A Study of Ordering Process in Low-Dimensional Hubbard Systems by Controlling Electron Correlation under High Pressure

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In order to make experimental research on the physics in low- imensional Hubbard model, we have studied magnetic bebavior of low-dimensional organic free radicals under hydrostatic pressure and magnetic field. Magnetic and thermal properties of two kinds of organic free radicals have been used first ; TANOL (2, 2, 6, 6-tetramethyl-4-pipreridinol-l-oxyl) and TPV (1, 3, 5-triphenylverdazyl), where the unpaired electrons are localized on NO bonds in the former compound and delocalized on the larger molecular orbitals in the latter. TAN0L has been revealed to be approximated magnetically as a typical one-dimensional Heisenberg system with localized quantum spins coupled with the exchange constant J/k_B =-5.2K. While, the experimental results of TPV can not be understood so reasonably on the localized model as to satisfy the possible paths of magnetic interactions such as railroad trestle lattice. It is suggested that organic free radicals offer promising real systems for getting the essential physical phenomena to check the model Hamiltonians for delocalized and frustrated quantum systems. The pressure effect on the magnetic behavior of p-Cl-BDPA has been also studied and the possibility of controlling the electron correlation by the pressurization has been examined.