ANISOTROPIC COOPERATIVE JAHN-TELLER EFFECT AND METAMAGNETISM IN PRCU2

We have studied the origin of the cooperative Jahn-Teller effect and t he metamagnetic transition in the orthorhombic compound PrCu2. We have determined the anisotropic ferroquadrupole phase diagram by measuring the specific heat and thermal expansion under magnetic fields. With i ncreasing the field, the Jahn-Teller transition temperature increases slightly for the field along a-and c-axes, while It decreases intensiv ely along the b-axis. The phase diagram can he explained quantitativel y by including the quadrupolar interaction of -K-JT (O-xy) O-xy in the crystalline electric field and Zeeman Hamiltonian. The metamagnetic t ransition, which brings about the conversion between the easy (a)- and hard (c)-axes magnetization, has been also studied through the magnet ostriction. This phenomenon is also explained quantitatively on the ba sis of the quadrupolar interaction of -K-M (O-2(2)) O-2(2). The sign o f (O-2(2)) is found to be negative in the state where the c-axis is th e hard one, while it changes from negative to positive above the metam agnetic transition field.