STIMULATED COOPERATIVE JAHN-TELLER EFFECT IN TMPO4

The idea of stimulating a cooperative Jahn-Teller effect in undercriti cal electronic systems using an external stress, which does not belong to the symmetry of the quadrupolar ordering, was recently suggested t heoretically. The rare-earth oxide compound TmPO4 (tetragonal zircon s tructure) is discussed here. The role of the external stress, a magnet ic field applied along the [100] axis, is to bring the delta-symmetry quadrupolar component to criticality by the reconstruction of both the levels spacings and the eigenfunctions. The thermodynamical propertie s are obtained by considering all the characteristics of the magnetoel astic couplings and of the crystalline electric field. The structural phase diagram exhibits reentrant transitions between the tetragonal an d orthorhombic symmetries. The measurements of the magnetic vector and of the magnetostriction induced by the magnetic field are well descri bed within this model. The simultaneous measurement of the components of the magnetic moment parallel and perpendicular to the field is an i nteresting experimental technique, efficient for the investigation of quadrupolar interactions.