Anisotropic Cooperative Jahn-Teller Effect and Metamagnetism in PrCu₂

We have studied the origin of the cooperative Jahn-Teller effect and the 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 increasing the field, the Jahn-Teller transition temperature increases slightly for the field along a- and c-axes, while it decreases intensively along the b-axis. The phase diagram can be explained quantitatively by including the quadrupolar interaction of -K_JT <O_xy> O_xy in the crystalline electric field and Zeeman Hamiltonian. The metamagnetic transition, which brings about the conversion between the easy (a)- and hard (c)-axes magnetization, has been also studied through the magnetostriction. This phenomenon is also explained quantitatively on the basis of the quadrupolar interaction of -K_M <O₂²> O₂². The sign of <O₂²> is found to be negative in the state where the c-axis is the hard one, while it changes from negative to positive above the metamagnetic transition field.