Study of the quantum properties and dissipative phenomena in molecular

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The magnetic properties of molecular clusters attract considerable interest for the new physics involved and for the potential applications. At low temperatures, these molecules act as individual quantum nanomagnets, enabling to probe, at the macroscopic scale, the crossover between quantum and classical physics. Of fundamental interest is the situation of degeneracy of two magnetic levels, where quantum mechanical phenomena such as tunneling or coherence can occur. The situation of degeneracy between levels raises fundamental problems of quantum dynamics. A crucial issue is the role played by the coupling between magnetic molecular levels and the environment such as phonons. The purpose of this project is to verify experimentally the characteristic quantum energy levels and discuss the possible interplay between the anomalous magnetic relaxation phenomena and the level-crossings. For this purpose, a variety of model substances were synthesized by our group and some of them were kindly … More supplied by chemists. The representatives are dimers (binuclear complexes) such as Cs3Cr2C19 with X=Cl, Br, trimers (trinuclear complexes) such as A3Cu3(PO4)4 with A=Ca, Sr, trimerized chain system 3CuCl2・2dx, polynuclear molecular magnets such as Mn12, Fe12, V15 and so on We applied various complementary techniques, including advanced crystal preparation, nuclear magnetic resonance, electron spin resonance, neutron scattering, muon spin relaxation, high field magnetization as well as susceptibility and specific heat measurements, in cooperation with many other research groups. A high-field magnetization technique was applied to study the field-dependent behaviors of various kind of quantum spin systems by applying high magnetic field up to 54 T at temperature down to 0.1 K.For example, we observed successive phase transitions at three restricted field regions up to 40T in the S=2/3 dimerized system Cs3Cr2Cl9. Also, in Fe12 molecular magnet which forms a finite chain ring consisting of 12 Fe3+(S=5/2) ions, we observed five magnetization steps with equal field spacing, arising from energy-level crossings for the discrete quantum states in the finite spin system. In these examples, we discussed the structure of the energy levels an d the relaxation phenomena due to quantum spin dynamics in the metastable states around the level-crossing fields. Also, a high-frequency electron spin resonance (ESR) technique was applied for our purpose, in the wide temperature range down to 0.5 K, using high frequency sources up to 800 GHz. Examples are for the S=1/2 Heisenberg anttiferromagnetic chain system Cu-benzoate, Cu(C6H5COO)2・3H2O and for the trimerized chain system 3CuCl2・2dx, in which ferromagnetic trimers consisting of three S=1/2 spins coupled antiferromagnetically in one dimension.These results partly have been published already. Less