of 0.21T, a self-avoid quantum walk will happen at the resonant field of 0.11T. This behavior may be used for detecting weak transient signals. Also as a real system demonstrating the self-avoid quantum random walk, it may provide some feasible way to build novel quantum algorithms to enhance the computing power.

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