Interplay between different degrees of freedom:: unusual physical phenomena in Ca3Ru2O7

Ca3Ru2O7, a Mott-like system with a narrow charge gap of 0.1 eV, is characterized by strong coupling between different degrees of freedom and various magnetic and electronic transitions that are abrupt and highly anisotropic. In this paper, we report a host of physical phenomena including: (1) A collapse of the c-axis lattice parameter at a metal-nonmetal transition, T-MI (=48 K), and a rapid increase of T-MI with low uniaxial pressure applied along the c-axis, suggesting that the shortening of the c-axis is critical to the antiferromagnetic nonmetallic state. (2) A 90degrees rotation of the easy-axis for magnetization in the vicinity of T-MI, implying a drastic change in spin-orbit coupling through the lattice degree of freedom. (3) Anomalous angular dependence of magnetization and resistivity, indicating an unusually large anisotropy and a strong spin-charge coupling. (4) Quantum oscillations in the gapped, nonmetallic state for 20 mK < T < 6.5 K; (5) Tunneling colossal magnetoresistance due in part to the magnetic valve-like effect, which yields a precipitate drop in resistivity. (6) Different in-plane anisotropies of colossal magnetoresistance and magnetization, suggesting that the electron scattering mechanism is not entirely driven by spins. It is believed that the unusually strong coupling between lattice, orbital, charge and spin degrees of freedom primarily dictates the exotic ground states. (C) 2003 Elsevier Science B.V. All rights reserved.